Influence of Convection on the Piston Effect

This paper answers some recent questions posed by heat propagation mechanisms in near-supercritical pure fluids under normal gravity conditions. The role of microgravity experiments in the discovery of a fourth temperature equilibration mode in hypercompressible fluids, called the piston effect, is described, as well as its basic mechanisms that are responsible for temperature equilibration on a much shorter time scale than heat diffusion. The question whether this mechanism still exists on the ground is then approached. The results of the numerical calculations which answer this question and the basic references are preseInted. It is emphasized, in particular, that, although the piston effect has been demonstrated by microgravity experiments, this effect appears to be the temperature equilibrating mechanism on ground, also leading to the striking evidence of a quasi-isothermal convection. Recent modeling in connection with experiments that suggested that the piston effect could be killed by convection shows, on the contrary, that a cooling piston effect, triggered by thermal plumes, prevents the bulk increase in temperature.