Improvement of impinging jet heat transfer model for thermal-hydraulic analysis in the vacuum vessel of a fusion reactor

Abstract

In-vessel Loss of Coolant Accident (In-vessel LOCA) in the vacuum vessel of a tokamak fusion reactor, where water is used as the primary coolant, presents a significant challenge. Coolant jets are expected to impinge on high-temperature plasma-facing components (PFCs), resulting in phase transitions on the PFCs surfaces that accelerate pressurization within the vacuum vessel and threaten the chamber's pressure limits. To enhance the predictive capability for pressurization caused by coolant impingement on high-temperature PFCs, the jet impingement heat transfer model must be evaluated and refined. In this study, data from the Ingress of Coolant Event (ICE) experimental setup conducted by the Japan Atomic Energy Research Institute (JAERI) are employed as a benchmark. Initially, a wall heat transfer model based on convective heat transfer is evaluated, revealing a substantial discrepancy between simulation results and experimental data due to the original model's lack of mechanistic considerations, particularly regarding the hysteresis region of impingement heat transfer. Subsequently, two typical jet impingement heat transfer models, developed through experimental and theoretical methods, are introduced. Evaluations indicate that both models underestimate the heat transfer efficiency during coolant impingement on the wall in a vacuum environment, with the Liu model exhibiting errors of approximately 15 % and 10 % for wall temperature and temperature change rate simulations, respectively. The maximum deviation in pressure prediction within the vacuum vessel exceeds 20 %. A theoretical analysis of Helmholtz instability of thin liquid film layer on high-temperature surface in a vacuum environment is then conducted. This analysis considers the enhanced generation of steam under vacuum conditions, leading to a higher steam jet area percentage. Perturbations in the thin liquid film affecting the high-temperature surface are shown to strengthen heat transfer. Based on these findings, a jet impingement heat transfer model tailored for vacuum environments is developed. Comparisons between simulation results and the ICE experimental data demonstrate that the modified model significantly improves predictive accuracy for wall temperature and its rate of change, reducing the pressure prediction error in the vacuum vessel to <10 %.