Mode transition in a standing-wave thermoacoustic engine: A numerical study

This study investigates the mode transition phenomenon in a standing-wave thermoacous-tic engine (TAE) by means of computational fluid dynamics (CFD). Firstly, the steady-state responses of the TAE at selected temperature ratios are examined via continuous wavelet transform. The bifurcation diagram and spectral map indicate that, as the temperature ratio increases, the TAE experiences a series of bifurcations, through which first-mode periodic oscillations, quasiperiodic oscillations and second-mode periodic oscillations occur. Sec-ondly, the TAE performances in the initial decay/build-up, nonlinear saturation and steady states are studied. The onset of the first and/or second acoustic mode is identified via dy-namic mode decomposition. The oscillation frequencies and growth/attenuation rates from CFD agree well with those from the reduced-order network model. Nonlinear mode com-petition takes place during saturation in which the growth of one acoustic mode is affected or even totally inhibited by the growth of the other. At steady state, periodic oscillations exhibit a closed loop in the phase space whilst quasiperiodic oscillations generate a torus. The time-averaged acoustic energy density, acoustic intensity and efficiency increase with increasing temperature ratio. Finally, parametric studies are conducted to investigate the effects of the gap between stack plates and stack position on mode transition. It is found that the TAE will exhibit second-mode oscillations if the stack is near the closed end or the gap is small. Results in this study indicate that mode transition could become a novel approach to match the TAE with external loads for higher electric power outputs. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigates the mode transition phenomenon in a standing-wave thermoacoustic engine using computational fluid dynamics (CFD). It is found that with increasing temperature ratio, the TAE undergoes a series of bifurcations, leading to different oscillation modes. Nonlinear mode competition occurs during saturation, and the acoustic energy density, intensity and efficiency increase with increasing temperature ratio.