Thermodynamic modeling results for a novel small satellite (SmallSat) Stirling Cryocooler, capable of delivering over 200 mW net cooling power at 80 K for less than 6 W DC input power, are used in this work as the basis for related pulse tube computational fluid dynamics (CFD) analysis. Industry and government requirements for SmallSat infrared sensors are driving the development of ever-more miniaturized cryocooler systems. Such cryocoolers must be extremely compact and lightweight, a challenge met by this research team through operating a Stirling cryocooler at a frequency of approximately 300 Hz. The primary advantage of operating at such a high frequency is that the required compression and expansion swept volumes are reduced relative to linear coolers operating at lower frequencies, which evidently reduces the size of the motor mechanisms and the thermodynamic components. In the case of a pulse tube cryocooler, this includes a reduction in diameter of the pulse tube itself. This unfortunately leads to high boundary layer losses, as the presented results demonstrate. Using a Stirling approach with a mechanical moving expander piston eliminates this small pulse tube loss mechanism, but other challenges are introduced, such as maintaining very tight clearance gaps between moving and stationary elements. This work focuses on CFD modelling results for a highly miniaturized pulse tube cooler.
CFD analysis of a pulse tube expander was performed to investigate the role of size on refrigeration losses inside the pulse tube component itself. As the pulse diameter shrinks, the boundary layer region becomes an increasing percentage of the overall cross section, ultimately compromising the simplified 1-D “gas piston” assumption to the point where the pulse tube no longer functions as a practical refrigeration device. This analysis was performed to explore where that occurs, starting from a dimeter of 10 mm and going down to 1 mm. The computational domain was constructed to focus on this pulse tube boundary layer loss mechanism; regenerator and other losses were not considered for this study.
Below demo clearly shows the effect of boundary layer as diameter decreases with constant length-diameter ration. For the 10 mm case, plug flow with minimal axial mixing is observed. The 7 mm case exhibits a mild increase in boundary layer influence with increasing evidence at 4 mm and a dominant influence at 1 mm.
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