| Gas wave refrigerator is suitable for gas expansion refrigeration process under high pressure and large liquid carrying conditions because of its simple structure and the advantages of allowing two-phase inlet.The core component of gas wave refrigerator is a pressure oscillation tube,in which the different energy levels of gasses are converted using the motion of shock waves and expansion waves.However,in industrial processes,the inlet gas of the gas wave refrigerator usually contains a large amount of condensable components,which are cooled and liquefied in the oscillation tube to form condensate.The condensate accumulates in the tube in large quantities due to uninterrupted inlet gas and long residence time,leading to reduced refrigeration efficiency and increased vibration of the oscillation tube,which may damage the oscillation tube in severe cases.Therefore,it is of great scientific significance and industrial application value to reveal the gas-liquid flow characteristics inside the tube and realize real-time detection for the high-frequency changing non-constant flow field inside the oscillation tube.This thesis takes the wave system and gas-liquid motion in the oscillation tube as the research object,and uses acoustic emission detection,pressure detection,high-speed camera technologies to analyze the development process and fluctuation characteristics of the shock wave in the oscillation tube,and reveal the motion behavior and dispersion law of liquid under the action of high-frequency pulsated gas flow,realizing the online detection of the morphology and content of the liquid in the oscillation tube.The main research achievements of this thesis are as follows:(1)Based on the verification of the sound-pressure consistency experiment,the mechanism of the acoustic signal generation in the oscillation tube is revealed,and the motion behavior of the wave system in the oscillation tube is quantitatively characterized by the effective analysis of the acoustic signal characteristics(e.g.,morphology,intensity,and spectral characteristics).The results show that the main moving wave system in the oscillation tube include the incident shock wave and expansion wave,and the pressure oscillation(pressure differential)during the motion of the wave system is the main source of the acoustic signal.When the acoustic signal morphology changes from a multi-peak distribution to a single strong sharp peak shape,it marks the shock wave formation.By quantitative portrayal of the acoustic signal morphology,the location of the convergence of the shock wave can be accurately identified based on the peak shape transformation,and the identification results are consistent with theoretical calculations.Furthermore,the energy of the acoustic signal comprehensively reflects the catch-up rate and intensity of the compression wave.The faster the compression wave catch-up rate,the higher the pressure value,the greater the shock wave/compression wave intensity,and the higher the energy of the acoustic signal.In addition,the frequency spectrum of the acoustic signal shows the oscillation characteristics of the shock wave and the expansion wave,both of which do not vary with the gas injection frequency,but show high frequency and intensity of the shock wave motion(acoustic signal frequency>30 kHz),while the expansion wave motion is relatively low frequency and low intensity(acoustic signal frequency 15～30 kHz).(2)The liquid motion under the action of high-frequency pulsated gas flow in the oscillation tube was explored,and the mechanism was revealed through the force analysis of the liquid.The results show that the liquid in the oscillation tube periodically moves toward the gas outlet in the form of droplets and liquid flow,and the motion of the liquid in a single cycle can be divided into three periods(flattening,spreadingcoalescing,and constrictive splitting).The force analysis revealed that the dominant forces of each period were the gas aerodynamic force,the gas aerodynamic force/viscous force,and the surface tension,respectively.Furthermore,as a macroscopic manifestation of liquid motion,the gas-liquid flow pattern in the oscillation tube undergoes the evolution process from stratified entrained flow to stratified flow along the tube.An obvious difference between the two types of flow patterns is the number of droplets and the proportion of medium droplets.The reason for the evolution of the flow pattern lies in the change of the gas aerodynamic force along the tube.As the distance from the gas injection increases,the gas aerodynamic force gradually decreases,and the position of the action shifts to the bottom of the pipe,leading to a lower probability of droplet fragmentation and an increased probability of coalescence.Finally,it is easier to form large droplets and liquid flow,therefore promoting the evolution of flow pattern.Finally,the gas-liquid flow patterns in the oscillating tube under different operating conditions are summarized through extensive experiments.(3)The influence of the introduction of liquid on the motion of the wave system in the oscillation tube was explored,and the identification of liquid,the prediction of gas-liquid flow patterns,and the detection of liquid content were realized by quantifying the difference in acoustic signals under different liquid content conditions.The results show that the presence of liquid could move the converging position of the shock wave toward the gas injection port.Due to the influence of liquid motion in the flattening period and the spreading-coalescing period,the oscillation in the pressure drop section is more significant,and the regularity of the acoustic signal decreases(the peak value fluctuate significantly).The identification of the liquid in the oscillation tube can be realized by using the standard deviation of the peak value of the acoustic signal,and the inflection point of the standard deviation can also be used to predict the transition position of the gas-liquid flow pattern.In addition,the existence of liquid will lead to the attenuation of the acoustic signal intensity and the increase of high-frequency components.At the same time,the introduction of liquid will cause a decrease in the strength of acoustic signals and an increase in high-frequency components.This is due to the dispersed liquid weakening the impact of the gas on the wall,while also increasing the gas-liquid-wall interaction(the effect of gas being conducted through the liquid to the wall).Based on this characteristic,it is possible to detect the liquid content inside the oscillation tube under low injection gas frequency conditions using highfrequency acoustic energy. |
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