Development and analysis of non-linearity in the pressure waves resulting from thermoacoustic heat engines

Thermoacoustic heat engines are intrinsically simple, reliable, environmentally friendly and reasonably efficient. The second law of thermodynamics dictates that higher conversion efficiencies necessitates high temperatures at the hot side of the stack for higher efficiencies. In this work, a thermoacoustic prototype was built and tested. Operating the engine at a temperature just above the onset temperature generates an acoustic wave at the fundamental mode (with no nonlinearity observed) as defined by the engine’s geometry with heat to acoustic conversion efficiency relatively low. As the temperature difference across the stack increases, the resulting acoustic wave distorts with observed nonlinearities. Yet, the heat to acoustic conversion efficiency improves. Further increase in the temperature difference across the stack increases the distortion in the wave with development of more nonlinearity. Moreover, the heat to acoustic conversion efficiency decreases. Numerical analysis of the resulting waves shows that the AC-coupled distorted waves can be numerically re-constructed by considering only the harmonics of the fundamental frequency of the wave, indicating that at the operation range considered harmonics were the only source of non-linearity.