The role of convection in gas and liquid phases at droplet evaporation

The article presents the measuring results of droplet velocity and temperature fields using non-contact optical methods: Particle Image Velocity (PIV), Planar Laser Induced Fluorescence (PLIF) and Thermal imager. The novelty of the work is that the influence of free convection in gas and liquid is investigated experimentally and theoretically and that the key criteria affecting heat and mass transfer are determined. The analysis of experimental data has shown that in the initial period of water drop evaporation, the predominant role in the heat exchange is played by the thermal Marangoni convection. However, for an aqueous salt solution, in spite of the strong influence of the surfactant, the dominant role passes to the solutal Marangoni convection (Mac). In the first seconds after the drop falling, convection and heat transfer in liquid are maximal. Under such conditions it is important to realize an accurate numerical simulation to assess the degree of wall cooling and calculate the non stationary evaporation. When simulating heat transfer, it is incorrect to neglect free convection in gas or liquid due to their strong nonlinear influence on each other. The heat exchange in the drop is extremely conservative to convection in the liquid (the Peclet number Pe = 100 and the Nusselt number Nu = 4).