Numerical simulation of heat transfer during growth of single vapor bubbles in nucleate boiling

Abstract

In the model presented in this paper the region around a single growing vapor bubble in nucleate boiling is subdivided into three parts: a small, ring-shaped zone between heating wall and bubble, called micro-region, the bubble itself, and its surrounding liquid, referred to as macro-region. As the micro-region is most important for heat transfer, a recently developed model put special emphasis on this region, and predicted heat transfer, bubble growth, and departure diameters of vapor bubbles for low to moderate heat fluxes fairly well. The current paper aims at modeling the macro-region in more detail than done before. For this purpose, Navier–Stokes equations for both vapor and liquid phase were solved with the aid of a finite element method. It turned out that the flow pattern in the liquid around a growing vapor bubble is determined by the movement of the bubble surface, but also by the vapor flow inside the bubble. The fluid in the macro-region transports cooler liquid towards the wall, thus increasing the heat transfer significantly. In regions farther away from the bubble, heat conduction prevails. Buoyancy was found to be of small influence compared to forced convection. Velocity and temperature fields, heat fluxes, bubble contours, and departure diameters were calculated for different fluids. The apparent contact angle is decisive for growth rate and departure diameter. Disturbances, caused by departing and ascending bubbles, are included into the calculations.