A numerical analysis of the evaporation process of small water droplets with diameters of 1 mm or less that are gently deposited on a hot isothermal solid surface has been performed. This study considers the internal fluid motion that occurs as a result of the thermocapillary convection in the droplet and it determines the effect of fluid motion on the heat transfer between the drop and the solid surface. This study is particularly relevant because the internal fluid motion has not been considered in previous numerical and analytical models presented in the literature. To assess the effects of internal fluid motion, the model results are compared to numerical results provided by a heat conduction model that neglects the fluid motion. The Navier-Stokes and Thermal Energy equations are solved using the Artificial Compressibility Method with Dual Time Stepping. Boundary-fitted grids are used to track the changes in the droplet surface shape during the evaporation process. The numerical simulations have demonstrated that the internal fluid motion provides vastly different temperature distributions in the drop compared to the results from the heat conduction model that neglects fluid motion. The evolution of the droplet geometry was simulated from an initial spherical-shaped cap until the contact angle was close to the receding contact angle.

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The effects of a small-scale, rectangular synthetic air jet on the local convective heat transfer from a flat, heated surface were measured experimentally. The synthetic jet impinges normal to the surface and induces small-scale motions by zero-net mass flux, time-periodic entrainment, and ejection of ambient air at frequencies whose periods are far higher than the characteristic thermal time scale. The velocity field between the jet orifice and the target plate is measured in planar cross sections using particle image velocimetry and is related to the local heat transfer from the plate. The present work suggests that synthetic jets can lead to substantial enhancement of the local heat transfer from heated surfaces by strong mixing that disrupts the surface thermal boundary layer. The dependence of the local heat transfer coefficient on the primary parameters of jet motion is characterized over a range of operating conditions.

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