Numerical studies on mode transition and performance of the thermoacoustic engine coupled with acoustic pressure amplifier tube and load

Acoustic pressure amplifier tubes (APAT) have shown excellent performance in thermoacoustic engine (TAE) systems when coupled with loads. Nonetheless, the oscillatory mode transition caused by the introduction of APAT and loads can significantly influence system performance. In order to investigate the effect of APAT dimensions and load impedances on the mode transition and system performance, the TAE system coupled with APAT and load was numerically solved by CFD software. Combined with the nonlinear dynamic methods, three oscillatory states are found in the coupled system: low frequency limit cycle oscillation, quasi-periodic oscillation, and high frequency limit cycle oscillation. The parameter selection scheme favoring the excitation of high frequency modes is also given. There are the optimal APAT length, diameter, position and acoustic resistance, to allow the load to obtain the maximum acoustic power. Comparing the system performance under different oscillatory modes, it is found that the existence of high frequency modes is helpful to take full advantage of the APAT's acoustic pressure amplification effect. Furthermore, the correspondence between the oscillatory modes, the pressure distributions and the stream directions are identified. Based on this, the principle of APAT's acoustic pressure amplification under different modes is explained, and the change tendency of high frequency and low frequency with APAT length and position is predicted reasonably. We believe that this work will provide a reference for the design and optimisation of the couple between TAE and load.