Effect of nozzle sizes on jet impingement heat transfer in He-cooled divertor

The use of impinging jets for divertor cooling in the conceptual fusion power plant is attracting much attention due to its very high heat removal capability and moderate pumping power requirement. The latest and the most advanced divertor concept is based on modular design cooled by helium impinging jets. To reduce the thermal stresses, the plasma-facing side of the divertor is build up of numerous small cooling fingers cooled by an array of helium jets. In this study the influence of nozzle sizes on the heat transfer and flow characteristics of such cooling finger is investigated numerically. The main objective is to find an optimal size and distribution of nozzle diameters in the jet array in which the heat transfer would be the highest possible at an acceptable pressure drop through the cooling finger. Prior to nozzle diameters modification, the simulation results for the reference finger geometry were validated against high heat flux experiments. A good agreement was obtained. The nozzle diameters were then modified at two different mass flow rates (13.5 g/s and 6.8 g/s per cooling finger). The most critical design parameter of interest was the maximum thimble temperature, which is limited by the melting temperature of the filler material in the brazed finger joint. It has been found that an optimal jet arrangement should have equal nozzle diameters to reach the highest thimble temperature decrease, while keeping the pressure drop within reasonable limits.