Microscopic modelling of boiling

Abstract

One of the main thermal-hydraulic challenges of LWR modelling is the prediction of boiling phenomena. This thesis describes numerical and analytical studies aimed at modelling the heat transfer and hydrodynamics of a single steam bubble during nucleate boiling of water, aiming both to improve our current understanding of the phenomena - of the evaporation process itself, and of nucleate boiling heat transfer - and to improve our ability to predict such phenomena, at both single bubble and component scales. Analytical and CFD studies of bubble formation are described. These require accurate representations of evaporation from the liquid microlayer at the bubble base. This vaporization has been investigated from a molecular point of view, with modelling base on kinetic theory, and an apparent inconsistency in measurements of microlayer evaporation during bubble formation has been resolved, and an improved understanding of the molecular mechanism of phase-change thereby gained. The importance of including this improved representation of the evaporation process in single-bubble CFD simulations has been demonstrated. Aiming to improve the closure relations employed for component-scale CFD simulation of boiling flows, interface-tracking modelling of bubble growth and release has been used. Single-bubble interface-tracking models have been developed in an attempt to quantify the transient conduction (“quenching”) component of nucleate boiling heat transfer, associated with bubble lift-off. These mechanistic models allowed detailed quantification of the complex physics associated with bubble growth, and with quenching of the dry area at the bubble base that takes place at bubble departure. A large discrepancy was observed between estimates of the quench heat transfer from these interface-tracking simulations, and that incorporated in the more approximate modelling embodied in the closure relations widely used in component scale CFD.