Dissertations / Theses on the topic 'CFD; Dynamics; Heat transfer'

Heat exchanger fouling is the deposition of material onto the heat transfer surface causing a reduction in thermal efficiency. A study using Computational Fluid Dynamics (CFD) was conducted to increase understanding of key aspects of fouling in desalination processes. Fouling is a complex phenomenon and therefore this numerical model was developed in stages. Each stage required a critical assessment of each fouling process in order to design physical models to describe the process???s intricate kinetic and thermodynamic behaviour. The completed physical models were incorporated into the simulations through employing extra transport equations, and coding additional subroutines depicting the behaviour of the aqueous phase involved in the fouling phenomena prominent in crystalline streams. The research objectives of creating a CFD model to predict fouling behaviour and assess the influence of key operating parameters were achieved. The completed model of the key crystallisation fouling processes monitors the temporal variation of the fouling resistance. The fouling rates predicted from these results revealed that the numerical model satisfactorily reproduced the phenomenon observed experimentally. Inspection of the CFD results at a local level indicated that the interface temperature was the most influential operating parameter. The research also examined the likelihood that the crystallisation and particulate fouling mechanisms coexist. It was found that the distribution of velocity increased the likelihood of the particulate phase forming within the boundary layer, thus emphasizing the importance of differentiating between behaviour within the bulk and the boundary layer. These numerical results also implied that the probability of this composite fouling was greater in turbulent flow. Finally, supersaturation was confirmed as the key parameter when precipitation occurred within the bulk/boundary layer. This investigation demonstrated the advantages of using CFD to assess heat exchanger fouling. It produced additional physical models which when incorporated into the CFD code adequately modeled key aspects of the crystallisation and particulate fouling mechanisms. These innovative modelling ideas should encourage extensive use of CFD in future fouling investigations. It is recommended that further work include detailed experimental data to assist in defining the key kinetic and thermodynamic parameters to extend the scope of the required physical models.

The main purpose of the dissertation is to contribute to the development of numerical techniques for computational heat transfer and fluid flow, suitable for low cost (loosely coupled) parallel computers. It is focused on implicit integration schemes, using finite control volumes with multigrid (MG) algorithms.

Natural convection in closed cavities is used as a problem model to introduce different aspects related with the integration of the incompressible Navier-Stokes equations, such as the solution of the pressure correction (or similar) equations that is the bottleneck of the algorithms for parallel computers. The main goal of the dissertation has been to develop new algorithms to advance in the solution of this problem rather than to implement a complete parallel CFD code.

An overview of different sequential multigrid algorithms is presented, pointing out the difference between geometric and algebraic multigrid. A detailed description of segregated ACM is given. The direct simulation of a turbulent natural convection flow is presented as an application example. A short description of the coupled ACM variant is given.

Background information of parallel computing technology is provided and the the key aspects for its efficient use in CFD are discussed. The limitations of low cost, loosely coupled cost parallel computers (high latency and low bandwidth) are introduced. An overview of different control-volume based PCFD and linear equation solvers is done. As an example, a code to solve reactive flows using Schwartz Alternating Method that runs particularly well on Beowulf clusters is given.

Different alternatives for latency-tolerant parallel multigrid are examined, mainly the DDV cycle proposed by Brandt and Diskin in a theoretical paper. One of its main features is that, supressing pre-smoothing, it allows to reduce the each-to-neighbours communications to one per MG iteration. In the dissertation, the cycle is extended to two-dimensional domain decompositions. The effect of each of its features is separately analyzed, concluding that the use of a direct solver for the coarsest level and the overlapping areas are important aspects. The conclusion is not so clear respect to the suppression of the pre-smoothing iterations.

A very efficient direct method to solve the coarser MG level is needed for efficient parallel MG. In this work, variant of the Schur complement algorithm, specific for relatively small, constant matrices has been developed. It is based on the implicit solution of the interfaces of the processors subdomains. In the implementation proposed in this work, a parallel evaluation and storage of the inverse of the interface matrix is used. The inner nodes of each domain are also solved with a direct algorithm. The resulting algorithm, after a pre-processing stage, allows a very efficient solution of pressure correction equations of incompressible flows in loosely coupled parallel computers.

Finally, all the elements presented in the work are combined in the DDACM algorithm, an algebraic MG equivalent to the DDV cycle, that is as a combination of a parallel ACM algorithm with BILU smoothing and a specific version of the Schur complement direct solver. It can be treated as a black-box linear solver and tailored to different parallel architectures.

The parallel algorithms analysed (different variants of V cycle and DDV) and developed in the work (a specific version of the Schur complement algorithm and the DDACM multigrid algorithm) are benchmarked using a cluster of 16 PCs with a switched 100 Mbits/s network.

The general conclusion is that the algorithms developed are suitable options to solve the pressure correction equation, that is the main bottleneck for the solution of implicit flows on loosely coupled parallel computers.

Designing buildings to use energy more efficiently can lead to lower energy costs, while maintaining comfort for occupants. Computational fluid dynamics (CFD) can be utilized to visualize and simulate expected flows in buildings and structures. CFD gives architects and designers the ability to calculate the velocity, pressure, and heat transfer within a building. Previous research has not modeled natural ventilation situations that challenge common design rules of thumb used for cross-ventilation and single-sided ventilation. The current study uses a commercial code (FLUENT) to simulate cross-ventilation in simple structures and analyzes the flow patterns and heat transfer in the rooms. In the Casa Giuliana apartment and the Affleck house, this study simulates passive cooling in spaces well-designed for natural ventilation. Heat loads, human models, and electronics are included in the apartment to expand on prior research into natural ventilation in a full-scale building. Two different cases were simulated. The first had a volume flow rate similar to the ambient conditions, while the second had a much lower flow rate that had an ACH of 5, near the minimum recommended value Passive cooling in the Affleck house is simulated and has an unorthodox ventilation method; a window in the floor that opens to an exterior basement is opened along with windows and doors of the main floor to create a pressure difference. In the Affleck house, two different combinations of window and door openings are simulated to model different scenarios. Temperature contours, flow patterns, and the air changes per hour (ACH) are explored to analyze the ventilation of these structures.
Master of Science

Computational Fluid Dynamics (CFD) is one of the fields that has strongly developed since the recent development of faster computers and numerical modeling. CFD is also finding its way into chemical engineering on several levels. We have used CFD for detailed modeling of heat and mass transfer in a packed bed. One of the major questions in CFD modeling is whether the computer model describes reality well enough to consider it a reasonable alternative to data collection. For this assumption a validation of CFD data against experimental data is desired. We have developed a low tube to particle, structured model for this purpose. Data was gathered both with an experimental setup and with an identical CFD model. These data sets were then compared to validate the CFD results. Several aspects in creating the model and acquiring the data were emphasized. The final result in the simulation is dependent on mesh density (model detail) and iteration parameters. The iteration parameters were kept constant so they would not influence the method of solution. The model detail was investigated and optimized, too much detail delays the simulation unnecessarily and too little detail will distort the solution. The amount of data produced by the CFD simulations is enormous and needs to be reduced for interpretation. The method of data reduction was largely influenced by the experimental method. Data from the CFD simulations was compared to experimental data through radial temperature profiles in the gas phase collected directly above the packed bed. It was found that the CFD data and the experimental data show quantitatively as well as qualitatively comparable temperature profiles, with the used model detail. With several systematic variances explained CFD has shown to be an ample modeling tool for heat and mass transfer in low tube to particle (N) packed beds.

In this thesis. a three dimensional heat transfer model of heated airflow through the upper human respiratory tract consisting of nasal, oral, trachea, and the first two generations of bronchi is developed using computational fluid dynamics simulation software. Various studies have been carried out in the literature investigating the heat and mass transfer characteristics in the upper human respiratory tract, and the study focuses on assessing the injury taking place in the upper human respiratory tract and identifying acute tissue damage based on level of exposure. The model considered is for the simultaneous oronasal breathing during the inspiration phase with high volumetric flow rate of 90/liters minute and a surrounding air temperature of 100 degrees centigrade. The study of the heat and mass transfer, aerosol deposition and flow characteristics in the upper human respiratory tract using computational fluid mechanics simulation requires access to a two dimensional or three dimensional model for the human respiratory tract. Depicting an exact model is a complex task since it involves the prolonged use of imaging devices on the human body. Hence a three dimensional geometric representation of the human upper respiratory tract is developed consisting of nasal cavity, oral cavity, nasopharynx, pharynx, oropharynx, trachea and first two generations of the bronchi. The respiratory tract is modeled circular in cross-section and varying diameter for various portions as identified in this study. The dimensions are referenced from the literature herein. Based on the dimensions, a simplified model representing the human upper respiratory tract is generated.This model will be useful in studying the flow characteristics and could assist in treatment of injuries to the human respiratory tract as well as help optimize drug delivery mechanism and dosages. Also a methodology is proposed to measure the characteristic dimension of the human nasal and oral cavity at the inlet/outlet points which are classified as internal measurements.

The main objective of this Master Thesis project is the development of a new methodology to perform Computational Fluid Dynamics (CFD) conjugate heat transfer simulations for internal combustion engines, at the Fluid and Combustion Simulations Department (NMGD) at Scania CV AB, Södertalje, Sweden. This new method allows to overcome the drawbacks identified in the former methodology, providing the ability to use the more advanced polyhedral mesh type to generate good quality grids in complex geometries like water cooling jackets, and integrating all the different components of the engine cylinder in one unique multi-material mesh. In the method developed, these advantages can be used while optimizing the process to perform the simulations, and obtaining improved accuracy in the temperature field of engine components surrounding the water cooling jacket when compared to the experimental data from Scania CV AB tests rigs. The present work exposes the limitations encountered within the former methodology and presents a theoretical background to explain the physics involved, describing the computational tools and procedures to solve these complex fluid and thermal problems in a practical and cost-effective way, by the use of CFD.A mesh sensitivity analysis performed during this study reveals that a mesh with low y+ values, close to 1 in the water cooling jacket, is needed to obtain an accurate temperature distribution along the cylinder head, as well as to accurately identify boiling regions in the coolant domain. Another advantage of the proposed methodology is that it provides new capabilities like the implementation of thermal contact resistance in periodical contact regions of the engine components, improving the accuracy of the results in terms of temperature profiles of parts like valves, seats and guides. The results from this project are satisfactory, providing a reliable new methodology for multi-material thermal simulations, improving the efficiency of the work to be performed in the NMGD department, with a better use of the available engineering and computational resources, simplifying all the stages of multi-material projects, from the geometry preparation and meshing, to the post-processing tasks.

In this study, forced cooling of heat sinks mounted on CPU&rsquo
s was investigated. Heat sink effectiveness, effect of turbulence models, effect of radiation heat transfer and different heat sink geometries were numerically analyzed by commercially available computational fluid dynamics softwares Icepak and Fluent. The numerical results were compared with the experimental data and they were in good agreement. Conjugate heat transfer is simulated for all the electronic cards and packages by solving Navier-Stokes equations. Grid independent, well converged and well posed models were run and the results were compared. The best heat sink geometry is selected and it is modified in order to have lower maximum temperature distribution in the heat sink.

During the impinging heat transfer, a jet of working fluid, either gas or liquid, will besprayed onto the heat transfer surface. Due to the high turbulence of the fluid, the heat transfer coefficient between the wall and the fluid will be largely enhanced. Previously, an impinging type solar receiver with a cylindrical cavity absorber was designed for solar dish system. However, non-uniform temperature distribution in the circumferential direction was found on absorber surface from the numerical model, which will greatly limit receiver's working temperature and finally affect receiver's efficiency. One of the possible alternatives to solve the problem is through modifying the roughness of the target wall surface. This thesis work aims to evaluate the possibility and is focusing on the study of heat transfer characteristics. The simulation results will be used for future experimental impinging solar receiver optimization work. Computational Fluid Dynamics (CFD) is used to model the conjugate heat transfer phenomenon of atypical air impinging system. The simulation is divided into two parts. The first simulation was conducted with one rib arranged on the target surface where heat transfer coefficient is relatively low to demonstrate the effects of rib shape (triangular,rectangular, and semi-circular) and rib height (2.5mm, 1.5mm, and 0.5mm). The circular rib with 1.5mm height is proved to be most effective among all to acquirerelatively uniform temperature distribution. In the second part, the amount of ribs is taken into consideration in order to reach more uniform surface heat flux. The target wall thickness is also varied to assess its influence.

The U.S. Evolutionary Power Reactor (EPR) is a new, large-scale pressurized water reactor made by AREVA NP Inc. Surrounding the core of this reactor is a steel wall structure sitting inside called the heavy reflector. The purpose of the heavy reflector is to reduce the neutron flux escaping the core and thus increase the efficiency of the reactor while reducing the damage to the structures surrounding the core as well. The heavy reflector is heated due to absorption of the gamma radiation, and this heat is removed by the water flowing through 832 cooling channels drilled through the heavy reflector. In this project, the temperature distribution in the heavy reflector was investigated to ascertain that the maximum temperature does not exceed the allowable temperature of 350 ºC, with the intent of modifying the flow distribution in the cooling channels to alleviate any hot spots. The analysis was conducted in two steps. First, the flow distribution in the cooling channels was calculated to test for any maldistribution. The temperature distribution in the heavy reflector was then calculated by simulating the conjugate heat transfer with this flow distribution as the coolant input. The turbulent nature of the flow through the cooling channels made the calculation of the flow distribution computationally expensive. In order to resolve this problem, a simplification method using the â equivalent flow resistanceâ was developed. The method was validated by conducting a few case studies. Using the simplified model, the flow distribution was calculated and was found to be fairly uniform. The conjugate heat transfer calculation was conducted. The same simplification method used in the flow distribution analysis could not be applied to this calculation; therefore, the computational cost of this model was reduced by lowering the grid density in the fluid region. The results showed that the maximum temperature in the heavy reflector is 347.7 ºC, which is below the maximum allowable temperature of 350 ºC. Additional studies were conducted to test the sensitivity of the maximum temperature with change in the flow distribution in the cooling channels. Through multiple calculations, the maximum temperature did not drop more than 3 ºC; therefore, it was concluded that the flow distribution in the cooling channels does not have significant effect on the maximum temperature in the heavy reflector.
Master of Science

Dense fluid-particulate systems are widely encountered in the pharmaceutical, energy, environmental and chemical processing industries. Prediction of the heat transfer characteristics of these systems is challenging. Use of a high fidelity Discrete Element Method (DEM) for particle scale simulations coupled to Computational Fluid Dynamics (CFD) requires large simulation times and limits application to small particulate systems.  The overall goal of this research is to develop and implement parallelization techniques which can be applied to large systems with O(105- 106) particles to investigate particle scale heat transfer in rotary kiln and fluidized bed environments. The strongly coupled CFD and DEM calculations are parallelized using the OpenMP paradigm which provides the flexibility needed for the multimodal parallelism encountered in fluid-particulate systems. The fluid calculation is parallelized using domain decomposition, whereas N-body decomposition is used for DEM. It is shown that OpenMP-CFD with the first touch policy, appropriate thread affinity and careful tuning scales as well as MPI up to 256 processors on a shared memory SGI Altix. To implement DEM in the OpenMP framework, ghost particle transfers between grid blocks, which consume a substantial amount of time in DEM, are eliminated by a suitable global mapping of the multi-block data structure. The global mapping together with enforcing perfect particle load balance across OpenMP threads results in computational times between 2-5 times faster than an equivalent MPI implementation. Heat transfer studies are conducted in a rotary kiln as well as in a fluidized bed equipped with a single horizontal tube heat exchanger. Two cases, one with mono-disperse 2 mm particles rotating at 20 RPM and another with a poly-disperse distribution ranging from 1-2.8 mm and rotating at 1 RPM are investigated. It is shown that heat transfer to the mono-disperse 2 mm particles is dominated by convective heat transfer from the thermal boundary layer that forms on the heated surface of the kiln. In the second case, during the first 24 seconds, the heat transfer to the particles is dominated by conduction to the larger particles that settle at the bottom of the kiln. The results compare reasonably well with experiments. In the fluidized bed, the highly energetic transitional flow and thermal field in the vicinity of the tube surface and the limits placed on the grid size by the volume-averaged nature of the governing equations result in gross under prediction of the heat transfer coefficient at the tube surface. It is shown that the inclusion of a subgrid stress model and the application of a LES wall function (WMLES) at the tube surface improves the prediction to within ± 20% of the experimental measurements.
Ph. D.

This master thesis was provided by Scania AB. The objective of this thesis was to modify an application in the free Computational Fluid Dynamics software OpenFOAM to be able to handle spray and wall film modeling of a Urea Water Solution together with Conjugate Heat Transfer. The basic purpose is to widen the knowledge of the vaporization process of a Urea Water Solution in the exhaust gas after treatment system for a diesel engine by using OpenFOAM. First, urea has been modeled as a very viscous liquid at low temperature to mimic the solidication process of urea. Second, the development of the new application has been done. At last, test simulations of a simple test case are performed with the new application. The results are then compared with simplied hand calculations to verify a correct behavior of certain exposed source terms. The new application is working properly for the test case but to ensure the reliability, the results need to be compared with another Computational Fluid Dynamics software or more preferable, real experiments. For more advanced geometries, the continued development presented last in this thesis is highly recommended to follow.

While the gas turbine research community is continuously pursuing development of higher cyclic efficiency designs by increasing the combustor firing temperatures and thermally resistant turbine vane / blade materials, a simultaneous effort to reduce the emission levels of high temperature driven thermal NOX also needs to be addressed. Lean premixed combustion has been found as one of the solutions to these objectives. However, since less amount of air is available for backside cooling of liner walls, it becomes very important to characterize the convective heat transfer that occurs on the inside wall of the combustor liners. These studies were explored using laboratory scale experiments as well as numerical approaches for several inlet flow conditions under both non-reacting and reacting flows. These studies may be expected to provide valuable insights for the industrial design communities towards identifying thermal hot spot locations as well as in quantifying the heat transfer magnitude, thus aiding in effective designs of the liner walls. Lean premixed gas turbine combustor flows involve strongly coupled interactions between several aspects of physics such as the degree of swirl imparted by the inlet fuel nozzle, premixing of the fuel and incoming air, lean premixed combustion within the combustor domain, the interaction of swirling flow with combustion driven heat release resulting in flow dilation, the resulting pressure fluctuations leading to thermo-acoustic instabilities there by creating a feedback loop with incoming reactants resulting in flow instabilities leading to flame lift off, flame extinction etc. Hence understanding combustion driven swirling flow in combustors continues to be a topic of intense research. In the present study, numerical predictions of swirl driven combustor flows were analyzed for a specific swirl number of an industrial fuel nozzle (swirler) using a commercial computational fluid dynamics tool and compared against in-house experimental data. The latter data was obtained from a newly developed test rig at Applied Propulsion and Power Laboratory (APPL) at Virginia Tech. The simulations were performed and investigated for several flow Reynolds numbers under non-reacting condition using various two equation turbulence models as well as a scale resolving model. The work was also extended to reacting flow modeling (using a partially premixed model) for a specific Reynolds number. These efforts were carried out in order investigate the flow behavior and also characterize convective heat transfer along the combustor wall (liner). Additionally, several parametric studies were performed towards investigating the effect of combustor geometry on swirling flow and liner hear transfer; and also to investigate the effect of inlet swirl on the jet impingement location along the liner wall under both non-reacting as well as reacting conditions. The numerical results show detailed comparison against experiments for swirling flow profiles within the combustor under reacting conditions indicating a good reliability of steady state modeling approaches for reacting conditions; however, the limitations of steady state RANS turbulence models were observed for non-reacting swirling flow conditions, where the flow profiles deviate from experimental observations in the central recirculation region. Also, the numerical comparison of liner wall heat transfer characteristics against experiments showed a sensitivity to Reynolds numbers. These studies offer to provide preliminary insights of RANS predictions based on commercial CFD tools in predicting swirling, non-reacting and reacting flow and heat transfer. They can be extended to reacting flow heat transfer studies in future and also may be upgraded to unsteady LES predictions to complement future experimental observations conducted at the in-house test facility.
Ph. D.

Natural convection is a phenomenon that occurs in a wide range of applications such as cooling towers, air conditioners, and power plants. Natural convection may be used in decay heat removal systems such as spent fuel casks, where the higher reliability inherent of natural convection is more desirable than forced convection. Passive systems, such as natural convection, may provide better safety, and hence have received much attention recently. Cooling of spent fuel rods is conventionally done using water as the coolant. However, it involves contaminating the water with radiation from the fuel rods. Contamination becomes dangerous and difficult for humans to handle. Further, the recent nuclear tragedy in Fukushima, Japan has taught us the dangers of contamination of water with nuclear radiation. Natural convection can perhaps significantly reduce the risk since it is self-sufficient and does not rely on other secondary system such as a blower as in cases of forced convection. The Utah State University Experimental Fluid Dynamics lab has recently designed an experiment that models natural convection using heated rod bundles enclosed in a rectangular cavity. The data available from this experiment provides and opportunity to study and validate computational fluid dynamics(CFD)models. The validated CFD models can be used to study multiple configurations, boundary conditions, and changes in physics(natural and/or forced convection). The results are to be validated using experimental data such as the velocity field from particle image velocimetry (PIV), pressure drops across various sections of the geometry, and temperature distributions along the vertically heated rods. This research work involves modeling natural convection using two-layer turbulence models such as k - ε and RST (Reynolds stress transport) using both shear driven (Wolfstein) and buoyancy driven (Xu) near-wall formulations. The interpolation scheme employed is second-order upwinding using the general purpose code STAR-CCM+. The pressure velocity coupling is done using the SIMPLE method. It is ascertained that turbulence models with two-layer formulations are well suited for modeling natural convection. Further it is established that k - ε and Reynolds stress turbulence models with the buoyancy driven (Xu)formulation are able to accurately predict the flow rate and temperature distribution.

Modern gas turbine blades and vanes are operated at temperatures above their material’s melting point. Active external and internal cooling are therefore necessary to reach acceptable lifetimes. One possible internal cooling method is called matrix cooling, where a matrix of intersecting cooling air channels is integrated into a blade or vane. To further increase the efficiency of gas turbines, the amount of cooling air must be reduced. Therefore it is necessary that heat transfer inside a cooling matrix is well understood. In the first part of the thesis, a methodology for estimating heat transfer in the flow of matrix cooling channels was established using Computational Fluid Dynamics. Two four-equation RANS turbulence models based on the k-ε turbulence model showed a good correlation with experimental results, while the k-ω SST model underpredicted the heat transfer significantly. For all turbulence models, the heat transfer showed high sensitivity towards changes in the numerical setup. For the k-ω SST turbulence model, the mesh requirements were deemed too computationally expensive and it was excluded from further investigations. As the second part of the thesis, a parameter study was conducted investigating the influence of several geometric parameters on the heat transfer in a cooling matrix. The matrix was simplified as a channel flow interacting with multiple crossing flows. The highest enhancement in heat transfer was seen with changes in taper ratio, aspect ratio and matrix angle. Compared to smooth pipe flow, an increase in heat transfer of up to 60% was observed. Rounded edges of the cooling channels showed a significant influence on the heat transfer as well. In contrast, no influence of the wall thickness on the heat transfer was observed. While no direct validation is possible, the base case and the parameter sweeps show a good correlation with similar cases found in the literature.

The high turbine inlet temperature of modern gas turbines poses a challenge to the material used in the turbine blading of the primary stages. Mechanical failure mechanisms are more pronounced at these high temperatures, setting the lifetime of the blade. It is therefore crucial to obtain accurate local metal temperature predictions of the turbine blade. Accurately predicting the external heat transfer coefficient (HTC) distribution of the blade is therefore of uttermost importance. At present time, Siemens Energy uses the boundary layer code TEXSTAN for this purpose. The limitations coupled to such codes however make them less applicable for the complex flow physics involved in the hot gas path of turbine blading. The thesis therefore aims at introducing CFD for calculating the external HTC. This includes conducting an extensive literature study to find and validate a suitable methodology. The literature study was centered around RANS modeling, reviewing how the calculation of the HTC has evolved and the performance of some common turbulence and transition models. From the literature study, the SST k − ω model in conjunction with the γ − Reθ transition model, the v2 − f model and the Lag EB k − ε model were chosen for the investigation of a suitable methodology. The validation of the methodology was based on the extensively studied LS89 vane linear cascade of the von Karman Institute. In total 13 test cases of the cascade were chosen to represent a wide range of flow conditions. Both a periodic model and a model of the entire LS89 cascade were tested but there were great uncertainties whether or not the correct flow conditions were achieved with the model of the entire cascade. It was therefore abandoned and a periodic model was used instead. The decay of turbulence intensity is not known in the LS89 cascade. This made the case difficult to model since the turbulence boundary conditions then were incomplete. Two approaches were attempted to handle this deficiency, where one was ultimately found invalid. It was recognized that the Steelant-Dick postulation could be used in order to find a turbulent length scale which when specified at the inlet, lead to fairly good agreement with data of the HTC. The validation showed that the SST γ − Reθ model performs relatively well on the suction side and in transition onset predictions but worse on the pressure side for certain flow conditions. The v2 − f model performed better on the pressure side and on a small portion of the suction side. Literature emphasized the importance of obtaining proper turbulence characteristics around the vane for accurate HTC-predictions. It was found that the results of the validation step could be closely coupled to this statement and that further work is needed regarding this. Further research must also be done on the Steelant-Dick postulation to validate it as a reliable method in prescribing the inlet length scale.

Under certain circumstances pulse combustors have been shown to improve both heat transfer and drying rate when compared to steady flow impingement. Despite this potential, there have been few investigations into the use of pulse combustor driven impingement jets for industrial drying applications. The research presented here utilized experimental and numerical techniques to study the heat transfer characteristics of these types of oscillating jets when impinging on solid surfaces and the heat and mass transfer when drying porous media. The numerical methods were extensively validated using laboratory heat flux and drying data, as well as correlations from literature. As a result, the numerical techniques and methods that were developed and employed in this work were found to be well suited for the current application. It was found that the pulsating flows yielded elevated heat and mass transfer compared to similar steady flow jets. However, the numerical simulations were used to analyze not just the heat flux or drying, but also the details of the fluid flow in the impingement zone that resulted in said heat and mass transport. It was found that the key mechanisms of the enhanced transfer were the vortices produced by the oscillating flow. The characteristics of these vortices such as the size, strength, location, duration, and temperature, determined the extent of the improvement. The effects of five parameters were studied: the velocity amplitude ratio, oscillation frequency, the time-averaged bulk fluid velocity at the tailpipe exit, the hydraulic diameter of the tailpipe, and the impingement surface velocity. Analysis of the resulting fluid flow revealed three distinct flow types as characterized by the vortices in the impingement zone, each with unique heat transfer characteristics. These flow types were: a single strong vortex that dissipated before the start of the next oscillation cycle, a single persistent vortex that remained relatively strong at the end of the cycle, and a strong primary vortex coupled with a short-lived, weaker secondary vortex. It was found that the range over which each flow type was observed could be classified into distinct flow regimes. The secondary vortex and persistent vortex regimes were found to enhance heat transfer. Subsequently, transition criteria dividing these regimes were formed based on dimensionless parameters. The critical dimensionless parameters appeared to be the Strouhal number, a modified Strouhal number, the Reynolds number, the velocity amplitude ratio, and the H/Dh ratio. Further study would be required to determine if these parameters offer similar significance for other configurations.

Efficient analysis and storage of data is an integral but often challenging task when working with computation fluid dynamics mainly due to the amount of data it can output. Methods centered around the proper orthogonal decomposition were used to analyze, compress, and model various simulation cases. Two different high-fidelity, time-accurate turbomachinery simulations were investigated to show various applications of the analysis techniques. The first turbomachinery example was used to illustrate the extraction of turbulent coherent structures such as traversing shocks, vortex shedding, and wake variation from deswirler and rotor blade passages. Using only the most dominant modes, flow fields were reconstructed and analyzed for error. The reconstructions reproduced the general dynamics within the flow well, but failed to fully resolve shock fronts and smaller vortices. By decomposing the domain into smaller, independent pieces, reconstruction error was reduced by up to 63 percent. A new method of data compression that combined an image compression algorithm and the proper orthogonal decomposition was used to store the reconstructions of the flow field, increasing data compression ratios by a factor of 40.The second turbomachinery simulation studied was a three-stage fan with inlet total pressure distortion. Both the snapshot and repeating geometry methods were used to characterize structures of static pressure fluctuation within the blade passages of the third rotor blade row. Modal coefficients filtered by frequencies relating to the inlet distortion pattern were used to produce reconstructions of the pressure field solely dependent on the inlet boundary condition. A hybrid proper orthogonal decomposition method was proposed to limit burdens on computational resources while providing high temporal resolution analysis.Parametric reduced order models were created from large databases of transient and steady conjugate heat transfer and airfoil simulations. Performance of the models were found to depend heavily on the range of the parameters varied as well as the number of simulations used to traverse that range. The heat transfer models gave excellent predictions for temperature profiles in heated solids for ambitious parameter ranges. Model development for the airfoil case showed that accuracy was highly dependent on modal truncation. The flow fields were predicted very well, especially outside the boundary layer region of the flow.

The European Union is planning to greatly decrease energy consumption during the coming decades. The ultimate goal is to create sustainable communities that are energy neutral. One way of achieving this challenging goal may be to use efficient hydronic (water-based) heating systems supported by heat pumps. The main objective of the research reported in this work was to improve the thermal performance of wall-mounted hydronic space heaters (radiators). By improving the thermal efficiency of the radiators, their operating temperatures can be lowered without decreasing their thermal outputs. This would significantly improve efficiency of the heat pumps, and thereby most probably also reduce the emissions of greenhouse gases. Thus, by improving the efficiency of radiators, energy sustainability of our society would also increase. The objective was also to investigate how much the temperature of the supply water to the radiators could be lowered without decreasing human thermal comfort. Both numerical and analytical modeling was used to map and improve the thermal efficiency of the analyzed radiator system. Analyses have shown that it is possible to cover space heat losses at low outdoor temperatures with the proposed heating-ventilation systems using low-temperature supplies. The proposed systems were able to give the same heat output as conventional radiator systems but at considerably lower supply water temperature. Accordingly, the heat pump efficiency in the proposed systems was in the same proportion higher than in conventional radiator systems. The human thermal comfort could also be maintained at acceptable level at low-temperature supplies with the proposed systems. In order to avoid possible draught discomfort in spaces served by these systems, it was suggested to direct the pre-heated ventilation air towards cold glazed areas. By doing so the draught discomfort could be efficiently neutralized.     Results presented in this work clearly highlight the advantage of forced convection and high temperature gradients inside and alongside radiators - especially for low-temperature supplies. Thus by a proper combination of incoming air supply and existing radiators a significant decrease in supply water temperature could be achieved without decreasing the thermal output from the system. This was confirmed in several studies in this work. It was also shown that existing radiator systems could successfully be combined with efficient air heaters. This also allowed a considerable reduction in supply water temperature without lowering the heat output of the systems. Thus, by employing the proposed methods, a significant improvement of thermal efficiency of existing radiator systems could be accomplished. A wider use of such combined systems in our society would reduce the distribution heat losses from district heating networks, improve heat pump efficiency and thereby most probably also lower carbon dioxide emissions.

QC 20131029

Efficient, accurate numerical simulation of coupled heat transfer and fluid dynamics systems continues to be a challenge. Direct numerical simulation (DNS) packages like FLU- ENT exist and are sufficient for design and predicting flow in a static system, but in larger systems where input parameters can change rapidly, the cost of DNS increases prohibitively. Major obstacles include handling the scales of the system accurately - some applications span multiple orders of magnitude in both the spatial and temporal dimensions, making an accurate simulation very costly. There is a need for a simulation method that returns accurate results of multi-scale systems in real time. To address these challenges, the Multi- Resolution Discontinuous Galerkin (MRDG) method has been shown to have advantages over other reduced order methods. Using multi-wavelets as the local approximation space provides an inherently efficient method of data compression, while the unique features of the Discontinuous Galerkin method make it well suited to composition with wavelet theory. This research further exhibits the viability of the MRDG as a new approach to efficient, accurate thermal system simulations. The development and execution of the algorithm will be detailed, and several examples of the utility of the MRDG will be included. Comparison between the MRDG and the "vanilla" DG method will also be featured as justification of the advantages of the MRDG method.

An accurate measurement of total temperature in turbomachinery flows remains critical for component life models and cycle performance optimization. While many techniques exist to measure these flows, immersed thermocouple based probes remain highly desirable due to well established practices for probe design and implementation in typical industrial flow applications. However, as engine manufacturers continue to push towards higher maximum cycle temperatures and smaller flow passages, the continued use of these probes requires new probe designs considering both improved sensor durability and measurement accuracy. Increased maximum temperatures introduce many challenges for total temperature measurements using conventional immersed probes, including increased influences of conduction, convection, and radiation heat transfer between the sensor, fluid and the surroundings due to large thermal gradients present in real turbomachinery systems. While these effects have been previously investigated, the available design models are very limited to specific geometries and flow conditions. In this Dissertation, a more fundamental understanding of the flow behavior around typical vented shield style total temperature probes as a function of probe geometry and operating condition is gained using results from high-fidelity Computational Fluid Dynamics simulations with Conjugate Heat Transfer. A parametric study was conducted considering three non-dimensional probe geometric ratios (vent location to shield length (0.029-0.806), sensor diameter to shield inner diameter (0.252-0.672), and shield outer diameter to strut/mount thickness (0.245-0.759)) and three operating conditions (total temperature (70, 850, 2500°F) and pressure (1, 1, 10 atm), respectively) at a moderate Mach number of 0.4. Results were further quantified in the form of new empirical correlations necessary for rapid thermal performance evaluations of current and future probe designs. Additionally, a new mixed-fidelity or Reduced Order Modeling technique was developed which allows the coupling of high fidelity surface heat transfer data from CFD with a generalized form of the 1-D conducting solid equations for evaluating radiation and transient influences on sensor performance. These new flow and heat transfer correlations together with the new Reduced Order Modeling technique developed here greatly enhance the capabilities of designers to evaluate performance of current and future probe designs, with higher accuracy and with significant reductions in computational resources.
Doctor of Philosophy
An accurate measurement of total temperature in turbomachinery flows remains critical for component life models and cycle performance optimization. While many techniques exist to measure these flows, immersed thermocouple based probes remain highly desirable due to well established practices for probe design and implementation in typical industrial flow applications. However, as engine manufacturers continue to push towards higher maximum cycle temperatures and smaller flow passages, the continued use of these probes requires new probe designs considering both improved sensor durability and measurement accuracy. Increased maximum temperatures introduce many challenges for total temperature measurements using conventional immersed probes, including increased influences of conduction, convection, and radiation heat transfer between the sensor, fluid and the surroundings due to large thermal gradients present in real turbomachinery systems. While these effects have been thoroughly described and quantified in the past, the available design models are very limited to specific geometries and flow conditions. In this Dissertation, a more fundamental understanding of the flow behavior around typical vented shield style total temperature probes as a function of probe geometry and operating condition is gained using results from high-fidelity Computational Fluid Dynamics simulations with Conjugate Heat Transfer (CHT) capabilities. Results were further quantified in the form of new empirical correlations necessary for rapid thermal performance evaluations of current and future probe designs. Additionally, a new mixed-fidelity or Reduced Order Modeling (ROM) technique was developed which allows the coupling of high fidelity surface heat transfer data from CFD with a generalized form of the 1-D conducting solid equations for readily predicting the impact of radiation environment and transient errors on sensor performance.

Defrosting in the refrigeration industry is used to remove the frost layer accumulated on the evaporators after a period of running time. It is one way to improve the energy efficiency of refrigeration systems. There are many studies about the defrosting process but none of them use computational fluid dynamics (CFD) simulation. The purpose of this thesis is (1) to develop a defrost model using the commercial CFD solver FLUENT to simulate numerically the melting of frost coupled with the heat and mass transfer taking place during defrosting, and (2) to investigate the thermal response of the evaporator and the defrost time for different hot gas temperatures and frost densities. A 3D geometry of a finned tube evaporator is developed and meshed using Gambit 2.4.6, while numerical computations were conducted using FLUENT 12.1. The solidification and melting model is used to simulate the melting of frost and the Volume of Fluid (VOF) model is used to render the surface between the frost and melted frost during defrosting. A user-defined-function in C programming language was written to model the frost evaporation and sublimation taking place on the free surface between frost and air. The model was run under different hot gas temperatures and frost densities and the results were analyzed to show the effects of these parameters on defrosting time, input energy and stored energy in the metal mass of the evaporator. The analyses demonstrate that an optimal hot gas temperature can be identified so that the defrosting process takes place at the shortest possible melting time and with the lowest possible input energy.

Carbon fiber reinforced polymers (CFRP) show an excellent balance between thermomechanical properties and low density, leading them to be the material of choice in many applications. In the past years, the use of polymer matrix composites has significantly increased in the aerospace, automotive and naval sectors. Correspondingly, the requirements on high performance and quality keep increasing with the market demand. This generates a big concern on material properties along with the production of a large number of components, minimizing cycles’ time. This document deals with the optimization of the distribution of composite components inside the autoclave obtaining therefore, the best configuration in order to achieve fair properties in all the elements in the shortest time possible. This study has been done along with the company Eligio Re Fraschini S.p.A., analyzing one of their autoclaves. The aim is to find the best configuration for a given repetitive production. The optimization procedure that has been followed consisted in, for each of the three different productions considered, assess a set of models representing contrasting scenarios, in order to understand the flow behaviour and find the optimal distribution. The outcomes for the different locations are obtained through simulations performed with Ansys Fluent, a computational fluid dynamic (CFD) software; the ideal configuration is the one maximizing the heat transfer through all the components, in an homogeneous way.

This is a thesis work proposed by Sweco System in order to carry out a study related to the heating system of a circular fuel oil storage tank or cistern. The study tank is a 23m diameter and 18m height with a storage capacity of around 7500m3 of Eo5 heavy fuel oil. The content ought to be at a minimum storage temperature of 50ºC so that the fuel oil is fluid enough and operation labors can be adequately performed. In fact, these types of heavy fuel oils have fairly high viscosities at lower temperatures and the heating and pumping system can be compromised at temperatures below the pour point. For this purpose a heating system is installed to maintain the fluid warm. So far the system was operated by an oil burner but there are plans to its replacement by a District Heating-heat exchanger combo. Thereby, tank heating needs, flow and thermal patterns and heat transfer within it are principally studied.

 

Tank boundaries are studied and their thermal resistances are calculated in order to dimension heat supply capacity. The study implies Finite Elements (Comsol Multiphysics) and Finite Volume (Fluent) analysis to work out some stationary heat transfer by conduction cases on some parts and thermal bridges present on these boundaries. Afterwards both cooling and heating processes of the fuel oil are studied using several strategies: basic models and Computational Fluid Dynamics (CFD). CFD work with Fluent is focused on optimizing inlet and outlet topologies. Understanding the cooling process is sought as well; Fluent CFD transient models are simulated in this way as well. Additionally the effect of filling levels is taken into account leading to a multiphase (fuel oil and air) flow cases where especially heating coupling of both phases is analyzed.

 

Results show that maximum heat supply needs are around 80kW when the tank temperature is around 60ºC and 70kW when it is around 50ºC. Expectedly the main characteristic of the flow turns out to be the buoyancy driven convective pattern. K-ε turbulence viscous models are applied to both heating and cooling processes showing thermal stratification, especially at the bottom of the tank. Hotter fluid above follows very complex flow patterns. During the heating processes models used predict fairly well mixed and homogenous temperature distribution regardless small stratification at the bottom of the tank. In this way no concrete inlet-outlet configuration shows clear advantages over the rest. Due to the insulation of the tank, low thermal conductivity of the fluid and vast amount of mass present in the tank, the cooling process is slow (fluid average temperature drops around 5.7 ºC from 60ºC in 15 days when the tank is full and ambient temperature is considered to be at -20ºC) and lies somewhere in the middle between the solid rigid and perfect mixture cooling processes. However, due to stratification some parts of the fluid reach minimum admissible temperatures much faster than average temperature does. On the other hand, as expected, air phase acts as an additional thermal resistance; anyhow the cooling process is still faster for lower filling levels than the full one.

El presente proyecto fue propuesto por Sweco Systems para llevar a cabo un estudio relacionado con el sistema de calefacción de una cisterna o tanque de almacenamiento de fuel oil circular. Dicho tanque tiene 23 m de diámetro  y 18 m de altura con una capacidad de almacenamiento de alrededor de 7500 m³ de Eo5 fuel oil pesado. El contenido mantenerse a una temperatura mínima de 50 ºC de manera que el fuel oil es suficientemente fluido para que las labores de operación puedan ser ejecutadas adecuadamente. De hecho, estos tipos de fuel oil pesado tienen altas viscosidades a bajas temperaturas y, por tanto, tanto los sistemas de calefacción y como el de bombeo pueden verse comprometidosr a temperaturas por debajo del pour point. Con este fin un sistema de calefacción es instalado para mantener el fluido suficientemente caliente. Hasta el momento, el sistema era operado por un quemador de fuel, sin embargo, hay planes que éste sea sustituido por un combo intercambiador de calor-District Heating. Por lo tanto, principalmente son estudiadas las necesidades de calefacción así como los flujos térmicos y fluidos.

Se estudian las fronteras del tanque, y sus respectivas resistencias térmicas son calculadas con el fin de dimensionar la capacidad necesaria de suministro de calor. El estudio implica Elementos Finitos (Comsol Multiphysics) y Volúmenes Finitos (Fluent) para elaborar análisis estacionarios de transferencia de calor por conducción en algunos casos. Existen puentes térmicos en las paredes y su importancia es también anallizada. Posteriormente se estudian tanto los procesos de calentamiento y enfriamiento del fuel oil utilizando diversas estrategias: modelos básicos y Dinámica de Fluidos Computacional (CFD). El trabajo con CFD se centra en la optimización de topologías de entradas y salidas del sistema. También es solicitado entender el proceso de enfriamiento; En este sentido, se simulan modelos CFD transitorios de Fluent. Además, el efecto de los niveles de llenado se tiene en cuenta dando lugar a estudios de flujo multifase (fuel oil y aire), haciendo hincapié en el análisis de acoplamiento de transferencia de calor entre las dos fases.

Los resultados muestran que las necesidades de calefacción máximas son de alrededor de 80kW cuando la temperatura del tanque es de alrededor de 60 º C y 70kW cuando está alrededor de 50 ºC. Como era de esperar, la principal característica de este tipo de flujos es la convección natural resultante de las fuerzas de flotabilidad. Se aplican modelos turbulentos k-ε a los procesos de calentamiento y enfriamiento, mostrando estratificación térmica, sobre todo en la parte inferior de la cisterna. El líquido más caliente que se sitúa encima muestra complejos patrones de flujo. Durante los procesos de calentamiento, los modelos utilizados predicen un buen mezclado y distribución homogénea de la temperatura independientemente de esta pequeña estratificación en la parte inferior de la cisterna. De esta manera, ninguna concreta configuración de entradas-salidas simuladas muestra claras ventajas sobre el resto. Debido al aislamiento de la cisterna, la baja conductividad térmica del fluido y la gran cantidad de masa presente en el tanque el proceso de enfriamiento es lento (la temperatura media del fluido desciende 5.7 º C desde 60 º C en 15 días cuando el tanque está lleno y la temperatura ambiente es de -20 º C) y se encuentra en algún lugar en medio de los procesos de enfriamiento del sólido rígido y perfecta mezcla. Sin embargo, debido a la estratificación, algunas partes el líquido alcanzan la temperatura mínima admisible mucho más rápido que la media de temperatura. Por otra parte, como se esperaba, la fase de aire actúa como una resistencia térmica adicional, de todos modos, el proceso de enfriamiento es aún más rápido para niveles de llenado más bajos que el lleno.

Thesis (MScEng)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: A high temperature reactor (HTR) generates heat inside of the reactor core through nuclear fission, from where the heat is transferred through the core and heats up the reactor pressure vessel (RPV). The heat from the RPV is transported passively through the reactor cavity, where it is cooled by the reactor cavity cooling system (RCCS), through the concrete confinement structure and ultimately into the environment. The concrete confinement structure can withstand temperatures of up to 65°C for normal operating conditions and temperatures of up to 125°C during an emergency. This project endeavours to research the heat transfer between an HTR’s RPV and the outside of the concrete confinement structure by utilising three investigative approaches: experimental, computational fluid dynamics (CFD) and analytical. The first approach, an experimental analysis, required the development of an experi- mental model. The model was used to perform experiments and gather temperature data that could be used to verify the accuracy of the CFD simulations. The second approach was a CFD analysis of the experimental model, and the external concrete temperatures from the simulation were compared with the temperatures measured with the experimen- tal model. Finally, an analytical analysis was performed in order to better understand CFD and how CFD solves natural convection-type problems. The experiments were performed successfully and the measurements taken were com- pared with the CFD results. The CFD results are in good agreement with the Dry experiments, but not with the Charged experiments. It was identified that the inaccurate results for the CFD simulations of the Charged experiments arose due to convective heat leakage through gaps in the heat shield and between the heat shield and the sides of the experimental model. A computer program was developed for the analytical analysis and it was established that the program could successfully solve the natural convection in a square cavity - as required.
AFRIKAANSE OPSOMMING: ’n Hoë temperatuur reaktor (HTR) genereer hitte binne die reaktor kern deur kernsplyting en die hitte word dan deur die kern versprei en verhit die reaktor se drukvat. Die hitte van die reaktor drukvat word dan passief deur die reaktorholte versprei, waar dit deur die reaktorholte se verkoelingstelsel afgekoel word, en deur die beton beskermingstruktuur gelei word en uiteindelik die omgewing bereik. Die beton beskermingstruktuur kan temperature van tot 65°C onder normale operasietoestande van die reaktor weerstaan, en temperature van tot 125°C tydens ’n noodgeval. Hierdie projek poog om die hitte-oordrag tussen ’n HTR-reaktor drukvat en die buitekant van die beton beskermingstruktuur te on- dersoek deur gebruik te maak van drie ondersoekbenaderings: eksperimenteel, numeriese vloei dinamika (NVD) en analities. Die eerste benadering, ’n eksperimentele analise, het die ontwikkeling van ’n eksper- imentele model vereis. Die model is gebruik om eksperimente uit te voer en temperatu- urmetings te neem wat gebruik kon word om die akkuraatheid van die NVD simulasies te bevestig. Die tweede benadering was ’n NVD-analise van die eksperimentele model, en die eksterne betontemperature verkry van die simulasies is vergelyk met die gemete temperature van die eksperimente. Uiteindelik is ’n analitiese analise uitgevoer ten einde NVD beter te verstaan en hoe NVD natuurlike konveksie-tipe probleme sal oplos. Die eksperimente is suksesvol uitgevoer en die metings is gebruik om die NVD resultate mee te vergelyk. Die NVD resultate van die Droë eksperimente het goeie akkuraatheid getoon. Dit was nie die geval vir die Gelaaide eksperimente nie. Daar is geïdentifiseer dat die verskille in resultate tussen die NVD en die eksperimente aan natuurlike konveksie hitte verliese deur gapings in die hitteskuld en tussen die hitteskuld en die kante van die eksperimentele model toegeskryf kan word. ’n Rekenaarprogram is geskryf vir die analitiese ontleding en die program kon suksesvol die natuurlike konveksie in ’n vierkantige ruimte oplos.

The main goal of this thesis project is to identify interesting concepts related to cooling of electrical motors and generators which could be evaluated using suitable computer simulation tools. As the project proceeded it was decided to focus on investigating how the air from a fan flows along the finned frame of a general purpose low voltage electrical machine, how the heat is transferred between the frame and the cooling air and what the temperature distribution looks like. It was also investigated if it is possible to make improvements in the effectiveness of the cooling without adding additional coolers. This investigation focused on varying the fin design and evaluating the resulting temperature distribution. Due to the complex nature of the simulations a segment, and not the full frame, was considered. Simulation model validation was performed through comparing air speed measurements that were performed on two different machines with the corresponding simulated air speed. The validation showed that good agreement between simulated and measured air speeds are obtained. The conclusion from the simulations is that slight modifications to the current fin design could increase the cooling effect of the finned surface. The air velocity measurements also indicate that the cooling of the machines surface could potentially be improved by small changes in the exterior of the frame.
Målet med detta examensarbete var att identifiera intressanta koncept relaterade till kylning av elektriska maskiner och generatorer, som kunde utvärderas med lämplig programvara för datorsimuleringar. Under projektets gång så bestämdes det att fokusera på hur luften från en fläkt flödar längs med en generell lågspänningsmaskin, hur värmen överförs från ramen till den omgivande luften och hur temperaturfördelningen ser ut. Det undersöktes även om det var möjligt att förbättra effektiviteten av kylningen utan att ansluta extra kylanordningar. Undersökningarna fokuserades på olika fendesigner och dess påverkan på värmefördelningen. På grund av simuleringarnas komplexitet så har simuleringarna endast utförts på ett segment istället för hela maskinen. Validering av simuleringarna utfördes genom att jämföra de simulerade lufthastigheterna med verklig lufthastighet som mättes på två maskiner i testmiljö. Valideringen visade att simuleringarna överensstämmer väl med de mätningar som utfördes. Slutsatsen utifrån simuleringarna är att mindre förändringar av fenornas nuvarande design kan förbättra fenornas kylningsförmåga. Mätningarna av lufthastigheten ger även indikationer på att kylningen av maskinens utsida eventuellt kan förbättras genom små förändringar av ramens exteriör.

The efficacy, reliability and versatility of the light emitting diode (LED) can outcompete most established light source technologies. However, they are particularly sensitive to high temperatures, which compromises their efficacy and reliability, undermining some of the technology s key benefits. Consequently, effective thermal management is essential to exploit the technology to its full potential. Thermal management is a well-established subject but its application in the relatively new LED lighting industry, with its specific constraints, is currently poorly defined. The question this thesis aims to answer is how can LED thermal management be achieved most effectively? This thesis starts with a review of the current state of the art, relevant thermal management technologies and market trends. This establishes current and future thermal management constraints in a commercial context. Methods to test and evaluate the thermal management performance of a luminaire system follow. The defined test methods, simulation benchmarks and operational constraints provide the foundation to develop effective thermal management strategies. Finally this work explores how the findings can be implemented in the development and comparison of multiple thermal management designs. These are optimised to assess the potential performance enhancement available when applied to a typical commercial system. The outcomes of this research showed that thermal management of LEDs can be expected to remain a key requirement but there are hints it is becoming less critical. The impacts of some common operating environments were studied, but appeared to have no significant effect on the thermal behaviour of a typical system. There are some active thermal management devices that warrant further attention, but passive systems are inherently well suited to LED luminaires and are readily adopted so were selected as the focus of this research. Using the techniques discussed in this thesis the performance of a commercially available component was evaluated. By optimising its geometry, a 5 % decrease in absolute thermal resistance or a 20 % increase in average heat transfer coefficient and 10 % reduction in heatsink mass can potentially be achieved . While greater lifecycle energy consumption savings were offered by minimising heatsink thermal resistance the most effective design was considered to be one optimised for maximum average heat transfer coefficient. Some more radical concepts were also considered. While these demonstrate the feasibility of passively manipulating fluid flow they had a detrimental impact on performance. Further analysis would be needed to conclusively dismiss these concepts but this work indicates there is very little potential in pursuing them further.

Jet engines often operate under dirty conditions where large amounts of particulate matter can be ingested, especially, sand, ash and dirt. Particulate matter in different engine components can lead to degradation in performance. The objective of this dissertation is to investigate sand transport and deposition in the internal cooling passages of turbine blades. A simplified rectangular geometry is simulated to mimic the flow field, heat transfer and particle transport in a two pass internal cooling geometry. Two major challenges are identified while trying to simulate particle deposition. First, no reliable particle-wall collision model is available to calculate energy losses during a particle wall interaction. Second, available deposition models for particle deposition do not take into consideration all the impact parameters like impact velocity, impact angle, and particle temperature. These challenges led to the development of particle wall collision and deposition models in the current study. First a preliminary simulation is carried out to investigate sand transport and impingement patterns in the two pass geometry by using an idealized elastic collision model with the walls of the duct without any deposition. Wall Modeled Large Eddy Simulations (WMLES) are carried to calculate the flow field and a Lagrangian approach is used for particle transport. The outcome of these simulations was to get a qualitative comparison with experimental visualizations of the impingement patterns in the two pass geometry. The results showed good agreement with experimental distributions and identified surfaces most prone to deposition in the two pass geometry. The initial study is followed by the development of a particle-wall collision model based on elastic-plastic deformation and adhesion forces by building on available theories of deformation and adhesion for a spherical contact with a flat surface. The model calculates deformation losses and adhesion losses from particle-wall material properties and impact parameters and is broadly applicable to spherical particles undergoing oblique impact with a rigid wall. The model is shown to successfully predict the general trends observed in experiments. To address the issue of predicting deposition, an improved physical model based on the critical viscosity approach and energy losses during particle-wall collisions is developed to predict the sand deposition at high temperatures in gas turbine components. The model calculates a sticking or deposition probability based on the energy lost during particle collision and the proximity of the particle temperature to the softening temperature. For validation purposes, the deposition of sand particles is computed for particle laden jet impingement on a coupon and compared with experiments conducted at Virginia Tech. Large Eddy Simulations are used to calculate the flow field and heat transfer and particle dynamics is modeled using a Lagrangian approach. The results showed good agreement with the experiments for the range of jet temperatures investigated. Finally the two pass geometry is revisited with the developed particle-wall collision and deposition model. Sand transport and deposition is investigated in a two pass internal cooling geometry at realistic engine conditions. LES calculations are carried out for bulk Reynolds number of 25,000 to calculate flow and temperature field. Three different wall temperature boundary conditions of 950 oC, 1000 oC and 1050 oC are considered. Particle sizes in the range 5-25 microns are considered, with a mean particle diameter of 6 microns. Calculated impingement and deposition patterns are discussed for different exposed surfaces in the two pass geometry. It is evident from this study that at high temperatures, heavy deposition occurs in the bend region and in the region immediately downstream of the bend. The models and tools developed in this study have a wide range of applicability in assessing erosion and deposition in gas turbine components.
Ph. D.

This thesis is based on the PhD work of investigating the Friction Stir Welding process (FSW) with numerical and Artificial Neural Network (ANN) modelling methods. FSW was developed at TWI in 1991. As a relatively new technology it has great advantages in welding aluminium alloys which are difficult to weld with traditional welding processes. The aim of this thesis was the development of new modelling techniques to predict the thermal and deformation behaviour. To achieve this aim, a group of Gleeble experiments was conducted on 6082 and 7449 aluminium alloys, to investigate the material constitutive behaviour under high strainrate, near solidus conditions, which are similar to what the material experiences during the FSW process. By numerically processing the experimental data, new material constitutive constants were found for both alloys and used for the subsequent FSW modelling work. Importantly no significant softening was observed prior to the solidus temperature. One of the main problems with numerical modelling is determining the values of adjustable parameters in the model. Two common adjustable parameters are the heat input and the coefficients that describe the heat loss to the backing bar. To predict these coefficients more efficiently a hybrid model was created which involved linking a conventional numerical model to an ANN model. The ANN was trained using data from the numerical model. Then thermal profiles were abstracted (summarised) and used as inputs; and the adjustable parameters were used as outputs. The trained ANN could then use abstracted thermal profiles from welding experiments to predict the adjustable parameters in the model. The first stage involved developing a simplified FE thermal model which represents a typical welding process. It was used to find the coefficients that describe the heat loss to the backing bar, and the amount of power applied in the model. Five different thermal boundary conditions were studied, including both convective and ones that included the backing bar with a contact gap conductance. Three approaches for abstracting the thermal curves and using as inputs to the ANN were compared. In the study, the characteristics of the ANN model, such as the ANN topology and gradient descent method, were evaluated for each boundary condition for understanding of their influences to the prediction. The outcomes of the study showed that the hybrid model technique was able to determine the adjustable parameters in the model effectively, although the accuracy depended on several factors. One of the most significant effects was the complexity of the boundary condition. While a single factor boundary condition (e.g. constant convective heat loss) could be predicted easily, the boundary condition with two factors proved more difficult. The method for inputting the data into the ANN had a significant effect on the hybrid model performance. A small number of inputs could be used for the single factor boundary condition, while two factors boundary conditions needed more inputs. The influences from the characteristics of the ANN model were smaller, but again thermal model with simpler boundary condition required a less complex ANN model to achieve an accurate prediction, while models with more complex boundary conditions would need a more sophisticated ANN model. In the next chapter, the hybrid method was applied to a FSW process model developed for the Flexi-stir FSW machine. This machine has been used to analyse the complex phase changes that occur during FSW with synchrotron radiation. This unique machine had a complex backing bar system involving heat transfer from the aluminium alloy workpiece to the copper and steel backing bars. A temperature dependent contact gap conductance which also depends on the material interface type was used. During the investigation, the ANN model topologies (i.e. GFF and MFF) were studied to find the most effective one. Different abstracting methods for the thermal curves were also compared to explore which factors (e.g. the peak temperature in the curve, cooling slope of a curve) were more important to be used as an input. According to close matching between the simulation and experimental thermal profiles, the hybrid model can predict both the power and thermal boundary condition between the workpiece and backing bar. The hybrid model was applied to six different travel speeds, hence six sets of heat input and boundary condition factors were found. A universal set was calculated from the six outcomes and a link was discovered between the accuracy of the temperature predictions and the plunge depth for the welds. Finally a model with a slip contact condition between the tool and workpiece was used to investigate how the material flow behaviour was affected by the slip boundary condition. This work involved aluminium alloys 6082-T6 and 7449-T7, which have very different mechanical properties. The application of slip boundary condition was found to significantly reduce the strain-rate, compared to a stick condition. The slip condition was applied to the Flexi-stir FSW experiments, and the results indicated that a larger deformation region may form with the slip boundary condition. The thesis successfully demonstrates a new methodology for determining the adjustable parameters in a process model; improved understanding of the effect of slip boundary conditions on the flow behaviour during FSW and insight in to the behaviour of aluminium alloys at temperatures approaching the solidus and high strain-rates.

[ES] Actualmente, los vehículos propulsados por motores de combustión interna alternativos (MCIA) constituyen uno de los mayores agentes contaminantes para el medio ambiente. En este sentido, ha existido una importante cooperación internacional para promulgar leyes que regulen las emisiones contaminantes. De manera que los fabricantes de coches han impulsado el desarrollo de tecnologías más limpias y amigables con el medio ambiente. Ante esta situación, ha surgido recientemente la electrificación, como uno de los proyectos más ambiciosos de la industria automotriz para los próximos años. Sin embargo, esta meta parece aún lejana en el horizonte. En tal sentido, la hibridación con motores térmicos y eléctricos parece ser el camino a seguir en el corto plazo. Por consiguiente, los MCIA seguirán siendo la principal fuente de propulsión terrestre durante los años venideros. Para mitigar los inherentes efectos contaminantes de los motores de combustión interna, se han propuesto diferentes tecnologías para desarrollar motores más eficientes. Entre ellas, la aplicación de recubrimientos térmicos en las paredes de la cámara de combustión apunta a reducir las pérdidas por calor en el motor, y así aumentar su eficiencia térmica. El objetivo principal de esta tesis es estudiar el impacto de aplicar recubrimientos térmicos en las paredes de la cámara de combustión en motores de combustión interna. En este sentido, determinar los flujos de calor experimentalmente a través de las paredes es complicado y no del todo fiables, debido a que dependen de la medición de las temperaturas de pared. Por este motivo, el CFD-CHT es utilizado. El primer paso fue validar la herramienta computacional que es utilizada para los cálculos en motores de combustión interna. Para ello se realizó un estudio preliminar en geometrías sencillas como una tubería circular o un canal rectangular. Se evaluaron los modelos de transferencia de calor y se determinó la relevancia de ciertos parámetros como la rugosidad. Para complementar el estudio, se realizó un análisis de las temperaturas en una geometría más realista como el pistón de un MCIA. Los valores de temperatura calculados por el software fueron casi iguales a las medidas experimentales. Por consiguiente, la fiabilidad de la herramienta computacional fue verificada. Seguidamente, se plantea una metodología para abordar al problema de modelar capas muy finas de recubrimientos térmicos en el espacio tridimensional. Para de esta manera poder simular las paredes recubiertas en la cámara de combustión. La metodología consiste en definir un material equivalente con un espesor y número de nodos que permitan un mallado computacionalmente realista. Para ello se utilizó un DoE en combinación con un análisis de regresión múltiple. Los primeros estudios se llevaron a cabo en un motor de gasolina. El modelado se llevó a cabo para dos configuraciones: motor con paredes metálicas y motor con pistón y culata recubiertos. A través de un análisis exhaustivo de la transferencia del calor, se evaluó el impacto que tenía aplicar el revestimiento térmico en el motor. La comparación con datos experimentales demuestran la utilidad del cálculo CHT para evaluar las pérdidas de calor en un MCIA. Sin embargo, ninguna mejora fue observada en el motor de gasolina debido al tipo de recubrimiento aplicado en las paredes de la cámara de combustión. Las simulaciones llevadas a cabo en el motor de gasolina permitieron determinar que los cálculos CHT son computacionalmente largos. En este sentido, una serie de estrategias diseñadas a optimizar los cálculos han sido analizadas con el fin de reducir los tiempos de cálculo. A través de este estudio, se encontró una metodología para optimizar la malla del dominio computacional. Esta última, emplea un refinamiento AMR basado en la distancia de pared. Este método es utilizado para modelar el impacto de aplicar un revestimiento tér
[CA] Actualment, els vehicles propulsats per motors de combustió interna alter- natius (MCIA) constitueixen un dels majors agents contaminants per al medi ambient. En aquest sentit, ha existit una important cooperació internacional per a promulgar lleis que regulen les emissions contaminants. De manera que els fabricants de cotxes han impulsat el desenvolupament de tecnologies més netes i amigables amb el medi ambient. Davant aquesta situació, ha sorgit recentment l'electrificació, com un dels projectes més ambiciosos de la indústria automotriu per als pròxims anys. No obstant això, aquesta meta sembla encara llunyana en l'horitzó. En tal sentit, la hibridació amb motors tèrmics i elèctrics sembla ser el camí a seguir en el curt termini. Per consegüent, els MCIA continuaran sent la principal font de propulsió terrestre durant els anys esdevenidors. Per a mitigar els inherents efectes contaminants dels motors de combustió interna, s'han proposat diferents tecnologies per a desenvolupar motors més eficients. Entre elles, l'aplicació de recobriments tèrmics en les parets de la cambra de combustió apunta a reduir les pèrdues per calor en el motor, i així augmentar la seua eficiència tèrmica. L'objectiu principal d'aquesta tesi és estudiar l'impacte d'aplicar reco- briments tèrmics en les parets de la cambra de combustió en motors de combustió interna. En aquest sentit, determinar els fluxos de calor experi- mentalment a través de les parets és complicat i no del tot fiable, pel fet que depenen del mesurament de les temperatures de paret. Per aquest motiu, el CFD-CHT (Computational fluid dynamics-Conjugate Heat Transfer) és utilitzat. El primer pas va ser validar l'eina computacional que és utilitzada per als càlculs en motors de combustió interna. Per a això es va realitzar un estudi preliminar en geometries senzilles com una canonada circular o un canal rectangular. Es van avaluar els models de transferència de calor i es va determinar la rellevància de certs paràmetres com la rugositat. Per a complementar l'estudi, es va realitzar una anàlisi de les temperatures en una geometria més realista com el pistó d'un MCIA. Els valors de temperatura calculats pel software van ser quasi iguals a les mesures experimentals. Per consegüent, la fiabilitat de l'eina computacional va ser verificada. Seguidament, es planteja una metodologia per a abordar el problema de modelar capes molt fines de recobriments tèrmics en l'espai tridimensional, per a d'aquesta manera poder simular les parets recobertes en la cambra de combustió. La metodologia consisteix a definir un material equivalent amb una grossària i nombre de nodes que permeten un mallat computacionalment realista. Per a això es va utilitzar un DoE (Design of experiments) en combinació amb una anàlisi de regressió múltiple. Els primers estudis es van dur a terme en un motor de gasolina. El mod- elatge es va dur a terme per a dues configuracions: motor amb parets metàl·liques i motor amb pistó i culata recoberts. A través d'una anàlisi exhaustiva de la transferència de la calor, es va avaluar l'impacte que tenia aplicar el revestiment tèrmic en el motor. La comparació amb dades experi- mentals demostren la utilitat del càlcul CHT per a avaluar les pèrdues de calor en un MCIA. No obstant això, cap millora va ser observada en el motor de gasolina a causa de la mena de recobriment aplicada en les parets de la cambra de combustió. Les simulacions dutes a terme en el motor de gasolina van permetre determinar que els càlculs CHT són computacionalment llargs. En aquest sentit, una sèrie d'estratègies dissenyades per a optimitzar els càlculs han sigut analitzades amb la finalitat de reduir els temps de càlcul. A través d'aquest estudi, es va trobar una metodologia per a optimitzar la malla del domini computacional. Aquesta última, empra un refinament AMR basat en la distància de paret.
[EN] Currently, vehicles powered by internal combustion engines (ICE) are targeted as contributing largely to environmental pollution. In this regard, there has been significant international cooperation to enact laws that regulate the polluting emissions. Hence, the car manufacturers have oriented efforts to the development of cleaner and more eco-friendly technologies. In order to face this situation, electrified vehicles have emerged as one of the most promising projects in the automotive industry for the coming years. However, this target still seems far on the horizon. In this sense, hybridization with thermal and electric engines seems to be the path to follow in the short term. Consequently, ICEs will continue to be one of the important sources of terrestrial propulsion in the coming years. To mitigate the inherent polluting effects of internal combustion engines, different technologies have been proposed to develop more efficient engines. Among them, the application of thermal coatings on the combustion chamber walls. This technology aims at reducing the heat losses in the engine, and thus increase its thermal efficiency. The main objective of this thesis is to study the impact of coating the combustion chamber walls of an engine on heat losses and thermal efficiency. The experimental definition of the heat fluxes through the walls is complex and not very reliable because it requires the measurement of wall temperatures. For this reason, CFD-CHT (Computational fluid dynamics-Conjugate Heat Transfer) is used. The first step was to validate the computational tool employed for CFD-CHT calculations in internal combustion engines. For this, a preliminary study in simple geometries such as a circular pipe or a rectangular channel was performed. Heat transfer models were evaluated and the relevance of certain parameters such as roughness was determined. To reinforce the study, a thermal analysis in a more realistic geometry such as the piston of a CI engine was carried out. The temperature values calculated by the software were almost the same as the experimental measurements. Consequently, the reliability of the computational tool was verified. Next, a methodology was proposed to address the problem of modeling very thin layers of thermal coating for three-dimensional CFD-CHT calculations. The methodology consists in defining an "equivalent material" with a thickness and number of nodes that allow a computationally realistic mesh. For this, a DoE in combination with a multiple regression analysis was employed. The first CFD-CHT simulations in ICEs were carried out for a gasoline engine. The study was performed for two configurations: metallic engine and engine with coated piston and cylinder head. An exhaustive heat transfer analysis was made in order to determine the impact of applying the thermal coating on the engine. Comparison with experimental data proved the suitability of the CHT calculations to evaluate heat losses in ICEs. However, no improvement on engine efficiency was observed in the gasoline engine due to the type of coating applied on the combustion chamber walls. Experience with the gasoline engine calculations showed that CHT calculations were very time consuming. In this regard, some strategies aimed at optimizing the calculations were analyzed in order to reduce calculation times. The most successful methodology was based on AMR cell refinement to optimize the mesh and reduce significantly the computational costs. This approach was used to study the impact of applying a new generation thermal coating on the piston top of a Diesel engine. The results obtained indicated that this type of coating allows for some improvement in the thermal efficiency of the engine without affecting its performance.
The author wishes to acknowledge the financial support received through contract FPI-2018-S2-1205 of the Programa para la Formación de Personal investigador (FPI) 2018 of Universitat Politècnica de València. Parts of the work presented in this thesis have received funding from the European Union’s Horizon 2020 research and innovation programme undergrant agreement No 724084.The author wishes to thank IFPEN for their permission to use their single cylinder engine geometry and experimental results, as well as Saint Gobain Research Provence for providing the coating characteristics.The respondent wants to express its gratitude to CONVERGENT SCIENCE Inc. and Convergent Science GmbH for their kind support for performingthe CFD-CHT calculations using CONVERGE software
Escalona Cornejo, JE. (2021). Modelling of Heat Losses through Coated Cylinder Walls and their Impact on Engine Performance [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/165244
TESIS

In this thesis, the ribbed ducts of the internal cooling passage in turbine blading are investigated to demonstrate the effects of high speed rotation. Rotation coupled with high temperature operating conditions alters the mean flow, turbulence, and heat transfer augmentation due to Coriolis and centrifugal buoyancy forces that arises from density stratification in the domain. Gas turbine engines operate in particle laden environments (sand, volcanic ash), and particulate matter ingested by the engine can make their way into the blade internal cooling passages over thousands of operating hours. These particulates can deposit on the walls of these cooling passages and degrade performance of the turbine blade. Large-Eddy Simulations (LES) with temperature dependent properties is used for turbulent flow and heat transfer in the ribbed cooling passages and Lagrangian tracking is used to calculate the particle trajectories together with a wall deposition model. The conditions used are Re=100,000, Rotation number, Ro = 0.0 and 0.2, and centrifugal Buoyancy parameters of Bo=0, 0.5, and 1.0. First, the independent effects of Coriolis and centrifugal buoyancy forces are investigated, with a focus on the additional augmentation obtained in heat transfer with the addition of centrifugal buoyancy. Coriolis forces are known to augment heat transfer at the trailing wall and attenuate the same at the leading wall. Phenomenological arguments stated that centrifugal buoyancy augments the effects of Coriolis forces in outward flow in the first pass while opposing the effect of Coriolis forces during inward flow in the second pass. In this study, it was found that in the first pass, centrifugal buoyancy had a greater effect in augmenting heat transfer at the trailing wall than in attenuating heat transfer at the leading wall. On the contrary, it aided heat transfer in the second half of the first pass at the leading wall by energizing the flow near the wall. Also, contrary to phenomenological arguments, inclusion of centrifugal buoyancy augmented heat transfer over Coriolis forces alone on both the leading and trailing walls of the second pass. Sand ingestion is then investigated, by injecting 200,000 particles in the size range of 0.5-175μm with 65% of the particles below 10 μm. Three duct wall temperatures are investigated, 950, 1000 and 1050 °C with an inlet temperature of flow and particles at 527 °C . The impingement, deposition levels, and impact characteristics are recorded as the particles move through the domain. It was found that the Coriolis force greatly increases deposition. This was made prevalent in the first pass, as 84% of the deposits in the domain occurred in the first pass for the rotating case, whereas only 27% of deposits occurred in the first pass for the stationary case with the majority of deposits occurring in the bend region. This was due to an increased interaction with the trailing wall in the rotating case whereas particles in the stationary case were allowed to remain in the mean flow and gain momentum, making rebounding from a wall during collision more likely than deposition. In contrast, the variation of wall temperatures caused little to no change in deposition levels. This was concluded to be a result of the high Reynolds number used in the flow. At high Reynolds numbers, the particles have a short residence times in the internal cooling circuit not allowing the flow and particles to heat up to the wall temperature. Overall, 87% of the injected particles deposited in the rotating duct whereas 58% deposited in the stationary duct.
Master of Science

The tips of unshrouded, high-pressure turbine blades are prone to significantly high heat loads. The gap between the tip and over-tip casing is the root cause of undesirable over-tip leakage flow that is directly responsible for high thermal material degradation and is a major source of aerodynamic loss within a turbine. Both must be minimised for the safe working and improved performance of future gas-turbines. A joint experimental and numerical study is presented to understand and characterise the heat transfer and aerodynamics of unshrouded blade tips. The investigation is undertaken with the use of a squealer or cavity tip design, known for offering the best overall compromise between the tip aerodynamics, heat transfer and mechanical stress. Since there is a lack of understanding of these tips at engine-realistic conditions, the present study comprises of a detailed analysis using a high-speed linear cascade and computational simulations. The aero-thermal performance is studied to provide a better insight into the behaviour of squealer tips, the effects of casing movement and tip cooling. The linear cascade environment has proved beneficial for its offering of spatially-resolved data maps and its ability to validate computational results. Due to the unknown tip gap height within an entire engine cycle, the effects of gap height are assessed. The squealer's aero-thermal performance has been shown to be linked with the gap height, and qualitative different trends in heat transfer are established between low-speed and high-speed tip flow regimes. To the author's knowledge, the present work is the first of its kind, providing comprehensive aero-thermal experimental research and a dataset for a squealer tip at engine-representative transonic conditions. It is also unique in terms of conducting direct and systematic validations of a major industrial computational fluid dynamics method for aero-thermal performance prediction of squealer tips at enginerepresentative transonic conditions. Finally, after recognising the highest heat loads are found on the squealer rims, a novel shaped squealer tip has been investigated to help improve the thermal performance of the squealer with a goal to improve its durability. It has been discovered that a seven percent reduction in tip temperature can be achieved through incorporating a shaped squealer and maximising the internal cooling performance.

De récentes études expérimentales ont montré que les valeurs usuelles du coefficient d’échange convectif sont différentes en présence de matériaux à changement de phase. Cette thèse de doctorat porte sur l'étude numérique des échanges convectifs fluide/paroi dans une cavité ouverte en régime dynamique. Plus précisément, les parois étudiées sont une paroi avec une capacité thermique et une paroi qui contient des matériaux à changement de phase. Trois modèles distincts ont été développés. Dans un premier temps un modèle (modèle 1) qui concerne l’interaction fluide-paroi à la surface d’une paroi résistive (temperature imposée) en régime laminaire stationnaire a été développé et validé. Les résultats ont été confrontés avec la littérature. Ensuite, les échanges convectifs à la surface d’une plaque capacitive (modèle 2) soumise à une rampe de température d’air ont été étudiés. Finalement, un troisième modèle (modèle 3) a été développé, à la suite du modèle 2. Ce dernier modèle concerne l’interaction fluide-paroi à la surface d’une paroi contenant des matériaux à changement de phase en régime dynamique. Les résultats obtenus révèlent des pics locaux du flux de chaleur au cours du temps. Ce fait témoigne du changement d’état à l’intérieur de la paroi qui contient le materiau à changement de phase. De plus, les courbes des coefficients d’échanges convectifs moyens révèlent la dépendance du coefficient d’echange convectif à la capacité thermique du materiau. Par conséquent, la présence des matériaux à changement de phase à l’intérieur d'une paroi influence l’évolution et la forme de la couche limite thermique
Recent experimental studies have shown that the usual values ​​of the convective heat transfer coefficient h are no longer valid in the presence of phase change materials. Three separate models were developed. Initially a model 1 which treats the fluid-wall (constant temperature) interaction in steady laminar flow has been developed and validated. Then, the wall with heat capacity (model 2) subjected to an air temperature ramp were studied. Finally, a third model (3) has been developed which treats the interaction fluid-wall which contains a phase change material. The results show local peaks of heat flow over time. This fact reflects the phase change inside the wall. Moreover, the curves of the convective heat transfer coefficient indicate the dependence of the coefficient h to the wall’s energy storage capacity. Therefore, the presence of the phase change materials within a wall effect and changes the shape of the thermal boundary layer

La trempe à l’air est largement utilisée dans les procédés de production de verre de sécurité. L’obtention d’une distribution de contraintes adéquate requiert un refroidissement intense et homogène à la fois, et ces deux propriétés sont difficiles à obtenir sur la courte durée de la trempe. Les batteries de jets utilisées dans la plupart des systèmes de trempe produisent un refroidissement adéquat mais souffrent d’inhomogénéité, à l’origine de défauts de trempe et de casse durant le processus.L’objectif de cette thèse est d’explorer des nouvelles configurations qui améliorent l’homogénéité du refroidissement en préservant son intensité. L’approche choisie consiste à implanter des jets rotatifs dans les réseaux de manière à accentuer le mélange des jets avant impact. Les études ont été menées principalement par simulation numérique, corroborées par des visualisations par enduit gras sur un banc d’essai dédié, conçu et réalisé dans le cadre de cette thèse.La première phase a été consacrée à la conception des générateurs de jets rotatifs et à l’étude de leur dynamique en mode isolé. Le développement d’une structure tourbillonnaire se formant à l’entrée de chaque lobe du dispositif de mise en rotation a été mis en évidence. L’interaction des jets rotatifs dans le réseau de refroidissement constitue la deuxième phase. Il apparait que la structure cellulaire du schéma d’impact n’est que marginalement perturbée par les jets rotatifs et que la présence de ces derniers n’influe que peu sur la dynamique de l’écoulement. Enfin, la modélisation détaillée des transferts de chaleur sur la plaque d’impact montre que les jets rotatifs ne contribuent que faiblement au refroidissement, mais que l’interférence avec le réseau de jets simples augmente légèrement le transfert de chaleur local au niveau de leur impact. Sans avoir obtenu les résultats escomptés, cette thèse a toutefois montré la complexité du système et le couplage fort entre les phases d’alimentation et d’évacuation de l’air de refroidissement
Air quenching is widely applied in security glass manufacturing processes. Proper residual stresses distribution requires strong and homogeneous cooling and both are difficult to achieve over the very short time of the tempering process. Jet arrays used in most processes provide with sufficient cooling but suffer from inherent inhomogeneity, leading to quality loss of the glass product and, in extreme cases, to unacceptable breaking numbers during production.The objective of the present study is to investigate ways to improve cooling homogeneity while maintaining efficiency. For this purpose, swirling jets are located inside the jet arrays to enhance jet mixing prior to impingement. Numerical simulation is performed, corroborated by oil flow visualization and a dedicated test bench has been designed and set up within the frame of this thesis.The first part was concerned with the design of swirlers and their dynamic behaviour in standalone mode. It has been shown that a vortex is forming at the inlet of each swirl compartment. Inserting the swirlers within jet arrays constitutes the seconf phase. It turns out that the cellular structure of the impingement pattern is only marginally affected by the swirlers, which have a weak influence on the flow dynamics. Last, the detailed heat transfer modeling on the impingement surface shows that the swirlers themselves do barely contribute to the overall cooling, while the coupling with the simple jet array slightly improves the local heat transfer close to the impingement area. Although the expected outcome was not achieved, this thesis showed the flow complexity as well as the strong coupling between the feeding and the exhaust phases experienced by the cooling air

This thesis is concerned with the development and implementation of computational fluid dynamics (CFD) based prediction methodologies for turbulent reacting flows with principal application to turbulent diffusion flame combustors. Numerical simulation of combustion problems involve strong coupling between chemistry, transport and fluid dynamics. The works accomplished in this study can be separated mainly into three distinct areas: i) assessment of the performance of turbulent combustion models and to implement suitable submodels for combustion and flame behaviour into CFD code; ii) Conducting CFD modelling of turbulent diffusion flames, radiation heat loss from combustion and flame zones; and iii) modelling of pollutants like NOx (oxides of nitrogen), identification of the effect of radiation heat loss on NOx formation. The combustion models studied are the flame-sheet, equilibrium, eddy break-up and laminar flamelet models. An in-house CFD code is developed and combustion models are implemented. The basic numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD (total variation diminishing) schemes. The combustion of different fuels ranging from simple H2/N2 and CO/H2/N2 to complex CH4/H2 are investigated for different inlet velocities and boundary conditions. The performances of the combustion models are analysed for these fuels. The configurations used for the validation and assessment of the combustion models are co-flowing jet flames and bluff body burner stabilized flames. The high quality experimental databases available from Sandia national laboratories, the University of Sydney and other reported measurements are used for the purpose of evaluating the combustion models. The predicted results demonstrate the effects of turbulent mixing and the effects of chemical reactions on the combustion models. The calculations show that all the combustion models like flame-sheet and equilibrium models are found to be inadequate even for the near equilibrium flames. Although the equilibrium chemistry model is capable of predicting the mixture fraction, temperature and concentrations of major and minor species, the predictive accuracy is found to be inadequate specially, when compared to the experimental data. In situations, where finite rate chemistry effects are important the laminar flamelet model is a good choice. The key contributions of this thesis are as follows: 1) Modification of in-house CFD code for turbulent reacting flow and development of CFD based iterative scheme for the turbulent diffusion flames to account for radiation heat loss from combustion and flame zones. 2) Thorough assessment of turbulent combustion modelling techniques for different cases of diffusion flames, demonstration of the importance of differential diffusion in the flamelet modelling of combustion and comprehensive validation 3) Demonstration of the importance of radiation heat loss in the modelling of turbulent combustion, implementation of radiation modelling in the three cases of diffusion flames and comprehensive validation of CFD based combustion radiation results. 4) Development of modelling strategy for the pollutants like oxides of nitrogen (NOx), implementation of NOx modelling in the different flames cases and identified the effect of radiation heat loss on NOx formation. The works addressed in this thesis are presented with the applications to turbulent diffusion flame combustors. However, these works can easily be extended to the industrial applications and applied to a large variety of other challenging domains.

Energy usage in buildings has become a major topic of research in the past decade, driven by the increased cost of energy. Designing buildings to use less energy has become more important, and the ability to analyze buildings before construction can save money in design changes. Computational fluid dynamics (CFD) has been explored as a means of analyzing energy usage and thermal comfort in buildings. Existing research has been focused on simple buildings without much application to real buildings. The current study attempts to expand the research to entire buildings by modeling two existing buildings designed for energy efficient heating and cooling. The first is the Viipuri Municipal Library (Russia) and the second is the Margaret Esherick House (PA). The commercial code FLUENT is used to perform simulations to study the effect of varying atmospheric conditions and configurations of openings. Three heating simulations for the library showed only small difference in results with atmospheric condition or configuration changes. A colder atmospheric temperature led to colder temperatures in parts of the building. Moving the inlet only slightly changed the temperatures in parts of the building. The cooling simulations for the library had more drastic changes in the openings. All three cases showed the building cooled quickly, but the velocity in the building was above recommended ranges given by ASHRAE Standard 55. Two cooling simulations on the Esherick house differed only by the addition of a solar heat load. The case with the solar heat load showed slightly higher temperatures and less mixing within the house. The final simulation modeled a fire in two fireplaces in the house and showed stratified air with large temperature gradients.
Master of Science

Thermal properties of complex suspension flows are investigated using numerical computations. The objective is to develop an efficient and accurate computational method to investigate heat transport in suspension flows. The method presented here is based on solving the lattice Boltzmann equation for the fluid phase, as it is coupled to the Newtonian dynamics equations to model the movement of particles and the energy equation to find the thermal properties. This is a direct numerical simulation that models the free movement of the solid particles suspended in the flow and its effect on the temperature distribution. Parallel implementations are done using MPI (message passing interface) method. Convective heat transfer in internal suspension flow (low solid volume fraction, φ<10%), heat transfer in hot pressing of fiber suspensions and thermal performance of particle filled thermal interface materials (high solid volume fraction, φ>40%) are investigated. The effects of flow disturbance due to movement of suspended particles, thermo-physical properties of suspensions and the particle micro structures are discussed.

Although the terms "Human Thermal Comfort" and "Indoor Air Quality (IAQ)" can be highly subjective, they still dictate the indoor climate design (HVAC design) of a building. In order to evaluate human thermal comfort and IAQ, one of three main tools are used, a) direct questioning the subjects about their thermal and air quality sensation (voting, sampling etc.), b) measuring the human thermal comfort by recording the physical parameters such as relative humidity, air and radiation temperature, air velocities and concentration gradients of pollutants or c) by using numerical simulations either including or excluding detailed thermo-physiological models. The application of the first two approaches can only take place in post commissioning and/or testing phases of the building. Use of numerical techniques can however be employed at any stage of the building design. With the rapid development in computational hard- and software technology, the costs involved in numerical studies has reduced compared to detailed tests. Employing numerical modelling to investigate human thermal comfort and IAQ however demand thorough verification and validation studies. Such studies are used to understand the limitations and application of numerical modelling of human thermal comfort and IAQ in indoor climates. This PhD research is an endeavour to verify, validate and apply, numerical simulation for modelling heat transfer and respiration of occupants in indoor climates. Along with the investigations concerning convective and radiation heat transfer between the occupants and their surroundings, the work focuses on detailed respiration modelling of sedentary human occupants. The objectives of the work have been to: verify the convective and radiation numerical models; validate them for buoyancy-driven flows due to human occupants in indoor climates; and apply these validated models for investigating human thermal comfort and IAQ in a real classroom for which field study data was available. On the basis of the detailed verification, validation and application studies, the findings are summarized as a set of guidelines for simulating human thermal comfort and IAQ in indoor climates. This PhD research involves the use of detailed human body geometries and postures. Modelling radiation and investigating the effect of geometrical posture has shown that the effective radiation area varies significantly with posture. The simulation results have shown that by using an effective radiation area factor of 0.725, estimated previously (Fanger, 1972) for a standing person, can lead to an underestimation of effective radiation area by 13% for the postures considered. Numerical modelling of convective heat transfer and respiration processes for sedentary manikins have shown that the SST turbulence model (Menter, 1994) with appropriate resolution of near wall region can simulate the local air velocity, temperature and heat transfer coefficients to a level of detail required for prediction of thermal comfort and IAQ. The present PhD work has shown that in a convection dominated environment, the detailed seated manikins give rise to an asymmetrical thermal plume as compared to the thermal plumes generated by simplified manikins or point sources. Validated simulation results obtained during the present PhD work have shown that simplified manikins can be used without significant limitations while investigating IAQ of complete indoor spaces. The use of simplified manikins however does not seem appropriate when simulating detailed respiration effects in the immediate vicinity of seated humans because of the underestimation in the amount of re-inhaled CO2 and pollutants from the surroundings. Furthermore, the results have shown that due to the simplification in geometrical form of the nostrils, the CO2 concentration is much higher near the face region (direct jet along the nostrils) as compared to a detailed geometry (sideways jet). Simulating the complete respiration cycle has shown that a pause between exhalation and inhalation has a significant effect on the amount of re-inhaled CO2. Previous results have shown the amount of re-inhaled CO2 to range between 10 - 19%. The present study has shown that by considering the pause, this amount of re-inhaled CO2 falls down to values lower than 1%. A comparison between the simplified and detailed geometry has shown that a simplified geometry can cause an underestimation in the amount of re-inhaled CO2 by more than 37% as compared to a detailed geometry. The major contribution to knowledge delivered by this PhD work is the provision of a validated seated computational thermal manikin. This PhD work follows a structured verification and validation approach for conducting CFD simulations to predict human thermal comfort and indoor air quality. The work demonstrates the application of the validated model to a classroom case with multiple occupancy and compares the measured results with the simulation results. The comparison of CFD results with measured data advocates the use of CFD and visualizes the importance of modelling thermal manikins in indoor HVAC design rather than designing the HVAC by considering empty spaces as the occupancy has a strong influence on the indoor air flow. This PhD work enables the indoor climate researchers and building designers to employ simplified thermal manikin to correctly predict the mean flow characteristics in indoor surroundings. The present work clearly demonstrates the limitation of the PIV measurement technique, the importance of using detailed CFD manikin geometry when investigating the phenomena of respiration in detail and the effect of thermal plume around the seated manikin. This computational thermal manikin used in this work is valid for a seated adult female geometry.