A new approach to predicting departure from Nucleate Boiling (DNB) from direct representation of boiling heat transfer physics

Abstract

Accurate prediction of the Departure from Nucleate Boiling (DNB) type of boiling crisis is essential for the design of Pressurized Water Reactors (PWR) and their fuel. Recent advances in the instrumentation of boiling experiments via infrared thermometry have provided new insights on DNB physics that were ignored in past modeling efforts. The growing consensus from experimental studies that DNB is caused by a microhydrodynamic phenomenon at the boiling surface, instead of the classical macrohydrodynamic scale effects, has yet to be formulated into a working model and applied for simulation of high pressure conditions representative of PWRs. While the idea of an energy imbalance between wetted (in contact with liquid) and dry (in contact with vapor) areas has been suggested by multiple experimental groups [1-6] as a triggering mechanism for DNB, incorporating this new understanding into a functional boiling model remains an open challenge.

Existing approaches to model DNB have not demonstrated the capability to generally "predict" the occurrence of the boiling crisis due to two main reasons: (1) the incomplete or inaccurate physical description of the phenomenon, and/or (2) the absence of consideration of parameters of influence, such as the contact angle or the cavity size distribution which affect DNB. Their modeling capabilities are typically limited to the thermal hydraulic conditions they were initially validated against and are not suitable when extrapolated to new untested conditions. The present work tackles the development of a new DNB model via a systematic approach that leverages understanding of the boiling crisis at the microscopic scale. From the hypothesized mechanism, a formulation is proposed and is then validated against high resolution data.

The concept of heat partitioning for nucleate boiling offers a general framework where the total heat removed at the boiling surface is split between contributions from individual heat transfer mechanisms. In this formulation, the boiling crisis is identified as the onset of instability in the energy balance at the boiling wall. The model formulation is extended to high pressure, representative of PWR conditions, and benchmarked against relevant DNB data, in comparison to the industry-standard W-3 correlation and CHF Lookup Table. The model is further adopted to evaluate the potential benefits of surface property alterations (contact angle, cavity distribution) in advanced nuclear fuel concepts aiming at delaying the boiling crisis limit.