Toward Mechanistic Wall Heat Flux Partitioning Model for Fully Developed Nucleate Boiling

Abstract

Mechanistic models developed to predict partial nucleate boiling are not adequate for fully developed nucleate boiling due to differences in the prevailing heat transfer governing mechanisms. In place of the mechanistic model, several empirical correlations and semi-mechanistic models have been proposed over the years for the prediction of fully developed nucleate boiling as presented in this study but they are unsuitable for use in computational fluid dynamics (CFD) code. Recently, the simulation of fully developed nucleate boiling has become much more practical because of advancement in a computational method that involves the coupling of the interface capturing method (for slug bubbles) with the Eulerian multi-fluid model (for dispersed spherical bubbles). Nonetheless, there is a need for a mechanistic closure law for the fully developed nucleate boiling phenomenon that would complement this advancement in CFD. Towards this end, a mechanistic wall heat flux partitioning model for fully developed nucleate boiling is proposed in this study. This model is predicated on the hypothesis that a high heat flux nucleate boiling is distinguished by the existence of a liquid macro-layer between the heated wall and the slug or elongated bubbles and that the macro-layer is interspersed with numerous high frequency nucleate small bubbles. With this hypothesis, the heat flux generated on the heated wall is partitioned into two parts: conduction heat transfer across the macro-layer liquid film thickness and evaporation heat flux of the microlayer of the nucleating small bubbles. The proposed model is validated against experimental data.