Thermal transport in sodium boiling flows for concentrating solar thermal applications
Creators: name: Iyer, Siddharth
Publisher: name: The Australian National University
Year: 2023

description: Understanding heat and mass transfer phenomena in nucleate boiling of liquid metals such as sodium is an emerging field of study, in particular for the development of next generation concentrating solar thermal power plants with boiling sodium as the heat transfer fluid. The research presented in this doctoral project is focused on advancing the knowledge of sodium boiling by developing comprehensive physics-based bubble growth models. Such models can highlight the governing heat transfer and hydrodynamic phenomena dominating the bubble growth process in sodium, thus aiding the development of efficient sodium boiling systems. In the first part of this work, two numerical heat transfer models are developed with the aim of quantifying the influence of heat transfer mechanisms on the growth of a bubble in sodium pool boiling. In the first model the governing mass, momentum and energy conservation equations are solved to compute the evaporative heat flux from a region where the liquid-vapour interface of the bubble meets the wall, referred to as the contact line region. The model accounts for the influence of an electron pressure component on the evaporation of the fluid film in the contact line region in sodium. The results show that for the same wall superheat, the heat flux from sodium is six times larger compared to a high Prandtl number fluid, here FC-72, due to the high thermal conductivity of the liquid metal. The second numerical model predicts the growth rate of a sodium bubble based on the heat transferred from a microlayer (which is a thin layer of fluid formed underneath a bubble), the thermal boundary layer, and the bulk liquid surrounding the bubble. The model accounts for the variation in the wall temperature below the bubble as the liquid in the microlayer and the thermal boundary layer evaporates. Predictions from the model for a bubble growing with a constant contact angle indicate that the microlayer evaporation is the dominant heat transfer mechanism during the initial phase of bubble growth after nucleation. In addition, a parametric study conducted to study the effect of wall superheat indicated that the larger the wall superheat, the larger is the growth rate and radius of a sodium bubble. The development of a comprehensive mechanistic bubble growth model accounting for the variation in the contact angle and the shape of a bubble is pursued next. The heat transfer model that was developed based on the evaporation of the microlayer in the first part of this project is coupled to a force and a contact angle sub-model to study the complete bubble growth process from nucleation to departure in pool boiling. A novel methodology is presented to approximate the balloon-like shape of a bubble prior to departure as a truncated sphere atop a conical bottleneck. The model is extensively verified and validated against high-fidelity CFD simulations and experimental data on pool boiling of water and methanol from literature, and shows good agreement. The validated mechanistic model is then used to simulate the bubble growth process in sodium and to investigate the effects of wall superheat, contact angle rate, bulk liquid temperature and the accommodation coefficient on the bubble growth and departure characteristics. It is found that a sodium bubble is typically large with departure radius on the order of a few centimetres. In addition, it is observed that smaller the wall superheat, the greater is the tendency of the bubble to have a balloon-like shape at departure.