Force balance model predictions of sliding bubbles velocity in vertical subcooled boiling flow

Abstract

In flow boiling systems, bubbles tend to slide along the wall as they depart from nucleation sites where they originate. This behavior has a profound impact on the wall heat transfer as evidenced in previous experiments. As a result, the heat transfer modeling associated with sliding bubbles has gained great attention within the CFD boiling heat transfer community. The modeling efforts are primarily aimed at improving the Heat Flux Partitioning (HFP) model by accounting for the heat transfer mechanisms introduced by sliding bubbles. The performance of the HFP model depends inherently on the accuracy of sub-models to predict the fundamental bubble parameters, such as bubble departure and lift-off diameters. Similarly, the accurate prediction of sliding bubble parameters, such as sliding bubble growth and sliding bubble velocity, is critical to improving the HFP model when taking into account the effect of sliding bubbles. Of the many sliding bubble parameters, this paper deals with the sliding bubble velocity. Specifically, a force balance model is presented and assessed in view of the sliding bubble velocity prediction. The experimental results from Maity (2000)[1] and Yoo et al. (2016)[2] are used to demonstrate the force balance model accuracy under various subcooled boiling conditions of both, water and refrigerant (NOVEC-7000) flowing upward in vertical channels. We performed a parametric study of the key sub-models (e.g., drag force, bubble growth) used in the force balance equation, together with a direct comparison with proposed models from other researchers. Then, a new force balance model is proposed which outperforms previous models and consistently well-predicts both sliding bubble velocity and growth.