Thermal-hydraulics modeling for prototype testing of the W7-X high heat flux scraper element

Abstract

Long-pulse operation of the Wendelstein 7-X (W7-X) stellarator experiment is scheduled to begin in 2020. During this operational phase the vessel will be equipped with water-cooled plasma facing components to allow for longer pulse durations. Certain simulated plasma scenarios have been shown to produce heat fluxes that could surpass the technological limits of the divertor target elements during steady-state operation. To reduce the heat load on the target elements, the addition of a "scraper element" (SE) is under investigation. The SE is composed of 24 water-cooled carbon fiber reinforced carbon composite monoblock units. Multiple full-scale prototypes have been tested in the GLADIS high heat flux test facility. Previous computational studies revealed discrepancies between the simulations and experimental measurements. In this work, single-phase thermal-hydraulics modeling was performed in ANSYS CFX to identify potential causes for such discrepancies. Possible explanations investigated were the effects of a non-uniform thermal contact resistance and a potential misalignment of the monoblock fibers. While the difference between the experimental and computational results was not resolved by a non-uniform thermal contact resistance, the computational results provided insight into the potential performance of a W7-X monoblock unit. Circumferential temperature distributions highlighted the expected boiling regions of such a unit. Furthermore, simulations revealed that modest angles of fiber misalignment in the monoblocks result in asymmetries at the unit edges and provide temperature differences similar to the experimental results.

Design and Test of Wendelstein 7-X Water-Cooled Divertor Scraper

“…The monoblock geometry always induces a thermal gradient between the outer edges and the centre of the CFC blocks. A misalignment in the orientation of the carbon fiber mesh during production of the CFC results in further asymmetries of the surface temperature [12], [13]. Therefore, the resulting surface temperature distribution is not uniform as expected from the Gaussian distributed heat load.…”