Surface microstructures for boiling and evaporation enhancement in falling films of low-viscosity fluids

Abstract

The effect of surface microstructures created by the precision method of processing plastic materials (method of deformational cutting) on heat transfer was studied on falling films of refrigerant mixture R114/R21. The binary mixture film flowed along the outer surface of the vertical heated cylinders in the laminar-wave regime within the range of the film Reynolds numbers from 300 to 1400. The heat flux density varied from zero to critical values. It has been shown that surfaces with semi-closed pores have an advantage in heat transfer enhancement at boiling (up to four times compared to a smooth surface), while micro-pin structures contribute to an increase at evaporation regime (up to two times compared to a smooth surface). Both types of microstructures demonstrate an increase in the values of critical heat flux in comparison with a reference smooth surface (more than two times) and significantly exceed the values obtained for some of modern commercially available surfaces of “GEWA” and “Turbo” series. The parameters of surface microstructures, most effective for heat transfer enhancement in various regimes, have been identified. The experimental results were compared with the available correlations for calculating the heat transfer coefficient and the critical heat flux.