Development of flow boiling mechanism model and its application to rectangular narrow channel under atmosphere pressure

摘要

Bubble sliding is an important phenomenon in subcooled flow boiling that contributes significantly to overall heat transfer, especially in geometrically constrained geometries. While the widely used wall boiling model which embedded in the CFD commercial software for subcooled boiling simulation, assumes that the bubble jumps directly into subcooled mainstream when its diameter grows to a critical value. While in narrow channel, almost all the bubbles slide along the heating surface before lift-off, the size increases continuously during the sliding process, which is greatly different from the assumption in the original wall boiling model. In this paper, heat transfer induced by bubble sliding is introduced to the original heat partitioning strategy. A force balance mechanism model validated by visualization experiments in narrow rectangular channel is applied to determine whether bubbles slide on the heating surface and when to detach from the nucleation site. The nucleate site density and bubble sliding velocity obtained from our previous experiment are also utilized in the new model. The applicability of the proposed model is demonstrated by comparing the calculated heat transfer coefficients with the experimental heat transfer coefficients. The wall temperature predictions of the new model are closer to experimental data than the original model, typically are close to the boiling region. The heat partitioning components show that the heat transfer due to bubble sliding is the highest fraction of surface heat dissipation, which is consistent with experiment observations. Finally, the wall heat flux partition, bubble sliding distance, bubble departure diameter and lift-off diameter are analyzed. The proposed model has been successfully applied to the analysis of flow and heat transfer characteristic of plate type fuel element in marine nuclear power system.