Matej Tekavčič

The applicability of wall-boiling computational fluid dynamics models to fusion-relevant high-heat-flux cooling conditions remains uncertain. Under highly subcooled, high-velocity flow, classical Rensselaer Polytechnic Institute (RPI) type formulations have been shown to produce non-physical thermal behaviour. In the present work, a wall-flux-partitioning model developed by Massachusetts Institute of Technology (MIT) is implemented (including the modifications) within the ANSYS Fluent two-fluid framework and assessed against smooth-bore High heAt loaD tESt (HADES) experiments conducted at CEA-IRFM (Institut de recherche sur la fusion magnétique, CEA, Cadarache, France), under fusion-relevant conditions. The model is compared directly with both the classical RPI formulation and a conservative single-phase simulation, which does not consider heat transfer increase due to nucleate boiling.The MIT model, based on a bubble sliding-conduction mechanism, provides significantly improved agreement with the measured thermocouple temperatures compared with the classical RPI formulation. To the authors’ knowledge, this work represents the first independent implementation and assessment of this MIT wall-boiling model outside its original development. It is evaluated under fusion-divertor-relevant cooling conditions. Such simulations remain rare because they combine high heat flux, high liquid velocity, strong subcooling, and numerically stiff near-wall phase-change behaviour. Beyond the direct comparison with HADES experimental data, the present work also analyses model-relevant quantities, including local wall-temperature redistribution, wall heat-flux redistribution, heat-flux partitioning, and the response of the underlying boiling parameters. The results show that the implemented model behaves physically consistently and captures the main measured thermal trends substantially better than the classical RPI formulation.A sensitivity study of the main wall-boiling parameters and selected bulk-flow two-phase parameters shows that, under fusion-relevant conditions, the predicted thermal response of the MIT model is governed by several coupled closure relations. The bubble departure diameter andbubble departure freqency are highly influential, but nucleation site density, wait time, and sliding-area modelling also affect the solution. The selected bulk two-phase parameters have an insignificant direct thermal influence, but still affect numerical robustness. These results suggest that future development of fusion condition relevant boiling models should primarily target improved bubble departure diameter and bubble departure frequency closures.

Efficient removal of extreme heat loads from plasma-facing components (PFCs) remains one of the key thermal-hydraulic challenges in fusion technology. In water-cooled divertors, operation in the convective boiling regime offers high heat removal capability, but predictive modelling remains limited by insufficient understanding of boiling under one-sided heating and high coolant velocities, resulting in conservative margins to the critical heat flux (CHF). The present paper describes a physics-guided strategy for developing predictive two-phase computational fluid dynamics (CFD) models for such conditions. The strategy combines three tightly connected elements: boiling mechanism identification through a scaled visualisation experiment, mechanistic closure development within a two-fluid CFD framework, and staged validation against realistic high heat flux (HHF) experiments. The experimental basis includes the FEDORA (Fusion Experiment for Divertor Optimization Research Applications) visualisation experiment, designed to provide local boiling observables in the flow channel under scaled divertor-relevant conditions, and the HHF facilities, which provide realistic validation environments. The current status of model development is most advanced at the level of wall heat flux partitioning. Our recent analyses show that classical quenching-based wall boiling formulations become physically inconsistent under fusion-relevant high flow velocity conditions, whereas a formulation based on sliding-conduction provides a more credible physical basis. This study summarizes the first assessment of that wall boiling framework across a broad fusion-relevant database, together with its first CFD implementation and assessment against HHF experiments in the HADES facility. The results support the proposed development direction, showing that improved wall boiling physics can substantially improve thermal predictions under divertor-relevant conditions, while bulk flow modelling and near-CHF vapour-structure transition remain the next required steps toward a fully predictive divertor-relevant boiling model.

The capability of the standard SST k-ω turbulence model for the prediction of jet impingement cooling characteristics using a coarse mesh is investigated. The discussion is based on a sensitivity study with five computational grids, differing from each other in topology and resolution. The analysis considers a hexagonal configuration of turbulent jets at the inlet Reynolds number equal to 20,000, with the distance between the nozzle and target plates equal to four nozzle diameters. The results of steady RANS simulations are validated against the time-averaged LES results and data from experiments. The mean heat transfer characteristics of turbulent impinging jets have been successfully reproduced with all tested grids, which indicates that for a rather accurate mean heat transfer prediction, it is not necessary to resolve all the small-scale flow features of impinging jets above the target plate.