A new mechanistic model for predicting flow boiling critical heat flux based on hydrodynamic instabilities

CHF is an important design parameter in two-phase thermal management systems utilizing flow boiling schemes. In this study, a new theoretical model for predicting the trigger mechanism for flow boiling CHF is presented. Flow visualization shows that flow boiling on a heated wall is sustained mostly by the region containing discrete vapor bubbles called the wetting front because it allows the heated wall continuous access to the bulk liquid. CHF is triggered when intense evaporation of the liquid in the wetting front causes combined effects of parallel-to-flow direction interfacial stability and perpendicular-to-wall direction Helmholtz instability generating a continuous vapor patch on the wetting front. The new mechanistic CHF model incorporates three sub-models: a separated flow model to calculate two-phase flow parameters, an interfacial instability model to calculate the critical wavelength of parallel-to-flow vapor wave, and a Helmholtz instability model to calculate the critical wavelength of perpendicular-to-wall liquid-vapor interface. The CHF model is used to predict experimental data for a rectangular channel with single-sided heating in Earth gravity and microgravity for a variety of operating conditions. The model predictions show good agreement with experimental data, evidenced by an MAE of 15.36%. (C) 2019 Elsevier Ltd. All rights reserved.