Computational Analysis of Heat Transfer Effect on Liquid–Vapor Phase Change and Boiling-Induced Phase Instabilities

Abstract

Liquid–vapor phase change (boiling/evaporation and condensation) is one of the most important phenomena in thermal management and many other applications of flow and heat transfer. Numerically, it poses challenge due to disparate densities of the phases, resulting in a stiff matrix. The presence of a third phase increases the numerical challenge. In the case of a primary domestic heater, water is heated by hot flue gas in a plate heat exchanger. It is important to avoid boiling of water in this heat exchanger and ensure a steady flow. In the present study, simplified plate heat exchanger geometry is used to predict the flow, heat transfer, and boiling in the heat exchanger in the given condition through CFD. Physically meaningful flow and phase change behavior could be predicted with moderate computational cost. Although it should be a steady process in absence of boiling, the formation of water vapor induces oscillation in the flow. It is observed that dead zones in the heat exchangers initiate local boiling and the vapor is periodically flushed out by the liquid. An oscillatory pattern is observed with a single dominant frequency, which is supported by experimental observations.