Background-oriented schlieren temperature field reconstruction method and device combined with laser absorption spectrum

Abstract

The present application relates to a kind of background guided schlieren temperature field reconstruction method and device combined with laser absorption spectrum, the method includes the following steps: step one, build the measuring device of background guided schlieren tomography, complete laser collimator alignment;Step two, kHz sawtooth wave signal is generated using function signal generator, the output wavelength range of tuning laser is scanned repeatedly the absorption spectral line of each gas component in flame;Step three, using multi-camera acquisition a group of schlieren image when temperature and density are uniformly distributed in the measurement region, and the schlieren image of the measurement region is synchronously collected when the temperature and density to be measured are non-uniformly distributed;Step four, move background pattern board through time-sharing mechanism;Step five, acquisition multi-wavelength laser scanning signal;Step six, according to laser intensity, Voigt line type fitting is carried out;Step seven, inversion operation is carried out, and the three-dimensional temperature field of measurement region is obtained.The method can obtain higher resolution three-dimensional temperature field distribution.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of flame temperature measurement, and particularly relates to a background-oriented schlieren temperature field reconstruction method and device combined with laser absorption spectroscopy. BACKGROUND

[0002] The research on combustion process is crucial in aviation, aerospace and energy engineering, and is of great significance to military equipment design, improvement of combustion efficiency and control of pollution gas. The combustion process usually involves complex turbulent flame structures, and the research region spans a large space and time scale and is accompanied by material mixing and heat transfer. In order to observe the propagation of the flame, the important features such as folds and vortex shedding and breaking, the quantitative diagnosis of the combustion flow field and the acquisition of the instantaneous three-dimensional physical parameter distribution are very important for the combustion research.

[0003] Background-oriented schlieren technology is a flow field measurement technology for detecting light deflection information. By combining multi-view setting and tomography technology, the refractive index field of three-dimensional unsteady flow can be reconstructed, and the density, temperature, flame peak surface geometry and other parameters can be obtained. Compared with other optical diagnosis methods, the measurement equipment is simple, only a camera and a schlieren pattern background plate are needed, and the requirements for the measurement environment are lower, so it has high application value. Since the technology uses multiple groups of images taken as reconstruction information, the sampling light information of each pixel can be extracted, so the number of voxels that can be divided in the measured region is large, and the spatial resolution of the reconstruction is high. The existing background-oriented schlieren technology can realize relatively accurate refractive index field reconstruction, but due to the lack of component concentration information, the uncertainty of the density field and temperature field inference is large.

[0004] Therefore, there is an urgent need for a background-oriented schlieren temperature field reconstruction method and device combined with laser absorption spectroscopy, which combines the high-resolution refractive index field reconstructed by background-oriented schlieren tomography and the component information obtained by laser absorption spectroscopy to obtain a high-resolution and relatively accurate temperature field distribution of the measured space. SUMMARY

[0005] The application is to solve the large conversion error from the refractive index field reconstructed by the background-oriented schlieren tomography to the temperature field, and proposes a background-oriented schlieren temperature field reconstruction method and device combined with laser absorption spectroscopy.

[0006] The application relates to a background-oriented schlieren temperature field reconstruction method combined with laser absorption spectroscopy, which comprises the following steps:

[0007] Step one, build a background-oriented schlieren tomography measurement device, calibrate the position of the measurement region, and complete the alignment of the laser collimator;

[0008] Step two, kHz sawtooth wave signal is generated by function signal generator, and the function signal generator is used to tune the output wavelength range of the laser, so that the output wavelength repeatedly scans the absorption spectrum of each gas component in the flame;

[0009] Step three, a group of schlieren images of uniform distribution of temperature and density in the measurement area are collected by using multiple cameras, and the schlieren images of non-uniform distribution of temperature and density in the measurement area are collected synchronously;

[0010] Step four, the background pattern plate is moved by the time-sharing mechanism;

[0011] Step five, the multi-wavelength laser scanning signal is collected by using the fiber-coupled detector and the data acquisition module;

[0012] Step six, Voigt line fitting is performed according to the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector;

[0013] Step seven, the reconstruction equation of the background-guided schlieren tomography is inverted by the computer to obtain the three-dimensional temperature field of the measurement area.

[0014] Further, in step one, the measurement device of the background-guided schlieren tomography includes a camera and a background pattern plate arranged at the focus of the lens, and the camera and the background pattern plate are arranged at a circular position centered on the measurement area; the internal and external parameters of the camera are calibrated; to measure the three-dimensional concentration information, the emitting end array of the laser collimator is arranged on the vertical plane around the camera lens, and the receiving end array of the laser collimator is arranged on the vertical plane behind the background plate.

[0015] Further, in step one, the setting method of the background-guided schlieren is that the background pattern plate illuminated by the light source is placed behind the measured non-uniform refractive index field, and the background pattern images with and without the non-uniform refractive index field are obtained by the camera.

[0016] Further, in step five, the method for collecting the multi-wavelength laser scanning signal is as follows: a plurality of wavelength range laser signals are used for scanning, a plurality of tunable lasers of different wavebands sequentially output a plurality of wavelength range laser signals, and the reconstruction of the concentration of each gas component utilizes two wavelength range laser absorption signals; each laser beam is divided into two beams by a beam splitter for measurement and reference respectively; the standard laser intensity and the laser intensity after the measurement area are recorded by using the fiber-coupled detector, the detected signals are collected by using the data acquisition module, and then post-processing is performed.

[0017] Further, in step six, the Voigt line profile fitting is performed between the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector, as follows: the laser intensity measured by the fiber-coupled detector varies with time in a single scanning period, and the laser intensity curve produces a dip at the selected absorption spectrum after passing through the absorption medium in the measurement region, and the relationship between the unabsorbed laser intensity and time is fitted by the laser intensity in the non-absorption region; the line profile shape of the spectral line is described by a linear function, and the spectral line broadening is divided into uniform broadening and non-uniform broadening, and for an actual absorption spectrum, both uniform broadening and non-uniform broadening exist, which is called Voigt line profile, and the fitting is performed between the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector.

[0018] Further, in step seven, the reconstruction equation of the background-oriented schlieren tomography is:

[0019]

[0020] where I u , I v and I t respectively represent the vectors composed of the gradients of the brightness of the feature points of the image in the u direction, the v direction and the time direction, n v represents the number of viewing angles of the camera arrangement, respectively represent the coordinate axis direction vectors of the image coordinate system corresponding to each viewing angle, and the passing distances of the M sampling light rays in the N voxels constitute the tomographic projection matrix S ∈ R M×N , R is a real set, D x , D y , D z respectively represent the finite difference matrices in the x, y and z directions.

[0021] The above formula is arranged into a linear equation set and the refractive index field is solved by a reconstruction algorithm;

[0022] The integral absorption coefficients of the divided grids in the measurement region are calculated by the reconstruction algorithm through the measured laser attenuation at two transition wavelengths v1 and v2 with different temperature dependences; assuming that each grid has the same pressure, the same molar fraction and the same laser beam path length, the ratio of the integral coefficients is approximated to the ratio of the line intensities, and the temperature is calculated:

[0023]

[0024] where h is the Planck constant, c is the speed of light, k is the Boltzmann constant, T0 is the reference temperature, α υ1 , α υ2 are the integral absorption coefficients at the transition wavelengths v1 and v2, E ν " i is the lower state energy of the transition, and Sνi is the line strength of the transition with central frequency v i , i = 1, 2;

[0025] The gas concentration is calculated as:

[0026]

[0027] where P represents the pressure, a υi is the integral absorption coefficient;

[0028] The variation of the spatial refractive index field is caused by the variation of temperature and composition, the relationship between density p and refractive index n can be expressed by the Gladstone-Dale equation as

[0029] n-1 = pG (4)

[0030] where G is the Gladstone-Dale constant, which is a function of gas composition and wavelength, and is calculated from the mole fraction and molar mass of different components of the gas;

[0031] On the basis of the reconstructed refractive index field, the refractive index is converted to density by the Gladstone-Dale equation:

[0032]

[0033] where G i is the Gladstone-Dale constant of a certain component of the mixed gas, X i is the mole fraction of a certain component of the mixed gas, and M i is the molar mass of a certain component of the mixed gas;

[0034] The density distribution of the mixed gas can be combined with the ideal gas state equation to obtain the temperature distribution:

[0035]

[0036] where R is the universal gas constant, and P is the pressure.

[0037] The present application also relates to a device for realizing the background-guided schlieren temperature field reconstruction method combined with laser absorption spectroscopy, which comprises a distributed feedback laser, a laser controller, a function signal generator, a background pattern plate, a laser collimator, a camera, a fiber-coupled detector and a computer.

[0038] The function signal generator generates a scanning signal to drive the laser controller, and the laser controller is used to control the distributed feedback laser to generate a required laser signal; the fiber-coupled detector is used to emit a laser transmission signal; and the computer is used to perform Voigt line type fitting calculation.

[0039] Advantages

[0040] The present application provides a background-oriented schlieren temperature field reconstruction method and device combined with laser absorption spectroscopy. The method can obtain more accurate temperature field estimation by combining laser absorption spectroscopy technology to obtain gas component concentration distribution information, and can obtain higher resolution three-dimensional temperature field distribution by obtaining high-resolution three-dimensional refractive index field through background-oriented schlieren tomography technology. BRIEF DESCRIPTION OF DRAWINGS

[0041] Fig. 1 It is a background-oriented schlieren optical path schematic diagram of the present application;

[0042] Fig. 2 It is a measuring device of the present application combined with background-oriented schlieren technology of laser absorption spectroscopy.

[0043] Explanation of reference numerals in the drawing:

[0044] A - image plane, B - focal plane, C - main surface, D - reconstruction area, E - background pattern, F - world coordinate system.

[0045] 1 - distributed feedback laser, 2 - laser controller, 3 - function signal generator, 4 - background pattern board, 5 - laser collimator, 6 - camera, 7 - fiber-coupled detector, 8 - computer. DETAILED DESCRIPTION

[0046] The following is combined Figs. 1-2 The present embodiment is specifically described.

[0047] Fig. 1 It is a background-oriented schlieren optical path schematic diagram, as shown in the figure, the image plane-A, the focal plane-B, the main surface-C, the direction deflection δ0 of the light after passing through the reconstruction area-D is expressed according to the deflection equation:

[0048]

[0049] A reconstruction area of a regular hexahedron is established in the flow area, and is uniformly divided into regular hexahedron voxels. The refractive index of the gaseous medium is approximately equal to 1, and the deflection equation is discretized and approximated:

[0050]

[0051] In the formula, e represents the world coordinate system (x, y, z), the world coordinate system-F, N is the number of voxels divided in the reconstruction area, Δs i represents the chord length of the light in voxel i. In this study, the camera is set to align the background pattern-E, which transforms the three-dimensional direction deflection δ0 of the light into the two-dimensional offset δ b :

[0052]

[0053] where d is the distance from the center of the reconstruction region to the background plate. In practical applications, in order to obtain accurate schlieren image information, the lens needs to be focused to the plane of the background plate, so the two-dimensional offset δ of the plane of the background plate can be obtained b converted into the pixel displacement δ in the camera image i , and finally the relationship between the deflection δ0 of the light passing through the reconstruction region and the image displacement δ i is established:

[0054]

[0055] where p is the physical length corresponding to an image pixel on the background plane.

[0056] Fig. 2 The measurement device of the present application combined with the background-oriented schlieren technique of laser absorption spectroscopy is shown in the figure:

[0057] The most commonly used laser source for laser absorption spectroscopy technology is a distributed feedback (DFB) type diode laser, which has the characteristics that the output wavelength and laser intensity both approximately linearly change with the input current. The measurement device mainly consists of a distributed feedback laser 1, a laser controller 2, a function signal generator 3, a function signal generator 4, a laser collimator 5, a camera 6, a fiber-coupled detector 7, and a computer 8 with a multifunctional data acquisition module. First, a kHz sawtooth wave signal is generated by the function signal generator to tune the output wavelength range of the laser, so that the output wavelength repeatedly scans the selected absorption line. The laser signal output by the laser is emitted by the laser collimator 5 and passes through the measurement region. The collimator and the photodetector receive the laser signal passing through the measurement region, and finally the measurement data is transmitted to the computer by using the multifunctional data acquisition module, and the computer performs the inversion operation of the measurement results. The laser intensity measured by the fiber-coupled detector changes with time within a single scanning period, which is the laser transmission signal. After passing through the absorbing medium in the measurement region, the laser intensity curve produces a depression at the selected absorption frequency spectrum, and the relationship between the laser intensity without absorption and time can be obtained by fitting the laser intensity in the non-absorbing region. The absorption spectrum of the molecule is not a single frequency, but occupies a certain spectral width. The spectral line width can be described by a linear function, and the spectral line broadening can be divided into uniform broadening and non-uniform broadening. For actual absorption spectrum, uniform broadening and non-uniform broadening exist at the same time, which is called Voigt line fitting, which can be fitted according to the laser intensity without absorption and the laser intensity measured by the fiber-coupled detector.

[0058] The measurement device of the background-oriented schlieren tomography includes a camera 6 and a background pattern plate 4 arranged at the focus of the lens, and the camera and the background plate are arranged at a circular position with the measurement region as the center. For measuring three-dimensional concentration information, the collimator array of the laser emitting end is arranged in a vertical plane around the camera lens, and the collimator array of the laser receiving end is arranged in a vertical plane behind the background plate.

[0059] The present application relates to a background-oriented schlieren temperature field reconstruction method combined with laser absorption spectroscopy, comprising the following steps:

[0060] Step one, build a measurement device of the background-oriented schlieren tomography, calibrate the position of the measurement region, and complete the alignment of the laser collimator.

[0061] The measurement device of the background-oriented schlieren tomography includes a camera and a background pattern plate arranged at the focus of the lens, and the camera and the background pattern plate are arranged at a circular position with the measurement region as the center. Calibrate the internal and external parameters of the camera; for measuring three-dimensional concentration information, the laser collimator array of the emitting end is arranged in a vertical plane around the camera lens, and the laser collimator array of the receiving end is arranged in a vertical plane behind the background plate.

[0062] The setting method of the background-oriented schlieren is that the background pattern plate illuminated by the light source is placed behind the measured non-uniform refractive index field, and the background pattern images in the presence and absence of the non-uniform refractive index field are obtained by the camera.

[0063] Step two, generate a kHz sawtooth wave signal using a function signal generator, and use the function signal generator to tune the output wavelength range of the laser to repeatedly scan the absorption spectral lines of each gas component in the flame.

[0064] Step three, use multiple cameras to collect a group of schlieren images with uniform distribution of temperature and density in the measurement region, and synchronously collect schlieren images with non-uniform distribution of temperature and density in the measurement region.

[0065] Step four, move the background pattern plate through a time-sharing mechanism.

[0066] Step five, use a fiber-coupled detector and a data acquisition module to collect multi-wavelength laser scanning signals.

[0067] The method for collecting multi-wavelength laser scanning signals is as follows: in order to obtain the concentration information of all gas components, a plurality of wavelength ranges of laser signals are used for scanning, a plurality of different waveband tunable lasers output a plurality of wavelength ranges of laser signals in turn, and the reconstruction of the concentration of each gas component utilizes two wavelength ranges of laser absorption signals; initially, the driving current is lower than the threshold value, and no laser is generated, and the signals detected during this period are recorded as background noise, with the increase of the driving current, a plurality of DFB lasers generate laser beams with a plurality of center wavelengths; each laser beam is divided into two beams by a beam splitter, and is used for measurement and reference respectively; the standard laser intensity and the laser intensity after the measured area are recorded by using a fiber-coupled detector respectively, the detected signals are collected by using a data acquisition module, and then post-processing is performed.

[0068] Step six, Voigt line fitting is performed according to the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector.

[0069] Voigt line fitting is performed according to the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector, and the method is as follows: the laser intensity measured by the fiber-coupled detector changes with time within a single scanning period, and the laser intensity curve after passing through the absorption medium in the measured area produces a depression at the selected absorption spectrum, and the relationship between the unabsorbed laser intensity and time is obtained by fitting the laser intensity of the non-absorption area; the absorption spectrum of a molecule is not a single frequency, but occupies a certain spectral width; the profile shape of the spectral line width is described by using a linear function, and the spectral line broadening is divided into uniform broadening and non-uniform broadening, for the actual absorption spectrum, uniform broadening and non-uniform broadening exist at the same time, which is called Voigt line, and fitting is performed according to the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector.

[0070] Step seven, the reconstruction equation of the background-oriented pattern shadow tomography is inverted by the computer, and the three-dimensional temperature field of the measured area is obtained.

[0071] The basic principle of the inversion calculation is that the change of the refractive index in the flow field causes the deflection of the light from its original path, and the pixel displacement appears between the distortion image of the background pattern and the reference image. According to the geometric optics equation, the path of the light is as shown in equation (9)

[0072]

[0073] where ds is the differential path length along the ray trajectory, x, y, z are the Cartesian coordinates of a point on the ray, and n is the refractive index. If the refractive index field is uniform, the right side of each of the equations in (11) is zero, and the path of the ray is a straight line. Further decomposition of the ray path equations into a set of first order differential equations is more suitable for numerical integration. The equations are shown in (12), where r is the local direction of propagation of the ray, and p is the position (x, y, z).

[0074]

[0075]

[0076] The ray deflection δ = dp / ds for background oriented schlieren measurements can be obtained by integrating (13), resulting in (14).

[0077]

[0078] Pixel displacement calculations in the projected images use either the cross-correlation algorithm in particle image velocimetry studies or the optical flow algorithm in computer vision studies. In particle image velocimetry applications, a pulsed laser illuminates tracer particles in the flow field under investigation, and particle field images are acquired synchronously with the laser pulses. To measure the particle displacement between images, the cross-correlation algorithm divides the images into diagnostic windows and determines the most likely displacement of a group of particles within the defined window. The maximum correlation location is identified and a sub-pixel estimate of the location is determined using interpolation. The measured displacement represents the average displacement in the diagnostic window. For two sequential images, the calculated pixel displacement is divided by the pulse time interval to calculate the velocity. The cross-correlation algorithm can also be used in background oriented schlieren techniques, where the background pattern is equivalent to the imaging particles in the particle image velocimetry flow field. The measured quantity is not a velocity field, but rather an image displacement resulting from a non-uniform refractive index field.

[0079] For the optical flow algorithm, the main assumption is that the intensity of the displaced feature remains constant, from which the optical flow constraint equation can be derived as shown in (15). I represents the image intensity, ξ, ψ represent the pixel coordinates, and δ ξ and δ ψ are the derivatives of the image position with respect to time, i.e., the pixel displacement between the reference and distorted images.

[0080]

[0081] The equation contains two unknowns δ ξ and δ ψDifferent optical flow algorithms introduce additional constraints to solve the unknown displacement values. Commonly used gradient-based algorithms include the Horn-Schunck method and the Lucas-Kanade method. Cross-correlation algorithms are easier to implement but can only obtain the average displacement based on the cross-correlation window. Using optical flow algorithms can obtain more accurate pixel displacements. Since optical flow algorithms require the assumption of constant image brightness, the range of background schlieren measurement using optical flow algorithms is limited, and the measurement environment should avoid factors that cause changes in the brightness of the schlieren image, such as contamination of the background plate, spontaneous light of the measured object, interference of smoke, etc.

[0082] The direction deflection δ0 of the light ray after passing through the reconstruction region is expressed according to the deflection equation as follows:

[0083]

[0084] A regular hexahedron reconstruction region is established in the flow region and is uniformly divided into regular hexahedron voxels. The refractive index of the gaseous medium is approximately equal to 1, and the formula is discretized and approximated as follows:

[0085]

[0086] In the formula, e represents the direction of the world coordinate system, N is the number of voxels divided in the reconstruction region, Δs i represents the chord length of the light ray in voxel i. By using the setting of aligning the camera with the background plane, the three-dimensional direction deflection δ0 of the light ray is transformed into the two-dimensional offset δ b on the background plate plane.

[0087]

[0088] In the formula, d is the distance from the center of the reconstruction region to the background plate, δ bu , δ bv are the offsets in the u and v directions on the background plate, δ 0x , δ 0y , δ 0z are the deflections of the light ray in the x, y, and z directions of the measurement region. In practical applications, in order to obtain accurate schlieren image information, the lens needs to be focused on the background plate plane, so the two-dimensional offset δ b on the background plate plane can be converted into the pixel displacement δ i in the camera image, and finally the relationship between the deflection δ0 of the light ray passing through the reconstruction region and the image displacement δ i is established:

[0089]

[0090] where Δp is the physical length corresponding to the image pixel on the background plane, δ iu , δ ivrespectively. The travel distance of M sampling rays in N voxels forms the tomographic projection matrix S ∈ R M×N , R is the real number set. The refractive index to be solved in all voxels of the reconstruction region forms the vector n ∈ R N , the first-order finite difference matrix D e is used to perform difference operation on n to obtain the refractive index gradient in the formula.

[0091] The two-dimensional displacement of feature points observed in consecutive images is called optical flow. Different images are associated by a specific invariant brightness, and the displacement of each feature point in the image is represented by a two-dimensional vector. The basic constraint equation of optical flow is based on the assumption that the brightness of the feature point in the image is constant and the displacement is small:

[0092]

[0093] The background-oriented schlieren technique calculates the corresponding image displacement by capturing the schlieren pattern deflected by the flow field and the original pattern. Since the brightness of the background pattern remains unchanged and the absorption and scattering of the medium are ignored, the brightness of the image at different times is equal and the displacement of the image pixels is small, the optical flow constraint equation can be introduced into the model. The background-oriented schlieren technique does not need to solve the velocity according to the time interval, only the displacement of the image feature points is needed, the time step can be set to a unit value, and formula (20) is simplified as:

[0094]

[0095] The reconstruction equation of the background-oriented schlieren tomography can be obtained by combining the above derivation:

[0096]

[0097] In the formula, I u , I v and I t respectively represent the gradient vectors of the brightness of the image feature points in the u direction, the v direction and the time direction, n v represents the number of viewing angles of the camera arrangement, respectively represent the coordinate axis direction vectors of the image coordinate system corresponding to each viewing angle, the travel distance of M sampling rays in N voxels forms the tomographic projection matrix S ∈ R M×N , R is the real number set, D x , D y , D z respectively represent the finite difference matrices in the x, y and z directions; the above formula can be arranged into the final matrix system An = b, and the refractive index field can be solved by the reconstruction algorithm.

[0098] For laser absorption spectroscopy measurements, when a collimated laser beam of frequency v passes through a medium with a total path length of L, the relationship between the transmitted intensity and the incident intensity is described by the Beer-Lambert law:

[0099]

[0100] In the formula, I t I0(ν) and I0(ν) are the transmitted laser intensity and the incident laser intensity, respectively; P(l), X(l), and T(l) are the local total pressure, mole fraction, and temperature of the absorbing material at position l along the beam path, respectively; φ(ν) is a normalized linear function that makes... The center frequency is v i The linear intensity S of the transition υi (T(l)) is a function of the local temperature T, as shown below:

[0101]

[0102] In the formula, h is Planck's constant; c is the speed of light; k is Boltzmann's constant; Q(T) is the partition function of the absorbing molecule; T0 is the reference temperature (usually 296 K); E ν " i It is the lower state energy of the transition; S νi (T) is the linear intensity at a local temperature T, S νi (T0) is the linear strength at the reference temperature.

[0103] The integral absorbance can be derived from equation (23):

[0104]

[0105] In the formula, the integral absorption coefficient α at coordinate position l νi (l) is defined as:

[0106] α νi (l)=P(l)X(l)S νi (T(l)) (21)

[0107] For typical combustion environments, the linear function φ(ν) is usually derived from the Voigt linear φ. V (ν) represents the shape of the spectral line produced by the convolution of the Lorentz and Doppler broadening mechanisms:

[0108]

[0109] In the formula, w G It is the full width at half maximum of Gaussian peak, w L It is the full width of the Lorenz half-peak.

[0110] By measuring at two transitions v1 and v2 with different temperature dependence, the integral absorption coefficients of the measurement region multi-partition grid are calculated by using a reconstruction algorithm. Assuming that each grid has the same pressure, the same mole fraction and the same laser beam path length, the ratio of integral coefficients can be reduced to the ratio of line intensity, and the temperature can be calculated:

[0111]

[0112] According to formula (26), the gas concentration can be further calculated:

[0113]

[0114] The change of spatial refractive index field is caused by the change of temperature and composition, and the relationship between density p and refractive index n can be expressed by the Gladstone-Dale equation as

[0115] n-1 = pG (25)

[0116] In the formula, G is the Gladstone-Dale constant, which is a function of gas composition and wavelength, and can be calculated from the mole fraction and molar mass of different components of the gas. The equation shows that the density of the measured gas can be calculated from the Gladstone-Dale constant and the refractive index of the measured gas, and the density can be converted to temperature by the ideal gas state equation.

[0117] On the basis of reconstructing the refractive index field, the refractive index can be converted to density by the Gladstone-Dale equation:

[0118]

[0119] Where G is the Gladstone-Dale constant of a certain component of the mixed gas, X is the mole fraction of a certain component of the mixed gas, and M is the molar mass of a certain component of the mixed gas. i i i

[0120] The density distribution of the mixed gas can be combined with the ideal gas state equation to obtain the temperature:

[0121]

[0122] In the formula, R is the universal gas constant, and P is the pressure.

[0123] ​​​The application also relates to a device for realizing the background-guided schlieren temperature field reconstruction method combined with laser absorption spectroscopy, which comprises a distributed feedback laser, a laser controller, a function signal generator, a background pattern plate, a laser collimator, a camera, a fiber-coupled detector and a computer.

[0124] The function signal generator generates a scanning signal to drive the laser controller, the laser controller is used for controlling the distributed feedback laser to generate a required laser signal; the fiber-coupled detector is used for emitting a laser transmission signal; and the computer is used for performing Voigt line type fitting calculation.

[0125] The above description of the application is only the preferred embodiment of the application, not for limiting the implementation of the application, and the person skilled in the art can easily make corresponding changes or modifications according to the main concept and spirit of the application, so the protection scope of the application should be subject to the protection scope required by the claims.

Claims

1. A method of background-oriented schlieren temperature field reconstruction combined with laser absorption spectroscopy, characterized in that, It comprises the following steps: Step one, build the measurement device of background-oriented schlieren tomography, calibrate the position of the measurement area, and complete the alignment of the laser collimator; Step two, generate kHz sawtooth wave signals using a function signal generator, and use the function signal generator to tune the output wavelength range of the laser to make the output wavelength repeatedly scan the absorption spectral lines of each gas component in the flame; Step three, use multiple cameras to collect a group of schlieren images when the temperature and density in the measurement area are uniformly distributed, and synchronously collect schlieren images when the temperature and density in the measurement area are non-uniformly distributed; Step four, move the background pattern plate through the time-sharing mechanism; Step five, use the fiber-coupled detector and the data acquisition module to collect multi-wavelength laser scanning signals; Step six, perform Voigt line fitting according to the laser intensity without absorption and the laser intensity measured by the fiber-coupled detector; Step seven, perform inversion operation on the reconstruction equation of background-oriented schlieren tomography by the computer to obtain the three-dimensional temperature field of the measurement area; The reconstruction equation of background-oriented schlieren tomography is: where I u , I v and I t denote the gradient vectors of the image feature point intensity in the u, v and t directions respectively, n v denotes the number of camera views arranged, denote the coordinate axis direction vectors of the image coordinate system corresponding to each view, and the traversal distances of M sampling rays in N voxels constitute the tomographic projection matrix S ∈ R M×N , where R is the real number set, D x , D y , D z denote the finite difference matrices in the x, y and z directions respectively; The above formula is arranged into a linear equation set and solved by a reconstruction algorithm to obtain the refractive index field; By measuring the laser attenuation at two transition wavelengths ν1 and ν2 with different temperature dependencies, the integral absorption coefficient of the grid in the measurement area is calculated using the reconstruction algorithm; assuming that each grid has the same pressure, the same molar fraction, and the same laser beam path length, the ratio of the integral absorption coefficient is approximated to the ratio of the line intensity, and the temperature is calculated: where h is Planck's constant, c is the speed of light, k is the Boltzmann constant, T0is a reference temperature, a υ1 , a υ2 are the integrated absorption coefficients at the transition wavelengths u1and u2, E" νi is the lower state energy of the transition, S νi is the line strength of the transition with a center frequency of v i , i = 1, 2. The gas concentration is calculated as: where P represents pressure, a υi is the integrated absorption coefficient; The change of the spatial refractive index field is caused by the change of the temperature and the component, and the relationship between the density ρ and the refractive index n can be expressed by the Gladstone-Dale equation as n-1=ρG (4) In the formula, G is the Gladstone-Dale constant, which is a function of the gas component and the wavelength, and is calculated from the molar fraction and molar mass of different components of the gas; On the basis of reconstructing the refractive index field, the refractive index is converted into the density by the Gladstone-Dale equation: where G i is the Gladstone-Dale constant for a component of the mixture gas, X i is the mole fraction of a component of the mixture gas, M i is the molar mass of a component of the mixture gas; The density distribution of the mixed gas can be combined with the ideal gas state equation to obtain the temperature distribution: In the formula, R is the universal gas constant, and P is the pressure.

2. The background oriented schlieren temperature field reconstruction method combined with laser absorption spectroscopy according to claim 1, characterized in that, In step one, the measurement device of background-oriented schlieren tomography comprises a camera and a background pattern plate arranged at the focus of the lens, and the camera and the background pattern plate are arranged at a circular position with the measurement area as the center; the internal and external parameters of the camera are calibrated; to measure the three-dimensional concentration information, the emission end array of the laser collimator is arranged on the vertical plane around the camera lens, and the receiving end array of the laser collimator is arranged on the vertical plane behind the background plate.

3. The background oriented schlieren temperature field reconstruction method combined with laser absorption spectroscopy according to claim 2, characterized in that, In step one, the setting method of the background-oriented schlieren is that the background pattern plate illuminated by the light source is placed behind the measured non-uniform refractive index field, and the background pattern images with and without the non-uniform refractive index field are obtained by the camera.

4. The background oriented schlieren temperature field reconstruction method combined with laser absorption spectroscopy according to claim 1, characterized in that, In step five, the method for collecting multi-wavelength laser scanning signals is as follows: a plurality of wavelength ranges of laser signals are used for scanning, a plurality of different waveband tunable lasers output a plurality of wavelength ranges of laser signals in turn, and the reconstruction of each gas component concentration utilizes two wavelength ranges of laser absorption signals; each laser beam is divided into two beams by a beam splitter for measurement and reference respectively; a fiber-coupled detector is used to record the standard laser intensity and the laser intensity after the measured area respectively, a data acquisition module is used to collect the detected signals, and then post-processing is performed.

5. The background oriented schlieren temperature field reconstruction method combined with laser absorption spectroscopy according to claim 1, characterized in that, In step six, the Voigt line fitting of the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector is as follows: the laser intensity measured by the fiber-coupled detector changes with time within a single scanning period, and the laser intensity curve after the absorption medium in the measurement area produces a concave at the selected absorption spectrum, and the relationship between the unabsorbed laser intensity and the time is fitted by the non-absorption area laser intensity; a line function is used to describe the profile shape of the spectral line width, the spectral line broadening is divided into uniform broadening and non-uniform broadening, for actual absorption spectrum, uniform broadening and non-uniform broadening exist at the same time, which is called Voigt line, and the unabsorbed laser intensity and the laser intensity measured by the fiber-coupled detector are fitted.

6. A device for reconstructing temperature field of background oriented schlieren using laser absorption spectroscopy, characterized in that, The device is used to realize the method of any one of claims 1 to 5, and the device comprises a distributed feedback laser, a laser controller, a function signal generator, a background pattern board, a laser collimator, a camera, a fiber-coupled detector, and a computer.

7. The apparatus for background oriented schlieren temperature field reconstruction combined with laser absorption spectroscopy according to claim 6, characterized in that, The function signal generator generates a scanning signal to drive the laser controller, and the laser controller is used to control the distributed feedback laser to generate a required laser signal; the fiber-coupled detector is used to emit a laser transmission signal; and the computer is used to perform Voigt line fitting calculation.

Images