Population balance modelling for subcooled boiling flow of liquid nitrogen in a vertical tube

Population balance equations combined with a three-dimensional two-fluid model were employed to predict subcooled boiling flow of liquid nitrogen in a vertical upward tube. Closure correlations describing bubble nucleation and departure on the heating surface is indispensable when modeling subcooled boiling flow using a two-fluid model. Due to the small latent heat of vaporization and surface tension, nucleation and departure of nitrogen vapor bubble has different characteristics to those of high-boiling liquids. Based on the mechanism of boiling heat transfer and the unique physical properties, some important bubble model parameters were modified to be applicable to the modeling of liquid nitrogen. In this study, some modified closure correlations of the bubble departure diameter, bubble departure frequency and density of the active nucleation sites were incorporated into the frame of the two-fluid model and the CFX code. The distribution patterns of different discrete bubble classes and void fraction in the wall-heated tube were systematically analyzed. The partition of the three components of wall heat flux along the tube and the instability of bubbly flow were also studied in the paper. Good agreement was achieved on the local heat transfer coefficient against experimental measurements, which demonstrated the accuracy of the improved model.