Two-dimensional impinging jet cooling of high heat flux surfaces in magnetic confinement fusion reactors

Abstract

The divertor surface of a magnetic confinement fusion reactor is exposed to strong radiation heating by high flux charged particles. According to the standard design for the ITER, the heat flux on the divertor surface averages 15 MW m −2 or more. In this study, cooling by a two-dimensional impinging jet flow is proposed to cool this surface. For an impinging jet flow on a flat heated surface, a high critical heat flux (CHF) is obtained only in the limited surface region where the jet flow hits directly. Outside this region, the CHF decreases abruptly with distance from the center. The main reason is that the pressure decreases abruptly away from the center region and the liquid flow is spread away from the heated surface region by the strong boiling. To overcome these difficulties, we propose that the impinging jet is applied to a heat transfer wall with a concave surface. In this study, the CHFs and the nucleate boiling curves for two-dimensional impinging jet cooling were first obtained as a function of the distance from the center using a thin copper foil heater designed on the plastic sheet. Experiments were done under various conditions of liquid subcooling, flow velocity and surface curvature. Empirical correlations for the CHF including these parameters were obtained. It is clear that impinging jet cooling of the curved surface is useful to keep the CHF in the downstream region high. Finally, application of the two-dimensional jet to the cooling of a fusion divertor surface is assessed. When the jet velocity is equal to 14.6 m −1 , and the liquid subcooling is 80 K, a two-dimensional jet is able to cool a curved surface area 50 mm wide with an average heat flux of 30 MW m −2 .