Specific for Near-Critical Fluids the Piston Effect

Specific for Near-Critical Fluids the Piston Effect 

In thermally non-homogeneous supercritical fluids, very intense convective motion can occur [Ij. Moreovei thermal transport measurements report a very fast heat transport although the heat diffusivity is extremely small. In 1985, experiments were performed in a sounding rocket in which the bulk temperature followed the wall temperature with a very short time delay [11]. This implies that instead of a critical slowing down of heat transport, an adiabatic critical speeding up was observed, although this was not interpreted as such at that time. In 1990 the thermo-compressive nature of this phenomenon was explained in a pure thermodynamic approach in which the phenomenon has been called adiabatic effect [12]. Based on a semi-hydrodynamic method [13] and numerically solved Navier-Stokes equations for a Van der Waals fluid [14], the speeding effect is called the piston effecf. The piston effect can be observed in the very close vicinity of the critical point and has some remarkable properties [1, 15]  

The piston effect is a thermoacoustic phenomenon. Acoustic compression waves are emitted at heated boundaries, which provoke a homogeneous increase in the bulk temperature. 

The piston effect is a fourth heat transport mechanism. Depending on the applied boundary conditions, the piston effect can transport heat from one side of a thermostat-controlled container to the other on a very short time scale. As the bulk phase is homogeneously heated by the piston effect, a boundary layer is formed at the thermostat-controlled wall. 

Temperature can propagate with the speed of sound. When the critical point is approached more closely than a crossover value given by asymptote analysis, the characteristic time of the piston effect does not monotonicaUy decrease to zero, but tends to reach a constant value, which is the characteristic acoustic time. For CO2 contained in a 10 mm long container set at 1 K above its critical temperature, the crossover value is some mK. At these conditions, the 

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