[pt] Um sistema térmico de armazenamento da energia com pasta de cristais de gelo foi desenvolvido para aplicações de condicionamento de ar e resfriamento de processos. O sistema usa um evaporador orbital de haste, um trocador de calor vertical do tipo tubo e carcaça com intensificação mecânica de transferência de calor. A pasta de gelo é produzida continuamente sem acumulação no evaporador e é compatível com unidades condensadoras convencionais, tanques de armazenamento e bombas. Soluções aquosas diluídas ou soluções inorgânicas de salmoura promovem a formação de cristais de gelo, e o gelo líquido resultante pode ser bombeado ou por gravidade alimentar um tanque de armazenamento. O circuito hidráulico de refrigeração (carga térmica) pode ser desacoplado da produção do gelo utilizando-se o tanque de armazenamento. O armazenamento de gelo líquido fornece temperaturas consistentemente baixas à medida que se derrete o gelo, que por sua forma pode ser derretido também muito rapidamente. Com suas altas temperaturas características de evaporação e elevados fluxos do calor, os sistemas de geração de gelo líquido apresentam potencial para reduzir significativamente os custos de capital inicial e operação, quando comparados com tecnologias de sistemas estáticos de gelo ou ice harvesting. / [en] New ice crystal slurry thermal energy storage (TES) system has been developed for both HVAC and process cooling applications. The system uses an orbital rod evaporator (ORE), a vertical shell-and-tube heat exchanger with mechanical heat transfer augmentation, as a dynamic ice maker to generate liquid ice. Ice forms continuously without accumulation in the ORE and is compatible with conventional condensing units, storage tanks, and pumps. Dilute glycol or inorganic brine solutions promote formation of ice crystals, and the resulting liquid ice may be pumped or gravity fed to a storage tank. The cooling load circuit can be hydraulically decoupled from ice production at the storage tank. Stored liquid ice provides consistently low solution supply temperatures over significant portions of the ice melt period and may be melted very rapidly. With its characteristic high evaporator temperatures and high heat fluxes, ORE TES systems have the potential for significantly lower capital and operating costs than static ice or ice harvesting technologies.

<div>Thermal management in next generation power electronic devices, radar applications and semiconductor packaging architectures is becoming increasingly challenging due to the need to reject localized high heat fluxes as well as large total powers. Air cooling has been considered as a simple and reliable method for thermal management compared to architectures that incorporate liquid cooling. However, air-cooled heat sinks typically require effective heat spreading to provide the requisite level of area enhancement to dissipate high heat fluxes. Compared to solid metallic heat spreaders, advanced heat sinks that incorporate two-phase heat transfer devices such as vapor chambers can significantly enhance the power dissipation capabilities in such configurations. Vapor chambers are devices that utilize evaporation/boiling processes within a sealed cavity to achieve efficient heat spreading. In high-heat-flux applications, boiling can occur within the internal wick structure of the vapor chamber at the location of the heat input (i.e., the evaporator). The maximum dryout heat flux and thermal resistance of the device is dictated by the resulting two-phase flow and heat transfer in the porous evaporator due to boiling. While various works in the literature have introduced new evaporator wick designs to improve the dryout heat flux during boiling, the enhancement is limited to small, millimeter scale hotspots or at a very high thermal resistance. In additixon, the effective design of such evaporator systems requires mechanistic models that can accurately predict the dryout limit and thermal performance. </div><div> This thesis first explores the usage of a novel ‘two-layer’ evaporator wick for passive high heat flux dissipation over large heater areas at a low thermal resistance. Moreover, a new mechanistic (first principles based) model framework is introduced for dryout limit and thermal performance prediction during boiling in capillary fed evaporators, by considering the resulting simultaneous flow of two phases (liquid and vapor) within the microscale porous media.</div><div> The novel two-layer wick concept uses a thick ‘cap’ layer of porous material to feed liquid to a thin ‘base’ layer through an array of vertical liquid-feeding ‘posts’. Vapor ‘vents’ in the cap layer allow for vapor formed during the boiling process (which is constrained to the base layer) to escape out of the wick. This two-layer structure decouples the functions of liquid resupply and capillary-fed boiling heat transfer, making the design realize high heat flux dissipation greater than 500 W/cm2 over large heat input areas of ~1 cm2. A reduced-order model is first developed to demonstrate the performance of a vapor chamber incorporating such a two-layer evaporator wick design. The model comprises simplified hydraulic and thermal resistance networks for predicting the capillary-limited maximum heat flux and the overall thermal resistance, respectively. The reduced-order model is validated against a higher fidelity numerical model and then used to analyze the performance of the vapor chamber with varying two-layer wick geometric feature sizes. The fabrication of the proposed two-layer wick is then presented. The thermal performance of the fabricated wicks is characterized using a boiling test facility that utilizes high speed visualization to identify the characteristic regimes of boiling operation in the wicks. The performance is also benchmarked to conventional single-layer wicks. </div><div> It is observed that single-layer wicks exhibit an unfavorable boiling regime where the center of the heater area dries out locally, leading to a high value of thermal resistance. The two-layer wicks avoid local dryout due to the distributed feeding provided by the posts and enhance the dryout heat flux significantly compared to single-layer wicks. A two-layer design that consists of a 10 × 10 array of liquid feeding posts provided a 400% improvement in the dryout heat flux. Following a parametric analysis of the effect of particle size, two-layer wicks composed of 180 – 212 µm particles and a 15 × 15 array of liquid feeding posts yielded a maximum heat flux dissipation of 485 W/cm2 over a 1 cm2 heat input area while also maintaining a low thermal resistance of only ~0.052 K/W. The effect of vapor venting and liquid-feeding areas is also experimentally studied. By understanding these effects, a parametrically optimized design is fabricated and shown to demonstrate an extremely high dryout limit of 512 W/cm2. We identify that the unique area-scalability of the two-layer wick design allows it to achieve an unprecedented combination of high total power and low-thermal-resistance heat dissipation over larger areas than was previously possible in the literature.</div><div> The results from the characterization of two-layer wicks revealed that the overall performance of the design was limited by the boiling process in the thin base wick layer. A fundamental model-based understanding of the resulting two-phase flow and heat transfer process in such thin capillary-fed porous media was still lacking. This lack of a mechanistic model precluded the accurate prediction of dryout heat flux and thermal performance of the two-layer wick. Moreover, such an understanding is needed for the optimal design of advanced hybrid evaporator wicks that leverage capillary-fed boiling. Despite the existence of various experimental works, there are currently no mechanistic approaches that model this behavior. To fill this unmet need, this thesis presents a new semi-empirical model for prediction of dryout and thermal resistance of capillary-fed evaporator systems. Thermal conduction across the solid and volumetric evaporation within the pores are solved to obtain the temperature distribution in the porous structure. Capillary-driven lateral liquid flow from the outer periphery of the evaporator to its center, with vapor flow across the thickness, is considered to obtain the local liquid and vapor pressures. Experiments are conducted on sintered copper particle evaporators of different particle sizes and heater areas to collect data for model calibration. To demonstrate the wider applicability of the model for other types of porous evaporators, the model is further calibrated against a variety of dryout limit and thermal resistance data collected from the literature. The model is shown to predict the experimentally observed trends in the dryout limit with mean particle/pore size, heater size, and evaporator thicknesses. This physics–based modeling approach is then implemented into a vapor chamber model to predict the thermal performance limits of air-cooled heat sinks with embedded vapor chambers. The governing energy and momentum equations of a low-cost analytical vapor chamber modeling approach is coupled with the evaporator model to capture the effect of boiling in the evaporator wick. An example case study illustrating the usage of the model is demonstrated and compared to a purely evaporation-based modeling approach, for quantifying the differences in dryout limit prediction, signifying the need to account for boiling in the evaporator wick. </div><div> The understanding gained from this thesis can be utilized for the prediction of dryout and thermal performance during boiling in capillary limited evaporator systems. The work also suggests the usage of a universal relative permeability correlation for the two-phase flow configuration studied herein for capillary-fed boiling, based on a wide calibration to experimental data. The modeling framework can also be readily leveraged to find novel and unexplored designs of advanced evaporator wicks. From an application standpoint, the new vapor chamber model developed here can be used for the improved estimation of performance limits specifically when high heat fluxes are encountered by the device. This will enable better and informed design of air-cooled heat sink architectures with embedded vapor chambers for high performance applications. </div><div><br></div>

O presente trabalho trata do estudo do modelo computacional, EVSIM, que permite a simulação e caracterização da transferência de calor e massa entre o ar úmido externo e o refrigerante em evaporadores utilizados em sistemas frigoríficos e condicionadores de ar domésticos. O modelo é capaz de levar em conta a distribuição do refrigerante ao longo da serpentina mesmo em circuitos complexos. O desempenho do evaporador é calculado através de uma análise tubo a tubo. Os cálculos em cada tubo são baseados no cômputo da distribuição do ar, da vazão mássica do refrigerante e dos estados termodinâmicos determinados para cada tubo, assim como dos processos de transferência de calor e massa em cada tubo, respectivamente. A principal vantagem do modelo está na análise termodinâmica local, e na consideração dos mecanismos de transferência de calor e equações de estado para diferentes substâncias. Este trabalho acrescenta em relação ao código inicialmente desenvolvido, diferentes correlações de transferência de calor para escoamento bifásico. Os resultados de simulação obtidos são coerentes com resultados experimentais e de razoável confiabilidade. / This work concerns about a study of the computational model, EVSIM, which allows simulating and characterizing the heat and mass transfer between the humid air and the refrigerant in evaporators used in frigorific and domestic air-conditioning systems. The model is able to consider the refrigerant distribution through the coil even in complex geometrical designs. The evaporator thermal performance is calculated through a tube-by-tube analysis. The computation in each tube is based on the air distribution calculation, refrigerant mass flow rate and the thermodynamic states determination for each tube, as well as on the computation of the heat and mass transfer processes, respectively. The main advantage in this model is the ability in performing local thermodynamic analysis, and in the consideration of heat transfer mechanisms and state equations for different substances. This work brings, in relation to the original code, an update on heat transfer correlations for simulating a two-phase flow heat transfer process. The results obtained are coherent with experimental data and show a reasonable accuracy.

Neste trabalho é apresentada a avaliação da inversão da sacarose em um sistema constituído de um Evaporador de filme descendente com promotor de película. Foi feita a modelagem da inversão considerando-se os balanços de massa e de entalpia e a cinética de inversão no tubo de evaporação. Os principais parâmetros considerados foram: constante cinética (k), taxa de evaporação(mv), concentração de açúcares totais (C), vazão do líquido no tubo de evaporação (q), pH, temperatura (T) e tempo de residência no tubo de evaporação (T). Usaram-se dois métodos para a resolução do modelo, a primeira através da integração numérica do modelo (Runge Kutta de 4ta ordem) e a outra através de equacionamento simplificado a partir valores médios das propriedades, pH, e calculando as constantes cinéticas nas temperaturas efetivas em cada zona do tubo de evaporação, Tefa (zona de aquecimento) e Tefe (zona de evaporação), possibilitando assim, a integração analítica do modelo. Foram calculados os valores do volume do líquido (V) através dos dois métodos. Os resultados foram expressos em função da vazão do líquido (q). Verificou-se que os resultados calculados pelos dois métodos foram muito próximos. Os modelos desenvolvidos podem ser aplicados para a otimização do processo de evaporação visando à minimização da inversão da sacarose. O método desenvolvido possibilita a determinação do tempo de residência no tubo de evaporação. / In this work the evaluation of sucrose inversion in a system formed by falling film Evaporator with Promoter of film is presented. The modeling was made considering: mass and enthalpy balances and the kinetic of inversion in the evaporation tube. The mean parameters investigated were: kinetic constant (k), evaporation rate (mv), concentration of total sugars (C), volumetric velocity of the liquid in the evaporation tube (q), pH, temperature (T) and residence time in the evaporation tube (T). Two methods was used to solve the model, the first by numeric integration (Runge Kutta 4th Order) and the other by analytic integration of simplified model considering mean values of properties and pH, and the kinetic constant calculates at effective temperatures in each zone of the evaporation tube, Tefa and Tefe, allowed the analytic integration of the model. The volume of liquid in the evaporation tube (V) was calculated by the two methods (numeric integration and simplified methods). The results were expressed as a function of liquid flow rate (q). It was verified that the values obtained by the two methods were very closed. These models can be used for the optimization of the evaporation process emphasizing the minimization of the sucrose inversion. The methodology could be used for the determination of the residence time in the evaporation tube.

Differential geometric nonlinear control of a multiple stage evaporator system of the liquor burning facility associated with the Bayer process for alumina production at Alcoa Wagerup alumina refinery, Western Australia was investigated.Mathematical models for differential geometric analysis and nonlinear controller synthesis for the evaporator system were developed. Two models, that were structurally different from each other, were used in the thesis for simulation studies. Geometric nonlinear control structure, consisting of nonlinear state feedback control laws and multi-loop single-input single-output proportional-integral controllers, were designed for the industrial evaporator system. The superiority of the geometric nonlinear control structure for regulatory control of the evaporator system was successfully demonstrated through computer simulations and real-time simulator implementation. The implementation trial has verified the practicality and feasibility of these type of controllers. It also re-solved some practical issues of the geometric nonlinear control structure for industrial control applications. In addition, the implementation trial also established a closer link between the academic nonlinear control theory and the industrial control practices.Geometric nonlinear output feedback controller, consisting of the geometric nonlinear control structure and reduce-order observer was proposed for actual plant implementation on the evaporator system on-site. Its superior performance was verified through computer simulations, but its feasibility on the evaporator system on-site has yet to be investigated either through simulator implementation or actual plant implementation. This investigation was not performed due to the time constraint on the preparation of this thesis and the inavailability of the plant personnel required for this implementation.Robust ++ / nonlinear control structures that are simple and computationally efficient have been proposed for enhancing the performance of geometric nonlinear controllers in the presence of plant/model mismatch and/or external disturbances. The robust nonlinear control structures are based on model error compensation methods. Robustness properties of the proposed robust nonlinear control structures on the evaporator system were investigated through computer simulations and the results indicated improved performance over the implemented geometric nonlinear controller in terms of model uncertainty and disturbance reductions.A software package was developed in MAPLE computing environment for the analysis of nonlinear processes and the design of geometric nonlinear controllers. This developed symbolic package is useful for obtaining fast and exact solutions for the analysis and design of nonlinear control systems. Procedures were also developed to simulate the geometric nonlinear control systems. It was found that MAPLE, while it is superior for the analyses and designs, is not viable for simulations of nonlinear control systems. This was due to limitation of MAPLE on the physical, or virtual, memory management. The use of both symbolic and numeric computation for solutions of nonlinear control system analysis, design and simulation is recommended.To sum up, geometric nonlinear controllers have been designed for an industrial multiple stage evaporator system and their simplicity, practicality, feasibility and superiority for industrial control practices have been demonstrated either through computer simulations or real-time implementation. It is hoped that the insights provided in this thesis will encourage more industry-based projects in nonlinear control, and thereby assist in closing the widening gap between academic nonlinear control theory and industrial control ++ / practice.Keywords: geometric nonlinear control, input-output linearization, multiple stage evaporator, robust geometric nonlinear control, control performance enhancement.

The high ozone depletion and global warming potentials of fluorocarbon refrigerants have resulted in prohibitions and restrictions in many markets. Hydrocarbon refrigerants have low environmental impacts and are successfully used in domestic refrigerators and car air conditioners but replacing fluorocarbons in centrifugal chillers for air conditioning applications is unknown. Hydrocarbon replacements need a heat transfer correlation for refrigerant in flooded evaporators and predictions for operating conditions, capacity and performance. Safety precautions for large quantities of hydrocarbon refrigerants are needed and control of overpressure in plantrooms requires accurate prediction. Reliable correlations exist for forced convection in a single phase flow, condensation outside tubes and evaporation off sprayed tubes. For flooded evaporators this thesis proposes a new correlation for forced convection boiling of any refrigerant. An enhancement factor is combined with a modified Chen coefficient using recent pool boiling and forced convection correlations outside tubes. This correlates within typically a factor of two to known boiling literature measurements for CFC-113, CFC-11, HCFC-123, HFC-134a and HC-601. The operating conditions, capacity and performance of replacement hydrocarbons in centrifugal chillers were predicted using fluorocarbon performance as a model. With the new heat transfer correlation hydrocarbon predictions for flooded evaporators were made. For any fluorocarbon refrigerant there exists a replacement mixture of hydrocarbons which with a rotor speed increase about 40% gives the same cooling capacity in the same centrifugal chiller under the same operating conditions. For example replacing HCFC-123 in a flooded evaporator with HC-601/602 [90.4/9.6] and increasing the rotor speed by 43% will increase the coefficient of performance by 4.5% at the same cooling capacity. The maximum plantroom overpressure considered was from leakage and ignition of a uniform air/refrigerant mixture with maximum laminar burning velocity. Flow was modelled using a turbulence viscosity due to Launder and Spalding and turbulent deflagration using a reaction progress variable after Zimont. These partial differential equations were solved approximately for two and three dimensional geometries using finite volume methods from the Fluent program suite. Simple overpressure predictions from maximum flame area approximations agreed with Fluent results within 13.7% promising safe plantroom design without months of computer calculation.

In this thesis, one of the current control algorithms for the R744 cycle, which tries tooptimize the performance of the system by two SISO control loops, is compared to acost-effective system with just one actuator. The operation of a key component of thissystem, a two stage orifice expansion valve is examined in a range of typical climateconditions. One alternative control loop for this system, which has been proposed byBehr group, is also scrutinized.The simulation results affirm the preference of using two control-loops instead of oneloop, but refute advantages of the Behr alternate control approach against one-loopcontrol. As far as the economic considerations of the A/C unit are concerned, usinga two-stage orifice expansion valve is desired by the automotive industry, thus basedon the experiment results, an improved logic for control of this system is proposed.In the second part, it is investigated whether the one-actuator control approach isapplicable to a system consisting of two parallel evaporators to allow passengers tocontrol different climate zones. The simulation results show that in the case of usinga two-stage orifice valve for the front evaporator and a fixed expansion valve forthe rear one, a proper distribution of the cooling power between the front and rearcompartment is possible for a broad range of climate conditions.

The orientation relationship and interfaces of PtSn4 and PtSn with the Pt (001) and (111) surfaces have been studied with transmission electron microscopy. Pt was evaporated onto the NaCl (001) and (111) surfaces to form epitaxial Pt thin films and Sn was evaporated onto the Pt films at different temperature to form PtSn4 and PtSn. Pt was evaporated onto the NaCl (001) and (111) surfaces at 350 ¢J to form epitaxial Pt thin films of [001] and [111] zone axes, respectively. Some grains are in random orientation and other as ring pattern. The grain size was at about 10-20 nm. Sn was evaporated onto the Pt surface at 150 ¢J to form PtSn4 and at 200 ¢J to form PtSn. No good orientation relationships were formed on both the PtSn4/ Sn and the PtSn/ Sn interfaces. Heterogeneous nucleation theory, predicts that PtSn should form before PtSn4, but PtSn4 was observed to the first to form. The possible reasons were discussed. Keywords: PtSn4/ Sn interface, PtSn/ Sn interface, orientation relationships, thin films, evaporator, transmission electron microscopy

Three experimental studies were conducted to quantify the effects of system cycling, evaporator airflow, and condenser coil fouling on the performance of residential air conditioners. For all studies, the indoor dry-bulb (db) temperature was 80°F (26.7°C) db. The cycling study consisted of twelve transient tests conducted with an outdoor temperature of 95°F (35°C) db for cycle times of 6, 10, 15, and 24 minutes. Indoor relative humidities of 40%, 50%, and 60% were also considered. The evaporator airflow study consisted of twenty-four steady-state tests conducted with an indoor condition of 67°F (19.4°C) wet-bulb (wb) for evaporator airflows ranging from 50% below to 37.5% above rated airflow. Outdoor temperatures of 85°F (29.4°C) db, 95°F (35°C) db, and 105°F (40.6°C) db were also considered. The coil fouling study used a total of six condensers that were exposed to an outdoor environment for predetermined amounts of time and tested periodically. Three of the condensers were cleaned and retested during the periodic testing cycles. Testing consisted of thirty-three steady-state tests conducted with an indoor condition of 67°F (19.4°C) wb for outdoor exposure times of 0, 2000, 4000, and 8000 hours. Outdoor temperatures of 82°F (27.8°C) db and 95°F (35°C) db were also considered.

Evaporation is used in the dairy industry to reduce the production costs of powder production (including milk powder) as it is more energy efficient to remove water by evaporation than by drying. There are significant economic reasons why gaining a greater understanding of the complex interactions occurring between the liquid and vapour phases in evaporators is advantageous. The multiphase flows in industrial dairy falling film evaporators were studied. Several computational fluid dynamic (CFD) models were created using Ansys CFX 10. Two case studies were chosen. The first case involved modelling the dispersed droplets that require separation from the water vapour evaporated from the feed of the evaporator. The CFD results were able to show that fouling was not caused by a lack of separation. The predicted separation agreed with experimental measurements. The atomisation process was found to be critical in the prediction of the separation. The atomisation process is not well understood and introduced the greatest error to the model. A plug flow assumption is currently used as a basis for the design the separators. The CFD solutions found no validity to this assumption. The second case study aimed to model and solve the distribution of the feed into the heat transfer tubes at the top of the falling film evaporators. The goal of this study was to be able to accurately predict wetting of the tubes. The volume of fluid (VOF) method using the continuum surface force method (CSF) to account for surface tension was chosen to model the system. The poor curvature estimate of the CSF method was found to produce parasitic currents that limited the stability of the solutions. Small VOF timesteps prevented the solver from diverging and the parasitic currents would oscillate the interface around the correct location. The small timesteps required significantly more computational power than was available and the model for the distribution process could not be solved. The CSF VOF method showed considerable promise, particularly because it can predict free surface topography without user input. There are still questions about numerical creeping of films, but the method was able to correctly predict several different surface tension and contact angle dominated film flows expected to be needed to accurately model the distribution of the falling film evaporator. Validated solutions of jet, meniscus, sessile, "overfall" and 3-D weir models were obtained and these agreed with published results in literature. A 2-D weir solution showed qualitative agreement with the expected form of the film. A 2-D hydraulic jump model without surface tension was created and agreed with experimental work in the literature to within 22%. The 3-D hydraulic jump solution only showed partial agreement with published experimental, the solutions were not mesh independent and not well converged so few conclusions can be drawn. The solutions of a rivulet model showed qualitative similarities with experimental work. The predicted wetting rate did not agree with values in the published literature because the spatial domain modelled was believed to be too narrow. An extended model of rivulet flow agreed with the idealised rivulet profile in literature and the predicted wetting rate agreed with some of the published literature. Again the solutions were not mesh independent so few conclusions can be confirmed.