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.

Subcooled boiling flows are encountered in most nuclear reactor configurations. Wall heat flux partitioning models form an integral part of the subcooled boiling formulations in CFD codes. These models attempt to describe the flow of heat from the wall into the fluid by dividing it according to several mechanisms of heat transfer. This work presents a one-dimensional evaluation of the wall heat flux partitioning model of Kurul and Podowski, also referred to commonly as the RPI model, which is used in the state-of-the-art codes of today. This model was assessed against the measurements of Okawa et al. for a vertically upward subcooled boiling flow of water at near atmospheric pressure. Although the predictions showed good agreement with the measured wall temperatures, significant discrepancies were observed in the predictions of the constituent sub-models that comprised the overall model. Prospects for improvement are discussed.

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