Assessment of semi-mechanistic bubble departure diameter modelling for the CFD simulation of boiling flows

Abstract

In Eulerian-Eulerian two-fluid computational fluid dynamic (CFD) models, increasingly often applied to the prediction of nucleate boiling in nuclear reactor thermal hydraulics, boiling at the wall is usually accounted for by partitioning the heat flux between the different mechanisms of heat transfer involved. Between the numerous closures required, the bubble departure diameter in particular has a significant influence on the predicted interfacial area concentration and void distribution within the flow. In the present work, and following evidence of the limited accuracy and reliability of the empirically-based correlations which are applied normally in CFD models, more mechanistic formulations of bubble departure have been introduced into the STAR-CCM+ code. The performance of these models, based on a balance of the hydrodynamic forces acting on a bubble, and their compatibility with existing implementations in a CFD framework, are assessed against two different data sets for vertically upward subcooled boiling flows. In general, a significant amount of modelling is required by these mechanistic models and some recommendations are made on different modelling choices. The model is extended to include a more physically-consistent coupled calculation of the frequency of bubble departure and the modelling of the local subcooling acting on the bubble cap is analyzed. In general, predictions of void distribution and wall temperature reach a satisfactory accuracy, even if numerous numerical and modelling uncertainties are still present. In view of this, several areas for future work and modelling improvement are identified.

Modelling of Boiling Flows for Nuclear Thermal Hydraulics Applications—A Brief Review

“…One of these approaches is the popular 'force-balance' method of Klausner [25][26][27][28][29], which, as indicated in Figure 6, computes the size of a bubble at departure from a solid surface based on evaluation of hydrodynamic (drag force F DF , lift force F L and liquid reaction to bubble expansion F H ), surface tension, gravity and wall-adhesion forces.…”

“…Recent efforts [27,35] aimed to improve CFD prediction of boiling heat transfer using data generated with semi-mechanistic bubble departure models. In [24] in particular, popular 'force balance' models were used to compute bubble departure diameters and used in CFD simulation of boiling flows.…”

“…Predicted bubble departure diameter (a) and wall temperature (b) in typical vertical subcooled flow boiling conditions, from Colombo et al[27]. Lines correspond to different methods to compute bubble departure diameters: Tolubinskiy et al correlation[23] (yellow lines), Kocamustafaogullari et al correlation[21] (green lines), Klausner et al model[25] (red lines), Sugrue et al model[28] (black lines), Yun et al model[34] (blue lines).…”

Numerical investigation of flow boiling characteristics of water in a rectangular microchannel

“…Bubbles are formed and rise from the bottom surface to the free surface in the microchannel. The quantity of the vapour fraction depends on the heat flux and mass flow rate [8,9]. Xin et al [10] predicted void fractions for two-phase flow by Lockhart-Martinelli correlation.…”

“…The simulations are accomplished with the commercial CFD software Ansys-Fluent where these VOF model, MM, EM are already incorporated. These model's continuity equation, momentum equation, energy equations are taken from a fluent theory guide[1][2][3][4][5][6][7][8][9][10] and presented here for quick assessment. Flow boiling models like VOF, MM, EM equations are expressed below:…”