Understanding triggering mechanisms for critical heat flux in pool boiling based on direct numerical simulations

Abstract

Boiling is a ubiquitous process in many applications including power generation, desalination, and high-heat flux electronic cooling. At the same time, boiling is a complicated physical process involving hydrodynamics and interfacial heat and mass transfer on multiple scales. One of the key limiting factors of boiling is the critical heat flux (CHF), beyond which a vapor blanket forms on the heating surface and catastrophic device burnout occurs. Yet, detailed understanding of the mechanism that triggers CHF remains elusive. In this paper, we elucidate the CHF mechanism by studying the evolution of wet/dry region on the heater surface using lattice Boltzmann simulations. We incorporate the equation of state for real gases in the liquid-vapor phase change model for direct numerical simulations of the CHF phenomenon. The results of this framework clarify the difference between the triggering mechanism of CHF and film boiling by analyzing the pool boiling curve. We demonstrate that the heat flux of the wet region on the heater surface increases while the wet area fraction decreases with superheat, leading to the CHF. We show that a vapor recoil force due to the interfacial heat and mass transfer plays an important role for the evolution of wet area fraction and therefore contributes to the occurrence of a second transition regime and CHF. Compared with previous CHF models which treat CHF as an isolated point on the boiling curve, this work elucidates the triggering mechanism of CHF from a perspective of the dynamic evolution of the wet/dry region with increasing superheat, which could potentially serve as a guideline for future CHF enhancement designs.