Thermal analysis and optimization of the calorimeter verification prototype of a negative ion-based neutral beam injection system

Abstract

The neutral beam needs to be cut off through the calorimeter before the neutral beam is injected into the Tokamak, so that the neutral beam injector can be debugged and tested independently of the Tokamak. The calorimeter can open or close the channel of the beam entering the Tokamak. The long pulse and strong heat flow of high-energy particles directly bombard the calorimeter, which needs to bear a power density of at least 6 MW/m2 which lasts 3600 s and circulates every half an hour, necessitating high requirements for the cooling and heat transfer capacity of the calorimeter. Therefore, optimization analysis must be performed for a single slender swirl tube to improve the heat transfer capacity of the CFETR NNBI calorimeter verification prototype. In this paper, the comprehensive heat transfer capacity is taken as the optimization index, and the key parameters are optimized and analyzed, including the shape of the swirl tube and the parameters of the tapes, such as the length, type, pitch, perforation scheme, number and placement direction. The simulation results show that by changing the heat exchange tube shape, the heat exchange capacity of oval tubes without tape is increased by 33.5% compared with that of circular tubes without tape. When the number of tapes or sides increases, the heat transfer efficiency increases only slightly. As the tape pitch decreases, the field synergy angle decreases, and the heat transfer effect improves. Comparing punched tapes with unperforated tapes, the heat transfer capacity of the inserted tapes with pitches of 28 mm and 15 mm is similar. The original swirl tube of the calorimeter verification prototype of the NNBI-designed cooling circular tube is inserted with full-length bilateral tape at a pitch of 56 mm. The resulting average wall temperature is 424 K. Finally, an oval tube is selected, with half-length bilateral perforated tape, the pressure inlet is set to 2 MPa, and the pressure outlet is set to 0.8 MPa, with a tape pitch of 20 mm. The resulting average wall temperature of the swirl tube is 404 K. Compared with the original model, the average wall temperature decreases by 11.4%, and the peak wall temperature of 601 K is lower than the softening temperature of 823 K of the CuCrZr wall material, which effectively improved the heat transfer performance. To prove the heat exchange effect of the improved structure, a velocity of 13 m/s is selected as the inlet boundary condition. Compared with the original model, the average wall temperature of the optimized model is reduced by 30.8%, which is consistent with the conclusion of a given inlet and outlet pressure. These simulation conclusions provide a reference for the future design of calorimeters that bear high current and long pulse bombardment.