Experimental Study On Gas Velocity And Concentration Measurement Based On Absorption Spectroscopy

With the increase of China's demand for clean energy and increasing attention to global climate change and ecological environment quality,the detection of trace gas pollutants has been paid more and more attention.At the same time,the measurement of airflow velocity is of great significance to the study of high-speed flow field in wind tunnel,engine,combustion field and other fields.Compared with the traditional gas measurement methods,absorption spectroscopy has several advantages,such as fast response,high sensitivity,online,real-time,etc.,and has developed into a mainstream trace gas detection method.In this paper,airflow velocity is measured based on absorption spectroscopy and doppler effect,CH₄ concentration is telemetered based on tunable laser absorption spectroscopy,and measurement of NOx concentration is designed and explored based on cavity attenuation phase shift spectroscopy.The main contents are as follows:Based on absorption spectroscopy and doppler effect,the velocity of the jet stream is measured.The characteristic spectral line near 1392nm of H₂O molecule is selected,and a high-speed airflow velocity measurement system is designed and built.Various experimental devices of the system are introduced,and the velocity of about 100m/s is measured and verified,with a relative error within 10.6%.Based on LabVIEW,the velocity measurement software is written to realize online real-time measurement of velocity,and the lower limit of the system was analyzed to be 25.9m/s.Based on the tunable laser absorption spectroscopy,the CH₄ concentration is telemetered.The direct absorption method and wavelength modulation technology are used to measure the CH₄ in situ of nine different concentrations from 100ppm to10000ppm.The relative errors are within 5.4%and 7.3%,respectively.The Fresnel lens system is designed and a test bench is built to conduct the simulated telemetry experiment of CH₄ concentration.The maximum relative error is 11.7%when the detection results are compared with the standard concentration,and the coincidence degree is high.The linear fitting coefficient is greater than 0.99.At the same time,the digital phase-lock program is written to demodulate the second harmonic,and the miniaturization and integration of the telemetry system are explored.Based on the cavity attenuation phase-shift spectroscopy technology,the principle and calculation method of measuring NOx concentration are introduced.The 420nm LED light source and reflector with a reflectivity up to 99.997%are selected.The confocal cavity with a cavity length of 50cm is designed as the measuring system.Optical path simulation is carried out based on ZEMAX software,lens system,cavity and diaphragm were designed,so that the light emitted by LED will converge on the detector after multiple reflections in the cavity,and the system is mathematically verified by ABCD matrix,and the system is preliminarily constructed.Through the above work,the design and verification of the device for on-line measurement of airflow velocity and trace gas concentration based on absorption spectroscopy were realized,laying a foundation for further development and improvement.