The formation of bubbles on a surface during evaporation markedly influences the surface heat transfer coefficient an‎d is essential for enhancing heat transmission from the surface. The emergence, expansion, an‎d dissipation of a bubble in this phenomenon directly affect the temperature distribution an‎d flow dynamics of the adjacent liquid, especially via alterations in the bubbleʹs contact angle. The contact angle is the angle created by the tangent to the bubble surface an‎d the solid surface at the three-phase contact point. Fluctuations in the contact angle during evaporation influence the magnitude of capillary forces an‎d the spatial arrangement of the thin liquid film underneath the bubble, significantly modifying the evaporation rate an‎d the local heat transfer coefficient. A significant challenge in this process is examining the dynamic behavior of the bubble while accounting for the continual variations in its contact angle during evaporation. This numerical study examines how the dynamic behavior of the bubble contact angle influences the accuracy of predicting the heat transfer coefficient during evaporation an‎d the timing of bubble departure. This research utilized the Volume of Fluid (VOF) approach to monitor the liquid-vapor interface, while the Lee phase-change model was applied to simulate mass transfer from the liquid to the vapor phase. The influence of a dynamic contact angle was eva‎luated against a static contact angle on the predictive accuracy of the heat transfer coefficient an‎d the dynamic characteristics of the bubble, including departure time. The impact of surface wettability on contact angle fluctuations, as well as the effects of heat flux an‎d bubble geometric parameters on departure time an‎d the heat transfer coefficient, were examined. Bubble evaporation was simulated in a two-dimensional axisymmetric domain under a constant heat flux at the base of the bubble. The simulation was performed under both dynamic an‎d static contact angle situations. A comparison of the heat transfer coefficient results with the Stephan an‎d Prosser correlation indicated that the predicted heat transfer coefficient in the dynamic contact angle scenario is approximately 37% greater than in the static contact angle scenario, with a deviation of 1.4% relative to the Stephan an‎d Prosser correlation. Additionally, in the scenario of dynamic contact angle, the geometric characteristics of the bubble during evaporation (diameter an‎d contact diameter) were forecasted with an error margin of less than 4% relative to experimental findings. The precision in forecasting values in the dynamic contact angle scenario is mostly attributable to the consideration of fluctuations in thin liquid layer thickness an‎d the ongoing transmission of capillary forces during bubble expansion. The findings demonstrated that the bubble contact angle is significantly affected by the wettability of the contact surface, resulting in distinct variations in the contact angle on hydro‎phobic versus hydro‎philic surfaces, which in turn leads to divergent alterations in the heat transfer coefficient for these surfaces. The results indicate that an augmentation in heat flux, along with a reduction in both the bubble diameter an‎d contact diameter, considerably enhances the heat transfer coefficient an‎d decreases the bubble departure time.

رامين كوهي كمالي

محسن دوازده امامي , علي انصاري