Transient behavior of thermocapillary convection in thin liquid film exposed to step laser heating

Abstract

Temporally developing thermocapillary convection induced by step laser heating of a thin liquid film has been studied numerically. Computations are performed using the commercial software STAR CCM+ version 2022.1. The liquid film of silicone oil (high Prandtl number fluid) is 60 mm in diameter and 3 mm in thickness. Flow characteristics related to surface velocity and surface temperature have been studied. Validation of the computations is achieved for the surface velocity, the velocity along thickness and the surface temperature through comparison with PIV and IR camera measurements. The laser-beam with a carbon dioxide gas laser (10.4 μm in wavelength) is used for heating. It is found that the temporally developing profile of surface velocity shows two local velocity peaks ($u_{S1}$ and $u_{S2}$) at two radial locations ($r_{S1}$ and $r_{S2}$) respectively. The first peak, $u_{S1}$, appearing due to the steep temperature gradient generated by laser-beam heating and its radial position, $r_{S1}$, do not change noticeably with time. On the other hand, the second peak, $u_{S2}$, travels radially outwards with decreasing magnitude in a self-propelling manner until its radial position, $r_{S2}$, approaches an asymptotic maximum. Detailed analysis of the coupling among radial temperature gradient, local pressure variation and local convective acceleration near the second peak reveals that hydrothermal mechanisms are responsible for self-propelling travel of $u_{S2}$. The transient behaviors of both primary and secondary velocity peaks are found to depend on the fluid viscosity and the laser-beam settings.