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

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 inhomoge-neous and asymmetrical at a layer height of 1.4 mm. At a layer height of 4-5 mm, the symmetry of the tem-perature 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.

Automated summary

This study experimentally investigated the heat flux, Nusselt number, heat transfer coefficient, and entropy generation under local laser heating at different water layer heights. The results showed that the temperature field on the liquid surface was inhomogeneous and asymmetrical at a layer height of 1.4 mm, while the symmetry of the temperature field increased at a layer height of 4-5 mm. The instantaneous velocity and vorticity fields on the liquid surface were studied using the optical method of Particle Image Velocimetry. It was found that local laser heating caused spatially uneven heat exchange due to the self-organization of various convective vortex structures.