CN108801496B - Path temperature histogram measurement system and method based on overlapping absorption spectrum - Google Patents
Path temperature histogram measurement system and method based on overlapping absorption spectrum Download PDFInfo
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Abstract
The invention provides a path temperature histogram measurement system and method based on an overlapped absorption spectrum. The invention obtains the temperature histogram of the absorption path by the following steps: the laser outputs laser with periodically changed wavelength, enters the detector through the absorption gas and the etalon respectively, and is processed from a detector signal to obtain a laser absorption spectrum; calculating theoretical laser absorption spectra at different temperatures by using a HITRAN database; and performing optimal square approximation on the measured laser absorption spectrum by using the theoretical laser absorption spectrum at different temperatures to obtain an approximation coefficient serving as a temperature histogram. The method can directly acquire the histogram distribution of the path temperature from the overlapped laser absorption spectrum, has high calculation speed and simple measuring device, improves the adaptability of the laser absorption spectrum temperature measurement technology, and has wide application prospect.
Description
Technical Field
The invention relates to a path temperature histogram measurement system and method based on an overlapped absorption spectrum, and belongs to the technical field of tunable diode laser absorption spectra. The method is used for calculating a path temperature distribution histogram by utilizing the optimal approximation of the absorption spectrum after the overlapping absorption spectrum is obtained by measuring with the single-path laser absorption spectrum technology.
Background
The gas parameter method of the laser absorption spectrum measurement technology is an important method for measuring the temperature of the combustion gas due to the characteristics of high speed, non-invasion and accurate measurement. Direct Absorption Spectroscopy (DAS) is the most commonly used method of realization that can directly measure the full-scale of the laser absorption spectrum. Bolshov M.A. et al, 2010, Applied Physics B, Vol.2, page 397-407, A paper for determining temperature and water vapor concentration in high temperature zone by tunable diode laser absorption spectroscopy (measurement of the temperature and water vapor concentration in a high temperature zone by a hot zone by tunable diode laser absorption spectroscopy) describes a typical process of direct absorption, where the laser output wavelength is continuously varied over a period, scanning the entire wavelength band, to obtain an absorption spectrum.
The temperature histogram on the absorption path is obtained from the absorption spectrum, and the absorption areas of a plurality of independent spectral lines are calculated. Sanders S.T. et al, 2001, published in Applied Optics 40, 24, and 4404-4415, the Diode-laser absorption Sensor for measuring Gas Temperature distribution on a single optical path (Diode-laser absorption Sensor for Line-of-Sight Gas Temperature Distributions), established an equation of absorption area and Temperature probability distribution of 16 oxygen spectral lines in the range of 200-700K, thereby solving the Temperature histogram on the absorption path. On the basis, in 2007 Liu X. et al, published in journal of the American Association of aviation and space, AIAAjournal, Vol.45, No. 2, p.411-419, the article "measuring Non-Uniform Temperature distribution by single-light-path absorption Spectroscopy" (Measurement of Non-Uniform Temperature distribution Using Line-of-Signal Absorption Spectroscopy) proposes two methods for measuring a single-light-path Temperature histogram, Temperature discretization and Temperature distribution fitting method. In either case, the integral absorption area of several independent spectral lines is obtained.
Under high pressure conditions, temperature histogram measurement methods based on independent spectral line integral absorption area are no longer applicable. This is because the collisional broadening effect of the lines at high pressure is quite significant, and adjacent absorption lines overlap, making it difficult to obtain the absorptance of the individual absorption lines. Nagali V. et al, 1997, published in "Applied Optics" 36, 9518-9527, entitled "diode-laser sensor Design for detecting water vapor in high pressure combustion gases" (Design of a diode-laser sensor to a monitor water vapor in high-pressure combustion gases), indicated that it was very difficult to obtain the integrated absorption area of the independent lines due to collisional broadening.
The theoretical laser absorption spectra at different temperatures are used as elements, the optimal square approximation is carried out on the overlapped laser absorption spectra, and a temperature histogram can be obtained. The path temperature histogram measurement method based on the overlapped spectral lines does not require obtaining the absorption rate of independent spectral lines, and therefore has good performance under high pressure conditions. The Chinese invention patent 'a single-light path multispectral-based gas temperature probability density distribution fitting reconstruction method' (patent number: 201710469033.4) constructs an integral equation set according to a plurality of absorption spectral line intensities and corresponding absorption rate integral measurement data, and converts the integral equation into a matrix equation by a surface fitting method. The invention can obtain the temperature probability density distribution on the path, but also obtains the integral absorption area of the independent spectral line from the laser absorption spectrum. The Chinese invention patent 'gas temperature probability density distribution reconstruction method based on single light path multispectral' (patent number: 201710469715.5) adopts a discretization method to organize integral equations formed by spectral line intensity and corresponding absorptivity integral measurement data into a matrix equation set, and solves to obtain path temperature probability density distribution. This invention also requires the integrated absorption area of the individual lines to be obtained and is not applicable in the case of overlapping lines.
Based on the above background, a path temperature histogram measurement method based on overlapping absorption spectra is invented herein. And an overlapped absorption curve is obtained through single optical path absorption spectrum measurement, so that an inner product space optimal approximation problem is established and converted into a solving problem of a linear equation set. The method does not require obtaining the integral absorption area of each spectral line, is suitable for processing the laser absorption spectrum with complex overlapping, and has the characteristics of quick and accurate calculation.
Disclosure of Invention
Aiming at single-light-path overlapped laser absorption spectrum, the invention discloses a path temperature histogram measurement system and method based on overlapped absorption spectrum.
The measuring system comprises a tunable diode laser, a laser controller, an optical fiber beam splitter, a collimating mirror, a photoelectric detector, an etalon, a data acquisition card, a computer and the like. The measuring method of the invention comprises the following steps:
step one, acquiring a laser absorption spectrum. And controlling the tunable laser to scan in a certain frequency range, and acquiring a laser signal passing through the absorption gas by using a photoelectric detector. The absorption rate, i.e. the negative logarithm of the ratio of the transmitted intensity to the incident intensity of the light passing through the gas, is calculated in combination with the signal obtained from the etalon, as shown in equation (1)
In the formula I (v), I0(v) and α (v) respectively represent the transmitted light intensity, incident light intensity and absorption rate when the laser frequency is v.
And step two, calculating theoretical absorption spectra corresponding to different temperatures. Let the temperature distribution on the absorption path be discretized into n temperature values T1,T2,…,TnAn absorption spectrum at each temperature, and the n absorption spectrum data are recorded as { α }1(ν),α2(ν),…,αn(v) }, which can be obtained by the formula (2)
Wherein T isiThe total gas pressure is P, the total gas pressure is X, and the total gas concentration is K spectral lines in the required waveband. Sj(Ti) Denotes the temperature TiThe line intensity of the jth line can be calculated by the formula (3)
Where h is the Planck constant, c is the speed of light, T0For reference temperature, 296K is generally taken, K is Boltzmann constant, E' is the low-level energy of the absorption transition, v0To absorb the wavelength at which the line lies, Q (T)i) Is at a temperature TiThe value of the time distribution function reflects the proportion of the low-energy-level particles in the absorption transition to all the particles.
Phi in the formula (2)i(v) represents a linear function of the ith absorption line, which can be obtained from equation (4)
Wherein Δ νCFor the Lorentz half-width of the absorption line, the value can be determined by equation (5)
Wherein P is the total pressure of the gas, TiIs the gas temperature, χiIs the concentration of the i-th substance in the gas, gammaiFor collisional broadening coefficient, T, due to disturbance of the substance0Is a reference temperature, niIs the coefficient of influence of temperature on half-width.
Δ ν in formula (4)DIs Gaussian half width and can be obtained by the formula (6)
In the formula TiIs the gas temperature v0The frequency of the absorption spectrum line, c is the speed of light, k is the boltzmann constant, and m is the molecular mass.
And step three, performing optimal square approximation on the laser absorption spectrum to obtain a temperature histogram. For discretized temperature series T1,T2,…,TnAnd setting the proportion of each temperature on the absorption path as a corresponding vector p ═ p1p2…pn]T. Solving for vector p requires constructing matrix G and column vector b, as shown in equation (7)
Wherein α (v) represents the absorption spectrum obtained in the first step, αi(v) and αj(v) the theoretical absorption spectrum { α) at different temperatures is obtained in the second step1(ν),α2(ν),…,αn(v) } the ith and jth laser absorption spectra. Solving the linear system of equations Gp-b yields the temperature histogram p, i.e.
p＝G-1b (8)
Drawings
Fig. 1 is a structural diagram of a measurement system, which is composed of the following parts: the device comprises a laser controller (101), a tunable diode laser (102), a fiber beam splitter (103), a collimating mirror (104), an etalon (105), photodetectors (106) and (107), a data acquisition card (108), a computer (109) and the like.
FIG. 2 shows the simulated water molecules at 7178-7187cm-1Laser absorption spectra, there is severe spectral overlap.
Fig. 3 is a comparison of the calculated temperature histogram with a preset temperature histogram.
FIG. 4 is a flow chart of a measurement method according to the invention
Detailed Description
The present invention is further illustrated by the following examples.
The water molecules obtained by simulation in this example are 7179-7184cm-1To find a temperature histogram on the path, comprising the steps of:
firstly, obtaining absorption spectrum data by a computer simulation method. The example used water molecules at 7178-7187cm-1The wave band, the temperature distribution is 300K to 800K, the total gas pressure is 1atm, the water molecule concentration is 5 percent, the absorption spectrum α (v) under the condition that the absorption distance is 1 meter is arranged at intervals of 0.01cm-1The sampling was performed once, and the absorption spectrum had a total of 501 data points. The ratio of each temperature in the absorption path is shown in table 1 below:
TABLE 1 simulation of the respective temperature ratios
The absorption spectrum has obvious line overlapping phenomenon, and the absorption spectrum under the condition is shown in figure 2.
Calculating theoretical absorption spectra corresponding to different temperatures, discretizing the temperature distribution on the path into 6 temperature values {300,400,500,600,700,800}, and recording the absorption spectra corresponding to the temperature values as { α }1(ν),α2(ν),…,α6(v) }, calculated by the formula (1). These absorption spectra are the same as those obtained in step one, and are all the sameThe sequence of 501 points is discretized in the calculation.
Wherein T isiIs the temperature corresponding to the absorption spectrum, the total gas pressure P is 1atm, and the water molecule concentration chi is 5%. Sj(Ti) Denotes the temperature TiThe line intensity of the jth line can be calculated by the formula (2)
Where h is the Planck constant, c is the speed of light, T0For reference temperature, 296K is generally taken, K is Boltzmann constant, E' is the low-level energy of the absorption transition, v0To absorb the wavelength at which the line lies, Q (T)i) Is at a temperature TiThe value of the time allocation function.
Phi in the formula (1)i(v) represents a linear function of the ith absorption line, which can be obtained from the formula (3)
Wherein Δ νCFor the Lorentz half-width of the absorption line, the value can be determined by equation (4)
Wherein P is the total pressure of the gas, TiIs the gas temperature, χiIs the concentration of the i-th substance in the gas, gammaiFor collisional broadening coefficient, T, due to disturbance of the substance0Is a reference temperature, niIs the coefficient of influence of temperature on half-width.
Δ ν in formula (4)DIs Gaussian half width and can be obtained by the formula (5)
In the formula TiIs the gas temperature v0The frequency of the absorption spectrum line, c is the speed of light, k is the boltzmann constant, and m is the molecular mass.
Solving the optimal square approximation problem of the absorption spectrum by using a linear equation system to obtain a histogram of the temperature distribution, wherein the { α is used1(ν),α2(ν),…,α6(v) } linear combinatorial approximation of six sequences α (v), p ═ p1p2… p6]TAre the coefficients of the best approximation. This problem is solved by constructing a matrix G and a column vector b shown in formula (6)
Thus, a 6X 6 matrix G and a 6X 1 vector b are obtained, and the ratio of each temperature is further obtained
p＝G-1b (7)
The temperature profiles found are shown in table 2 below:
the temperature ratios obtained in Table 2
Therefore, the simulated preset temperature histogram is basically consistent with the obtained temperature histogram, the relative error does not exceed 2%, and the comparison between the two is shown in figure 3.
Claims (2)
1. A path temperature histogram measurement method based on overlapping absorption spectrum, the system for realizing the method comprises a tunable diode laser, a laser controller, an optical fiber beam splitter, a collimating mirror, a photoelectric detector, an etalon, a data acquisition card and a computer; the tunable diode laser outputs laser with wavelength changing along with time under the control of the laser controller; laser is divided into two paths through an optical fiber beam splitter: one path of light is changed into space light after passing through a collimating mirror, passes through a gas to be detected and enters a detector A, and the other path of light passes through an etalon and enters a detector B; comparing the change curve of the wavelength obtained by the detector B along with the time with the absorption intensity obtained by the detector A in a computer to obtain an overlapped laser absorption spectrum, and then performing optimal square approximation on the overlapped absorption spectrum by taking the theoretical absorption spectrum at the selected discretization temperature as a substrate to obtain a temperature histogram on an absorption path.
2. A path temperature histogram measurement method based on overlapping absorption spectra as claimed in claim 1, wherein the measurement method comprises the steps of:
step one, acquiring a laser absorption spectrum; controlling a tunable laser to scan in a certain frequency range, and obtaining a laser signal passing through the absorption gas by using a photoelectric detector; the absorption rate, i.e. the negative logarithm of the ratio of the transmitted intensity to the incident intensity of the light passing through the gas, is calculated in combination with the signal obtained from the etalon, as shown in equation (1)
In the formula I (v), I0(v) and α (v) respectively represent the transmission light intensity, incident light intensity and absorption rate when the laser frequency is v;
calculating theoretical absorption spectra corresponding to different temperatures; let the temperature distribution on the absorption path be discretized into n temperature values T1,T2,…,TnAn absorption spectrum at each temperature, and the n absorption spectrum data are recorded as { α }1(ν),α2(ν),…,αn(v) }, which can be obtained by the formula (2)
Wherein T isiThe gas temperature, the total gas pressure and the gas concentration are P and x, and k spectral lines are in total in the obtained waveband; sj(Ti) Denotes the temperature TiThe line intensity of the jth line can be calculated by the formula (3)
Where h is the Planck constant, c is the speed of light, T0For reference temperature, 296K is generally taken, K is Boltzmann constant, E' is the low-level energy of the absorption transition, v0To absorb the wavelength at which the line lies, Q (T)i) Is at a temperature TiThe value of the time allocation function reflects the proportion of low-energy-level particles in the absorption transition to all particles;
phi in the formula (2)i(v) represents a linear function of the ith absorption line, which can be obtained from equation (4)
Wherein Δ νCFor the Lorentz half-width of the absorption line, the value can be determined by equation (5)
Wherein P is the total pressure of the gas, TiIs the gas temperature, χiIs the concentration of the i-th substance in the gas, gammaiFor collisional broadening coefficient, T, due to disturbance of the substance0Is a reference temperature, niThe influence coefficient of temperature on the half width;
Δ ν in formula (4)DIs Gaussian half width and can be obtained by the formula (6)
In the formula TiIs the gas temperature v0The frequency of an absorption spectrum line, c is the speed of light, k is a Boltzmann constant, and m is the molecular mass;
performing optimal square approximation on the laser absorption spectrum to obtain a temperature histogram; for discretized temperature series T1,T2,…,TnEvery temperature is set in the absorption pathThe ratio of upper is formed into a corresponding vector p ═ p1p2… pn]T(ii) a Solving for vector p requires constructing matrix G and column vector b, as shown in equation (7)
Wherein α (v) represents the absorption spectrum obtained in the first step, αi(v) and αj(v) the theoretical absorption spectrum { α) at different temperatures is obtained in the second step1(ν),α2(ν),…,αn(v) } wherein the ith and jth laser absorption spectra; solving the linear system of equations Gp-b yields the vector p, i.e.
p＝G-1b (8)
Each item in the vector p is the proportion of different temperatures in the absorption path; thus, the temperature { T } is obtained1,T2,…,TnCorresponding ratio p1,p2,…,pnAnd completing the solution of the temperature histogram.
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| CN109556731A (en) * | 2018-12-13 | 2019-04-02 | 北京航空航天大学 | A kind of path temperature histogram measurement System and method for based on neural network and laser absorption spectrum |
| CN113252204B (en) * | 2021-04-01 | 2022-04-01 | 北京航空航天大学 | Laser absorption spectrum temperature measurement method based on amplitude modulation |
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