A model on bubble dynamics in liquid film boiling

Liquid film boiling, where bubbles are generated together with evaporation in several hundred micrometers thick liquid film, has attracted great interest recently due to its potential in dissipating high heat flux with low superheat. However, the existing models on bubble dynamics based on pool boiling are not suitable for predicting the bubble behaviors in liquid film boiling. Here, we develop a theoretical model to study bubble dynamics (including nucleation, growth, and departure) in liquid film boiling on the horizontal surface. By considering the evaporation atop the liquid film surface, we solve the transient heat conduction in the liquid film, and then derive the waiting period for bubble nucleation. The bubble growth rate is computed by taking into account evaporation from both superheat liquid layer and microlayer. Bubble departure diameter is obtained by considering the surface tension force atop the liquid film and reconstructing the pendant bubble shape based on Young-Laplace equation. It is shown that when liquid film thickness reduces, the bubble waiting period increases, while the bubble growth period and growth rate decrease. By predicting the heat transfer based on bubble dynamics, we find that the enhanced heat transfer in liquid film boiling, compared to pool boiling, mainly benefits from the significantly increased bubble departure frequency due to reduced departure diameter.