Nucleate boiling heat transfer and critical heat flux (CHF) from micro-pit surfaces

•Micro-pit surface is able to reduce boiling incipience superheat, and improve nucleate boiling heat transfer and CHF.•Boiling enhancement has weak dependence on pit diameter, but increases monotonously with decreasing pit depth.•Optimum pit-to-pit spacing for maximum boiling enhancement is identical to capillary length.•A correlation devised for predicting heat transfer coefficient on micro-pit surfaces is proposed. This paper studies pool boiling enhancement on the micro-pit surfaces. Microscale pits fabricated on plain surface are able to reduce boiling incipience superheat, and improve both nucleate boiling heat transfer coefficient and critical heat flux (CHF). Boiling enhancement magnitudes have a weak dependence on the micro-pit diameter, but increase monotonously with decreasing the pit depth. There exists an optimum pit-to-pit spacing for the maximum boiling enhancements, which is virtually identical to bubble departure diameter, and estimated using the capillary length. The major mechanism behind is that this spacing is favorable for alleviating hydrodynamic instabilities induced by the counterflow between liquid inflow and vapor outflow. The highest heat transfer coefficient and CHF using water as fluid under the present conditions are 70.0 kW/m2K and 165.7 W/cm2, improved by 58.8% and 33.7% compared to the plain surface, respectively, at micro-pit diameter of 100 μm, depth of 100 μm and spacing of 2.5 mm. Finally, a correlation devised for predicting the nucleate boiling heat transfer coefficient on the micro-pit surfaces by incorporating dimensionless pit depth is proposed, with MAE of 8.3%.