ID 350

Study of the heat transfers and vapor film thickness at the drop impact in the film boiling regime

 

Abstract:

This study deals with the impact of a water droplet onto a hot solid surface in the film boiling regime. A model is proposed for the heat transfer across the vapor layer and the evolution of its thickness. Among the main assumptions, heat transfers are supposed to be predominant in the direction normal to the solid surface, the thickness of vapor film is uniform and there is a quasi-steady Poiseuille flow in the vapor layer. Employing heat energy and momentum balances allows to deduce the time evolution of the vapor film. For water droplet injected with a significant subcooling temperature (a few °C), the model predicts that the liquid heating represents almost all of the heat transfer from the wall and its rate governs almost all of the evolution of the vapor thickness. Hence, the impact velocity and droplet diameter have no influence on the film thickness. The model is validated against experimental data obtained for millimeter-sized water droplets impinging onto a heated sapphire window. IR thermography is used to record the thermal response of the solid surface during the drop impact. An Inverse Heat Conduction Problem is solved to reconstruct the heat flux from the solid surface. The vapor film thickness is then estimated from the heat flux assuming a linear temperature profile within the vapor layer. The liquid heating is characterized using a laser-induced fluorescence imaging technique. Comparisons for a large set of impact conditions show a good agreement between the model and the experiments.