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We report on the analysis of the response of a near-critical fluid sample cell submitted to heating at different distances from the critical temperature Tc, in the Mir station microgravity environment. We recall the hydrodynamic and thermodynamic bases of the mechanisms of fast adiabatic heating, also called "thermalization by Piston Effect" (PE). We give a phenomenological approach of these mechanisms in a highly compressible critical fluid submitted to heating until a steady state is reached, and present the main results of numerical experiments in a Van der Waals fluid. We then use this phenomenological understanding to analyze an experimental case of heating in reduced gravity onboard Mir, using the Alice 2 facility between 1996 and 1999 (Cassiopea to Perseus missions). We have measured the characteristic time of the PE as a function of the critical temperature distance, and investigated the crossover behavior from the pure adiabatic regime for a surrounding wall of infinite conductivity, to the bottleneck conductivity regime induced by a real cell. Close to Tc, we have evidenced the key influence of the geometry of the cell and of the thermal properties of the wall materials.

Close to the liquid gas critical point, the linear treatment of the symmetrical one-component Φ4model to observe the fluid-restricted universality of the subclass of pure fluids is reversed. The comparison with the fitting results obtained from the recent applications of the crossover description to CO2, CH4, C2H4, C2H6, R134a, SF6, and H2O confirms that the dimensionless characteristic two scale factors involved in this description are: (a) the critical compressibility factor and (b) the slope at the critical point of the reduced potential $$\frac{P}{T}\frac{{T_c }}{{P_c }}$$ along the critical isochore. For the two-phase domain along the critical isochore, a precise formulation for the extension range of the fluid-restricted universality is given in terms of the reduced scaling size $$l^{* - } = \frac{{\xi ^ - }}{{a_c }}$$ of the critical density fluctuations, expressed as a function of the dilated scaling field which measures the distance to the critical point below Tc. The explicit definition of the microscopic length scale $$a_c = (\frac{{k_B T_c }}{{P_c }})^{\frac{1}{3}} $$ , which characterizes the short-range of the microscopic interaction, gives a correlative estimation of the crossover domain when $$\xi ^ - \sim a_c $$ .

An experimental study of the thermal response to a stepwise rise of the wall temperature of two-phase near-critical SF6in low gravity for an initial temperature ranging from 0.1 to 10.1 K from the critical temperature is described. The change in the vapor temperature with time considerably exceeds the change in the wall temperature (overheating by up to 23% of the wall temperature rise). This strong vapor overheating phenomenon results from the inhomogeneous adiabatic heating process occurring in the two-phase near-critical fluid while the vapor bubble is thermally isolated from the thermostated walls by the liquid. One-dimensional numerical simulations of heat transfer in near-critical two-phase 3He confirm this explanation. The influence of heat and mass transfer between gas and liquid occurring at short time scales on the thermal behavior is analyzed. A model for adiabatic heat transfer, which neglects phase change but accounts for the difference between the thermophysical properties of the vapor and those of the liquid, is presented. A new characteristic time scale of adiabatic heat transfer is derived, which is found to be larger than that in a one-phase liquid and vapor.

When coexisting gas and liquid phases of a pure fluid are heated through their critical point, large-scale density fluctuations make the fluid extremely compressible and expandable and slow the diffusive transport. These properties lead to perfect wetting by the liquid phase (zero contact angle) near the critical temperature Tc. However, when the system's temperature Tis increased to Tc, so that it is slightly out of equilibrium, the apparent contact angle is very large(up to 110°), and the gasappears to “wet” the solid surface. These experiments were performed and repeated on several missions on the Mir space station using the Alice-II instrument, to suppress buoyancy-driven flows and gravitational constraints on the liquid–gas interface. These unexpected results are robust, i.e., they are observed under either continuous heating (ramping) or stepping by positive temperature quenches, for various morphologies of the gas bubble and in different fluids (SF6and CO2). Possible causes of this phenomenon include both a surface-tension gradient, due to a temperature gradient along the interface, and the vapor recoil force, due to evaporation. It appears that the vapor recoil force has a more dominant divergence and explains qualitatively the large apparent contact angle far below Tc.

Abstract: A strong laboratory vortex is generated in a cylindrical cell using a rotating disk and stretched by pumping the fluid out through a hole in the centre of the top of the cell. The velocity field is measured by means of laser Doppler anemometry and Doppler ultrasonic anemometry which are both non intrusive methods. The vortex exhibits a slight precession which induces temporal fluctuations of the velocity at the measurement point. Due to the centrifugal force, the tracers concentrate in a tubular region around the vortex, leading to spatial variations of the measurement counting rate. Under these two effects, the probability density function (PDF) of the one point velocity exhibits a strong non-Gaussian behaviour. In order to access the details of the velocity profile of the vortex in its own system of reference, the influence of the vortex precession, of the spatial variations of the concentration in tracers and of the intrinsic measurement dispersion is investigated and a model is proposed. It allows to recover statistically the characteristics of the vortex and to deduce the trajectory of its centre from the instantaneous velocity profiles.

Une expérience de spectroscopie THz résolue en temps nous a permis d'étudier l'évolution des propriétés optiques du CO2dans les phases liquide, gazeuse et critique. Cette évolution est directement reliée à celle des interactions dans le milieu et leur analyse présente de multiples intérêts tant en physique fondamentale qu'en physique appliquée.