Ultrasonic thermoacoustic prime movers.

Thermoacoustic devices that generate sound with frequencies in the near ultrasonic range (∼21 kHz) have been developed. These devices consist of a wave resonator that is 4.1 mm in length, with hot and cold heat exchangers made of copper mesh placed at the resonator’s midpoint. Between these is sandwiched a stainless steel stack; the working fluid is air at 1 atmosphere. Heat is injected to the hot heat exchanger, creating a temperature gradient across the stack. When this exceeds a critical temperature gradient, acoustic oscillations ensue with intensities over 140 dB. The performance of a given engine depends drastically on the alignment achieved in the assembly process. Due to this variability, temperature differences at onset ranged from 85°C to 180°C. In order to increase positive feedback in the systems, single engines were coupled to a resonant cavity, the end of which was fitted with a pressure transducer. The use of a cavity also provides an acoustic coupling mechanism in which two ultrasonic engines can be attached to the same cavity, allowing for the investigation of synchronization of these devices in this regime. [Research supported by the U.S. Army Space and Missile Defense Command/U.S. Army Forces Strategic Command.]