In recent years carbon dioxide (CO2, R-744)has moved to the foreground as an environmentally friendly alternative to commonly used CFCs and HFCs, which are being phased out due to its high ozone depleting and global warming potentials. R-744 is not only environmentally friendly but due to its unique properties, it is also ideally suited for the use in heat pump water heaters. High cycle efficiencies are achievable even at high hot water temperatures. The high cycle efficiency not only leads to energy and cost savings but also ties in with the drive for implementation of energy saving measures in South Africa. It is therefore paramount to continue development and implementation of R-744 in heat pump water heaters. Optimizing the cycle efficiency is only possible if detailed component simulation models, taking these unique properties of R-744 into account, are available. The purpose of this study therefore was to develop a detail simulation model of a concentric tube-in-tube water-to-refrigerant evaporator, as well as a fin-and-tube air-to-refrigerant evaporator model. Data from the North-West University R-744 heat pump test bench were used to verify the tube-in-tube evaporator simulation model. The discrepancies in the cooling capacity between the simulation and test bench can be attributed to the presence of lubricant in the system.The fin-and-tube model was verified by testing it against the NIST program EVAP-COND (NIST 2010). Overall there was good agreement between the results of the two programs, with EVAP-COND predicting a lower cooling capacity(6% to 14%) and and a higher pressure refrigerant pressure drop (30% to 50%). It was found that both the heat transfer correlation of Jung et al. (1989) and the pressure drop correlation of Choi et al. (1999) are able to predict the experimental values accurately and are valid for use in both the evaporator models developed. To demonstrate the use of the detail evaporator fin-and-tube model, an evaluation of the different tube geometries, commercially available in South Africa, for use with R-744 fin-and-tube evaporators was done. For a fin-and-tube evaporator it was found that the most cost effective option is to use ⅜" (10.05 mm)copper tubes and the least effective is " (12.6 mm) stainless steel tubes. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

In recent years carbon dioxide (CO2, R-744)has moved to the foreground as an environmentally friendly alternative to commonly used CFCs and HFCs, which are being phased out due to its high ozone depleting and global warming potentials. R-744 is not only environmentally friendly but due to its unique properties, it is also ideally suited for the use in heat pump water heaters. High cycle efficiencies are achievable even at high hot water temperatures. The high cycle efficiency not only leads to energy and cost savings but also ties in with the drive for implementation of energy saving measures in South Africa. It is therefore paramount to continue development and implementation of R-744 in heat pump water heaters. Optimizing the cycle efficiency is only possible if detailed component simulation models, taking these unique properties of R-744 into account, are available. The purpose of this study therefore was to develop a detail simulation model of a concentric tube-in-tube water-to-refrigerant evaporator, as well as a fin-and-tube air-to-refrigerant evaporator model. Data from the North-West University R-744 heat pump test bench were used to verify the tube-in-tube evaporator simulation model. The discrepancies in the cooling capacity between the simulation and test bench can be attributed to the presence of lubricant in the system.The fin-and-tube model was verified by testing it against the NIST program EVAP-COND (NIST 2010). Overall there was good agreement between the results of the two programs, with EVAP-COND predicting a lower cooling capacity(6% to 14%) and and a higher pressure refrigerant pressure drop (30% to 50%). It was found that both the heat transfer correlation of Jung et al. (1989) and the pressure drop correlation of Choi et al. (1999) are able to predict the experimental values accurately and are valid for use in both the evaporator models developed. To demonstrate the use of the detail evaporator fin-and-tube model, an evaluation of the different tube geometries, commercially available in South Africa, for use with R-744 fin-and-tube evaporators was done. For a fin-and-tube evaporator it was found that the most cost effective option is to use ⅜" (10.05 mm)copper tubes and the least effective is " (12.6 mm) stainless steel tubes. / Thesis (MIng (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013.

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.

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.

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.

[pt] É desenvolvido um modelo de simulação de evaporadores automotivos tipo placa (Brazed Laminated Plate), para o resfriamento de ar úmido. O modelo estudado adota o método de análise local, onde o trocador de calor é dividido em um número de elementos de troca de calor, para os quais as equações de troca de calor e de conservação de energia são aplicadas. Para efeito da determinação do coeficiente local de troca de calor e do fator de atrito, o modelo considera duas regiões na troca de calor. Pelo lado do refrigerante foi considerada uma região bifásica (líquido - vapor) e outra, de superaquecimento (vapor). No lado do ar têm-se duas condições: com a superfície do evaporador seca ou molhada. Para cada zona foram levados em conta diferentes mecanismos de transferência de calor. Também foi estudada a queda de pressão do lado do refrigerante e do lado do ar, utilizando, para tal, as respectivas correlações de queda de pressão. Para a simulação foram consideradas definidas as condições de entrada dos fluídos e a geometria do evaporador. Um programa foi desenvolvido em FORTRAN para calcular os estados termodinâmicos de saída dos dois fluidos. As propriedades do refrigerante foram calculadas utilizando o software REFPROP versão 7.0, desenvolvido no NIST, EUA, o que permitiu a modelagem do trocador de calor operando com uma vasta gama de refrigerantes. Novos refrigerantes, H e 1234yf, ainda não constantes da biblioteca REFPROP v07, foram também testados. Os resultados da simulação foram comparados com dados experimentais disponíveis. / [en] A simulation model for automotive brazed laminate plate evaporators, for humid air cooling, was developed. The model adopts the local analysis method, where the heat exchanger is divided into a number of elemental control volumes, for which the equations of heat transfer and conservation of energy are applied. In order to determine the local heat transfer coefficient and friction factor, the model considers different regions in the heat exchanger. In the refrigerant side, two-phase or superheated flows were considered. In the air side, dry or wet surface conditions were modeled. For each zone different mechanisms of heat transfer and pressure drop were taken in account. The simulation model assumed prescribed evaporator geometry and inlet conditions for both fluids. A computer program was developed in FORTRAN to calculate overall thermodynamic outlet states of both fluids. Refrigerant properties were calculated using the software REFPROP version 7.0, developed by the NIST, U.S.A.. It allowed the modeling of the evaporator, operating with a vast range of refrigerants. New refrigerants, fluids H and 1234yf, (still not available in REFPROP v07 libraries) also were tested. The results of the simulation were compared with available experimental data.

Cubic boron nitride (cBN) synthesis has gained lot of interest during the past decade as it offers outstanding physical and chemical properties like high hardness, high wear resistance, and chemical inertness. Despite of their excellent properties, every application of cBN is hindered by high compressive stresses and poor adhesion. The cost of equipment is also high in almost all the techniques used so far. This thesis deals with the synthesis of cubic phase of boron nitride on Si (100) wafers using electron beam evaporator, a low cost equipment that is capable of depositing films with reduced stresses. Using this process, need of ion beam employed in ion beam assisted processes can be eliminated thus reducing the surface damage and enhancing the film adhesion. Four sets of samples have been deposited by varying substrate temperature and the deposition time. scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) techniques have been used to determine the structure and composition of the films deposited. X-ray diffraction (XRD) was performed on one of the samples to determine the thickness of the film deposited for the given deposition rate. Several samples showed dendrites being formed as a stage of film formation. It was found that deposition at substrate temperature of 400oC and for a period of one hour yielded high quality cubic boron nitride films.

[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. / [CAT] 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 no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/71357 / TESIS

The main goal of the master thesis is an evaluation of a software package Smap 3D Plant Design in design of evaporation technology, specifically 3D of the pipeline network and its subsequent documentation. The thesis describes evaporation technology in general, then the evaporator testing site in Laboratory of Energy Intensive Processes in NETME Centre is presented, where pipeline network needs to be designed. In the next part a reference 3D model was created as well as a documentation using SolidWorks routing. Subsequently, an equivalent 3D model was created using Smap 3D Plant Design – Piping and a documentation was created using Smap 3D Plant Design – Isometric. Finally, these two methods of creating pipeline were compared to each other, and recommended method was chosen.

The purpose of this Diploma Thesis was the construction design of the steam boiler for fytomass. For the specified parameters of biomass have been gradually implemented stoichiometric calculations of which are further based calculation of enthalpies of combustion gas. In the next part have been dealt with heat balance of the boiler, the efficiency of the boiler, recirculation of exhaust gases and the temperature of the combustion gases in outlet from fire. Thereinafter, the proposal of particular heat surfaces of steam boiler was solved, so that the resulting number and size correspond to the desired parameters. The parameters are temperature, pressure and the amount of steam.