Study On The Propagation And Evolution Principles In Non-Homogeneous Acoustic Media

Compared with other energy conversion and utilization technology, thermoacoustics is a new field which needs further research. Thermoacoustic device has comparatively simple configuration, high reliability, long duration and is environmentally friendly; it also has the potential to be compacted and lightened and the capacity of utilizing low-grade thermal energy. The device configuration and performance have been advanced and improved in these several decades. Moreover, linear thermoacoustic theory has been well established. However, there still exist some important issues not studied adequately, which leads to the weak understanding about non-linear process or effect in real system, such as onset, mode evolution, acoustic stream, loop flow and harmonics. And this cognitive limitation restricts the constructor to adopt appropriate approaches to increase the specific power density, improve the stability and advance the efficiency. Therefore this thesis focuses on the thermoacoustic stability and finite amplitude acoustic field in non-homogeneous media to reveal the propagation and evolution principles in such media, under which the related engineering application is discussed.In the first part of this thesis, the propagation principles of small amplitude sound in tube and experimental technology related to thermoacoustic engineering have been refined and concluded. Specifically, the derived method of the wave equation is proposed and the mathematical forms of complex density and compressibility keeping unchangeable under the present thermoacoustic condition are demonstrated. The influence of viscosity and thermal release on various thermoacoustic elements is analyzed. Also the proper combination of thermal release with viscosity release can reinforce the thermoacoustic effect. Active control principle in acoustics is analyzed and its potential contribution to Stirling device is predicted, then, the performance parameters of the combined noise absorption system have been calculated in nondimensional way. The working mechanism of such system is demonstrated by equivalent network topology.In order to reveal the predominant physical mechanism of onset process, the frequency-domain method, the thermoacoustic oscillator, and time-field dynamic method describing the evolution and the coexistence of modes are originally proposed. In frequency-domain method, thermoacoustic engines are regarded as thermoacoustic oscillators consisting of the active network and the passive network. Accordingly, the two-port Y-parameter for relevant component is derived, and standing wave and traveling wave thermoacoustic engine are described by the negative-resistance and feedback model, respectively. The relevant two-port network topology is given as well. The startup criteria for thermoacoustic oscillators are obtained using Nyquist instability criterion. Moreover, by topological graphs it is verified that standing wave engines would start up in a negative-resistance state and there would exist high frequency modes in thermoacoustic-Stirling engines. By investigating into the frequency response of thermoacoustic system, this method proposed can achieve such an objective that these effects of operating and structural parameters of engine on startup modes and startup temperature can be revealed in an analytical way. On the other hand, based on the mode synchronization of different locations in acoustic field during the evolution process, the multimode oscillation in nonhomogenous media reduced to multimode vibration of particles and then the initial-boundary value problem reduced to initial value problem by the method of separation of variables. From dynamic viewpoint, the relevant bifurcation types of some multimode effects (such as coexistence, oppression, evolution and so on) are identified. The orbit stability of the "limit cycle" and its scientific implication are analyzed by using the method of multiple scales. The dynamic equations of two coupled modes are brought forth based on the Van der Pol equation which was used to describe the self-excited vibration. Also the quantification method for such equation parameters is given. And then the capacity of the equations is demonstrated with the help of the Maccari's work. In addition, on the basis of the self excitation shown by thermoacoustic onset, the feature of forced self-excited oscillation system, frequency capture, can be utilized to guide the construction of high-level power thermoacoustic-oscillator array.In the last part, the engineering requirements for the computational mode of the finite amplitude acoustic field in non-homogeneous media are specified. And the disadvantage of the traditional acoustic method dealing with finite amplitude problem is pointed out, which lies in the mismatch of the perturbation expansion series and harmonic expansion series. Then this mismatch leads to the failure of the consistency between dimensional analysis and Fourier analysis. Thus an alternative approach, Lattice Gas Automaton, is applied to simulate the thermoacoustic device which is a typical non-homogeneous media. In the simulation, the collision rules of multi-body are defined; the thermoacoustic onsets under different temperature parameters are shown; the interior limitation of the mode is also mentioned.In the thesis, the author explores some fundamental and important scientific problem in thermoacoustics and proposed some original ideas, models and predictions. More in-depth research and relevant engineering applications can be fulfilled based on the present work.