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• A mechanistic model is proposed to study the bubble growth process in sodium pool boiling. • The model couples a heat transfer, force and contact angle sub-model to compute the bubble growth rate and departure radius. • Bubbles in sodium tend to be large with departure radius of the order of centimeters. • Lower the wall superheat, greater is the tendency of a sodium bubble to have a balloon-like shape at departure. This work presents a mechanistic model to simulate the growth of a sodium bubble from nucleation to departure in sodium pool boiling. A previously developed and validated heat transfer sub-model is coupled to a force balance sub-model to predict the growth rate and departure radius of a sodium bubble. The model accounts for the change in the contact angle of a bubble as it grows, and the shrinkage of the bubble base prior to departure. The developed model is used to quantify and analyse the heat transfer from different regions, i.e. the microlayer, the macrolayer, the thermal boundary layer and the bulk liquid surrounding the bubble. In addition, bubble growth rate and departure radius are calculated for different values of wall superheat, rate of change of contact angle and bulk liquid temperature. It is found that the departure radius of a sodium bubble is on the order of a few centimetres and the wall superheat has a significant influence on the shape of a sodium bubble at departure. [ABSTRACT FROM AUTHOR]

A mechanistic model to study the growth of a bubble from nucleation to departure in pool boiling is proposed in this work taking into account the change in shape of a bubble as it grows. The model consists of three sub-models: (a) a heat transfer sub-model to compute the bubble growth rate based on the evaporation of the microlayer, the macrolayer, the thermal boundary layer and the bulk liquid surrounding the bubble; (b) a force sub-model to calculate the forces acting on a bubble; and (c) a contact angle and bottleneck sub-model to account for the change in shape of the bubble. Analysis of past experiments on bubble growth in nucleate pool boiling has shown that the shape of a bubble transitions from a truncated sphere to a balloon-like shape before departure. In the present work, a novel method to model this balloon-like shape of the bubble as a truncated sphere atop a conical bottleneck is presented. The model is validated against high-fidelity CFD simulations and pool boiling experiments of water and methanol from literature. In particular, the bubble departure time, wall temperature, bubble shape and microlayer profile obtained from the model are in good agreement with experimental and CFD results. • A mechanistic model is developed to study the growth of a bubble from nucleation to departure in pool boiling. • A balloon-like shape of the bubble prior to departure is modelled as a truncated sphere atop a conical bottleneck. • Bubble growth rate is modelled based on the heat transferred from different mechanisms. • Good agreement between the modelled contact radius with experimental data is demonstrated. • Transition in bubble shape is qualitatively well-predicted by the model. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Contact line region in sodium boiling flows is studied; • Compared contact line characteristics of high and low Prandtl number fluids; • Analyzed factors characterizing sodium boiling-high superheat, electron pressure; • Evaporative heat flux decreases with increase in electron pressure. 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. [ABSTRACT FROM AUTHOR]