The Influence of Local Heating on Heat Transfer in a Thin Liquid Layer

In recent years, specialists have paid significant attention to development of heat transfer enhancement methods that make it possible to reduce energy consumption and increase the heat transfer coefficient. Application of such methods is of special importance if there are high local heat fluxes and compact sizes of heat transfer devices. The use of local heating makes it possible to decrease energy consumption and enhhance the transfer processes. However, many mechanisms of heat transfer and ways for improving it still remain insufficiently understood. In the present study, the effect the laser heating power and its modulation frequency (local heating) have on heat transfer in a 1.4–5.1 mm thick liquid layer was considered. In carrying out the study, the particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods were applied for measuring velocity fields, and infrared recording was used for analyzing the temperatre fields. In the laser power modulation mode (0.05–1 Hz), high heat transfer intensity is retained when the average power is decreased by 40%. With such mode, a flow with numerous small vortices is generated, but the convection velocity in the liquid layer horizontal section decreases insignificantly (by 6–7%). The average convection velocity depends on the laser power oscillation frequency. With increasing the laser impact power by a factor of 2.5, the heat transfer coefficient increases by 20–25%. An analysis of the velocity field correlation function has revealed that with increassing the liquid layer height and decreasing the laser power, the flow becomes more chaotic in nature, which points to a change of the flow regimes. The temperature field and its nonuniformity depend essentially on the layer height and heating power. With a small layer height (1.4 mm), the heated area is localized in the center, and the longitudinal temperature gradient is maximal, whereas with a large height (5.1 mm) the temperture is distributed along the layer radius more uniformly.