| In recent years,with the rapid development of industry and the improvement of people’s economic level,the problems of resources and environment are becoming more and more serious.Among them,the greenhouse effect will cause the environmental problems such as sea level rise,climate anomaly,sea storm increase,land drought and crop yield decline,and have become the focus of world attention.CO2 is one of the main gases that cause greenhouse effect,mainly from industrial production,winter heating and automobile exhaust.Therefore,CO2 emission reduction and recycling has become the focus of current research.Among them,the monitoring of CO2 emissions is a particularly important issue.In view of the existing gas detection technology,tunable diode laser absorption spectroscopy(TDLAS)has a strong environmental adaptability,high sensitivity,fast response speed,non-intrusive,not only can be used for gas detection in the atmospheric environment,but also coal-fired boiler,engine combustion chamber and has wide application prospect in flow field diagnosis.This paper takes CO2 as the target gas,research on CO2 encountered in actual measurement in the application of using TDLAS:(1)Solid optical etalon is an important tool for TDLAS to measure gas parameters,with precise free spectral range(FSR)of the solid etalon optical is difficult to manufacture and has high cost,and the FSR is difficult to determine precisely.The frequency response characteristics of the distributed feedback laser can not be accurately obtained,which brings errors to the calibration-free wavelength modulation(CF-WMS)in TDLAS.(2)TDLAS generally uses wavelength modulation combined with two-line method to measure the temperature of gas,which is easy to produce frequency crosstalk and reduce time resolution in frequency division and time division multiplexing,and it has some limitations in practical applications.(3)Measurement of TDLAS on the concentration of CO2 is the main line average measurement,while study on the measurement of C02 concentration in two-dimensional heterogeneous environment of combustion flow field is less.Moreover,the absorption of CO2 in the near infrared band is weak and it is easily disturbed by H2O in the combustion flow field.Because the tunable range of DFB laser is small,it is difficult to scan the absorption peaks of CO2 and H2O at the same time.Therefore,it is not suitable to obtain the integral absorption area A by multi peak fitting method to obtain the concentration of CO2.In view of the above problems,the main contents of this paper are as follows:1.The problem of obtaining the frequency response characteristics of distributed feedback lasers through solid optical etalon,to solve this problem presents a simple method to determine the frequency response characteristics of distributed feedback(DFB)laser.The accuracy of the acquired v(t)was verified by measuring the concentration of CO2 by CF-WMS,and the applicable conditions of the method were analyzed.When the temperature is known,in a certain concentration range,the background subtracted peak-normalized 2f signal(R2f/p)is not sensitive to the concentration,using least squares fitting R2f/p to obtain the frequency response.The R(50)transition of the CO2 at 5007.787cm-1 is selected to measure the concentration of carbon dioxide,and the results show that the maximum relative deviation is-1.40%,which verifies the feasibility of the method.2.A calibration-free one-line measurement method based on wavelength modulation spectroscopy(WMS)is proposed.Simultaneous measurement of gas temperature and concentration with only one absorption line can be realized.The one-line method is verified by experiments using the R(50)transition of CO2 at 5007.787 cm-1,which is the one of the main products in the combustion process.The selected transition absorption line has the following properties:the lineshape of the background subtracted peak normalized second harmonic signal(R2f/p)mainly depends on the temperature;And the amplitude of the background subtracted first harmonic normalized second harmonic signal(R2f/1f)is proportional to the concentration of gas.Therefore,the temperature can be extracted from R2f/p,and then the CO2 concentration can be extracted by using R2f/1f.3.A reconstruction method of concentration field in non-uniform flow field based on TDLAS is proposed.The measurement of CO2 concentration distribution at the exit of aeroengine combustor is realized.The results show that the R(50)transition of CO2 has the following characteristics:the amplitude of R2f/1f at the center frequency approximately satisfies the linear superposition relation.According to this characteristic,the CO2 concentration field reconstruction of inhomogeneous flow field is carried out by using the absorption information of a single absorption line and ART(Algebraic Reconstruction,Technique).Through numerical simulation,the anti noise ability of projecting light at 2 angles and 38 angles is compared.On this basis,the measurement of CO2 concentration field at the exit of aeroengine combustor has been realized. |
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