Mechanism And Optimization Of Acoustic Impedance Matching Of Looped Travelling-Wave Thermoacoustic Engine

Thermoacoustic heat engine is a type of energy conversion machine,converting thermal energy into mechanical energy or consuming mechanical energy to pump heat,with the outstanding merits of high reliability,long operating life and environmental friendliness.Over the past three decades,significant progresses have been made in both theoretical study and experimental prototypes.However,the potential industrial applications of the thermoacoustic engines face many difficulties,considering the unavoidabled competition with traditional thermal engines.The thermoacoustic engines powered by low-grade heat sources are regarded as a prospective field for commercializing the thermoacoustic technique,and have been focused on in recent years by many researchers.The looped travelling-wave thermoacoustic engine has been expected to realize efficient thermoacoustic conversion and acoustic wave propagation,thus the potential utilization of low-grade thermal energy.To further explore the mechanism of acoustic impedance matching,to reduce the onset temperature,and to improve the thermal efficiency of the looped thermoacoustic engine powered by low-grade heat,the following theoretical and experimental studies have been carried out in the present work:(1)The mechanism of the phase adjustment with the compliance/resistance tube in the looped thermoacoustic engine is clarified,and the phase adjustment effect is verified by theoretical analysis and experimental study.Two phase adjustors(i.e.,compliance tube and resistance tube),which can help to form quasi-soft/hard boundary,are used to create the appropriate acoustic field in the looped thermoacoustic engine.A single-stage looped thermoacoustic engine is constructed to compare the performance when no phase adjustor,a compliance tube or a resistance tube is adopted,respectively.The experimental results show that the onset temperature can be dramatically reduced by installing the compliance tube or the resistance tube at proper position.The lowest onset temperature is 40? for the case with a compliance tube(the corresponding temperature difference is 31?),using CO2 of 2.37 MPa as working fluid.(2)Single-stage,two-stage,three-stage and four-stage looped thermoacoustic engines capable of utilizing low-grade heat are designed and constructed.There are four pure travelling-wave points(where the phase difference between the pressure and velocity amplitudes is zero)in the typical acoustic field of a looped thermoacoustic engine.The distance of the adjacent points is approximately 1/4 wavelength.The acoustic fields at all of the four points satisfy the needs for efficient thermoacoustic conversion.Based on this deduction and the proposed phase adjustors,four types of thermoacoustic engines which can utilize low-grade heat are designed and constructed,among which the single-stage,two-stage and three-stage configurations are asymmetric,which breaks the restriction of symmetric configuration of previous systems.The lowest onset temperature obtained in the experiments is as low as 29?(the corresponding temperature difference is 17?),which can be achieved in both three-stage and four-stage looped thermoacoustic engines,with CO2 of 1 MPa or 1.5 MPa as the working fluid.This is the lowest onset temperature ever achieved,compared with the results of thermoacoustic systems with regular dimensions reported in the literatures.(3)Based on the looped thermoacoustic engine,the applicability of the thermoacoustic electric generation or thermoacoustic refrigeration driven by low-grade heat is verified.The three-stage looped thermoacoustic engine is coupled with a linear alternator to form a thermoacoustic electric generation system.Upon optimization,a thermal-to-electric efficiency of 1.51%is achieved from the experiment when the hot end temperature is 120?.Moreover,a looped travelling-wave thermoacoustic refrigerator is proposed.Simulation results indicate that a looped travelling-wave thermoacoustic refrigerator can be driven by heat source between 210? and 250?,with the ambient temperature of 30?.The achieved cooling temperature is-3°C,with the overall coefficient of performance above 0.4 and the relative Carnot coefficient of performance over 13%.