A Simulation of Bubbly Flow Regime in Boiling Process by Comparing the Numerical Models of VOF and RPI Boiling

Boiling process in a heated tube is commonly used in different industries such as electronic equipment cooling, power plant, and air conditioning systems. Despite the significance of thoroughly and separately analyzing of heat transfer in different two-phase flow regimes encountered in boiling process, just a few simulations have been conducted. That is because of the lack of proper understanding of the many numerical methods that are now in use and their relative efficacy under various circumstances. This leads to dispersed effort and the application of disparate numerical methods, which incurs significant computational expenses. In this study, Eulerian-Eulerian approach was used to simulate the bubbly flow, which includes vapor bubbles in the rising water flow within a vertical tube. In order to identify the optimal numerical model and the extent of application of available numerical models in the simulation of bubbly flow, volume of fluid (VOF) and Eulerian boiling model of Rensselaer Polytechnic Institute (RPI) models were compared and evaluated. Results demonstrated that while the RPI boiling model results are more appropriate for estimating the heat transfer coefficient and wall temperature in this regime, the VOF model is more effective than the RPI model at simulating the regime, bubble formation and interface between phases. Moreover, RPI model was used to examine how changes in wall heat flux and inlet mass flow rate affected effective parameters. Results revealed that in the bubbly flow regime, a 100% increase in wall heat flux relative to its original value of 5000 W/m2, resulted in a 150% increase in the outlet vapor quality, a 75% rise in temperature difference between the wall and the saturation temperature, and a 20.8% increase in the mean wall heat transfer coefficient. Furthermore, by increasing the inlet mass flow rate, the nucleate boiling zone increases and the outlet vapor quality decreases.