FLOW BOILING HEAT TRANSFER OF REFRIGERANTS THROUGH INCLINED TUBE

Abstract

The industrial adaptation to new alternative refrigerants is now one of the major driving forces in two-phase flow and evaporation heat transfer exploration. However, the study of boiling and two-phase flow phenomena of refrigerants is still in its infancy. This field of research provides many opportunities to study new bailiwicks. The present research involved a comprehensive experimental study on the flow evaporation heat transfer coefficient and the associated frictional pressure drop for refrigerants. Hydrochlorofluorocarbon (HCFC) refrigerants have adverse environmental impacts like ozone depletion at earth stratosphere and also the global warming apart from their all over adoption in most of the refrigeration applications due to theier inherent thermo-physical properties and excellent thermodynamic performances. It is obvious to incur possible appropriate alternate to substitute HCFC, mainly R22 to meet environmental sanctuary to protect earth’s biota as refrigeration has become indispensable to modern life. By the international proscription of such refrigerants and substantial mesmerisms from the universally adopted agencies and programs, suitable refrigerants, such as, pure fluids R134a and R600a, and refrigerant mixtures like R410A and R407C have been preferred for this present study. To summarize the current canvas in this newly developing field and to identify the prospect of future research as well, a comprehensive research of the characteristics of these environment friendly refrigerants and their comparative experimental results of heat transfer performances during evaporation, this thesis is especially aimed to contribute to a better understanding of the inherent physical phenomena in flow boiling heat transfer in small channels. For this purpose, well specified heat transfer experiments have been executed for the flow boiling inside a single, inclined tube of 7 mm internal diameter with five different inclinations ranging from 0o to 90o in upward directional flow. Additionally, flow visualizations have been carried out to illuminate the II two phase flow behavior and describing the variation of the boiling heat transfer characteristics with the flow pattern transition. Through high speed flow visualization under non adiabatic conditions, four flow patterns have been observed: stratified flow, intermittent flow, stratified-wavy flow and annular flow under the test parameters such as: the mass velocities in the range of 100 to 400 kg/m2s; the heat flux of the range of 3-10 kW/m2, and the vapor quality ranges between 0.10 to 0.95. Two phase flow pattern observations are explained in the flow pattern maps as the plots between the mass velocity versus vapor quality or with respect to non-dimensional parameters. Two-phase flow boiling heat transfer and pressure drop results from the present study have been compared with the prediction correlations available in the literature. The best agreements are obtained with the correlations by Liu and Winterton (1991) and Wojtan et al. (2005) for horizontal and inclined evaporator tube. However, better correlations to correctly predict the flow boiling heat transfer coefficient and pressure drop in inclined small tubes including those for horizontal and vertical conditions for different test refrigerants are proposed. The correlation predicts the 91% data of current investigation in the error band of ±20%.