Assessment of different RANS turbulence models in mini-channels for the cooling of MW-class gyrotron resonators

The adoption of high-speed water flow in mini-channels is a viable option for the cooling system of the resonant cavity in gyrotrons, which are a candidate technology for the external plasma heating in nuclear fusion reactors. The evaluation of the performance of such mini-channel cooling system is a combined fluid dynamics and heat transfer phenomenon which seeks more attention to a highly accurate computational analysis. In this study, a computational-based comparative platform is proposed to evaluate the performance and fidelity of the applied turbulence models which are utilized to study the mini-channel cavity cooling systems in gyrotrons. A full-size mock-up of the gyrotron resonator equipped with mini-channels has been realized and tested in 2019 by THALES to check its total pressure drop applying a wide range of water flow rates, including that available for the gyrotron operation. In parallel, a numerical model of the mock-up has been developed using the commercial software STAR-CCM+, and simulations have been performed using different RANS turbulence closures, and namely: SST k −ω, realizable k − ε and Lag EB k − ε. The detailed comparison of the computed hydraulic characteristics (i.e., a range of pressure drop measurements at different flow rates) to the set of measured values has been addressed using a multivariate metric to assess the performance of different turbulence models in pure hydraulic simulations. This comparative platform reveals a significant clarified difference in fidelity among the RANS models. Based on the performed comparative studies against the entire set of available experimental data, the Lag EB k − ε closure provides the best performance among the other turbulence models and can be applied for the future studies of the mini-channel cavity cooling systems of the gyrotron resonators.