Micro-scale heat transfer modelling of the contact line region of a boiling-sodium bubble

The use of boiling liquid metals such as sodium is attractive for providing a near-isothermal heat source for engineering applications. However, previous use of boiling sodium as a coolant in nuclear reactors and as a heat transfer fluid in solar thermal applications has shown that the boiling process is unstable. To stabilise the flow, it is imperative to gain a better understanding of the boiling phenomena. An integral part of the boiling process is the evaporation of the region where the liquid-vapour interface meets the heater wall, referred to as the contact line region. The heat transfer modelling of this region formed below a single bubble in nucleate pool boiling of sodium is considered in this study. A contact line model previously developed for high Prandtl number flows is extended by including the effect of an electron pressure component which is unique to liquid metals. The assumptions made in the model are critically assessed to determine their validity for modelling micro-scale evaporation in sodium. The model was used to show that the evaporative heat flux from the contact line region in sodium can be up to six times larger compared to a high Prandtl number fluid FC-72 for a superheat of 15 K, owing to the high thermal conductivity of sodium. Furthermore, a study on the influence of specific characteristics of sodium — high boiling superheat and presence of an electron pressure — showed that the evaporative heat flux increases with increasing superheat and decreases with increasing electron pressure.