Simulation of Boundary Layer Effects in the Pulse Tube of a Miniature Cryocooler
Conrad, T. J.
Ghiaasiaan, S. M. (Seyed M.)
Kirkconnell, Carl Scott
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As pulse tube cryocoolers are miniaturized, boundary layer effects in the pulse tube may become more important than they are for larger refrigerators. Nearly uniform flow in the pulse tube is necessary for efficient cooling, and this condition is compromised as the pulse tube diameter becomes smaller relative to the thermal and viscous boundary layer thicknesses. As a result, miniature pulse tube cryocoolers are likely to experience enhanced acoustic streaming losses compared to larger PTC’s. This acoustic streaming results from thermal and viscous interactions between the working fluid and the pulse tube walls. The thermal and viscous penetration depths and their magnitudes relative to the pulse tube diameter and wall thickness are therefore important parameters for this phenomenon. A parametric study of the effects of the pulse tube diameter, scaled to a non-dimensional value by the relevant boundary layer thicknesses, on acoustic streaming in the pulse tube was performed using CFD modeling. The effect of the operating frequency was also considered through the frequency dependence of the viscous and thermal penetration depths. Temperature dependant material properties were included in the CFD models because they play an important role in acoustic streaming. Results indicated that close attention must be paid to the sizes of the boundary layers relative to the pulse tube physical dimensions when designing miniature pulse tube cryocoolers.