CFD Prediction of High-Pressure Subcooled Boiling Flow with Semi-Mechanistic Bubble Departure Diameter Modelling

Most water-cooled nuclear reactors experience some subcooled boiling during normal operation and all will under fault conditions. In the current work, the commercial CFD code STAR-CCM+ is applied to predict the popular case of Bartolomei & Chanturiya for a vertically upward subcooled boiling flow of water in a uniformly heated pipe at 45 bar. Special emphasis is placed on the prediction of the diameter of bubbles that depart from the heated wall. This is an important closure relation that exerts a large influence on the source terms for vapor generation and interfacial area transport, and consequently on the predicted void distribution. To this end, recent approaches based on a more intricate semi -mechanistic analysis of forces acting on the bubble were introduced into the established CFD modelling framework. The behaviour and capability of this 'force -balance' style of approach was compared against conventional approaches. Agreement with the available void and wall superheat measurements was found to be acceptable. The force -balance method predicted departure diameters that were comparable to those predicted using the existing state-of-the-art correlations (0.01 0.1 mm) and in good agreement with the best available optical measurements under these conditions. The results also suggested that the balance of forces might be dominated by the effects of drag rather than lift at the conditions of interest, causing bubbles to slide along the wall rather than to lift off from it. Several numerical and modelling uncertainties are highlighted. Prospects for modelling improvement and suggestions for future work are identified.