An enhanced model for the prediction of slug flow boiling heat transfer on a downward-facing heated wall

Abstract

This paper presents a mechanistic wall boiling model for the prediction of flow boiling on a downward-facing oriented heating surface containing sliding slug bubbles on the heating surface at the pre-critical heat flux condition. Essentially, this boiling model is an extension of the well-known RPI boiling model which was initially developed based on dispersed small spherical bubbles on the heated wall. The fundamental assumption of the RPI model sharply contrasts with the real situation during flow boiling on a heated wall facing downward because of the existence of large-scale interface bubbles (vapor slugs) that are virtually attached (separated from the wall by a thin liquid film) to the wall due to buoyancy effect. Therefore, the existing wall boiling model might not be adequate for a situation like this because the heat transfer governing mechanisms of vapor slug (deformable shape) is dissimilar to that of dispersed spherical bubbles. In this article, a liquid film conduction model that represents the wall heat transfer of slug bubble is coupled with the existing wall boiling model and the new extended wall heat flux partitioning model is administered within the solution framework of hybrid multiphase flow model in OpenFoam. The hybrid model combines Eulerian multi-fluid model (for the dispersed regime) and volume of fluid model (for the large-scale interface vapor slug regime). With the hybrid multiphase flow model, different multiphase morphologies coexisting in the flow are duly accounted for. The extended wall boiling model shows an improved prediction of the experimental data during flow boiling on a downward-facing heated surface when compared with the prediction obtained with the existing wall boiling models.