SUMMARY

The proposed research addresses the effect of asymmetric gravitational body force on Rayleigh-Bénard instability during oscillatory flow of a cryogen in a pulse tube cryocooler (PTC). PTCs are among the most attractive choices of refrigerators for space applications which require cooling in the temperature range of 4 K to 123 K. The pulse tube, along with an impedance network consisting of an inertance tube and surge volume, replaces the expander piston in a typical Stirling-cycle cooler, invoking out-of-phase pressure and mass flow oscillations with no moving parts in the cold section. Terrestrial applications of PTCs, which are widespread and include cooling infrared sensors and superconducting materials, reveal a fundamental flaw. Most PTCs only function properly in one stable orientation, with the cold end of the device pointed downward with respect to gravity. Changing the orientation of the cold head often leads to a catastrophic loss of cooling, rendering the entire cryocooler system inoperable. Previous research indicates that cooling loss is likely a result of the convective instability known as the Rayleigh-Bénard instability, due to non-uniform density gradients within the pulse tube. The ensuing secondary flow mixes the cryogen and causes a thermal short-circuit between the warm and cold heat exchangers of the cryocooler. The proposed research aims to verify and quantify the convective instability through a combined experimental and modeling study. A fully-instrumented PTC cold head assembly will be designed and fabricated. Pressure, velocity, and heat loading measurements will be made under prototypical cryogenic conditions in various orientations. CFD simulations will be performed in tandem and in support of the experiments. The resulting data will be used for model development and validation of system-level CFD simulations. The experimentally verified CFD model will be used to predict the onset of convective instability for various cold head geometries as a function of flow oscillation frequency, average temperature, temperature gradient, length, diameter, oscillation amplitude, and tilt angle. The results of the computational model will be compared to current analytical models in order to suggest modifications that may improve their accuracy, facilitating more robust pulse tube design methods.