Fundamental measurements in standing-wave and traveling-wave thermoacoustics

The periodic compressions and expansions of the gas in an acoustic wave, combined with heat exchange with external reservoirs, generate a rich variety of thermoacoustic processes. A new class of heat pumps and prime-movers make use of these effects. In a traditional standing-wave thermoacoustic device, the energy conversion occurs in the stack, influencing the device efficiency. A major part of the dissertation focuses on fundamental measurements of stack properties. Different geometries and nonlinear effects are studied with a versatile volume-modulation technique yielding the complex compressibility, C(ω) (equivalent to the cycle P-V diagram), for a variety of temperature gradients and frequencies. The thermoacoustic function F_T, describing the fluid oscillatory temperature field in the stack, can be calculated from compressibility measurements with a zero temperature gradient. When a gradient is imposed on the stack, thermoacoustic gain can be empirically determined from the compressibility, for any stack geometry.; The method was first tested with a circular geometry for which theoretical calculations were possible. Good agreement was found between theory and experiment. The technique was further adapted to look at the effect of high displacement amplitudes relative to the stack length, beyond the range of validity of the linear theory.; The measurements were extended to look at pin-array stacks, which are particularly interesting because of their convex geometry, which enhances the ratio of useful power to viscous dissipation. Pins parallel and perpendicular to the acoustic axis were studied. The results of the zero temperature gradient experiment tested theoretical assumptions regarding thermal and viscous effects. With a nonzero gradient, thermoacoustic gain was measured and found to be the same for both geometries.; Traveling-wave thermoacoustics is discussed in the last chapter. Earlier work showed that amplification of pure traveling waves is surmounted by viscous losses, unless the acoustic impedance in the regenerator is increased. Traveling and standing waves were produced in a resonator in a controlled ratio, with the regenerator placed at the velocity node of the standing wave. Traveling-wave amplification across the regenerator was significantly improved by the addition of the standing wave component. An optimal standing-wave-ratio, specific to a given regenerator/heat-exchanger geometry was observed.