Simulation of void fraction profile evolution in subcooled nucleate boiling in a vertical annulus with a bubble-tracking model

A three-dimensional bubble-tracking model of subcooled nucleate boiling flow in a vertical channel at low-pressure conditions is proposed, with specific application to the case of boiling in an annulus with a central heating rod. In the model, vapour is distributed in the liquid in the form of individually tracked bubbles. The overall behaviour of the liquid-vapour system results from motion, interaction, coalescence and boiling mechanisms prescribed mostly at the level of bubbles. The wall heat transfer coefficient and the wall temperature are calculated from one-dimensional correlations. The partitioning of the heat flux, which is consumed for bubble nucleation and heating of the liquid, varies along the flow and depends on bubble size as well as on local flow conditions. Bubbles are nucleated with constant frequencies at fixed nucleation sites randomly distributed over the heated surface. Liquid temperature profiles at different axial locations are determined from steady-state energy balances. The nucleation site density is determined from a balance between vapour generation rate, bubble departure sizes and nucleation frequencies. After nucleation, bubbles slide on the heated surface, detach and then gradually migrate into the low-temperature region away from the heated surface, where they eventually condense. Both bubble detachment and migration are modelled probabilistically. Bubble lateral migration is restricted by the lift force due to the liquid velocity gradient. The proposed model was applied to experiments on subcooled boiling that were carried out at Purdue University (USA) by Bartel [1]. A good agreement between measured and calculated void fraction profiles at different axial locations was obtained.