Fluctuation before onset of oscillations in a thermoacoustic device

Abstract

The onset of oscillations in a resonant device where a temperature gradient produces a sound is a fundamentally interesting problem as it consists of a transition where random gas motion, when biased, produces an almost pure sound. This was studied in small 1/4-wave resonant tubes containing a fibrous stack across which a temperature gradient was maintained by means of heat exchangers at each end. The temperature gradient biases the random motion of the gas to trigger a standing wave in the resonator. The acoustic spectrum was measured as the temperature gradient approached the onset for oscillations. Factors affecting the transition to oscillation consist of stack gain, quality factor of the resonator, magnitude of the temperature gradient, and acoustic load. The directed diffusion of the gas along the stack leads to a series of sharp pressure pulses whose stochastic behavior triggers resonant oscillations in the tube. Onset temperature differences less than 50 °C have been observed and this could further be reduced by a suitable choice of parameters affecting the onset. This is an example of a thermal ratchet engine biased by a temperature gradient.