Mechanisms of transitions to film boiling at CHFs in subcooled and pressurized liquids due to steady and increasing heat inputs

Experimental and theoretical studies published recently on the transitions to film boiling from non-boiling regimes or nucleate boiling at critical heat flux, CHFs, on a solid surface in water and wetting liquids such as ethanol, liquid nitrogen and liquid helium I due to steady and transient heat inputs for wide ranges of subcooling and pressure are reviewed. A large part of the transition processes to film boiling at CHF highly subcooled liquids at high pressure measured was not explained by the mechanism of heat transfer crisis based on the hydrodynamic instability (HI) model. Another mechanism assumed is that the transition occurs due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities on the solid surface in non-wetting and wetting liquids. The measured all transition processes for wide ranges of subcooling and pressure in the liquids due to steady and transient conditions were explained by the HI and HSN models for lower and higher subcooling, respectively. A HSN theoretical model based on the HSN surface superheat obtained experimentally was derived. The vapor behavior on the solid surface at the points in the processes measured was observed using a high-speed video camera. The assumption of the HSN model was verified theoretically and photographically. (C) 2000 Elsevier Science S.A. All rights reserved.