Evaporation Momentum Force and Its Relevance to Boiling Heat Transfer

Abstract

As a liquid evaporates into its vapor, the vapor phase leaves at a higher velocity than the approaching liquid and exerts a net momentum force on the evaporating interface. This force is especially relevant in the contact line region where liquid temperature is higher than the bulk liquid, and local saturation temperature is reduced due to curvature effects. These factors result in an increased evaporative flux resulting in higher evaporation momentum force that can influence the interface motion and bubble trajectory. This force provides a new mechanism for enhancing boiling heat transfer by altering the individual bubble trajectory. In microchannels, it can lead to flow instability. These effects are critically evaluated in this paper and their relevance to bubble growth and heat transfer phenomena during pool and flow boiling is presented. Two nondimensional groups K1 and K2, respectively, representing the ratio of evaporation momentum force to inertia and surface tension forces, have been used in modeling heat transfer and interface motion. Evaporation momentum force has been successfully applied in modeling critical heat flux (CHF) in pool and flow boiling, analyzing instability during flow boiling in microchannels, controlling individual bubble motion, and enhancing CHF and heat transfer coefficient (HTC) during boiling on flat surfaces as well as tubular geometries.