Thermalhydraulic optimization of hypervapotron geometries for first wall applications

Plasma disruptions and Edge Localized Modes (ELMS) may result in transient heat fluxes as high as 5 MW/m² on portions of the ITER first wall (FW). To accommodate these heat loads, roughly 50% of the first wall will have Enhanced Heat Flux (EHF) panels equipped with water-cooled hypervapotron heat sinks. Recent advances in computational fluid dynamics (CFD) enable designers to predict thermal performance even under transient two-phase flow conditions. The challenge is to design a heat sink that operates well under nominal 0.5 MW/m² conditions, but still has enough design margin to accommodate off-normal events. In this article, we present the results of a CFD study to investigate the tooth height and backchannel depth of 50-mm-wide hypervapotrons with 3-mm-pitch and 3-mm side slots as proposed for the fingers in the EHF FW panels. The typical EHF panel contains approximately 40 hypervapotron fingers connected to a common manifold. The water inlet temperature is 70°C at a pressure of 2.7 MPa and a mass flow rate of 0.435 kg/s per finger. The heated surface of the CuCrZr hypervapotron fingers are armored with 8-mm-thick beryllium tiles of various areas. The standard design with 4-mm-high teeth and a 5-mm-backchannel is compared to a more optimal case with 2-mm-high teeth and a 3-mm-backchannel under nominal heat loads and single-phase flow conditions. Better heat transfer in the latter case and the smaller backchannel permit a factor of two reduction in the required mass flow while maintaining the same beryllium armor surface temperatures near 130°C. The shallow teeth and smaller back channel allow the 40 fingers in a typical panel to flow in parallel and simplify the water circuit. The two hypervapotron designs are then compared during off-normal loading and two-phase flow. The design with 2-mm teeth has a 3.5% higher beryllium surface temperature of 648°C. This study highlights the necessary compromise between design marg- - in during transient events, effective heat transfer under nominal conditions and the simplicity needed in the water circuit design.