Book Description

The post-critical heat flux (post-CHF) heat transfer is a complex two-phase flow phenomenon divided in two regions: a so-called 'transition boiling' regime characterized by the rapid degradation of heat transfer from a heated surface to a liquid/vapor mixture, and a so-called 'stable film boiling' regime characterized by heat transfer from the heated surface to the vapor. The limit between these two regimes is usually referred as the minimum film boiling temperature (TMFB), and has been interpreted as a temperature from which the liquid no longer can contact the heated surface. To better understand these phenomena, experiments were performed at LWR conditions in the High Pressure Heat Transfer Facility at the University of Wisconsin-Madison. These experiments and subsequent prototypic experiments from the KATHY facility, in Germany, contribute to the limited experimental database on this topic. KATHY data were collected under BWR conditions during Anticipate Transient Without Scram that leads to cycles of dryout and rewet events and ultimately to a failure to rewet of at least one thermocouple. The data were analyzed using a 2D inverse heat transfer computational tool to obtain the surface heat fluxes and temperatures during post-CHF. It was observed that the transition to the film boiling heat transfer regime is a hydrodynamic mechanism dependent on both the pressure and the mass flux, while the local quality affects the mechanism of film formation between a film dryout at high qualities and bubbles coalescence at low qualities. The homogeneous nucleation temperature, which is a thermodynamic mechanism, acts as a lower bound for this transition temperature. The analysis also quantifies different transient boiling curves for flow regimes that differ qualitatively and quantitatively from the typical boiling curve considered in steady-state two-phase heat transfer analysis. The results provide insights into boiling heat transfer and they can change the way we currently model the two-phase post-CHF.