| Tunable diode laser absorption spectroscopy（TDLAS）is a modern non-invasive optical measurement and diagnostic technique emerging in recent years.TDLAS-based measurement system has the advantages of high sensitivity,fast time response,strong anti-interference ability and simple measurement equipment,so it is widely used in the fields of combustion diagnosis and high-precision measurement of gas pollutants.wavelength modulation spectrocopy（WMS）method based on harmonic detection can effectively suppress the interference of low-frequency noise,and gradually becomes the mainstream technical route of TDLAS technology.This dissertation addresses three key problems encountered in the practical measurement application of TDLAS technology:（1）Voigt function is the key to the calculation of absorption spectral lineshape.However,the Voigt function approximation algorithm based on strict integral estimation has high accuracy but low computational efficiency,which does not meet the demand for online measurement of highly dynamic objects such as transient combustion.The Voigt function approximation algorithm commonly used in TDLAS technology is faster in computation but less accurate.（2）Among the two technical routes of TDLAS,the theoretical model of direct absorption spectroscopy is simple but sensitive to the baseline;the use range of Fourier analysis model of wavelength modulation spectroscopy has no clear conclusion,and the modulation parameter selection lacks theoretical basis.（3）The existing calibration-free WMS measurement algorithms often cannot balance the two requirements of fast calculation speed and low dependence on the spectral database.To address the above problems,this dissertation develops the harmonic detection theory of wavelength modulation spectroscopy through comprehensive theoretical and experimental studies,and proposes corresponding calibration-free wavelength modulation spectroscopy measurement algorithms for different measurement needs such as gas concentration monitoring and high-temperature multiparameter testing.The related research results are successfully applied to the field testing of high-temperature gas parameters of aero-engines,and good results are achieved.The main work and innovation points of the paper are as follows:1.A high precision and fast calculation method for Voigt function and its half width at half maximum intensity is proposed.By means of infinite series and Chebyshev’s best consistent approximation theory,a high precision approximation scheme for the half width at half maximum intensity of Voigt function composed of elementary functions is established,and the computational accuracy is better than 10-17.By means of the extended analysis of the exponential function expansion,a fast approximation scheme for Voigt function is proposed,which is very convenient for applications involving large-scale computation of Voigt linear forms or multiple iterations.By estimating the imaginary part of the Fadeeva function with high accuracy,an approximation scheme for the Voigt function/complex error function with small imaginary parameter is proposed,which solves the problem of large computational errors of the Gauss-dominated Voigt function by existing methods,and the accuracy of the approximation scheme is better than 10-15 for y≤0.1.This work provides a basis for improving the accuracy of TDLAS.2.A baseline free wavelength scan direct absorption spectroscopy method is proposed.The method achieves the separation of the absorption spectral signal from the baseline/background signal in the frequency domain by adding Nuttall windows to the transmitted laser light intensity signal in the time domain,which solves the problem of baseline sensitivity of the conventional direct absorption spectroscopy method.The range of use of the Fourier model for fixed-point wavelength modulation spectroscopy is given.An in-depth theoretical and numerical simulation study is carried out to address the problem of using the fixed-point wavelength modulation spectroscopy Fourier model（Fixed-WMS）to analyze the scanned wavelength modulation spectrocopy in the existing literature,and it is pointed out for the first time that the errors arising from the description of the scanned wavelength modulation spectrocopy using the Fixed-WMS model can be neglected only when the scan factor SF≤0.025.The use range of the Fixed-WMS model is clarified,and theoretical guidance is provided for the selection of the measurement parameters of wavelength modulation spectrocopy.3.A gas parameter measurement method based on the height-width feature of second harmonic is proposed.The method is suitable for gas concentration measurement under strong absorption because it is computationally small and does not need to use too many a priori spectral parameters;to address the problem of low signal-to-noise ratio of the width parameter in the method,an improved method of calibration-free wavelength modulation spectrometry based on the second-fourth harmonic combination is proposed,which does not need to scan the shape of the whole absorption spectrum and has a wider application.The limit of detection of the improved method can reach 4.28 ppm at an absorption range of 20 cm.4.A calibration-free wavelength modulation spectrocopy measurement method based on non-zero even harmonics is proposed.The method solves the problem of simultaneous measurement of gas temperature-concentration parameters by deriving the harmonic analysis under Voigt line functions.The selected H2O molecular line pairs(7185.60 cm-1&7444.36 cm-1)were subjected to numerical simulations and high-temperature laboratory experiments.The simulation results show that the relative error is less than 0.5%in the temperature range of 500 K-1250 K,and the algorithm is more robust when more harmonic combinations are used.The high-temperature experiments show that the relative error of the maximum temperature measurement is 3.6%in the temperature range of 773 K-1273 K.The computational efficiency of the algorithm is improved by at least two orders of magnitude compared to conventional waveform fitting algorithms.5.A calibration-free wavelength modulation spectroscopy method based on the power spectral density of modulated laser signals is proposed.The method is able to accurately estimate the environmental noise based on the sparse characteristics of the power spectral density,thereby simultaneously improving the accuracy and sensitivity of the measurement system.Compared with the conventional wavelength modulation spectroscopy method,the method improves the detection sensitivity of the measured CH4 absorption by 2.64 times at an integration time of 1 second.In addition,this dissertation shows for the first time that the data averaging method used by default in the conventional method can only improve the sensitivity but not the accuracy of the measurement,because noise causes pseudo-enhancement of the harmonic signal.These results are of fundamental significance and provide ideas for further in-depth research and system performance optimization of wavelength modulation spectroscopy.6.Finally,the experimental study of aero-engine high-temperature gas parameter measurement based on wavelength modulation spectroscopy was carried out.The test results of the conventional waveform fitting algorithm on three typical test pieces,including supercombustion ram engine test pieces,standing vortex combustion chambers,and additive combustion chambers,are analyzed.The application of the calibration-free wavelength modulation spectroscopy measurement algorithm based on non-zero even harmonic peak heights proposed in this dissertation to the measurement of gas parameters at the exit of a full-size aero-engine combustion chamber is briefly presented.The experimental results show that the calibration-free wavelength modulation spectroscopy based on harmonic detection has a broad application prospect in the field of aero-engine combustion diagnosis because of its high sensitivity,high selectivity and fast response. |
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