Heat Transfer Performance of Cu-Cr-Zr Tube with Swirl Insert Under Cyclic Thermal Loading in Monoblock Divertor

Abstract

Development and optimization of the plasma-facing components for the fusion reactors ITER and DEMO are necessary for sufficient heat removal because of the high heat fluxes in these systems. In this work, we consider the heat transfer performance of the Cu-Cr-Zr alloy tube with a swirl (twisted tape) insert within a monoblock divertor experiencing cyclic thermal loading expected during ITER operating conditions. Thermal loading is examined up to 2000 cycles, leading to increased tube surface roughness and decreased tube thermal conductivity. A simplified model of thermal-hydraulic performance is used that accounts for forced convection in the swirled flow, conduction through the Cu-Cr-Zr tube, and tube fouling (surface roughness and thermal conductivity changes). From our work, it is found that the overall heat transfer rate of the tube is enhanced with increased thermal loading over a wide range of Reynolds numbers (i.e., flow rates). This is due to the increase of convective heat transfer from turbulence enhancement induced by increasing surface roughness. However, the increase in surface roughness also leads to an increase in pressure losses in the system, requiring increased pumping power to maintain flow rates. We consider the heat transfer rate at equivalent pumping power (quantified by the overall enhancement ratio) and find it has a complicated dependence on Reynolds number and the number of thermal loading cycles. In particular, we see that for a Reynolds number of 1 000 000, the overall enhancement ratio is decreased by up to 9% at 2000 loading cycles. Such a decrease could meaningfully impact the operations of ITER or DEMO, requiring additional pumping input to maintain sufficient heat removal. This suggests the need for further investigation of the thermal-hydraulic performance of plasma-facing components, including the full monoblock assembly, after many loading cycles.