Heat transfer enhancement for flow boiling of zeotropic mixtures with regulating liquid mass transfer resistance

Abstract

The mass transfer resistance is a significant element leading to heat transfer degradation of zeotropic mixtures in the flow boiling. In this paper, the heat transfer enhancement of zeotropic mixtures in the flow boiling was investigated through regulating the mass transfer resistance by constructing reasonable surface structure. The heat and mass transfer, pressure drop characteristics of stratified bubble flow for R134a/R245fa zeotropic mixtures in a rectangular channel with multi-cone staggered spoiler porous structure were studied experimentally. The bottom of channel was sintered with multi-cone staggered porous layer with thickness/height/diameter of 1/3/2 mm, respectively. The experiments were carried out under an inlet evaporation temperature of 20 ℃, and the ranges for heat and mass fluxes were 10–150 kW/m2 and 100–150 kg/(m2·s), individually. Compared to the rectangular channel with smooth porous structure, liquid mass transfer resistance of zeotropic mixtures in the rectangular channel with multi-cone staggered porous structure was decreased obviously. The decrease degree for Mixture 2 was largest, while the maximum average decrease was 89.61 % at G = 100 kg/(m2·s). The heat transfer coefficients were obviously improved and heat transfer enhancement increased gradually especially under conditions of high heat flux. The enhancement degree for Mixture 2 was the largest, while the maximum average increase was 26.05 % at G = 100 kg/(m2·s). The pressure drop increased significantly, and the increasing degree for Mixture 2 was smallest, while the minimum average increase was 38.18 % at G = 150 kg/(m2·s). Multi-cone staggered structure was helpful to reduce liquid mass transfer resistance and improve heat transfer performance for zeotropic mixture. The zeotropic mixture with higher liquid mass transfer resistance would have better heat transfer intensify and smaller increase of pressure drop with the help of multi-cone staggered structure.