Heat Removal Performance of a Helium Gas Flow in Unidirectional Porous Copper Pipes with Spatially Gradating Pore Structure for Nuclear Fusion Divertors

Unidirectional porous copper pipes with spatially graded pore structures are introduced to develop gas-cooled divertors with high energy efficiency. First, six types of grading-pore structures were evaluated using a two-dimensional (2-D) simulation of heat conduction to determine the optimum pore structure. Then, the actual cooling performances of the representative porous pipes, which were proposed by the 2-D simulation, were evaluated using a three-dimensional (3-D) thermofluid simulation. The 2-D simulation of heat conduction verified that the pore diameter distribution of a suitable pore structure decreased spatially in the radial direction. The 3-D thermofluid simulations demonstrated that heat conduction toward the pipe inlet on the upstream side prevented a temperature increase in the porous copper pipe on the downstream side. Although this study uses simulation systems with simple pore shape and boundary conditions, it can evaluate heat transfer performance. Consequently, the gradating pore structure achieved an average heat transfer coefficient of 14 400 W/m²∙K⁻¹, which was 20% higher than that of a conventional pipe with uniform pores. Furthermore, the pumping power required for divertor cooling was reduced by approximately 5%. Future works are simulations under actual inlet conditions and one-sided heat flux of 10 MW/m² for thermal stress evaluation as well as effect evaluation of pore shape and surface irregularities on cooling performance by simulation and experiment.