R245fa vertical downward flow boiling heat transfer characteristics in a micro/nanoporous surface coating tube

This research employed a combination of sintering and electrochemical deposition methods to prepare a micro/nanoporous coating on the inner wall of stainless-steel tube with an inner diameter of 10 mm. A vertical downward flow boiling experiment was conducted using R245fa as the working fluid to compare the flow boiling heat transfer characteristics between the smooth tube and the enhanced tube. The experimental parameters were the saturation pressure was maintained at 0.6 MPa, mass fluxes ranged from 200-700 kg/m?s, heat fluxes varied from 5-75 kW/m?, and inlet vapor qualities spanned from 0 to 1. The findings indicated that both tubes exhibited only annular flow and dryness under the vertically downward flow conditions. The mass fluxes had a limited effect on heat transfer performance. However, as the inlet vapor quality increased, the boiling heat transfer coefficients exhibited a tendency to increase and then decrease, with the heat transfer coefficients of the enhanced tube outperforming those of the smooth tube. This phenomenon can primarily be attributed to the increased density of nucleation sites and superior surface wettability compared to that of the smooth tube. The enhancement factor for heat transfer and the performance evaluation coefficient achieved values of 1.884 and 1.762, respectively.