Thermoacoustic engines with near-critical working fluids

Thermoacoustic engines are devices in which heat is converted into acoustic oscillations, which can be tuned to mimic the thermodynamic cycle executed by mechanical motion such as that of a piston. In classical thermoacoustics, the working fluid is a gas far from the critical point. Herein, we extend the scope of working fluids to theoretically examine thermoacoustic conversion with fluids near their critical points. First, a "short engine"approximation is used, in which the acoustic field is assumed to be uniform and standing-wave dominated. This is then followed by a full-scale model of a standing-wave engine. Both models are investigated via the numerical solution of the equations for linear thermoacoustics, supplemented by the non-ideal equation of state for all fluid properties. The numerical model was first validated against published experimental results, and then used to project performance characteristics under various conditions. Results demonstrate that under operating conditions close to the critical point, thermoacoustic conversion can be enhanced; however, acoustic dissipation also increases, resulting in a trade-off between the larger output power of the engine (at a lower temperature difference) and the efficiency, which decreases. Importantly, the sub-critical region (i.e., at a pressure slightly lower than the critical pressure) yields better performance than the supercritical region in terms of both power output and efficiency. A potential application of near-critical thermoacoustic engines is low-grade heat recovery, due to the lower temperature difference required to drive the engine compared to classical engines.