Vapor space condensation with surface modifications

Abstract

For the completely hydrophilic surface, the highest heat transfer coefficient was about 12400 W/m2K and that of the completely hydrophobic surface was around 31500 W/m2K, which is about 2.5 times higher than that for the hydrophilic surface with all other parameters unaltered. On the other hand, the surface with a pattern of distinct hydrophobic and hydrophilic regions showed that heat transfer coefficients were higher than that of the hydrophilic surface. However, depending on the type of patterns on the condensation surface, the heat transfer coefficients were either higher or lower than that of the completely hydrophobic surface. Among all the surfaces, the highest heat transfer coefficient (34000 W/m2K) was observed the patterned surface with hydrophilic island diameter of 0.25 mm. With the patterned and the completely hydrophobic surfaces, the heat transfer coefficient was observed to increase significantly with mass flux, while for the completely hydrophilic surface, the heat transfer coefficient was observed to be affected much less by the mass flux. In all cases, the heat transfer coefficients increased with increasing heat flux and decreased with increasing wall sub-cooling. The effect of average quality of the steam showed little effect on the heat transfer coefficients for the hydrophilic surface while an increasing trend of heat transfer coefficient was observed with increasing quality for the completely hydrophobic and patterned surfaces.

An experimental study of condensation heat transfer was carried out on a 25.4 mm diameter surface using steam as the condensing fluid. Various surface conditions were studied: completely hydrophilic, completely hydrophobic, and surfaces with patterns of distinct hydrophilic and hydrophobic regions. For the all the patterned surfaces, the surface area of the distinct regions of hydrophilic regions was 25% and that of the completely hydrophobic region was 75%. The effects of inlet vapor velocity, mass flux, and hydraulic diameter on the heat transfer coefficients were investigated. The inlet vapor velocity was varied from about 0.05 m/s to about 5 m/s and the hydraulic diameter was varied from 4.5 mm to 32.5 mm. Depending on the surface condition, the heat transfer coefficients showed different responses to the varying parameters of the experiments.