Liquid cooled small channel heat sinks have become a promising heat dissipation method for future high power electrical devices. Traditional mini and microchannel heat sinks consist of a single layer of low-aspect ratio rectangular channels. The alternative new heat sinks are fabricated by stacking many channels together to create multiple layer channels. These multilayer heat sinks can achieve high heat flux due to high heat transfer coefficients from small channels and large surface area from multilayer structure. In this research, multilayer copper and silicon carbide (SIC) minichannel heat sinks were tested in single-phase flow. It was shown that multilayer heat sinks have significant advantages over single-layer equivalents with reductions both in thermal resistance and pressure drop. A 3-D resistance network model for single and multilayered heat sinks was developed and validated. Parametric study and optimization on copper and SiC heat sinks with respect to channel geometries, number of layers, and heat sink conductivity were conducted by using the model.Both copper and SiC heat sinks were also tested in two-phase flow. In experiments, the multilayer copper heat sinks achieved smaller average surface temperature than their single-layer counterpart at low heat flux. However the multilayer copper heat sinks gradually lost stability at high heat flux, which lead to increased surface temperature. The redistribution of flow in different layers caused by pressure discrepancy in different layers was believed to be the cause. A three-zone model, which dividing the flow in small channels into three distinguishing parts: single-phase flow, subcooled boiling flow, and saturated boiling flow, was proposed to describe the different two-phase flow regimes. In each zone, the local heat transfer coefficient was computed by corresponding correlation. Several boiling correlations combined with the resistance network model were used to compute the heat sink surface temperature distributions, which were compared with experimental results. It was found the classical boiling correlations for macro channels are not suitable for the minichannels, frequently overestimating the boiling heat transfer coefficient. Boiling correlations for small channels are more consistent with experimental data and the predictions of Yu's correlation match the experimental results best.

Thermoelectric generator (TEG) is a solid state technology based on the Seebeck effect that can generate electrical power from waste heat. For continuous electrical power generation heat exchangers are integrated into the “cold side” and the “hot side” of the TEG such that a temperature difference across the TEG can be established and maintained. This thesis will focus on characterizing two different flat-plate cold side heat exchanger prototypes specifically designed for dissipating the thermal loads from TEG modules. The majority of TEGs modules available have a flat geometry design and a square shape with typical dimension of 40 mm × 40 mm or 56 mm × 56 mm. To maximize the net electrical power generated by the TEGs the cold side heat exchanger is required to have uniform surface temperature distribution, and excellent heat transfer performance with minimum pressure drop. To achieve the previously mentioned requirements, two flat-plate heat exchanger prototypes having two distinct heat transfer techniques were investigated. Each heat exchanger is designed to accommodate an array of 14 TEG modules arranged in two parallel rows with 7 TEGs per row a typical arrangement for large waste energy harvesting applications. The first heat exchanger prototype utilized single-phase forced convection through 140 minichannels (1 mm × 1 mm × 90 mm long) as a heat transfer technique. The second prototype utilized 14 liquid jets, 3 mm in diameter and 40.3 mm apart, impinging on a flat surface located 5 mm above. Each impinging jets was positioned at the centre of the TEG cooling area. An experimental facility was constructed in order to test the minichannels heat exchanger and the impinging jets thermally and hydrodynamically. The heat transfer, pressure drop and temperature distribution results were compared to determine the most appropriate cold side heat exchanger prototype for the TEG POWER system. The TEG POWER system is a waste heat recovery system designed to recoup waste heat from the exhaust gases of commercial pizza ovens. The TEG POWER system is capable of harvesting waste thermal energy produced by an establishment and utilize it for electrical power generation and thermal storage purposes. Heat transfer results indicated that for a given mass flow rate the minichannels heat exchanger has better heat transfer performance compared to the impinging jets heat exchanger. The minichannels heat exchanger design had a thermal conductance of 238 W/C at 0.19 kg/s coolant flow rate compared to 111 W/C for the impinging jets heat exchanger. The total pressure drop and the minor losses for each heat exchanger prototype were measured experimentally. For the minichannels heat exchanger, the total pressure drop is 23.3 kPa at flow rate of 0.235 kg/s. Comparatively, the total pressure drop for the impinging jets heat exchanger was 27.4 kPa at the same flow rate. Fittings losses for the minichannels heat and impinging jets heat exchanger were found to be 50% and 80% respectively. The maximum total measured drop corresponded to pumping power requirements of 5.7 W and 6.8 W for the minichannels and impinging jets heat exchanger respectively. Local and average temperature distributions and their influence on the electrical power generated were studied for both heat exchanger prototypes. It was found that the minichannels heat exchanger offers more uniform surface temperature distribution per row of TEGs compared to the impinging jets heat exchanger. Therefore the minichannels heat exchanger is well suited for cooling two rows of TEGs simultaneously. Based on the thermal and hydrodynamics comparison results the minichannels heat exchanger prototype is recommended for implementation in the TEG POWER system. / Thesis / Master of Applied Science (MASc)

Air cooled heat exchangers are used in many technology applications. Compactness, functionality and prize is decision maker for many future customers. Current development of compact heat exchangers is evident. Mostly used technology - expanded copper tube on aluminium fin is fluently changed by all aluminium heat exchangers with sophisticated heat exchange surface. Heating, Ventilating and Air-Conditioning with Refrigeration industry (HVAC-R) uses more and more extrude aluminium profiles as the main structure for all size heat exchangers. Similarly to Automotive industry HVAC-R uses minichannels as the basis of their products. It is not a new technology, it is technology proved through many years use in car industry. It is also true that current well known minichannel technology is not the last step. New designs and production lines are currently developed as well. All these new technologies have one unsolved thing together – how will be the new technologies easily computed through models. Current correlations which were used till now are obsolete, because of system miniaturization mainly. It is not easy to adapt old macro channel correlations on minichannel or microchannels where is not possible to diminish capillarity effects, different flow structures, etc. All these difficulties causes in model changes, software changes or iterative procedures upgrades. It is also evident that a lot of experimental exercises have to be done to examine heat exchanger behaviour ad characteristics. Because of precise computation of minichannel heat exchangers need or its geometry optimization is dissertation theme aimed to compare computation methodologies. PhD thesis is also aimed to check functionality of proposed computation system used simple similarity model. All proposed methods are compared to experimentally measured data and as the outcome of thesis are these data used to develop new computation basis.

Mini-/microchannel components have received attention over the past few decades owing to their compactness and superior thermal performance. Microchannel heat sinks are typically manufactured through traditional manufacturing practices (milling and sawing, electrodischarge machining, and water jet cutting) by changing their components to work in microscale environments or microfabrication techniques (etching and lost wax molding), which have emerged from the semiconductor industry. An extrusion process is used to produce multiport minichannel-based heat exchangers (HXs). However, geometric manufacturing limitations can be considered as drawbacks for all of these techniques. For example, a complex out-of-plane geometry is extremely difficult to fabricate, if not impossible. Such imposed design constraints can be eliminated using additive manufacturing (AM), generally known as three-dimensional (3D) printing. AM is a new and growing technique that has received attention in recent years. The inherent design freedom that it provides to the designer can result in sophisticated geometries that are impossible to produce by traditional technologies and all for the redesign and optimization of existing models. The work presented in this thesis aims to investigate the thermal performance of enhanced minichannel HXs manufactured via metal 3D printing both numerically and experimentally. Rectangular winglet vortex generators (VGs) have been chosen as the thermal enhancement method embedded inside the flat tube. COMSOL Multiphysics, a commercial software package using a finite element method (FEM), has been used as a numerical tool. The influence of the geometric VG parameters on the heat transfer and flow friction characteristics was studied by solving a 3D conjugate heat transfer and laminar flow. The ranges of studied parameters utilized in simulation section were obtained from our previous interaction with various AM technologies including direct metal laser sintering (DMLS) and electron-beam melting (EBM). For the simulation setup, distilled water was chosen as the working fluid with temperaturedependent thermal properties. The minichannel HX was assumed to be made of AlSi10Mg with a hydraulic diameter of 2.86 mm. The minichannel was heated by a constant heat flux of 5 Wcm−2 , and the Reynolds number was varied from 230 to 950. A sensitivity analysis showed that the angle of attack, VG height, VG length, and longitudinal pitch have notable effects on the heat transfer and flow friction characteristics. In contrast, the VG thickness and the distance from the sidewalls do not have a significant influence on the HX performance over the studied range. On the basis of the simulation results, four different prototypes including a smooth channel as a reference were manufactured with AlSi10Mg via DMLS technology owing to the better surface roughness and greater design uniformity. A test rig was developed to test the prototypes. Owing to the experimental facility and working fluid (distilled water), the experiment was categorized as either a simultaneously developing flow or a hydrodynamically developed but thermally developing flow. The Reynolds number ranged from 175 to 1370, and the HX was tested with two different heat fluxes of 1.5 kWm−2 and 3 kWm−2 . The experimental results for the smooth channel were compared to widely accepted correlations in the literature. It was found that 79% of the experimental data were within a range of ±10% of the values from existing correlations developed for the thermal entry length. However, a formula developed for the simultaneously developing flow overpredicted the Nusselt number. Furthermore, the results for the enhanced channels showed that embedding VGs can considerably boost the thermal performance up to three times within the parameters of the printed parts. Finally, the thermal performance of the 3D-printed channel showed that AM is a promising solution for the development of minichannel HXs. The generation of 3D vortices caused by the presence of VGs ii can notably boost the thermal performance, thereby reducing the HX size for a given heat duty.

[EN] In the first part of the current thesis, two fundamental numerical models (Fin2D-W and Fin1D-MB) for analyzing the air-side performance of minichannel evaporators were developed and verified. The Fin2D-W model applies a comprehensive two-dimensional scheme to discretize the evaporator. On the other hand, the Fin1D-MB model is based on the one-dimensional fin theory in conjunction with the moving boundaries technique along the fin height. The first objective of the two presented models is to identify and quantify the most influential phenomena encountered in the process of cooling and dehumidification. The second objective is to study the impact of the classical modeling assumptions on the air-side performance of minichannel evaporators. Different comparative studies between the traditional Effectiveness-NTU approach and the proposed numerical models were implemented to achieve the mentioned goals. The results revealed that the modeling assumptions which have the most significant impacts on the heat and mass transfer rates are: the uniform air properties along the fin height, adiabatic-fin-tip at half the height, and negligence of partial dehumidification scenarios. These widely used assumptions resulted in substantial deviations in total heat transfer rate, up to 52%, between the Effectiveness-NTU approach and Fin2D-W model. In the second part of the thesis, the Fin1D-MB model was integrated into the IMST-ART® simulation tool to evaluate the global performance of minichannel evaporators (air- and refrigerant-side). The Fin1D-MB model was selected because of its simplicity, calculation speed, and reasonable solution accuracy relative to the Fin2D-W model. The validation of the complete Fin1D-MB model was conducted against many experimental data and numerical models available in the literature. The validation process was achieved for different heat exchanger geometries, refrigerants, and operating conditions. The results showed that for the R134a minichannel evaporators studied, the Fin1D-MB model successfully predicted the Inlet refrigerant and outlet air temperatures, cooling capacity, and refrigerant-side pressure drop within error bands of ±0.5 ºC, ±5%, and ±20%, respectively. For the CO2 (R744) minichannel evaporator studied, the presented model estimated the cooling capacity and outlet air temperature within error bands of ±10% and ±1.0 ºC, respectively. Regarding the CO2 pressure drop, the Fin1D-MB model generally underpredicted the pressure drop values compared to the experimental data, with a maximum deviation of 11 kPa. / [ES] En la primera parte de la tesis actual, dos modelos numéricos fundamentales (Fin2D-W y Fin1D-MB) para analizar el lado del aire de los evaporadores de minicanales se han desarrollado y verificado. El modelo Fin2D-W aplica un esquema detallado de dos dimensiones para discretizar el evaporador mientras que el modelo Fin1D-MB se basa en la teoría de la aleta unidimensional junto con la técnica de fronteras móviles para el lado del aire. El primer objetivo de los dos modelos presentados es identificar y cuantificar los fenómenos más influyentes encontrados en el proceso de enfriamiento y deshumidificación. El segundo objetivo es estudiar el impacto de las hipótesis comúnmente usadas en el modelado de la transmisión de calor del aire de los evaporadores de minicanales. Se implementaron diferentes estudios comparativos entre el enfoque tradicional Effectiveness-NTU y los modelos numéricos propuestos para alcanzar los objetivos mencionados. Los resultados muestran que las hipótesis que provocan una mayor desviación con respecto a la solución detallada en la transferencia de calor y masa son: propiedades de aire uniforme a lo largo de la altura de la aleta, extremo adiabático de aleta a mitad de su longitud, y no contemplar el supuesto de deshumidificación parcial en la aleta. Estas hipótesis ampliamente utilizadas han resultado en errores importantes en la transferencia de calor total, hasta un 52%, entre el enfoque Effectiveness-NTU y el modelo Fin2D-W. En la segunda parte de la tesis, el modelo Fin1D-MB se integró en la herramienta de simulación IMST-ART® para evaluar el rendimiento global de los evaporadores de minicanales (en el lado del aire y del refrigerante). El modelo Fin1D-MB se seleccionó gracias a su simplicidad, velocidad de cálculo, y solución de una precisión razonable relativa al modelo Fin2D-W. Se realizó una validación del modelo completo Fin1D-MB con la ayuda de datos experimentales y modelos numéricos ya disponibles en la literatura. El modelo se ha validado para diferentes geometrías de intercambiadores de calor, refrigerantes y condiciones de funcionamiento. Los resultados han mostrado que para los evaporadores de minicanales funcionando con el refrigerante R134a, el modelo Fin1D-MB predice de manera correcta las temperaturas de entrada del refrigerante y de salida del aire, la capacidad de enfriamiento, y la caída de presión del lado de refrigerante dentro de las bandas de error de ±0.5 ºC, ±5%, y ±20%, respectivamente. Para el evaporador de minicanales con CO2 (R744) estudiado, el modelo estima la capacidad de refrigeración y la temperatura de salida del aire dentro de las bandas de error de ±10% y ±1.0 ºC, respectivamente. En cuanto a la caída de presión de CO2, el modelo Fin1D-MB generalmente predice a la baja los valores de la caída de presión en comparación con los datos experimentales, con una desviación máxima de 11 kPa. / [CA] A la primera part de la tesi actual, dos models numèrics fonamentals (Fin2D-W i Fin1D-MB) per analitzar el costat de l'aire dels evaporadors de minicanals s'han desenvolupat i verificat. Al model Fin2D-W s'aplica un esquema detallat de dues dimensions per discretitzar l'evaporador mentre que al model Fin1D-MB es basa en la teoria d'aleta unidimensional juntament amb la tècnica de frontera mòbil per al costat de l'aire. El primer objectiu dels dos models presentats és identificar i quantificar els fenòmens més influents trobats en el procés de refredament i deshumidificació. El segon objectiu és estudiar l'impacte de les hipòtesis comunament utilitzades en el modelatge de la transmissió de calor de l'aire dels evaporadors de minicanals. Es van implementar diferents estudis comparatius entre l'enfocament tradicional Effectiveness-NTU i els models numèrics proposats per assolir els objectius esmentats. Els resultats mostren que les hipòtesis que provoquen una major desviació respecte a la solució detallada a la transferència de calor i massa són: propietats d'aire uniforme al llarg de l'altura de l'aleta, extrem adiabàtic d'aleta a la meitat de la seua longitud, i no contemplar el supòsit de deshumidificació parcial en l'aleta. Aquestes hipòtesis àmpliament utilitzades donen errors importants en la transferència de calor total, fins a un 52%, entre l'enfocament Effectiveness-NTU i el model Fin2D-W. A la segona part de la tesi, el model Fin1D-MB es va integrar en l'eina de simulació IMST-ART® per avaluar el rendiment global dels evaporadors de minicanals (al costat de l'aire i del refrigerant). El model Fin1D-MB es va seleccionar gràcies a la seva simplicitat, velocitat de càlcul, i solució d'una precisió raonable relativa al model Fin2D-W. Es va realitzar una validació del model complet Fin1D-MB amb l'ajuda de dades experimentals i models numèrics ja disponibles a la literatura. El model s'ha validat per a diferents geometries d'intercanviadors de calor, refrigerants i condicions de funcionament. Els resultats mostren que per als evaporadors de minicanals funcionant amb el refrigerant R134a, el model Fin1D-MB prediu de manera correcta les temperatures d'entrada del refrigerant i de sortida de l'aire, la capacitat de refreda-ment, i la caiguda de pressió del costat de refrigerant dins de les bandes d'error de ±0.5 ºC, ±5%, i ±20%, respectivament. Per a l'evaporador de minicanals amb CO2 (R744) estudiat, el model estima la capacitat de refrigeració i la temperatura de sortida de l'aire dins de les bandes d'error de ±10% i ±1.0 ºC, respectivament. Pel que fa a la caiguda de pressió de CO2, el model Fin1D-MB generalment prediu a la baixa els valors de la caiguda de pressió en comparació amb les dades experimentals, amb una desviació màxima d'11 kPa. / Hassan, AHA. (2016). Development and Validation of a Minichannel Evaporator Model under Dehumidification [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/71357

This PhD work has been dedicated to the improvement of the modelling of air condensers of both round tube and fins (RTPFs) and Minichannel technologies. The calculation platform employed is IMST-ART. This is a dedicated software for the design of refrigeration, air-conditioning and heat pump equipment following the vapor compression cycle. The model implemented in IMST-ART for condensers and evaporators is the combination of a segment-by-segment approach with the numerical method SEWTLE (Semi Explicit method for Wall Temperature Linked Equations) for the solution of the resulting system of equations. The target of the first part of this thesis was the comparative analysis of the empirical correlations aimed at the evaluation of the heat transfer coefficients and pressure drop in both the air and refrigerant sides of a condenser. The Literature review pointed out the presence of many studies concerning the condensation modelling. Therefore, after selecting the most interesting to compare, the first objective of this first part of the PhD became the identification of a suitable methodology for defining the best combination of correlations for the estimation of the thermo-hydraulic performance of the condensers. After an in-depth analysis of different possibilities, a well-defined methodology was identified as the best for the purpose. In the thesis, it was successfully applied to the identification of the best set of correlations for the heat transfer coefficients and friction factors for both the round-tube and minichannel condensers. The second part of the PhD was targeted to the improvement of the condensation modelling. In particular, the attention was focused on the analysis of the phenomena taking place at the beginning of the condensation process, when the superheated vapor finds the wall of the condenser being at a temperature lower than the refrigerant saturation temperature, i.e. convective condensation superheated vapor zone (CSH zone). It is well known that, in this zone, the condensation starts with some kind of droplet/thin film condensation on the walls. Afterwards, the bulk of the refrigerant flow reaches the saturation temperature and the condensation occurs at saturated conditions. Hence, the PhD thesis has been dedicated to the implementation in the general model for condensers (in IMST-ART software) of this CSH zone, which it was found to have an important effect on the prediction of the wall temperatures distribution in the tested air condensers. Two different numerical solutions were implemented and compared, i.e. Temperature and Enthalpy approaches, and validated against experimental results. Prediction results are very similar, thus the Enthalpy approach was selected because it required lower computational time. The final part of thesis was oriented towards the study of the effect of airflow maldistribution on the performance of air condensers. An innovative experimental methodology for generating and measuring any uneven air velocity profile at the inlet of a heat exchanger was first developed in a dedicated wind tunnel and then applied for the analysis of the performance degradation of one sample of condenser of each RTPFs and Minichannel technologies. Three different velocity profiles were produced and tested along a wide set of operating conditions, including different refrigerant charges and hence subcoolings. The experimental results showed that, although differences in wall temperature distribution were significant, the effect of air maldistribution on the performance of the two tested condensers was small. The improved model was validated against the experimental results and also showed little effect on condenser performance. Finally, the agreement between the results of the simulation and the experimental results was very satisfactory. / Este trabajo de doctorado se ha dedicado a la mejora del modelado de condensadores de aire, con tecnología de tubos y aletas o minicanales. La plataforma de software empleada es IMST-ART, que es un software dedicado a asistir el diseño de equipos de refrigeración, aire acondicionado y bomba de calor, basados en el ciclo de compresión de vapor. El modelo de IMST-ART para condensadores y evaporadores se basa en una aproximación segmento a segmento combinada con el método numérico SEWTLE (Semi Explicit method for Wall Temperature Linked Equations) para la solución del sistema de ecuaciones resultante. El objetivo de la primera parte de esta tesis fue el análisis comparativo de las correlaciones empíricas destinadas a evaluar los coeficientes de transferencia de calor y la caída de presión para condensadores de aire, tanto para el lado del aire como para el del refrigerante. La revisión de la Literatura mostró la existencia de numerosos estudios sobre el modelado de la condensación en este tipo de intercambiadores. Por lo tanto, después de la selección de las correlaciones más interesantes a comparar, el primer objetivo de esta primera parte de la tesis resultó el encontrar la metodología más adecuada para la identificación de cuáles eran las correlaciones que mejor estimaban el comportamiento termo-hidráulico de los condensadores. Después de un análisis en profundidad de diferentes posibilidades, se encontró la metodología claramente más adecuada y se pasó a aplicarla a la identificación del mejor conjunto de correlaciones para los coeficientes de transferencia de calor y factores de fricción para condensadores de aire. La segunda parte del doctorado se dirigió a la mejora del modelado del comienzo del proceso de condensación cuando el vapor sobrecalentado encuentra la pared del condensador a una temperatura que está por debajo de la temperatura de saturación del refrigerante en lo que se puede denominar como condensación convectiva en la zona de vapor sobrecalentado (zona CSH). Es bien sabido que la condensación comienza en esta zona con algún tipo de condensación de gotas/película delgada sobre las paredes antes de que el núcleo del flujo de refrigerante alcance la temperatura de saturación y la condensación se produzca en condiciones saturadas. La segunda parte del doctorado se ha dedicado a la implementación en el modelo general de condensadores (en el software IMST-ART) de esta zona CSH, que se encontró que tenía un efecto importante en la predicción de la distribución de las temperaturas de la pared en los condensadores de aire ensayados. Se implementaron y compararon dos soluciones numéricas diferentes, denominados aproximación de temperatura y aproximación de entalpía respectivamente, y se validaron por comparación con resultados experimentales. La predicción resultó ser muy similar con ambas aproximaciones por lo que finalmente se seleccionó la aproximación de entalpía por ser considerablemente más rápida. La parte final de la tesis se orientó hacia el estudio del efecto de la mala distribución del flujo de aire en el rendimiento de los condensadores de aire. Para este fin se desarrolló una metodología experimental innovadora capaz de generar y medir cualquier perfil de velocidad de aire no uniforme a la entrada de un intercambiador de calor. El desarrollo se llevó a cabo primero en un túnel de viento específicamente dedicado a este propósito y luego se aplicó para el análisis de la degradación de las prestaciones de dos muestras de condensador de cada una de las tecnologías estudiades: RTPFs y Minicanal. Mediante la metodología desarrollado se generaron tres perfiles de velocidad diferentes que se ensayaron a lo largo de un amplio conjunto de condiciones de funcionamiento, incluyendo diferentes cargas de refrigerante y, por tanto, grados de subenfriamiento en el refrigerante. Los resultados experimentales mostraron que el efecto de la mala distrib / Aquest treball de doctorat s'ha dedicat a la millora de la modelització de condensadors d'aire, amb tecnologia de tubs i aletes o minicanals. La plataforma de software emprada és IMST-ART, que és un software dedicat a assistir el disseny d'equips de refrigeració, aire condicionat i bomba de calor, basats en el cicle de compressió de vapor. El model de IMST-ART per condensadors i evaporadors es basa en una aproximació segment a segment combinada amb el mètode numèric SEWTLE (Semi Explicit method for Wall Temperature Linked Equations) per a la solució del sistema d'equacions resultant. L'objectiu de la primera part d'aquesta tesi va ser l'anàlisi comparativa de les correlacions empíriques destinades a avaluar els coeficients de transferència de calor i la caiguda de pressió per condensadors d'aire, tant per al costat de l'aire com per al del refrigerant. La revisió de la Literatura va mostrar l'existència de nombrosos estudis sobre la modelització de la condensació en aquest tipus d'intercanviadors. Per tant, després de la selecció de les correlacions més interessants a comparar, el primer objectiu d'aquesta primera part de la tesi va resultar el trobar la metodologia més adequada per a la identificació de quines eren les correlacions que millor estimaven el comportament termo-hidràulic dels condensadors. Després d'una anàlisi en profunditat de diferents possibilitats, es va trobar la metodologia clarament més adequada i es va passar a aplicar-la a la identificació del millor conjunt de correlacions per als coeficients de transferència de calor i factors de fricció per condensadors d'aire. La segona part del doctorat es va dirigir a la millora de la modelització del començament del procés de condensació quan el vapor sobreescalfat troba la paret del condensador a una temperatura que està per sota de la temperatura de saturació del refrigerant, en el que es pot denominar com condensació convectiva a la zona de vapor sobreescalfat (zona CSH). És ben sabut que la condensació comença en aquesta zona amb algun tipus de condensació de gotes/pel·lícula sobre les parets abans que el nucli del flux de refrigerant arribi a la temperatura de saturació i la condensació es produeixi en condicions saturades. La segona part del doctorat s'ha dedicat a la implementació en el model general de condensadors (en el programari IMST-ART) d'aquesta zona CSH, que es va trobar que tenia un efecte important en la predicció de la distribució de les temperatures de la paret en els condensadors d'aire assajats. Es van implementar i van comparar dues solucions numèriques diferents, denominades aproximació de temperatura i aproximació d'entalpia respectivament, i es van validar per comparació amb resultats experimentals. La predicció va resultar ser molt semblant amb les dues aproximacions pel que finalment es va seleccionar l'aproximació d'entalpia per ser considerablement més ràpida. La part final de la tesi es va orientar cap a l'estudi de l'efecte de la mala distribució del flux d'aire en el rendiment dels condensadors d'aire. Amb aquesta finalitat es va desenvolupar una metodologia experimental innovadora capaç de generar i mesurar qualsevol perfil de velocitat d'aire no uniforme a l'entrada d'un intercanviador de calor. El desenvolupament es va dur a terme primer en un túnel de vent específicament dedicat a aquest propòsit i després es va aplicar per a l'anàlisi de la degradació de les prestacions de dues mostres de condensador de cadascuna de les tecnologies estudiades: RTPFs i Minicanal. Mitjançant la metodologia desenvolupada es van generar tres perfils de velocitat diferents que es van assajar al llarg d'un ampli conjunt de condicions de funcionament, incloent càrregues diferents de refrigerant i, per tant, graus de subrefredament en el refrigerant. Els resultats experimentals van mostrar que l'efecte de la mala distribució de l'aire en les prestacions dels dos condensadors provats va / Pisano, A. (2017). Analysis of the condensation process and air maldistribution in finned tube and minichannel condensers [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/86182

Submitted by Maicon Juliano Schmidt (maicons) on 2015-05-25T14:45:18Z No. of bitstreams: 1 Bruno de Sá Beckerle.pdf: 3991624 bytes, checksum: 500b993c0f3a1c3a8c3abae05ed51c5e (MD5) / Made available in DSpace on 2015-05-25T14:45:18Z (GMT). No. of bitstreams: 1 Bruno de Sá Beckerle.pdf: 3991624 bytes, checksum: 500b993c0f3a1c3a8c3abae05ed51c5e (MD5) Previous issue date: 2015-01-05 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Este trabalho apresenta um estudo experimental da transferência de calor e queda de pressão na ebulição do isobutano, R-600a, em um tubo composto por 7 mini canais paralelos, cujo diâmetro hidráulico é de 1,47 mm. Os testes em ebulição foram realizados com uma temperatura de saturação de 22 ºC e pressão de saturação de 302 kPa, com velocidade mássica entre 50 e 200 kg/(m²s) e fluxos de calor na seção de testes entre 7 e 40 kW/m². Com os testes realizados verificou-se que o coeficiente de transferência de calor aumenta conforme o incremento do fluxo de calor e velocidade mássica, sendo que esta última tem maior influência para baixos títulos de vapor. O coeficiente de transferência de calor atingiu valores máximos próximos a 3.200 W/(m²K) para a condição de maior vazão e fluxo de calor. A queda de pressão aumentou com o incremento da velocidade mássica e título de vapor em todos os testes, enquanto que o fluxo de calor apresentou influência na queda de pressão apenas nas maiores velocidades mássicas. A perda de pressão por atrito representou até 93,7% da perda total. Também foram analisados os padrões de escoamento, sendo observados os padrões de bolhas isoladas, bolhas alongadas, intermitente e anular, sendo que o padrão de bolhas isoladas foi observado para baixos fluxos e títulos de vapor, e o padrão anular mostrou-se presente para títulos superiores a 0,13. / This work presents an experimental study of heat transfer and drop pressure in flow boiling of the isobutane, R-600a, in a 7 mini channel of 1,47 mm hydraulic diameter. The tests were performed a boiling with a saturation temperature of 22 °C and saturation pressure of 302 kPa, with a mass velocity between 50 and 200 kg/(m²s) and heat fluxes in the test section between 7 and 40 kW/m². In the tests, it was found that the heat transfer coefficient increases with increasing heat flux and mass velocity has more influence at low quality. The heat transfer coefficient achieved values around 3.200 W/(m²K) for the condition of greater flow and heat flux. The drop pressure was increase by increasing the mass velocity and quality in all tests, while the heat flow have any influence on the drop pressure to the greater mass velocity. The friction drop pressure represented to 93.7% of the total loss. As well analyzed the flow patterns, and observing the patterns of isolated bubbles, plugs/slugs, intermittent and annular, and that the pattern of isolated bubbles were observed for low quality and heat flux and the annular pattern was present for quality from 0.13.

Recentemente, observa-se o crescimento do número de trocadores de calor baseados em microcanais devido a necessidade de transferência de elevadas taxas de calor utilizando dispositivos compactos. Tubos de calor, trocadores de calor compactos para equipamentos eletrônicos e controle térmico de satélites, sistemas de condicionamento de ar para automóveis, escritórios e residências são exemplos de aplicações para condensação em canais de diâmetro reduzido. No entanto, na literatura encontra-se reduzido número de estudos experimentais tratando da condensação no interior de canais com diâmetros inferiores a 3 mm, os quais geralmente envolvem refrigerantes com elevado potencial de aquecimento global (GWP). Neste contexto, o presente estudo apresenta uma revisão crítica da literatura envolvendo critérios de transição entre padrões de escoamento, perda de pressão por atrito e coeficiente de transferência de calor durante a condensação no interior de canais convencionais e de micro-escala (minicanais). Levantou-se resultados para o gradiente de pressão por atrito e coeficiente de transferência de calor em aparato experimental localizado na Universidade de Pádua (Università Degli Studi di Padova) para os fluidos refrigerante R1234ze(E) e R32 (GWP de 550 e 6, respectivamente), temperatura de saturação de 40°C, fluxo de calor até 35 kW/m², grau de sub-resfriamento da parede entre 2 e 10 K, velocidade mássicas entre 55 e 275 kg/m²s e título de vapor de 0 a 1. Os dados foram levantados em seção de teste composta por 36 minicanais com diâmetro hidráulico de 1,6 mm e geometria retangular, com o efeito de resfriamento obtido através de água resfriada escoando em contra-corrente ao refrigerante. Os dados experimentais levantados para o gradiente de pressão por atrito e o coeficiente de transferência de calor foram comparados com métodos de previsão da literatura, concluindo que as correlações propostas por Jige, Inoue e Koyama (2016) fornecem as melhores previsões. O comportamento do coeficiente de transferência de calor foi analisado com foco nos mecanismos físicos predominantes durante a condensação. A partir desta análise concluiu-se o predomínio de efeitos de tensão superficial em velocidades mássicas reduzidas e de arrasto em velocidades mássicas elevadas. Este estudo também apresenta uma avaliação comparativa do desempenho dos refrigerantes R1234ze(E) e R32 em relação ao R134a (GWP de 1300) baseada na taxa de transferência de calor por unidade de potência de bombeamento e no potencial de transferência de calor, conforme o critério proposto por Cavallini et al. (2010). Esta análise revelou o desempenho superior para o refrigerante R32 seguido do R134a, com o R1234ze(E) apresentando o pior resultado, independentemente da velocidade mássica. / Recently, micro-scale channels are increasingly being used to combine high heat transfer rates and high degree of compactness. Condensation inside small diameter channels can be found in several applications such as heat pipes, thermal management of electronic equipments, spacecraft thermal control, automotive and residential air conditioning systems, heat pumps and refrigeration systems. However, despite of its importance, few studies concerning condensation inside minichannels (DH < 3 mm) involving low GWP (Global Warming Potential) refrigerants are found in the literature. In this context, initially, this study presents a critical review on the literature involving transition criteria on two-phase flow patterns for micro- and macro-scale conditions, frictional pressure drop and heat transfer coefficient during condensation inside channels. Experimental results for frictional pressure gradient and heat transfer coefficient obtained in apparatus located at the University of Padua (Università Degli Studi di Padova) are carefully analysed. The database includes results for the refrigerants R1234ze(E) and R32 (GWP of 550 and 6, respectively), saturation temperature of 40°C, heat flux up to 35 kW/m², fluid and wall temperature diference up to 10 K, mass velocity in the range of 55 to 275 kg/m²s and vapor quality between 0 and 1. The test section is composed of 36 rectangular minichannels with hydraulic diameter of 1.6 mm. The refrigerant is cooled by water flowing. From a comparison of experimental data for frictional pressure drop and heat transfer coefficient, and prediction methods available in literature, the methods proposed by Jige, Inoue e Koyama (2016) were ranked as the best ones. During the data analyses, focus was put on in order to relate the heat transfer coefficient behavior with the prevailing mecanisms during condensation. Based on this carefull analysis, the predominance of surface tension effects was pointed out under conditions of low mass velocities and condensation inside rectangular minichannels. On the other hand, for high mass velocities shear stress effects prevailed. Also, it has been presented a comparative evaluation of the performance of the refrigerants R1234ze(E), R32 and R134a (GWP of 1300) based on the following criteria: (i) heat transfer rate per unit of power pumping; and (ii) a penalty factor based on the heat transfer potential proposed by Cavallini et al. (2010). According to this evaluation, independently of the mass velocity, the refrigerant R32 was ranked as the one presenting the best performance, followed by R134a ranked as the second best. The refrigerant R1234ze(E) provided the worst performance among them all.

For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was not well studied, and in the scientific literature there were large discrepancies between results reported by different investigators. The present thesis aims to add to the knowledge of the fundamentals of single- and two-phase flow heat transfer and pressure drop in narrow channels, thereby aiding in the development of this new, interesting technology with the possibility of decreasing the size of electronics through better cooling, and of increasing the energy efficiency of thermal processes and thermodynamic cycles through enhanced heat transfer. A comprehensive experimental single-phase flow and saturated flow boiling heat transfer and pressure drop study has been carried out on vertical stainless steel tubes with inner diameters of 1.700, 1.224 and 0.826 mm, using R-134a as the test fluid. The heat transfer and pressure drop results were compared both to conventional correlations developed for larger diameter channels and to correlations developed specifically for microscale geometries. Contrary to many previous investigations, this study has shown that the test data agree well with single-phase heat transfer and friction factor correlations known to be accurate for larger channels, thus expanding their ranges to cover mini/microchannel geometries. The main part of the study concerns saturated flow boiling heat transfer and pressure drop. Tests with the same stainless steel tubes showed that the heat transfer is strongly dependent on heat flux, but only weakly dependent on mass flux and vapor fraction (up to the location of dryout). This behavior is usually taken to indicate a dominant influence of nucleate boiling, and indicates that the boiling mechanism is strongly related to that in nucleate boiling. The test data for boiling heat transfer was compared to several correlations from the literature, both for macro- and mini-channels. A new correlation for saturated flow boiling heat transfer of refrigerant R-134a correlation was obtained based on the present experimental data. This correlation predicts the presented data with a mean absolute deviation of 8%. The frictional pressure drop results were compared to both macro- and mini channel correlations available from the literature. The correlation suggested by Qu and Mudawar (2003) gave the best prediction to the frictional two-phase pressure drop within the studied ranges. A unique visualization study of saturated flow boiling characteristics in a vertical 1.332 mm inner diameter quartz tube, coated with a transparent heater has also been conducted. The complete evaporation process in a heated circular mini-channel has been studied visually in detail using high speed CCD camera. The study revealed the developments of the flow patterns and the behavior from bubble nucleation to the dry out of the liquid film. The bubble departure frequency, diameter, growth rate, and velocity were determined by analyzing the images. Finally, a flow pattern map for boiling flow in microchannels has been developed based on the test data. / QC 20100812

Recentemente, observa-se o crescimento do número de trocadores de calor baseados em microcanais devido a necessidade de transferência de elevadas taxas de calor utilizando dispositivos compactos. Tubos de calor, trocadores de calor compactos para equipamentos eletrônicos e controle térmico de satélites, sistemas de condicionamento de ar para automóveis, escritórios e residências são exemplos de aplicações para condensação em canais de diâmetro reduzido. No entanto, na literatura encontra-se reduzido número de estudos experimentais tratando da condensação no interior de canais com diâmetros inferiores a 3 mm, os quais geralmente envolvem refrigerantes com elevado potencial de aquecimento global (GWP). Neste contexto, o presente estudo apresenta uma revisão crítica da literatura envolvendo critérios de transição entre padrões de escoamento, perda de pressão por atrito e coeficiente de transferência de calor durante a condensação no interior de canais convencionais e de micro-escala (minicanais). Levantou-se resultados para o gradiente de pressão por atrito e coeficiente de transferência de calor em aparato experimental localizado na Universidade de Pádua (Università Degli Studi di Padova) para os fluidos refrigerante R1234ze(E) e R32 (GWP de 550 e 6, respectivamente), temperatura de saturação de 40°C, fluxo de calor até 35 kW/m², grau de sub-resfriamento da parede entre 2 e 10 K, velocidade mássicas entre 55 e 275 kg/m²s e título de vapor de 0 a 1. Os dados foram levantados em seção de teste composta por 36 minicanais com diâmetro hidráulico de 1,6 mm e geometria retangular, com o efeito de resfriamento obtido através de água resfriada escoando em contra-corrente ao refrigerante. Os dados experimentais levantados para o gradiente de pressão por atrito e o coeficiente de transferência de calor foram comparados com métodos de previsão da literatura, concluindo que as correlações propostas por Jige, Inoue e Koyama (2016) fornecem as melhores previsões. O comportamento do coeficiente de transferência de calor foi analisado com foco nos mecanismos físicos predominantes durante a condensação. A partir desta análise concluiu-se o predomínio de efeitos de tensão superficial em velocidades mássicas reduzidas e de arrasto em velocidades mássicas elevadas. Este estudo também apresenta uma avaliação comparativa do desempenho dos refrigerantes R1234ze(E) e R32 em relação ao R134a (GWP de 1300) baseada na taxa de transferência de calor por unidade de potência de bombeamento e no potencial de transferência de calor, conforme o critério proposto por Cavallini et al. (2010). Esta análise revelou o desempenho superior para o refrigerante R32 seguido do R134a, com o R1234ze(E) apresentando o pior resultado, independentemente da velocidade mássica. / Recently, micro-scale channels are increasingly being used to combine high heat transfer rates and high degree of compactness. Condensation inside small diameter channels can be found in several applications such as heat pipes, thermal management of electronic equipments, spacecraft thermal control, automotive and residential air conditioning systems, heat pumps and refrigeration systems. However, despite of its importance, few studies concerning condensation inside minichannels (DH < 3 mm) involving low GWP (Global Warming Potential) refrigerants are found in the literature. In this context, initially, this study presents a critical review on the literature involving transition criteria on two-phase flow patterns for micro- and macro-scale conditions, frictional pressure drop and heat transfer coefficient during condensation inside channels. Experimental results for frictional pressure gradient and heat transfer coefficient obtained in apparatus located at the University of Padua (Università Degli Studi di Padova) are carefully analysed. The database includes results for the refrigerants R1234ze(E) and R32 (GWP of 550 and 6, respectively), saturation temperature of 40°C, heat flux up to 35 kW/m², fluid and wall temperature diference up to 10 K, mass velocity in the range of 55 to 275 kg/m²s and vapor quality between 0 and 1. The test section is composed of 36 rectangular minichannels with hydraulic diameter of 1.6 mm. The refrigerant is cooled by water flowing. From a comparison of experimental data for frictional pressure drop and heat transfer coefficient, and prediction methods available in literature, the methods proposed by Jige, Inoue e Koyama (2016) were ranked as the best ones. During the data analyses, focus was put on in order to relate the heat transfer coefficient behavior with the prevailing mecanisms during condensation. Based on this carefull analysis, the predominance of surface tension effects was pointed out under conditions of low mass velocities and condensation inside rectangular minichannels. On the other hand, for high mass velocities shear stress effects prevailed. Also, it has been presented a comparative evaluation of the performance of the refrigerants R1234ze(E), R32 and R134a (GWP of 1300) based on the following criteria: (i) heat transfer rate per unit of power pumping; and (ii) a penalty factor based on the heat transfer potential proposed by Cavallini et al. (2010). According to this evaluation, independently of the mass velocity, the refrigerant R32 was ranked as the one presenting the best performance, followed by R134a ranked as the second best. The refrigerant R1234ze(E) provided the worst performance among them all.