Thermoacoustic boundary layers near the liquid-vapor critical point

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Abstract
We measure and calculate the sound attenuation within thermoacoustic boundary layers between solid surfaces and xenon at its critical density as the reduced temperature approaches zero. ( is the critical temperature.) Using the known thermophysical properties of xenon, we predict that the attenuation at the boundary first increases approximately as and then saturates when the effusivity of the xenon exceeds that of the solid. [The effusivity is , where is the isobaric specific heat and is the thermal conductivity.] The model correctly predicts the quality factors of resonances measured in a stainless steel resonator ; it also predicts the observed increase of the , by up to a factor of 8, when the resonator is coated with a polymer . The test data span the frequency range and the reduced temperature range . We also predict that the thickness of the thermal boundary layer in the xenon decreases approximately as until . ( is the bulk viscosity, is the heat capacity ratio, and is the speed of sound.) Still closer to , becomes complex and its magnitude increases. These predictions concerning have not yet been tested. We deduce accurate values for the heat capacity and thermal conductivity for xenon in the range .