Enhancing R245fa flow boiling heat transfer in a micro/nano-porous surface coating tube with varying particle sizes for low-grade energy utilization

The effect of pore size on heat transfer performance is investigated by preparing a multi-scale micro/nano-structured porous coating on the inner surface of conventional heat exchange tubes using sintering and electroplating method. Comparative experiments were conducted with varying porous coating pore diameters, utilizing R245fa as the working fluid in the stainless-steel tube with an inner diameter of 10 mm. Flow visualization revealed three flow patterns: stratified flow, annular flow, and dry-out, occurring in all experimental tubes with similar distributions. Analysis of operating parameters revealed that mass flux had a limited influence on the heat transfer coefficient, while inlet vapor quality had a significant effect with the heat transfer coefficient first increasing and then decreasing. The porous coating, particularly in the SET-150, exhibited excellent liquid absorption capacity, facilitating liquid film migration from the bottom to the top of the tube during stratified flow and enhancing re-wetting in annular flow. These processes significantly contributed to improved heat transfer performance. The enhancement factor of SET-150 was 1.57 times that of SET-50 and 1.21 times that of SET-100, confirming its superior heat transfer performance. The maximum enhancement factor and performance evaluation criterion were 4.06 and 3.98, respectively.