Influence of thermocapillary forces on the characteristics of falling liquid films

Abstract

The evolution of three-dimensional waves into rivulet structures during heating of a vertically falling water film for Reynolds numbers 70,105,and 150 was experimentally studied. A high-speed high-resolution infrared camera was used to study the interaction of hydrodynamic waves with thermocapillary instabilities. The liquid film thickness was measured by the Laser-Induced Fluorescence (LIF) method. The analysis and generalization of the obtained data together with the previous results in the range of Re = = 20–500 were carried out. The main attention was directed to the investigation of the rivulet flow formation when small and moderate temperature gradients are reached on the surface of the liquid film.The experimental data on the distance between the rivulets and the factors influencing them were analyzed in detail. The experimental data were generalized and it was confirmed that the distances between the rivulets do not depend on the distance along the heater, the heat flux, and the Reynolds number of the film.An analysis of root-mean-square temperature fluctuations depending on the distance along the heater for different values of the heat flux and the Reynolds number of the film was made.Fourier spectra were calculated at several points on the liquid film surface. The dynamics and spatial structure of temperature fluctuations are studied using Dynamic Mode Decomposition (DMD) analysis.The movement of large waves, which propagate at a frequency of approximately 1–2 Hz, has been studied. It was shown that if a part of a large wave moves in the interrivulet region, then the thermocapillary forces directed to the crest of this wave, although they lead to its deceleration, cannot move it into the region of the rivulet. In this case, an intensive thinning of the liquid film occurs in front of the wave, and with an increase in the Reynolds number of the film, the modified Marangoni number increases.It has been established that, at low heat flux, the deceleration of waves propagating along the liquid film surface along the heater occurs only in the interrivulet region, when, as at high heat flux, all waves are decelerated due to thermocapillary forces.It is shown that the length of the initial thermal region sharply decreases with an increase in the heat flux and the development of thermocapillary instability in the region of the upper edge of the heater.