Boiling Heat Transfer through Vertical Stagnant Water Annulus - Approaches for Modelling of Subcooled Critical Heat Flux

This work is devoted to the improvement and development of approaches to model boiling heat transfer in stagnant vertical water annuli. Experimental rigs for the BR2 reactor often contain long vertical annular fluid layers with a hot component at the interior and an outer wall which is cooled by the reactor primary water. The hot component wall temperature is limited thanks to the boiling process ensuring design simplicity. However, the critical heat flux (CHF) is of major importance for design optimization and safety evaluation. Therefore, this work aids quantifying the degree to which confinement and subcooling contribute to CHF degradation. Based on prevailing studies and experimental data of SCK-CEN, first an analytical analysis is made. Condensation of steam on the cold wall is shown to dominate heat transfer for gaps that are small compared to the space occupied by the vapor. Therefore, a simple analytical model is proposed which is based on a condensation correlation. On the other hand, heat transfer at elevated pressures and for large gaps is more similar to subcooled pool boiling conditions. Hence, a subcooling correction is included as a correction on a classical pool boiling correlation. Different conservative assumptions ensure that both formulations give rise to an underestimation of the measured CHF while preserving accuracy which is interesting for (nuclear) safety studies. Finally, different non-dimensional numbers, which facilitate characterization for a larger range of working conditions are discussed. In the second part of this work the three-dimensional two-fluid code NEPTUNE_CFD has been compared with the measurements of SCK-CEN near CHF. The lack of a wall condensation model and the inherent difficulty of modeling natural circulation makes NEPTUNE_CFD inapplicable for the majority of the measured data. Hence, the validity of the observation that condensation is of importance for subcooled enclosed boiling heat transfer is enforced. For the remaining cases, temperature profiles correspond well with measurement data while the bulk temperature calculation is in accordance with the analytical estimations. Additionally, new insights with respect to non-measured parameters like velocities and void fractions are obtained. Finally, inspection of temperatures in the near heated wall region is employed as a method to investigate CHF. Hence, NEPTUNE_CFD is a promising tool despite the areas for future improvements with respect to wall condensation and natural circulation that were identified.