Rozprawy doktorskie na temat „Cooling Applications”

Magnetic cooling is an emerging refrigeration technology with potential to surpass the performance of vapour compression devices. It has been successfully applied in the cryogenic temperature ranges, where magnetic cooling gas liquefiers surpass the performance of conventional liquefaction systems. Magnetic refrigeration technology is based on the magnetocaloric effect, a characteristic present in all magnetic materials and alloys. In magnetic thermodynamic cycles, magnetization of a magnetocaloric material is equivalent to the compression of a gas, while demagnetization is equivalent to expansion of a gas, with a subsequent diminution of the entropy. In this thesis, the applicability of this technology to the domestic environment is reviewed. First, the thermodynamics of magnetic refrigeration are explored. Then, a comprehensive review of magnetocaloric materials suitable for use at room temperature is presented. To ascertain the state of the art, the most relevant prototypes and their performances have been described. Concluding the documentation, a survey on the existing mathematic models has been performed, that provided the foundation to create a Matlab model of a magnetic refrigeration device. To gain greater insight on the internal working of these devices, a representative room temperature cooling device has been modelled, and used to simulate a magnetic refrigerator and room air conditioner. Its performance has been analysed and compared with that of vapour compression devices. Also, the influence of parameters such as magnetic field applied, temperature span, refrigerant fluid and different regenerator configurations has been investigated.

The technology and applications of evaporative cooling to provide human comfort in buildings is not new and has been used in different places based on different methods and materials. Conventional air conditioning systems overshadowed the application of evaporative cooling for buildings despite their ozone layer depletion. Evaporative cooling using porous ceramic evaporators were experimentally investigated. Encouraging results in terms of temperature reduction and cooling effectiveness were reported. In this work also thermoelectric unit was integrated in to the evaporative cooling system containing porous ceramic evaporators. The warm inlet air cooled in the evaporative cooling chamber was passed over the hot-side fins of the thermoelectric cooling device to act as a better heat sink. Typical test results showed that the cold side temperature of thermoelectric unit was 5Deg.C lower and the hot side was 10Deg.Clower, respectively when operated on the wet and dry porous ceramics evaporative cooling chamber. Direct evaporative cooling is often associated with the rise in relative humidity which may result in uncomfortable feeling due to unwanted increase in moisture. Indirect evaporative cooling offers a solution but still requires improvements in the effectiveness. There is also need for using cheap and readily available materials for the construction, requiring simple fabrication technology without very complex engineering infrastructure. Most widely used common fibrous materials have very limited capillary effect. So a periodic water spray system with an automatic control is required for running the cooler which adds to the power consumption, rise in operation costs as well as construction and operational difficulties. As a compromise using horizontal arrangement was considered. Use of pump for supplying water required to moisten the evaporative cooling surface was eliminated. The system was constructed and tested under varying temperature, relative humidity and air flow rates. Results showed significant temperature reduction accompanied with acceptable increase in relative humidity. Temperature drop of 6-10Deg.C between the inlet and outlet temperatures of the product or supply air was recorded. Increase in relative humidity of the supply air was 6 - 10% less than the working air. Application of this novel system was demonstrated in the parasol self-cooling arrangement. The fibre tube vaporative cooler has the potential of cooling a building space to the acceptable comfort limits. The application of porous ceramics for building space cooling, integrating the system to be used as a heat sink and the use of horizontal fibre tubes for evaporative cooling are all novel ideas in this field of research. Other novel features also include the ability to minimise energy consumption by eliminating common methods of continuous water circulation.

Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in numerous industrial sectors. Recently submicron and high aspect ratio particles (nanoparticles and nanotubes) were introduced into the heat transfer fluids to enhance the thermal conductivity of the resulting nanofluids. The aim of this project was to investigate the physico-chemical properties of nanofluids synthesized using submicron and high aspect ratio particles suspended in heat transfer fluids .

The Vlasov technique is used to model the electron cooling force. Limitations of the applicability of the method is obtained by considering the perturbations of the electron plasma. Analytical expressions of the electron cooling force, valid beyond the Coulomb logarithm approximation, are derived and compared to numerical calculations using adaptive Monte Carlo integration. The calculated longitudinal cooling force is verified with measurements in CELSIUS. Transverse damping rates of betatron oscillations for a nonlinear cooling force is explored. Experimental data of the transverse monochromatic instability is used to determine the rms angular spread due to solenoid field imperfections in CELSIUS. The result, θrms= 0.16 ± 0.02 mrad, is in agreement with the longitudinal cooling force measurements. This verifies the internal consistency of the model and shows that the transverse and longitudinal cooling force components have different velocity dependences. Simulations of electron cooling with applications to HESR show that the momentum reso- lution ∆p/p smaller than 10−5 is feasible, as needed for the charmonium spectroscopy in the experimental program of PANDA. By deflecting the electron beam angle to make use of the monochromatic instability, a reasonable overlap between the circulating antiproton beam and the internal target can be maintained. The simulations also indicate that the cooling time is considerably shorter than expected.

The overall goals of the present PhD research are to explore the giant magnetoimpedance (GMI) and giant magnetocaloric (GMC) effects in functional magnetic materials and provide guidance on the optimization of the material properties for use in advanced magnetic sensor and refrigeration applications. GMI has attracted growing interest due to its promising applications in high-performance magnetic sensors. Research in this field is focused on the development of new materials with properties appropriate for practical GMI sensor applications. In this project, we have successfully set up a new magneto-impedance measurement system in the Functional Materials Laboratory at USF. We have established, for the first time, the correlation between sample surface, magnetic softness, critical length, and GMI in Co-based amorphous ribbon materials, which provide a good handle on selecting the suitable operating frequency range of magnetic materials for GMI-based field sensor applications. The impact of field-induced magnetic anisotropy on the GMI effect in Co-based nanocrystalline ribbon materials has also been investigated, providing an important understanding of the correlation between the microstructure, magnetic anisotropy, and GMI in these materials. We have shown that coating a thin layer of magnetic metal on the surface of a magnetic ribbon can reduce stray fields due to surface irregularities and enhance the magnetic flux paths closure of the bilayer structure, both of which, in effect, increase the GMI and its field sensitivity. This finding provides a new way for tailoring GMI in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors. We have also introduced the new concepts of incorporating GMI technology with superparamagnetic nanopthesiss for biosensing applications and with carbon nanotubes for gas and chemical sensing applications. GMC forms the basis for developing advanced magnetic refrigeration technology and research in this field is of topical interest. In this project, we have systematically studied the ferromagnetism and magnetocaloric effect in Eu8Ga16Ge30 clathrate materials, which are better known for their thermoelectric applications. We have discovered the GMC effect in the type-VIII clathrate and enhanced refrigerant capacity in the type-I clathrate. We have successfully used the clathrates as excellent host matrices to produce novel Eu8Ga16Ge30-EuO composite materials with desirable properties for active magnetic refrigeration technologies. A large refrigerant capacity of 794 J/kg for a field change of 5 T over a temperature interval of 70 K has been achieved in the Eu8Ga16Ge30-EuO composite with a 40%-60% weight ratio. This is the largest value ever achieved among existing magnetocaloric materials for magnetic refrigeration in the temperature range 10 K - 100 K. The excellent magnetocaloric properties of the Eu8Ga16Ge30-EuO composites make them attractive for active magnetic refrigeration in the liquid nitrogen temperature range.

The content of this Master thesis is the characterization of three different cross unmixed flow heat exchangers. All of the heat exchangers have different inner geometries and dimensions. In order to perform the characterization of these heat exchangers, measurements of heat transfer were done under different conditions: five different temperatures at the inlet of the sorption side, different mass flow for both inlet sides of the heat exchangers.The heat transfer measurements were done with and without applying indirect evaporative cooling in order to find out the influence of indirect evaporative cooling. This research was done with the objective to find out which heat exchanger presents the best performance. The purpose is to install the heat exchanger in the novel solar driven open air SorLuKo system. This system was developed in Fraunhofer ISE and works under the same principe as the ECOS system. The main objective of the SorLuKo system is to dehumidify and cool a dwelling or small office.

White dwarfs are the most common stellar evolutionary end-point. Moreover, they can be considered as reliable cosmic clocks to infer the age of a wide variety of stellar populations, including globular and open clusters. Consequently, there is a considerable interest in the study of white dwarf cooling models. In this thesis we used two different approaches. From a theorical perspective, we computed a set of new cooling white dwarfs sequences which incorporates the most up-to-date physical inputs for precision white dwarf cosmochronology and for asteroseismological studies of ZZ Ceti stars. Moreover, we studied the role of 22Ne diffusion in the evolution of white dwarf stars with high-metallicity progenitors. Our evolutionary sequences have been self-consistently evolved from the zero age main sequence to the white dwarf stage. Our calculations include: nuclear burning at the very early phases of white dwarf evolution (which is important to determine the final thickness of the hydrogen-rich envelope), diffusion and gravitational settling (which are important to shape the profiles of the outer layers), accurate neutrino emission rates (which control the cooling at high luminosities), a correct treatment of crystallization and phase separation of carbon and oxygen (which dominate the cooling times at low luminosities), a very detailed equation of state (which is important in all the evolutionary phases), and improved non-gray model atmospheres (which allow for a precise determination of white dwarf colors and outer boundary conditions for the evolving models). From an applied point of view, we use a Monte Carlo simulator that employs our up-to-date evolutionary cooling sequences for white dwarfs. From this and the observations obtained by Hubble Space Telescope of NGC 6791, a nearby metal-rich open cluster, we obtain important conclusions. NGC 6791 is a well studied metal-rich open cluster ([Fe/H]¿ 0.4) that it is so close to us that can be imaged down to luminosities fainter than that of the termination of its white-dwarf cooling sequence, thus allowing for an in-depth study of its white dwarf population. We constrain important properties of this cluster stellar population, such as the age, or the existence of a putative population of massive helium core white dwarfs among other aspects. Some of our main findings can be summarized as follows. With respect to the computation of new cooling sequences for hydrogen-rich DA white dwarfs (Renedo et al. 2010) We correctly reproduced the observed initial-to-final mass relationship of white dwarfs with solar metallicity progenitors. We calculated the energy released from 22Ne sedimentation and we confirm this energy release strongly delays the cooling. The precise value of the delays depends on the mass of the white dwarf, its luminosity and on the metal content. We also solved the age discrepancy between the main sequence turn-off age (~ 8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (~ 6 Gyr). Finally we found that the fraction of non-DA white dwarfs in this particular cluster is surprinsingly small, on the order of 6%.

Thesis (Nav. E. and S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 65-66).
This thesis utilizes concepts taken from the NAVSEA Design Practices and Criteria Manualfor Surface Ship Freshwater Systems and other references to create a Cooling System Design Tool (CSDT). With the development of new radars and combat system equipment on warships comes the increased demand for the means to remove the heat generated by these power-hungry systems. Whereas in the past, the relatively compact Chilled Water system could be tucked away where space was available, the higher demand for chilled water has resulted in a potentially exponential growth in size and weight of the components which make up this system; as a result, the design of the cooling systems must be considered earlier in the design process. The CSDT was developed to enable naval architects and engineers to better illustrate, early in the design process, the requirements and characteristics for the Chilled Water system components. Utilizing both Excel and Paramarine software, the CSDT rapidly creates a visual model of a Chilled Water system and conducts pump, damage, cost, weight, and volume analyses to assist in further development and design of the system. Several case studies were run to show the capability and flexibility of the tool, as well as how new electronic and mecahnical systems can affect the parameters of the Chilled Water system.
by Ethan R. Fiedel.
Nav.E.and S.M.

Multiphase and multicomponent flows are frequently encountered in the cooling applications due to combined heat transfer and phase change phenomena. Two-fluid and homogeneous mixture models are chosen to numerically study these flows in the cooling phenomena. Therefore this work is divided in two main parts. In the first part, a two-fluid model algorithm for free surface flows is presented. The two fluid model is usually used as a tool to simulate dispersed flow. With its extension, it may also be applied to large interface (separated) flow. In the second part, the homogeneous mixture model for the multicomponent flow is employed to solve evaporation problems. Finally the simulation is focused on the mixed transitional or turbulent flow with and without evaporation. In detail, this thesis consists of six chapters. The first chapter is devoted to an introduction to the two-fluid and homogeneous mixture models employed in the multiphase/multicomponent flow. The multiphase classification is explained and the previous works on the two models are reviewed. The second chapter is mainly focused on the application of the Fractional step method algorithm in the two-fluid model. In addition, the Conservative Level Set method(interface sharpening) is applied to overcome the weakness of the two-fluid model (numerical diffusion of the interface), which is often encountered in the simulations using this model. With the proposed algorithm, the two-fluid model suitable for the dispersed flow is extended to the separated flow. The homogeneous mixture model is introduced in the third chapter. As an application of this model, different evaporation cases have been tested. A hydrodynamically fully developed laminar flow in a horizontal duct is firstly studied. It is used to verify the model in a laminar flow considering constant physical properties. Water falling films are often applied to enhance the heat transfer. Therefore the second case analyzes the natural convection in a cavity with liquid film (assuming variable physical properties), and validates the falling film model. Finally, a third case is focused on mixed convective flow interacting with a water falling liquid film. The effects of heat flux on the evaporation rate and the flow structure are investigated employing numerical experiments. In the fourth chapter, the laminarization phenomena of turbulent forced flow in a vertical pipe with constant heat flux is studied. These studies validate the prediction ability of large eddy simulation in this complex situation. Afterwards additional cases in a long vertical pipe (100 times diameters) are conducted and the results are compared with the existing experimental data. Throughout the whole pipe, the flow state follows a complicated process, which includes turbulent-laminar and laminar-turbulent transitions. This problem is of great significance in industrial applications for it may result in the enhancement or impairment of heat transfer. Based on the previous verification of the model in turbulent and transitional flow, the simulation of the cooling in a uniformly heated vertical tube is conducted in the fifth chapter with an ascending flow of air and a falling film. This case also involves the transitional complex flow and boundary conditions of falling film with simultaneous heat/mass transfer. The variable factors affecting the evaporation and thermal efficiency have been analyzed. In Appendix C, as an application in engineering of the work developed within the thesis, a series of flows in a complex geometry of a refrigerator chamber without or with fins are simulated to obtain their effects on the flow distribution and mixing feature. In the last chapter, the main conclusions are summarized and the future works are listed.
Debido a la transferencia de calor y de cambio de fase, fenomenología multifase y multicomponente se encuentra en las aplicaciones de refrigeración. Dependiendo de la estructura de interfaz multifase pueden clasificarse como flujo separado (flujo estratificado), de transición o mezclado flujo y flujo disperso. Dependiendo de los diferentes estados de flujo de dos fases, se deben aplicar diferentes modelos. La presente tesis se centra principalmente en flujo separado. Modelos de mezcla homogénea de dos fluidos se emplean para simular fenómenos de enfriamiento en multifase. Este trabajo se divide en dos partes principales. En la primera parte, un algoritmo de modelo de dos fluidos de la superficie libre se presenta. El modelo de dos fluidos se utiliza generalmente como una herramienta para simular flujo disperso. En la segunda parte, el modelo de mezcla homogénea para flujo multicomponente se emplea para resolver el problemas de evaporación. Finalmente se simulan flujos turbulentos con influencia de la fuerza de flotabilidad. El objetivo a largo plazo es acoplar los dos modelos, que podrían resolver todos los regímenes de flujo y tendrian aplicación en problemas industriales. La presente tesis se compone de seis capítulos. El primer capítulo está dedicado a una introducción a los modelos de mezcla homogénea de dos fluidos empleados en el flujo multifásico / multicomponente. La clasificación de múltiples fases se explica y se revisa la bibliografia existente. El segundo capítulo se centra principalmente en la aplicación del Fractional Step Method en bifasico. Con el algoritmo propuesto, el modelo de dos fluidos adecuado para el flujo disperso se extiende al flujo separado. El modelo para mezcla homogénea se introduce en el tercer capítulo con las mismas ecuaciones de masa, cantidad de movimiento, energía y concentración. Se aplica en casos de evaporación y condensación. El flujo laminar completamente desarrollado en un conducto horizontal se estudia en primer lugar considerando propiedades físicas constantes para verificar el modelo en un flujo laminar. La simulacion de convección natural en una cavidad con propiedades físicas variables y película de líquido se realiza numéricamente para validar el modelo de película descendente. Finalmente, el flujo de convección mixta se investiga en un film descendente. Los efectos de los diferentes parámetros en la evaporación y el flujo son investigados mediante experimentos numéricos. En el cuarto capítulo, la laminarización de flujo turbulento forzado en un tubo vertical con flujo de calor constante se estudia para validar la capacidad de predicción de los modelos LES en el flujo de convección mixta de transición turbulenta-laminar con fuerte fuerza de flotación. Se llevan a cabo estudios numericos y los resultados se comparan con los datos experimentales existentes. A lo largo de toda el conducto, el estado de flujo sigue un proceso complicado, que incluye turbulencia.

Thesis: S.B., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 53-54).
Motors used to actuate powered prostheses generally under-utilize their torque capacity due to thermal limitations of the windings. This thesis investigates the effectiveness of increasing the rate of heat transfer away from the windings in order to enable running motors at higher currents than their rated maximum levels, thus raising this torque saturation limit. Simulation models and physical prototypes based on the RCTiger 100KV U8 brushless outrunner motor were built to observe the temperature of the windings as constant current was applied to the motor. The addition of a fan-based active cooling system allowed the motor to run at 142% of its maximum continuous current rating for up to 56 seconds before winding temperatures exceeded 550 C, and limited temperature increase in simulation to 26' above ambient temperature when the full 35A stall current was applied for one second. Although the simplified circuit model was not able to fully capture nonlinear thermal behavior of the motor, simulations were able to predict approximate heating time constants and time duration before windings reached threshold temperature for current ranges 5- 15A. Experimental and simulation results support the hypothesis that active cooling enables motors to run at their full torque potential for short periods of time, which holds promise for the use of cooling mechanisms in prosthetic applications.
by Ava E. Chen.
S.B.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.
Includes bibliographical references (p. 133-136).
In this thesis, an inverter-driven variable speed scroll compressor is tested on a de-superheater test stand to determine its performance in areas of low-lift and low compressor speed. The goal is to adapt this test stand so that it could be used to test a reciprocating compressor in this region. A control program is written to maintain a constant saturated suction temperature, suction temperature, and saturated discharge temperature. The program was able to maintain control with errors of ±0.2 °C at most points. At each test point, refrigerant mass flow rate, compressor input power, and discharge temperature is monitored. The amount of heat removed by the condenser was within 7% of the compressor input power and the inverter efficiency was within 5% of the compressor input power for all test points. The inverter efficiency is lowest at low speed. The isentropic efficiency is found to drop off significantly for low pressure ratios. A similar drop off is not expected for reciprocating compressors, so a model for reciprocating compressors is developed. The model is able to predict refrigerant mass flow rate and compressor input power as a function of shaft speed as well as suction and discharge pressures and temperatures. The model is able to accurately predict the mass flow rate with an RMS error within 0.5% and for the power model, the RMS errors are within 3.6%. The mass flow model is found to perform well when extrapolated into lower speed ranges with RMS errors remaining below 0.5%.
by Ryan Alexander Willingham.
S.M.

International Energy Agency Solar Heating & Cooling (IEA SHC) programme states the fact that space/water heating and cooling demand account for over 75% of the energy consumed in single and multi-family homes. Solar energy technology can meet up to 100% of this demand depending on the size of the system, storage capacity, the heat load and the region’s climate. Solar thermal collectors are particular type of heat extracting devices that convert solar radiation into thermal energy through a transport medium or flowing fluid. Although hybrid PV/T or thermal-alone systems offer some advantages to improve the solar heat utilisation, there are a few technical challenges found in these systems in practice that prevented wide-scale applications. These technical drawbacks include being expensive to make and install, inability of switching already-built photovoltaic (PV) systems into PV/T systems, architectural design etc. The aims of this project, therefore, were to investigate roof integrated solar thermal roof collectors that properly blend into surrounding thus avoiding ‘add on’ appearance and having a dual function (heat absorption and roofing). Another objective was to address the inherent technical pitfalls and practical limitations of conventional solar thermal collectors by bringing unique, inexpensive, maintenance free and easily adaptable solutions. Thus, in this innovative research, unique and simple building integrated solar thermal roof collectors have been developed for heating & cooling applications. The roof systems which mainly based on low cost and structurally unique polyethylene heat exchanger are relatively cost effective, competitive and developed by primarily exploiting components and techniques widely available on the market. The following objectives have been independently achieved via evaluating three aspects of investigations as following: • Investigation on the performance of poly heat exchanger underneath PV units • Investigation on the performance of a Building Integrated PV/T Roof ‘Invisible’ Collector combined with a liquid desiccant enhanced indirect evaporative cooling system • Investigation on the build-up and performance test of a novel ‘Sandwich’ solar thermal roof for heat pump operation These works have been assessed by means of computer simulation, laboratory and field experimental work and have been demonstrated adequately. The key findings from the study confirm the potential of the examined technology, and elucidate the specific conclusions for the practice of such systems. The analysis showed that water temperature within the poly heat exchanger loop underneath PV units could reach up to 36°C and the system would achieve up to 20.25% overall thermal efficiency. Techno-economic analysis was carried out by applying the Life Cycle Cost (LCC) method. Evaluations showed that the estimated annual energy savings of the overall system was 10.3 MWh/year and the cost of power generation was found to be £0.0622 per kWh. The heat exchanger loop was coupled with a liquid desiccant enhanced indirect evaporative cooling unit and experimental results indicated that the proposed system could supply about 3 kW of heating and 5.2 kW of cooling power. Lastly, the results from test of a novel solar thermal collector for heat pump operation presented that the difference in water temperature could reach up to 18°C while maximum thermal efficiency found to be 26%. Coefficient Performance of the heat pump (COPHP) and overall system (COPSYS) averages were attained as COPHP=3.01 and COPSYS=2.29, respectively. An economic analysis pointed a minimum payback period of about 3 years for the system.

The increasing density of transistors in electronic components is leading to an inexorable rise in the heat dissipation that must be achieved in order to preserve reliability and performance. Hence, improving the thermal management of electronic devices is a crucial goal for future generations of electronic systems. Therefore, a complementary experimental and numerical investigation of single-phase water flow and heat transfer characteristics of the benefits of employing three different configurations of serpentine minichannel heat sink (MCHS) designs has been performed, to assess their suitability for the thermal management of electronic devices. These heat sinks are termed single (SPSMs), double (DPSMs) and triple path serpentine rectangular minichannels (TPSMs), and their performance is compared, both experimentally and numerically, with that of a design based on an array of straight rectangular minichannels (SRMs) in terms of pressure drop (ΔP), average Nusselt number (Nuavg) and total thermal resistance (Rth). The results showed that the serpentine channel bends are very influential in improving heat transfer by preventing both the hydrodynamic and thermal boundary layers from attaining a fully-developed state. The SPSM design provides the most effective heat transfer, followed by the DPSM and TPSM ones, both of which out-performed the SRM heat sink. The SPSM heat sink produced a 35% enhancement in Nuavg and a 19% reduction in Rth at a volumetric flow rate (Qin) of 0.5 l/min compared to the conventional SRM heat sink. These improvements in the heat transfer are, however, achieved at the expense of significantly larger ΔP. It was found that the incorporation of serpentine minichannels into heat sinks will significantly increase the heat-removal ability, but this must be balanced with the pressure drop requirement. Therefore, an experimental and numerical investigation of the benefit of introducing chevron fins has been carried out to examine the potential of decreasing pressure drop along with further thermal enhancement. This novel design is found to significantly reduce both the ΔP across the heat sink and the Rth by up to 60% and 10%, respectively, and to enhance the Nuavg by 15%, compared with the SPSM heat sink without chevron fins. Consequently, the design of the SPSM with and without chevron fins was then optimised in terms of the minichannel width (Wch) number of minichannels (Nch) and chevron oblique angle (θ). The optimisation process uses a 30 (without chevron fins) and 50 (with chevron fins) point Optimal Latin Hypercubes Design of Experiment, generated from a permutation genetic algorithm, and accurate metamodels built using a Moving Least Square (MLS) method. A Pareto front is then constructed to enable the compromises available between designs with a low pressure drop and those with low thermal resistance to be explored and appropriate design parameters to be chosen. These techniques have then been used to explore the feasibility of using serpentine MCHS and heat spreaders to cool GaN HEMTs.

Nowadays, the Data Center industry is playing a leading role in the world economic development and it is growing rapidly and constantly. Beside this, it has become more concerned with energy consumption and the associated environmental effects. Since about half of the total energy consumption in a typical Data Center is devoted to cooling the IT equipment, energy efficiency must be the primary focus in the design and management of the cooling infrastructure. In this Thesis, we consider the problem of optimizing the operation of cooling systems in Data Centers. The main objective is that of maximizing the energy efficiency of the systems, while provisioning the required cooling demand. For this purpose, we propose a two-layer hierarchical control approach, where a supervisory high-level layer determines the optimal set-points for the local low-level controllers. The supervisory layer exploits an Extremum Seeking model-free optimization algorithm, which ensures flexibility and robustness against changes in the operating conditions. In particular, a Newton-like Phasor-based Extremum Seeking scheme is presented to improve the convergence properties and the robustness of the algorithm. The proposed control architecture is tested in silico in optimizing the operation of an Indirect Evaporative Cooling system and a Liquid Immersion Cooling unit. Simulations are performed by exploiting First-Principle Data-Driven models of the considered systems and the results demonstrate the effectiveness of the proposed approach.

Current state-of-the-art thermoregulatory models do not predict body temperatures with the accuracies that are required for the development of automatic cooling control in liquid cooling garment (LCG) systems. Automatic cooling control would be beneficial in a variety of space, aviation, military, and industrial environments for optimizing cooling efficiency, for making LCGs as portable and practical as possible, for alleviating the individual from manual cooling control, and for improving thermal comfort and cognitive performance. In this study, we adopt the Fiala thermoregulatory model, which has previously demonstrated state-of-the-art predictive abilities in air environments, for use in LCG environments. We validate the numerical formulation with analytical solutions to the bioheat equation, and find our model to be accurate and stable with a variety of different grid configurations. We then compare the thermoregulatory model s tissue temperature predictions with experimental data where individuals, equipped with an LCG, exercise according to a 700 W rectangular type activity schedule. The root mean square (RMS) deviation between the model response and the mean experimental group response is 0.16°C for the rectal temperature and 0.70°C for the mean skin temperature, which is within state-of-the-art variations. However, with a mean absolute body heat storage error (e_BHS_mean) of 9.7 W·h, the model fails to satisfy the ±6.5 W·h accuracy that is required for the automatic LCG cooling control development. In order to improve model predictions, we modify the blood flow dynamics of the thermoregulatory model. Instead of using step responses to changing requirements, we introduce exponential responses to the muscle blood flow and the vasoconstriction command. We find that such modifications have an insignificant effect on temperature predictions. However, a new vasoconstriction dependency, i.e. the rate of change of hypothalamus temperature weighted by the hypothalamus error signal (DThy·dThy/dt), proves to be an important signal that governs the thermoregulatory response during conditions of simultaneously increasing core and decreasing skin temperatures, which is a common scenario in LCG environments. With the new DThy·dThy/dt dependency in the vasoconstriction command, the e_BHS_mean for the exercise period is reduced by 59% (from 12.9 W·h to 5.2 W·h). Even though the new e_BHS_mean of 5.8 W·h for the total activity schedule is within the target accuracy of ±6.5 W·h, e_BHS fails to stay within the target accuracy during the entire activity schedule. With additional improvements to the central blood pool formulation, the LCG boundary condition, and the agreement between model set-points and actual experimental initial conditions, it seems possible to achieve the strict accuracy that is needed for automatic cooling control development.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering PhD

Gas turbines have become an intricate part of today's society. Besides powering practically all 200,000+ passenger aircraft in use today, they are also a predominate form of power generation when coupled with a generator. The fact that they are highly efficient, and capable of large power to weight ratios, makes gas turbines an ideal solution for many power requirement issues faced today. Designers have even been able to develop small, micro-turbines capable of producing efficient portable power. Part of the turbine's success is the fact that their efficiency levels have continuously risen since their introduction in the early 1800's. Along with improvements in our understanding and designs of the aerodynamic components of the turbine, as well as improvements in the areas of material design and combustion control, advances in component cooling techniques have predominantly contributed to this success. This is the result of a simple thermodynamic concept; as the turbine inlet temperature is increased, the overall efficiency of the machine increases as well. Designers have exploited this fact to the extent that modern gas turbines produce rotor inlet temperatures beyond the melting point of the sophisticated materials used within them. This has only been possible through the use of sophisticated cooling techniques, particularly in the 1st stage vanes and blades. Some of the cooling techniques employed today have been internal cooling channels enhanced with various features, film and showerhead cooling, as well as internal impingement cooling scenarios. Impingement cooling has proven to be one of the most capable heat removal processes, and the combination of this cooling feature with that of channel flow, as is done in impingement channel cooling, creates a scenario that has understandably received a great deal of attention in recent years. This study has investigated several of the unpublished characteristics of these impingement channels, including the channel height effects on the performance of the channel side walls, effects of bulk temperature increase on heat transfer coefficients, circumferential heat variation effects, and effects on the uniformity of the heat transfer distribution. The main objectives of this dissertation are to explore the various previously unstudied characteristics of impingement channels, in order to sufficiently predict their performance in a wide range of applications. The potential exists, therefore, for a designer to develop a blade with cooling characteristics specifically tailored to the expected component thermal loads. Temperature sensitive paint (TSP) is one of several non-intrusive optical temperature measurements techniques that have gained a significant amount of popularity in the last decade. By employing the use of TSP, we have the ability to provide very accurate (less than 1 degree Celsius uncertainty), high resolution full-field temperature measurements. This has allowed us to investigate the local heat transfer characteristics of the various channel surfaces under a variety of steady state testing conditions. The comparison of thermal performance and uniformity for each impingement channel configuration then highlights the benefits and disadvantages of various configurations. Through these investigations, it has been shown that the channel side walls provide heat transfer coefficients comparable to those found on the target surface, especially at small impingement heights. Although the side walls suffer from highly non-uniform performance near the start of the channel, the profiles become very uniform as the cross flow develops and becomes a dominating contributor to the heat transfer coefficient. Increases in channel height result in increased non-uniformity in the streamwise direction and decreased heat transfer levels. Bulk temperature increases have also been shown to be an important consideration when investigating surfaces dominated by cross flow heat transfer effects, as enhancements up to 80% in some areas may be computed. Considerations of these bulk temperature changes also allow the determination of the point at which the flow transitions from an impingement dominated regime to one that is dominated by cross flow effects. Finally, circumferential heat variations have proven to have negligible effects on the calculated heat transfer coefficient, with the observed differences in heat transfer coefficient being contributed to the unaccounted variations in channel bulk temperature.
Ph.D.
Department of Mechanical, Materials and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering PhD

This thesis presents an in-depth study into the characterization and enhancement of a metastable neon laser cooled and trapped atomic beam. The apparatus consists of a standard Zeeman slowed atomic beam loaded into a magneto-optical trap and was designed for applications to electron scattering experiments and photoionization. The efficiency of the metastable neon atomic source was investigated to determine the ideal cathode type for maximum metastable production and optimal atomic beam velocity haracteristics. A series of characterization measurements were performed on the MOT, and the trap volume and population were investigated for a range of trapping and slowing laser intensities and detunings, together with the MOT and Zeeman slower magnetic fields. The volume measurements were compared to standard Doppler theory and it was found that the Doppler model inadequately explained the trap behaviour. It was found that the MOT population characteristics were governed by two processes: two-body losses that limit the trap population at high densities, and the efficiency of the atom capture process which limits the operational range of the MOT over the various parameters. The trap temperature was determined to be 1.3mK via a time-of-flight technique. This was nearly twice that predicted by Doppler theory and the lack of agreement once again suggests the inadequacies in the Doppler theory to correctly model the experiment. The application of the MOT to the photoionization cross-section measurement of the (2p53p)3D3 state of neon was investigated. The MOT decay technique was utilized to measure cross-section values of o351 = 2.9+0.2 -0.3 x 10 -18cm2 and o363 = 3.1 +0.3 -0.4 x 10-18cm2 at the wavelengths of 351nm and 363nm respectively. This is an increase in accuracy of around a factor of five from previous measurements and it was found that the results agreed well with the values predicted by current theories.

Efficient heat transfer technologies are essential for magnetically confined fusion reactors; this applies to both the current generation of experimental reactors as well as future power plants. A number of High Heat Flux devices have therefore been developed specifically for this application. One of the most promising candidates is the HyperVapotron, a water cooled device which relies on internal fins and boiling heat transfer to maximise the heat transfer capability. Over the past 30 years, numerous variations of the HyperVapotron have been built and tested at fusion research centres around the globe resulting in devices that can now sustain heat fluxes in the region of 20 – 30MW/m2 in steady state. Unfortunately, there have been few attempts to model or understand the internal heat transfer mechanisms responsible for this exceptional performance with the result that design improvements are traditionally sought experimentally which is both inefficient and costly. This thesis seeks to develop an engineering model of the HyperVapotron device using commercial Computational Fluid Dynamics software. To establish the most appropriate modelling choices, in-depth studies were performed examining the turbulence models (within the Reynolds Averaged Navier Stokes framework), near wall methods, grid resolution and boiling submodels. Validation of the models is accomplished via comparison with experimental results as well as high order Implicit Large Eddy Simulation methods. It is shown that single phase cavity flows and their related heat transfer characteristics (time-averaged) can be accurately captured if the SST k-omega turbulence model is employed using a fine nearwall grid throughout the cavity (e.g. y+ < 1 throughout). Separately, multiphase solutions with tuned wall boiling models also showed reasonable agreement with experimental data for vertical boiling tubes. As more complex multiphase HyperVapotron models were constructed, it became clear that there is an intrinsic incompatibility between the fine grids required for the single phase heat transfer predictions and the coarser grids plus wall functions required by the boiling model. Ultimately, the full 3D solution was based on the coarser grids as the fall off in accuracy in single phase heat transfer only becomes significant for HyperVapotron designs with deeper cavities. Since it is also shown here that deeper cavities are generally less efficient, these grid induced errors become less relevant if the primary objective is to find optimised performance.Comparing the CFD solutions with HyperVapotron experimental data suggests that a RANS-based, multiphase model is indeed capable of predicting performance over a wide range of geometries and boundary conditions. Whilst a definitive set of design improvements is not defined here, it is expected that the methodologies and tools developed will enable designers of future High Heat Flux devices to perform significant virtual prototyping before embarking on the more costly build and test programmes.

The effect of doping on the crystal structure, magnetic, magnetocaloric and transport properties of MnM′Ge (M′ = Ni, Co) intermetallic compounds and NiMnX (X = Sn, In) Heusler alloys have been studied by room temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. The studied magnetic systems include Ni1-xCrxMnGe1.05 (0 ≤ x ≤ 0.120), Mn1-xAlxCoGe (0 ≤ x ≤ 0.05), MnCo1-xZrxGe (0.01 ≤ x ≤ 0.04), Mn1-xAgxCoGe (0.01 ≤ x ≤ 0.10), Ni50-xRxMn35Sn15 (x = 0, 1 and R = La, Pr, Sm), Ni43-xRxMn46Sn11 (x = 0, 1 and R = Pr, Gd, Ho, Er), and Ni50Mn35In15-xBix (0 ≤ x ≤ 1.5).A temperature induced first-order structural transition characterized by a change in crystal structure from high temperature austenite phase (AP) with Ni2In-type Hexagonal structure to low temperature martensite phase (MP) with TiNiSi-type orthorhombic structure was observed at T = TM (martensitic transition temperature) in some of the MnM′Ge-based compounds. The partial substitution of doping elements such as Cr, Al, Zr, and Ag resulted in a decrease in TM and at certain concentration, TM was found to decrease below / coincide with the ferromagnetic transition temperature (TC) of AP. Therefore, such system show a first-order magnetostructural transition (MST).In Ni1-xCrxMnGe1.05, a MST from antiferromagnetic (AFM) orthorhombic to ferromagnetic (FM) hexagonal phase was observed for 0.105 ≤ x ≤ 0.120. Both direct and inverse MCE were observed in this compound. The peak values of the magnetic entropy change (ΔSMpeak ) in the vicinity of TC for ΔH = 5T were found to be 4.5 J/kg K, 5.6 J/Kg K, and 5.1 J/Kg K for x = 0.105, 0.115, and 0.120 respectively. A magnetic field-induced transition from an AFM to a FM state in the martensite structure was observed in annealed Ni0.895Cr0.105MnGe1.05 melt-spun ribbons, which led to a coupled MST from a FM martensite to a PM austenite phase with a large change in magnetization. As a result of the field-induced MST, a large ΔSMpeak value of 16.1 J kg-1 K-1 (which is about a four times larger than the bulk) and Refrigeration capacity (RC-1) =144 J kg-1 at μ0∆H = 5 T was found. It was also found that the ribbon samples showed excellent magnetic reversibility that is important for application. MCE parameters, adiabatic temperature change (∆Tad) and |〖∆S〗_M |, with maximum value of ~ 2.6 K (µoH = 10 T) and 4.4 J kg-1 K-1(µo∆H = 5 T), respectively, were observed in the vicinity of TC. The ∆Tad (T) curves obtained for µoΔH = 10 T and magnetization isotherms were found to be completely reversible, which indicates the reversibility of the MCE in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ∆Tad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µoH)2/3 near TC in accordance with Landau’s theory of phase transitions.In MnCoGe compounds doped with Al, Zr, and Ag, a tunable MST from the paramagnetic hexagonal to ferromagnetic orthorhombic phase was observed. The maximum ΔSM values of about 18, 7.2, and 22 J kg-1 K-1for ∆H = 5T at TM was observed for Al, Zr, and Ag doped compounds, respectively. The corresponding maximum value of RC was found to be (303, 266, and 308) JKg-1.The new compounds containing low concentration of rare earth (R) metals: Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, with R = La, Pr, Sm, Gd, Ho, Er and Ni50Mn35In15-xBix were synthesized. The compounds crystallized in the cubic L21 austenite phase (AP) or a mixture of AP and low temperature martensitic phase (MP) at room temperature. For Ni50-xRxMn35Sn15 and Ni43-xRxMn46Sn11 alloys, TM shifted towards higher temperature with rare-earth doping, thus stabilizing the MP at higher temperature. A maximum shift in TM by ~ 60-62 K relative to the parent compound (TM = 190-195 K) was observed for the Ni49LaMn35Sn15 and Ni42PrMn46Sn11. TM shifted towards lower temperature if Bi is placed in In position in Ni50Mn35In15-xBix. A maximum shift of ~ 36 K was detected for x = 1.5. Abnormal shifts in TC and TM to higher temperatures were observed at high field for Bi concentration ≥ 0.5.The ground state magnetization decreased with the rare-earth doping and increasing Bi content. The compounds exhibit both inverse and normal magnetocaloric effects. Large values of ∆SM = 12 (Ni49PrMn35Sn15), 32 Jkg-1K-1(Ni42PrMn46Sn11), 28 Jkg-1K-1 (Ni42GdMn46Sn11), 25 Jkg-1K-1 (Ni42HoMn46Sn11), 40 J/kg K (Ni50Mn35In15) and 34 J/kg K (Ni50Mn35In15-xBix, x = 0.25) were found at TM for ∆H = 5T that can be tuned in a wide temperature range. RC values ranging from 267-336 Jkg-1 at TC, 182 -250 Jkg-1 at TM and 144-165 Jkg-1 at TC were found with ∆H = 5T for Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, and Ni50Mn35In15-xBix, respectively. Significant magnetoresistance (MR) values of -30%, -20 % and -30% were observed in Ni49LaMn35Sn15, Ni42GdMn46Sn11, and Ni50Mn35In14.5Bi0.5 compounds, respectively, at TM and ∆H = 5T. A large exchange bias effect with HEB ~ 1.1 kOe at 10 K was observed for the Ni42PrMn46Sn11 compound in its MP. Thus, the pronounced multifunctional properties such as shape memory effects, MCE, EB, and MR make these new systems promising for the ongoing development of magnetocaloric and multifunctional technologies.

[EN] This PhD study deals with the modelling of an adsorption system designed to provide air conditioning for vehicles, and is driven by the waste heat available from the water/glycol cooling circuit of the engine. The system is based on the sequential heating/cooling of two sorption beds containing a solid sorption material which desorbs or adsorbs water vapour. The condensation of the vapour is carried out by a cooling circuit while the subsequent evaporation of the condensed liquid is employed to produce the cooling effect, generating chilled water, which is then employed to cool down the air of the cabin. The developed model is fully dynamic and is based on zero-dimensional lumped parameter models for all the necessary components of the overall system including the engine, the beds, the heating circuit, the cooling circuit, the chilled water circuit and the vehicle cabin. The sorption bed model takes into account the non-equilibrium of the adsorption and desorption processes and is able to work with any kind of adsorbent materials, but the study has been restricted to silica gel and zeolite which are among the most appropriate materials for this application. The model is employed to simulate a standard driving cycle of a vehicle, evaluating the instantaneous available heat from the engine cooling system and the dynamic behaviour of the described sorption A/C system, resulting in the estimation of the evolution of the cabin temperature along the cycle. The model of the overall system has been developed under the MATLAB Simulink programming environment. The model of the adsorption system has been first validated against experimental results, showing its excellent capabilities to predict the dynamic behaviour of the system. The model was then used to analyse the influence of the main design parameters of the bed and the main operation parameters on the system's performance: cooling capacity and coefficient of performance (COP). This was done in order to provide rules for the optimal design and operation of this kind of systems. Finally, the model has been employed to analyse the overall system (engine, adsorption system, heating and cooling circuits, chilled water circuit and cabin) performance along a standard driving cycle, under various operation strategies with regards to the initial state of the adsorbent material in the beds, and operation conditions both for a car and a truck. The results show the difficulties of activating the system at the initial periods of the cycle, when the engine is warming up, and the difficulties to synchronise the operation of the system with the availability of waste energy. They also highlight the limitation in capacity of the designed system, showing that it would not able to fulfil the comfort requirements inside the cabin in hot days or after soaking conditions. Part of this PhD study was carried out in the frame of an R&D project called "Thermally Operated Mobile Air Conditioning Systems - TOPMACS", financially supported by the EU under the FP6 program, which was devoted to the evaluation of the feasibility and performance of potential sorption system solutions for the air conditioning of vehicles.
[ES] Esta tesis doctoral se centra en el modelado de un sistema de adsorción diseñado para proporcionar aire acondicionado de vehículos a partir del calor residual disponible en el circuito de refrigeración de agua/glicol del motor. El sistema se basa en el calentamiento/enfriamiento secuencial de dos reactores que contienen un material adsorbente sólido que desorbe o absorbe vapor de agua. La condensación del vapor se lleva a cabo mediante un circuito de refrigeración, mientras que la posterior evaporación del agua condensada se emplea para producir agua fría, que se emplea finalmente en enfriar el aire de la cabina. El modelo desarrollado es completamente dinámico y se basa en modelos cero dimensionales de parámetros concentrados, para todos y cada uno de los componentes del sistema global incluyendo el motor, los reactores, el circuito de calentamiento, el circuito de enfriamiento, el circuito de agua fría y la cabina del vehículo. El modelo del reactor contempla el no equilibrio de los procesos de adsorción o desorción y es capaz de trabajar con cualquier par de materiales adsorbentes. No obstante el estudio se ha restringido a gel de sílice y zeolita que se encuentran entre los materiales más adecuados para esta aplicación. El modelo se emplea para simular un ciclo de conducción estándar del vehículo, evaluando el calor disponible instantáneamente en el sistema de refrigeración del motor, y el comportamiento dinámico del sistema descrito adsorción-Aire Acondicionado, permitiendo como resultado principal la estimación de la evolución de la temperatura de la cabina a lo largo el ciclo. El modelo del sistema global se ha desarrollado en el marco del entorno de programación MATLAB Simulink. El modelo del sistema de adsorción se ha validado primero contra resultados experimentales demostrando las excelentes capacidades del modelo para predecir el comportamiento dinámico del sistema. A continuación, el modelo se ha aplicado para analizar la influencia de los principales parámetros de diseño del reactor, y de los principales parámetros de operación, sobre el rendimiento del sistema: la capacidad y coeficiente de operación (COP), con el fin de proporcionar directrices para el diseño y operación óptima de este tipo de sistemas. Por último, el modelo ha sido empleado para analizar el funcionamiento y prestaciones del sistema en su conjunto (motor, sistema de absorción, los circuitos de calefacción y refrigeración, circuito de agua fría, y la cabina) a lo largo de un ciclo de conducción estándar, bajo diferentes estrategias de operación en lo que se refiere al estado inicial del material adsorbente en los reactores, y las condiciones de operación, para el caso de un coche, y para el de un camión. Los resultados muestran las dificultades de la activación del sistema en los periodos iniciales del ciclo, cuando el motor se está calentando, y las dificultades para sincronizar el funcionamiento del sistema con la disponibilidad de energía térmica excedente del motor, así como la limitación en la capacidad de enfriamiento del sistema diseñado, que no resulta capaz de satisfacer los requerimientos mínimos de confort dentro de la cabina en los días calurosos o de enfriarlo con suficiente rapidez cuando el vehículo ha estado estacionado bajo el sol durante varias horas. Parte de este estudio de doctorado se ha llevado a cabo en el marco de un proyecto de I + D denominado " Thermally Operated Mobile Air Conditioning Systems - TOPMACS", financiado parcialmente por la UE en el marco del programa FP6, y que perseguía la evaluación de la viabilidad y el potencial de aplicación de soluciones de sistemas de adsorción activadas por el calor residual del motor para el aire acondicionado de vehículos.
[CAT] Aquesta tesi doctoral es centra en el model d'un sistema d'adsorció dissenyat per a proporcionar aire acondicionat a vehicles a partir de la calor residual disponible al circuit de refrigeració d'aigua / glicol del motor. El sistema es basa en l'escalfament / refredament seqüencial de dos reactors que contenen un material adsorbent sòlid que desorbeix o absorbeix vapor d'aigua. La condensació del vapor es porta a terme mitjançant un circuit de refrigeració, mentre que la posterior evaporació de l'aigua condensada s'utilitza per a produir aigua freda, que s'empra finalment en refredar l'aire de la cabina. El model desenvolupat és completament dinàmic i es basa en models zero dimensionals de paràmetres concentrats, per a tots i cada un dels components del sistema global incloent el motor, els reactors, el circuit d'escalfament, el circuit de refredament, el circuit d'aigua freda i la cabina del vehicle. El model del reactor contempla el no equilibri dels processos d'adsorció o desorció i és capaç de treballar amb qualsevol parell de materials adsorbents. No obstant això, l'estudi s'ha restringit a gel de sílice i zeolita que es troben entre els materials més adequats per a aquesta aplicació. El model s'utilitza per a simular un cicle de conducció estàndard del vehicle, avaluant la calor disponible instantàniament en el sistema de refrigeració del motor, i el comportament dinàmic del sistema descrit Adsorció-Aire Acondicionat, permetent com a resultat principal l'estimació de l'evolució de la temperatura de la cabina al llarg del cicle. El model del sistema global s'ha desenvolupat en l'entorn de programació MATLAB Simulink. El model del sistema d'adsorció s'ha validat primer amb resultats experimentals demostrant les excel¿lents capacitats del model per a predir el comportament dinàmic del sistema. A continuació, el model s'ha aplicat per analitzar la influència dels principals paràmetres de disseny del reactor, i dels principals paràmetres d'operació, sobre el rendiment del sistema: la capacitat i coeficient d'operació (COP), amb la finalitat de proporcionar directrius per al disseny i operació òptima d'aquest tipus de sistemes. Finalment, el model ha estat utilitzat per analitzar el funcionament i prestacions del sistema en el seu conjunt (motor, sistema d'absorció, els circuits de calefacció i refrigeració, circuit d'aigua freda, i la cabina) al llarg d'un cicle de conducció estàndard, sota diferents estratègies d'operació pel que fa a l'estat inicial del material adsorbent en els reactors, i les condicions d'operació, per al cas d'un cotxe, i per al d'un camió. Els resultats mostren les dificultats de l'activació del sistema en els períodes inicials del cicle, quan el motor s'està escalfant, i les dificultats per sincronitzar el funcionament del sistema amb la disponibilitat d'energia tèrmica excedent del motor, així com la limitació en la capacitat de refredament del sistema dissenyat, que no resulta capaç de satisfer els requeriments mínims de confort dins de la cabina en els dies calorosos o de refredar amb suficient rapidesa quan el vehicle ha estat estacionat sota el sol durant diverses hores. Part d'aquest estudi de doctorat s'ha dut a terme en el marc d'un projecte d'I + D denominat "Thermally Operated Mobile Air Conditioning Systems - TOPMACS", finançat parcialment per la UE en el marc del programa FP6, i que perseguia l'avaluació de la viabilitat i el potencial d'aplicació de solucions de sistemes d'adsorció activats per la calor residual del motor per a l'aire condicionat de vehicles.
Verde Trindade, M. (2015). MODELLING AND OPTIMIZATION OF AN ADSORPTION COOLING SYSTEM FOR AUTOMOTIVE APPLICATIONS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/54120
TESIS

This thesis presents an in-depth study into the characterization and enhancement of a metastable neon laser cooled and trapped atomic beam. The apparatus consists of a standard Zeeman slowed atomic beam loaded into a magneto-optical trap and was designed for applications to electron scattering experiments and photoionization. The efficiency of the metastable neon atomic source was investigated to determine the ideal cathode type for maximum metastable production and optimal atomic beam velocity haracteristics. A series of characterization measurements were performed on the MOT, and the trap volume and population were investigated for a range of trapping and slowing laser intensities and detunings, together with the MOT and Zeeman slower magnetic fields. The volume measurements were compared to standard Doppler theory and it was found that the Doppler model inadequately explained the trap behaviour. It was found that the MOT population characteristics were governed by two processes: two-body losses that limit the trap population at high densities, and the efficiency of the atom capture process which limits the operational range of the MOT over the various parameters. The trap temperature was determined to be 1.3mK via a time-of-flight technique. This was nearly twice that predicted by Doppler theory and the lack of agreement once again suggests the inadequacies in the Doppler theory to correctly model the experiment. The application of the MOT to the photoionization cross-section measurement of the (2p53p)3D3 state of neon was investigated. The MOT decay technique was utilized to measure cross-section values of o351 = 2.9+0.2 -0.3 x 10 -18cm2 and o363 = 3.1 +0.3 -0.4 x 10-18cm2 at the wavelengths of 351nm and 363nm respectively. This is an increase in accuracy of around a factor of five from previous measurements and it was found that the results agreed well with the values predicted by current theories.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Science
Full Text

This thesis presents a new technique for measuring total absolute collision cross sections. Using this technique, the total absolute collision cross sections were determined for neon in the (3s)3P2 metastable state with ground state thermal atoms and molecules. A magneto-optical trap (MOT) is used in this technique which infers the cross sections via the measurement of population dynamics within the MOT to determine the collision cross section. This technique is capable of providing benchmark measurements of total absolute collision cross sections. The measurements are unique for the low average collision energy which ranges between 11meV and 27meV for the dierent collision species and relatively low uncertainty of approximately 9.4%. The measurements were for neon in the (3s)3P2 metastable state with He, Ne, Ar, H2, O2, N2 and CO2. The measured cross sections were respectively 160±20Å2, 500±50Å2, 840±80Å2, 230±20Å2, 1000±100Å2, 1300.0±100Å2, 830±80Å2. The measurements made using this technique have small uncertainties, of the order of 10% of the measured cross section. As Ne* does not have the energy to ionize He, the Ne*-He collision was entirely elastic and the validity of this technique was conrmed by comparing the experimental result for this collision with an approximation for the elastic collision cross section based on van der Waals forces. The calculation based on this approximation yielded an elastic cross-section of 168.88Å2 for the Ne*-He system. This theoretical value compared favourably and within the uncertainties of the experimental measurement for the Ne*-He collision of 160±20Å2. To be able to perform these investigations a rebuild and partial redesign of the Griffith University metastable neon trapping apparatus was necessary and was included in this work.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Biomolecular and Physical Sciences
Science, Environment, Engineering and Technology
Full Text

Increased consciousness of the environmental problems has aroused people’s interest of renewable energy systems, especially the application of green features in buildings. The demand for air conditioning / cooling in domestic and non-domestic buildings is rising throughout the world; this increases the reliance on conventional fuels and the global warming effect from greenhouse gas emissions. Passive cooling and energy efficient design can substantially reduce reliance on fuel based heating and cooling. Passive and Hybrid Downdraught Cooling, in different forms, is now technically viable in many parts of the world. This has been established through a combination of research projects. In some hot arid regions, a major part of the energy consumed consists of air-conditioning requirements. Alternative methods, using passive cooling techniques, can assist in reducing the conventional energy consumption in buildings. Evaporative cooling, which can be tracked back several hundreds of years in ancient Egypt and Persia [1–3], is one of the most effective strategies, because of the enormous latent heat needed for evaporation of water. Green features are architectural features used to mitigate migration of various air-borne pollutants and transmission of air from outside to indoor environment in an advantageous way [9]. The reduction of fossil fuel consumption and the associated decrease in greenhouse gas emissions are vital to combat global warming and this can be accomplished, in part, by the use of natural ventilation. To assess the performance of several innovative cooling systems devices and to develop improved models for more established technology, quantitative measurement of output was necessary. This was achieved in this study by the development of simply constructed low energy cooling systems which were calibrated by the innovative use of wind and water as a source. These devices were found to be consistent and accurate in measuring the temperature and cooling load from a number of devices. There were some problems in the original evaporative units. Therefore, a number of modifications have to be made to enhance the systems performance. The novel Windcatcher – PEC cooling system was assessed and different cooling loads were achieved.

Experimental measurements and numerical simulations have been performed on copper and aluminum microchannel heatsinks of 300, 420, 500, and 900 &
#956
m channel widths. The heatsinks have been designed specifically for use with T/R (transmit/receive) module cooling applications of military phased array radars. An analytical calculation was also performed to aid in the design methodology. Distilled water was used as the coolant with flow rates ranging from 0.50 lpm (liters per minute) to 1.00 lpm. Local heat fluxes as high as 100 W/cm2 were tested. Upon completion of the experiments, the thermally best performing specimen, the 300 &
#956
m copper specimen, yielded a maximum temperature rise of 26.1 °
C between the heat load and coolant inlet, at a coolant flow rate of 1.00 lpm and local heat flux of 100 W/cm2, leading to a thermal resistance of 0.63 °
C/W. The pressure drop measured across the heatsink under these conditions was 0.030 bar. Numerical simulations were carried out using the commercial Computational Fluid Dynamics (CFD) software FLUENT®
. Effects of thermal interface layers and heat spreading due to the localized heat load were investigated. Simulation results for temperature were seen to agree fairly well with experimental data as long as thermal interface layers were accounted for. The study showed that the T/R modules of military phased array radars, dissipating as high as 100 W/cm2 locally, could be cooled within the limits of the harsh environmental conditions required of military applications with moderate pressure drops.

An enhanced-modularity thermally driven chemical heat pump was conceptualised as a second generation product for various heating and cooling applications with special emphasis on solar applications. The typical characteristics of the absorption heat pump were studied and the key performance parameters were selected for further investigation. An experimental test rig was constructed to allow for the testing of each component’s performance characteristics with special attention being paid to the ability to interchange components to test various configurations as well as to the facilitation of standardised relatively rapid testing. The heat transfer coefficient of the condenser/evaporator was found to be between 260 and 300 W/m2-°C during evaporation and between 130 and 170 W/m2-°C during condensation. Salt type has major impact on the system’s cooling power and cooling energy with the LiBr and water sorption pair having a 62% higher cooling/heating power than LiCl with the same matrix type and thickness. Matrix types and sorption pairs were compared with regards to the principal parameters of power and energy density with results ranging from 60 to 163 Wh/litre. The final section of the study tackled the theoretical foundation behind the system processes with modelling and simulation of the processes and comparison with the experimental data. The model makes the foundation of the continuous development of a more detailed and accurate physical model to enhance the design and optimisation process of the system.

Thesis (Ph. D.) -- University of Maryland, College Park, 2008.
Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

In this thesis we consider an efficient cooling schedule for a mean field annealing (MFA) algorithm. We combine the MFA algorithm with microcanonical simulation (MCS) method and propose a new algorithm called the microcanonical mean field annealing (MCMFA) algorithm. In the proposed algorithm, the cooling speed is controlled by the current temperature so that the amount of computation in MFA can be reduced without a degradation of performance. Unlike that produced by MFA, the solution quality produced by MCMFA is not affected by the choice of the initial temperature. Properties of MCMFA are analyzed and simulated with Hopfield neural networks (HNN). In order to compare MCMFA with MFA, we apply both algorithms to three problems namely, the graph bipartitioning problem, the traveling salesman problem and the weighted matching problem. Simulation results show that MCMFA produces a superior performance to that of MFA.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2008.
Dr. David Gerlach, Committee Member ; Dr. Samuel Graham, Committee Member ; Dr. Minami Yoda, Committee Member ; Dr. Yogendra Joshi, Committee Chair.

Exploiting solar energy technology for both heating and cooling purposes has the potential of meeting an appreciable portion of the energy demand in buildings throughout the year. By developing an integrated, multi-purpose solar energy system, that can operate all twelve months of the year, a high utilisation factor can be achieved which translates to more economical systems. However, there are still some techno-economic barriers to the general commercialisation and market penetration of such technologies. These are associated with high system and installation costs, significant system complexity, and lack of knowledge of system implementation and expected performance. A sorption heat pump module that can be integrated directly into a solar thermal collector has thus been developed in order to tackle the aforementioned market barriers. This has been designed for the development of cost-effective pre-engineered solar energy system kits that can provide both heating and cooling. This thesis summarises the characterisation studies of the operation of individual sorption modules, sorption module integrated solar collectors and a full solar heating and cooling system employing sorption module integrated collectors. Key performance indicators for the individual sorption modules showed cooling delivery for 6 hours at an average power of 40 W and a temperature lift of 21°C. Upon integration of the sorption modules into a solar collector, measured solar radiation energy to cooling energy conversion efficiencies (solar cooling COP) were between 0.10 and 0.25 with average cooling powers between 90 and 200 W/m2 collector aperture area. Further investigations of the sorption module integrated collectors implementation in a full solar heating and cooling system yielded electrical cooling COP ranging from 1.7 to 12.6 with an average of 10.6 for the test period. Additionally, simulations were performed to determine system energy and cost saving potential for various system sizes over a full year of operation for a 140 m2 single-family dwelling located in Madrid, Spain. Simulations yielded an annual solar fraction of 42% and potential cost savings of €386 per annum for a solar heating and cooling installation employing 20m2 of sorption integrated collectors.

Cold gases of atoms and molecules provide a system for the exploration of a diverse set of physical phenomena. For example, cold gasses of magnetically and electrically polar atoms and molecules are ideal systems for quantum simulation and quantum computation experiments, and cold gasses of large polar molecules allow for novel spectroscopic techniques. Buffer-gas cooling is a robust and widely applicable method for cooling atoms and molecules to temperatures of approximately 1 Kelvin. In this thesis, I present novel applications of buffer-gas cooling to obtaining gases of trapped, ultracold atoms and diatomic molecules, as well as the study of the cooling of large organic molecules. In the first experiment of this thesis, a buffer-gas beam source of atoms is used to directly load a magneto-optical trap. Due to the versatility of the buffer-gas beam source, we obtain trapped, sub-milliKelvin gases of four different lanthanide species using the same experimental apparatus. In the second experiment of this thesis, a buffer-gas beam is used as the initial stage of an experiment to directly laser cool and magneto-optically trap the diatomic molecule CaF. In the third experiment of this thesis, buffer-gas cooling is used to study the cooling of the conformational state of large organic molecules. We directly observe conformational relaxation of gas-phase 1,2-propanediol due to cold collisions with helium gas. Lastly, I present preliminary results on a variety of novel applications of buffer-gas cooling, such as mixture analysis, separation of chiral mixtures, the measurement of parity-violation in chiral molecules, and the cooling and spectroscopy of highly unstable reaction intermediates.
Chemical Physics

This work describes the development of Metal Organic Frameworks based adsorption system for producing ice, cooling, ice slurry and potable water using a CP0-27(Ni) and potable/sea water as working pair. Also a novel vacuum based direct freezing technique has been developed in the evaporator of the adsorption ice making system. In this new technique, cooling is generated in the evaporator to produce three outputs, namely, ice, ice slurry and cooling by lowering the freezing point of water using sea salt. The ice and ice slurry are produced in the evaporator during the adsorption-evaporation process, while the cooling process is produced by circulating the water/antifreeze to be cooled in the evaporator. Moreover, the usage of seawater as refrigerant offers producing fresh water as a fourth output in the condenser. A Single and a double bed CP0-27(Ni) MOF adsorption systems were developed to investigate the effect of this novel technique on the system performance in terms of producing of the four outputs. Compared to published literature, the proposed technology showed significantly higher Specific Daily Ice Production of 3 times those reported in literature with additional outputs of ice slurry, cooling and distilled water.

Large-Eddy Simulations (LES or wall-resolved LES, WRLES) has been used extensively in capturing the physics of anisotropic turbulent flows. However, near wall turbulent scales in the inner layer in wall bounded flows makes it unfeasible for large Reynolds numbers due to grid requirements. This study evaluates the use of a wall model for LES (WMLES) on a channel with rotation at ã Reã _b = 34,000 from ã Roã _b = 0 to 0.38, non-staggered 90° ribbed duct with rotation at ã Reã _b = 20,000 from ã Roã _b = 0 to 0.70, stationary 45° staggered ribbed duct at ã Reã _b = 49,000, and two-pass smooth duct with a U-bend at ã Reã _b = 25,000 for ã Roã _b = 0 to 0.238 against WRLES and experimental data. In addition, for the two-pass smooth duct with a U-bend simulations, the synthetic eddy method (SEM) is used to artificially generate eddies at the inlet based on given flow characteristics. It is presented that WMLES captures the effects of Coriolis forces and predicts mean heat transfer augmentation ratios reasonably well for all simulations. The alleviated grid resolution for these simulations indicates significant reductions in resources, specifically, by a factor of 10-20 in non-staggered 90° ribbed duct simulations. The combined effects of density ratio, Coriolis forces, with SEM for the inlet turbulence, capture the general trends in heat transfer in and after the bend.
Master of Science

The demand for more power and performance from electronic equipment has constantly been growing resulting in an increased amount of heat dissipation from these devices. Thermal management of high-density power control units for hybrid electric vehicles is one such application. Over the last few years, the performance of this power control unit has been improved and size has been reduced to attain higher efficiency and performance causing the heat dissipation as well as heat density to increase significantly. However, the overall cooling system has remained unchanged and only the heat exchanger corresponding to the power control unit (PCU) has been improved. This has allowed the manufacturing costs to go down. Efforts are constantly being made to reduce the PCU size even further and also to reduce manufacturing costs. As a consequence, heat density will go up (~ 200 – 250 W/cm2) and thus, a better high performance cooler/heat exchanger is required that can operate under the existing cooling system design and at the same time, maintain active devices temperature within optimum range (<120 – 125 °C) for higher reliability. The aim of this dissertation was to study the various cooling options based on jet impingement, mini-channel, ribbed mini-channel, phase change material and double sided cooling configurations for application in hybrid electric vehicle and other similar consumer products and perform parametric and optimization study on selected designs. Detailed experimental and computational analysis was performed on different cooling designs to evaluate overall performance. Severe constraints such as choice of coolant, coolant flow-rate, pressure drop, minimum geometrical size and operating temperature were required for the overall design. High performance jet impingement based cooler design with incorporated fin-like structures induced swirl and provided enhanced local heat transfer compared to traditional cooling designs. However, the cooling scheme could manage only 97.4% of the target effectiveness. Tapered/nozzle-shaped jets based designs showed promising results (~40% reduction in overall pressure drop) but were not sufficient to meet the overall operating temperature requirement. Various schemes of mini-channel arrangement, which were based on utilizing conduction and convection heat transfer in a conjugate mode, demonstrated improved performance over that of impingement cooling schemes. Impingement and mini-channel based designs were combined to show high heat transfer rates but at the expense of higher pressure drops (~5 times). As an alternate, mini-channel based coolers with ~1.5 mm size channels having trip strips or ribs were studied to accommodate the design constraints and to enhance local as well as overall heat transfer rates and achieve the target operating temperature. A step by step approach to the development of the heat exchanger is provided with an emphasis on system level design. The computational based optimization methodology is confirmed by a fabricated test bed to evaluate overall performance and compare the predicted results with actual performance. Additionally, one of the impingement based configuration (Swirl-Impingement-Fin) developed during the course of this work was applied to the internal cooling of a turbine blade trailing edge and was shown to enhance the thermal performance by at least a factor of 2 in comparison to the existing pin-fin technology for the conditions studied in this work.
Ph. D.

Nanotechnology has made it possible to create materials with unique properties. This development offers new opportunities and overcomes challenges for many thermal transport applications. Yet, it requires a more fundamental scientific understanding of nanoscale transport. This thesis emphasizes how simulation, mathematical, and numerical methods can lead to more grounded studies of nanoscale thermal transport for biological and physical applications. For instance, magnetic fluid hyperthermia (MFH), an emerging cancer treatment, is a noninvasive method to selectively destroy a tumor by heating a ferrofluid-impregnated malignant tissue with minimal damage to the surrounding healthy tissue. We model the problem by considering an idealized spherical tumor that is surrounded by healthy tissue. The dispersed magnetic nanoparticles in the tumor are excited by an AC magnetic field to generate heat. The temperature distribution during MFH is investigated through a bioheat transfer relation which indicates that the P\'eclet, Joule, and Fourier numbers are the more influential parameters that determine the heating during such a thermotherapy. Thus, we show that a fundamental parametric investigation of the heating of soft materials can provide pathways for optimal MFH design. Since ferrofluid materials themselves play a key role in heating, we examine six materials that are being considered as candidates for MFH use. These are simulated to investigate the heating of ferrofluid-loaded tumors. We show that iron-platinum, magnetite, and maghemite are viable MFH candidates since they are able to provide the desired heating of a tumor which will destroy it while keeping the surrounding healthy tissues at a relatively safe temperature. Recent advances in the synthesis and nanofabrication of electron devices have lead to diminishing feature sizes. This has in turn increased the power dissipation per unit area that is required to cool the devices, leading to a serious thermal management challenge. The phonon thermal conductivity is an important material property because of its role in thermal energy transport in semiconductors. A higher thermal conductivity material is capable of removing more heat since higher frequency phonons are able to travel through it. In this thesis, the effects of surface stress on the lattice thermal conductivity are presented for a silicon nanowire. Based on a continuum approach, a phonon dispersion relation is derived for a nanowire that is under surface stress and the phonon relaxation time is employed to subsequently determine its thermal conductivity. The surface stress is found to significantly influence the phonon dispersion and thus the Debye temperature. Consequently, the phonon thermal conductivity decreases with increasing surface stress. Different magnitudes of surface stress could arise from various material coatings and through different nanofabrication processes, effects of which are generally unclear and not considered. Our results show how such variations in surface stress can be gainfully used in phonon engineering and to manipulate the thermal conductivity of a nanomaterial. The thermal transport during thermoelectric cooling is also an important property since thermoelectric devices are compact, reliable, easy to control, use no refrigerants and require lower maintenance than do more traditional refrigeration devices. We focus on the Thomson effect that occurs when there is a current flow in the presence of a temperature gradient in the material, and investigate its influence on an intrinsic silicon nanowire cooler. The temperature dependence of the Thomson effect has a significant influence on the cooling temperature. We also consider thermal nonequilibrium between electrons and phonons over the carrier cooling length in the nanowire. The results show that a strong energy exchange between electrons and phonons lowers the cooling performance, suggesting useful strategies for thermoelectric device design.
Ph. D.

This thesis is divided into four distinct chapters all linked by the topic of spray cooling. Chapter one gives a detailed categorization of future and current spray cooling applications, and reviews the major advantages and disadvantages that spray cooling has over other high heat flux cooling techniques. Chapter two outlines the developmental goals of spray cooling, which are to increase the output of a current system and to enable new technologies to be technically feasible. Furthermore, this chapter outlines in detail the impact that land, air, sea, and space environments have on the cooling system and what technologies could be enabled in each environment with the aid of spray cooling. In particular, the heat exchanger, condenser and radiator are analyzed in their corresponding environments. Chapter three presents an experimental investigation of a fluid management system for a large area multiple nozzle spray cooler. A fluid management or suction system was used to control the liquid film layer thickness needed for effective heat transfer. An array of sixteen pressure atomized spray nozzles along with an imbedded fluid suction system was constructed. Two surfaces were spray tested one being a clear grooved Plexiglas plate used for visualization and the other being a bottom heated grooved 4.5 x 4.5 cm2 copper plate used to determine the heat flux. The suction system utilized an array of thin copper tubes to extract excess liquid from the cooled surface. Pure water was ejected from two spray nozzle configurations at flow rates of 0.7 L/min to 1 L/min per nozzle. It was found that the fluid management system provided fluid removal efficiencies of 98% with a 4-nozzle array, and 90% with the full 16-nozzle array for the downward spraying orientation. The corresponding heat fluxes for the 16 nozzle configuration were found with and without the aid of the fluid management system. It was found that the fluid management system increased heat fluxes on the average of 30 W/cm2 at similar values of superheat. Unfortunately, the effectiveness of this array at removing heat at full levels of suction is approximately 50% & 40% of a single nozzle at respective 10[degrees]C & 15[degrees]C values of superheat. The heat transfer data more closely resembled convective pooling boiling. Thus, it was concluded that the poor heat transfer was due to flooding occurring which made the heat transfer mechanism mainly forced convective boiling and not spray cooling. Finally, Chapter four gives a detailed guide for the design and construction of a high heat flux heater for experimental uses where accurate measurements of surface temperatures and heat fluxes are extremely important. The heater designs presented allow for different testing applications; however, an emphasis is placed on heaters designed for use with spray cooling.
M.S.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Mechanical Engineering

Recently, the electronic industry has had to face the issue of dissipation of high heat fluxes, as they have increased due to miniaturization of electronics devices, in order to keep chip junction temperature below a fixed value to guarantee reliability and to avoid damage. This issue has also involved the aeronautics industry, which has for the most part favored air cooling systems; however, they are no longer suitable to meet avionics requirements and thus of power electronics. In fact, as avionics demands higher heat fluxes, power electronics, that is the set of transformers, converters, rectifiers etc., demands higher heat flow rates compared with traditional systems, as a consequence of the current design propensity for substituting hydraulic and pneumatic devices with electric systems, with accompanying increases in generated and transformed electric power in aircraft. The present work proposes a review of cooling techniques which can be suitable to be employed in high flux electronic components, meeting the requirement for compact cooling systems. The study focuses on cold plate heat exchangers, in consideration of their versatility, moderate cost, and the cooling capacity which can be attained by using these types of systems. Particularly, mini and micro channel cold plates have been investigated, with parallel channel arrangement, which are easy to produce and cost effective, and which can be used both in liquid loops and in mini vapor compression systems, as evaporators. To allow a careful and proper performance evaluation of such cooling systems, a new simulation model has been developed, as described in detail in the present work, which does not neglect the thermal conduction on the heat exchanger walls (unlike common numerical codes), then allowing a complete evaluation of the temperature profile along the cold plate base, on the surface in contact with the electronic component. Furthermore, this numerical code allows one to properly design cold plates in consideration of both heat flux and the fluid employed. In fact, the present work proposes a review of a large number of fluids, including saline solutions, hydrocarbon and aromatics blends (PAO) and fluorine compounds, both single phase and two phase, which have been considered for electronics cooling purposes, and whose properties have been taken from the open scientific literature or private communications with the manufacturers. Among all the considered fluids, particular attention has been focused on a new low GWP refrigerant, the hydro flour olefin R1234yf, since it is proposed as a candidate to substitute R134a in air conditioning systems in the automotive industry. As the more recent developments in the aeronautics industry consider the use of vapor compression systems aboard new aircrafts, mainly for air conditioning purposes but potentially also for electronics, test results are proposed in the present work, relating to several laboratory tests carried out to evaluate air conditioning system performance both with R134a and R1234yf. Finally, the appendix reports laboratory test results for systems working with carbon dioxide, as it is still considered as a candidate substitute for traditional refrigerants. Two test campaign results are illustrated: in the first one, the thermal conduction effect along fins in a finned coil heat exchanger has been analyzed, and in the second one, capillary tube behavior has been assessed when used as a throttling device.
Nel corso degli ultimi anni l’industria dell’elettronica ha dovuto affrontare il problema della dissipazione di elevati flussi termici specifici, aumentati in seguito ai processi di miniaturizzazione che interessano tutti i dispositivi elettronici, in modo da contenere la temperatura dei chip entro limiti che ne garantiscano l’integrità e l’affidabilità. Tale problema ha pertanto riguardato anche l’industria aeronautica, che fino a questo momento ha privilegiato sistemi di raffreddamento ad aria, attualmente non più in grado di rispondere alle esigenze non solo dell’avionica, ma anche dell’elettronica di potenza. In fatti, se l’avionica impone dei maggiori flussi termici specifici, l’elettronica di potenza, vale a dire il complesso dei sistemi di conversione e distribuzione della potenza elettrica, impone flussi termici complessivi maggiori rispetto a quelli tradizionali, in conseguenza dell’attuale ricerca di un maggior impiego di componenti elettici in sostituzione di quelli idraulici o pneumatici, con conseguente incremento di potenza elettrica generata e trasformata nel velivolo. Il presente lavoro propone una sintesi delle tecniche di raffreddamento di possibile impiego per componenti elettronici ad alto flusso termico specifico, e che permettono inoltre di ottenere un sistema più compatto, in linea con le esigenze del settore. Particolare rilievo viene dato allo scambiatore di tipo cold plate, per la sua versatilità, il suo costo contenuto, e la capacità di raffreddamento che può essere conseguita con l’uso di tale sistema. In particolare, il tipo di cold plate considerato è a mini o micro canali, disposti e alimentati in parallelo, pertanto di realizzazione piuttosto semplice, e che sono utilizzati sia in sistemi ad anello liquido sia come evaporatori in cicli frigoriferi miniaturizzati. Per consentire una corretta e approfondita valutazione delle prestazioni di questa tipologia di scambiatori, è stato realizzato un modello di simulazione numerica, dettagliatamente descritto nel presente lavoro, che non trascura la conduzione termica lungo le pareti metalliche dello scambiatore, a differenza di quanto accade per i modelli tradizionali, e che pertanto permette di analizzare il profilo di temperatura che si realizza alla base dello scambiatore, vale a dire lungo la superficie di contatto con il componente elettronico. Grazie a questo modello è possibile inoltre eseguire un corretto dimensionamento in considerazione sia dei flussi termici imposti, sia del fluido da utilizzare. Infatti, il presente lavoro propone una vasta rassegna di fluidi, tra cui soluzioni saline, idrocarburi e miscele di composti aromatici (PAO) e composti fluorurati, sia monofase che bifase, presi in esame per il raffreddamento di elettronica, e le cui principali proprietà sono state ricavate dalla letteratura scientifica o da comunicazioni private con i produttori. Tra i vari fluidi considerati, è parso importante concentrare l’attenzione su di un nuovo fluido frigorigeno, R1234yf, in quanto candidato a sostituire R134a nelle applicazioni di condizionamento dell’aria nell’industria automobilistica. Poiché i recenti sviluppi dell’industria aeronautica prevedono l’impiego di cicli frigoriferi a bordo dei nuovi velivoli, utilizzati per il sistema di condizionamento ma potenzialmente anche per elettronica, si è ritenuto opportuno presentare i risultati di laboratorio conseguiti durante un’intensa campagna di prove condotte per valutare le prestazioni di un impianto di condizionamento, in cui è stato impiegato sia R134a sia R1234yf. In appendice sono riportati infine i risultati conseguiti nello studio di sistemi ad anidride carbonica, fluido tuttora ritenuto potenziale sostituto di refrigeranti tradizionali. In particolare si riportano i risultati conseguiti in due distinte campagne di prove: nella prima è stato esaminato l’effetto della conduzione termica lungo le alette di uno scambiatore ad aria, mentre nella seconda è stato valutato il comportamento di un capillare utilizzato come organo di laminazione.