A BUBBLE-LAYER-BASED MECHANISTIC MODEL FOR THE SLIGHTLY SUBCOOLED FLOW BOILING IN VERTICAL TUBES

Abstract

Bubble behaviors and interactions are quite complex in the slightly subcooled flow boiling including sliding, lift-off and condensation. Currently, the predicting of this process mainly relies on numerical simulations combined with bubble-behavior-related sub-models, or a combination of one-dimensional empirical correlations without considering bubble behaviors. In contrast, the theoretical analysis is still lacking. Therefore, the present paper aims to develop a mechanistic model for this process which divides the flow field into the bubble layer region and the core region based on a bubble layer adjacent to the wall. Bubble behaviors in each region, mass and energy exchanges at the interfaces of different regions, and variations of parameters along the channel direction are considered and analyzed through a new set of two-dimensional steady-state conservation equations of mass and energy combined with bubble-behavior-related sub-models. Based on this model, void fraction and liquid temperature distributions are obtained which are verified with experimental results with fairly good accuracy. In addition, distributions of velocity and mass flow rate in each region in the axial direction, and mass flow rate between different regions in the radial direction are also obtained and analyzed. In comparison, the present model can reveal more detailed information than one-dimensional correlations and provides a simple, fast and stable method for the predictions of subcooled flow boiling especially for industrial applications.