Condensation of refrigerants in a horizontal tube in annular flow regime

Presently, the global warming due to depletion of the ozone layer in the atmosphere is one of the major environmental problems in the world. The chlorofluorocarbon (CFC) refrigerants which are commonly used in refrigeration devices are believed to be the major responsible agents in many environmental issues. Following the signing of the Montreal Protocol, many research studies have been initiated on possible alternative refrigerants. The present doctoral thesis is an experimental study on pressure drops and convective heat transfer coefficients of isobutane, a possible refrigerant, in horizontal tubes. The thesis is composed of seven major parts. In the introduction section, the reasons for initiating this study, as well as the goal of the work are presented. The second part involves a discussion of the properties of isobutane utilized in this work and mentions the desired characteristics of refrigerants, in general. The third part is a literature survey on the topic. It is necessary to clearly determine the flow pattern in two-phase flows in order to evaluate the heat convection coefficient and the pressure drop. Thus, it is for this reason that the data reported in the literature on the so-called void fraction, which is an influential parameter in determinations of pressure drops, heat convection coefficients and the flow patterns, were thoroughly searched. In the last part of this section, the limited studies on the fluid R600a were mentioned. The fourth part of this work is devoted to the experimental work carried out. In this connection, stages in the preparation of the experimental setup, its components, the methods of analyses and the uncertainties were discussed. The fifth part is concerned with the evaluation of the experimental data obtained. The experiments were carried out in a smooth pipe, with an inner diameter of 4 mm, contained within the experimental setup. It is possible to divide this study into two parts as thermal and pressure drop analyses. The thermal analyses involved transfer of vapors of the refrugerant to the test area at various saturation temperatures, with varying mass flow rates and vapor qualities to detemine the heat convection coefficients and to obtain Nu (0.04599 Reeq 0.8525) versus vapor quality graphs. The designated value of Nu is based on the experimental results obtained in condensation of R600a within smooth horizontal pipes. This value of Nu is consistent with the accepted value in the literature. The pressure drop analyses involved similar experiments at various saturation temperatures with varying mass flow rates and vapor qualities. The data obtained from experiments were used to calculate pressuere drops due to friction and to correlate friction coefficients in twophase flow to Reeq. The pressure drops depending on mass flow rates and friction coefficients were individually calculated and their effects on the total pressure were presented graphically. The sixth part of this work was concerned with the determination of one-phase liquid flow which is equivalent to temperature and velocity distributions of liquid film in two-phase circular flow. Thus, a model was developed which enables the calculation of convection heat transfer coefficients based on equations for one-phase liquid. The calculated heat convection coefficient values obtained using this model, which was basically aimed to establish a relationship between pressure drops and heat convection coefficients, were highly compatible ( ± 10% ) with the experimental values. Keywords: Two phase flow, smooth pipe, condensation, alternative refrigerant