Investigation On Vapor-liquid Phase-change Travelling-wave Thermoacoustic Engine Operating At Near Room Temperatures

Thermoacoustic engine can convert thermal energy into acoustic power with the merits of simple construction and high reliability,showing promising prospects in the applications such as refrigeration and power generation.By introducing vapor-liquid phase change process into a thermoacoustic engine,the onset temperature of the engine can be significantly decreased.However,the available vapor-liquid phase-change thermoacoustic engines can not yet be driven by the near room temperature heat sources,while their pressure amplitude and output acoustic power are still relatively low.The present work focuses on the following studies to further explore the mechanism of vapor-liquid phase change in strengthening the thermoacoustic conversion process,to realize the near room temperature onset and to improve the efficiency of the vapor-liquid phase-change thermoacoustic engine driven by low-grade heat sources.(1)Thermodynamic analysis was carried out for vapor-liquid phase-change travelling-wave thermoacoustic engine to preselect candidate working fluids.Two thermodynamic models were developed for the thermoacoustic systems with pure working fluids and mixtures,respectively.The calculation results showed that different pure working fluids,azeotropic mixtures and zeotropic mixtures have marked effect on the net work per unit mass and net work per unit volume,while their thermal efficiencies change little when adopting different working fluids.Upon the preselection on the basis of thermodynamic models,six pure working fluids,including three azeotropic mixtures and nine zeotropic mixtures,were screened as the candidate working fluids.By adopting suitable working fluids,the vapor-liquid phase-change travelling-wave thermoacoustic engine can obtain higher net work per unit volume,thermal efficiency and relative Carnot efficiency than the travelling-wave thermoacoustic engine with gas as working fluid.(2)An acoustic-electric analogy model was developed to study and optimize the vapor-liquid phase-change travelling-wave thermoacoustic engine.The acoustic-electric analogy model included the heat loss due to viscous dissipation and the compliance caused by thermal-relaxation effect,with the influence of the thermophysical properties of regenerator material also considered.Compared with the previous linear temperature profile heat exchanger model,the present model can make better predictions of both onset temperature difference and resonant frequency.The influence of the diameters and lengths of the feedback connection tube,displacer cylinder,power cylinder,vapor connection tube and load tube on the onset temperature difference and resonant frequency was then numerically studied.In the investigated range,a smaller power cylinder diameter,a longer power cylinder length,a larger load tube diameter and a shorter load tube length are preferred for reducing the onset temperature difference.Besides,a larger load tube diameter,a shorter load tube length and a smaller displacer cylinder diameter can achieve higher resonant frequency.(3)A vapor-liquid phase-change travelling-wave thermoacoustic engine was designed and built,aiming for near room temperature onset.A screen-stacked regenerator was inserted between the heater and the cold heat exchanger,while a gas reservoir was introduced at the load tube.The onset,stable operation and damping processes were then analyzed.Hysteresis phenomenon was found during the onset and damping processes of the vapor-liquid phase-change thermoacoustic engine.Besides,the effects of the structural parameters of regenerator and phase adjustment unit as well as the working fluid on the onset temperature difference,damping temperature difference and resonant frequency were experimentally studied.Upon optimization,an onset temperature difference as low as 7.1°C(the corresponding hot temperature is 18.1°C)was achieved,which was the lowest one ever reported in the literatures.(4)Performance of the vapor-liquid phase-change travelling-wave thermoacoustic engine was experimentally investigated and feasibility of utilizing low-grade heat was verified.The influences of the regenerator material,phase adjustment unit and working fluid on the pressure ratio,acoustic power,thermal-to-acoustic efficiency and relative Carnot efficiency were experimentally studied.A higher regenerator available heat capacity,a longer liquid column length inside load tube and a larger gas reservoir volume can help to improve the output performance.The vapor-liquid phase-change travelling-wave thermoacoustic engine can operate stably in the temperature difference range of 80-160°C,with different pure working fluids,azeotropic mixtures and zeotropic mixtures as working fluids.The highest relative Carnot efficiency of 19.3% was achieved with R1234ze(E)as working fluid at a temperature difference of 139°C,while the corresponding thermal-to-acoustic efficiency and acoustic power were 6.3% and 11.3 W.This relative Carnot efficiency was the highest ever achieved,compared with those of the vapor-liquid phase-change thermoacoustic systems reported in the literatures.