Effect of gas mixture on temperature and mass streaming in a phase-change thermoacoustic engine

Wet thermoacoustic engines (WTEs) are simple, robust energy conversion devices that employ a condensable vapor to generate acoustic power from heat. While previous studies focused on the potential of WTE technology to convert low-grade heat to power, this work focuses on the WTE's dependence on the working gas mixture, unique mass transport, and low working temperature properties, which make it a potential candidate for thermally driven separation processes, e.g., distillation and water desalination. In a set of experiments, the steady-state behavior of a water-based, standing-wave WTE was studied. The results highlight a remarkable trait of this system—a WTE is capable of producing a substantial flux of water vapor while exhibiting an operating hot-side temperature significantly lower than the boiling point. This “limiting temperature” is independent of the heat input to the system—theoretically predicted in systems that have no external “load” that consumes the acoustic power. In addition, the results suggest that the characteristics of the working gas play an important role in determining the device behavior. It was shown that in the tested range, a heavier, more complex-structured gas has significantly improved the steady-state performance of the engine. Specifically, a WTE working with SF₆ as its working gas exhibited a vapor mass flux nearly three times greater than simple evaporation at the same heating power while maintaining a temperature as low as 57 °C. The link between gas characteristics and engine behavior is outlined. In conclusion, potential applications of the device and future research avenues are discussed.