Research On Coupling Structure Of Quartz Tuning Fork And Resonant Tube In QEPAS Detection System Of Methane

Methane is widely used in many fields,such as aerospace engineering,seabed exploration,medical research and industrial production.However,methane has the characteristics of inflammable and explosive,and can be mixed with other organic matter in the air to form an explosive mixed gas.At the same time,the increase of methane concentration in the air will aggravate the greenhouse effect.Therefore,real-time and accurate detection of methane gas concentration is of great significance in ensuring industrial production,people ’s safety and environmental protection.Quartz enhanced photoacoustic spectroscopy detection technology is a technology based on photoacoustic effect,which has many advantages such as low detection cost and large detection range.In the quartz enhanced photoacoustic spectroscopy detection system,the detection module composed of a tuning fork and a resonant tube is one of the important detection units.The structure and parameters of the detection module coupling directly affect the sensitivity of the entire detection system.Therefore,this thesis takes the detection module composed of tuning fork and resonant tube as the research object to optimize the research,and builds a methane gas detection system based on quartz enhanced photoacoustic spectroscopy.The main research work is as follows:Firstly,the theory of gas molecular absorption is studied,and the line shape of gas absorption spectrum is analyzed.The excitation process of photoacoustic signal is studied in detail.Based on this,the principle of photoacoustic spectroscopy and the theory of quartz enhancement technology are analyzed,and the wavelength modulation technology is studied.Secondly,the acoustic impedance of the resonant tube is analyzed,and the length range of the resonant tube is calculated based on this.With the help of finite element method and COMSOL Multiphysics software,the coupling model of resonant tube and tuning fork in quartz enhanced photoacoustic spectroscopy detection system is established.The coaxial and off-axis coupling structures are modeled respectively,and the sound pressure of the two is simulated.Taking the coaxial coupling structure model as the research object,the length and inner diameter of the resonant tube and the horizontal distance from the tuning fork are studied.Taking the off-axis coupling structure model as the research object,the length of the resonant tube,the horizontal distance from the tuning fork and the optimal incident height of the laser are studied.The influence of different parameters on the amplitude of the tuning fork is obtained by simulation calculation.The analysis results obtain the optimization parameters that maximize the amplitude of the tuning fork under the two coupling structures.Finally,the inner diameter of the coaxial coupling resonant tube is 0.4mm,the length of the resonant tube is 5mm,and the horizontal distance between the resonant tube and the tuning fork is 10μm.The length of the off-axis coupling resonant tube is 8mm,the horizontal distance between the resonant tube and the tuning fork is 30μm,and the optimal incident height of the laser is 0.7mm from the top of the tuning fork finger.Finally,the gas molecular absorption lines and experimental equipment were selected.Combined with the simulation optimization results,a quartz enhanced photoacoustic spectroscopy detection system was established to detect methane gas,and the performance of the detection system was analyzed.