On physical considerations regarding development and validation of Heat Flux Partitioning models: Application to vertical boiling flows simulations

TL;DR

This work addresses the challenge of accurately predicting heat transfer in vertical flow boiling by developing a physically grounded Heat Flux Partitioning framework that explicitly accounts for bubble sliding and coalescence. The authors couple a mechanistic bubble departure/slide model with a lift-off diameter correlation and a comprehensive set of closures for nucleation-site density, wait times, and growth, while introducing a coalescence-driven evaporation contribution and a vapor-conduction term. A rigorous validation strategy is employed, including detailed comparisons to Kossolapov data and a broad set of wall-heat-flux measurements, revealing that separate validation of closure laws is essential to avoid compensating errors and to reliably capture the partitioning among liquid convection, quenching, evaporation, and dry-wall conduction. The results demonstrate that the proposed framework can outperform traditional models in predicting wall heat flux across varied pressures and conditions, while highlighting remaining sensitivities to the contact angle, hysteresis, and growth constant, and underscoring the need for targeted experiments to further constrain these parameters. The study reinforces the importance of physically consistent, thoroughly validated HFP formulations for reliable CFD simulations of wall boiling and bubble dynamics near heated walls.

Abstract

As part of a critical assessment of wall boiling modeling through Heat Flux Partitioning approach, a new model dedicated to vertical boiling flows is proposed, with a revisited partitioning including a boiling heat flux related to bubble coalescence. Closure laws include a recent model for bubble dynamics, a new correlation for the bubble maximum lift-off diameter, and comprehensive selection of existing models for nucleation site density and bubble wait time. Each formulation are compared to relevant existing data from the literature in order to emphasize the importance of separate validation in such a modeling framework. The whole model is then confronted to detailed wall boiling experiments to simultaneously compare boiling curve predictions along other physical parameters such as boiling time scales (bubble growth, transient conduction, bubble wait), nucleation frequency, or nucleation site density. Finally, validation against wall heat flux measurements in various conditions are used to assess the model accuracy and further discuss the limits of the Heat Flux Partitioning approach.

Figures (19)

• Figure 1: Sketch of the HFP model.
• Figure 2: Single-phase wall temperature predictions using Gnielinski correlation with $\pm 3$K error bars. Colorbar represents the bulk flow Reynolds number.
• Figure 3: Sketch of the force balance model for bubble departure and sliding. Adapted from Favre et al. favre_updated_2023.
• Figure 4: Lift-off diameter reduced correlation, $\pm 50\%$ errors bars are indicated.
• Figure 5: Wait time data