Heat transfer and flow pattern map development of R134A in a U-bend tube for flow boiling evaporation

Abstract

This study presents flow boiling evaporation heat transfer and flow patterns development of R134a in a U-bend tube with the curvature ratio of 2.55 for the downward flow orientation. The volume of fluid (VOF) multiphase model in the computational fluid dynamics (CFD) code Ansys Fluent® was utilized and incorporated with phase change model for volume fraction tracking of each of the phases. The simulations were conducted under these operating conditions: (mass fluxes from 200 to 600 kg/m2s, vapour qualities from 0.01–0.9 and 10 kW/m2 of heat flux). The relationships of mass flux with respect to vapour quality on heat transfer and frictional pressure drop in the U-bend tube were analysed. The effect of mass flux with respect to vapour quality on heat transfer coefficient showed that heat transfer coefficient increases as mass flux increases and with increase in vapour qualities at constant mass flux which later decreases at the higher vapour qualities due to dry-out. The frictional pressure drop was found to increase as mass flux and vapour quality increases as a result of increase in an interfacial shear stress of liquid and vapour phases. At higher vapour qualities, frictional pressure drops decrease due to drastic reduction of the liquid film. The influence of the bend was determined by the development of flow pattern map before and after the bend. The numerical computation results agrees with what is obtained in the open literature.