Development of mechanistic approach for wall heat flux partitioning in subcooled boiling flows
Creators: name: Promtong, Machimontorn
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401204
subject: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)
classificationCode: 401706
subject: Numerical modelling and mechanical characterisation
classificationCode: 490399
subject: Numerical and computational mathematics not elsewhere classified

description: The wide range of industrial applications involved with boiling flows promotes the necessity of establishing fundamental knowledge of boiling flow phenomena. For this purpose, in the past several decades, a number of experimental and numerical studies have been performed to elucidate the underlying physics of this complex flow. 
This research focused on introducing and developing a Computational Fluid Dynamics (CFD) technique to predict the subcooled boiling flow. Instead of using the traditionally empirical correlations, a proposed mechanistic approach (originally formulated by Yeoh et al. [1]), consisting of fractal analysis model, force balance method, and mechanistic frequency model, was simultaneously proposed and coupled with the Eulerian-Eulerian Two-Fluid framework for capturing the complex heat and mass transfers especially at the nucleation sites of vapour bubble generation on the heated surface. A Population Balance Method (PBM), named the Multiple Size Group (MUSIG) model, was also introduced to handle bubble interactions and predict bubble size distribution. 
At the early stage, the mechanistic approach was introduced to investigate the boiling flow under low pressure (1.37-1.97 bars) operations. The approach’s prediction accuracy was evaluated using a modelling assumption of bubble sliding along the wall before lifting off, which is usually found in boiling flow. By accommodating the variable materials, like Wet-Steam (IAPWS-IF97), the actual capability in terms of the approach’s prediction accuracy could be assessed, and the realistic phenomenon of subcooled boiling flow could be explored from this modelling. This validation study was performed over a wide range of flow conditions. As a result, simulations of the subcooled boiling flow by using the constant-property liquid and the Wet-Steam both showed good agreement for the void fraction with Lee’s experiments [2]. While introducing them for predicting Yun’s cases [3, 4] (the height of heated-section is almost twice of Lee’s cases), the void fraction and the bubble Sauter Mean Diameter (SMD) of the Wet-Steam cases had better agreement with the experimental data. This investigation could reveal an outstanding performance of introducing the Wet-Steam together with the mechanistic wall portioning model for predicting the subcooled boiling flow. 
Afterward, the mechanistic wall boiling models were further introduced to elucidate the subcooled boiling flow at elevated pressure (4.97-9.49 bars). Since the more accurate predicted results had been found in the case using the Wet-Steam properties, in this evaluation study, the variable properties were again introduced for the realistic simulation. Existing experimental data at elevated pressure (Ozar’s experiments [5]) were chosen to evaluate the accuracy of the presented mechanistic approach. The void fraction and Interfacial Area Concentration (IAC) are in good agreement with the experiments; however, the bubble velocity and bubble Sauter Mean Diameter (SMD) are over-predicted. This over-prediction may be caused by a consideration of only dispersed and spherical bubbles (Group-1) in the simulations. In reality, the merging with neighbouring bubbles before detachment from the heated rod may lead to bigger bubbles (Group-2) and lower bubble velocities. Hence, the effect of turbulence models and the development of PBM (by embedding Group-2 bubbles in the modelling) may improve the current predictions of bubble velocity. 
At the later stage, the mechanistic approach was further developed to consider more essential physics of bubble dynamics, including i) conditions of sliding, ii) merging during sliding, and iii) merging without sliding, which usually happens along the vertical hot surface before leaving the nucleation site.