Quantifying Heat Transfer Rates in Porous Media with Near-Critical Fluids

New engines and power cycles are needed to provide power and cooling for an extended Venus landing, increasing mission times from hours to weeks. State of the art power cycles being developed at NASA to accomplish this goal include systems with a porous media regenerator and fluids at supercritical conditions. For example, researchers at NASA’s Glenn Research Center are proposing to utilize supercritical carbon dioxide, available in the Venus atmosphere. However, this working fluid may experience various states, and heat transfer at these conditions has not been studied in detail, particularly in porous media. Conduction, convection, and anomalous heat transfer (e.g. piston effect or reduced heat flux despite enhanced fluid mixing) can occur together, particularly with the fluid near its critical point. The proposed basic research will study these heat transfer and fluid flow processes with nuclear magnetic resonance imaging, focusing on a fluid near its critical point. At the fluid’s critical temperature and pressure, the fluid properties are most divergent and the influence of anomalous heat transfer is highest. The resulting data will provide a strong basis for understanding the fundamentals so more advanced systems can be optimized in the future. The work will use supercritical hexafluoroethane (C2F6) as the fluid (T_critical = 293 K and P_critical = 30.4 bar), and a packed bed of encapsulated wax particles will provide the template porous medium. This arrangement allows the NMR experiments to measure heat transfer processes via 1H NMR in the wax and flow via 19F NMR in the fluid. The coupling of these measurements is not available via other experimental techniques. A range of experimental temperatures and pressures are proposed within sub-, near-, and supercritical conditions.

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