ID 409

Heat flux during a drop impact onto a hot, dry solid surface

 

Abstract:

The outcome from an isothermal single drop impact onto a solid surface without phase change is determined by impact parameters, liquid material properties, substrate morphology and wettability. A drop impact onto a hot surface causes additional effects such as the Marangoni flows, evaporation, nucleate boiling, film boiling and other thermodynamic effects. By changing the surface temperature and impact parameters the observed phenomena of a drop impact can be manipulated and various thermodynamic and hydrodynamic regimes can be observed. The heat flux during drop spreading on a wall depends mainly on the substrate temperature and on the instantaneous boiling regime. Better knowledge on the heat transfer will allow to develop reliable models for spray cooling. The focus of this experimental study is the estimation of the heat flux between the hot surface and the impacting drop depending on the observed hydrodynamic and thermodynamic regime. Drop impact has been observed from the side by using a high-speed video system. The temperature distribution of the hot surface has been measured from below by using a high-speed infrared camera. Therefore an IR transparent sapphire substrate was used as hot surface and coated on top with a high emissivity coating. By varying the impact speed and surface temperature different hydrodynamic (deposition, rebound and breakup) and thermodynamic regimes (evaporation, nucleate boiling, transitional boiling and film boiling) have been characterized and the relationship between heat flux and regimes has been described. Furthermore, the appearance of partial wetting and the Leidenfrost effect have been observed within single regimes of the drop impact.