Interaction of 3D waves with thermocapillary structures in a heated liquid film

The evolution of three-dimensional waves into thermocapillary-wave structures at heating a vertically falling water film with Reynolds number of 50 was studied experimentally. A high-resolution, high-speed infrared camera and fluorescence thickness measurement method were used to study of interaction of hydrodynamic disturbances with thermocapillary instabilities. The thickness and temperature fields of the film surface were measured synchronously. The amplitudes and velocities of waves, amplitudes of rivulet deflection, temperature fluctuations on the liquid film surface, frequency spectra and pulsation energies were determined. The method of dynamic mode decomposition (DMD) was used to calculate characteristic frequencies (global frequencies) in the flow and reveal spatial distributions of thickness and temperature fluctuations (dynamic modes) evolving in the flow with a characteristic frequency. It is shown that the main deformation of wave fronts occurs because of their interaction with a thermocapillary structure, which formed in the residual layer near the leading edge of the heater. The high temperature gradients of up to 10-20 K/mm were observed in this region. In this case, the wave amplitudes increase in the initial region of heating under the action of thermocapillary forces directed against the film flow. Thermocapillary structures transform into another type in the lower part of the heater, where the small values of temperature gradients were observed. (C) 2020 Elsevier Ltd. All rights reserved.