Boiling of single bubbles on slippery surfaces: a direct numerical approach

Boiling and spray cooling are highly effective thermal management strategies that exhibit a strong dependence on surface characteristics such as wettability. In nucleate boiling, increased surface wettability typically delays the onset of nucleate boiling, increases nucleation site density, reduces bubble departure diameter and increases critical heat flux, thereby enhancing heat transfer. In spray cooling, higher wettabilities improve liquid spreading and rewetting of the heated surface, leading to more uniform cooling and higher critical heat flux. Moreover, higher droplet velocities promotes greater droplet breakup and spreading, which improves surface coverage and enhances the overall heat transfer rate.

This work is composed of direct numerical simulations that use an in-house TPLS solver based on the Cahn-Hilliard phase-field method in order to alleviates stress discontinuities at the three-phase contact line. Dynamic contact lines are implemented, in conjunction with a phase change model, utilising a geometric formulation that imposes a specified micro-contact angle within a hysteresis window at the solid-fluid interface. This allows for the direct numerical modeling of boiling on slippery surfaces.

Our simulations present the heat transfer coefficient as a function of bubble population density and wettability for nucleate pool boiling, with an elucidation of the transitional boundary between nucleate and film boiling. For droplet impact, a loosely coupled conjugate heat transfer solver is used in addition to the prescribed model in order to investigate the effects of Weber number, wettability and heat flux on the surface temperature and heat transfer coefficient, with results compared to experimental data.