Heat transfer incipience of capillary-driven liquid film boiling

High-power electronic devices bring both unprecedented challenges and great opportunities to two-phase cooling technologies for efficient thermal management. Capillary-driven liquid film boiling with wicking structures combines the advantages of capillary evaporation and nucleate boiling, and thus could be one of the most promising cooling methods. However, a comprehensive understanding of liquid film boiling mechanism is lacking. For example, the current prediction of liquid film boiling incipience is based on the bubble nucleation model utilized in pool boiling, which assumes that bubbles nucleate inside the thermal boundary layer with linear temperature distribution due to heat conduction. In liquid film boiling, heat transfer processes are quite different, including both heat conduction inside the liquid film and evaporation at the liquid-vapor interface. In this work, based on the thermal resistance network considering evaporation at thin-film region near the tri-phase line, we develop a model to predict liquid film boiling incipience. Both visualization and heat transfer experiments are conducted using copper micromesh to validate this model. The model predictions of critical liquid film thickness, surface superheat, and heat flux at the onset of nucleate boiling (ONB) are compared with the experimental data on various wicking structures in the literature, including silicon micropillar, copper micropowder, and copper micromesh. The results show that the heat flux and superheat reduce as the wick thermal conductivity decreases, the wick thickness increases, and the wick porosity increases.