Impedance and stability analysis of two thermoacoustic prime movers.

Abstract

Thermoacoustic heat engines can be used to produce sound from heat and to transport heat using sound. The air-filled prime mover studied is a quarter wavelength resonator that produces sound at nominally 116 Hz for a temperature difference of ΔT=176 K. Specific acoustic impedance at the mouth of the prime mover was measured as a function of the temperature difference between the hot and cold heat exchangers. The real part of the impedance changes sign for sufficiently large temperature differences, indicating the possibility of sound production. The theoretical expression for radiation impedance of an open pipe was overlaid on the impedance curves. The operating point was confirmed from the intersection of experimental and theoretical impedance curves. The stability curve was computed for a helium-filled prime mover as a function of the ambient pressure, P0, for the first two modes. The first mode has a minimum ΔT=154.6 K for P0=173 kPa and the second mode for ΔT=455.4 K for P0=200 kPa. The resonant frequencies of these modes are approximately 310 and 610 Hz. The quality factor, which characterizes the exponential decay or initial exponential growth of an initial perturbation, was computed as a function of ΔT at fixed P0=173 kPa for ΔT well in excess of the onset temperature. [Work supported by ONR.]