The flow and heat transfer in the wedge-shaped liquid film formed during the growth of a vapour bubble

Abstract

After providing simple equations that should govern the behaviour of a wedge-shaped liquid film sandwiched between the vapour in a growing bubble and the heated wall, we discuss some questions that may be important for modelling heat transfer in nucleate boiling. Considered are the curvature of the film surface and the liquid flow in the film, for example. We further examine the consequences of some simplifications adopted in models from the literature. Our analysis indicates that the bubble interface is concave-convex in the region of the three-phase-line (TPL). The convex curvature is caused by strong evaporation at the tip of the liquid wedge under no slip condition of the liquid on the wall surface. The ultra-thin layer of this wedge adhering to the wall is practically motionless and hence incapable of supplying the quantity of liquid to the film tip necessary for evaporation to occur rapidly at a fixed TPL. Based on this fact, we are led to the conclusion that, under common boiling conditions, the TPL and the film interface are not motionless, as was assumed in very recent models of bubble growth. These models are shown in the present paper to be inconsistent. Namely, the liquid flow orthogonal to the heated surface is ignored within the hydrodynamics, but this flow is indeed included into the energy balance of the models.