Flow instabilities in refractory metal, porous media, helium-cooled plasma facing components

Past numerical investigations of the performance of porous media to enhance heat transfer in helium-cooled devices neglected the susceptibility of multi-channel heat sinks to parallel flow instabilities even though experimental evidence suggests it may be a problem for narrow channel devices. In previous work, our simulations have shown that helium micro-jets do not experience changes in flow distribution due to non-uniform heating. However, jets are difficult to fabricate for large area refractory metal components. The same is not true for narrow channel devices filled with porous media. Although these refractory devices are easier to fabricate, the effects of downstream hot gas expansion can influence the incoming flow distribution in multi-channel configurations. In this article, we review the experimental data and illustrate how the ideal gas law can predict this behavior using computational fluid dynamics. The modeling can reveal the subset of conditions that will lead to deleterious flow mal-distributions in multi-channel geometries containing porous media. Such phenomena rarely occur in devices with large channels made of high thermal conductivity materials, but are easily produced in small-channel refractory metal devices. In these devices, flow mal-distributions result from highly localized heat fluxes due to off-normal transient events or from non-uniformity in the heat flux profile at leading edges and divertor strike-points. Unfortunately, the nominal flow conditions compatible with efficient Brayton cycle power conversion favor the low flow rate, high delta-T devices that are most vulnerable to instabilities and flow bypass. Our simulations of narrow-channel tungsten devices revealed that a 33% mass flow mal-distribution could be produced easily by applying a 30 MW/m² heat flux over a small area above a few channels. This can lead to even larger temperature gradients on the surface compared to the same uniform heat flux over a much larger area. The highly peaked temperature distributions will produce higher thermal stresses in the faceplate and limit the lifetime of the device. Modeling insights allow the designer to optimize the porous media properties, channel configuration and manifolding to mitigate these effects and to produce more robust, easily manufactured, gas-cooled refractory metal heat sinks.