Thermoacoustic transport in supercritical fluids at near-critical and near-pseudo-critical states

Thermoacoustic wave induced transport in carbon dioxide near its critical and pseudo-critical states is investigated numerically. A real-fluid computational fluid dynamic model has been developed considering all of the relevant fluid property variations (including bulk viscosity) near the critical and pseudo-critical states. The predicted results provide interesting details regarding the thermal transport mechanisms at near-critical and pseudo-critical state fluids. As a layer of supercritical fluid (near the critical or the pseudo-critical states) is heated rapidly, the combination of very high thermal compressibilities and vanishingly small thermal diffusivities affect the thermal energy propagation, leading to the formation of acoustic waves as carriers of thermal energy (the so called piston effect). The results show that under the same temperature perturbation at the boundary, the induced acoustic field becomes stronger as the critical point or the corresponding pseudo-critical state is approached. The heating rate, at which the boundary temperature is raised, is a key factor in the generation of these acoustic waves. We also study the effect of critically diverging bulk viscosity and different rates of boundary heating on the temperature equilibration mechanism near the critical point. An application of the piston effect in near-critical and near-pseudo-critical fluids for effective thermal transport over a long distance is demonstrated for a supercritical heat pipe.