Research On Enhancement Method Of Gas Detection Performance Based On Resonant Photoacoustic Cells

Trace gas detection plays an important role in many industries.For example,the in-situ detection of nitrogen dioxide(NO2)in the atmospheric environment is conducive to determining the source of pollution and achieving emission control.The concentration detection of the end-expiratory carbon dioxide(CO2)is conducive to analyzing the lung ventilation situation of patients.The quantitative detection of dissolved methane(CH4)and acetylene(C2H2)in transformer oil is conducive to early fault diagnosis and maintenance of the power systems.Based on the photoacoustic effect,photoacoustic spectroscopy has the advantages of good selectivity,strong reliability,and fast response,and is widely used in various gas detection fields.Resonant photoacoustic cells are the place where the photoacoustic effect occurs,which can accumulate sound energy and amplify the photoacoustic signal.In practical detection,high-performance resonant photoacoustic cells should be designed based on the specific detection requirements of the measured gas such as specific light sources,response capabilities,minimum detection limits,and normalized noise equivalent absorption coefficients.The research objective of this thesis is to improve the photoacoustic detection performance and achieve high-sensitivity gas detection.In response to the shortcomings of the existing structure of photoacoustic cells and the utilization efficiency of excitation light,two types of photoacoustic cells are designed and optimized based on the acoustic mode characteristics of acoustic tubes,with differential noise suppression and optical power enhancement as the starting points.On this basis,a high-performance photoacoustic cell with differential and optical power enhancement is developed based on a Helmholtz resonator.For the coherent noise in the photoacoustic detection,an active noise suppression method was proposed to improve the detection signal-to-noise ratio and signal-to-background ratio,and further improve the gas detection performance of the photoacoustic sensor.The main research contents and achievements of the thesis are summarized as follows:(1)The effects of various geometric parameters of a differential double-tubes photoacoustic cell on resonance frequency,sound pressure inside the cell,and quality factor was researched,to optimize and improve the detection performance of this type photoacoustic cell.The thermal decomposition and adsorption effects of NO2 were analyzed and verified,and the optimal detection temperature of the photoacoustic cell was determined,which achieved high-precision online detection of NO2.According to the acoustic mode characteristics of the double-free-ends acoustic tube,a differential double-tubes photoacoustic cell was designed based on the principle of a differential circuit.Through finite element simulation,the geometric parameters of the photoacoustic cell were optimized.When a 500 mW low-cost laser diode was used as the excitation light source,the minimum detection limit of 124 ppt for NO2 gas was achieved within only a 5-s detection time.Besides,the temperature control model of the photoacoustic cell was constructed,and the thermal decomposition and adsorption effects of NO2 were analyzed and verified.The optimal detection temperature of the photoacoustic cell was determined through experiments.Based on this,a fully automatic NO2 in-situ detection system was built,and the online detection of NO2 in the atmospheric environment was simulated.(2)Two types of cylindrical cavity-acoustic tube coupled photoacoustic cells were developed.Based on the resonance of the acoustic tube,the intracavity optical power enhancement methods were designed,and the detection performance of CO2 with different light sources excitation was improved.A diffuse reflective sphere-acoustic tube coupled photoacoustic cell was designed,and the equivalent optical power in the ultraviolet to mid-infrared bands was further enhanced.Based on the acoustic mode characteristics of the single-free-end acoustic tube,two types of optical power-enhanced column-tube coupled photoacoustic cells were developed.A intracavity multiple reflection mode was designed by changing the incidence angle of the collimated laser light source,which reduced the minimum detection limit of CO2 by～14 times compared to the traditional mode.For the non-collimated low-cost LED light sourcelthe minimum detection limit was reduced to the order of 10-4.A sphere-tube coupled photoacoustic cell based on polytetrafluoroethylene diffuse reflection sphere absorption cavity was developed,and sub-ppm level detection sensitivity was achieved with water vapor humidification to enhance relaxation rate.(3)A optical power-enhanced differential Helmholtz photoacoustic cell was designed,the equivalent optical power was raised and the photoacoustic signal was amplified,while the incoherent noise was suppressed.The photoacoustic cell had a high conversion ability of light energy-sound energy,and the rapid detection of the measured gases with high sensitivity and ultra-low noise was realized.Based on the Helmholtz resonator,a Helmholtz photoacoustic cell with the characteristics of optical power enhancement and differential noise suppression was developed.Through the optimization of parameters and structure,the damping against low-frequency noise and differential characteristics of the photoacoustic cell were guaranteed,while the detection performance was improved,and the gas replacement time was shortened.For CH4 gas,when the excitation light was a 6 mW low-power near-infrared laser,the minimum detection limit of 177 ppb and the ultra-low normalized noise equivalent absorption coefficient of 10-10-level were achieved within only a 1-s detection time.(4)An active suppression method for coherent noise was proposed,based on the principle of differential and acoustic active noise suppression,which reduced the multiple reflection noise caused by the wavelength-untunable light source in the mode of optical power enhancement.The detection signal-to-noise/signal-to-background ratio was improved and the gas detection performance of the photoacoustic sensor was further enhanced.Because the wavelength untunable light source could only be intensity modulated,the solid photoacoustic effect would be triggered while realizing optical power enhancement,and coherent multiple reflection noise would be generated.Such noise could not be suppressed by differential technology.An excitation light source with a wavelength in the strong absorption band of the measured gas was used to generate a photoacoustic signal with superimposed multiple reflection noise.A compensation light source with a wavelength in the non-absorption band of the measured gas was used to generate compensation noise in the opposite phase to the multiple reflection noise.The two noises formed interference cancelation,which reduced the total noise value.For C2H2 gas,the minimum detection limit and normalized noise equivalent absorption coefficient were in the order of ppb and 10-10,respectively.