THERMAL RESPONSE OF A FLUID NEAR ITS CRITICAL-POINT - HE-3 AT T-GREATER-THAN-T-C

The local density response is studied in a simple fluid near the liqui d-vapor critical point, subjected to temperature oscillations of its c ontainer. This investigation provides a new approach in the study of t he adiabatic energy transfer (''piston effect'') in the fluid. The den sity response function Z(F)(omega, epsilon, z) is calculated for He-3 in the absence of stratification, where omega is the angular frequency , epsilon = (T - T-c)/T-c the reduced temperature, T-c = 3.316 K the c ritical temperature, and z the vertical position in the container. Exp eriments are described where the density is measured by two superposed capacitive sensors in a cell of 3.5 mm height, and where the temperat ure oscillation frequency f = 2 omega/2 pi is varied between 10(-4) an d 2 Hz. Over the experimental range 5 x 10(-4) < 5 x 10(-2) there is i n general reasonable agreement between predictions and experiments. Th e systematic departures might be accounted for by deviations from 1D g eometry, which were not included in the calculations. Over the frequen cy and reduced temperature ranges, the damping effect from the critica l bulk viscosity is predicted to be too small to be detectable. The ob served effect of the stratification and its frequency dependence in Z( F) are briefly discussed. In the appendix, the predicted critical acou stic attenuation from the bulk viscosity is compared with published da ta, the effect from finite thermal conductivity of the fluid container plates and also the corrections to Z(F) for the effects of the cell s idewalls are calculated.