Evaluation of heat dissipation phenomena in dispersed flow film boiling using a mechanistic model and different correlations for droplet impact heat transfer

Dispersed-flow film boiling (DFFB) is a flow regime found in several engineering applications, like cryogenic and energy industries or nuclear reactors in accidental conditions, comprising a continuous steam phase with dispersed droplets (with a high void fraction). This type of flow is rather complex because of its non-equilibrium nature, both thermal and dynamic, which makes it challenging to evaluate the many heat transfer paths involved. This study utilizes an in-house mechanistic code called NECTAR to simulate DFFB and compares the results with new experimental data for an internal steam-droplet flow in a vertical tube with high droplet volume fraction and elevated steam temperature. The experimental data were obtained for a variety of droplets’ mass flow rates, steam mass flow rates, and different tube diameters. While correlations of heat transfer for single-phase flow were validated, especially concerning wall-to-steam convection and radiation, there remain uncertainties in the wall-to-droplet heat transfer correlations. Therefore, we compared simulation results using different correlations specifically designed for droplet-impact heat transfer, recognizing the distinctions between these approaches. The validated simulation results provide insights into the intricate thermohydraulic factors involved in DFFB, especially regarding the contribution of each heat transfer path. For instance, the results show that droplet impact on heated walls, which has been neglected in several past models, can contribute up to 50% to the heat dissipation.