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Can the throttling of a perfect gas through a free molecular orifice produce a cooling effect?

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6 Author(s)
Young, Robert M. ; Northrop Grumman Corp., Electronic Systems, P.O. Box 1521, Baltimore, Maryland 21901 ; Braggins, Timothy T. ; Kirby, C.F. ; Adam, J.Douglas
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A novel refrigeration cycle using throttling through free molecular orifice(s) was explored experimentally and theoretically, attempting to exploit the heat transfer that occurs by the entropy shift across a discontinuous system formed by a wall containing tens of thousands of micron or submicron free molecular throttling (FMT) orifices. Single stage free molecular throttle cooling is compared in terms of figure of merit and coefficient of performance (COP) against the analogous isothermal entropy shift cooling effect in thermoelectric junctions. Unlike thermoelectric materials, free molecular throttling is predicted to perform increasingly better as the temperature drops below 200 K. Experimentally, the authors have shown in three rounds of progressively smaller dimensions and hence larger heat flux vector magnitude that the simple, isothermal movement of heat, taken in analogy with thermoelectric (Peltier junction) refrigerators, is not valid. By using this information, the authors then show that solid state heat conduction across the orifice wall controls the degree of temperature reduction. This was described in a new dimensionless number which gives the relative importance of the backward thermal leakage due to solid state conduction through the wall thickness, against that of the FMT heat flow, and is thus the means by which a designer would direct the design space toward a functional and practical wall. The largest theoretical temperature drop per stage was found to be ΔT=T0/6, and thus staging would be required to reach cryogenic temperatures from ambient. Single stage FMT refrigeration was derived theoretically to have a peak second law efficiency of 5%–10% of the Carnot COP. The low pressures involved and lack of the regenerative heat exchanger in this new cycle means that it is scalable to very small dimensions, and indeed, since it operates in a regime where the charac- - teristic size of the orifices must be smaller than the mean free path length of the gas molecules, it is already scaled to microscopic dimensions. FMT cooling would appear to have future applications in subwatt heat lift systems with the ultimate lowest temperature of ∼10 K being only dictated by the perfect gas limit of He.

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Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films  (Volume:26 ,  Issue: 4 )