ICMS ' 2000 The Physics of Boiling at Burnout
The basic elements of a new experimental approach for the investigation of burnout in pool boiling are presented. The approach consists of the combined use of ultrathin (nano-scale) heaters and high speed infrared imag|ng of the heater temperature pattern as a whole, in conjunction with highly detailed control and characterization of heater morphology at the nano and micron scales. It is shown that the burnout phenomenon can be resolved in both space and time. Ultrathin heaters capable of dissipating power levels, at steady-state, of over 1 MW/m 2 are demonstrated. A separation of scales is identified and it is used to transfer the focus of attention from the complexity of the two-phase mixing layer in the vicinity of the heater to a micron-scaled microlayer and nucleation and associated film-disruption processes within it.
A Theoretical Analysis of Heat Transfer in Natural Convection and in Boiling

 A theoretical analysis of natural convection and of boiling heat transfer is presented. Instead of approaching these problems according to a conventional concept, a short-cut method is introduced by the application of wave motion. Above the heating surface a boundary layer is assumed whose thickness depends upon the heat flow. Inside this layer there is wave motion, stable in the lower part but unstable in the upper. It has been recognized that, if there is a temperature gradient across a stratum of liquid, a wave motion will occur. Each loop of this wave can initiate vortexes in the natural-convection process and also can facilitate the formation of bubbles in boiling. A unified formula is thus obtained for convection and for boiling. The calculated results agree excellently with experiments as conducted by previous investigators. This paper does not profess to be a complete treatise, but shows how the concept of wave motion can be made of use in giving a new picture of these heat-transfer processes.
The coolability limits of a reactor pressure vessel lower head
Configurations II and III of the ULPU experimental facility are described, and results from a comprehensive set of experiments are provided. The facility affords full-scale simulations of the boiling crisis phenomenon on the hemispherical lower head of a reactor pressure vessel submerged in water, and heated internally. Whereas Configuration I experiments (published previously) established the lower limits of coolability under low submergence, pool-boiling conditions, with Configuration II we investigate coolability under conditions more appropriate to practical interest in severe accident management; that is, heat flux shapes (as functions of angular position) representative of a core melt contained by the lower head, full submergence of the reactor pressure vessel, and natural circulation. Additionally, with Configuration III, we examine the effect of a channel-like geometry created by the reactor vessel thermal insulation. Critical heat fluxes as a function of the angular position on the lower head are reported and related to the observed two-phase flow regimes.