Effects of geometric structures on the sub-Kelvin porous media heat exchanger

A dilution refrigerator is an essential apparatus for achieving sub-Kelvin temperatures. Porous-media heat exchangers used inside dilution refrigerators at temperatures below 100 mK are crucial components that determine their optimal performance. In this study, a numerical model of a porous-media heat exchanger is developed. The local thermal non-equilibrium heat transfer model captures the Kapitza heat transfer between solid and fluid components, whereas the coupled Brinkman-Forchheimer and Navier-Stokes equations describe fluid flows in porous media. Results indicate that a liquid gap can enhance the thermal conduction between the flow channel and porous media, thus improving the heat-exchange efficiency of the porous media. For a constant-volume and -thickness porous media, the most suitable length-to-width ratio is 5. The concentrated phase exhibits a lower outlet temperature, whereas the dilute phase exhibits an appropriate pressure drop. For a constant-volume and -length porous media, the optimal width-to-thickness ratio is 10, which provides the best thermal and hydrodynamic performances. For a full-width liquid gap, increasing its thickness enhances both the thermal and hydrodynamic performances but also increases the usage of 3He. Under a constant volume of liquid gap, a partialwidth liquid gap can improve the heat-exchange efficiency, thus achieving better thermal and hydrodynamic performances than the conventional full-width one.