Entropy Generation Analysis and Optimum Tube Length of Two-Phase Flow Evaporator Tube

Due to the dependence of boiling heat transfer rate on heat flux, saturation pressure, mass flux and refrigerant quality, efforts made to increase heat transfer rate often cause further drop in refrigerant saturation pressure. In this study, entropy generation mechanisms associated with two-phase flow in smooth evaporator tube subject to constant surface heat flux are investigated. Refrigerant flow was considered annular, steady and one-dimensional. Separated fluid model was applied and flow path discretised. The governing equations were solved for fluid properties and heat transfer coefficient at each discretised section using Newton-Raphson finite difference numerical technique. Entropy generation analysis was also conducted and the evaporator tube parameters optimized subject to pressure drop and volume constraints. It is shown that tube dimensions of length and diameter which result in minimum entropy generation rate may not necessarily be the optimum tube parameters. Optimum tube parameter is obtained that satisfies all given conditions.