Experimental and numerical investigation of HyperVapotron heat transfer

Abstract

The divertor first wall and neutral beam injection (NBI) components of tokamak devices require high heat flux removal up to 20–30 MW m−2 for future fusion reactors. The water cooled HyperVapotron (HV) structure, which relies on internal grooves or fins and boiling heat transfer to maximize the heat transfer capability, is the most promising candidate. The HV devices, that are able to transfer large amounts of heat (1–20 MW m−2) efficiently, have therefore been developed specifically for this application. Until recently, there have been few attempts to observe the detailed bubble characteristics and vortex evolvement of coolant flowing inside their various parts and understand of the internal two-phase complex heat transfer mechanism behind the vapotron effect. This research builds the experimental facilities of HyperVapotron Loop-I (HVL-I) and Pressure Water HyperVapotron Loop-II (PWHL-II) to implement the subcooled boiling principle experiment in terms of typical flow parameters, geometrical parameters of test section and surface heat flux, which are similar to those of the ITER-like first wall and NBI components (EAST and MAST). The multiphase flow and heat transfer phenomena on the surface of grooves and triangular fins when the subcooled water flowed through were observed and measured with the planar laser induced fluorescence (PLIF) and high-speed photography (HSP) techniques. Particle image velocimetry (PIV) was selected to reveal vortex formation, the flow structure that promotes the vapotron effect during subcooled boiling. The coolant flow data for contributing to the understanding of the vapotron phenomenon and the assessment of how the design and operational conditions that might affect the thermal performance of the devices were collected and analysed. The subcooled flow boiling model and methods of HV heat transfer adopted in the considered computational fluid dynamics (CFD) code were evaluated by comparing the calculated wall temperatures with the experimentally measured values. It was discovered that the bubble and vortex characteristics in the HV are clearly heavily dependent on the internal geometry, flow conditions and input heat flux. The evaporation latent heat is the primary heat transfer mechanism of HV flow under the condition of high heat flux, and the heat transfer through convection is very limited. The percentage of wall heat flux going into vapour production is almost 70%. These relationships between the flow phenomena and thermal performance of the HV device are essential to study the mechanisms for the flow structure alterations for design optimization and improvements of the ITER-like devices' water cooling structure and plasma facing components for future fusion reactors.