Dynamic temperature propagation in a pure fluid near its critical point observed under microgravity during the German Spacelab Mission D-2

Diverging thermodynamic and transport properties of a near critical fluid have significant effects on heat and mass transport; the effects are still under investigation. These effects become visible especially in experiments carried out under microgravity conditions because in the absence of gravity, buoyancy induced convection is suppressed. According to the ‘‘critical slowing down’’ hypothesis, even longer thermal relaxation times are predicted for experiments under reduced gravity conditions than for that on earth. Early experiments under microgravity showed a fast dynamic temperature propagation. More recently, this effect has been called the ‘‘piston effect,’’ or ‘‘critical speeding up.’’ The final thermodynamic equilibration of temperature and density is still very slow according to a diffusion process of heat and mass. During the German Spacelab Mission D-2 in 1993, the calorimeter HPT-HYDRA, which was designed for measurements of the specific heat at constant volume ${\mathit{c}}_{\mathit{V}}$, was also used for investigations of the dynamic temperature propagation. The spherical cell filled with ${\mathrm{SF}}_6$ at critical density was pulsewise heated, which caused a temperature increase in the fluid of about 10 mK. The temperature propagation in the fluid was monitored with a resolution of 10 μK at three different radial positions in the fluid and at the wall of the cell. The experiments were carried out at 39 different temperatures in the region of 0.03 KT-${\mathit{T}}_{\mathit{C}}$‖${\mathit{T}}_{\mathit{C}}$, the temperature difference between the wall and the fluid decreases to zero in both the one phase and two phase regions. The experimental results in both the one phase region and two phase regions are in remarkably good agreement with numerical simulations and confirm the fast temperature propagation, which is predicted by the ‘‘piston effect’’ model quantitatively. In addition the phase separation process during cooling is explained with this model.