Light field tomography system and flame three-dimensional temperature field reconstruction method

Abstract

The invention discloses a light field tomography system and a flame three-dimensional temperature field reconstruction method, and belongs to the technical field of flame temperature measurement. The problem that the temperature field reconstruction error of an existing light field tomography method is large is solved. The method comprises the following steps: firstly, shooting a light field image of a measured flame by using a light field tomography system, dividing a space region where the measured flame is located into micro-element bodies, and then performing flame radiation ray tracing from each pixel point on the light field image of the measured flame; establishing a radiation transfer equation set according to a tracing result of flame radiation light, obtaining a reconstruction equation set according to the established radiation transfer equation set, and solving the reconstruction equation set based on multi-point accurate temperature measurement value information of a thermocouple to obtain black body spectrum radiation intensity of each infinitesimal body of the flame; and finally, converting the black body spectrum radiation intensity of each infinitesimal body into the temperature of each infinitesimal body to realize temperature field reconstruction. The method can be applied to flame three-dimensional temperature field reconstruction.

Description

Technical Field

[0001] The invention belongs to the technical field of flame temperature measurement, and in particular relates to a light field tomography system and a flame three-dimensional temperature field reconstruction method. Background Art

[0002] Combustion diagnostic technology uses optical and electrical means to detect characteristic parameters such as temperature, flow rate, pressure and concentration of fuel during the combustion process, providing important information support for in-depth understanding of the mechanism and characteristics of a combustion process. Combustion diagnostic technology is widely used in energy, chemical, material and environmental fields. Temperature is an important parameter reflecting the combustion process. Accurate measurement of the combustion temperature field plays an important guiding role in actual production processes such as pollutant emission control, design optimization of combustion equipment, and combustion process control. Therefore, the research on high-precision flame temperature measurement methods has important application value. Thermocouples use the functional relationship between the contact electromotive force generated when different metals or alloys come into contact and the contact point temperature to measure temperature. It has the advantages of simple structure, reliable measurement, and large temperature measurement range. It can be used to measure the temperature of a single point of the flame.

[0003] Since light field tomography can simultaneously record the intensity and direction of flame radiation, it only needs to arrange a few measurement angles to reconstruct the three-dimensional radiation intensity distribution of the flame. However, the sampling light corresponding to the light field image cannot pass through the entire measurement area. The light field image is only a projection of the three-dimensional distribution of space under a limited angle of view, and the reconstruction information obtained from it is limited. Therefore, the solution of the equation group in the reconstruction calculation is seriously ill-conditioned, and the reconstruction error is large. In order to solve the problem of large temperature field reconstruction error of the existing light field tomography method, it is very necessary to propose a new flame three-dimensional temperature field reconstruction method. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of large temperature field reconstruction error in the existing light field tomography method, and to propose a light field tomography system and a flame three-dimensional temperature field reconstruction method.

[0005] The technical solution adopted by the present invention to solve the above technical problems is:

[0006] According to one aspect of the present invention, a light field tomography system is provided, the light field tomography system comprising a main lens, a microlens array, a relay lens and an image detector; wherein:

[0007] The relay lens is composed of two identical lenses, and the main lens, the microlens array, the relay lens and the image detector share a common optical axis, that is, the center of the main lens, the center of the microlens array, the center of the relay lens and the center of the image detector are located on the same straight line;

[0008] The light field tomography system is placed horizontally above the flame to be measured.

[0009] Furthermore, the image detector is an RGB camera.

[0010] Furthermore, the microlens array is an array composed of a plurality of convex lenses.

[0011] According to another aspect of the present invention, a method for reconstructing a three-dimensional temperature field of a flame is provided, the method specifically comprising the following steps:

[0012] Step 1: adjusting the main lens to make the light field tomography system focus on the flame to be measured, and using the light field tomography system to capture the light field image of the flame to be measured;

[0013] Step 2: Use a thermocouple located obliquely above the flame to obtain the flame's random N c The temperature of each point is obtained, and the temperature of each point is corrected to obtain the corrected temperature of each point;

[0014] Step 3: The space area where the flame to be measured is located is taken as the measurement area, and the measurement area is divided into various micro-elements;

[0015] According to the relative position between the light field tomography system and the measurement area, flame radiation ray tracing is performed starting from each pixel point on the measured flame light field image;

[0016] Step 4: Establish a reconstruction equation group based on the radiation transfer equation and the flame radiation ray tracing results, and solve the reconstruction equation group based on the corrected temperature of each point in step 2 to obtain the blackbody spectral radiation intensity of each flame microelement;

[0017] Step 5: Convert the blackbody spectral radiation intensity of each microelement into the temperature of each microelement in the measurement area to complete the reconstruction of the three-dimensional temperature field of the flame.

[0018] Furthermore, the obtained temperatures at each point are corrected, specifically:

[0019] Take any point of the flame as an example

[0020]

[0021] Where T is the corrected temperature of the point, T c is the thermocouple measurement value at that point, T s is the ambient temperature, h c is the convective heat transfer coefficient, ε is the thermocouple emissivity, and σ is the Stefan-Boltzman constant.

[0022] Furthermore, the radiation transfer equation is specifically:

[0023] Considering the absorption, emission and scattering of the radiation transmission path medium:

[0024]

[0025] Among them, I bλ (s,s i ′) is the blackbody radiation intensity of the flame at position s and in the direction s′, I bλ (s) is the blackbody radiation intensity of the flame at position s, β λ (s) is the attenuation coefficient, unit is m -1 , κ λ (s) is the absorption coefficient, in m -1 , σ sλ (s) is the scattering coefficient, in m -1 ,Φ(s′ i ,s′) is s′ i The scattering phase function of the flame radiation incident from the direction to the s′ direction, Ω i is the integration interval;

[0026]

[0027] Where C1 is the first radiation constant, C2 is the second radiation constant, λ is the wavelength of the flame radiation, T(s) is the temperature of the flame at position s, and e is the base of the natural logarithm;

[0028] According to formula (8), the steady-state radiation transfer equation of the microelement is:

[0029]

[0030] Among them, I λ is the spectral radiation intensity of the flame radiation, I bλ is the blackbody spectral radiation intensity of the flame microelement, τ is the optical thickness of the flame radiation light passing through the flame microelement;

[0031] τ=κ·δ (11)

[0032] Among them, κ is the absorption coefficient of the flame microelement through which the flame radiation passes, and δ is the geometric length of the flame microelement through which the flame radiation passes.

[0033] Furthermore, the reconstruction equations are established according to the radiation transfer equation and the flame radiation ray tracing results, specifically:

[0034] For any pixel point m on the measured flame light field image, the total number of flame micro-elements that the radiation light corresponding to the pixel point m passes through is recorded as n, and the radiation transmission equation of the flame radiation light corresponding to the pixel point m is:

[0035]

[0036] Among them, I λ (m) is the spectral radiation intensity of the flame radiation corresponding to pixel m, I bλ (i) is the blackbody spectral radiation intensity of the i-th flame element, i = 1, 2, ..., n, A m,i is the calculation coefficient;

[0037] Then the reconstruction equations are formed according to the radiation transfer equations of all flame radiation rays;

[0038] Ax=b (13)

[0039] Among them, b is the vector composed of the spectral radiation intensity of the flame radiation light corresponding to each pixel, x is the vector composed of the blackbody spectral radiation intensity of each flame microelement, and A is the calculation coefficient matrix.

[0040] Furthermore, the calculation coefficient A m,i for:

[0041]

[0042] Among them, τ j It is the optical thickness of the jth flame element through which the flame radiation light corresponding to pixel point m passes.

[0043] Furthermore, the blackbody spectral radiation intensity of each flame microelement is obtained by solving the reconstruction equations. The specific process is:

[0044] Convert the corrected temperature of each point in step 2 into the blackbody spectral radiation intensity of the microelement to which each point belongs:

[0045]

[0046] Where ε is the blackbody emissivity, I′ λ is the blackbody spectral radiation intensity corresponding to the point with temperature T, where T is the blackbody temperature value;

[0047] Taking the blackbody spectral radiation intensity at each point as a priori term, the objective function for solving the reconstruction equations is:

[0048]

[0049] Among them, 0 represents a column vector composed of 0 elements equal to the number of micro-elements, 1 represents a column vector composed of 1 elements equal to the number of micro-elements, α and β are constraint weight coefficients, L is the Laplace matrix, and R is the temperature correction matrix;

[0050]

[0051] Among them, x t,k Indicates the blackbody spectral radiation intensity corresponding to the true temperature value of the kth thermocouple temperature measurement point, k = 1, 2, ..., N c , N c is the total number of thermocouple temperature measurement points.

[0052] Furthermore, the heat transfer correlation equation between the convective heat transfer coefficient and the Nusselt number is:

[0053]

[0054] Where Nu is the Nusselt number, d is the diameter of the thermocouple wire, and k is the thermal conductivity of the gas.

[0055] The beneficial effects of the present invention are:

[0056] The present invention first uses a light field tomography system to capture a light field image of the flame to be measured, divides the spatial region where the flame to be measured is located into various micro-elements, and then traces the flame radiation rays from each pixel point on the light field image of the flame to be measured, and establishes a radiation transmission equation group based on the tracing results of the flame radiation rays, and obtains a reconstruction equation group based on the established radiation transmission equation group, and solves the reconstruction equation group based on the multi-point precise temperature measurement value information of the thermocouple to obtain the blackbody spectral radiation intensity of each micro-element of the flame, and finally converts the blackbody spectral radiation intensity of each micro-element into the temperature of each micro-element to achieve temperature field reconstruction. Compared with the existing light field tomography temperature field reconstruction method, the present invention combines the multi-point precise temperature measurement value information of high-precision thermocouples on the basis of the light field image, and can obtain a more accurate temperature field reconstruction result, which greatly reduces the error of temperature field reconstruction. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a schematic diagram of a measurement system used in the present invention;

[0058] Figure 1 The measurement system in the embodiment is composed of a high-precision thermocouple (numbered F in the figure) and a light field tomography system (numbered E in the figure) installed on a translation table; the measurement point position of the high-precision thermocouple is controlled by a translation table in three dimensions of x, y, and z (the three-dimensional translation tables are represented by numbers C, A, and B in the figure, respectively), and the arrangement position of the light field tomography system is determined according to the size of the measurement object and the focal length of the lens, so that the measurement area (the area where the flame in the burner D is located) fills the frame of the image detector as much as possible;

[0059] Figure 2 is a schematic structural diagram of the light field tomography system of the present invention;

[0060] The light field tomography system consists of a main lens 3, a microlens array 2, a relay lens 5 and an image detector 1; the relay lens 5 is an optical system composed of two identical lenses, and its optical principle is to generate an upright and enlarged virtual image of a close-up object, i.e., the actual measurement object, at a farther distance, so that the image formed by the microlens array 2 can be imaged twice, and finally imaged on the image detector 1. In order to obtain high-precision light field data and improve the spatial resolution of light field tomography reconstruction, a large target surface and high-resolution image detector is used. In order to increase the angular range of the sampled radiation light, when the focal length of the main lens 3 remains unchanged, the aperture diameter of the main lens 3 is increased to reduce the F number of the main lens. The main lens 3 uses a lens with a variable focal length and an adjustable F number to achieve F number matching. In order to match the F number of the main lens 3 with the microlens array 2, the F number of the microlens array 2 should be equal to or slightly greater than the F number of the main lens 3. In order to reduce the influence of the relay lens 5 on the imaging quality, a lens with a larger aperture, i.e., a smaller F number and smaller aberration is selected. The parameters and position of each component can be adjusted or replaced independently, and the optical parameters can be flexibly changed to adapt to objects of different distances and sizes;

[0061] Figure 3 It is a schematic diagram of the light path during ray tracing;

[0062] In order to establish the relationship between the flame radiation intensity value and the grayscale value of the image imaged by the image detector, the flame radiation ray is traced starting from the pixel in the image detector 1 (numbered C1 in the figure) to determine the trajectory of the flame radiation light in the flame. The direction of the flame radiation beam received by the pixel is determined by the microlens center (numbered D1 in the figure) and the pixel center (numbered C1 in the figure). The sampling light is traced by using the conjugate relationship between the microlens array 2 and the object plane 4 with respect to the main lens 3, the thin lens imaging model and the geometric similarity theorem. Finally, the relationship between the internal radiation intensity of the flame and the radiation intensity of the light received by the image detector 1 is established based on the flame radiation transmission equation. The ray tracing method uses the optical center of the main lens 3 as the coordinate origin and the normal of the image detector as the positive direction of the z-axis to establish a Cartesian right-handed coordinate system. The distance between the object plane 4 and the main lens 3 is denoted as l OE1 , the distance between the microlens array 2 and the image detector 1 is recorded as l A1B1 , the distance between the microlens array 2 and the main lens 3 is recorded as l OB1 , the focal length of the microlens array is recorded as f MLA , the focal length of the main lens 3 is recorded as f. The coordinates of each pixel on the image of the image detector 1 are recorded as (x i ,y i ), and (x i ,y i ) corresponds to the center coordinate of the microlens marked as (x m ,y m), the intersection coordinate of the pixel flame radiation light and the main lens 3 is marked as (x j ,y j ), the optical center coordinate of the main lens 3 is marked as (x O ,y O );

[0063] According to the geometric similarity theorem, we can get:

[0064]

[0065] The intersection coordinates can be obtained from the pixel coordinates and the microlens center coordinates:

[0066]

[0067] The microlens array surface is conjugate with the object plane with respect to the main lens. From the thin lens imaging model and the geometric similarity theorem, we can get:

[0068]

[0069] The object point coordinates can be obtained from the microlens center coordinates and the main lens center coordinates:

[0070]

[0071] Figure 4 It is a flow chart of a flame three-dimensional temperature field reconstruction method of the present invention;

[0072] First, adjust the main lens 3 of the light field tomography system to focus on the flame to be measured, and shoot and save the flame light field image. Calculate the spectral radiation intensity value of the corresponding flame radiation light according to the pixel gray value; establish a coordinate system to determine the relative position of the measurement area and the measurement device, divide the measurement area into a suitable number of micro-elements and number the micro-elements. Start ray tracing from the pixels in the image obtained by imaging with the image detector 1, determine the trajectory of the sampling light in the measurement area (that is, determine the micro-element through which the sampling light passes), calculate the micro-element number it passes through and the path length in the micro-element; use the spectral radiation intensity values ​​of several flame radiation lights, the ray tracing calculation results and the thermocouple temperature measurement results to obtain the reconstruction equation group, solve the equation group according to the reconstruction algorithm, and obtain the blackbody spectral radiation intensity of each flame micro-element; convert the blackbody spectral radiation intensity of each micro-element in the measurement area into the temperature of each micro-element in the measurement area according to Planck's law. DETAILED DESCRIPTION

[0073] Specific implementation method 1: Combination Figure 2 This embodiment is described as a light field tomography system, which includes a main lens 3, a microlens array 2, a relay lens 5 and an image detector 1; wherein:

[0074] The relay lens 5 is composed of two identical lenses, and the main lens 3, the microlens array 2, the relay lens 5 and the image detector 1 share a common optical axis, that is, the center of the main lens 3, the center of the microlens array 2, the center of the relay lens 5 and the center of the image detector 1 are located on the same straight line;

[0075] The light field tomography system is placed obliquely above the flame to be measured in the horizontal direction (i.e., in the measuring state, the line connecting the center of the main lens 3 of the light field tomography system, the center of the microlens array 2, the center of the relay lens 5 and the center of the image detector 1 is parallel to the horizontal direction).

[0076] The relay lens 5 can generate an upright and enlarged virtual image of a close-up object, that is, the actual measurement object, at a farther distance, and perform secondary imaging on the image formed by the microlens array 2, and finally form an image on the image detector 1. Figure 1 As shown, the light field tomography system can be fixed by a fixing device. The fixing device is mainly composed of a cage plate and a cage rod. The center of the cage plate has a through hole for installing optical elements, and the four corners of the cage plate have through holes for fixing and passing through the cage rods. The four sides of the cage plate can be connected to other optical platforms by installing bolts and connecting rods and other fasteners to achieve the adjustment of the imaging system posture. The cage plate can slide freely along the cage rods to adjust the position of the optical elements on the cage plate. In order to increase the sampling angle range of the flame radiation light, the aperture of the main lens is increased, that is, the F number is reduced. A main lens with adjustable focal length and F number is selected to adapt to different measurement requirements and achieve F number matching. In order to match the F number of the main lens with the microlens, the F number of the microlens should be equal to or slightly larger than the F number of the main lens. In order to reduce the influence of the relay lens on the final imaging quality, a lens with adjustable F number and small aberration is selected to match the size of the microlens and the image detector. The cage structure can ensure that the components of the imaging system are coaxial, and each component can be independently adjusted or replaced to adapt to measurement objects of different distances and sizes.

[0077] Specific implementation method 2: This implementation method is different from specific implementation method 1 in that the image detector 1 is an RGB camera.

[0078] The other steps and parameters are the same as those in the first embodiment.

[0079] Specific implementation method three: This implementation method is different from specific implementation methods one or two in that the microlens array 2 is an array composed of a plurality of convex lenses.

[0080] The other steps and parameters are the same as those in the first or second embodiment.

[0081] In this embodiment, the convex lenses constituting the microlens array 2 are tiny lenses, and the size of the tiny lenses can be 10 mm×10 mm, but is not limited to 10 mm×10 mm, and the specific number can be selected according to actual conditions. Moreover, the array direction of the microlens array 2 is perpendicular to the line connecting the centers of each component. In addition, the lenses included in the main lens 3 and the relay lens 5 are thin lenses (specifically convex lenses).

[0082] Specific implementation method four: Combination Figure 1 and Figure 4 The present embodiment describes a flame three-dimensional temperature field reconstruction method, the method specifically comprising the following steps:

[0083] Step 1: adjusting the main lens 3 to make the light field tomography system focus on the flame to be measured, and using the light field tomography system to capture the light field image of the flame to be measured;

[0084] Step 2: Use a thermocouple located obliquely above the flame to obtain the flame's random N c The temperature of each point is measured (the height, level and depth of the thermocouple are controlled by an optical translation stage to measure the temperature of different points. For the temperature measurement of high-temperature flame objects, R-type bare wire thermocouples can be used), and the obtained temperature of each point is corrected to obtain the corrected temperature of each point;

[0085] Step 3: The space area where the flame to be measured is located is taken as the measurement area, and the measurement area is divided into various micro-elements;

[0086] According to the relative position between the light field tomography system and the measurement area, flame radiation ray tracing is performed starting from each pixel point on the measured flame light field image;

[0087] Step 4: Establish a reconstruction equation group based on the radiation transfer equation and the flame radiation ray tracing results, and solve the reconstruction equation group based on the corrected temperature of each point in step 2 to obtain the blackbody spectral radiation intensity of each flame microelement;

[0088] Step 5: Convert the blackbody spectral radiation intensity of each microelement into the temperature of each microelement in the measurement area to complete the reconstruction of the three-dimensional temperature field of the flame.

[0089] Step 5 is based on the relationship between the blackbody spectrum radiation intensity of the microelement and the temperature of the microelement. The method for obtaining the relationship between the blackbody spectrum radiation intensity of the microelement and the temperature of the microelement is:

[0090] Step 1: Determine the temperature range of the blackbody furnace calibration according to the temperature range of the flame to be measured (the calibrated temperature range should be slightly larger than the temperature range of the flame to be measured);

[0091] Step 2, setting the temperature interval, determining each temperature measurement point of the blackbody furnace calibration according to the temperature interval, and the temperature difference between adjacent temperature measurement points is equal to the set temperature interval;

[0092] Step 3, a blackbody furnace light field image is captured at each temperature measurement point, a polynomial fitting is performed on the grayscale of the blackbody furnace light field image and the temperature of the blackbody furnace, and a relationship between the grayscale of the blackbody furnace light field image and the temperature of the blackbody furnace is established;

[0093] Step 4: Similarly, the method proposed in the present invention can be used to calculate the blackbody spectral radiation intensity, and establish the relationship between the blackbody furnace light field image grayscale and the blackbody spectral radiation intensity;

[0094] Step 5: Based on the results of step 3 and step 4, the relationship between the blackbody spectrum radiation intensity of the microelement and the temperature of the microelement can be obtained:

[0095] Specific implementation method 5: This implementation method is different from the specific implementation method 4 in that the obtained temperature of each point is corrected, specifically:

[0096] Take any point of the flame as an example

[0097]

[0098] Where T is the corrected temperature of the point, T c is the thermocouple measurement value at that point, T s is the ambient temperature, h c is the convective heat transfer coefficient, ε is the thermocouple emissivity, and σ is the Stefan-Boltzman constant.

[0099] The other steps and parameters are the same as those in the fourth embodiment.

[0100] For different situations, the calculation formula of Nusselt number is different. For example, when RePr≤0.2, the calculation method of Nusselt number is (using the Churchill and Bernstein fluid transversely swept single tube formula):

[0101]

[0102] Where Re = ul / v, Pr = v / a, u is the fluid velocity, l is the characteristic length, i.e. the outer diameter of the thermocouple measuring point, v is the kinematic viscosity, and a is the thermal diffusivity. The qualitative temperature is (t w +t ∞ ) / 2,t w is the surface temperature of the thermocouple, t ∞ is the flame temperature.

[0103] Specific implementation method 6: This implementation method is different from specific implementation method 4 or 5 in that the radiation transfer equation is specifically:

[0104] Since the exposure time of the image detector is extremely short, the change of the measured distribution over time can be ignored. Therefore, the radiation transfer process inside the flame can be described by the steady-state radiation transfer equation.

[0105] Considering the absorption, emission and scattering of the radiation transmission path medium:

[0106]

[0107] Among them, I bλ (s,s′ i ) is the blackbody radiation intensity of the flame at position s and in the s′ direction, I bλ (s) is the blackbody radiation intensity of the flame at position s, β λ (s) is the attenuation coefficient, unit is m -1 , κ λ (s) is the absorption coefficient, in m -1 , σ sλ (s) is the scattering coefficient, in m -1 ,Φ(s′ i ,s′) is s′ i The scattering phase function of the flame radiation incident from the direction to the s′ direction, Ω i is the integration interval;

[0108]

[0109] Where C1 is the first radiation constant, C2 is the second radiation constant, λ is the wavelength of the flame radiation, T(s) is the temperature of the flame at position s, and e is the base of the natural logarithm;

[0110] Since the scattering of flame soot is much smaller than the absorption, the flame scattering is ignored. According to formula (8), the steady-state radiation transfer equation of the microelement is obtained as follows:

[0111]

[0112] Among them, I λ is the spectral radiation intensity of the flame radiation, I bλ is the blackbody spectral radiation intensity of the flame microelement, τ is the optical thickness of the flame radiation light passing through the flame microelement;

[0113] τ=κ·δ (11)

[0114] Among them, κ is the absorption coefficient of the flame microelement through which the flame radiation passes, and δ is the geometric length of the flame microelement through which the flame radiation passes.

[0115] The other steps and parameters are the same as those in the fourth or fifth embodiment.

[0116] The cone angle of the flame radiation imaging beam received by a single pixel in the image detector of the light field tomography system is very small. The flame radiation light can be sampled through the center of the microlens and the center of the pixel to approximately represent the direction of the flame radiation imaging beam received by the pixel. The flame radiation intensity received by a single pixel is the intensity of the flame radiation sampling light, so that the relationship between the flame radiation intensity value and the pixel grayscale value in the image detector imaging is established through the flame radiation light. From the image detector to the inside of the flame, the flame radiation light is traced starting from the pixel to determine the trajectory of the flame radiation light in the flame. The specific ray tracing process is as follows: Figure 3 shown.

[0117] Specific implementation method 7: This implementation method is different from any one of specific implementation methods 4 to 6 in that the reconstruction equation group is established based on the radiation transfer equation and the flame radiation ray tracing results, specifically:

[0118] For any pixel point m on the measured flame light field image, the total number of flame micro-elements passed by the radiation light corresponding to the pixel point m is recorded as n. According to the order in which the radiation light passes through each flame micro-element, the flame micro-elements passed through are numbered 1, 2, ..., n in sequence. Then the radiation transmission equation of the flame radiation light corresponding to the pixel point m is:

[0119]

[0120] Among them, I λ (m) is the spectral radiation intensity of the flame radiation corresponding to pixel m, I bλ (i) is the blackbody spectral radiation intensity of the i-th flame element, i = 1, 2, ..., n, A m,i is the calculation coefficient;

[0121] Then the reconstruction equations are formed according to the radiation transfer equations of all flame radiation rays;

[0122] Ax=b (13)

[0123] Among them, b is the vector composed of the spectral radiation intensity of the flame radiation light corresponding to each pixel, x is the vector composed of the blackbody spectral radiation intensity of each flame microelement, A is the calculation coefficient matrix, the dimension of A is M×N, and N is the total number of microelements.

[0124] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 6.

[0125] Specific implementation eight: This implementation differs from any one of specific implementations four to seven in that the calculation coefficient A m,ifor:

[0126]

[0127] Among them, τ j It is the optical thickness of the jth flame element through which the flame radiation light corresponding to pixel point m passes.

[0128] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 7.

[0129] After all the measured values ​​are obtained according to the above method, the equations can be solved. The tomography problem is ill-posed, that is, there may not be a unique solution that satisfies the equations. The underdetermined problem has infinite candidate solutions, and each candidate solution can satisfy the equations without residuals. Moreover, the solution of this type of inverse problem is highly sensitive to small disturbances such as noise in the initial data. In order to improve the stability of solving the inverse problem of tomography, a regularization method can be used to provide additional information for solving the tomography problem. In actual measurement applications, tomography is affected by errors and noise. The initial reconstruction equations are shown in equation (15), where e represents the measurement noise and error, and the rank of the matrix A depends on the setting of the measurement optical path and the discretization method of the measurement area. One method to solve this linear system of equations is the least squares method, that is, minimizing the squared L2 norm. The goal of this method is to minimize the difference between the estimated value Ax and the measured data b. However, the least squares inversion will amplify the model error in the matrix A and the noise in b into non-physical disturbances in x. Although the least squares method has greater fidelity to the measured data, the amplification of noise usually leads to unstable reconstruction results. Therefore, additional information must be added to the reconstruction process to obtain a true estimate of the solved field.

[0130] Ax+e=b (15)

[0131] One way to solve this problem is to introduce a minimization objective function consisting of two components, including the least squares term and the regularization penalty term. As shown in Equation (16), the first term represents the least squares criterion, and the second term is composed of the regularization parameter α and the prior information about the solution composition.

[0132]

[0133] The Tikhonov regularization method uses the L2 norm of the squared spatial gradient of the reconstructed distribution As a priori, where D is a linear smoothing operator. In discrete form, the Tikhonov regularization method essentially uses the finite difference form to obtain the sum of the square differences of the differences between adjacent values. This method usually uses the discrete Laplace operator Δ. The minimization objective function of Tikhonov regularization is shown in Equation (17).

[0134]

[0135] In the formula, 0 represents a column vector of zero elements equal to the number of voxels, and Δ is a Laplace matrix, which represents the relationship between adjacent voxels, i.e., the smoothness condition. The Tikhonov regularization coefficient λ can be determined by the L-curve method, by calculating the difference between the values ​​of λ and the and The curve is obtained. A point that maximizes the curvature of the curve is selected as parameter λ, and the selected parameter λ meets two of the criteria.

[0136] Specific implementation method 9: This implementation method is different from any one of specific implementation methods 4 to 8 in that the blackbody spectral radiation intensity of each flame microelement is obtained by solving the reconstruction equation group. The specific process is:

[0137] Convert the corrected temperature of each point in step 2 into the blackbody spectral radiation intensity of the microelement to which each point belongs:

[0138]

[0139] Where ε is the blackbody emissivity, I′ λ is the blackbody spectral radiation intensity corresponding to the point with temperature T, where T is the blackbody temperature value;

[0140] Taking the blackbody spectral radiation intensity at each point as a priori term, the objective function for solving the reconstruction equations is:

[0141]

[0142] Among them, 0 represents a column vector composed of 0 elements equal to the number of micro-elements, 1 represents a column vector composed of 1 elements equal to the number of micro-elements, α and β are constraint weight coefficients, L is the Laplace matrix, which represents the relationship between adjacent voxels, i.e., the smoothness condition, and R is the temperature correction matrix, which represents the relationship between the temperature estimate and the true temperature value, i.e., the multi-point constraint condition;

[0143]

[0144] Among them, x t,k Indicates the blackbody spectral radiation intensity corresponding to the true temperature value of the kth thermocouple temperature measurement point, k = 1, 2, ..., N c , N c is the total number of thermocouple temperature measurement points.

[0145] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 8.

[0146] The present invention uses high-precision thermocouples to measure the precise temperature of multiple points while capturing light field images, and fuses light field information with the precise temperature information of multiple points during temperature field reconstruction calculation to obtain a more accurate temperature distribution.

[0147] Specific embodiment 10: This embodiment is different from any one of specific embodiments 4 to 9 in that the convective heat transfer coefficient depends on the gas flow characteristics of the flame, and the heat transfer correlation formula between the convective heat transfer coefficient and the Nusselt number is:

[0148]

[0149] Where Nu is the Nusselt number, d is the diameter of the thermocouple wire, and k is the thermal conductivity of the gas.

[0150] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 9.

[0151] The above calculation examples of the present invention are only used to explain the calculation model and calculation process of the present invention in detail, and are not intended to limit the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A light field tomography system, characterized in that: The light field tomography system comprises a main lens (3), a microlens array (2), a relay lens (5) and an image detector (1); wherein: The relay lens (5) is composed of two identical lenses, and the main lens (3), the microlens array (2), the relay lens (5) and the image detector (1) share a common optical axis, that is, the center of the main lens (3), the center of the microlens array (2), the center of the relay lens (5) and the center of the image detector (1) are located on the same straight line; The light field tomography system is placed horizontally above the flame to be measured.

2. The optical field tomography system according to claim 1, characterized in that: The image detector (1) is an RGB camera.

3. The optical field tomography system according to claim 2, characterized in that: The microlens array (2) is an array composed of a plurality of convex lenses.

4. A flame three-dimensional temperature field reconstruction method based on a light field tomography system according to claim 1, characterized in that: The method specifically comprises the following steps: Step 1: adjusting the main lens (3) so that the light field tomography system focuses on the flame to be measured, and using the light field tomography system to capture a light field image of the flame to be measured; Step 2: Use a thermocouple located obliquely above the flame to obtain the flame's random N c The temperature of each point is obtained, and the temperature of each point is corrected to obtain the corrected temperature of each point; Step 3: The space area where the flame to be measured is located is taken as the measurement area, and the measurement area is divided into various micro-elements; According to the relative position between the light field tomography system and the measurement area, flame radiation ray tracing is performed starting from each pixel point on the measured flame light field image; Step 4: Establish a reconstruction equation group based on the radiation transfer equation and the flame radiation ray tracing results, and solve the reconstruction equation group based on the corrected temperature of each point in step 2 to obtain the blackbody spectral radiation intensity of each flame microelement; Step 5: Convert the blackbody spectral radiation intensity of each microelement into the temperature of each microelement in the measurement area to complete the reconstruction of the three-dimensional temperature field of the flame.

5. The flame three-dimensional temperature field reconstruction method of the optical field tomography system according to claim 4 is characterized in that: The correction of the obtained temperature at each point is specifically as follows: Take any point of the flame as an example Where T is the corrected temperature of the point, T c is the thermocouple measurement value at that point, T s is the ambient temperature, h c is the convective heat transfer coefficient, ε is the thermocouple emissivity, and σ is the Stefan-Boltzman constant.

6. The flame three-dimensional temperature field reconstruction method of the optical field tomography system according to claim 5 is characterized in that: The radiation transfer equation is specifically: Considering the absorption, emission and scattering of the radiation transmission path medium: Among them, I bλ (s,s i ′) is the blackbody radiation intensity of the flame at position s and in the direction s′, I bλ (s) is the blackbody radiation intensity of the flame at position s, β λ (s) is the attenuation coefficient, unit is m -1 , κ λ (s) is the absorption coefficient, in m -1 , σ sλ (s) is the scattering coefficient, in m -1 ,Φ(s i ′,s′) is s i The scattering phase function of the flame radiation incident from the ′ direction to the s′ direction, Ω i is the integration interval; Where C1 is the first radiation constant, C2 is the second radiation constant, λ is the wavelength of the flame radiation, T(s) is the temperature of the flame at position s, and e is the base of the natural logarithm; According to formula (8), the steady-state radiation transfer equation of the microelement is: Among them, I λ is the spectral radiation intensity of the flame radiation, I bλ is the blackbody spectral radiation intensity of the flame microelement, τ is the optical thickness of the flame radiation light passing through the flame microelement; τ=κ·δ (11) Among them, κ is the absorption coefficient of the flame microelement through which the flame radiation passes, and δ is the geometric length of the flame microelement through which the flame radiation passes.

7. The flame three-dimensional temperature field reconstruction method of the optical field tomography system according to claim 6 is characterized in that: The reconstruction equations are established based on the radiation transfer equation and the flame radiation ray tracing results, specifically: For any pixel point m on the measured flame light field image, the total number of flame micro-elements that the radiation light corresponding to the pixel point m passes through is recorded as n, and the radiation transmission equation of the flame radiation light corresponding to the pixel point m is: Among them, I λ (m) is the spectral radiation intensity of the flame radiation corresponding to pixel m, I bλ (i) is the blackbody spectral radiation intensity of the i-th flame element, i = 1, 2, ..., n, A m,i is the calculation coefficient; Then the reconstruction equations are formed according to the radiation transfer equations of all flame radiation rays; Ax=b (13) Among them, b is the vector composed of the spectral radiation intensity of the flame radiation light corresponding to each pixel, x is the vector composed of the blackbody spectral radiation intensity of each flame microelement, and A is the calculation coefficient matrix.

8. The flame three-dimensional temperature field reconstruction method of the optical field tomography system according to claim 7 is characterized in that: The calculation coefficient A m,i for: Among them, τ j It is the optical thickness of the jth flame element through which the flame radiation light corresponding to pixel point m passes.

9. The flame three-dimensional temperature field reconstruction method of the optical field tomography system according to claim 8, characterized in that: The blackbody spectral radiation intensity of each flame microelement is obtained by solving the reconstruction equation group. The specific process is: Convert the corrected temperature of each point in step 2 into the blackbody spectral radiation intensity of the microelement to which each point belongs: Where ε is the blackbody emissivity, I′ λ is the blackbody spectral radiation intensity corresponding to the point with temperature T, where T is the blackbody temperature value; Taking the blackbody spectral radiation intensity at each point as a priori term, the objective function for solving the reconstruction equations is: Among them, 0 represents a column vector composed of 0 elements equal to the number of micro-elements, 1 represents a column vector composed of 1 elements equal to the number of micro-elements, α and β are constraint weight coefficients, L is the Laplace matrix, and R is the temperature correction matrix; Among them, x t,k Indicates the blackbody spectral radiation intensity corresponding to the true temperature value of the kth thermocouple temperature measurement point, k = 1, 2, ..., N c , N c is the total number of thermocouple temperature measurement points.

10. The flame three-dimensional temperature field reconstruction method of the optical field tomography system according to claim 9, characterized in that: The heat transfer correlation between the convective heat transfer coefficient and the Nusselt number is: Where Nu is the Nusselt number, d is the diameter of the thermocouple wire, and k is the thermal conductivity of the gas.

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