Mechanistic modeling of nucleate boiling

Abstract

The existing nucleation site density models assume that boiling occurs when the wall temperature reaches the fluid saturation temperature. This assumption is not consistent with the fact that wall superheat of a few degrees is always observed at the onset of nucleate boiling. A new model for nucleation site density is developed to take into account the non-uniform temperature in the liquid near the wall, which allows one to physically impart the onset of nucleation into the nucleation site density model.

Accurate prediction of nucleate boiling in reactor coolant channels is an important factor in the analysis of the operating conditions of both pressurized water reactors (PWRs) and boiling water reactors (BWRs). Its importance arises from purely thermal-hydraulic considerations (thermal limits, CRUD formation, etc.) and the impact of void reactivity feedback on core neutronics. In the simulation of nucleate boiling, the two-fluid model, a macroscopic formulation of two-phase systems based on appropriate averaging of local instantaneous balance equations, is widely used. The evaluation of averaged interfacial mass and energy transfer in the two-fluid model at the boundary of the physical domain has been commonly based on empirical correlations deduced from selected experimental data, such as those for nucleation site density and wall superheat, which introduces considerable uncertainties into the predictions of the overall two-fluid model.

The focus of this work is on the development of mechanistic models for wall superheat and nucleation site density. The major parts include: an improved wall heat flux partitioning model, new mechanistic models for the bubble ebullition cycle at low and high heat fluxes, and a new physically consistent model for nucleation site density. These models have been validated against experimental data and good agreement has been reached.

The improved wall heat flux partitioning model, derived from the integral energy balance equation, accounts for the condensation effect on the cap of the bubble. This effect is important at high subcooling, at which bubbles may collapse before departure, or maintain a certain size on the wall. At a relatively low subcooling, bubbles depart from the wall and the condensation effect is relatively small. Instead of averaging the local instantaneous energy balance equation for the bubble ebullition cycle, wall heat flux partitioning models are based on using the average (constant) wall surface temperature as a common reference to evaluate the heat flux components of the overall wall heat flux. However, in various time- and space-resolved boiling experiments, the wall surface temperature has been observed to fluctuate during the bubble ebullition cycle. Considering a non-linear variation of the quenching component in the wall heat flux partitioning models with wall superheat, the existing models are not fully consistent with the underlying physical phenomena or with the two-fluid model formulation. New mechanistic models for the bubble ebullition cycle at low and high heat fluxes, based on the local instantaneous energy balance equation, are proposed to capture the transient wall superheat fluctuations in the bubble influence area, which can be averaged to obtain the wall superheat needed in the two-fluid model.