Disk impact on a boiling liquid: Dynamics of the entrapped vapor pocket

TL;DR

This study experimentally investigates how entrapped vapor pockets under a impacting disk influence impact dynamics, revealing temperature and velocity-dependent behaviors, vapor pocket collapse mechanisms, and effects of disk tilt on impact pressure.

Contribution

It provides new insights into vapor pocket dynamics during impact, highlighting the effects of temperature, impact velocity, and disk tilt on vapor entrapment and collapse processes.

Findings

Vapor pocket retraction is slow at high temperature and low velocity.

Rapid vapor pocket collapse occurs at lower temperatures and higher velocities due to condensation.

Disk tilt affects vapor entrapment and impact pressure distribution.

Abstract

Upon the impact of a flat disk on a boiling liquid, i.e., a liquid that is in thermal equilibrium with its own vapor, a thin vapor layer is entrapped under the disk. Due to the tendency of vapor to undergo phase change under pressure variation upon impact, the dynamics of this entrapped vapor pocket are different from those of a non-condensable air pocket. In this work, we experimentally investigate the dynamics of the entrapped vapor pocket, more specifically its time evolution and its subsequent influence on the hydrodynamic loads at different equilibrium ambient temperatures and impact velocities. We find that the retraction of the vapor pocket at high ambient temperature and small impact velocity is slow, occurring from the disk edge, and driven by the dynamic pressure $\rho_{\text{L}}U_0^2$. In contrast, at lower ambient temperatures and large impact velocities, after a short initial stage, the vapor pocket will collapse rapidly due to condensation. This scenario is confirmed by conducting experiments where, by heating the disk, the vapor pocket collapse is observed to slow down. We attribute this to the vaporization of liquid near the three-phase contact line region that frustrates the condensation process and reduces the impact pressure on the disk. The violent collapse of the vapor pocket may impart additional instantaneous momentum, but the overall pressure and force impulses are still found to be closely associated with the liquid added mass. Finally, we found that at a high tilt angle of the disk, the three-phase contact line movement over the disk surface may hinder the proper entrapment and compression of the vapor pocket, which results in a lower central impact pressure as rapid condensation at the central disk region does not occur.