Experimental study of the effect of surface roughness on the heat transfer characteristics of subcooled flow boiling in a narrow rectangular channel

•Analyzed effect of surface roughness on subcooled flow boiling through visualization.•Large surface roughness shifts the axial position of the ONB forward.•Modified empirical equation for subcooled boiling in a narrow rectangular channel. Due to the size limitation, the heat transfer characteristics of subcooled flow boiling in the narrow rectangular channel are different from the conventional channels. Surface roughness directly affects bubble formation and detachment in subcooled boiling. This study presents a visualization study on the effect of surface roughness on the heat transfer characteristics and the onset of nucleate boiling (ONB) in narrow rectangular channels, including roughness of 0.34 μm, 1.2 μm, and 2.5 μm. The results indicate that surface roughness can enhance the heat transfer between the fluid and the wall and reduce wall temperature. The heat transfer coefficient in the subcooled flow boiling increases nonlinearly with surface roughness. Large surface roughness can shift the axial position of the ONB forward and reduce the wall heat flux required for subcooled flow boiling. An empirical correlation for predicting the subcooled flow boiling is modified based on Hsu’s nucleation criterion and the experimental data.