Novel Solids Flow Concepts Proposed for Regolith Heat Exchanger and Conveyors

This paper relates a story about efforts to develop a heat transfer concept which proved to be unfeasible, and how the search for alternatives to solve the heat transfer challenges led to new solids conveying concepts with several potential applications. Under vacuum conditions the effective bulk conductivity of loose regolith (k_eff ∼ 0.001 W/m-K) is nearly 100 times lower than it is when an interstitial gas, at atmospheric pressure, is present. After evaluating alternatives, two new, solids conveying technologies were proposed to increase convective solids motion, and provide better conductive and more direct radiative heat-flow paths in a novel coaxial, counter-current, centrifuging-flow screw-like conveyor. A compact modular design, with the option for separate optimization in different temperature zones, was proposed that could utilize radiation-dominated or conduction-dominated modes when each is most effective. In the proposed design, mechanically-induced solids convection would maximize heat flow into and through the flowing regolith. The heat-exchanger was not selected for funding, but one of the proposed conveying techniques, a flexible screw conveyor, may be able to provide a robust method for transfer of regolith from mining or collection points to final utilization locations without the need for wheeled vehicles to make multiple trips. For shorter distances, the other new technique, a centrifuging step-screw conveyor, offers new options for robust vertical or horizontal transfer of granular solids independent of gravity. The analysis behind the centrifuging step-screw conveyor is described in this paper.