| Thermoacoustic engine can convert thermal energy into acoustic power with the merits of simple construction,high reliability and environmental benignity.Compared with the conventional fluid-based thermoacoustic engines,solid-state thermoacoustic engines proposed in recent years have the potential to obtain higher energy density,providing a new technical route for thermoacoustics.However,the research on solid-state thermoacoustic engine is still in the preliminary stage,and its working mechanism and prototype still need to be further investigated.As one of the most significant characteristics of thermoacoustic engines,an in-depth study of acoustic characteristics is crucial for understanding the working mechanism of thermoacoustic conversion and guiding the design of thermoacoustic engines.Aiming to deepen the understanding of the working mechanism and guide the development of the prototype,the present work focuses on the acoustic characteristics analysis of solid-state thermoacoustic engines based on fluid-solid analogy,and the main research work is carried out as follows:1.The mechanism affecting the acoustic characteristics of the fluid-based and solid-state thermoacoustic engines was analogically analyzed to lay the theoretical foundation for the acoustic characteristics adjustment.Based on the wave equations,acoustic model was developed for fluid-based and solid-state thermoacoustic engines with different combinations of impedance boundary conditions,including inductive end,capacitive end,zero-impedance end,and infinite-impedance end,respectively.The comparison showed that due to the similarity in the wave equations,fluid-based and solid-state thermoacoustic engines have similar acoustic characteristics when the same combination of impedance boundary conditions is prescribed.For the thermoacoustic engine with a prescribed structure and size,the dimensionless angular frequency and acoustic field are only dependent on the characteristic impedance of the working media and the acoustic impedance at the ends of the system.The adjustable range of the dimensionless angular frequency and acoustic field can be changed by selecting different combinations of impedance boundary conditions,and the acoustic characteristics can be adjusted by varying the acoustic impedance at the boundaries.2.The feasibility of acoustic characteristics adjustment and its effectiveness in optimizing the performance were experimentally verified based on a fluid-based thermoacoustic engine.The combination of capacitive and inductive ends was adopted to build the fluid-based thermoacoustic engine,where the compliance tube and the liquid column were used to realize the capacitive and inductive ends,respectively,and the acoustic impedance was adjusted by changing their lengths.The procedure of acoustic characteristic adjustment was provided and the experiments were carried out.The results showed that the measured frequencies were in good agreement with the calculated ones,verifying the feasibility of the acoustic characteristic adjustment.On this basis,the influence of acoustic field and resonance frequency on the onset and damping performance as well as the output performance were investigated.Through optimizing the acoustic characteristics,the onset and damping temperature differences were decreased by 66.1% and 67.7%,respectively,and the acoustic power,thermoacoustic conversion efficiency,and relative Carnot efficiency were increased by 4.9 times,5.1 times,and 5.4 times,respectively.3.Thermoacoustic model of a solid-state thermoacoustic engine was developed based on the transfer matrix method to study the effect of acoustic characteristics on the performance.The linear thermoacoustic theory for solids was established and analogized to that for fluids,showing that both have a similar form.On this basis,the transfer matrix method used in simulating the fluid-based thermoacoustic engines was employed to develop the thermoacoustic model of the solid-state thermoacoustic engine,and the influence of the acoustic field and resonance frequency on the amplitude growth ratio were investigated.The results indicated that the acoustic field determines the magnitude and direction of the lowest temperature gradient to strengthen the thermoacoustic oscillation.A high growth ratio is obtained at a low angular frequency and an acoustic field around 1/8 wavelength from the stress antinode.According to the acoustic model,only the combination of capacitive and inductive ends can simultaneously satisfy the aforementioned acoustic characteristics,and the selection of this combination helps to improve the amplitude growth ratio of the solid-state thermoacoustic engine.4.The prototype of a solid-state thermoacoustic engine was designed and constructed to verify the feasibility of acoustic characteristics adjustment.The prototype was designed in terms of impedance boundary conditions,cross-section shape and material of solid rod.The combination of capacitive and inductive ends was adopted,where the spring load and the mass load were used to realize the capacitive and inductive ends,respectively.The acoustic impedance was adjusted by changing the stiffness and the mass of the loads.A metal rod with annular crosssection shape was selected as the medium for the thermoacoustic conversion.On this basis,the experimental system was constructed.The procedure of acoustic characteristic adjustment was introduced and the experiments were carried out.The results showed that the maximum deviation between the measured frequencies under different acoustic fields did not exceed 4.1%,and the measured and calculated frequencies showed in good agreement,verifying the feasibility of the acoustic characteristic adjustment. |
