Calibration-free flame atomic ratio detection method based on laser-induced breakdown spectroscopy

Abstract

The invention discloses a calibration-free flame atomic ratio detection method based on laser-induced breakdown spectroscopy, and relates to the technical field of optical combustion detection.The calibration-free flame atomic ratio detection method comprises the steps that a laser-induced breakdown spectroscopy system is built; carrying out a laser-induced breakdown spectroscopy collection test on the target measurement position of the flame by adopting a laser-induced breakdown spectroscopy system to obtain an original spectral signal; calculating the temperature of the laser-induced plasma based on the original spectral signal, and verifying that the laser-induced plasma is in a local heat balance state; calculating a partition function of the target atoms in the laser-induced plasma based on the temperature of the laser-induced plasma and the types of the target atoms; and calculating the flame atomic ratio based on the original spectral signal, the laser-induced plasma temperature and the partition function of the target atom in the laser-induced plasma. The method has the advantages that the flame atomic ratio in the wide laser energy range can be accurately measured without depending on a calibration curve, and meanwhile, the reliability of a measurement result is ensured by verifying the plasma state.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical combustion detection, and particularly relates to a calibration-free flame atomic ratio detection method based on laser-induced breakdown spectroscopy. BACKGROUND

[0002] Combustion diagnosis technology is a key means to study combustion mechanism, optimize combustion process and control pollutant emission. Traditional contact measurement methods will interfere with the combustion flow field, while non-contact optical measurement technology has become an important development direction of combustion diagnosis due to its advantage of not interfering with the combustion process. Laser-induced breakdown spectroscopy (LIBS) technology can realize rapid detection of element composition by focusing laser to generate plasma and analyzing the characteristic spectral lines emitted during the cooling process, and has unique advantages in flame atomic ratio measurement.

[0003] Atomic ratio, as a core parameter reflecting the local equivalence ratio distribution of the flame and the carbon soot generation characteristics, is of great significance to the understanding of the combustion process. The existing flame atomic ratio measurement methods based on LIBS technology generally rely on the pre-established atomic ratio calibration curve, and this method has obvious limitations. First, the laser energy has a significant impact on the plasma temperature and atomic spectral line intensity, and the calibration results under a single laser energy are difficult to adapt to the changing working conditions in the actual combustion environment. Second, repeated calibration experiments are needed under different laser energies, which not only increases the experimental complexity, but also introduces systematic errors. In addition, the traditional method lacks verification of the local thermal equilibrium state of the plasma, which may reduce the reliability of the measurement results.

[0004] Although existing research attempts to improve measurement accuracy by correcting the impact of laser energy on plasma temperature, these methods still cannot fundamentally get rid of the dependence on atomic ratio calibration experiments. Especially in the application scenarios of wide laser energy range, the existing technology is difficult to guarantee the stability and accuracy of the measurement results. Therefore, developing a flame atomic ratio detection method that does not require calibration experiments, can adapt to a wide range of laser energies, and can accurately verify the plasma state, has become a technical problem to be solved in the field.

[0005] In view of the above problems, the existing technology needs to be improved. SUMMARY

[0006] Therefore, the present application provides a calibration-free flame atomic ratio detection method based on laser-induced breakdown spectroscopy, which has the advantages of being able to accurately measure the flame atomic ratio in a wide range of laser energies without relying on calibration curves, and ensuring the reliability of the measurement results by verifying the plasma state.

[0007] The application provides a kind of calibration-free flame atomic ratio detection method based on laser-induced breakdown spectroscopy, comprising the following steps: S1, build a laser-induced breakdown spectroscopy system;The laser-induced breakdown spectroscopy system includes a pulsed laser and a fiber spectrometer; S2, the laser-induced breakdown spectroscopy system is used to carry out laser-induced breakdown spectroscopy collection test on the target measurement position of flame;Specifically: focus on the target measurement position of flame by pulsed laser and output pulsed laser to generate laser-induced plasma; synchronously collect the original spectrum signal emitted by the laser-induced plasma containing the characteristic spectrum line of the target atom by the fiber spectrometer; S3, based on the original spectrum signal, calculate the temperature of the laser-induced plasma, and verify that the laser-induced plasma is in a local thermal equilibrium state; S4, based on the temperature of the laser-induced plasma and the type of the target atom, calculate the partition function of the target atom in the laser-induced plasma; S5, based on the original spectrum signal, the temperature of the laser-induced plasma and the partition function of the target atom in the laser-induced plasma, calculate the flame atomic ratio.

[0008] In an alternative embodiment, the S1 comprises: S11, determine the flame atomic ratio, and the type of the target atom in the flame atomic ratio; S12, according to the type of the target atom, determine the selection of the laser pulser and the fiber spectrometer;The selection criteria meet: The laser energy of the pulsed laser generates a laser-induced plasma at the target measurement position of flame that meets the local thermal equilibrium state; The spectral measurement range of the fiber spectrometer covers the characteristic spectrum line wavelength of the target atom; S13, set the specific parameter values of laser energy, collection delay time and fiber spectrometer exposure time;The setting criteria meet: the original spectrum signal is not less than 50% of the range of the fiber spectrometer; S14, use a high-temperature blackbody furnace to calibrate the thermal radiation intensity of the fiber spectrometer, obtain a thermal radiation intensity calibration coefficient, which is used to convert the original spectrum signal into a radiation intensity signal, and its expression is: Wherein, is the original spectrum signal, is the thermal radiation intensity calibration coefficient, t is the exposure time, is the radiation intensity signal.

[0009] In an alternative embodiment, the S3 comprises: S31, converting the original spectrum signal into a radiation intensity signal according to a thermal radiation intensity calibration coefficient; S32, deducting a background radiation intensity signal from the radiation intensity signal to obtain an atomic spectrum line signal; S33, calculating a spectral peak area of a characteristic spectrum line of a target atom based on a kind of the target atom and a broadening of the atomic spectrum line signal, to obtain a characteristic spectrum line radiation intensity signal; S34, selecting an atomic kind with multiple characteristic spectrum lines from the characteristic spectrum line radiation intensity signal, drawing a curve through a preset relationship, and fitting a slope q e / k B T p to obtain a laser-induced plasma temperature; The preset relationship is: wherein, is a wavelength; is a characteristic spectrum line radiation intensity signal of the atom; h is a Planck constant, q e is an elementary charge, k B is a Boltzmann constant, is a light speed, N is a total number of atoms, T p is a laser-induced plasma temperature, Z is a partition function of the atom related to T p , , and are degeneracy, Einstein coefficient and energy level of the atom obtained by querying an atomic spectrum database, respectively.

[0010] In an alternative embodiment, in the S32, the deducting a background radiation intensity signal comprises: identifying a local minimum point in the radiation intensity signal; linearly interpolating the local minimum point; using a polynomial to fit the interpolated local minimum value to obtain a background radiation intensity signal; subtracting the background radiation intensity signal from the radiation intensity signal to obtain an atomic spectrum line signal.

[0011] In an optional implementation, in S3, verifying that the laser-induced plasma is in a state of local thermal equilibrium includes: Based on H atoms Stark broadening of characteristic spectral lines is used to calculate the actual electron density of laser-induced plasma. Theoretical electron density was calculated based on the McWhirter standard. When the actual electron density is greater than the theoretical electron density, it is verified that the laser-induced plasma is in a state of local thermal equilibrium.

[0012] In one optional implementation, S4 specifically includes: S41. The atomic energy levels are grouped into three lumped energy levels, namely the ground state energy level, the low excited state energy level generated by valence electron rearrangement, and the high excited state energy level defined by the cutoff criterion. S42. Based on the type of target atom, the statistical weight of each lumped energy level, the average excitation energy, and the temperature of the laser-induced plasma, calculate the partition function of the target atom in the laser-induced plasma.

[0013] In one optional implementation, the calculation expression for the partition function of the target atom in the laser-induced plasma is: in, G 0、 G L , G H These are the statistical weights for the ground state energy level, the lower excited state energy level, and the higher excited state energy level, respectively. and These represent the average energies of the lower and higher excited state levels relative to the ground state level, respectively. k B Boltzmann's constant, T p The temperature of the laser-induced plasma; The expressions for the statistical weights and average excitation energies of the lower excited state energy levels are as follows: in, g i The first in the low excited state energy level i The statistical weights of each energy level E i The first in the low excited state energy level i The energy of each energy level; low represents all energy levels in the lower excited state. The expressions for the statistical weights and average excitation energies of the higher excited states are as follows: wherein, g core is the statistical weight of the ground state of the atom composed of the nucleus and the inner-shell electrons, I is the ionization energy of the target atom, I H is the Rydberg constant, is the effective nuclear charge number, and , n 1 is the principal quantum number of the highest occupied energy level in the ground state electronic configuration of the atom plus 1, n 2 is the maximum principal quantum number; the maximum principal quantum number n 2 is obtained by rounding after satisfying the first condition; the first condition is: wherein, is the ionization energy reduction amount.

[0014] In an alternative embodiment, in the S5, the calculation expression of the flame atomic ratio is: wherein, is the characteristic spectral line radiation intensity signal of the atom A; is the characteristic spectral line radiation intensity signal of the atom B ; , are the wavelengths of the atom A and the atom B , respectively; , are the degeneracy of the atom A and the atom B , respectively; , are the Einstein coefficients of the atom A and the atom B , respectively; , are the energy levels of the atom A and the atom B , respectively; q e is the elementary charge; k B is the Boltzmann constant; T p is the laser-induced plasma temperature; N A 、N B are the total number of the atom A and the atom B , respectively,Z A 、Z B the partition function of atom A and atom B .

[0015] From the above, the application provides a kind of based on laser-induced breakdown spectroscopy's free calibration flame atomic ratio detection method, by setting up laser-induced breakdown spectroscopy system acquisition original spectrum signal, combined with plasma temperature calculation and local thermal equilibrium verification, directly deduce flame atomic ratio, without relying on calibration experiment, with the advantages of wide laser energy range, improve measurement accuracy and reliability. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the drawings needed in the specific embodiments or prior art description will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0017] Figure 1 is a flowchart of a kind of based on laser-induced breakdown spectroscopy's free calibration flame atomic ratio detection method according to the embodiments of the present application; Figure 2 is the structure principle diagram of laser-induced breakdown spectroscopy system according to the embodiments of the present application; Figure 3 is the radiation intensity signal diagram before and after deducting background radiation intensity signal according to the embodiments of the present application; Figure 4 is the curve diagram according to the embodiments of the present application ; Figure 5 is the N and O atom partition function diagram under different laser-induced plasma temperatures according to the embodiments of the present application; Figure 6 is the N / O atomic ratio result diagram of ethylene back diffusion flame axis under air atmosphere according to the embodiments of the present application. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical scheme and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0019] Therefore, as Figure 1 shown, the present application proposes a laser-induced breakdown spectroscopy-based calibration-free flame atomic ratio detection method, including the following steps: Step S1, building a laser-induced breakdown spectroscopy system; the laser-induced breakdown spectroscopy system includes a pulsed laser and a fiber spectrometer, as Figure 2 shown.

[0020] Step S2, using the laser-induced breakdown spectroscopy system to perform laser-induced breakdown spectroscopy acquisition test on the target measurement position of the flame; specifically: focusing and outputting pulsed laser on the target measurement position of the flame by the pulsed laser to generate laser-induced plasma; synchronously collecting the original spectral signal emitted by the laser-induced plasma and containing the characteristic spectral line of the target atom by the fiber spectrometer.

[0021] Step S3, based on the original spectral signal, calculating the temperature of the laser-induced plasma, and verifying that the laser-induced plasma is in a local thermal equilibrium state.

[0022] Step S4, based on the temperature of the laser-induced plasma and the type of the target atom, calculating the partition function of the target atom in the laser-induced plasma.

[0023] Step S5, based on the original spectral signal, the temperature of the laser-induced plasma, and the partition function of the target atom in the laser-induced plasma, calculating the flame atomic ratio.

[0024] Wherein, the laser-induced breakdown spectroscopy (LIBS) system refers to a device that generates plasma by focusing pulsed laser and synchronously collects its radiation spectrum, which can be specifically realized by combining an energy-adjustable Nd: YAG laser with a high-resolution fiber spectrometer, and its function is to generate stable plasma and capture characteristic spectral lines in a wide laser energy range. Laser-induced plasma temperature calculation refers to solving plasma thermodynamic parameters by multi-spectral line Boltzmann slope method, which can be specifically realized by calculating electron density by spectral line Stark broadening to verify the local thermal equilibrium condition, which provides a temperature benchmark for atomic energy state distribution. Partition function calculation refers to merging atomic energy levels into three "collective energy levels", and calculating the statistical weight and average excitation energy of each collective energy level. Flame atomic ratio calculation refers to the result after rearranging the calculation formula of spectral line intensity ratio into the ratio of target atom number, which realizes the direct physical derivation of atomic ratio.

[0025] By the technical solution, the application realizes the non-calibration measurement of the flame atomic ratio, adapts to the detection requirements under different laser energy conditions, and solves the measurement error problem caused by the limitation of the calibration experiment of the traditional method. The method can be directly applied to the combustion diagnosis under the variable laser energy working condition, and provides a reliable technical means for the in-situ detection of the atomic ratio in a complex combustion environment.

[0026] In an optional embodiment, step S1 comprises: Step S11, determining the flame atomic ratio and the type of target atom in the flame atomic ratio.

[0027] Step S12, determining the selection of the laser pulse device and the fiber spectrometer according to the type of target atom; the selection standard satisfies: The laser energy of the pulse laser generates a laser-induced plasma at the flame target measurement position to meet the local thermal equilibrium state. The spectral measurement range of the fiber spectrometer covers the characteristic spectral line wavelength of the target atom.

[0028] Step S13, setting specific parameter values of the laser energy, the acquisition delay time and the exposure time of the fiber spectrometer; the setting standard satisfies: the original spectral signal is not less than 50% of the range of the fiber spectrometer.

[0029] Step S14, using a high-temperature blackbody furnace to calibrate the thermal radiation intensity of the fiber spectrometer, obtaining a thermal radiation intensity calibration coefficient for converting the original spectral signal into a radiation intensity signal, and the expression is: Wherein, is the original spectral signal, is the thermal radiation intensity calibration coefficient, t is the exposure time, is the radiation intensity signal.

[0030] The determination of the target atomic species refers to the determination of two or more element types to be compared in the flame to be measured, such as carbon and oxygen, which can be specifically achieved by analyzing the composition of the fuel and the oxidizer, the combustion reaction equation or the pre-experiment spectrum, and provides a targeted basis for subsequent equipment selection. The selection of the equipment according to the target atom refers to the selection of the spectral instrument according to the wavelength range of the atomic characteristic spectrum, for example, for the oxygen atom 777.415 nm spectrum line, a spectral instrument covering the range of 750-800 nm is selected, and the laser output energy range is selected according to the flame environment, for example, a 50-200 mJ adjustable laser is used to ensure that the plasma meets the thermal equilibrium condition. The setting of the parameter value refers to adjusting the laser energy to, for example, 100 mJ, setting the delay time to, for example, 1 μs, and setting the exposure time to, for example, 2 ms, so that the spectral signal intensity is in the linear response interval of the instrument. The thermal radiation calibration refers to generating a standard radiation signal at a known temperature by using a blackbody furnace, and calculating the response coefficient of each wavelength by linear regression, for example, obtaining a calibration curve at a blackbody temperature of 1800 K to eliminate the influence of the sensitivity difference of the instrument itself on the measurement.

[0031] Specifically, the method ensures that the target atomic spectrum line is completely captured and the plasma is in an analyzable state through targeted equipment selection, and realizes accurate conversion of the original signal to the radiation intensity through thermal radiation calibration. For example, when measuring the carbon-oxygen atomic ratio, a spectral instrument covering the characteristic spectrum lines of the carbon atom 833.51 nm and the oxygen atom 777.41 nm is selected, and the laser energy is adjusted to make the plasma electron density reach the local thermal equilibrium threshold, and the response deviation of the spectral instrument at different wavelengths is eliminated through blackbody furnace calibration, finally providing a reliable data basis for atomic ratio calculation.

[0032] Through the above technical solutions, the present application effectively overcomes the calibration result deviation caused by the change of laser energy, and avoids systematic errors in the conversion process of the spectral signal. For the change of atomic species and the amount of substance in different combustion scenes, the method guarantees the universality of the measurement through equipment selection and parameter setting, and the thermal radiation calibration step converts the original signal to absolute radiation intensity, so that the data under different experimental conditions are directly comparable, and a reliable foundation is provided for the calibration-free atomic ratio detection.

[0033] In an optional implementation, step S3 comprises: S31, converting the original spectral signal to a radiation intensity signal according to the thermal radiation intensity calibration coefficient.

[0034] S32, deducting the background radiation intensity signal from the radiation intensity signal to obtain an atomic spectrum line signal.

[0035] S33, calculating the spectral peak area of the characteristic spectrum line of the target atom based on the type of the target atom and the broadening of the atomic spectrum line signal, to obtain a characteristic spectrum line radiation intensity signal.

[0036] S34, selecting an atomic species with multiple characteristic spectral lines from the characteristic spectral line intensity signal, drawing a curve through a preset relationship formula, and fitting a slope q e k B T p to obtain a laser-induced plasma temperature.

[0037] The preset relationship formula is: wherein, is a wavelength; is an atomic characteristic spectral line intensity signal; h is a Planck constant, q e is a unit electric charge, k B is a Boltzmann constant, is a light speed, N is a total number of atoms, T p is a laser-induced plasma temperature, Z is a partition function of the atom related to T p , , and are degeneracy, Einstein coefficient and energy level of the atom obtained by querying an atomic spectral database, respectively.

[0038] wherein, the thermal radiation intensity calibration coefficient refers to a conversion coefficient obtained by calibrating the fiber spectrometer by the high-temperature black body furnace, and specifically can be realized by a proportional relationship between the standard radiation signal emitted by the high-temperature black body furnace and the signal collected by the fiber spectrometer, for eliminating the influence of the response difference of the spectrometer on the original spectrum signal and ensuring the physical consistency of the radiation intensity signal.

[0039] The background radiation intensity signal deduction refers to a process of separating the continuous background radiation from the total radiation signal, and specifically can be realized by a method combining local minimum point identification, linear interpolation and polynomial fitting, by identifying the local minimum point in the spectrum signal and fitting the background radiation trend, to effectively suppress the interference of the flame continuous radiation and the instrument noise on the atomic spectral line signal.

[0040] The spectrum peak area calculation refers to an integral processing on the broadened atomic characteristic spectral line, and specifically can be realized by a Gaussian function fitting or a direct numerical integral method, by converting the discrete spectral line intensity data into integral area values to eliminate the error caused by the radiation intensity quantization due to the spectral line broadening effect.

[0041] ​​The multi-feature spectral line temperature fitting refers to temperature inversion using the relationship between the radiation intensity of multiple spectral lines of the same atom and the energy level. Specifically, the wavelength, intensity, degeneracy, Einstein coefficient and energy level parameter of the multiple characteristic spectral lines can be processed by using a logarithmic linear regression method to realize self-consistent calculation of the plasma temperature by establishing a logarithmic linear relationship between the spectral line intensity and the energy level and fitting the slope. The number of the multiple spectral lines of the same atom should be greater than or equal to 3.

[0042] Through the above technical solution, the present application solves the problem of inaccurate temperature measurement caused by background radiation interference and single spectral line error, and realizes reliable calculation of the plasma temperature in a wide laser energy range. By improving the accuracy of obtaining the temperature parameter in the flame atomic ratio detection, reliable thermodynamic state parameters are provided for the calibration-free atomic ratio calculation.

[0043] In an optional embodiment, in step S32, the background radiation intensity signal is deducted, including: Identifying the local minimum points in the radiation intensity signal.

[0044] Linearly interpolating the local minimum points.

[0045] Using a polynomial to fit the interpolated local minimum values to obtain the background radiation intensity signal.

[0046] Subtracting the background radiation intensity signal from the radiation intensity signal to obtain the atomic spectral line signal.

[0047] The local minimum point refers to a position in the radiation intensity signal at which the adjacent spectral intensities are greater than the position. Linear interpolation refers to linearly connecting the region between the local minimum points according to the wavelength interval of the spectrometer, which is used to maintain the spatial continuity of the background radiation. Polynomial fitting refers to curve fitting of the interpolated local minimum points using a fourth-order polynomial, and the polynomial coefficients can be solved by using the least squares method, which is used to eliminate local errors caused by spectrometer noise or abnormal points.

[0048] In an optional embodiment, in step S3, it is verified that the laser-induced plasma is in a local thermal equilibrium state, including: Calculating the actual electron density of the laser-induced plasma based on the H atom characteristic spectral line Stark broadening; Calculating the theoretical electron density based on the McWhirter criterion; When the actual electron density is greater than the theoretical electron density, it is verified that the laser-induced plasma is in a local thermal equilibrium state.

[0049] Specifically, the H atom The characteristic Stark line is taken as the analysis object, the quantitative relationship between its broadening characteristics and electron density is used, the actual electron density is calculated by measuring the full width at half maximum of the spectrum line and substituting into the Stark broadening formula. Meanwhile, the theoretical electron density is calculated based on the McWhirter standard calculation theory. By comparing the actual measured value with the theoretical threshold value, when the actual electron density exceeds the theoretical value, it indicates that the electron collision frequency is sufficient to maintain that the number of excited state particles obeys the Boltzmann distribution, so as to determine that the plasma satisfies the local thermal equilibrium state. The determination method directly depends on the physical characteristics of the plasma, without relying on external calibration data.

[0050] It should be noted that the theoretical electron density n e,t The following conditions need to be met: Wherein, M is a coefficient related to the atomic species, T p is the temperature of the laser-induced plasma, is the energy gap between the ground state and the low-energy state.

[0051] Through the above technical solution, the plasma state verification without calibration experiment is realized, and the effectiveness of the theoretical basis of atomic ratio calculation is ensured. The spectrum characteristics are directly related to the plasma state parameters through the physical model, avoiding the system error introduced by the calibration experiment, and at the same time, the detection requirements under wide range of laser energy conditions are met. Based on The selective analysis of the spectrum characteristics effectively utilizes the universal existence of hydrogen element, and enhances the applicability of the method in various combustion environments.

[0052] In an optional embodiment, step S4 specifically comprises: Step S41, the atomic energy levels are merged into three lumped energy levels, which are the ground state energy level, the low excited state energy level and the high excited state energy level.

[0053] Step S42, based on the type of the target atom, the statistical weight of each lumped energy level, the average excitation energy and the temperature of the laser-induced plasma, the partition function of the target atom in the laser-induced plasma is calculated.

[0054] Through the above technical solution, the stable calculation of the atomic partition function in the complex combustion environment is realized, and the calculation divergence problem caused by the large number of energy levels in the traditional method is solved. Through dynamic energy level grouping and parameter fusion, the partition function calculation can adapt to the change of the plasma temperature induced by different laser energies, providing a theoretical calculation basis for the calibration-free flame atomic ratio detection. The scheme avoids the strict limitation of the traditional calibration experiment on the measurement conditions, and can directly establish the quantitative relationship between the spectrum signal and the physical parameters, which is suitable for the combustion diagnosis scene under wide range of laser energy.

[0055] In one alternative implementation, the expression for calculating the partition function of the target atom in the laser-induced plasma is: in, G 0、 G L , G H These are the statistical weights for the ground state energy level, the lower excited state energy level, and the higher excited state energy level, respectively. and These represent the average excitation energies of the lower and higher excited state levels relative to the ground state level, respectively. k B Boltzmann's constant, T p The temperature of the laser-induced plasma; The expressions for the statistical weights and average excitation energies of the lower excited state energy levels are as follows: in, g i The first in the low excited state energy level i The statistical weights of each energy level E i The first in the low excited state energy level i The energy of each energy level; low represents all energy levels in the lower excited state. The expressions for the statistical weights and average excitation energies of the higher excited states are as follows: in, g core The statistical weights of the atomic real ground state, composed of the atomic nucleus and inner-shell electrons. I The ionization energy of the target atom, I H For Rydberg constant, The effective nuclear charge number, and , n 1 represents the principal quantum number of the highest occupied energy level in the ground state electronic configuration of an atom, plus 1. n 2 is the maximum principal quantum number; Maximum principal quantum number n 2 is obtained by rounding down after satisfying the first condition; The first condition is: in, This is to reduce the amount of ionization energy.

[0056] In an alternative embodiment, in step S5, the calculation expression of the flame atomic ratio is: wherein, is the characteristic spectral line radiation intensity signal of atom A; is the characteristic spectral line radiation intensity signal of atom B ; , are the wavelengths of atom A and atom B , respectively; , are the degeneracy of atom A and atom B , respectively; , are the Einstein coefficients of atom A and atom B , respectively; , are the energy levels of atom A and atom B , respectively; q e is the elementary charge; k B is the Boltzmann constant; T p is the laser-induced plasma temperature; N A 、N B are the total number of atom A and atom B , respectively, Z A 、Z B are the partition functions of atom A and atom B , respectively.

[0057] wherein, the calculation expression of the flame atomic ratio can be derived from the ratio of the characteristic spectral line radiation intensity signals of the target atoms. The calculation of the characteristic spectral line radiation intensity signals of the target atoms is expressed as: (This embodiment takes atom A and atom B as an example for illustration, and other atoms are calculated in the same way).

[0058] wherein, h is the Planck constant, is the spectral line frequency of atom A , is the spectral line frequency of atom B .

[0059] By the technical solution, the application realizes the calibrated accurate calculation of the flame atomic ratio, and the atomic ratio result can be obtained directly based on the spectrum signal and the plasma parameter without relying on the calibration experiment. By dynamically correlating the temperature and the atomic energy level distribution, the measurement conditions of a wide laser energy range are adapted, and the measurement error caused by the change of the laser energy is avoided. By introducing the partition function and the atomic inherent parameter, the physical rigor of the atomic ratio calculation is improved, and the measurement accuracy in different combustion scenes is ensured.

[0060] To verify the technical effect of the application, in this embodiment, the N / O atomic ratio distribution of ethylene counter diffusion flame in air atmosphere is detected.

[0061] Step 1: Build a laser-induced breakdown spectroscopy system: The selected Nd: YAG pulse laser has a rated output power of 325 mJ, and is equipped with an electric attenuator to adjust the laser energy. The rated output wavelength is 532 nm, and the maximum laser pulse frequency is 10 Hz. After the pulsed laser beam passes through the beam expander and the convex lens, the laser-induced plasma is generated at the target flame position. The laser energy can ensure that the laser-induced plasma generated at any position of the target flame is in a local thermal equilibrium state.

[0062] The characteristic spectrum line of N atom is located at 746.83 nm, and the characteristic spectrum line of O atom is located at 777.41 nm. In addition, the spectrum line for calculating the electron density of the plasma is located at 656.28 nm. The spectral measurement range of the fiber spectrometer is selected as 640-860 nm, which meets the measurement requirements of the target atomic spectrum line signal.

[0063] The laser energy range set in the experiment measurement is 100-200 mJ. The spectrum radiation signal emitted by the laser-induced plasma contains atomic spectrum line signals and background radiation signals. In order to reduce the influence of the background radiation on the atomic spectrum line signal as much as possible, the acquisition delay time of the fiber spectrometer is set to 1 , and the exposure time is set to 3 ms.

[0064] The fiber spectrometer is calibrated by a high-temperature blackbody furnace with known emissivity to establish the quantitative relationship between the spectrum signal and the radiation intensity signal. Taking the wavelength 710.19 nm as an example, the thermal radiation intensity calibration coefficient can be obtained by formula (1) as 0.075596.

[0065] (1) Step 2: Laser-induced breakdown spectroscopy acquisition test of the target position of the flame: ​A pulsed laser is focused at the measurement location of the flame target and outputs a pulsed laser to generate laser-induced plasma. A fiber optic spectrometer is then used to simultaneously acquire the raw spectral signal emitted by the laser-induced plasma, which contains the characteristic spectral lines of the target atoms.

[0066] Step 3: Calculation of laser-induced plasma temperature and verification of local thermal equilibrium: The original spectral signal is converted into a radiation intensity signal using equation (1). Then, by identifying the local minimum points of the radiation intensity signal and performing linear interpolation and fourth-order polynomial fitting on them sequentially, the background radiation intensity signal is obtained. Subtracting the background radiation intensity from the spectral radiation intensity yields the atomic spectral line signal, such as... Figure 3 As shown. Because atomic spectral lines exhibit a broadening effect, the area of ​​the peak where the target atomic spectral line is located is calculated and used as the radiation intensity signal of the atomic characteristic spectral line for subsequent calculations.

[0067] Based on equation (2) The curve is used to solve for the temperature of the laser-induced plasma. The characteristic spectral radiation intensities of O atoms at wavelengths of 715.67 nm, 777.41 nm, and 844.68 nm are substituted into equation (2) to plot the curve. Boltzmann diagram, such as Figure 4 As shown, and for the slope - q e / k B T p Linear fitting was performed, and the plasma temperature corresponding to the laser energy range used in the experiment was found to be 9250-10050 K.

[0068] (2) Using 656.28 nm Stark broadening of spectral lines The actual electron density of the laser-induced plasma is evaluated as shown in Equation (3). Stark broadening of spectral lines Subtract Doppler broadening from spectral linewidth (5 nm) Naturally widened and instrument expansion The remaining broadening calculations are shown in equations (4)-(6), yielding a Stark broadening of 3.88 nm and an actual electron density of 6.41 × 10⁻⁶. 17 cm -3 .

[0069] (3) (4) (5) (6) where, is the central wavelength of the spectral line 656.28 nm, M is the mass of H atom, is the Einstein coefficient, c is the speed of light. For the fiber-optic spectrometer, is the linear dispersion, w slit is the maximum width of the slit, is the spectral resolution.

[0070] The electron density required for the laser-induced plasma to satisfy the local thermal equilibrium state is calculated by the McWhirter criterion, as shown in equation (7): (7) where, is the energy gap between the ground state and the first excited state of H atom. The electron density required for the laser-induced plasma to satisfy the local thermal equilibrium state is calculated to be 1.74 x 1010cm-3, which is much smaller than the actual electron density, i.e., the laser-induced plasma produced in the experiment satisfies the local thermal equilibrium state. 17 cm -3

[0071] Step 4, calculation of the atomic partition function: The partition function Z is the weighted sum of the Boltzmann factors at all energy levels, as shown in equation (7). Although its definition is simple, in the process of solving the partition function, the statistical weight g will increase with the increase of the principal quantum number n, which will lead to the divergence of the summation. Therefore, in the actual calculation of the partition function, a cutoff criterion must be set to avoid its divergence. Specifically, the atomic energy levels are grouped into three "collective energy levels": the ground state level, the low-energy excited state level produced by the valence electron rearrangement, and the high-energy excited state level defined by the cutoff criterion. At this time, the partition function calculation is shown in equation (7): n (7) For N and O atoms, the ground states are 2s 2 2p 3 ( 4 S°) and 2s 2 2p 4 ( 3 P), respectively, and the statistical weights g0of the ground state levels are 4 and 9, respectively. G The statistical weights g of the low-energy excited state levels are

[0072] G ​​​L and average energy Solve using equations (8)-(9).

[0073] (8) (9) Specifically, the low excited state energy levels of the N atom are considered to be 2s. 2 2p 3 (2D°) and 2s 2 2p 3 ( 2 P°), the statistical weights and average energies of its low excited state energy levels are 16 and 2.831 eV, respectively. The low excited state energy levels of the O atom are considered to be 2s. 2 2p 4 ( 1 D) and 2s 2 2p 4 ( 1 The statistical weights and average energies of its low excited state energy levels were obtained as 6 and 2.338 eV, respectively.

[0074] Statistical weights of higher excited state energy levels G H and average energy Solve using equations (10)-(12): (10) (11) (12) For the N atom, the statistical weight g of its real ground state core The principal quantum number is 9, representing the first layer above the valence electron shell. n 1 is 3, n 2 is 20, ionization energy I The value is 14.543 eV, and the statistical weights and average energies of its higher excited states are 51570 and 14.457 eV, respectively. For the O atom, the statistical weight g of its atomic real ground state is... core The principal quantum number of the first layer above the valence electron shell is 4. n 1 is 3, n 2 is 20, ionization energy I The value is 13.518 eV, and the statistical weights and average energies of its high excited state energy levels are 22920 and 13.433 eV, respectively.

[0075] In summary, substituting into equation (7) yields the partition functions of N and O atoms at different laser-induced plasma temperatures, as follows: Figure 5 As shown.

[0076] Step 5, Calculation of Atomic Fractions: The N and O atomic characteristic line intensities, the laser-induced plasma temperature, and the atomic partition functions obtained above are substituted into Equation (13) to obtain the atomic fractions of the target flame. N N / N O The atomic fraction results are shown in Table 1. Figure 6

[0077] (13) wherein, I H is the characteristic line intensity signal of H atom; I O is the characteristic line intensity signal of O atom; λ H and λ O are the wavelengths of H atom and O atom, respectively; g H and g O are the degeneracy of H atom and O atom, respectively; A H and A O are the Einstein coefficients of H atom and O atom, respectively; E H and E O are the energy levels of H atom and O atom, respectively; q H and q O are the elementary charges of H atom and O atom, respectively; k B is the Boltzmann constant; T is the laser-induced plasma temperature; N H and N O are the total number of H atom and O atom, respectively, q e q H and q O are the elementary charges of H atom and O atom, respectively; k B k B is the Boltzmann constant; T p T is the laser-induced plasma temperature; N N 、N O N H and N O are the total number of H atom and O atom, respectively, Z N 、Z O q H and q O are the elementary charges of H atom and O atom, respectively;

[0078] Although the embodiments of the present application have been described with reference to the accompanying drawings, various modifications and changes can be suggested to one skilled in the art, and it is intended that the present application encompass such modifications and changes as fall within the scope of the appended claims.​

Claims

1. A method for calibration-free flame atomic ratio detection based on laser-induced breakdown spectroscopy, characterized in that, The method comprises the following steps: S1, establishing a laser-induced breakdown spectroscopy system; the laser-induced breakdown spectroscopy system comprises a pulsed laser and a fiber spectrometer; S2, using the laser-induced breakdown spectroscopy system to perform laser-induced breakdown spectroscopy acquisition test on a target measurement position of a flame; specifically: focusing and outputting pulsed laser on the target measurement position of the flame by the pulsed laser to generate laser-induced plasma; synchronously collecting original spectral signals emitted by the laser-induced plasma and containing characteristic spectral lines of target atoms by the fiber spectrometer; S3, based on the original spectral signals, calculating the temperature of the laser-induced plasma and verifying that the laser-induced plasma is in a local thermal equilibrium state; S4, based on the temperature of the laser-induced plasma and the type of the target atoms, calculating the partition function of the target atoms in the laser-induced plasma; S5, based on the original spectral signals, the temperature of the laser-induced plasma and the partition function of the target atoms in the laser-induced plasma, calculating the atomic ratio of the flame.

2. The method of claim 1, wherein, The S1 comprises: S11, determining the atomic ratio of the flame and the type of the target atoms in the atomic ratio of the flame; S12, according to the type of the target atoms, determining the selection of the pulsed laser and the fiber spectrometer; the selection standard satisfies: the laser energy of the pulsed laser generates laser-induced plasma at the target measurement position of the flame to meet the local thermal equilibrium state; the spectral measurement range of the fiber spectrometer covers the characteristic spectral line wavelength of the target atoms; S13, setting specific parameter values of the laser energy, the acquisition delay time and the exposure time of the fiber spectrometer; the setting standard satisfies: the original spectral signals are not less than 50% of the range of the fiber spectrometer; S14, using a high-temperature blackbody furnace to calibrate the thermal radiation intensity of the fiber spectrometer to obtain a thermal radiation intensity calibration coefficient, which is used to convert the original spectral signals into radiation intensity signals, and the expression is: wherein is the original spectral signal, is the thermal radiation intensity calibration coefficient, t is the exposure time, is the radiation intensity signal.

3. The method of claim 2, wherein, The S3 comprises: S31, converting the original spectral signals into radiation intensity signals according to the thermal radiation intensity calibration coefficient; S32, deducting a background radiation intensity signal from the radiation intensity signals to obtain an atomic spectral line signal; S33, based on the type of the target atoms and the broadening of the atomic spectral line signal, calculating the spectral peak area of the characteristic spectral line of the target atoms to obtain a characteristic spectral line radiation intensity signal; S34, selecting atomic species with multiple characteristic spectral lines from the characteristic spectral line intensity signal, drawing a curve through a preset relationship formula, and fitting a slope q e / k B T p , obtaining the laser-induced plasma temperature;​ The preset relationship is: wherein is the wavelength; is the characteristic spectral line radiation intensity signal of the atom; h is Planck's constant, q e is the elementary charge, k B is Boltzmann's constant, is the speed of light, N is the total number of atoms, T p is the laser induced plasma temperature, Z is the partition function of the atom related to T p , and are the degeneracy, Einstein coefficient and energy level of the atom, respectively, obtained by querying an atomic spectral database.​ 4. The method of claim 3, wherein, In the S32, the deduction of the background radiation intensity signal comprises: identifying a local minimum point in the radiation intensity signal; linearly interpolating the local minimum point; using a polynomial to fit the interpolated local minimum value to obtain a background radiation intensity signal; subtracting the background radiation intensity signal from the radiation intensity signal to obtain an atomic spectral line signal.

5. The method of claim 1, wherein, In the S3, the verification that the laser-induced plasma is in a local thermal equilibrium state comprises: Based on H atoms The actual electron density of the laser-induced plasma is calculated from the characteristic line Stark broadening. calculating a theoretical electron density based on the McWhirter standard; when the actual electron density is greater than the theoretical electron density, verifying that the laser-induced plasma is in a local thermal equilibrium state.

6. The method of claim 1, wherein, The S4 specifically comprises: S41, merging the atomic energy levels into three collective energy levels, respectively, a ground state energy level, a low excited state energy level generated by valence electron rearrangement, and a high excited state energy level defined by a cutoff criterion; S42, calculating a partition function of the target atom in the laser-induced plasma based on a kind of the target atom, a statistical weight of each collective energy level, an average excitation energy, and a temperature of the laser-induced plasma.

7. The method of claim 6, wherein, The calculation expression of the partition function of the target atom in the laser-induced plasma is: wherein, G 0, G L , G H respectively the statistical weights of the ground state energy level, the low excited state energy level and the high excited state energy level, and respectively the average energies of the low excited state energy level and the high excited state energy level compared to the ground state energy level, k B is the Boltzmann constant, T p is the laser induced plasma temperature; The calculation expression of the statistical weight and the average excitation energy of the low excited state energy level is: wherein g i is the statistical weight of the i th energy level in the low excited state energy levels, E i is the energy of the i th energy level in the low excited state energy levels; low is all of the energy levels in the low excited state energy levels. The calculation expression of the statistical weight and the average excitation energy of the high excited state energy level is: wherein, g core Z is the atomic number of the target atom, I Ei is the ionization energy of the target atom, I H R is the Rydberg constant, Zeff is the effective nuclear charge, and , n 1 is the principal quantum number of the highest occupied energy level in the atomic ground state electronic configuration plus 1, n 2 is the maximum principal quantum number; the maximum principal quantum number n 2 is obtained by rounding off after satisfying the first condition; The first condition is: wherein, is the amount of decrease in ionization energy.

8. The method of claim 1, wherein, In the S5, the calculation expression of the flame atomic ratio is: wherein is the characteristic spectral line radiation intensity signal of atom A; is the characteristic spectral line radiation intensity signal of atom B ; , are the wavelengths of atom A and atom B , respectively; , are the degeneracy of atom A and atom B , respectively; , are the Einstein coefficients of atom A and atom B , respectively; , are the energy levels of atom A and atom B , respectively; q e is the elementary charge; k B is the Boltzmann constant; T p is the laser induced plasma temperature; N A 、N B are the total number of atom A and atom B , respectively, Z A 、Z B are the partition functions of atom A and atom B , respectively.

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