Thermal-acoustic-electrical Coupling And Conversion Mechanisms Of A Traveling-wave Thermoacoustic Power Generation System

Traveling-wave thermoacoustic power generation systems have great advantages of high efficiency, high reliability, simple structure, low cost and capability of ultilizing low-grade thermal energy, showing a promising prospect in applications such as solar power generation, industrial waste heat recovery, combined cooling heating and power, and distributed energy supply, etc. Traveling-wave thermoacoustic power generation system is a strongly coupled nonlinear thermoacoustic and electromechanical system. The in-depth understanding of the mechanisms of the coupled oscillation and energy conversion is of great importance for the stable and efficient operations of the system. However, it still lacks a comprehensive study on the above key issues at present. The characteristics of the coupling and the matching between thermoacoustic engines and linear alternators have not yet been clearly addressed, and the overall performance should be further improved. The present work is devoted to revealing the coupling and conversion mechanisms by the following theoretical and experimental studies:1. A time-domain network model for the simulations of transient process of traveling-wave thermoacoustic systems is developed and built. Numerical simulations and experimental verifications of the onset characteristics of a traveling-wave thermoacoustic engine are conducted.Thermoacoustic onset is one of the fundamental problems in thermoacoustics. Accurately simulating onset process and predicting onset temperature and frequency are very meaningful for revealing onset mechanism and decreasing onset temperature. In this work, a time-domain network model is built for traveling-wave thermoacoustic systems. The viscous and heat transfer terms in the one-dimensional governing equations for alternating flow are deduced from linear thermoacoustic theory. The thermoacoustic effects including resistance, inertance, compliance and thermal-relaxation, are included in all acoustic components. The time-domain governing equations of porous media is adopted for regenerator. Numerical simulations of the onset characteristics of a traveling-wave thermoacoustic engine are carried out based on the proposed model. The dynamic pressure waveform evolutions, onset temperatures, frequencies, quality factors and the distributions of main physical parameters are obtained. It is shown that the thermal-relaxation effect in regenerator remarkably increases the system dissipations, and has a great influence on thermoacoustic onset process. The experimental validations of onset temperatures, frequencies, and quality factors indicate that the proposed model is capable of accurately predicting the onset characteristics, and describing the acoustic power generation and dissipation in the traveling-wave thermoacoustic engine below the onset. It is convenient for the present model to introduce mechanical moving components and thus to be further extended for the dynamic simulations of multi-physical fields coupling thermoacoustic systems.2. Numerical simulations and experimental investigations of the coupled oscillation processes of the double acoustic sources composed of a traveling-wave thermoacoustic engine and a linear alternator are first carried out. The mechanisms of the beating coupled oscillations are revealed. The factors that affect the oscillation frequencies as well as their distributions are obtained and clearly analyzed.The unstable beating oscillation, which is a type of periodical energy transfer process between thermoacoustic acoustic source and mechanical oscillator acoustic source, may occur in traveling-wave thermoacoustic power generation systems with multiple resonant components at certain circumstances. The integrated transient model of the double acoustic sources system is built by coupling the time-domain network of the traveling-wave thermoacoustic engine and the time-domain kinetic model of the linear alternator. The dynamic response characteristics and the frequency response curves of the thermoacoustic acoustic source under different excitation frequencies are obtained with the excitation of the linear alternator acoustic source. The calculated and experimental results indicate that thermoacoustic engine has sensitive selectivity of the excitation frequency, and beating phenomenon occurs at the initial stage of the excitation if the excitation frequency is far away from the resonant frequency. Numerical simulation and experimental investigation on the beating oscillation process in the free damping of the double acoustic sources system are then conducted. It is shown that beating effect results from the competition of the double acoustic sources with similar intensities. The two similar frequencies occurred in the system are separately controlled by the thermoacoustic and linear alternator acoustic sources. The intensity of thermoacoustic acoustic source is enhanced with the increase of heating temperature, and becomes dominant until the system oscillates at the frequency controlled by the thermoacoustic acoustic source. Changing moving mass, spring stiffness, and the length of resonator can change the beating frequencies but cannot eliminate the beating effect. The relationships between the two frequencies and the above three parameters are represented by hyperbolic curves, the asymptotic lines of which are the resonant frequencies of the thermoacoustic engine and the linear alternator. This research is instructive in the design of the matching between thermoacoustic engines and their loads. It is also meaningful for the synergetic and stable operation of complex thermoacoustic systems with multiple acoustic sources.3. The basic principle for the acoustic impedance matching of the traveling-wave thermoacoustic power generation system is proposed, and the mechanism of acoustic matching is fully explained. An effective acoustic matching between traveling-wave thermoacoustic engine and linear alternators is finally realized.The acoustic matching between traveling-wave thermoacoustic engines and linear alternators is critical to the efficient thermal-acoustic-electrical energy conversion. However, the underlying mechanism has not been fully uncovered from the intrinsic viewpoint of acoustic impedance, and effective approaches to realize the acoustic matching have not yet been proposed. Inspired by the impedance matching principle in an ac power circuit, this dissertation systematically proposes the basic principle and the approach for the acoustic impedance matching of the thermoacoustic power generation system. The output acoustic impedance of thermoacoustic engine and the input acoustic impedance of linear alternators are seperately calculated and analyzed. The acoustic impedance requirements for the optimal working conditions and the methods to modulate the acoustic impedances are then obtained. Acoustically well matched condition is finally achieved by coupling traveling-wave thermoacoustic engine and linear alternator at the optimal acoustic impedance. The results also indicate that the output acoustic impedance of thermoacoustic engine can be remarkably changed by changing the output position, which further affects the acoustic matching of the system. The input acoustic impedance of linear alternators can be effectively modulated by changing operating frequency, capacitance, and load resistance to the ranges that meet the requirements of the thermoacoustic engine, which realizes the acoustic matching between them. When the working gas is helium of3.16MPa and the coupling position is at the resonator, acoustically matched condition for the thermoacoustic power generation system is realized. The maximum electric power of750.4W and the highest thermal-to-electric efficiency of0.163are achieved. It clearly shows that one of the most critical factors for the efficient operation of the thermoacoustic power generation system is the well matched acoustic impedance. This study sheds light on the way to solve the matching problems for thermoacoustic power generation systems, as well as other acoustic systems, such as thermoacoustic refrigeration, pulse tube refrigeration, etc.