Abstract Spray cooling is frequently encountered in a number of engineering applications. An experimental study was made to investigate the effect of liquid sprays used to cool a hot surface. Both pure water and R-134a were served as a working medium sprayed from a single circular nozzle onto a Pt plated surface of an electrically heated surface. Spray cooling tests were performed for steady state and transient experiment. Cooling characteristics curves were obtained over a range of Weber number(Water¡G80¡B148¡B231¡AR-134a¡G50¡B96¡B152),pressure drop of liquid(0.1Mpa¡B0.15Mpa¡B0.2Mpa),degree of subcooling (Water¡G55¢J¡B60¢J¡AR-134a¡G2¢J¡B4¢J) and initial temperature(Water¡G240¢J¡AR-134a¡G60¢J). Thermal design data of high performance as well as more and further physical insight of the above-stated spray cooling heat transfer can be acquired. The results will hopefully be helpful not only for the academia but for the industry.

An examination of various methods for mass flow rate measurements was undertaken to evaluate their applicability for measuring refrigerant R-134a two-phase mass flow in refrigeration and air-conditioning equipment. An experimental apparatus was constructed to generate the required two-phase flow conditions. A turbine and a venturi flowmeter were used together with either a capacitance transducer or a gamma densitometer to determine the two-phase mass flow rate. The time-averaged void fraction was measured using a capacitance transducer and a gamma densitometer. Their measurements were in good agreement. Hence, for mass flow rate measurements, the capacitance transducer was used as the void fraction meter because of its ease of operation. A number of models were used to combine the output of either the turbine flowmeter or the venturi flowmeter, with the void fraction measurement to estimate the mass flow rate. It was found that, within the range of experimental data tested in the present work, the venturi flowmeter, in conjunction with Chisholm's model, provided the best agreement with the experimental results. / Thesis / Master of Engineering (MEngr)

Apresenta um estudo teórico-experimental da ebulição convectiva do fluido refrigerante R-134a no interior de tubos lisos. Os parâmetros físicos disponíveis para medida foram: pressão, temperatura, vazão de refrigerante e potência de aquecimento, os quais, juntamente com o registro fotográfico, foram utilizados para caracterizar os padrões de escoamento e as transições, investigando-se os efeitos do diâmetro do tubo, da velocidade mássica e do fluxo de calor sobre a perda de pressão e a transferência de calor. Os principais padrões de escoamento visualizados foram: o intermitente, o anular e o estratificado, nos quais constatou-se que, as transições são governadas, principalmente, pelos efeitos da velocidade mássica e do diâmetro do tubo. Dentre estes padrões de escoamento, o anular e o estratificado foram modelados analiticamente. O modelo para o escoamento anular foi utilizado na obtenção de correlações para o fator de atrito interfacial e para espessura do filme de líquido. O modelo para o escoamento estratificado foi dividido em duas partes, uma destinada a obter a configuração da interface, a qual se mostrou côncava e a outra destinada à determinação dos fatores de atrito líquido-parede e interfacial os quais foram correlacionados / The research reports a theoretical and experimental study of convective boiling of refrigerant R-134a in smooth tubes. Tests have been carried out to measure the following physical parameters at the test section: mass flow rate, pressure and pressure drop, refrigerant and surface temperatures and the electrical power. In addition to these parameters, a photographic study has been carried out from pictures taken at the test section exit in order to determine the flow regimes that intervene under the imposed operating conditions. Effects over the pressure drop and heat transfer of the mass flow rate, heat flux, quality, and tube diameter have been investigated. Three flow regimes have been found: the intermitent, the stratified and the annular. Flow regime transitions are apparently governed by the mass velocity and tube diameter. The annular and the stratified flow regimes have been semi-empirically modeled using a mechanistic approach. The annular flow model has been applied to develop correlations for two important physical parameters: the interfacial friction factor and the film thickness. Through the stratified model, the shape of the interface has been evaluated along with correlations for the liquid to wall and interface friction factors

Apresenta um estudo teórico-experimental da ebulição convectiva do fluido refrigerante R-134a no interior de tubos lisos. Os parâmetros físicos disponíveis para medida foram: pressão, temperatura, vazão de refrigerante e potência de aquecimento, os quais, juntamente com o registro fotográfico, foram utilizados para caracterizar os padrões de escoamento e as transições, investigando-se os efeitos do diâmetro do tubo, da velocidade mássica e do fluxo de calor sobre a perda de pressão e a transferência de calor. Os principais padrões de escoamento visualizados foram: o intermitente, o anular e o estratificado, nos quais constatou-se que, as transições são governadas, principalmente, pelos efeitos da velocidade mássica e do diâmetro do tubo. Dentre estes padrões de escoamento, o anular e o estratificado foram modelados analiticamente. O modelo para o escoamento anular foi utilizado na obtenção de correlações para o fator de atrito interfacial e para espessura do filme de líquido. O modelo para o escoamento estratificado foi dividido em duas partes, uma destinada a obter a configuração da interface, a qual se mostrou côncava e a outra destinada à determinação dos fatores de atrito líquido-parede e interfacial os quais foram correlacionados / The research reports a theoretical and experimental study of convective boiling of refrigerant R-134a in smooth tubes. Tests have been carried out to measure the following physical parameters at the test section: mass flow rate, pressure and pressure drop, refrigerant and surface temperatures and the electrical power. In addition to these parameters, a photographic study has been carried out from pictures taken at the test section exit in order to determine the flow regimes that intervene under the imposed operating conditions. Effects over the pressure drop and heat transfer of the mass flow rate, heat flux, quality, and tube diameter have been investigated. Three flow regimes have been found: the intermitent, the stratified and the annular. Flow regime transitions are apparently governed by the mass velocity and tube diameter. The annular and the stratified flow regimes have been semi-empirically modeled using a mechanistic approach. The annular flow model has been applied to develop correlations for two important physical parameters: the interfacial friction factor and the film thickness. Through the stratified model, the shape of the interface has been evaluated along with correlations for the liquid to wall and interface friction factors

[pt] A tecnologia automotiva moderna alcança progressos consideráveis tanto no aspecto mecânico como funcional, com esses avanços tecnológicos, o maior progresso foi feito na estrutura dos automóveis e dos sistemas de ar condicionado. O presente trabalho é desenvolvido considerando a importância da utilização de gás refrigerante, já que se tornou um assunto ambiental de grande importância pelo fato de ter um papel na destruição da camada de ozônio. Foi construída uma bancada experimental que consta de duas câmaras isoladas para a colocação do equipamento, instrumentos, dispositivos de medição e geração de carga térmica, em seguida foi montado um condicionador de ar automotivo composto por componentes originais do sistema de condicionamento de ar de um automóvel para simular funcionamentos, com a finalidade de avaliar os parâmetros de funcionamento afetados quando são utilizadas diferentes cargas de gás refrigerante. Foram realizados testes controlando a temperatura e umidade constantes no habitáculo automotivo (câmara 1), para variações de temperatura na câmara (2), da vazão mássica no sistema e variações de torque e velocidade, no sistema motor - compressor, com diferentes quantidades de gás refrigerante originando situações de insuficiência de carga, carga adequada e sobre carga. Para o controle da carga térmica dentro das câmaras foram utilizados controladores de potência e um software aplicativo. Os parâmetros de operação do equipamento foram obtidos e arquivados mediante um sistema automático de aquisição de dados. Os resultados experimentais mostraram o comportamento real do ciclo de refrigeração, a queda de pressão nas linhas de descarga e de sucção, assim como no condensador e no evaporador. Apresenta-se o sub-resfriamento do refrigerante na saída do condensador bem como o superaquecimento na sucção. Verifica-se uma tendência politrópica, no lugar do processo isentrópico do ciclo ideal. Esta divergência do ciclo faz com que a temperatura de descarga do compressor (T2) seja elevada. A carga de gás refrigerante exerce influência no desempenho de um sistema de condicionamento de ar e os resultados do presente trabalho concordaram bem com as expectativas teóricas do problema considerado. / [en] The modern automotive technology reaches considerable progress s in the mechanical aspect as in the functional, with these technological advances, the biggest progress was made in the structure of the automobiles and in the air conditioning systems. Considering the importance of the use of refrigerant, the present work is developed, because the refrigerant gas became a great importance ambient subject for its paper in the ozone layer destruction. An experimental bench was constructed; it consists of two isolated chambers, where equipment, instruments, devices of measurement and thermal load generation were installed. After that, an automotive air conditioner was mounted, made up of original components from an automobile air conditioning system to simulate operation, with the purpose to evaluate the operation parameters affected when using different refrigerant loads. Tests with constant temperature and humidity in the simulated automotive chamber (1) for variations of chamber (2) temperature, of mass outflow in the system and variations of torque and speed in the motor-compressor system were performed with different refrigerating amounts creating situations of insufficient load, adequate load and over load. For the chambers thermal load controlling, power controlling and appropriate software were used. The equipment operational parameters had been gotten and filed by means of an automatic data acquisition system. The experimental results had shown the real behavior of the refrigeration cycle, the pressure drop in the discharge and suction lines, as well as in the condenser and the evaporator. Also the refrigerant sub-cooling in the exit of the condenser, the gas overheating in the suction, the polytrophic trend instead of the ideal cycle isentropic process was observed. This divergence from the ideal cycle makes the discharge compressor temperature (T2) was higher. The refrigerant gas charge influences the performance of an air conditioning system and the present work results agreed well with the theoretical expectations for the considered problem.

This study focuses on researching the performance of CO2 heat pumps in different real-time applications and in some studies, it compares the performance to synthetic and other natural refrigerants based on heat pump data provided from buildings. The research on the performance of the CO2 heat pump is performed based on Sweden's climatic conditions. The study consists of four different case studies each focusing on the CO2 heat pump used for four different buildings. The first study evaluates the performance of air source CO2 heat pump installed in a residential building and performs cost benefit in comparison to district heating energy consumption. The second study investigates the performance of the air source CO2 heat pump for the district heating application and compares the same with other refrigerant heat pumps. The refrigerants compared with include Ammonia (R-717), Propane (R-290), R-134a (1,1,1,2-tetrafluoroethane). The third study examines the performance of air source CO2 heat pumps in a commercial building with the field measured data obtained directly from the heat pump sensors through the online portal “itop”. The fourth study analyses the performance of a CO2 heat pump with that of a propane (R-290) heat pump for a commercial swimming pool application.  The study is performed using a simulation model created using Microsoft Excel Sheets and Cool Prop add-in, a thermophysical property database. The simulation model makes use of formulae of heat pumps to analyse the performance of the heat pump systems. The climatic data for Stockholm is taken from ASHRAE IWEC 2 database. The results of the study show advantages of CO2 heat pumps when used for combined purposes like space heating, space cooling and domestic hot water over the heat pumps using other refrigerants for their operation, as these refrigerants when operated at high condensation temperature led to low Coefficient of Performance (COP). The first study on residential building CO2 heat pumps showed a cost savings of about 116,000 kr per year even in high-pressure operations concerning the annual cost of district heating, which is about 30% of the total cost district heating with auxiliary equipment. The study also examined the energy saving over the usage of an ejector used in the heat pump which reached an average energy saving of 8%. The second study shows the dominance of the performance of CO2 over other refrigerants for district heating purposes. The third study indicates the performance of the CO2 heat pump in the application using real-time measure data. The fourth study illustrates an increase in overall COP of about 10% from the CO2 heat pump in comparison to that of propane refrigerant for swimming pool application. These results show that when the domestic hot water demand is higher, the CO2 heat pump performs better than other refrigerants specifically because the COP of other refrigerants is lower at high condensation temperatures. / Denna studie fokuserar på att undersöka prestandan hos CO2-värmepumpar i olika realtidsapplikationer och i vissa studier jämför den prestandan med syntetiska och andra naturliga köldmedier baserat på värmepumpsdata från byggnader. Forskningen kring CO2-värmepumpens prestanda utförs utifrån Sveriges klimatförhållanden. Studien består av fyra olika fallstudier som var och en fokuserar på CO2-värmepumpen som används för fyra olika byggnader. Den första studien utvärderar prestandan hos luftkällans CO2-värmepump installerad i ett bostadshus och ger kostnadsfördelar jämfört med energiförbrukningen för fjärrvärme. Den andra studien undersöker prestandan hos luftkällans CO2-värmepump för fjärrvärmeapplikationen och jämför densamma med andra köldmedievärmepumpar. Köldmedierna jämfört med inkluderar ammoniak (R-717), propan (R-290), R-134a (1,1,1,2-tetrafluoretan). Den tredje studien undersöker prestandan hos luftkällans CO2-värmepumpar i en kommersiell byggnad med fältuppmätta data som erhålls direkt från värmepumpens sensorer via onlineportalen "itop". Den fjärde studien analyserar prestandan hos en CO2-värmepump med den hos en propan (R-290) värmepump för en kommersiell simbassängapplikation. Studien utförs med hjälp av en simuleringsmodell skapad med Microsoft Excel Sheets och Cool Prop-tillägget, en termofysisk egenskapsdatabas. Simuleringsmodellen använder formler för värmepumpar för att analysera värmepumpsystemens prestanda. Klimatdata för Stockholm är hämtade från databasen ASHRAE IWEC 2.  Resultaten av studien visar fördelarna med CO2-värmepumpar när de används för kombinerade ändamål som rumsuppvärmning, rumskylning och tappvarmvatten jämfört med värmepumpar som använder andra köldmedier för sin drift, eftersom dessa köldmedier när de används vid hög kondensationstemperatur ledde till låg koefficient prestanda (COP). Den första studien om bostadshus CO2-värmepumpar visade en kostnadsbesparing på cirka 116 000 kr per år även i högtrycksdrift avseende den årliga kostnaden för fjärrvärme, vilket är cirka 30 % av den totala kostnaden för fjärrvärme med hjälputrustning. Studien undersökte också energibesparingen jämfört med användningen av en ejektor som används i värmepumpen som nådde en genomsnittlig energibesparing på 8 %. Den andra studien visar dominansen av CO2s prestanda över andra köldmedier för fjärrvärmeändamål. Den tredje studien indikerar CO2-värmepumpens prestanda i applikationen med hjälp av mätdata i realtid. Den fjärde studien illustrerar en ökning av den totala COP på cirka 10 % från CO2-värmepumpen jämfört med den för propan-köldmedium för simbassängapplikationer. Dessa resultat visar att när efterfrågan på tappvarmvatten är högre presterar CO2-värmepumpen bättre än andra köldmedier, särskilt eftersom COP för andra köldmedier är lägre vid höga kondensationstemperaturer.

No description available.

Eco-friendly non-CFC refrigerants were introduced in the Air Conditioning and Refrigeration industry during the last few years to reduce damage to the stratospheric ozone layer. The HFC refrigerant R-134a, which has zero Ozone Depletion Potential (ODP), is being used extensively as a replacement for R-12 and also in some centrifugal chillers as a replacement for R-11. However, the disadvantage of R-134a is its comparatively high global warming potential (GWP). Owing to energy crisis and also to reduce the indirect warming impact resulting from electrical energy usage, the new refrigeration systems should be operated at the lowest possible condensing temperatures. In view of this, several active and passive techniques for augmentation of condensation heat transfer and reduction of condensation temperature are gaining increasing attention. Passive augmentation methods are more popular than active ones. To this end, micro-finned tubes of various geometrical shapes are being explored for compact heat exchangers in the refrigeration industry as the best choice. Towards understanding the enhancement in condensation heat transfer coefficients in micro-finned tubes, a test facility has been fabricated to measure the condensing coefficients for R-134a refrigerant. Condensation experiments have been conducted on single plain and finned tubes of outer diameter 19 mm with a refrigerant saturation temperature of 400C and tube wall temperatures 350C, 320C, 300C and 280C respectively. Water is used as the cooling medium inside the tubes with the flow rate varying from 180 lph to 600 lph. The condensing coefficient typically ranged from 0.9 – 1.4 kW/(m2 K) for plain tubes and from 4.2 to 5.8 kW/(m2 K) for the finned tubes. The results of the plain v tube are found to compare favourably with the Nusselt’s theory, leading to a validation of the experimental procedure. Upon comparing the results of finned and plain tubes, it is found that provision of fins result in an enhancement factor of 3.6 to 4.6 in the condensation heat transfer coefficients. This level of enhancement is larger than that resulting from the enhanced surface area of the finned tube surface, suggesting that, apart from the extended area, the surface tension forces play an important role in the augmentation process by driving the condensate from the fin crests to the valleys in between the fins. The measured augmentation factors have also been cross-checked using the Wilson plot method. Detailed error analysis has been performed to quantify the uncertainty in the condensation heat transfer coefficient. The performance of a bank of tubes has been determined based on the measurements carried out on practical condensers of two large chillers with refrigerating capacities of 500 TR and 550 TR. On comparing the finned tube bank results and the single finned tube results, it is found that the average condensation heat transfer coefficient in a bank of tubes having N rows varies as N ¯1/6. The deterioration is in agreement with the relation proposed by Kern.