Adiabatic versus conductive heat transfer in off-critical SF6 in the absence of convection.

The process of adiabatic heating of compressible fluids [piston effect (PE)] has been investigated in ${\mathrm{SF}}_{6}$ at off-critical density \ensuremath{\rho}\ensuremath{\approxeq}1.27${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ near the coexistence temperature in the absence of convection. The temperature response of the fluid to an internal heat pulse has been recorded at different distances from the heat source confirming a (spatially) homogeneous temperature rise outside an expanding boundary layer during heating. This process can be distinguished from the following conductive heat transfer when the energy contained in the boundary layer diffuses. This observation is confirmed by both experiment and calculations. During and after the heating process, (P,\ensuremath{\rho},T) data of the fluid behaves according to a given equation of state at equilibrium because hydrodynamic velocities remain small. The isentropic character of the PE was confirmed by both calculations from the pressure and density measurements. The presented experimental results were obtained on ESA's critical point facility (CPF) during the Spacelab IML-2 mission in July 1994. \textcopyright{} 1996 The American Physical Society.