Study of thermoacoustic engines operating at frequencies between 2 kHz and 25 kHz

The research in this dissertation deals with the development of high frequency thermoacoustic heat engines that convert heat to sound. They are important for applications in thermoacoustic coolers and heat pumps, energy converters, and for the production of high intensity coherent sound. The emphasis in this work is on high frequency operation covering the frequency range of 2 kHz to 21 kHz. The devices are resonant and hence their size scales inversely with operating frequency; they ranged in length from 8 cm to 3 mm. This type of engine is a self-sustained oscillator where positive feedback determines the threshold level for acoustic oscillations. It consists of a resonator, working gas, a stack of plates, and a heat exchanger at each end of the stack. A study of this provides data on the onset of oscillations and on the importance of the standing wave ratio in the resonator which depends on the ratio of resonator length to its diameter. Since oscillations are initiated by a temperature gradient along the stack which is between two heat exchangers, this study has investigated the role of positive feedback in determining the threshold for onset of oscillations. By a careful choice of geometric parameters the temperature difference for onset of oscillations can be varied by more than a factor of 10. Modeling of this self-sustained oscillator driven by heat input has provided a quantitative description of the steady-state acoustic oscillations, from their initiation to maximum output. At equilibrium the acoustic output from this device is a balance between heat input and losses such as viscous, thermal, and radiative.; The transition from random acoustic fluctuations to coherent sound at the threshold was investigated from the point of view of a non-equilibrium phase transition. A similar approach had been taken in studies of lasers. The study in the acoustic case consisted of the measurement and analysis of acoustic pressure oscillations inside the device resonator as the temperature difference along the stack was increased gradually. Auto-correlation techniques were used in search of acoustic wave coherence as the phase transition was crossed from the disordered state of fluctuations to the ordered state where coherent sound was produced. As a result of this study, small-temperature differences for onset heat devices were developed for sound production from heat. This is particularly important for applications of this type of device where large temperature differences cannot be tolerated, as for examples in computers and other electronic systems. One application of the device developed here is in the thermoacoustic cooler where it can perform as a high power acoustic driver (it can produce sound levels above 160 dB); it can also be the basic element for energy conversion. There is a variety of other applications using this type of oscillator, which works from the acoustic range to the ultrasonic range. Moreover, the study presented here opens the technology for miniaturizing thermoacoustic engines.