Heat Transport by the Piston Effect: Experiments

Abstract

Heat transfer experiments are presented where heat pulses are produced by a point-like thermistor at the center of a thermostated cell. This configuration allows the theoretical analyses of the piston effect mechanism to be tested. It is observed, first, a hot boundary layer, developing at the heat source, which shows large coupled density-temperature inhomogeneities. This part relaxes by a diffusive process, whose density and temperature relaxations are slowed down close to the critical point. During heating, the dynamics of the bulk fluid part, which remains uniform in temperature and density, is accelerated near the critical point and governed by the characteristic time of the piston effect. At the thermostated walls, a slightly cooler boundary layer forms, simultaneously. It cools down the bulk by also a piston effect mechanism. The final sample cell relaxation to the temperature and density equilibration is governed by the time of the thermal diffusion, which corresponds to the slowest mechanism. Comparison with a one-dimensional model shown good agreement with experimental results when the characteristic length of the three-dimensional sample cell is obtained from a pancake cell model. A brief illustration of the situation in the presence of gravity is also given.