Analysis of transient thermally-induced convection of supercritical helium in a conduit

The author investigates the thermally induced transient flow and heat transfer in cable-in-conduit conductors (CICCs) cooled with initially stagnant supercritical helium. Prior studies have demonstrated that thermally induced flow results in a high heat-transfer coefficient and a large thermal stability margin. Accurate prediction of the stability margin requires a quantitative description of the variation of heat transfer with heat flux. This study focuses on the heat-transfer/heat-flux relationship and on the basic mechanisms governing the early stages of transient heating. For modeling the problem is divided into two parts: a low-heat-flux regime and a high-heat-flux regime. Results from the models indicate that the heat-transfer coefficient exhibits a minimum, as observed experimentally in previous study, and that, in general, heat transfer can be considerably enhanced by using a high compressible fluid like supercritical helium. For low heat fluxes, heat transfer decreases with increasing heat flux because the thermal boundary layer is thickened by the expansion-driven velocity directed away from the heat transfer surface. For high heat fluxes, heat transfer increases with increasing heat flux because the thermal boundary layer thickness is reduced by the action of the expansion-driven axial velocity