Analysis and performance of a large thermoacoustic engine

Abstract

Measurements and analysis of a 13-cm-diam thermoacoustic engine are presented. At its most powerful operating point, using 13.8-bar helium, the engine delivered 630 W to an external acoustic load, converting heat to delivered acoustic power with an efficiency of 9%. At low acoustic amplitudes, where (linear) thermoacoustic theory is expected to apply, measurements of temperature difference and frequency agree with the predictions of theory to within 4%, over conditions spanning factors of 4 in mean pressure, 10 in pressure amplitude, 6 in frequency, and 3 in gas sound speeds. But measurements of the square of pressure amplitude versus heater power differ from the predictions of theory by 20%, twice the estimated uncertainty in the results. At higher pressure amplitudes (up to 16% of the mean pressure), even more significant deviation from existing thermoacoustic theory is observed. Several causes of this amplitude-dependent deviation are identified, including resonance-enhanced harmonic content in the acoustic wave, and a new, first-order temperature defect in thermoacoustic heat exchangers. These causes explain some, but not all, of the amplitude-dependent deviation of the high-amplitude measurements from existing (linear) theory.