Temperature concentration discrete imaging method combined with wavelength modulation large absorption correction

Abstract

The invention provides a temperature concentration discrete imaging method combined with wavelength modulation large absorption correction, which comprises the following steps: laser after wavelength modulation is split, passes through a detected area and is received by a detector to obtain a normalized second harmonic signal of multi-light-path transmitting light intensity; according to the temperature concentration range and the light path layout, establishing a temperature concentration discrete reconstruction model, dividing a long light path into a plurality of small sections and then overlapping the small sections again, and correcting the reconstruction model during large absorption; and performing iterative solution according to the corrected model and the normalized second harmonic measurement value to obtain an initial reconstruction result of a temperature concentration histogram, obtaining a normalized second harmonic signal meeting a linear superposition relation after each light path is corrected by combining a large absorption correction method, and performing iterative solution to obtain temperature and concentration distribution in the to-be-measured area. According to the method, spectral information is effectively utilized, the number of constraint equations is increased, the combustion field parameter reconstruction accuracy and the anti-noise capability are improved, and the method has important application prospects in the field of combustion imaging monitoring.

Description

Technical Field

[0001] The present invention relates to a temperature concentration discrete imaging method combined with wavelength modulation large absorption correction, which can realize high-precision solution of combustion field gas temperature and concentration distribution under large absorption conditions, and belongs to the field of laser spectral tomography. Background Art

[0002] Combustion monitoring can provide a basis for combustion process research, combustion efficiency improvement, pollutant emission reduction, and combustion chamber design optimization. Temperature and component concentration, as the core parameters of combustion monitoring, can reflect combustion efficiency and combustion state. Therefore, the measurement of temperature and component concentration distribution in the combustion field is of great significance. Laser Absorption Spectroscopy (LAS) is based on the Beer-Lambert law and measures the relationship between the transmittance of laser intensity through the measured field and the measured gas parameters. Due to its advantages such as fast response speed, high sensitivity, and non-invasive measurement, it is currently widely used in combustion process monitoring and combustion field parameter distribution imaging measurement process.

[0003] Laser absorption spectroscopy technology includes two typical measurement methods, namely direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). In the DAS method, the laser output laser is periodically modulated in a certain waveform and then incident on the combustion field to be measured. The photodetector receives the transmitted light intensity and linearly fits the non-absorption position to obtain the reference signal, i.e., the baseline, so as to calculate the absorption spectrum. The amplitude, half-width, shape, etc. of the absorption spectrum are related to parameters such as temperature, component concentration, and pressure. In an article entitled “VCSEL-based, high-speed, in situ TDLAS for in-cylinder water vapor measurements in IC engines” published in Optics Express, Vol. 21, No. 17, pp. 19951-19965 in 2013, Witzel et al. used a vertical cavity surface emitting laser as a light source to obtain the direct absorption spectrum of water vapor in the cylinder of an internal combustion engine, and measured the water vapor concentration using a single spectrum line. When the water vapor concentration was 1.3% by volume, it was demonstrated that the measurement accuracy was 0.046%. In 2005, Liu et al. published an article entitled "Development of a tunable diode laser sensor for measurements of gas turbine exhaust temperature" in Applied Physics B-Lasers and Optics, Vol. 82, No. 3, pp. 469-478. They used a tunable diode laser as a light source to obtain the integral absorption rate on two absorption lines, and solved the path temperature based on their ratio, i.e., the two-color method. The accuracy of the sensor was verified using a heatable gas chamber. The average deviation between the temperature calculation result under the optimally selected spectrum line and the thermocouple was 2K. The exhaust temperature of the gas turbine was measured using this sensor, and the result was highly consistent with that of the K-type thermocouple. After using a multi-angle sensor to obtain the absorption spectra of multiple rays passing through the combustion field under test, the linear tomography algorithms such as SART and Landweber can be combined with the two-color method to achieve the inversion solution of the parameter distribution of the combustion field.In 2015, Liu et al. published an article entitled "Development of a fan-beam TDLAS-based tomographic sensor for rapid imaging of temperature and gas concentration" in Optics Express, Vol. 23, No. 17, pp. 22494-22511. They developed a five-angle fan-beam TDLAS tomographic sensor based on the direct absorption method, and used the improved Landweber algorithm combined with the two-color method to measure the temperature and concentration distribution of water vapor in the measured area. In 2017, Jing et al. published an article titled "A Reconfigurable Parallel Data Acquisition System for Tunable Diode Laser Absorption Spectroscopy Tomography" in IEEE Sensors Journal, Vol. 17, No. 24, pp. 8215-8223. They designed a parallel data acquisition system for a five-angle fan-beam TDLAS tomography sensor to measure the flame temperature and water vapor concentration of an acoustically excited Bunsen burner. It has high temporal and spatial resolution, and the acoustically excited frequency components can be clearly and accurately extracted from the reconstruction results. In 2023, Gao et al. published an article titled "Radial Basis Function Coupled SART Method for Dynamic LAS Tomography" in Volume 72 of IEEE Transactions on instrumentation and measurement. They used a five-angle fan-beam TDLAS tomography sensor to obtain absorption spectra on multiple optical paths, used a compact radial basis function to fit the gas parameter distribution in the measured area, and used the classical iterative method SART to iteratively solve the fitting coefficients to achieve high spatial resolution imaging of temperature and water vapor concentration. In order to avoid anomalies and artifacts caused by the ratio calculation of the two-color method, a hyperspectral imaging system and a nonlinear image reconstruction algorithm can also be introduced. The temperature and concentration are directly used as unknown quantities, and the gas parameters of the measured area are reconstructed through the optimization algorithm.In 2013, Ma et al. published an article entitled "50-kHz-rate 2Dimaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectraltomography" in Optics Express, Vol. 21, No. 1, pp. 1152-1162. They used a Fourier mode-locked laser as a light source to build a multispectral tomography system with 2 angles and 30 optical paths to obtain the laser absorption spectrum of the J85 gas turbine engine outlet plane and obtain the dynamic change of its temperature and water vapor concentration over time. However, the nonlinear image reconstruction algorithm has a large amount of calculation, a long calculation time, and a high system cost. In addition, the reconstruction of combustion field parameters can be realized based on histogram information. The prior knowledge of combustion field parameters is used to discretize the gas temperature and component concentration distribution into a finite number of temperature-concentration pairs, and the gas parameters are reconstructed by inverting the absorption path length of the temperature-concentration pairs on multiple optical paths. Chinese patent CN 110514622 B "A two-dimensional temperature and concentration reconstruction system and method based on histogram information" (patent number: 201910782513.5) proposes a two-dimensional temperature and concentration reconstruction system and method based on histogram information. The reconstruction system includes a laser control and generation module, an optical fiber beam splitter, a sensor for beam conversion and detector reception, a Mach-Zehnder interferometer and a photodetector for wavenumber conversion, a data acquisition system, a computer, etc.; the reconstruction method is to select a finite number of temperature values ​​and concentration values ​​according to the temperature and concentration reconstruction range, establish a model between the projected absorption rate on each optical path and the temperature concentration histogram, and at the same time establish a model between the multi-optical path temperature concentration histogram and the two-dimensional temperature and concentration distribution according to the optical path layout in the field to be measured, and iteratively solve the above two models to realize the reconstruction of the temperature concentration distribution. This method can effectively utilize the absorption spectrum information and improve the reconstruction quality of the temperature concentration distribution.In 2023, Qiu et al. published an article entitled "A Binary Valued Reconstruction Algorithm for Discrete TDLAS Tomography of Dynamic Flames" in Volume 72 of IEEE Transactions on Instrumentation and Measurement. They used a five-angle fan beam sensor to obtain absorption spectra on multiple paths, combined with a temperature concentration reconstruction algorithm based on histogram information to obtain the distribution results of combustion field gas parameters. This system and method were used to measure the flame parameter distribution of McKenna burners and acoustically excited Bunsen burners, respectively. Compared with the colorimetric method, imaging results with higher accuracy and fewer artifacts can be obtained. The DAS method is simple to operate, the principle is easy to understand, no calibration process is required, it is convenient for practical application, and a complete absorption spectrum can be obtained. However, this method has problems such as inaccurate baseline fitting and spectral linear fitting under noise interference, resulting in errors in parameter solution, and is not suitable for on-site measurement of combustion fields under harsh industrial annular conditions.

[0004] During the laser absorption spectrum measurement process, optical noise is usually located in the low-frequency section. The modulation and demodulation technology is used to move the absorption information to the high-frequency section, thereby improving the signal-to-noise ratio of the measurement signal and reducing the impact of noise on the measurement results. In the WMS method, the laser scanning method is a superposition of wavelength linear scanning and high-frequency sinusoidal modulation. By orthogonally demodulating the transmitted light intensity collected by the photodetector, the higher-order harmonic spectrum results after the normalization of the first harmonic spectrum are obtained, and the gas parameters are solved according to its peak value, spectral shape, etc. The WMS method does not require baseline fitting, is less affected by noise, and is suitable for measurements in industrial field environments. In 2009, Rieker et al. published an article entitled “Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments” in Applied Optics, Vol. 48, No. 29, pp. 5546-5560. They used the first harmonic signal 1f of the transmitted laser intensity to normalize the second harmonic signal 2f to obtain the 2f / 1f signal, and compared the ratio of the 2f / 1f signals on the two spectral lines with the results obtained by previous simulation to achieve calibration-free temperature concentration measurement. This method was used to measure H2O and CO2 in scramjet engines, and the signal-to-noise ratio of the calibration-free WMS measurement results was 4 times that of the direct absorption spectroscopy measurement results. In 2016, Goldenstein et al. published an article entitled "Fiber-coupled diode-laser sensors for calibration-free stand-off measurements of gas temperature, pressure, and composition" in Applied Optics, Vol. 3, Issue 55, Pages 479-484. They developed a single-ended fiber-coupled laser absorption sensor that emits and receives wavelength-modulated light reflected from a natural surface, and extracts its normalized second harmonic spectrum for least squares fitting to obtain the spectral line integral area, collision width and center frequency, thereby solving the gas parameters. This sensor was used to measure the temperature and H2O concentration of an unstable propane flame under a shorter absorption path, and to measure CH4 in the environment under a longer absorption path, and relatively accurate results were obtained.By obtaining the normalized second harmonic spectrum of the transmitted light intensity of two different spectral lines on multiple optical paths, the distribution of the normalized second harmonic spectrum peak can be inverted using imaging algorithms such as SART and Landweber, and the distribution of parameters such as temperature and concentration in the measured field can be obtained based on the colorimetric search method. In 2020, Fisher et al. published an article entitled "A Custom, High-Channel Count Data Acquisition System for Chemical Species Tomography of Aero-Jet Engine Exhaust Plumes" in IEEE Transactions on Instrumentation and Measurement, Vol. 2, No. 69, pp. 549-558. They combined WMS technology with tomography technology and designed a 6-angle 126-optical path parallel beam imaging sensor, which achieved the measurement of CO2 concentration distribution in the cross section of the tail nozzle of an aircraft engine. However, since only one absorption line was scanned, its temperature distribution result could not be obtained. In 2020, Huang et al. published an article titled "Frequency-Division Multiplexing and Main Peak Scanning WMS Method for TDLAS Tomography in Flame Monitoring" in IEEE Transactions on Instrumentation and Measurement, Issue 69, Volume 11, Pages 9087-9096. They used a five-angle sector sensor to scan the two absorption lines of water vapor, and used the improved Landweber algorithm to reconstruct the normalized second harmonic spectrum peak distribution in the area of ​​interest, and reconstructed the temperature and concentration based on the temperature and concentration lookup table obtained in advance by simulation.In 2020, Zhao et al. published an article titled "A WMS Based TDLAS Tomographic System for Distribution Retrievals of Both Gas Concentration and Temperature in Dynamic Flames" in IEEE Sensors Journal, Issue 20, Volume 8, Pages 4179-4188. They used a five-angle sector sensor to obtain the normalized second harmonic spectrum peak of the transmitted light intensity on multiple optical paths, used the SART algorithm to obtain the distribution of the normalized second harmonic spectrum peak in the imaging area, and used a lookup table to obtain the temperature and concentration distribution results. In 2022, Gao et al. published an article titled "Sparse Zernike Fitting for Dynamic LAS Tomographic Images of Temperature and Water Vapor Concentration" in the 71st issue of IEEE Transactions on Instrumentation and Measurement. They used Zernike polynomials to fit the normalized second harmonic spectrum peak distribution in the measured area, used the SART algorithm to solve the fitting coefficients, and used a lookup table to obtain the temperature and water vapor concentration distribution. In the process of looking up the temperature, this type of method usually involves calculating the ratio of the normalized second harmonic peaks of two spectral lines, which is more sensitive to noise and has poor noise resistance. At the same time, in the imaging process, only the peak of the normalized second harmonic spectrum is usually used for solution, and the spectral information is not effectively utilized.

[0005] In the case of large absorption, the normalized second harmonic spectrum no longer has the characteristic of being integrable along the path, and the above-mentioned traditional imaging model has nonlinear errors. In 2011, Peng et al. published an article entitled "Calibration-free wavelength modulated TDLAS under high absorbance conditions" in Optics Express, Vol. 23, No. 19, pp. 23104-23110. They calculated the normalized second harmonic spectrum by performing a second-order Taylor expansion on the transmittance, established the relationship between the normalized second harmonic result and the gas concentration at this time, and measured the NH3 concentration in the gas chamber under large absorption. This method improves the measurement accuracy of gas concentration under large absorption, but it is only for concentration calculation when the pressure, temperature, and absorption path length are known. At this time, the normalized second harmonic spectrum has a nonlinear relationship with the path length, which is difficult to use in the tomography process. In 2023, Li Shaomin and others published an article entitled "High-absorption Methane Gas Measurement Based on Improved Wavelength Modulation Spectroscopy Technology" in Volume 1, Issue 72 of "Acta Physica Sinica". The output laser was divided into a non-absorption reference path and an absorbed measurement path, and the harmonic demodulation phase of the measurement signal was determined according to the non-absorption light intensity. In this way, the demodulated second harmonic maintains a linear relationship with the concentration even under high absorption. However, its demodulation phase is not constant and requires real-time solution and measurement. It cannot be obtained in advance through calibration. When performing multi-optical path measurements, the demodulation phase of each optical path needs to be calculated in real time, and the operation and calculation are relatively complex.

[0006] Based on the above background, this paper proposes a temperature concentration discrete imaging method combined with wavelength modulation large absorption correction. According to the temperature concentration range and optical path layout, a temperature concentration discrete reconstruction model is established. Since the normalized second harmonic signal does not satisfy the linear superposition relationship under large absorption conditions, the long optical path is divided into multiple small segments that meet the small absorption conditions. The normalized second harmonic spectrum of each small segment is calculated and re-superimposed to correct the reconstruction model; according to the corrected model and the normalized second harmonic measurement value, the initial reconstruction result of the temperature-concentration histogram is obtained by iterative solution, so as to obtain the normalized second harmonic signal after large absorption correction of each optical path, and the temperature and concentration distribution in the measured area is obtained by iterative solution again. Compared with the traditional WMS tomography model, the effective use of spectral information increases the number of independent equations, reduces the underdetermination of the problem, and improves the accuracy and anti-noise ability of the reconstruction results. At the same time, the normalized second harmonic under large absorption conditions is corrected, which reduces the reconstruction error caused by nonlinearity in the imaging model, and provides a new method for imaging the temperature and component concentration of the combustion field under large absorption conditions. Summary of the invention

[0007] 1. Technical issues to be resolved

[0008] The present invention proposes a temperature-concentration discrete imaging method combined with wavelength modulation large absorption correction, aiming to solve the problems of poor accuracy and low noise resistance in the colorimetric imaging process, as well as the nonlinear problem of the wavelength modulation tomography model under large absorption.

[0009] (II) Technical solution

[0010] The present invention relates to a temperature concentration discrete imaging method combined with wavelength modulation large absorption correction, which mainly comprises the following steps:

[0011] Step 1: In the wavelength modulation method, the wave number v(t) output by the laser is expressed as:

[0012]

[0013] in, is the laser center wave number, a md is the modulation depth, ω=2πf is the modulation angular frequency, f is the modulation frequency, and the model of the incident laser intensity I0(t) is

[0014]

[0015] in, Wave number The laser intensity at , i0 and i2 are the linear and nonlinear intensity modulation amplitudes, respectively. and is the phase shift between the corresponding intensity modulation and frequency modulation, and the transmittance is expressed as:

[0016]

[0017] If a path satisfies the small absorption condition, that is, α(v)≤Δ, the transmittance is approximated by the first-order Taylor expansion:

[0018]

[0019] Since the transmission coefficient τ[v(t)] is periodic, it can be expanded into a Fourier series:

[0020]

[0021] Among them, the Fourier coefficient for:

[0022]

[0023] Therefore, under conditions of small absorption, the Fourier coefficients The transmitted light intensity I can be integrated along the path by using phase-locked amplification and low-pass filtering. t (t) Orthogonal demodulation is performed to obtain its first harmonic and second harmonic, and finally its normalized second harmonic spectrum is obtained as follows:

[0024]

[0025] Therefore, under the condition of small absorption, the normalized second harmonic spectrum S 2f / 1f It also has the path-integrable property;

[0026] Step 2: Use a multi-angle sensor to obtain the transmitted light intensity on Q laser paths, and perform orthogonal demodulation to obtain the normalized second harmonic spectrum on Q optical paths, and obtain the measured multi-optical path normalized second harmonic spectrum matrix S Q×K , where K is the number of normalized second harmonic spectrum points; according to the estimation of the measured field parameters, the measured field parameters are discretized into M temperature-concentration pairs {T1,X1},{T2,X2},…{T M ,X M}, forming the gas parameter matrix U M×2 :

[0027]

[0028] Calculate the i-th temperature-concentration pair {T i ,X i Normalized second harmonic spectrum R under unit path length i , forming the normalized second harmonic basis matrix R M×K ; Under the condition of small absorption, the normalized second harmonic spectrum signal has the characteristic of being integrable along the path, so the normalized second harmonic spectrum S on the qth optical path q for:

[0029]

[0030] Among them, hq ,i is the ith temperature-concentration pair {T i ,X i}, and write equation (9) in matrix form:

[0031] S Q×K =H Q×M ·R M×K (10)

[0032] Among them, H Q×M In order to characterize the histogram matrix of each temperature-concentration absorption length, the measured field is discretized into D grids, and the length w of the qth laser passing through the jth grid is calculated according to the optical path layout. q,j , forming the sensitivity matrix WQ×D , the path length on the qth light path is equal to the sum of the absorption paths occupied by all temperature-concentration pairs, that is:

[0033]

[0034] Write equation (11) in matrix form:

[0035] H Q×M ·e M×1 =W Q×D ·e D×1 (12)

[0036] Among them, e M×1 and e D×1 is a column vector with all elements equal to 1. Combining equations (10) and (12), the multi-light path histogram inversion model is:

[0037] H Q×M ·(R M×K e M×1 )=(S Q×K W Q×D ·e D×1 ) (13)

[0038] In each grid of the measured area, the gas parameters are discretized into one of M temperature-concentration pairs, using the 0-1 matrix Y D×M represents the temperature-concentration distribution. If element y j,i is 1, it means that the gas parameter in the jth grid is the temperature concentration pair {T i ,X i}, if it is 0, it means that the gas parameters in this grid are not {T i ,X i};

[0039] The absorption path length hq of the ith temperature-concentration on the qth light path ,i It is expressed as:

[0040]

[0041] By writing equation (14) in matrix form, we can obtain the discrete imaging model of temperature and gas concentration distribution:

[0042] H Q×M =W Q×D ·Y D×M (15)

[0043] Step 3: When the small absorption condition is not met, that is, α(v)>Δ, the normalized second harmonic spectrum no longer has the characteristic of being integrable along the path, and the above linear model (13) is no longer valid, and a large absorption correction is required; the absorption path is divided into n small segments of equal length, so that each small segment meets the small absorption condition, that is, α(v) / n≤Δ, and the normalized second harmonic spectrum in each small segment is calculated. The sum of the two is taken as the normalized second harmonic spectrum of the entire absorption path after correction

[0044]

[0045] In the case of large absorption, the modified path-normalized second harmonic spectrum projection matrix is And the modified normalized second harmonic basis matrix In between, we can also establish the multi-light path histogram inversion model as shown in formula (13) and the temperature and gas concentration distribution discrete imaging model as shown in formula (15):

[0046]

[0047] H Q×M =W Q×D ·Y D×M (18)

[0048] The joint iterative reconstruction algorithm is used to solve equation (17) to obtain the histogram matrix H Q×M , and formula (18) to obtain the gas temperature concentration distribution matrix Y D×M , complete the solution of the inversion imaging model after large absorption correction.

[0049] Step 4: Corrected normalized second harmonic basis matrix It can be obtained from simulation, and the corrected path normalized second harmonic spectrum projection matrix It cannot be measured directly. It is necessary to solve the correction coefficient according to the absorption on each optical path and calculate the normalized second harmonic spectrum projection matrix S on multiple optical paths based on the measured multi-path transmission light intensity. Q×K , solve the following matrix equation to obtain the preliminary results of the multi-path temperature-concentration histogram matrix:

[0050]

[0051] Calculate the i-th temperature-concentration pair {T i ,X i Absorption rate per unit path length under} i , forming the absorption rate basis matrix α M×N , where N is the wave number length of the absorbance, thus calculating the absorbance projection results A on multiple light pathsQ×N :

[0052] A Q×N =H Q×M α M×N (20)

[0053] Among them, A Q×N The qth row a q is the absorption rate projection on the qth optical path. According to the incident light intensity I0 of the optical path and the absorption rate projection result on the optical path, the transmitted light intensity I on the optical path is calculated. t :

[0054] I t =I0exp(-a q ) (twenty one)

[0055] Calculate the normalized second harmonic spectrum by directly performing orthogonal demodulation based on the transmitted light intensity The absorption path is divided into multiple small segments, each of which satisfies the small absorption condition. The normalized second harmonic spectrum in each small segment is calculated and superimposed to obtain the normalized second harmonic spectrum on the entire path after large absorption correction. Calculate the correction factor on this optical path:

[0056]

[0057] Calculate the normalized second harmonic spectrum of the optical path after large absorption correction

[0058]

[0059] Forming the multi-path corrected normalized second harmonic spectrum projection matrix after large absorption correction The final result of the multi-path temperature-concentration histogram matrix is ​​obtained by iteratively solving equation (17): Iteratively solve equation (18) to obtain the gas temperature concentration distribution matrix Y D×M , gas temperature distribution T in the measured field D×1 and concentration distribution X D×1 for:

[0060] (T D×1 X D×1 )=Y D×M ·U M×2 (twenty four)

[0061] This enables the inversion solution of the gas temperature and concentration distribution results after large absorption correction.

[0062] (III) Beneficial effects

[0063] The beneficial effect of the present invention is to propose a temperature concentration discrete imaging method combined with wavelength modulation large absorption correction, establish a temperature concentration discrete reconstruction model according to the temperature concentration range and the optical path layout; in view of the problem that the normalized second harmonic signal does not satisfy the linear superposition relationship under large absorption conditions, after dividing the long optical path into multiple small absorption sections, the normalized second harmonic spectrum in each small section is calculated and superimposed as the result of the corrected entire path, thereby establishing a reconstruction model under large absorption conditions; according to the preliminary results of the multi-path temperature-concentration histogram matrix obtained by the initial reconstruction, the corrected multi-path normalized second harmonic spectrum projection matrix is ​​determined, and the final result of the multi-path temperature-concentration histogram matrix and the temperature and concentration distribution in the area to be measured are reconstructed again. This method reduces the nonlinear error in the WMS image reconstruction process and improves the quality of the reconstructed image. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Attached Figure 1 :The specific implementation diagram of this method

[0065] Attached Figure 2 :Original temperature and water vapor concentration distribution diagram (a) temperature distribution (b) concentration distribution

[0066] Attached Figure 3 :Imaging results with and without large absorption correction (a) Temperature result without large absorption correction (b) Concentration result without large absorption correction (c) Temperature result with large absorption correction (d) Concentration result with large absorption correction DETAILED DESCRIPTION

[0067] Reference Figure 1 This is a specific implementation diagram of this method. Figure 2 is the original temperature and water vapor concentration distribution, Figure 3 The following are the imaging results with and without large absorption correction. The specific operation steps combined with an example are as follows:

[0068] Step 1: The laser output by the tunable laser after wavelength modulation enters the measured field after splitting. The transmitted light intensity is received by the photodetector to form 120 light paths. The transmitted light intensity is orthogonally demodulated by phase-locked amplification and low-pass filtering to obtain its first harmonic and second harmonic, and finally obtain its normalized second harmonic spectrum to form the multi-light path measurement normalized second harmonic spectrum matrix S 120×1202 , and its normalized second harmonic spectrum length corresponds to 7184.5cm -1 ~7186cm -1 and 7443.5cm -1 ~7445cm -1There are 1202 points in total within the wavenumber range; according to the estimation of the measured field parameters, the measured field parameters are discretized into 10 temperature concentration pairs: {300K, 0.02}, {511.1K, 0.04}, {722.2K, 0.06}, {933.3K, 0.08}, {1144.4K, 0.10}, {1355.6K, 0.12}, {1566.7K, 0.14}, {1777.8K, 0.16}, {1988.9K, 0.18}, {2200K, 0.20}, forming the gas parameter matrix U 10×2 ; Divide the region of interest into 40×40 grids, totaling 1600 grids, and calculate the length of each light ray passing through each grid according to the optical path layout to form the sensitivity matrix W 120×1600 ;

[0069] Step 2: When the small absorption condition is not met, that is, α(v)>0.05, the normalized second harmonic spectrum no longer has the characteristic of being integrable along the path. The absorption path is divided into n small segments of equal length, so that the small absorption condition is met in each small segment, that is, α(v) / n≤0.05, and the normalized second harmonic spectrum in each small segment is calculated. The sum of them is taken as the normalized second harmonic spectrum on the entire absorption path

[0070]

[0071] Calculate the i-th temperature-concentration pair {T i ,X i Normalized second harmonic spectrum after large absorption correction under unit path length Form the modified normalized second harmonic basis matrix Solving the following matrix equations yields the preliminary results of the multi-path temperature-concentration histogram matrix:

[0072]

[0073] Among them, e 10×1 and e 1200×1 is a column vector with all elements equal to 1;

[0074] Calculate the i-th temperature-concentration pair {T i ,X i Absorption rate α when passing through unit path length under i , forming the absorption rate basis matrix α 10×16800 , the wave number length of the absorption rate is 16800 points, so as to calculate the absorption rate projection result A on multiple light paths 120×16800 :

[0075] A 120×16800 =H120×10 α 10×16800 (3)

[0076] Among them, A 120×16800 The qth row a q is the absorption rate on the qth optical path. The transmitted light intensity I on the optical path is calculated based on the incident light intensity I0 and the absorption rate projection result on the optical path. t :

[0077] I t =I0exp(-a q ) (4)

[0078] Calculate the normalized second harmonic spectrum by directly performing orthogonal demodulation based on the transmitted light intensity The absorption path is divided into several small segments, and the normalized second harmonic spectrum is calculated and superimposed to obtain the normalized second harmonic spectrum after large absorption correction. Calculate the correction factor on this optical path:

[0079]

[0080] Calculate the normalized second harmonic spectrum of the optical path after large absorption correction

[0081]

[0082] Forming the multi-path corrected normalized second harmonic spectrum matrix after large absorption correction Solve the multi-path histogram inversion model after large absorption correction and obtain the final result of the multi-path temperature-concentration histogram matrix

[0083]

[0084] Will Substitute the temperature and gas concentration distribution discrete imaging model shown in the following equation for iterative inversion to solve the gas temperature concentration distribution matrix Y 1600×10 :

[0085]

[0086] Calculate the gas temperature distribution T in the measured field 1600×1 and concentration distribution X 1600×1 :

[0087] (T 1600×1 X 1600×1 )=Y 1600×10 ·U 10×2 (9)

[0088] The above description of the present invention and its embodiments is not limited thereto, and the accompanying drawings are only one embodiment of the present invention. Without departing from the purpose of the present invention, any structure or embodiment similar to the technical solution designed without creativity shall fall within the protection scope of the present invention.

Claims

1. A temperature concentration discrete imaging method combined with wavelength modulation large absorption correction, characterized in that: The temperature and two-dimensional distribution of gas molecule concentration in the measured area are measured using a multi-angle laser absorption spectrum imaging sensor. The wavelength-modulated laser passes through the measured area after spectroscopy and is received by the detector to obtain the transmitted light intensity on multiple optical paths. After orthogonal demodulation, the multi-path normalized second harmonic spectrum projection matrix is ​​obtained. The length of each laser path passing through each grid is obtained according to the laser optical path layout of the sensor to form a sensitivity matrix. Based on a rough estimate of the target gas temperature and concentration range, the gas parameters are discretized into multiple temperature-concentration pairs, and the absorptivity of each temperature-concentration pair per unit length is calculated. The transmitted light after absorption per unit length of each temperature-concentration pair is obtained according to the incident laser intensity. Strong, after orthogonal demodulation, the normalized second harmonic spectrum of the temperature-concentration pair per unit length is obtained to form a normalized second harmonic basis matrix, and a multi-path temperature-concentration histogram inversion model and a two-dimensional temperature concentration distribution discrete imaging model under small absorption conditions are established; for large absorption conditions, the normalized second harmonic spectrum in the wavelength modulation method no longer satisfies the linear superposition law, and a large absorption correction method is used to reduce the nonlinearity in the model: the absorption path length is divided into multiple small segments of equal length, and the absorption in each small segment satisfies the small absorption condition and the linear superposition relationship. The normalized second harmonic spectrum under each small segment is calculated and the sum is taken as the corrected normalized second harmonic spectrum on the entire absorption path, which also satisfies the linear superposition relationship; The normalized second harmonic spectrum projection matrix after large absorption correction and the normalized second harmonic basis matrix are used to establish a multi-path histogram inversion model and a discrete imaging model of temperature and gas concentration distribution under large absorption conditions. In order to perform large absorption correction on the multi-path normalized second harmonic projection measurement results, the multi-path histogram inversion model is first solved based on the normalized second harmonic spectrum projection matrix of each light path before correction and the corrected normalized second harmonic basis matrix to obtain the preliminary result of the multi-path temperature-concentration pair histogram matrix, thereby determining the large absorption correction coefficient on each light path, and then calculating the normalized second harmonic spectrum projection matrix after large absorption correction. The multi-path histogram inversion model is solved for the second time to obtain the final result of the multi-path temperature-concentration pair histogram matrix, and the discrete imaging model of temperature and gas concentration distribution is solved to obtain the temperature and concentration distribution of the measured field.

2. The temperature concentration discrete imaging method combined with wavelength modulation large absorption correction according to claim 1 is characterized in that The gas temperature concentration distribution in the measured area is discretized into multiple temperature-concentration pairs, and a multi-light path histogram inversion model under small absorption conditions and a discrete imaging model of temperature and gas concentration distribution are established; a large absorption correction method is used to reduce the nonlinearity in the model, and the absorption path length is divided into small absorption segments of equal length. The sum of the normalized second harmonic spectra under each small segment is taken as the result on the entire path, and a corrected multi-light path histogram inversion model and a discrete imaging model of temperature and gas concentration distribution under large absorption conditions are established, which specifically includes the following steps: Step 1: In the wavelength modulation method, the wave number v(t) output by the laser is: in, is the laser center wave number, a md is the modulation depth, ω=2πf is the modulation angular frequency, f is the modulation frequency, and the model of the incident laser intensity I0(t) is in, Wave number The laser intensity at , i0 and i2 are the linear and nonlinear intensity modulation amplitudes, respectively. and is the phase shift between the corresponding intensity modulation and frequency modulation, and the transmittance is expressed as: If a path satisfies the small absorption condition, that is, α(v)≤Δ, the transmittance is approximated as a first-order Taylor expansion: Since the transmission coefficient τ[v(t)] is periodic, it can be expanded into a Fourier series: Among them, the Fourier coefficient for: Under the condition of small absorption, the Fourier coefficient The transmitted light intensity I can be integrated along the path by using phase-locked amplification and low-pass filtering. t (t) Orthogonal demodulation is performed to obtain its first harmonic and second harmonic, and finally its normalized second harmonic spectrum is obtained as follows: Under the condition of small absorption, the normalized second harmonic spectrum S 2f / 1f It also has the path-integrable property; Step 2: Use a multi-angle sensor to obtain the transmitted light intensity on the Q laser paths, and obtain the normalized second harmonic spectrum on the optical path according to the above calculation process to obtain the measured normalized second harmonic spectrum projection matrix S Q×K , where K is the number of normalized second harmonic spectrum points; according to the estimation of the measured field parameters, the temperature and gas concentration are discretized into M temperature-concentration pairs {T1,X1},{T2,X2},…{T M ,X M }, forming the gas parameter matrix U M×2 : Calculate the i-th temperature-concentration pair {T i ,X i Normalized second harmonic spectrum R of the transmitted light intensity after the laser passes through a unit path length under} i , forming the normalized second harmonic basis matrix R M×K ; Under the condition of small absorption, the normalized second harmonic spectrum signal has the characteristic of being integrable along the path, so the normalized second harmonic spectrum S on the qth optical path q for: Among them, hq ,i is the ith temperature-concentration pair {T i ,X i }, and write equation (9) in matrix form: S Q×K =H Q×M ·R M×K (10) Among them, H Q×M is a histogram matrix that characterizes the absorption length occupied by each temperature-concentration pair. The measured field is discretized into D grids, and the length w of the qth laser passing through the jth grid is calculated according to the optical path layout. q,j , forming the sensitivity matrix W Q×D , the path length on the qth light path is equal to the sum of all temperature-concentration absorption paths on the path, that is: Write equation (11) in matrix form: A Q×M ·have been M×1 =W Q×D ·have been D×1 (12) Among them, e M×1 and e D×1 is a column vector with all elements equal to 1. Combining equations (10) and (12), a multi-light path histogram inversion model is formed: H Q×M ·(R M×K e M×1 )=(S Q×K W Q×D ·e D×1 ) (13) In each grid of the measured area, the gas parameters are discretized into one of M temperature-concentration pairs, using the 0-1 matrix Y D×M represents the temperature-concentration distribution. If element y j,i is 1, it means that the gas parameter in the jth grid is the temperature concentration pair {T i ,X i }, if it is 0, it means that the gas parameters in this grid are not {T i ,X i }; The absorption path length hq of the ith temperature-concentration on the qth light path ,i It is expressed as: By writing equation (14) in matrix form, we can obtain the discrete imaging model of temperature and gas concentration distribution: H Q×M =W Q×D ·Y D×M (15) Step 3: When the small absorption condition is not met, that is, α(v)>Δ, the normalized second harmonic spectrum no longer has the characteristic of being integrable along the path, and the above linear model (13) is no longer valid, and a large absorption correction is required; the absorption path is divided into n small segments of equal length, so that each small segment meets the small absorption condition, that is, α(v) / n≤Δ, and the normalized second harmonic spectrum in each small segment is calculated. The sum of the two is taken as the corrected normalized second harmonic spectrum on the entire absorption path In the case of large absorption, the modified path-normalized second harmonic spectrum projection matrix is And the modified normalized second harmonic basis matrix The multi-light path histogram inversion model of formula (13) and the discrete imaging model of temperature and gas concentration distribution of formula (15) are also established: H Q×M =W Q×D ·Y D×M (18) The joint iterative reconstruction algorithm is used to solve equation (17) to obtain the histogram matrix H Q×M , and formula (18) to obtain the gas temperature concentration distribution matrix Y D×M , complete the solution of the inversion imaging model after large absorption correction.

3. The temperature concentration discrete imaging method combined with wavelength modulation large absorption correction according to claim 1, characterized in that: First, the normalized second harmonic spectrum projection result of each optical path is calculated according to the measured transmitted light intensity, and the multi-path histogram inversion model is solved in combination with the corrected normalized second harmonic basis matrix to obtain the preliminary result of the multi-path temperature-concentration pair histogram matrix, thereby determining the maximum absorption correction coefficient on each optical path, and calculating the path normalized second harmonic spectrum projection matrix after the large absorption correction, and the multi-path histogram inversion model is solved for the second time in combination with the corrected normalized second harmonic basis matrix to obtain the final result of the multi-path temperature-concentration pair histogram matrix, and the discrete imaging model of temperature and gas concentration distribution is solved to obtain the temperature and concentration distribution of the measured field, which specifically includes the following steps: Step 1: Calculate the normalized second harmonic spectrum projection matrix S on multiple light paths based on the measured multi-light path transmission intensity Q×K , solve the following matrix equation to obtain the preliminary results of the multi-path temperature-concentration histogram matrix: Calculate the i-th temperature-concentration pair {T i ,X i Absorption rate per unit path length under} i , forming the absorption rate basis matrix α M×N , where N is the wave number length of the absorbance, so as to calculate the absorbance projection result A on multiple light paths Q×N : A Q×N =H Q×M ·a M×N (20) Among them, A Q×N The qth row a q is the absorption rate projection on the qth optical path. The transmitted light intensity I on the optical path is calculated based on the incident light intensity I0 and the absorption rate projection result on the optical path. t : I t =I0exp(-a q ) (21) Calculate the normalized second harmonic spectrum by directly performing orthogonal demodulation based on the transmitted light intensity The absorption path is divided into multiple small segments, each of which satisfies the small absorption condition. The normalized second harmonic spectrum in the small segment is calculated and superimposed to obtain the normalized second harmonic spectrum after large absorption correction on the entire path. Calculate the correction factor on this optical path: Calculate the normalized second harmonic spectrum of the optical path after large absorption correction Forming the multi-path normalized second harmonic spectrum projection matrix after large absorption correction The final result of the multi-path temperature-concentration histogram matrix is ​​obtained by iteratively solving equation (17): Iteratively solve equation (18) to obtain the gas temperature concentration distribution matrix Y D×M , gas temperature distribution T in the measured field D×1 and concentration distribution X D×1 for: (T D×1 X D×1 )=Y D×M ·U M×2 (24) This enables the inversion of the gas temperature and concentration distribution results after large absorption correction, reducing the imaging error caused by the nonlinear error in the wavelength modulation imaging model.

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