A Unified Wall Heat Flux Partitioning Model from Nucleate to Transition Boiling

This study develops a new wall heat flux partitioning model to extend the predictive capability for flow boiling. The model incorporates bubble merger behavior and accounts for wall dry areas during transition boiling. Based on the distinct features of nucleate and transition boiling, the heated wall is divided spatially into four zones: non-bubble influenced areas, active nucleation site areas, bubble sliding areas, and dry areas. Temporally, the bubble nucleation cycle is partitioned into growth time, waiting time, and a period dominated by transient heat conduction. The new model also accounts for the effects of bubble merger on nucleation site density and sliding heat transfer, with modifications made to the nucleation site density and sliding heat transfer models. Within the framework of the Eulerian-Eulerian two-fluid approach, this study applies both the new wall heat flux partitioning model and the RPI model to predict classical flow boiling experimental values under various conditions. Results show that for wall heat transfer, the new model predicts wall superheat with a deviation within 21% from experimental data, demonstrating significant improvement over the RPI model. The study also finds that the new model can predict the Critical Heat Flux (CHF) in flow boiling. By incorporating heat transfer mechanisms in dry areas, the model successfully extends its predictive range from nucleate boiling into the transition boiling regime. The predicted CHF values from the new model show a deviation within 17% from experimental data. For vapor phase distribution, the predictions of the new model agree well with experimental data.