Heat transfer modelling of an isolated bubble in sodium pool boiling

Abstract

Sodium tubular boiler receivers in concentrated solar thermal power plants are a viable option to provide near-isothermal heat for industrial applications. Accurately predicting the boiling behaviour in the receivers is imperative to improve the performance of these plants. A physics-based reduced-order bubble growth model is developed in this work to gain a fundamental understanding of the boiling process in the receiver. The model predicts the growth rate of an isolated bubble in a sodium pool based on heat transferred from the microlayer, the macrolayer, the thermal boundary layer and the bulk liquid surrounding the bubble. A transient 2D conduction equation is solved to model the cooling of the wall below the bubble due to evaporation of the microlayer and the macrolayer. The model is used to study the growth of a sodium bubble in a liquid pool and analyse the relative contribution of different heat transfer mechanisms to the growth process. It is found that the microlayer heat transfer is the dominant mechanism controlling bubble growth in sodium. Furthermore, a parametric study of the effect of wall superheat, contact angle and temperature of the bulk liquid on the bubble growth process in sodium shows that the bubble size increases with increasing superheat, contact angle and bulk liquid temperature.