A method to suppress harmonics in standing-wave thermoacoustic engines

Abstract

Harmonic generation in thermoacoustic engines is regarded as a non-linear loss mechanism that extracts acoustic power from the fundamental wave into harmonics. It is shown that the use of specifically designed insert that limits the gas flow area over a very-limited part of the resonator can suppress this non-linear loss mechanism causing a significant increase in the generated acoustic power in the fundamental mode. In this experimental work, a standing-wave thermoacoustic engine is built and operated without inserts and with inserts of different shapes, porosities, and thicknesses. The self-generated dynamic pressure waves are captured under different operating conditions and then are decomposed into a fundamental component and harmonics. Results for different inserts are presented and discussed. All inserts caused lower harmonic content with respect to the no-insert case but inserts of low gas flow area cause severe flow blockage and thus a severe reduction in the produced acoustic powers. Results analyze the relationships between the generated acoustic power in the fundamental mode and the amplitudes of the dynamic pressures of the fundamental, first harmonic, and second harmonics. The blockage effects caused by the insert are discussed.