Response of unsteady thermocapillary convection to local heating at specific depths in a thin liquid film

A hydrothermal wave (HTW) refers to a flow pattern that arises during unsteady thermocapillary convection, negatively impacting the purity of single crystals during zone melting. This study utilized active control through localized heating to reduce HTW amplitude, akin to noise cancellation. The effectiveness of this method was evaluated both experimentally and numerically by adjusting the heating depth in a thin liquid film. Silicone oil served as the test fluid, and the liquid film was rectangular in shape. In the case of linear time-periodic heating, a line-shaped heater applied a constant maximum heat input, synchronized with the HTW's period. Experimental results indicated that HTW amplitude significantly depends on the heating depth of the liquid film. Downstream of the HTW, temperature amplitude could be reduced by up to 60%, whereas upstream, it could be amplified. In numerical simulations, HTW reduction and amplification were linked to substantial changes in the streamwise temperature gradient on the free surface, rather than to local cancellation of the temperature disturbance. Unlike the results from single-pulse heating control, the amplitude ratio showed a strong dependence on whether the heat input significantly affected the streamwise thermal and flow fields.