The best working gases for thermoacoustic refrigeration have high ratios of specific heats and low Prandtl numbers. These properties can be optimized by the use of a mixture of light and heavy noble gases. In this paper it is shown that light noble gas-heavy polyatomic gas mixtures can result in useful working gases. In addition, it is demonstrated that the onset temperature of a heat driven prime mover can be minimized with a gas with large Prandtl number and small ratio of specific heats. The gas properties must be optimized for the particular application of thermoacoustics; it cannot be assumed that high specific heat ratio and low Prandtl number are always desirable.

A theoretical analysis of radial wave thermoacoustic engines in cylindrical resonators is developed. Impedance and pressure translation equations are presented for open sections of the resonator and for heat exchangers. Coupled first-order differential equations are given for pressure and impedance in the temperature gradient supporting engine section ͑stack͒. These quantities are used to calculate heat and work flows and to predict engine performance. Theory and design of a variable quality factor resonator for enhanced photoacoustic spectroscopy are presented. The short stack approximation is developed for the radial geometry and is used along with plane-wave equations to compare refrigerator performance for these two geometries. Results of the comparison are that engines in the plane-wave geometry are better overall refrigerators when maximizing the coefficient of performance and cooling capacity together.

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Thermoacoustic engines are placed in resonant cavities for Q amplification. Variations in the cross-sectional area of the resonator serve to reduce device volume and to minimize nonlinear distortion by detuning higher harmonics. In the case of a thermoacoustic sound source, these cross-sectional variations in the resonator area may be carried to an extreme such that the resonator approaches the Helmholtz limit. This limit produces a dimensionally compact, low-frequency thermoacoustic sound source. A thermoacoustic sound source in the Helmholtz limit has been constructed. The measured particle velocity and acoustic pressure in the device will be compared to Helmholtz idealization. [Work supported by ONR.]

The purpose of this research is to branch out from thermoacoustics in the plane wave geometry to study radial wave thermoacoustic engines. The radial wave prime mover is described. Experimental results for the temperature difference at which oscillations begin are compared with theoretical predictions. Predictive models often assume a uniform pore size and temperature continuity between the stack and heat exchangers; however, stacks of nonuniform pore size and temperature discontinuities between the stack and heat exchangers are common imperfections in experimental devices. The radial engine results are explained using a theoretical model which takes into account these prevalent construction flaws. Theory and experiment are shown to be in agreement after the complications are included. Spectral measurements show that an additional feature of the radial geometry is the anharmonicity of the resonant modes which significantly reduces nonlinear harmonic generation.