Experimental study of critical heat flux in pool boiling using visible-ray optics

Abstract

Liquid, vapor and their dynamics on boiling surfaces were directly observed by a simple visible-ray optics with a sub-millisecond time scale. The observation results revealed that the critical heat flux (CHF) was triggered by a localized dry region. The fast and dense nucleation of vapor bubbles, and liquid supplied to the dry regions by bubbles, afford the boiling process with extremely high heat transport capability. However, the intensive nucleation of the bubbles in a narrow region, and their coalescence, generated a large, irregularly shaped dry region at high heat flux that functioned to impede heat transfer between the boiling surface and fluid. Sequential images showing the phase distribution revealed that the gravity-driven liquid flow was obstructed by the excessive evaporation occurring around the local dry region that triggered the CHF condition. We propose a CHF triggering mechanism based on gravity and vapor recoil force balance. Under the conditions of saturated water boiling under 1 atm of pressure, these forces balance were balanced when the temperature of the dry region was approximately 130°C. The temperature limit corresponding to the non-wetting of a local dry spot was found to be in agreement with previous studies and our simulation results for transient thermal conduction on a boiling surface. © 2021