Droplet boiling on heated surfaces with various wettabilities

Abstract

Single droplet boiling is a basic problem for spray cooling which is an effective technology for high heat flux applications. Visualizations were used to investigate the effect of surface wettability on single droplet boiling with measurements of the droplet evaporation times. Four boiling regimes were identified on the hydrophilic surface and the bare aluminum surface, including nucleate boiling, nucleate boiling with violent atomization, transition boiling and film boiling. The nucleate boiling with violent atomization regime was not seen on the hydrophobic surface and the superhydrophobic surface. The transition boiling regime was not seen on the superhydrophobic surface. The Leidenfrost temperatures were approximately 220 °C on the hydrophilic surface, 200 °C on the bare aluminum surface, 170 °C on the hydrophobic surface and 170 °C on the superhydrophobic surface. The effect of surface wettability on the Leidenfrost temperature was discussed using the potential barrier existing at the transition to film boiling. A droplet film boiling model was proposed to relate the droplet evaporation time, initial droplet size and surface superheat with a semi-empirical correlation developed for the droplet evaporation time. The correlation agrees well with the current measured droplet evaporation times for film boiling and with data in the literature.