Non-contact gas pressure measurement method based on dual-wavelength absorption

Abstract

The invention relates to the technical field of gas pressure detection, in particular to a non-contact gas pressure measurement method based on dual-wavelength absorption. The method comprises the following steps: acquiring two characteristic absorption wavelengths lambda 1 and lambda 2 of gas to be measured and absorbance alpha 1 and alpha 2 of the gas to be measured at the temperature T and the wavelengths lambda 1 and lambda 2, calculating Lorentz full width at half maximum, performing Voigt linear fitting on the measured absorbance alpha 1 and alpha 2 respectively, and finally obtaining Lorentz full width at half maximum w1 and w2 of the absorbance alpha 1 and alpha 2 through Voigt linear fitting. And calculating the gas pressure P. According to the method, the dependence on the concentration in pressure measurement is eliminated by utilizing two absorption lines of gas molecules, and the measurement method is easily combined with an existing dual-wavelength absorption non-contact gas temperature measurement method to realize simultaneous measurement of the temperature, the pressure and the concentration of the gas.

Description

Technical Field

[0001] The invention relates to the technical field of gas pressure detection, and in particular to a non-contact gas pressure measurement method based on dual-wavelength absorption. Background Art

[0002] In pursuit of higher thermodynamic efficiency, advanced combustion systems usually work in high temperature and high pressure environments. Pressure is one of the important parameters for studying the performance of combustion systems. At present, the measurement of pressure is mainly based on traditional contact sensors such as piezoelectric, piezoresistive, and capacitive sensors. After intervening in the combustion flow field, contact sensors will interfere with the flow field and affect the measurement results. In addition, due to material limitations, contact sensors are not resistant to high temperatures and it is difficult to directly measure the pressure in the high temperature area of ​​the combustion flow field in situ. For example, the operating temperature of advanced high-temperature pressure sensors abroad is only ~700°C, while the combustion temperature of advanced fifth-generation and sixth-generation aircraft engines abroad is as high as 2000K or above. In addition, the frequency response of contact pressure sensors is limited, and the pressure pulsation frequency in advanced combustion systems can even reach tens of kHz. Traditional pressure sensors are difficult to capture these rapidly changing characteristics.

[0003] As an optical measurement technology, absorption spectroscopy has the advantages of non-invasive, in-situ multi-parameter measurement, and high speed, showing a broad prospect for pressure measurement. Relevant combustion temperature and concentration measurement technologies and their applications have been developed at home and abroad. There are two existing gas pressure measurement methods based on direct absorption spectroscopy: First, absorption spectrum shape fitting: the measured absorption spectrum is fitted to the theoretical absorption spectrum using the nonlinear least squares method. During the fitting process, the pressure value is continuously iterated until the sum of square errors between the measured absorption spectrum and the theoretical absorption spectrum is minimized, thereby measuring the gas pressure; second, the pressure is derived from the Lorentz half-width at half-height (wL) of the absorption spectrum combined with the absorption line broadening parameter.

[0004] The existing gas pressure measurement method based on direct absorption spectroscopy must be based on the known concentration, or it is necessary to decouple pressure and concentration at the same time. The measurement accuracy of pressure and concentration is mutually restricted and affected by the accuracy of absorption line broadening parameters. Therefore, the existing absorption spectroscopy pressure measurement method is difficult to meet the needs of practical applications, and new measurement methods need to be developed. Summary of the invention

[0005] In order to overcome the dependence of gas pressure measurement on concentration in the prior art, a non-contact gas pressure measurement method based on dual-wavelength direct absorption spectroscopy is provided.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A non-contact gas pressure measurement method based on dual-wavelength absorption, which is implemented based on data measured by a measurement system containing a high-temperature and high-pressure absorption cell, and includes the following steps:

[0008] S1. Calibration coefficients a(T) and b(T),

[0009] S2. Obtain the absorbance α1 and α2 of the target gas molecules at characteristic absorption wavelengths λ1 and λ2 at temperature T by direct absorption spectroscopy technology;

[0010] S3. Calculate the Gaussian half-width at half-maximum wG of the target gas molecules at wavelengths λ1 and λ2 when the temperature is T. The Gaussian half-width at half-maximum wG calculation formula is:

[0011]

[0012] Where v0 represents the absorption wavelength of the target gas, in cm -1 , M represents the molar mass of the target gas molecule;

[0013] S4. Fix the Gaussian half-width at half-height wG calculated in step S3, and use the Voigt line type to fit the absorbances α1 and α2, respectively, to obtain the Lorentzian half-width at half-height w1 and w2 of α1 and α2;

[0014] S5. Calculate the gas pressure P of the target gas according to the following formula:

[0015] P = a(T) × w1 + b(T) × w2;

[0016] Where a(T) and b(T) are coefficients for calculating pressure, in bar / cm -1 .

[0017] Preferably, in step S2, the selection of wavelengths λ1 and λ2 is determined based on the absorption line intensity and spectral line separation degree of the target gas molecules at the wavelengths, and the selected wavelengths λ1 and λ2 enable the target gas molecules to produce detectable light absorption under the measurement conditions.

[0018] Preferably, in step S4, the fitting methods of absorbance α1 and α2 are the same, and the specific method of fitting absorbance α1 is:

[0019] S41. Obtaining initial values ​​of fitting parameters, specifically: setting the initial value of the Y-axis offset y0 of the absorbance α1 to the minimum value of the absorbance α1, setting the initial value of the X-axis offset xc of the absorbance α1 to the X-axis coordinate value corresponding to the maximum value of the absorbance α1, setting the initial value of the integral area A of the absorbance α1 to the integral value of the absorbance α1 with respect to the X-axis, and setting the initial value of the Lorentz half-width at half-height wL of the absorbance α1 to the Lorentz half-width at half-height w1 of the absorbance α1;

[0020] S42. Use the Voigt function to calculate the Lorentz half-height and half-width wL. Specifically, calculate the Voigt function:

[0021] f=y0+(f L *f G )(x);

[0022] where f L is the Lorentz function, f G is a Gaussian function, * indicates f L and f G Perform convolution operation; f L and f G The calculation formula is as follows:

[0023]

[0024] In the formula, exp represents the exponential operation with the natural constant e as the base.

[0025] The absorbance α2 can be fitted using the same method, which will not be described here.

[0026] Preferably, the calibration method of coefficients a(T) and b(T) comprises the following steps:

[0027] S11. Set the temperature of group i, i=0,1,2,…,N,N≥5; the pressure of group j, j=0,1,2,…,M,M≥5;

[0028] S12. Obtain the temperature T by direct absorption spectroscopy i , pressure is P j When the absorbance of the gas molecules at wavelengths λ1 and λ2 is α 1ij , α 2ij ;

[0029] S13. Calculate the temperature as T i When the gas molecules are located at the Gaussian half-height and half-width at wavelengths λ1 and λ2;

[0030] S14. Fix the Gaussian half-height and half-width obtained in step S13 and use the Voigt line type to calculate the absorbance α 1ij , α 2ij Perform fitting and obtain α 1ij , α 2ij The Lorentz half-height and half-width w 1ij 、w 2ij ;

[0031] S15. Determine whether j is equal to M: If not, then j = j + 1, and return to step S12; if so, then according to P j =a(T i )×w 1ij+b(T i )×w 2ij , (j=0,1,2,…,M), and use nonlinear curve fitting to obtain the temperature T i The coefficient a(T i ) and b(T i );

[0032] S16. Determine whether i is equal to N: If not, then i=i+1, and return to step S12; if so, then obtain a(T) through polynomial fitting. i ) and T i and b(T i ) and T i The relationship between T and B is obtained through the relationship between T and B.

[0033] Preferably, the nonlinear curve is fitted using the least squares method.

[0034] The beneficial effects of the present invention are:

[0035] The calculation formula for the Lorentz half-width and half-height of the absorbance α1 and α2 of gas molecules at wavelengths λ1 and λ2 is:

[0036] W1=PXγ self,1 +P(1-X)γ mix,1

[0037] W2=PXγ self,2 +P(1-X)γ mix,2

[0038]

[0039] Where X is the gas concentration, γ is the collision broadening parameter of gas molecules, which is an inherent property of gas molecules and can be obtained by querying the HITRAN2020 database or by experimental measurement. The subscripts self, mix, 1, and 2 represent self-broadening, total broadening of mixed gas (excluding the gas molecules to be measured), and wavelength λ1 and λ2, respectively.

[0040] By combining the above formulas 1 and 2 and eliminating X, we can obtain a dual-wavelength absorption gas pressure measurement formula that is independent of concentration, as shown in formula 3.

[0041] Obviously, coefficients a and b are a combination of γ. When the uncertainty of γ provided by the database is small, it can be calculated by the γ provided by the database according to Formula 3; when the uncertainty of γ provided by the database is large, it can be obtained through experimental measurement according to the method provided in this application.

[0042] This application uses two absorption lines of gas molecules to eliminate the dependence on concentration in pressure measurement: since the measurement of gas molecule collision broadening parameter γ requires precise control of gas concentration, the actual measurement is affected by the uncertainty of gas concentration. The experimental measurement of a(T) and b(T) can be calibrated according to formula 3. This process does not require knowing the gas concentration, which has the advantage of eliminating the influence of concentration uncertainty.

[0043] The calibration of the coefficients a(T) and b(T) is easier to calibrate experimentally: this is because the gas concentration needs to be precisely controlled when measuring the collision broadening parameter γ of gas molecules, while the measurement of the coefficients a(T) and b(T) does not require the control of the gas concentration and is experimentally simpler.

[0044] It is easy to combine with the existing dual-wavelength absorption non-contact gas temperature measurement method to achieve simultaneous measurement of gas temperature, pressure and concentration: At present, the temperature measurement technology based on absorption spectrum generally adopts the dual-line ratio method, that is, the temperature is calculated by the ratio of the integrated area of ​​the absorbance at two wavelengths. The dual-wavelength pressure measurement method proposed in this application also uses two wavelengths and can be used for dual-line ratio method temperature measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 A schematic diagram of a measurement system used in an embodiment of the present application;

[0046] Figure 2 A schematic diagram of the method flow of this application;

[0047] Figure 3 A schematic diagram of coefficients a(T) and b(T) calibrated in the embodiment of the present application;

[0048] Figure 4 This is a schematic diagram of the absorbance measured in the examples of the present application. DETAILED DESCRIPTION

[0049] The technical solution of the present application is described in more detail below with reference to the embodiments and drawings:

[0050] Example 1

[0051] This application is based on the data measured by the measurement system containing a high temperature and high pressure absorption cell. The structural diagram of the measurement system is shown in Figure 1 shown.

[0052] The measurement system includes: light source 1 (nominal wavelength 1343nm, used to detect H2O molecules 7444.3cm -1 absorption nearby), light source 2 (nominal wavelength 1392nm, used to detect H2O molecules 7185.6cm -1Nearby absorption), laser controller (self-developed, used to control the temperature and current of light source 1 and light source 2, as well as the timing), beam combiner (used to couple the lasers emitted by light source 1 and light source 2), collimator (used to collimate the coupled laser), high-temperature and high-pressure absorption cell (self-developed, used to provide a high-temperature and high-pressure gas environment), photodetector (used to receive optical signals and perform photoelectric conversion), acquisition card (used to collect and digitize electrical signals), and computer (used for data processing).

[0053] In the present application, light source 1 and light source 2 may be two lasers with different nominal wavelengths, or a single light source that can simultaneously cover at least two characteristic absorption lines of the gas to be measured, such as a supercontinuum light source, a VCSEL, a frequency comb, etc. The choice of photodetector may also be replaced by a spectrometer, etc., depending on the type of light source.

[0054] like Figure 2 As shown, this application includes the following steps:

[0055] S1. Calibration coefficients a(T) and b(T): a(T) and b(T) are combined coefficients, in bar / cm -1 , which means that at two wavelengths (λ1, λ2), the pressure is measured using the method of the present application, and the pressure coefficient is calculated from the Lorentz half-width at half-height.

[0056] S11. Set the temperature T of the high temperature and high pressure absorption cell to 673.15K, set five pressure groups (pressure P is set to 1 bar, 2 bar, 3 bar, 4 bar, and 5 bar respectively), and measure the corresponding H2O molecules at 7444.3 cm -1 and 7185.6cm -1 Direct absorption spectral signal nearby;

[0057] S12. Preprocess the measured direct absorption spectrum signal to obtain the H2O molecule at 7444.3cm -1 and 7185.6cm -1 The absorbance signal at T and P corresponds to the absorbance signal at T and P;

[0058] S13. Calculate the H2O molecule at 7444.3 cm -1 Gaussian half-height and half-width at wG=0.0163cm -1 , and 7185.6cm -1 Gaussian half-height and half-width at wG=0.01574cm -1 ;

[0059] S14. Perform Voigt line fitting on the absorbance signal obtained, and fix the Gaussian half-height and half-width in the fitting to the calculation result of step S13, and obtain the H2O molecule at 7444.3cm -1and 7185.6cm -1 The corresponding Lorentz half-height and half-width at T and P are w1 and w2 (under the conditions of 673.15K and 1 bar, w1 = 0.05071 cm -1 , w2=0.07833cm -1 );

[0060] S15. According to P = a(T) × w1 + b(T) × w2, use nonlinear curve fitting to obtain the coefficient a(T) at a temperature of 673.15K i ) and b(T i );

[0061] S16. Set the temperature T to 773.15K, 873.15K, 973.15K, 1123.15K, and 1273.15K, respectively, and repeat the above steps S11-S15, respectively, to obtain the coefficients a(T) and b(T) at the corresponding temperatures.

[0062] Finally, the coefficients a(T) at 673.15K, 773.15K, 873.15K, 973.15K, 1123.15K, and 1273.15K were obtained using a third-order polynomial. i ) and b(T i ) are fitted to obtain the coefficients a(T) and b(T) for any temperature in the temperature range of 673.15K to 1273.15K.

[0063] like Figure 3 The coefficients a(T) and b(T) calibrated in this embodiment are shown. The third-order polynomial coefficients of a(T) are 223.0486, -0.3017, -6.313E-5, and 9.08E-8, respectively; the third-order polynomial coefficients of b(T) are 33.6978, -0.3677, 6.538E-4, and -2.6473E-7, respectively. That is, a(T) = 223.0486-0.3017×T-6.313E-5×T 2 +9.08E-8×T 3 , b(T)=33.6978-0.3677×T+6.538E-4×T 2 -2.6473E-7×T 3 Based on this relationship, a(T) and b(T) at any temperature can be calculated.

[0064] S2. Set the temperature T of the high temperature and high pressure absorption cell to 1273.15K, the pressure P to 4.32bar, and measure the corresponding H2O molecules at 7444.3cm -1 and 7185.6cm -1The direct absorption spectrum signal near 7444.3cm is obtained by preprocessing the measured direct absorption spectrum signal. -1 and 7185.6cm -1 The absorbance α1 and α2 at Figure 4 As shown;

[0065] S3. Calculate the H2O molecule at 7444.3cm -1 Gaussian half-height and half-width at wG=0.0224207cm -1 , 7185.6cm -1 Gaussian half-height and half-width at wG=0.0216416cm -1 ;

[0066] S4. Perform Voigt line fitting on the absorbance signal obtained. The half-height and half-width of the Gaussian in the fitting are fixed to the calculation result of step S3, and the H2O molecule is obtained at 7444.3cm -1 and 7185.6cm -1 The Lorentz half-height and half-width at w1 = 0.16763 cm -1 、w2=0.21653cm -1 ;

[0067] S5. According to the relationship between a(T) and b(T) calibrated in S1, it is calculated that a(T) = -76.0083 bar / cm at 1273.15 K -1 , b(T)=78.9987bar / cm -1 ; According to P = a(T) × w1 + b(T) × w2, the calculated pressure is 4.3643 bar, with an error of 1.025%.

[0068] The Voigt linear function is the convolution of the Gaussian linear function and the Lorentzian linear function; in this application, in theory, Rautian, Galatry, speed-dependent Vogit, Hartmann-Tran linear functions can also be used to fit the observed data (absorbance signal) to obtain w1 and w2, because these linear functions are essentially extensions of the Voigt linear function. The so-called "nonlinear curve fitting" in step S15 is a commonly used method in data analysis. It is a relatively broad concept used to fit nonlinear models to observed data. All methods that can achieve nonlinear curve fitting, such as least squares method, nonlinear least squares method, Bayesian inference method, genetic algorithm, etc., should be included in the "nonlinear curve fitting" of this patent.

[0069] The above implementation modes are only used to illustrate the technical solutions of the present invention, but not to limit the present invention; the above-mentioned references are only used to supplement the prior art, but not to limit the present invention; although the present invention has been described in detail with reference to the above-mentioned implementation modes, those skilled in the art should understand that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A non-contact gas pressure measurement method based on dual-wavelength absorption, which is implemented based on data measured by a measurement system containing a high-temperature and high-pressure absorption cell, and is characterized in that: The following steps are involved: S1. Calibration coefficients a(T) and b(T); S2. Obtain the absorbance α1 and α2 of the target gas molecules at characteristic absorption wavelengths λ1 and λ2 at temperature T by direct absorption spectroscopy technology; S3. Calculate the Gaussian half-width at half-maximum wG of the target gas molecules at wavelengths λ1 and λ2 when the temperature is T. The Gaussian half-width at half-maximum wG calculation formula is: Where v0 represents the absorption wavelength of the target gas, in cm -1 , M represents the molar mass of the target gas molecule; S4. Fix the Gaussian half-width at half-height wG calculated in step S2, and use the Voigt line type to fit the absorbances α1 and α2, respectively, to obtain the Lorentzian half-width at half-height w1 and w2 of α1 and α2; S5. Calculate the gas pressure P of the target gas according to the following formula: P = a(T) × w1 + b(T) × w2; Where a(T) and b(T) are coefficients for calculating pressure, in bar / cm -1 .

2. A non-contact gas based on dual wavelength absorption as claimed in claim 1, wherein a(T) and b(T) are coefficients for calculating pressure, and the unit is bar / cm -1 A pressure measurement method, characterized in that In step S2, the selection of wavelengths λ1 and λ2 is determined according to the absorption line intensity of the target gas molecules at the wavelengths. The selected wavelengths λ1 and λ2 enable the target gas molecules to produce detectable light absorption under the measurement conditions.

3. A non-contact gas pressure measurement method based on dual-wavelength absorption as claimed in claim 1, characterized in that: In step S4, the fitting method of absorbance α1 and α1 is the same, and the specific method of fitting absorbance α1 is: S41. Obtaining initial values ​​of fitting parameters, specifically: setting the initial value of the Y-axis offset y0 of the absorbance α1 to the minimum value of the absorbance α1, setting the initial value of the X-axis offset xc of the absorbance α1 to the X-axis coordinate value corresponding to the maximum value of the absorbance α1, setting the initial value of the integral area A of the absorbance α1 to the integral value of the absorbance α1 with respect to the X-axis, and setting the initial value of the Lorentz half-width at half-height wL of the absorbance α1 to the Lorentz half-width at half-height w1 of the absorbance α1; S42. Use the Voigt function to calculate the Lorentz half-height and half-width wL. Specifically, calculate the Voigt function: f=y0+f L (x)*f G (x); where f L is the Lorentz function, f G is a Gaussian function, x refers to the wave number, and the unit is cm -1 , * indicates f L and f G Perform convolution operation; f L and f G The calculation formula is as follows: In the formula, exp represents the exponential operation with the natural constant e as the base.

4. A non-contact gas pressure measurement method based on dual-wavelength absorption as claimed in claim 1, characterized in that: The calibration method of the coefficients a(T) and b(T) comprises the following steps: S11. Set the temperature of group i, i=0,1,2,…,N,N≥5; the pressure of group j, j=0,1,2,…,M,M≥5; S12. Obtain the temperature T by direct absorption spectroscopy i , pressure is P j When the absorbance of the gas molecules at wavelengths λ1 and λ2 is α 1ij , α 2ij ; S13. Calculate the temperature as T i When the gas molecules are located at the Gaussian half-height and half-width at wavelengths λ1 and λ2; S14. Fix the Gaussian half-height and half-width obtained in step S13 and use the Voigt line type to calculate the absorbance α 1ij , α 2ij Perform fitting and obtain α 1ij , α 2ij The Lorentz half-height and half-width w 1ij 、w 2ij ; S15. Determine whether j is equal to M: If not, then j = j + 1, and return to step S12; if so, then according to P j =a(T i )×w 1ij +b(T i )×w 2ij , (j=0,1,2,…,M), and use nonlinear curve fitting to obtain the temperature T i The coefficient a(T i ) and b(T i ); S16. Determine whether i is equal to N: If not, then i=i+1, and return to step S12; if so, then obtain a(T) through polynomial fitting. i ) and T i and b(T i ) and T i The relationship between T and B is obtained through the relationship between T and B.

5. A non-contact gas pressure measurement method based on dual-wavelength absorption as claimed in claim 4, characterized in that: The nonlinear curves were fitted using the least squares method.

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