Dynamically Similar Studies of the Thermal Performance of Helium-Cooled Finger-Type Divertors With and Without Fins

Experimental studies based upon dynamic similarity have been used to evaluate the thermal performance of several modular helium-cooled tungsten divertor designs, including a configuration similar to the helium-cooled modular divertor with multiple jets (HEMJ). Until recently, all of these experiments used air, instead of helium, as the coolant. The average Nusselt number and loss coefficient were determined from cooled surface temperature and pressure drop data. Correlations were developed for the Nusselt number and loss coefficient as a function of the Reynolds number then used to predict the thermal performance of the divertor under prototypical conditions when cooled with high-temperature, high-pressure helium. Recently, experiments were performed using helium and argon to confirm the dynamic similarity assumption. The results indicated that the previous experiments with air, which were performed at the prototypical nondimensional coolant mass flow rate, or Reynolds number, did not account for the differences in the fraction of the incident power conducted through the walls of the divertor versus that convected, i.e., removed, by the coolant.

Dimensional analysis and numerical simulations suggest that for a given divertor geometry this fraction can be characterized by the ratio of the thermal conductivities of the divertor material and the coolant. Nusselt number correlations were developed to include the effect of the thermal conductivity ratio. Based on these correlations, the predicted maximum heat flux values that can be accommodated by the HEMJ-like configuration are reduced by [approximately]20% from previous estimates. The results also suggest that the maximum heat flux that can be accommodated by this design can be increased by as much as 19% by adding an array of cylindrical pin fins on the cooled pressure boundary. However, as expected, adding the fins increases the pumping power for the coolant by [approximately]16%. As a fraction of maximum total incident thermal power, however, the pumping power decreases by 2% when the fins are added due to the significant increase in the maximum heat flux.