Experimental study of falling film evaporation heat transfer on superhydrophilic horizontal-tubes at low spray density

Abstract

This paper presents a series of experiments concerning heat transfer in a vertical row of horizontal-tube falling film evaporator. By visualization and heat transfer experiments, the effects of hydrophilic modification on the inter-tube flow patterns and evaporation heat transfer performance on different tubes, the superhydrophilic, hydrophilic and plain tubes, were investigated. The visualization results indicated that four flow patterns, droplet, droplet/jet, jet and jet/sheet mode was achieved in this study and the hydrophilic modification has little effect on flow mode transition. Experiments have been indicated that the superhydrophilic and hydrophilic tubes were completely wetted while the plain tubes were partially wetted at low spray density. The heat transfer results demonstrated that superhydrophilic tubes could maintain a high heat transfer coefficient at a wide range of flow rates (Res from 60 to 700). While, high enhancement was achieved at low spray density, at which the plain tube was not be wetted. When Res reached 350, three surfaces had the similar heat transfer performance. Simultaneously, the heating water inlet temperature had little effect on heat transfer coefficient for superhydrophilic tubes and had a negative influence on plain tubes and hydrophilic tubes at low spray density.