Heat Transfer in Porous Media with Fluids near the Critical Point

Heat transfer in porous media with supercritical fluids is an emerging field with applications in separations, reactors, and power cycles. Specific to NASA, researchers at Glenn Research Center are proposing to use supercritical carbon dioxide on machinery related to landing on the surface of Venus. However, the fundamental heat transfer processes of these fluids have not been studied in detail, particularly in porous media. The proposed basic research will study these processes with nuclear magnetic resonance imaging and computational fluid dynamics, focusing on a fluid near its critical point. Preliminary results show that in addition to traditional heat conduction and convection mechanisms, a fluid near the critical point can transfer heat with an acoustic process called the piston effect. The work will use supercritical hexafluoroethane (C2F6) as the fluid (Tcritical = 293 K and Pcritical = 30.4 bar) and the porous medium will be a packed bed of encapsulated wax. This arrangement allows the NMR experiments to measure heat transfer processes via 1H NMR in the wax and fluid flow via 19F NMR in the fluid. The coupling of these measurements is not available via other experimental techniques. A selected range of experimental temperatures and pressures are proposed within sub-, near-, and supercritical conditions to be compared to traditional CFD models that focus only on conduction and convection. Thus, by comparing the experimental results to the numerical results, the difference in heat transfer rates can be attributed and quantified as the piston effect.

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