Thèses sur le sujet « CFD modelling for gas coolers »

Carbon dioxide (CO2) is a natural, low cost refrigerant with good thermo-physical properties. CO2 is a good alternative for replacing HFC refrigerants that possess high global warming potential and reducing the direct impacts of refrigeration systems on the environment. However, CO2 refrigeration systems operate at relatively high condenser/gas cooler pressures and this imposes special design and control considerations. The gas cooler is a very important part of the system and can have significant influence on its performance. In sub-critical operation, good gas cooler/condenser design can reduce the condenser pressure and delay switching to supercritical operation which increases system efficiency. In supercritical operation optimum design and control can enable the system to operate at pressures that maximise system efficiency. In air cooled systems, gas coolers/condensers are of the finned-tube type. This type of heat exchanger is well established in the HVAC and refrigeration industries. The large changes in the CO2 properties in the gas cooler, however, during supercritical operation impose special design and manufacturing considerations. This research project considered the influence of the unique heat transfer characteristics of CO2 on the design and performance of finned tube air cooled condensers/gas coolers for CO2 refrigeration applications. A combined experimental and modelling approach using Computational Fluid Dynamics (CFD) was employed. A CO2 condenser/gas cooler test facility was developed for the experimental investigations. The facility employs a ‘booster’ hot gas bypass CO2 refrigeration system, with associated condenser/gas cooler test rig and evaporator load simulation facility. A series of experimental tests were carried out with two gas coolers which incorporated horizontal and horizontal-vertical slit fins and was obtained adequate experimental data concerning gas cooler performance. CFD modelling was used to study the performance of the gas coolers. The model was validated against test results and was shown to predict the air outlet temperature and heat rejection of the gas cooler with an accuracy of within ±5%. The model was subsequently used to evaluate the effect of a fin slit between the 1st and 2nd row of tubes of the gas cooler as well as a vertical slit on the 1st row before the last tube of the section. The results showed a 6%-8% increase in the heat rejection rate of the gas cooler compared to the performance without the horizontal slit. The vertical slit in the fin of the last tube has resulted in an additional increase in heat rejection over and above that for the horizontal slit of 1%-2%. CFD modelling was also used to investigate the variation of the refrigerant side, air side and overall heat transfer coefficient along the heat exchanger. The results showed that the refrigerant heat transfer coefficient increases with the decreasing of bulk refrigerant temperature and reaches its maximum when the specific heat of the refrigerant is highest. Furthermore, increasing the refrigerant mass flux, increases the refrigerant side heat transfer coefficient and heat rejection. This can reduce the size of the gas cooler for a given capacity at the expense of higher pressure drop and compressor power consumption. Air side and overall heat transfer coefficient correlations were developed for the specific gas cooler designs which were investigated and showed the heat transfer coefficients increase with increasing Reynolds Number.

The objective of this study was the development of a systems-CFD model of the PBMR reactor that used the minimum number of grid points to achieve grid independence. The number of grid points is reduced by increasing the accuracy of the discretisation scheme in the reactor model. Any reduction in the number of grid points leads to an increase in calculation speed, which is critical for systems simulation codes that are used for optimisation or transient simulations. While some previous reactor models had been developed for systems simulation codes, their discretisation schemes have not been optimised to use the minimum number of grid points and some heat transfer phenomena were neglected without knowing the effect. Therefore, there was a need to optimise discretisation schemes as well as investigate the effect of including certain heat transfer mechanisms. Modelling methods for several phenomena were developed and implemented in a reactor model in the Flownex systems simulation code, which is used to simulate the PBMR. Subjects of investigation included pebble bed convection discretisation, fuel sphere discretisation, the effect of the radiation heat transfer modelling approach as well as conjugate conduction and radiation across the helium riser channels in the reactor side reflector. After testing the phenomenological models in isolation, the comprehensive reactor model was tested by simulating the SANA experiment and HTR-10 reactor experiments published by the IAEA. Several sensitivity studies were performed to assess the effect of physical as well as numerical parameters. Two reactor discretisation schemes were also evaluated, namely the control-volume based scheme and the element-based scheme. The control-volume based scheme was found to provide a simpler and more intuitive framework for implementing mathematical models, but not to increase accuracy directly. The most significant finding was that the newly developed second-order accurate convection heat transfer scheme gives the greatest improvement in calculation speed by requiring the least number of pebble bed increments. The other important finding was that the methods currently used in many reactor simulation codes for fuel sphere discretisation and radiation heat transfer approximation are appropriate and give adequate accuracy.
Thesis (Ph.D. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2007

In this thesis a numerical study has been undertaken to investigate turbulent flow and heat transfer in a number of flow problems, representing the gas-cooled reactor core flows. The first part of the research consisted of a meticulous assessment of various advanced RANS models of fluid turbulence against experimental and numerical data for buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of nuclear reactors, both presently-operating Advanced Gas-cooled Reactors (AGRs) and proposed ‘Generation IV’ designs. For this part of the project, an in-house code (‘CONVERT’), a commercial CFD package (‘STAR-CD’) and an industrial code (‘Code_Saturne’) were used to generate results. Wide variations in turbulence model performance were identified. Comparison with the DNS data showed that the Launder-Sharma model best captures the phenomenon of heat transfer impairment that occurs in the ascending flow case; v^2-f formulations also performed well. The k-omega-SST model was found to be in the poorest agreement with the data. Cross-code comparison was also carried out and satisfactory agreement was found between the results.The research described above concerned flow in smooth passages; a second distinct contribution made in this thesis concerned the thermal-hydraulic performance of rib-roughened surfaces, these being representative of the fuel elements employed in the UK fleet of AGRs. All computations in this part of the study were undertaken using STAR-CD. This part of the research took four continuous and four discrete design factors into consideration including the effects of rib profile, rib height-to-channel height ratio, rib width-to-height ratio, rib pitch-to-height ratio, and Reynolds number. For each design factor, the optimum configuration was identified using the ‘efficiency index’. Through comparison with experimental data, the performance of different RANS turbulence models was also assessed. Of the four models, the v^2-f was found to be in the best agreement with the experimental data as, to a somewhat lesser degree were the results of the k-omega-SST model. The k-epsilon and Suga models, however, performed poorly. Structured and unstructured meshes were also compared, where some discrepancies were found, especially in the heat transfer results. The final stage of the study involved a simulation of a simplified 3-dimensional representation of an AGR fuel element using a 30 degree sector configuration. The v^2-f model was employed and comparison was made against the results of a 2D rib-roughened channel in order to assess the validity and relevance of the precursor 2D simulations of rib-roughened channels. It was shown that although a 2D approach is extremely useful and economical for ‘parametric studies’, it does not provide an accurate representation of a 3D fuel element configuration, especially for the velocity and pressure coefficient distributions, where large discrepancies were found between the results of the 2D channel and azimuthal planes of the 3D configuration.

Stationary gas turbines manufacturers and operators are under constant scrutiny to both reduce environmentally harmful emissions and obtain efficient combustion. Numerical simulations have become an integral part of the development and optimisation of gas turbine combustors. In this thesis work, the gas turbine combustion process is analysed in two parts, a study on air-fuel mixing and turbulent combustion. For computational fluid dynamic analysis work the open-source CFD code OpenFOAM and STAR-CCM+ are used. A fuel jet injected to cross-flowing air flow is simplified air-fuel mixing arrangement, and this problem is analysed numerically in the first part of the thesis using both Reynolds Averaged Navier Stokes (RANS) method and Large Eddy Simulation (LES) methods. Several turbulence models are compared against experimental data in this work, and the complex turbulent vortex structures their effect on mixing field prediction is observed. Furthermore, the numerical methods are extended to study twin jets in cross-flow interaction which is relevant in predicting air-fuel mixing with arrays of fuel injection nozzles. LES methods showed good results by resolving the complex turbulent structures, and the interaction of two jets is also visualised. In this work, all three turbulent combustion regimes non-premixed, premixed, partially premixed are modelled using different combustion models. Hydrogen blended fuels have drawn particular interest recently due to enhanced flame stabilisation, reduced CO2 emissions, and is an alternative method to store energy from renewable energy sources. Therefore, the well known Sydney swirl flame which uses CH4: H2 blended fuel mixture is modelled using the steady laminar flamelet model. This flame has been found challenging to model numerically by previous researchers, and in this work, this problem has been addressed with improved combustion modelling approach with tabulated chemistry. Recognizing that the current and future gas turbine combustors operate on a mixed combustion regime during its full operational cycle, combustion simulations of premixed/partially premixed flames are also performed in this thesis work. Dynamical artificially thickened flame model is implemented in OpenFOAM and validated using propagating and stationary premixed flames. Flamelet Generated Manifold (FGM) methods are used in the modelling of turbulent stratified flames which is a relatively new field of under investigation, and both experimental and numerical analysis is required to understand the physics. The recent experiments of the Cambridge stratified burner are studied using the FGM method in this thesis work, and good agreement is obtained for mixing field and temperature field predictions.

This Technology in Industry Fellowship (TIF) funded Masters Project was structured around Computational Fluid Dynamics (CFD) modelling for Lemar Environmental Ltd (Lemar). This study is a component of a larger programme that is being undertaken by Lemar concerning the vitrification process. The modelling has built on an initial model developed by CSIRO for Lemar and has been carried out under the direction of Canterbury University. The modelling involved computer simulations and detailed comparisons of the gas flow for both high and low swirl vanes, in both the steady state and transient modes. The output of this activity; velocity profiles (tangential and axial), vorticity, as well as particle tracking (in steady state mode only) were compared to literature and evaluated for both scenarios. As the study was restricted to the gas flow in transient mode, no recommendations and extrapolated modifications to the burner geometry and plant equipment can be made as they have to be verified by the particle motion within the gas flow. The steady state particle simulations obtained through this project, did not provide sufficient evidence to conclude that particles attach to the outer wall and only demonstrated the influences that the high and low swirl had on the particles. Further investigations of transient particle tracking would provide an overall interpretation as to whether or not the dried sludge particles bounced or stuck to the viscous slag layer and a commentary as to their movement in the chamber. Lemar's strategic vitrification programme is still active and the resulting redesign process is nearing completion and modifications to the plant are expected to be finalised by January 2008. Following extensive testing by Lemar it is understood that they would be looking to seek venture capital in order to progress the project to the market. In order for the final stage of the sewage sludge vitrification plant project to commence, Lemar has been in consultation with subject matter experts in the field, as well as undertaking trials on the plant, computer modelling and research into both the technical and international marketing prospects for the combustion technology. The detailed analysis and research undertaken through the CFD modelling conducted for this Project, recommends that Lemar conducts further CFD modelling to investigate transient particle tracking before any plant or geometry modifications are proposed and undertaken in order to optimise the ash capture which is a key output of the vitrification process.

Methane and oxygen are a promising propellant combination in future rocket propulsion engines mainly due to its advantages like reusability and cost reduction. In order to have a comprehensive understanding of this propellant combination extensive research work is being done. Especially, for reusable rocket engines the thermal calculations become vital as an effective and efficient cooling system is crucial for extending the engine life. The design of cooling channels may significantly be influenced by radiation. Within the framework of this thesis, the gas radiation heat transfer is modelled for CFD simulation of rocket thrust chambers and analysed for the 𝐶𝐻4/𝑂2 fuel combination. The radiation is modelled within ArianeGroup’s in-house spray combustion CFD tool - Rocflam3, which is used to carry out the simulations. Radiation properties can have strong influence for certain chemical compositions, especially 𝐶𝑂2 and 𝐻2𝑂 which are the products of the 𝐶𝐻4 and 𝑂2 combustion. A simplified gas radiation transport equation is implemented along with various spectral models which compute the gas emissivity for higher temperature. Also, Rocflam-II code which has an existing gas radiation model is used to compare and validate the simplified model. Finally the combination of the convective and radiative heat transfer values are compared to the experimental test data. In contrast to the previously existing emissivity models with a certain temperature limit, the model used here enables the inclusion for the total emissivity of 𝐶𝑂2 and 𝐻2𝑂 for temperatures up to 3400 K and hence more appropriate for hydrocarbon combustion in space propulsion systems. It turns out that the gas radiation is responsible for 2-4% of the total heat flux for a 𝐶𝐻4/𝑂2 combustion chamber with maximum integrated temperature of 2700 K. The influence of gas radiation would be greater than 4% respective of the integrated temperature. Gas radiation heat flux effects are higher in stream-tube combustion zone compared to the other sections of the thrust chamber. The individual contribution of radiative heat flux by 𝐶𝑂2 was noted to be 1.5-2 times higher than that to 𝐻2𝑂. It was shown that the analytically derived simplified expression for gas radiation along with the various spectral models had reasonable approximation of the measured radiation. The estimated radiation was correct to the measured radiation from the Rocflam-II model for a temperature range of 400-3400 K.
Metan och syre är en lovande kombination av drivmedel i framtida raketframdrivningsmotorer främst på grund av dess fördelar som återanvändbarhet och kostnadsminskning. För att få en omfattande förståelse av denna drivmedelkombination görs ett omfattande forskningsarbete. Speciellt för återanvändbara raketmotorer blir värmeberäkningarna viktiga eftersom ett effektivt och effektivt kylsystem är avgörande för att förlänga livslängden på motorn. Utformningen av kylkanaler kan betydligt påverkas av strålning. Inom ramen för denna avhandling modelleras gasstrålningsvärmeöverföringen för CFD-simulering av rakettryckkamrar och analyseras för 𝐶𝐻4/𝑂2 -bränslekombinationen. Strålningen är modellerad i ArianeGroup’s egen förbränning CFD-verktyg - Rocflam3, som används för att utföra simuleringarna. Strålningsegenskaper kan ha starkt inflytande för vissa kemiska kompositioner, särskilt 𝐶𝑂2 och 𝐻2𝑂 som är produkterna från förbränningen 𝐶𝐻4 och 𝑂2. En förenklad gasstrålningstransportekvation implementeras tillsammans med olika spektralmodeller som beräknar gasemissiviteten för högre temperatur. Dessutom används Rocflam-II-kod som har en befintlig gasstrålningsmodell för att jämföra och validera den förenklade modellen. Slutligen jämförs kombinationen av konvektiva och strålningsvärmeöverföringsvärden med de experimentella testdata. Till skillnad från de tidigare existerande utsläppsmodellerna med en viss temperaturgräns möjliggör modellen som används här att inkludera den totala emissiviteten för 𝐶𝑂2 och 𝐻2𝑂 för temperaturer upp till 3400 K och därmed mer lämplig för kolväteförbränning i rymdframdrivningssystem. Det visar sig att gasstrålningen svarar för 2-4% av det totala värmeflödet för en 𝐶𝐻4/𝑂2 förbränningskammare med maximal integrerad temperatur på 2700 K. Påverkan av gasstrålning skulle vara större än 4% av den integrerade temperaturen. Effekter på värmeströmning av gasstrålning är högre i strömrörs förbränningszon jämfört med de andra sektionerna av tryckkammaren. Det individuella bidraget från strålningsvärmeflöde med 𝐶𝑂2 noterades vara 1.5-2 gånger högre än det 𝐻2𝑂. Det visades att det analytiskt härledda förenklade uttrycket för gasstrålning tillsammans med de olika spektralmodellerna hade en rimlig tillnärmning av det uppmätta strålning. Den uppskattade strålningen var korrekt den uppmätta strålningen från Rocflam-II-modellen för ett temperaturintervall på 400-3400 K.

Methane and oxygen are a promising propellant combination in future rocket propulsion engines mainly due to its advantages like reusability and cost reduction. In order to have a comprehensive understanding of this propellant combination extensive research work is being done. Especially, for reusable rocket engines the thermal calculations become vital as an effective and efficient cooling system is crucial for extending the engine life. The design of cooling channels may significantly be influenced by radiation. Within the framework of this thesis, the gas radiation heat transfer is modelled for CFD simulation of rocket thrust chambers and analysed for the 𝐶𝐻4/𝑂2 fuel combination. The radiation is modelled within ArianeGroup’s in-house spray combustion CFD tool - Rocflam3, which is used to carry out the simulations. Radiation properties can have strong influence for certain chemical compositions, especially 𝐶𝑂2 and 𝐻2𝑂 which are the products of the 𝐶𝐻4 and 𝑂2 combustion. A simplified gas radiation transport equation is implemented along with various spectral models which compute the gas emissivity for higher temperature. Also, Rocflam-II code which has an existing gas radiation model is used to compare and validate the simplified model. Finally the combination of the convective and radiative heat transfer values are compared to the experimental test data. In contrast to the previously existing emissivity models with a certain temperature limit, the model used here enables the inclusion for the total emissivity of 𝐶𝑂2 and 𝐻2𝑂 for temperatures up to 3400 K and hence more appropriate for hydrocarbon combustion in space propulsion systems. It turns out that the gas radiation is responsible for 2-4% of the total heat flux for a 𝐶𝐻4/𝑂2 combustion chamber with maximum integrated temperature of 2700 K. The influence of gas radiation would be greater than 4% respective of the integrated temperature. Gas radiation heat flux effects are higher in stream-tube combustion zone compared to the other sections of the thrust chamber. The individual contribution of radiative heat flux by 𝐶𝑂2 was noted to be 1.5-2 times higher than that to 𝐻2𝑂. It was shown that the analytically derived simplified expression for gas radiation along with the various spectral models had reasonable approximation of the measured radiation. The estimated radiation was correct to the measured radiation from the Rocflam-II model for a temperature range of 400-3400 K.
Metan och syre är en lovande kombination av drivmedel i framtida raketframdrivningsmotorer främst på grund av dess fördelar som återanvändbarhet och kostnadsminskning. För att få en omfattande förståelse av denna drivmedelkombination görs ett omfattande forskningsarbete. Speciellt för återanvändbara raketmotorer blir värmeberäkningarna viktiga eftersom ett effektivt och effektivt kylsystem är avgörande för att förlänga livslängden på motorn. Utformningen av kylkanaler kan betydligt påverkas av strålning. Inom ramen för denna avhandling modelleras gasstrålningsvärmeöverföringen för CFD-simulering av rakettryckkamrar och analyseras för 𝐶𝐻4/𝑂2 -bränslekombinationen. Strålningen är modellerad i ArianeGroup’s egen förbränning CFD-verktyg - Rocflam3, som används för att utföra simuleringarna. Strålningsegenskaper kan ha starkt inflytande för vissa kemiska kompositioner, särskilt 𝐶𝑂2 och 𝐻2𝑂 som är produkterna från förbränningen 𝐶𝐻4 och 𝑂2. En förenklad gasstrålningstransportekvation implementeras tillsammans med olika spektralmodeller som beräknar gasemissiviteten för högre temperatur. Dessutom används Rocflam-II-kod som har en befintlig gasstrålningsmodell för att jämföra och validera den förenklade modellen. Slutligen jämförs kombinationen av konvektiva och strålningsvärmeöverföringsvärden med de experimentella testdata. Till skillnad från de tidigare existerande utsläppsmodellerna med en viss temperaturgräns möjliggör modellen som används här att inkludera den totala emissiviteten för 𝐶𝑂2 och 𝐻2𝑂 för temperaturer upp till 3400 K och därmed mer lämplig för kolväteförbränning i rymdframdrivningssystem. Det visar sig att gasstrålningen svarar för 2-4% av det totala värmeflödet för en 𝐶𝐻4/𝑂2 förbränningskammare med maximal integrerad temperatur på 2700 K. Påverkan av gasstrålning skulle vara större än 4% av den integrerade temperaturen. Effekter på värmeströmning av gasstrålning är högre i strömrörs förbränningszon jämfört med de andra sektionerna av tryckkammaren. Det individuella bidraget från strålningsvärmeflöde med 𝐶𝑂2 noterades vara 1.5-2 gånger högre än det 𝐻2𝑂. Det visades att det analytiskt härledda förenklade uttrycket för gasstrålning tillsammans med de olika spektralmodellerna hade en rimlig tillnärmning av det uppmätta strålning. Den uppskattade strålningen var korrekt den uppmätta strålningen från Rocflam-II-modellen för ett temperaturintervall på 400-3400 K.

The aim of this thesis is three-fold: i) to investigate the performance of both the Eulerian-Lagrangian model and the Eulerian-Eulerian model to simulate the turbulent gas-particle flow; ii) to investigate the indoor airflows and contaminant particle flows using the Eulerian-Lagrangian model; iii) to develop and validate particle-wall collision models and a wall roughness model for the Eulerian-Lagrangian model and to utilize these models to investigate the effects of wall roughness on the particle flows. Firstly, the Eulerian-Lagrangian model in the software package FLUENT (FLUENT Inc.) and the Eulerian-Eulerian model in an in-house research code were employed to simulate the gas-particle flows. The validation against the measurement for two-phase flow over backward facing step and in a 90-degree bend revealed that both CFD approaches provide reasonably good prediction for both the gas and particle phases. Then, the Eulerian-Lagrangian model was employed to investigate the indoor airflows and contaminant particle concentration in two geometrically different rooms. For the first room configuration, the performances of three turbulence models for simulating indoor airflow were evaluated and validated against the measured air phase velocity data. All the three turbulence models provided good prediction of the air phase velocity, while the Large Eddy Simulation (LES) model base on the Renormalization Group theory (RNG) provided the best agreement with the measurements. As well, the RNG LES model is able to provide the instantaneous air velocity and turbulence that are required for the evaluation and design of the ventilation system. In the other two-zone ventilated room configuration, contaminant particle concentration decay within the room was simulated and validated against the experimental data using the RNG LES model together with the Lagrangian model. The numerical results revealed that the particle-wall coll ision model has a considerable effect on the particle concentration prediction in the room. This research culminates with the development and implementation of particle-wall collision models and a stochastic wall roughness model in the Eulerian-Lagrangian model. This Eulerian-Lagrangian model was therefore used to simulate the gas-particle flow over an in-line tube bank. The numerical predictions showed that the wall roughness has a considerable effect by altering the rebounding behaviours of the large particles and consequently affecting the particles motion downstream along the in-line tube bank and particle impact frequency on the tubes. Also, the results demonstrated that for the large particles the particle phase velocity fluctuations are not influenced by the gas-phase fluctuations, but are predominantly determined by the particle-wall collision. For small particles, the influence of particle-wall collisions on the particle fluctuations can be neglected. Then, the effects of wall roughness on the gas-particle flow in a two-dimensional 90-degree bend were investigated. It was found that the wa ll roughness considerably altered the rebounding behaviours of particles by significantly reducing the 'particle free zone' and smoothing the particle number density profiles. The particle mean velocities were reduced and the particle fluctuating velocities were increased when taking into consideration the wall roughness, since the wall roughness produced greater randomness in the particle rebound velocities and trajectories.

Fluidized bed technology is employed in a wide range of industrial applications, covering the pharmaceutical, food, chemical and petrochemical industries as well as the mining and power generation industries. However in most industrial applications of fluidization, the suspension consists of non-spherical particles of different diameters and sometimes-different densities. Computational Fluid Dynamics modelling has been recognized by several in academia and industry as an indepensible tool to study multiphase systems including fluidization. CFD models that describe gas solid flow systems can be formulated at different levels of mathematical detail. The use of the Eulerian-Eulerian approach has been considered as the highest possible level of continuum modelling where both the fluid and particle phase are treated as interpenetrating continua and mass and momentum conservation equations are solved for each phase. The Eulerian-Eulerian approach has been successfully used by many researchers for tackling problems relating to the modelling of gas-solid fluidized beds coupled with using the kinetic theory of granular flow for the description of the solid phase as derived from the kinetic theory of gases. However, most of the CFD investigations carried out to date have been limited to the study of the fluidization behaviour of mono-component gas-solid systems of modelling type materials (e.g. Ballotini). The aim of this thesis is to address the computational modelling of mono-component and binary gas-solid fluidized beds with particular focus placed on industrial materials. This work, sponsored by Huntsman Tioxide, is concerned with the titanium refining industry where a bubbling fluidized bed is used for extracting titanium from naturally occurring ore. The refining process begins in a fluidized bed with the chlorination of titanium rich rutile ore which is composed of many constituents. Due to the size and density differences of all the feedstock components used in the process, there are industrial concerns about the pervasiveness of dead zones within the fluidized bed as a result of feed stock segregation. Thus, the objective of the work is to develop a model capable of predicting the degree of mixing and segregation in the fluid bed system. To this end, the following powders, slag, natural and synthetic rutile, belonging to the Geldart Group B classification and used as feedstock in the Huntsman Tioxide chlorination process, were provided for the experimental and computational investigations in this project. This work presents a new hydrodynamic model for the CFD simulations of the mono-component and binary industrial materials using a commercial code (CFX4.4). The modelling development allowed the assessment of suitable governing equations for the description of the internal stress relevant to the solid phase(s), the fluid-particle and particle-particle interphase exchange terms. For the mono-component systems, a new expression for the fluid-particle interaction term has been developed based on the fluid bed elasticity concept originally proposed by Wallis (1969). Consequently, the procedure followed to obtain a stability criterion was re-examined analytically and subsequently numerical simulations were performed to validate the ability of the model to predict the fluidization behavior of the materials investigated. As part of the development, a comparison was conducted between the model proposed in this thesis and the granular kinetic theory model in order to assess the impact of the collisional stresses on the numerical predictions. The new modelling approach was subsequently extended to the modelling of binary systems using the three fluid approach, where a separate momentum equation is solved for the fluid and each solid phase. This part of the study also assessed the effect of the particle- particle drag force on the dynamics of the binary system by comparing three different closures available in literature and catering for this contribution against a reference test case where such contribution was not accounted for. Similar to the approach followed for the mono-component systems, a sensitivity analysis on the effect of the collisional stress on the simulations of the binary systems was also performed. Furthermore, a sensitivity analysis on grid and time step resolution was also carried out. Results of these analyses enabled the qualitative and quantitative numerical investigation into the mixing and segregation behaviour of the binary mixture of the industrial materials provided for this project. In this investigation, three different average compositions, corresponding to the average mass fraction of jetsam particles of 0.25, 0.50, 0.75 in the bed were considered, so that the hydrodynamic behavior of three binary mixtures in all was studied. In addition, a new fluid-particle interaction force closure for well mixed binary systems based on the two-fluid approach, where mixture continuity and momentum equations are employed in the description of the solid phases, was also derived and corresponding CFD simulations are carried out to assess the reliability of the derived mixture models.

Abstract In ladle metallurgy, gas stirring and the behaviour of the slag layer are very important for alloying and the homogenization of the steel. When gas is injected through a nozzle located at the bottom of the ladle into the metal bath, the gas jet exiting the nozzle breaks up into gas bubbles. The rising bubbles break the slag layer and create an open-eye. The size of the open-eye is very important as the efficiency of the metal-slag reactions depend on the interaction between the slag and steel created during the stirring process, and information about the position and size of the open-eye is important for effective alloying practice. Moreover, the open-eye has an effect on the energy balance since it increases heat losses. In this study, experimental measurements and numerical simulations were performed to study the effect of different operating parameters on the formation of the open-eye and mixing time in a water model and industrial ladle. Experimental measurements were performed to study the effect of the gas flow rate, slag layer thickness, slag layer densities and number of porous plugs in a 1/5 scale water model and in a 150-ton steelmaking ladle. For numerical modelling, a multi-phase volume of fluid (VOF) model was used to simulate the system including the behaviour of the slag layer. The numerical simulation of the open-eye size and mixing time was found to be in good agreement with the experimental data obtained from the water model and data obtained from the industrial measurements
Tiivistelmä Senkkametallurgiassa kaasuhuuhtelu ja kuonakerroksen käyttäytyminen ovat tärkeitä teräksen seostamisen ja homogenisoinnin näkökulmasta. Senkan pohjalla sijaitsevasta suuttimesta puhallettava kaasu hajoaa kupliksi, jotka rikkovat kuonakerroksen ja muodostavat avoimen silmäkkeen. Avoimen silmäkkeen koko on yhteydessä voimakkaampaan kuonan emulgoitumiseen, joka tehostaa metallisulan ja kuonan välisiä reaktioita. Tietoa avoimen silmäkkeen paikasta ja koosta tarvitaan myös tehokkaaseen seostuspraktiikkaan. Avoin silmäke vaikuttaa lisäksi prosessin energiataseeseen lisäten sen lämpöhäviöitä. Tässä tutkimuksessa tutkittiin kokeellisesti ja laskennallisesti erilaisten operointiparametrien vaikutusta avoimen silmäkkeen muodostumiseen vesimallissa ja terässenkassateollisessa senkassa. Kokeellisia mittauksia tehtiin kaasuhuuhtelun, kuonakerroksen paksuuden, ja suuttimien määrän vaikutuksen tutkimiseksi 1/5-mittakaavan vesimallissa ja 150 tonnin terässenkassa. Numeerisessa mallinnuksessa systeemin ja siihen lukeutuvan kuonakerroksen käyttäytymisen simuloimiseen käytettiin volume of fluid (VOF) –monifaasimenetelmää. Avoimen silmäkkeen kokoon ja sekoittumisaikaan liittyvien numeeristen simulointien havaittiin vastaavan hyvin vesimallista ja teollisista mittauksista saatua kokeellista aineistoa

Nowadays, the frequent occurrence of hydrocarbon leaks underlies the need for investigating every possible contribution to the fire and explosion risk in the oil and gas industry. Hence, controlling the potential ignition sources is paramount for ensuring tolerable risk levels. Among potential ignitors, gas turbines (GT) are regarded as one of the main contributors when employed for mechanical-drive in the process area of offshore facilities even though their behaviour when acting as live sources has never been analysed in detail. This study aimed at investigating both the role that GTs play on the fire and explosion risk and the effect of further controlling ignition barrier elements by examining a real case of study. The Kamaleon FireEx™ – Risk and Barrier Management tool was used since this couples a CFD-based description of transient releases dispersion to the ignition probability modelling. After a first optimisation of the grid resolution, a selection procedure was developed to define a base case risk level. This result was analysed to highlight the time-window where ignitions occur and used as terms of comparison in the following sensitivity studies. First, the contribution that the installation of a GT carries along was quantified. The effect of an alternative ignition probability modelling, which underlies either a detailed representation of the GT behaviour when acting as an ignition source or the activation of an inert gas injecting system, was investigated thereafter. Results show the need for a quick response to obtain a risk reduction. Then, the influence of a shield wall that was built around the GT air-intakes was analysed. A covering effect was observed, which involved a reduction in the risk. Finally, since the wall introduced even a delaying effect, it was investigated whether this may combine with the alternative ignition model. Results showed a further risk reduction and the dominant role that physical barriers play in this context.

Utsläpp från bilar har spelat stor roll de senaste decennierna. Detta har lett till ökad användning av Otto gasmotorer som använder naturgas som bränsle. Nya motordesigner behöver optimeras för att förbättra motorens effektivitet. Ett effektivt sätt att göra detta på är genom användningen av simuleringar för att minska ledtiden i motorutvecklingen. Verifiering och validering av simuleringarna spelar stor roll för att bygga förtroende för och förutsägbarhet hos simuleringsresultaten. Syftet med detta examensarbete är att föreslå förbränningsmodellparametrarna efter utvärdering av olika kombinationer av förbrännings- och tändmodeller för Otto förbränning, vad gäller beräkningstid och noggrannhet. In-cylindertrycksspår från simulering och mätning jämförs för att hitta den bästa kombinationen av förbrännings- och tändmodell. Inverkan av tändtid, antal motorcykler och randvillkor för simuleringsresultatet studeras också. Resultaten visar att ECFM-förbränningsmodellen förutsäger simuleringsresultaten mer exakt när man jämför med mätningarna. Effekten av tändningstiden på olika kombinationer av förbrännings- och tändningsmodell utvärderas också. Stabiliteten hos olika förbränningssimuleringsmodeller diskuteras också under körning för fler motorcykler. Jämförelse av beräkningstid görs även för olika kombinationer av förbrännings- och tändmodeller. Resultaten visar också att flamspårningsmetoden med Euler är mer känslig för cellstorlek och kvalitet hos simuleringsnätet, jämfört med övriga studerade modeller. Rekommendationer och förslag ges om nät- och simulerings-inställningar för att prediktera förbränningen på ett så bra sätt som möjligt. Några möjliga förbättringsområden ges som framtida arbete för att förbättra noggrannheten i simuleringsresultaten.
Emission from automobiles has been gaining importance for past few decades. This has gained a lot of impetus in search for alternate fuels among the automotive manufacturers. This led to the increase usage of Otto gas engine which uses natural gas as fuel. New engine designs have to be optimized for improving the engine efficiency. This led to usage of virtual simulations for reducing the lead time in the engine development. The verification and validation of actual phenomenon in the virtual simulations with respect to the physical measurements was quite important.  The aim of this master thesis is to suggest the combustion model parameters after evaluating various combination of combustion and ignition models in terms of computational time and accuracy. In-cylinder pressure trace from the simulation is compared with the measurement in order to find the nest suited combination of combustion and ignition models. The influence of ignition timing, number of engine cycles and boundary conditions on the simulation results are also studied. Results showed that ECFM combustion model predicts the simulation results more accurately when compare to the measurements. Impact of ignition timing on various combination of combustion and ignition model is also assessed. Stability of various combustion simulation models is also discussed while running for more engine cycles. Comparison of computational time is also made for various combination of combustion and ignition models. Results also showed that the flame tracking method using Euler is dependent on the mesh resolution and the mesh quality.  Recommendations and suggestions are given about the mesh and simulation settings for predicting the combustion simulation accurately. Some possible areas of improvement are given as future work for improving the accuracy of the simulation results.

A low-emissions power generator comprising a solid oxide fuel cell coupled to a gas turbine has been developed by Rolls-Royce Fuel Cell Systems. As part of the cycle, a fraction of the unreacted fuel (the off-gas) and oxidizer streams is reacted in a burner, which is the main source of pollutant formation. In this thesis a computational model of the burner has been developed which captures the formation of NOx and the oxidation of CO. This model gives accurate predictions at low computational cost, making it suitable for use as a design tool in future burner design optimization through parametric studies. A key factor in increasing computational efficiency was the development of a reduced H2/CO/N2 kinetic mechanism; from a starting mechanism of 30 species to 10 and 116 reactions to 6. The results of laminar opposed-flow diffusion flames have been used to validate the reduced mechanism. Several different turbulent combustion models have been evaluated by creating an interface between the reduced kinetic mechanism and the commercial CFD solver FLUENT. Comparison of model predictions with well-characterized turbulent syngas flames, which share a similar fuel composition to the experimental work conducted on the off-gas burner, shows acceptable agreement. These studies have demonstrated the sensitivity of modelling constants. Improved predictions were achieved by calibrating these constants and including radiative heat losses. Following suitable modification to reflect the predominantly laminar flow present in the current burner design, the relevant modelling approaches were applied to the off-gas burner. Comparison was made to previous detailed measurements, showing that the important trends of NOx and CO are captured in general. The model was extended to high pressure conditions, similar to those in the actual off-gas burner, with the emissions predictions within design limits. The outcome of this work is a fast, accurate design tool for CFD which has capabilities to simulate beyond the laminar burner studied here. It may be applied to more general types of off-gas/syngas burners where turbulence-chemistry interaction is expected to be more significant.

This study involves the modeling of small cold-gas (N2) thrusters nozzle and plume flows, their interactions with spacecraft surfaces and the induced pressure environment. These small cold-gas thrusters were used for pitch, yaw and roll control and were mounted on the bottom of the conical Environmental Monitor Payload (EMP) suborbital spacecraft. The pitch and yaw thrusters had 0.906 mm throat diameter and 4.826 mm exit diameter, while the roll thrusters had 1.6 mm throat diameter and 5.882 mm exit diameter. During thruster firing, at altitudes between 670 km and 1200 km, pressure measurements exhibited non-periodic pulses (Gatsonis et al., 1999). The pressure sensor was located inside the EMP and was connected to it's sidewall with a 0.1-m long, 0.022-m diameter tube and the pressure pulses appeared instantaneously with the firings for thrusters without a direct line-of-sight with the sensor entrance. Preliminary analysis showed that the plume of these small EMP thrusters undergoes transition from continuous to rarefied. Therefore, nozzle and plume simulations are performed using a combination of Navier-Stokes and Direct Simulation Monte Carlo codes. This study presents first a validation of the Navier-Stokes code Rampant used for the continuous EMP nozzle and plume simulations. The first Rampant validation example involves a two-dimensional axisymetric freejet expansion and is used to demonstrate the use of Bird's breakdown parameter. Results are compared favorably with those of Bird (1980) obtained through the method of characteristics. The second validation example involves three-dimensional plume simulations of a NASA thruster. This nitrogen nozzle has a throat diameter of 3.18 mm, an exit diameter of 31.8 mm, half-angle of 20 degrees, stagnation temperature of 699 K, stagnation pressure of 6,400 Pa. Simulation results are compared favorably with previous Navier-Stokes and Direct Simulation Monte Carlo numerical work. The third validation example involves three-dimensional simulations of Rothe's (1970) nozzle that has a throat diameter of 2.5 mm, an exit diameter of 20.3 mm, half-angle of 20 degrees, operating at stagnation temperature of 300 K and pressure of 1975 Pa. Numerical results also compared favorably to experimental data. The combined Navier-Stokes/DSMC approach and the EMP simulation results are presented and discussed. The continuous part of the EMP nozzle and plume flow is modeled using the three-dimensional Navier-Stokes Rampant code. The Navier-Stokes domain includes the geometry of the nozzle and the EMP base until transition of the continuous flow established by Bird's breakdown parameter. The rarefied part of the plume flow is modeled using the Direct Simulation Monte Carlo code DAC. Flowfield data obtained inside the breakdown surface from the Navier-Stokes simulation are used as inputs to the DSMC simulations. The DSMC domain includes the input surface and the EMP spacecraft geometry. The combined Navier-Stokes/DSMC simulations show the complex structure of the plume flow as it expands over the EMP surfaces. Plume reflection and backflow are demonstrated. The study also summarizes findings presented by Gatsonis et al. (2000), where the DSMC predictions at the entrance of the pressure sensor are used as inputs to a semi-analytical model to predict the pressure inside the sensor. It is shown that the pressure predictions for the pitch/yaw thrusters are close to the measurements. The plume of a pitch or yaw thruster reaches the pressure sensor after expanding on the EMP base. The pressure predicted for the roll thruster is larger that the measured. This is attributed to the uncertainty in the roll thruster location on the EMP base resulting, in the simulation, in a component of direct flow to the sensor.

A BLEVE is a physical explosion characterized by the sudden expansion of a liquefied gas under pressure and the vapor space above it. In this work, the analysis of a set of water BLEVE experiments was carried out both in terms of data processing and numerical modelling. The main purpose of the project was to investigate safety implications of a pipe rupture containing superheated water that may affect a steam generation system in a nuclear or chemical plant. The experimental campaign consisted in 27 explosive tests in which an instantaneous depressurization of the content was enabled by the use of a calibrated rupture disk. A flange calibrated for different dimensions of the releasing orifice was incorporated in the prototype to replicate a pipe failure for various nominal sizes. The analysis primarily focused on the pressure field distribution generated in the surroundings, in the form of multiple shock waves. First observations came directly from high-speed pressure data recorded, showing a high directionality of the blast, stronger in the vertical direction, and the independence of the lead shock on the initial liquid fill level. The intensity of the overpressure of the lead shock was found to be increasingly correlated with the opening size. Available theoretical methods were used to preliminarily estimate the first overpressure peak. Models based on real gas behaviour and adiabatic irreversible expansion gave the best approximation of the vertical overpressure, providing an energy conversion factor (energy contributing to the blast overpressure over the total expansion energy) comparable with values found in the literature. A few CFD simulations were then performed under a shock tube configuration to validate the widely accepted assumption that the lead pressure peak is exclusively depending on the expansion of the pressurized vapor space.

Cette thèse s’inscrit dans le projet d’ASTRID au CEA Cadarache. L’objectif de cette thèse est de développer un outil numérique capable de décrire finement la structure du jet de gaz sous-détendu dans du sodium en fonction du débit de la fuite de gaz. Dans un jet de gaz dans un liquide, il existe une couche de gouttelettes issues du liquide et une couche des bulles de gaz. Le modèle de Baer-Nunziato est alors choisi car c’est le modèle le plus adapté pour cette étude. De plus, la diffusion visqueuse, l’échange de quantité de mouvement entre les deux phases sont également pris en compte. Le modèle développé et ses schémas numériques sont implémentés dans le logiciel CANOP, qui permet de générer le raffinement de maillage adaptatif et de calculer en parallèle. La capacité du code à reproduire jets de gaz sous-détendu est validée. La structure du jet monophasique est cohérente avec les corrélations empiriques, la diffusion visqueuse raccourci la taille de du disque de Mach en raison du changement du comportement au niveau d’injecteur. Pour un jet diphasique, les corrélations empiriques obtenues à partir des jets monophasiques ne sont pas adaptées. Cet outil numérique reste à améliorer par une prise en compte d’une distribution non-uniforme des tailles des phases dispersées et de la fragmentation des phases dispersées etc
This thesis is in the frame of study of the energy conversion system for ASTRID project. The objective of this thesis is to provide a numerical tool that enables to describe the structure of under-expanded gas jet as a function of the flow rate of the gas leak. In a gas jet submerged into a liquid, there is a layer of liquid droplets issue from the entrainment by the gas and a layer of gas bubbles. The Baer-Nunziato model is then chosen as the starting model. In addition, the viscous diffusion, the momentum exchange between the two fluids are taken into account in the present work. The developed model and its numerical schemes are implemented in the CANOP software, which enables to generate the adaptive mesh refinement and to compute in parallel. The capability of the code to reproduce under-expanded gas jets is validated. The structure of the monophasic jet is consistent with the empirical correlations; the viscous diffusion shortens the size of the Mach disk due to the change of flow behavior inside nozzle. For a two-phase jet, the empirical correlations obtained from monophasic jets are not suitable anymore. This numerical tool remains to be improved in some directions, for example, to take the distribution of non-uniform bubbles and break-up of particles owning to the shock waves into consideration

[ES] El principal desafío en los motores turbina de gas empleados en aviación reside en aumentar la eficiencia del ciclo termodinámico manteniendo las emisiones contaminantes por debajo de las rigurosas restricciones. Ésto ha conllevado la necesidad de diseñar nuevas estrategias de inyección/combustión que operan en puntos de operación peligrosos por su cercanía al límite inferior de apagado de llama. En este contexto, el concepto Lean Direct Injection (LDI) ha emergido como una tecnología prometedora a la hora de reducir los óxidos de nitrógeno (NOx) emitidos por las plantas propulsoras de los aviones de nueva generación. En este contexto, la presente tesis tiene como objetivos contribuir al conocimiento de los mecanismos físicos que rigen el comportamiento de un quemador LDI y proporcionar herramientas de análisis para una profunda caracterización de las complejas estructuras de flujo de turbulento generadas en el interior de la cámara de combustión. Para ello, se ha desarrollado una metodología numérica basada en CFD capaz de modelar el flujo bifásico no reactivo en el interior de un quemador LDI académico mediante enfoques de turbulencia U-RANS y LES en un marco Euleriano-Lagrangiano. La resolución numérica de este problema multi-escala se aborda mediante la descripción completa del flujo a lo largo de todos los elementos que constituyen la maqueta experimental, incluyendo su paso por el swirler y entrada a la cámara de combustión. Ésto se lleva a cabo través de dos códigos CFD que involucran dos estrategias de mallado diferentes: una basada en algoritmos de generación y refinamiento automático de la malla (AMR) a través de CONVERGE y otra técnica de mallado estático más tradicional mediante OpenFOAM. Por un lado, se ha definido una metodología para obtener una estrategia de mallado óptima mediante el uso del AMR y se han explotado sus beneficios frente a los enfoques tradicionales de malla estática. De esta forma, se ha demostrado que la aplicabilidad de las herramientas de control de malla disponibles en CONVERGE como el refinamiento fijo (fixed embedding) y el AMR son una opción muy interesante para afrontar este tipo de problemas multi-escala. Los resultados destacan una optimización del uso de los recursos computacionales y una mayor precisión en las simulaciones realizadas con la metodología presentada. Por otro lado, el uso de herramientas CFD se ha combinado con la aplicación de técnicas de descomposición modal avanzadas (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificación numérica de los principales modos acústicos en la cámara de combustión ha demostrado el potencial de estas herramientas al permitir caracterizar las estructuras de flujo coherentes generadas como consecuencia de la rotura de los vórtices (VBB) y de los chorros fuertemente torbellinados presentes en el quemador LDI. Además, la implementación de estos procedimientos matemáticos ha permitido tanto recuperar información sobre las características de la dinámica de flujo como proporcionar un enfoque sistemático para identificar los principales mecanismos que sustentan las inestabilidades en la cámara de combustión. Finalmente, la metodología validada ha sido explotada a través de un Diseño de Experimentos (DoE) para cuantificar la influencia de los factores críticos de diseño en el flujo no reactivo. De esta manera, se ha evaluado la contribución individual de algunos parámetros funcionales (el número de palas del swirler, el ángulo de dichas palas, el ancho de la cámara de combustión y la posición axial del orificio del inyector) en los patrones del campo fluido, la distribución del tamaño de gotas del combustible líquido y la aparición de inestabilidades en la cámara de combustión a través de una matriz ortogonal L9 de Taguchi. Este estudio estadístico supone un punto de partida para posteriores estudios de inyección, atomización y combus
[CA] El principal desafiament als motors turbina de gas utilitzats a la aviació resideix en augmentar l'eficiència del cicle termodinàmic mantenint les emissions contaminants per davall de les rigoroses restriccions. Aquest fet comporta la necessitat de dissenyar noves estratègies d'injecció/combustió que radiquen en punts d'operació perillosos per la seva aproximació al límit inferior d'apagat de flama. En aquest context, el concepte Lean Direct Injection (LDI) sorgeix com a eina innovadora a l'hora de reduir els òxids de nitrogen (NOx) emesos per les plantes propulsores dels avions de nova generació. Sota aquest context, aquesta tesis té com a objectius contribuir al coneixement dels mecanismes físics que regeixen el comportament d'un cremador LDI i proporcionar ferramentes d'anàlisi per a una profunda caracterització de les complexes estructures de flux turbulent generades a l'interior de la càmera de combustió. Per tal de dur-ho a terme s'ha desenvolupat una metodología numèrica basada en CFD capaç de modelar el flux bifàsic no reactiu a l'interior d'un cremador LDI acadèmic mitjançant els enfocaments de turbulència U-RANS i LES en un marc Eulerià-Lagrangià. La resolució numèrica d'aquest problema multiescala s'aborda mitjançant la resolució completa del flux al llarg de tots els elements que constitueixen la maqueta experimental, incloent el seu pas pel swirler i l'entrada a la càmera de combustió. Açò es duu a terme a través de dos codis CFD que involucren estratègies de mallat diferents: una basada en la generación automàtica de la malla i en l'algoritme de refinament adaptatiu (AMR) amb CONVERGE i l'altra que es basa en una tècnica de mallat estàtic més tradicional amb OpenFOAM. D'una banda, s'ha definit una metodologia per tal d'obtindre una estrategia de mallat òptima mitjançant l'ús de l'AMR i s'han explotat els seus beneficis front als enfocaments tradicionals de malla estàtica. D'aquesta forma, s'ha demostrat que l'aplicabilitat de les ferramente de control de malla disponibles en CONVERGE com el refinament fixe (fixed embedding) i l'AMR són una opció molt interessant per tal d'afrontar aquest tipus de problemes multiescala. Els resultats destaquen una optimització de l'ús dels recursos computacionals i una major precisió en les simulacions realitzades amb la metodologia presentada. D'altra banda, l'ús d'eines CFD s'ha combinat amb l'aplicació de tècniques de descomposició modal avançades (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificació numèrica dels principals modes acústics a la càmera de combustió ha demostrat el potencial d'aquestes ferramentes al permetre caracteritzar les estructures de flux coherents generades com a conseqüència del trencament dels vòrtex (VBB) i dels raigs fortament arremolinats presents al cremador LDI. A més, la implantació d'estos procediments matemàtics ha permès recuperar informació sobre les característiques de la dinàmica del flux i proporcionar un enfocament sistemàtic per tal d'identificar els principals mecanismes que sustenten les inestabilitats a la càmera de combustió. Finalment, la metodologia validada ha sigut explotada a traves d'un Diseny d'Experiments (DoE) per tal de quantificar la influència dels factors crítics de disseny en el flux no reactiu. D'aquesta manera, s'ha avaluat la contribución individual d'alguns paràmetres funcionals (el nombre de pales del swirler, l'angle de les pales, l'amplada de la càmera de combustió i la posició axial de l'orifici de l'injector) en els patrons del camp fluid, la distribució de la mida de gotes del combustible líquid i l'aparició d'inestabilitats en la càmera de combustió mitjançant una matriu ortogonal L9 de Taguchi. Aquest estudi estadístic és un bon punt de partida per a futurs estudis de injecció, atomització i combustió en cremadors LDI.
[EN] Aeronautical gas turbine engines present the main challenge of increasing the efficiency of the cycle while keeping the pollutant emissions below stringent restrictions. This has led to the design of new injection-combustion strategies working on more risky and problematic operating points such as those close to the lean extinction limit. In this context, the Lean Direct Injection (LDI) concept has emerged as a promising technology to reduce oxides of nitrogen (NOx) for next-generation aircraft power plants In this context, this thesis aims at contributing to the knowledge of the governing physical mechanisms within an LDI burner and to provide analysis tools for a deep characterisation of such complex flows. In order to do so, a numerical CFD methodology capable of reliably modelling the 2-phase nonreacting flow in an academic LDI burner has been developed in an Eulerian-Lagrangian framework, using the U-RANS and LES turbulence approaches. The LDI combustor taken as a reference to carry out the investigation is the laboratory-scale swirled-stabilised CORIA Spray Burner. The multi-scale problem is addressed by solving the complete inlet flow path through the swirl vanes and the combustor through two different CFD codes involving two different meshing strategies: an automatic mesh generation with adaptive mesh refinement (AMR) algorithm through CONVERGE and a more traditional static meshing technique in OpenFOAM. On the one hand, a methodology to obtain an optimal mesh strategy using AMR has been defined, and its benefits against traditional fixed mesh approaches have been exploited. In this way, the applicability of grid control tools available in CONVERGE such as fixed embedding and AMR has been demonstrated to be an interesting option to face this type of multi-scale problem. The results highlight an optimisation of the use of the computational resources and better accuracy in the simulations carried out with the presented methodology. On the other hand, the use of CFD tools has been combined with the application of systematic advanced modal decomposition techniques (i.e., Proper Orthogonal Decomposition and Dynamic Mode Decomposition). The numerical identification of the main acoustic modes in the chamber have proved their potential when studying the characteristics of the most powerful coherent flow structures of strongly swirled jets in a LDI burner undergoing vortex breakdown (VBB). Besides, the implementation of these mathematical procedures has allowed both retrieving information about the flow dynamics features and providing a systematic approach to identify the main mechanisms that sustain instabilities in the combustor. Last, this analysis has also allowed identifying some key features of swirl spray systems such as the complex pulsating, intermittent and cyclical spatial patterns related to the Precessing Vortex Core (PVC). Finally, the validated methodology is exploited through a Design of Experiments (DoE) to quantify the influence of critical design factors on the non-reacting flow. In this way, the individual contribution of some functional parameters (namely the number of swirler vanes, the swirler vane angle, the combustion chamber width and the axial position of the nozzle tip) into both the flow field pattern, the spray size distribution and the occurrence of instabilities in the combustion chamber are evaluated throughout a Taguchi's orthogonal array L9. Such a statistical study has supposed a good starting point for subsequent studies of injection, atomisation and combustion on LDI burners.
Belmar Gil, M. (2020). Computational study on the non-reacting flow in Lean Direct Injection gas turbine combustors through Eulerian-Lagrangian Large-Eddy Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/159882
TESIS

Yes
A novel Euler-Lagrangian (EL) computational uid dynamics (CFD) nite volume-based model to simulate the gas mixing of sludge for anaerobic digestion is developed and described. Fluid motion is driven by momentum transfer from bubbles to liquid. Model validation is undertaken by assessing the ow eld in a labscale model with particle image velocimetry (PIV). Conclusions are drawn about the upscaling and applicability of the model to full-scale problems, and recommendations are given for optimum application.

Research Doctorate - Doctor of Philosophy (PhD)
This thesis describes a study in which the aim was to develop an improved method for computational fluid dynamics (CFD) modelling of gas-liquid flow in mechanically-stirred tanks. Stirred tanks are commonly used in the process industries for carrying out a wide range of mixing operations and chemical reactions, yet considerable uncertainties remain in design and scale-up procedures. Computational modelling is of interest since it may assist in investigating the detailed flow characteristics of stirred tanks. However, as shown by a review of the literature, a range of limitations have been evident in previously published modelling methods. In the development of the modelling method, single-phase liquid flow was firstly considered, as a basis for extension to multiphase flow. A finite volume method was used to solve the equations for conservation of mass and momentum, in conjunction with the k-epsilon turbulence model. Simulation results were compared with experimental measurements for tanks stirred by a Rushton turbine and by a Lightnin A315 impeller. Comparison was made between different methods which account for impeller motion. Accuracy was assessed in terms of the prediction of velocities, power and flow numbers, the presence of trailing vortices, pressures around the impeller, and the turbulent kinetic energy and dissipation rate. The effect of grid density was investigated. For gas dispersion in a liquid, the modelling method employed the Eulerian-Eulerian two-fluid equations, again in conjunction with the k-espilon turbulence model. The correct specification of the equations was firstly reviewed. Different forms of the turbulent dispersion force were compared. For the drag force, it was found that existing correlations did not properly account for the effect of turbulence in increasing the bubble drag coefficient. By analysing literature data, a new equation was proposed to account for this increase in drag. For the prediction of bubble size, a bubble number density equation was introduced, which takes into account the effects of break-up and coalescence. The modelling method also allows for gas cavity formation behind impeller blades. Simulations of gas-liquid flow were again carried out for tanks stirred by a Rushton turbine and by a Lightnin A315 impeller. Again, the impeller geometry was included explicitly. A series of simulations were carried out to test the individual effects of various alternative modelling options. With the final method, based on developments in this study, simulation results show reasonable overall agreement in comparison with experimental data for bubble size, gas volume fraction, overall gas holdup and gassed power draw. In comparison to results based on previously published modelling methods, a significant improvement has been demonstrated. However, a number of limitations have been identified in the modelling method, which can be attributed either to the practical limitations on computer resources, or to a lack of understanding of the underlying physics. Recommendations have been made regarding investigations which could assist with further improvement of the CFD modelling method.