The behavior of heat transfer and entropy in a thin layer of liquid under laser heating

Abstract

Experimental studies of heat flux, Nusselt number, heat transfer coefficient and entropy generation under local laser heating in the range of water layer heights from 1.4 to 5 mm have been carried out. The laser heats a graphite spot on the glass wall, which warms up the liquid. To date, most studies in the fields of the velocity rotor and entropy generation have been carried out theoretically, and in the absence of surface Marangoni forces. There is extremely little experimental data on the measurement of these fields, especially at non-stationary three-dimensional heat exchange. The temperature field on the free surface of the liquid layer is strongly inhomogeneous and asymmetrical at a layer height of 1.4 mm. At a layer height of 4–5 mm, the symmetry of the temperature field increases. Using the optical method of Particle Image Velocimetry, instantaneous fields of velocity and vorticity have been studied. As the layer height increases, the characteristic size of vortex structures on the liquid surface rapidly decreases with a sharp decrease in the correlation function of the vortex flow. Local laser heating of the layer creates a spatially uneven heat exchange, which is formed during self-organization of various convective vortex structures. For the first time it is shown that the behavior of entropy generation during local heating of the layer differs from the case with uniform heating of the wall. The transition of a symmetric velocity field from two vortices to an asymmetric field with many vortices leads to a decrease in entropy generation with an increase in the Rayleigh and Marangoni numbers. The heat transfer coefficient in the liquid decreases with increasing layer height from 1.4 mm to 5 mm.