An Experimental Investigation of the Effect of Jet-to-Surface Distance on the Thermal Hydraulics of the Helium-Cooled Modular Divertor with Multiple Jets

The helium-cooled modular divertor with multiple jets (HEMJ) can potentially accommodate the large steady-state heat fluxes expected in future long-pulse magnetic fusion reactors. This work, which is part of the joint US-Japan PHENIX collaboration, describes recent results on a single HEMJ “finger” unit obtained in a helium loop operating at prototypical pressures of ~10 MPa. A new heater was used to increase the maximum coolant inlet temperature  ≤  400°C (vs. the prototypical value of 600°C) at incident heat fluxes   ≤  4.5 MW/m² at these elevated temperatures. The effect of varying the jet-to-impingement surface separation distance H from 0.47 mm to 1.49 mm was also studied for mass flow rates   ≤  8 g/s. Numerical simulations of this HEMJ test section were also performed to obtain local information that could not be measured in the experiments.

Varying H within this range appears to have little effect on both the dimensionless heat transfer coefficient, or Nusselt number , and the dimensionless pressure drop across the HEMJ, or loss coefficient . The experimental measurements do, however, give lower after re-calibration of the differential pressure transducer; these results are now in better agreement with numerical predictions compared with previous experimental data. The experimental results obtained at higher and for are, however, lower than those predicted by a correlation for obtained from extensive measurements taken at lower temperatures in the same facility. These initial results require further examination because they are contradicted by the numerical predictions. If these results are valid, they suggest that the maximum heat flux that can be accommodated by a divertor module may be lower than expected.