Thermoacoustic boundary layers near the liquid-vapor critical point

We measure and calculate the sound attenuation within thermoacoustic boundary layers between solid surfaces and xenon at its critical density rhoc as the reduced temperature tau identical with (T- Tc)/Tc approaches zero. (Tc is the critical temperature.) Using the known thermophysical properties of xenon, we predict that the attenuation at the boundary first increases approximately as tau(-0.6) and then saturates when the effusivity of the xenon exceeds that of the solid. [The effusivity is epsilon identical with (rhoCPlambdaT)(1/2), where CP is the isobaric specific heat and lambdaT is the thermal conductivity.] The model correctly predicts (+/-1.0%) the quality factors Q of resonances measured in a stainless steel resonator (epsilon(ss) =6400 kg K(-1) s(-5/2)); it also predicts the observed increase of the Q, by up to a factor of 8, when the resonator is coated with a polymer (epsilon(pr) =370 kg K(-1) s(-5/2)). The test data span the frequency range 0.1<f<7.5 kHz and the reduced temperature range 10(-3) <tau< 10(-1). We also predict that the thickness deltaT of the thermal boundary layer in the xenon decreases approximately as tau(0.4) until 2pifgammazeta/(rhoc2) approximately 0.5. (zeta is the bulk viscosity, gamma is the heat capacity ratio, and c is the speed of sound.) Still closer to Tc, deltaT becomes complex and its magnitude increases. These predictions concerning deltaT have not yet been tested. We deduce accurate values for the heat capacity CV and thermal conductivity lambdaT for xenon in the range 10(-3) <tau< 10(-1).