Single-line excitation OH plane laser-induced fluorescence temperature measurement system and method

Abstract

The invention provides a single-line excitation OH plane laser-induced fluorescence temperature measurement system and method, and belongs to the technical field of aero-engine special tests, the system comprises an Nd: YAG laser, a sheet light shaping lens group, two ICCD cameras and a data acquisition device, the laser outputs quadruplicated pulse laser, the sheet light shaping lens group is arranged at the output end of the laser, and the data acquisition device is arranged at the output end of the laser. Laser sheet light is output through the sheet light shaping lens group to illuminate a to-be-detected area in the flame and reacts with OH groups in the flame to generate fluorescent light; the cameras are respectively arranged opposite to flames, are respectively in communication connection with the data acquisition device, are respectively provided with a band-pass optical filter with different bandwidths, and are used for synchronously measuring fluorescence to obtain a bispectral fluorescence image, and the data acquisition device is used for carrying out data processing on the image to realize two-dimensional temperature distribution measurement. Through the processing scheme provided by the invention, the measurement system configuration and the data processing flow are remarkably simplified, and the light spot quality and the signal-to-noise ratio are improved.

Description

Technical Field

[0001] The present application relates to the technical field of special tests for aircraft engines, and in particular to a single-line excited OH planar laser-induced fluorescence temperature measurement system and method. Background Art

[0002] Aircraft engines are a concentrated reflection of the comprehensive national strength, industrial base, and technological advancement of today's major nations, and they are a crucial strategic guarantee for national security and great power status. The combustion chamber is a core component of air-breathing engines. The energy released during the combustion process within the combustion chamber is the direct source of engine power. Experimental analysis of the temperature distribution within the combustion zone is crucial for revealing and explaining the flow and combustion mechanisms in the combustion chamber, improving the combustion chamber design system, and enhancing the level of combustion chamber design.

[0003] Traditional contact-based experimental measurement techniques use thermocouples, which are invasive and spatially limited. Laser measurement technology is non-invasive and capable of two-dimensional measurements, making it a key measurement method in current combustion experimental research. Compared to temperature measurement techniques such as Rayleigh scattering, Raman scattering, and absorption spectroscopy, planar laser-induced fluorescence (PLIF) offers advantages such as strong signal strength, two-dimensional field results, and the ability to simultaneously measure multiple parameters, making it crucial in testing aircraft engine combustion components.

[0004] The basic principle of PLIF measurement is to use a planar laser to excite specific molecules or free radicals in the combustion field to transition from the ground state to the excited state. The molecules that enter the high energy level of the excited state are unstable and transition to the low energy level, emitting a fluorescence signal with spectral characteristics. In the temperature measurement of the combustion field, the dual-line PLIF temperature measurement method using the OH radical energy level distribution as a temperature marker is widely used. Its basic principle is: a pair of excitation wavelengths are used to generate broadband fluorescence signals from two different low energy levels of OH; under the assumption of thermodynamic equilibrium, the ratio of fluorescence intensity is proportional to the relative population of the selected low energy level, and the relative population of different low energy levels satisfies the Boltzmann distribution and is a function of temperature. Therefore, by comparing the fluorescence intensities of two different low energy levels, a two-dimensional temperature distribution can be obtained. Ideally, the two low energy level to high energy level transitions used have the same high energy level to avoid differences in fluorescence yield. The most commonly used OH excitation scheme for this method is A 2 Σ + ←X 2The π electron vibration excitation bands AX(1, 0) and AX(0, 0) correspond to excitation wavelengths near 283 nm and 310 nm, respectively. Two Nd:YAG solid-state lasers, combined with two dye lasers, are typically used to achieve tunable wavelengths near these two bands to meet the wavelength requirements of specific excitation lines. This excitation method has the following problems: It requires two laser sources, resulting in high equipment cost and complexity; The high-quality beam output by the Nd:YAG laser decreases in energy and beam quality after being tuned by the dye laser, resulting in a low signal-to-noise ratio for the fluorescence image; This spectral band has a high absorption efficiency for OH radicals. However, due to the high mole fraction of OH radicals in the combustion reaction zone and high-temperature zone, the laser attenuates significantly along the high-pressure flame, resulting in weak laser penetration and a limited measurable flame size. Summary of the Invention

[0005] In view of this, and focusing on the need to measure the non-uniform temperature field at the combustion chamber outlet in the development of aircraft engines, the embodiments of the present application provide a single-line excitation OH planar laser-induced fluorescence temperature measurement system and method, which at least partially solves the problems of the dual-line OH-PLIF temperature measurement method in the prior art, such as complex laser system, low signal-to-noise ratio, severe attenuation along the way, and limited measurable flame size.

[0006] In a first aspect, an embodiment of the present application provides a single-line excitation OH planar laser-induced fluorescence temperature measurement system, the system comprising an Nd:YAG laser, a sheet light shaping lens group, two ICCD cameras, and a data acquisition device, wherein the Nd:YAG laser outputs a pulsed laser that has undergone quadruple frequency processing, the sheet light shaping lens group is disposed at the output end of the Nd:YAG laser, and the central axis of the sheet light shaping lens group coincides with the central axis of the output end of the Nd:YAG laser. The laser sheet light output by the sheet light shaping lens group illuminates a region to be measured in a flame, and reacts with OH radicals in the flame to generate OH radical fluorescence; the two ICCD cameras are disposed opposite the flame, and the two ICCD cameras are respectively communicatively connected to the data acquisition device, a bandpass filter is disposed in each of the two ICCD cameras, and each bandpass filter has a different bandwidth. The two ICCD cameras synchronously measure the OH radical fluorescence to obtain a dual-spectrum fluorescence image, and the data acquisition device acquires the dual-spectrum fluorescence image, and two-dimensional temperature distribution measurement is achieved through data processing.

[0007] According to a specific implementation of the embodiment of the present application, the wavelength of the pulsed laser after quadruple frequency processing is 266.1 nm, the pulse frequency is 10 Hz, the laser beam diameter is 12 mm, and the laser energy is 20 mW.

[0008] According to a specific implementation of an embodiment of the present application, the light shaping lens assembly includes a plano-concave spherical lens and a plano-convex cylindrical lens, the plano-concave spherical lens is arranged toward the Nd:YAG laser, and the plano-convex cylindrical lens is arranged toward the flame.

[0009] According to a specific implementation of the embodiment of the present application, the thickness of the laser light sheet output by the light sheet shaping lens assembly is 0.2-1 mm, and the height is not less than 10 mm.

[0010] According to a specific implementation of the embodiment of the present application, the laser sheet light output by the light sheet shaping lens assembly has a thickness of 1 mm and a height of 15 mm.

[0011] According to a specific implementation of the embodiment of the present application, the bandwidths of the bandpass filters in the two ICCD cameras are 280 nm and 310 nm respectively.

[0012] In a second aspect, embodiments of the present application further provide a single-line excitation OH plane laser-induced fluorescence temperature measurement method, the method employing the single-line excitation OH plane laser-induced fluorescence temperature measurement system as described in any embodiment of the first aspect, the method comprising: Control the Nd:YAG laser to output pulsed laser after quadruple frequency processing; The pulsed laser passes through the light-sheet shaping lens group to form a laser sheet light to illuminate the area to be measured in the flame, and reacts with the OH radical in the flame to produce OH radical fluorescence; Two ICCD cameras were used to synchronously measure the OH group fluorescence to obtain dual-spectrum fluorescence images. The data acquisition device acquires the dual-spectrum fluorescence image and realizes two-dimensional temperature distribution measurement through data processing.

[0013] According to a specific implementation of an embodiment of the present application, the two-dimensional temperature distribution measurement is achieved through data processing, including: Preprocessing of dual-spectrum fluorescence images; The pre-processed fluorescence image is registered in pixel space using a camera calibration plate, and grayscale interpolation is performed based on the nearest neighbor interpolation method to generate a new dual-spectrum fluorescence grayscale image. Perform pixel-by-pixel fluorescence signal intensity division on the new dual-spectrum fluorescence grayscale image to obtain the fluorescence signal intensity ratio of each pixel; The temperature calibration coefficient is calibrated, and the temperature of each pixel is obtained based on the calibrated temperature calibration coefficient and the ratio of the fluorescence signal intensity of each pixel to form a final two-dimensional temperature distribution image.

[0014] According to a specific implementation of the embodiment of the present application, the temperature of each pixel is calculated as follows: , Where T is temperature, C is the temperature calibration coefficient, R is the fluorescence signal intensity ratio, and E2 and E3 are the nearby states of the OH group indirectly excited by collision.

[0015] According to a specific implementation of the embodiment of the present application, the calibration of the temperature calibration coefficient adopts experimental calibration or simulation calibration. The experimental calibration comprises: placing the tip of a B-type platinum-rhodium thermocouple at the central axis of the combustion flame, recording the temperature change of the flame, obtaining a thermocouple experimental calibration coefficient, and using the thermocouple experimental calibration coefficient as the temperature calibration coefficient; The simulation calibration includes: using combustion dynamics simulation software CHEMKIN to perform numerical calculation on the flame adiabatic temperature, obtaining an adiabatic temperature calibration coefficient, and using the adiabatic temperature calibration coefficient as the temperature calibration coefficient.

[0016] Beneficial effects: The single-line excitation OH planar laser-induced fluorescence temperature measurement system and method in the embodiments of this application uses the quadrupled frequency of an Nd:YAG laser as the excitation wavelength for OH free radicals and uses the ratio of the fluorescence signals of two spectral bands as a sensitive temperature indicator to measure two-dimensional temperature distribution. This method differs significantly from existing OH-PLIF temperature measurement methods in its OH radical excitation mechanism. Compared with existing dual-line excitation mechanisms, this method significantly reduces the complexity of the measurement system and expands its applicability in high-pressure environments.

[0017] Existing OH-PLIF temperature measurement methods utilize a dual-line excitation mechanism, requiring a solid-state laser in conjunction with a dye laser to adjust the laser wavelength. This results in a complex laser system, low output energy, poor spot quality, and a low signal-to-noise ratio. Furthermore, the fluorescence signal ratio is affected by fluctuations in the dual-line excitation laser energy and the uneven distribution of the light sheet energy. Therefore, the measurement system design requires a device to monitor both the laser energy fluctuations and the light sheet energy distribution in real time, and relevant corrections must be applied to the measured data to reduce measurement errors, further increasing the complexity of the measurement system and data processing.

[0018] The present invention utilizes a single-line excitation mechanism, selecting the P1(10) excitation line of the AX(2,0) band, which is located precisely at the frequency quadrupling of the Nd:YAG laser. Therefore, OH-PLIF excitation can be accomplished using only a single solid-state laser. This significantly simplifies the laser system, resulting in high output energy, excellent spot quality, and a high signal-to-noise ratio. Under this single-line excitation mechanism, the dual-band fluorescence signal ratio is unaffected by fluctuations in the excitation laser energy and uneven distribution of the light sheet energy, significantly simplifying the measurement system configuration and data processing flow.

[0019] In addition, OH radicals are the most important free radicals in the combustion process and are an important means of characterizing the high-temperature zone of the flame. They have a large mole fraction in the flame. The existing dual-line excitation mechanism uses the excitation lines of the AX(1,0) and AX(0,0) bands of the OH radical, which have a high absorption efficiency for OH radicals and a relatively high signal-to-noise ratio under normal-pressure flames. However, due to the large loss of laser energy along the way, only a very small flame area can be measured under high pressure (1 MPa) conditions, and the signal attenuation caused by the loss of laser energy along the way cannot be avoided. The present invention uses the P1(10) excitation line of the AX(2,0) band of the OH radical. Compared with the existing dual-line excitation mechanism, it has a lower absorption efficiency for OH radicals and a smaller loss of laser energy along the way. It can achieve measurement of a larger flame area under higher pressure, greatly expanding the application potential of the OH-PLIF temperature measurement method in high-pressure extreme environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0021] Figure 1 Schematic diagram of a single-line excited OH planar laser-induced fluorescence temperature measurement system according to an embodiment of the present invention; Figure 2 Schematic diagram of the process of single-line excitation OH plane laser-induced fluorescence temperature measurement method according to one embodiment of the present invention; Figure 3 FIG. 4 is a schematic diagram of a four-level model of the interaction between laser and OH according to an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The embodiments of the present application are described in detail below with reference to the accompanying drawings.

[0023] The following describes the embodiments of the present application through specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The present application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present application. It should be noted that, in the absence of conflict, the features in the following embodiments and embodiments can be combined with each other. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of this application.

[0024] It should be noted that various aspects of the embodiments within the scope of the appended claims are described below. It should be apparent that the aspects described herein can be embodied in a wide variety of forms, and any specific structure and / or function described herein is merely illustrative. Based on this application, it should be understood by those skilled in the art that an aspect described herein can be implemented independently of any other aspect, and two or more of these aspects can be combined in various ways. For example, any number of aspects described herein can be used to implement an apparatus and / or practice a method. In addition, other structures and / or functionalities other than one or more of the aspects described herein can be used to implement this apparatus and / or practice this method.

[0025] It should also be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present application. The illustrations only show components related to the present application and are not drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component can be changed at will, and the component layout type may also be more complicated.

[0026] Additionally, in the following description, specific details are provided to provide a thorough understanding of the examples. However, one skilled in the art will appreciate that the aspects described can be practiced without these specific details.

[0027] The applicant has found that if only one solid-state laser is used to excite OH-PLIF in order to overcome the above shortcomings, the excitation wavelength of OH-PLIF must be consistent with the wavelength of the solid-state laser. The fundamental frequency of the Nd:YAG laser itself is around 1064 nm, and its fourth harmonic frequency is about 266 nm. By analyzing the OH-based absorption spectrum near this wavelength, it can be found that the P1(10) absorption peak of the OH-based AX(2, 0) band is located at 266.1 nm, which is exactly within the fourth harmonic frequency range of the Nd:YAG sub-level transition. Therefore, this absorption peak can be used to excite OH-PLIF, which can effectively solve the problems of complex laser systems and poor beam quality. At the same time, the absorption intensity of the P1(10) absorption peak is lower than the intensity of the absorption peaks near 283 nm and 310 nm, which can effectively overcome the problem of laser attenuation along the way.

[0028] Based on the above content, the embodiment of the present application provides a single-line excitation OH plane laser induced fluorescence temperature measurement system and method, which is described below with reference to Figures 1 to 3 Provide a detailed description.

[0029] In a first aspect, an embodiment of the present application provides a single-line excitation OH planar laser-induced fluorescence temperature measurement system, the system comprising an Nd:YAG laser, a sheet light shaping lens group, two ICCD cameras, and a data acquisition device, wherein the Nd:YAG laser outputs a pulsed laser that has undergone quadruple frequency processing, the sheet light shaping lens group is disposed at the output end of the Nd:YAG laser, and the central axis of the sheet light shaping lens group coincides with the central axis of the output end of the Nd:YAG laser. The laser sheet light output by the sheet light shaping lens group illuminates a region to be measured in a flame, and reacts with OH radicals in the flame to generate OH radical fluorescence; the two ICCD cameras are disposed opposite the flame, and the two ICCD cameras are respectively communicatively connected to the data acquisition device, a bandpass filter is disposed in each of the two ICCD cameras, and each bandpass filter has a different bandwidth. The two ICCD cameras synchronously measure the OH radical fluorescence to obtain a dual-spectrum fluorescence image, and the data acquisition device acquires the dual-spectrum fluorescence image, and two-dimensional temperature distribution measurement is achieved through data processing.

[0030] Specifically, the PLIF temperature measurement system of the OH excitation and detection scheme proposed in this application is as shown in the attached Figure 1 As shown in the figure. After the Nd:YAG laser undergoes frequency quadrupling, it outputs a 266.1 nm laser. The laser sheet light formed by the light-sheet shaping lens assembly illuminates the area of ​​interest in the flame, and reacts with the OH groups in the flame to produce fluorescence. Two ICCD cameras equipped with narrow-band filters are used to synchronously measure the OH group fluorescence to obtain a dual-spectrum fluorescence image. The image is acquired by a data acquisition device, and two-dimensional temperature distribution measurement is achieved through data processing. During the measurement process, the laser and ICCD camera are synchronously controlled by a computer through a signal / delay generator. The flame is generated by a burner, and a variety of combustion conditions are achieved through a flow meter and a control unit, enabling temperature measurement under different conditions.

[0031] In one embodiment, the light source is an Nd:YAG laser, which is composed of a laser emitter, a controller, and a cooling water tank. The laser outputs a pulsed laser with a wavelength of 1064 nm. The pulsed laser has a wavelength of 266.1 nm after frequency quadrupling, a pulse frequency of 10 Hz, a laser beam diameter of 12 mm, and a laser energy of 20 mW.

[0032] In one embodiment, the light shaping lens assembly includes a plano-concave spherical lens and a plano-convex cylindrical lens, the plano-concave spherical lens is arranged toward the Nd:YAG laser, and the plano-convex cylindrical lens is arranged toward the flame.

[0033] By properly placing the positions of the plano-concave spherical lens and the plano-convex cylindrical lens, the thickness of the laser light sheet output by the light sheet shaping lens assembly is 0.2-1 mm, and the height is not less than 10 mm.

[0034] For example, the focal length of the plano-concave spherical lens is -400 mm, and the focal length of the plano-convex cylindrical lens is 200 mm. By properly placing the plano-concave spherical lens and the plano-convex cylindrical lens, the laser sheet output by the light shaping lens assembly has a thickness of 1 mm and a height of 15 mm.

[0035] In one embodiment, the bandwidths of the bandpass filters in the two ICCD cameras are 280 nm and 310 nm respectively.

[0036] In practice, fluorescence signal acquisition utilizes two ICCDs coupled with UV lenses. Bandpass filters with central wavelengths of 280 nm and 310 nm, respectively, are fitted to eliminate the effects of stray light such as Mie and Rayleigh scattering during fluorescence measurement. Timing adjustments to the image intensifiers can reduce detection system acquisition noise, improve the signal-to-noise ratio, and minimize errors.

[0037] Since the OH-based fluorescence signal only exists for tens of nanoseconds, the pulse triggering of the Nd:YAG laser and the pulse acquisition of the ICCD camera should be kept synchronized. The signal triggering can be performed separately through the signal / delay generator to achieve synchronous control of the entire measurement system.

[0038] Secondly, refer to Figure 2 The present application also provides a method for measuring temperature using single-line excitation OH plane laser-induced fluorescence temperature, wherein the method uses the single-line excitation OH plane laser-induced fluorescence temperature measurement system as described in any embodiment of the first aspect, and the method includes: Control the Nd:YAG laser to output pulsed laser after quadruple frequency processing; The pulsed laser passes through the light-sheet shaping lens group to form a laser sheet light to illuminate the area to be measured in the flame, and reacts with the OH radical in the flame to produce OH radical fluorescence; Two ICCD cameras were used to synchronously measure the OH group fluorescence to obtain dual-spectrum fluorescence images. The data acquisition device acquires the dual-spectrum fluorescence image and realizes two-dimensional temperature distribution measurement through data processing.

[0039] In one embodiment, the two-dimensional temperature distribution measurement is achieved through data processing, including: Preprocessing of dual-spectrum fluorescence images; The pre-processed fluorescence image is registered in pixel space using a camera calibration plate, and grayscale interpolation is performed based on the nearest neighbor interpolation method to generate a new dual-spectrum fluorescence grayscale image. Perform pixel-by-pixel fluorescence signal intensity division on the new dual-spectrum fluorescence grayscale image to obtain the fluorescence signal intensity ratio of each pixel; The temperature calibration coefficient is calibrated, and the temperature of each pixel is obtained based on the calibrated temperature calibration coefficient and the ratio of the fluorescence signal intensity of each pixel to form a final two-dimensional temperature distribution image.

[0040] Specifically, the calculation formula for the temperature of each pixel is: , Where T is temperature, C is the temperature calibration coefficient, R is the fluorescence signal intensity ratio, and E2 and E3 are the nearby states of the OH group indirectly excited by collision.

[0041] In one embodiment, the temperature calibration coefficient is calibrated by experimental calibration or simulation calibration. The experimental calibration comprises: placing the tip of a B-type platinum-rhodium thermocouple at the central axis of the combustion flame, recording the temperature change of the flame, obtaining a thermocouple experimental calibration coefficient, and using the thermocouple experimental calibration coefficient as the temperature calibration coefficient; The simulation calibration includes: using combustion dynamics simulation software CHEMKIN to perform numerical calculation on the flame adiabatic temperature, obtaining an adiabatic temperature calibration coefficient, and using the adiabatic temperature calibration coefficient as the temperature calibration coefficient.

[0042] In specific implementation, the data processing flow is as follows Figure 2 As shown in the figure, the fluorescence image is first preprocessed, focusing on image noise reduction. Then, using a camera calibration plate, the fluorescence images acquired by the two ICCD cameras are pixel-space matched, and grayscale interpolation is performed based on the nearest neighbor interpolation method to generate a new fluorescence grayscale image. A pixel-by-pixel division operation is performed on the registered dual-spectral fluorescence image to obtain a fluorescence signal intensity ratio image. The temperature calibration coefficient is calibrated based on the actual thermocouple temperature measurement or simulation results, and finally a two-dimensional temperature distribution image is obtained.

[0043] The temperature calibration coefficient can be calibrated using two methods: experimental calibration and simulation calibration. The experimental calibration uses a B-type platinum-rhodium thermocouple, which has the characteristics of a high temperature measurement upper limit, high accuracy, a wide temperature measurement range, and a long service life. The tip of the thermocouple is placed at the combustion flame to record the temperature changes of the flame. Considering that the heat loss at the edge of the flame increases the volatility of the temperature measurement results, selecting the center axis of the flame for temperature measurement can obtain a result as close to the true value as possible, which is used to obtain the experimental calibration coefficient of the thermocouple. The simulation calibration uses the combustion dynamics simulation software CHEMKIN to numerically calculate the adiabatic temperature of the flame. For symmetrically structured burners, a one-dimensional model can be used to calculate the axial temperature distribution of the burner, thereby obtaining the adiabatic temperature calibration coefficient. Finally, based on the obtained calibration coefficient and the dual-spectral fluorescence ratio image, the two-dimensional temperature distribution is calculated.

[0044] Refer to the following Figure 3, the test principle of this application is explained in detail.

[0045] Usually the generation and detection of OH radical fluorescence signals are mainly based on the ground state 2 π and excited states 2 Σ + The existing temperature measurement methods mostly use the 283 nm and 310 nm excitation mechanisms, and the corresponding energy level transitions are A 2 Σ + ←X 2 The AX(1, 0) and AX(0, 0) bands of Π, that is, the bands from the ground state X 2 The ν''=0 vibration energy level of Π moves to the excited state A 2 Σ + The present invention proposes an OH-based AX(2, 0) band excitation mechanism, which uses a laser sheet of a specific frequency to make OH transition from the ground state X 2 The ν''=0 vibration energy level of Π moves to the excited state A 2 Σ + According to this excitation mechanism, a four-level model of the interaction between laser and OH is established, as shown in the attached Figure 3 As shown in the figure, E0(ν'', J'') is the ground state vibration and rotation energy level, E1(ν', J'), E2(ν', J'), and E3(ν', J') represent the excited state vibration and rotation energy levels; B 01 and B 10 are the Einstein stimulated absorption and emission coefficients, ρ ν is the laser energy density; A i represents the spontaneous radiation transition probability, R is the collision relaxation rate, and Q is the electron quenching rate; Q ion and Q pre represent ionization and pre-dissociation respectively. When the laser energy density is ρ ν , wavelength is λ ν The laser light is incident on the combustion field, and the laser is stimulated to absorb (B 01 ρ ν ) and stimulated emission (B 10 ρ ν ) coupled to the OH ground state X 2 π and the first excited electronic state A 2 Σ + There are two specific vibration-rotation energy levels E0 (ν'', J'') and E1 (ν', J'). At the same time, during the excitation process, various molecular collision energy exchange processes will occur, including rotational and vibrational energy transfer (R) and electronic energy transfer or quenching (Q) caused by inelastic collisions between OH and other molecules. In addition, ionization (Q ion ) and predissociation (Qpre Finally, the initially excited state (E1) and the nearby states (E2 and E3) indirectly excited by collisions emit fluorescence to generate PLIF signals (A i ).

[0046] According to the above process, the energy level rate equations are established to solve the excited state particle population. Consider the measurement area V of the detection optical device i opt and collection efficiency η i opt (depending on factors such as spectral filtering, time gating, photocathode quantum efficiency, and enhancer gain), the fluorescence intensity I2 of E2 state radiation and the fluorescence intensity I3 of E3 state radiation can be expressed as: (1) (2) Where: h is Planck's constant; ν2 and ν3 represent the fluorescence radiation frequencies of the E2 and E3 states, corresponding to the fluorescence wavelengths λ2 and λ3 respectively; A2 and A3 represent the spontaneous radiation transition probabilities of the E2 and E3 states; N2 and N3 represent the particle populations of the E2 and E3 states. Under steady-state conditions, N2 and N3 satisfy the Boltzmann distribution: (3) (4) Where: N0 is the number density of OH ground state particles before laser action; J'2 and J'3 are the rotational energy level quantum numbers of the E2 and E3 states; k is the Boltzmann constant; T is the temperature; f2 and f3 are the Boltzmann fractions on the rotational energy levels J'2 and J'3, respectively.

[0047] Substitute equations (3) and (4) into equations (1) and (2) respectively, and calculate the fluorescence signal ratio R: (5) The temperature measurement formula can be expressed as: (6) Here, C can be considered a temperature calibration coefficient, determined by both the experimental system and the two excited-state energy levels. It can be estimated and calibrated through theoretical calculations and is often used as an experimental calibration constant to eliminate systematic errors. Equation (6) shows that the intensity ratio of the two fluorescence signals at steady state is a sensitive indicator of temperature and can be used to derive the two-dimensional temperature field distribution.

[0048] Differentiating both ends of equation (6), we can get the temperature measurement sensitivity: (7) Equation (7) reflects the impact of the measurement error of the fluorescence intensity ratio on the temperature measurement error. The higher the temperature, the greater the impact; the larger the energy level difference |E2-E3|, the smaller the error impact. Compared with the traditional OH excitation and detection mechanism, the fluorescence detected by this method comes from different vibrational energy levels of the excited state, rather than different rotational energy levels of the same vibrational energy level. The larger energy level difference helps reduce measurement error.

[0049] According to the OH fluorescence excitation scheme proposed in the present invention, the Nd:YAG laser is quadrupled (λ ν = 266.1 nm) to excite OH groups A 2 Σ + (ν'=2)←X 2 The P1(10) rotational excitation line of the Π(ν''=0) AX(2, 0) electronic vibration band. The OH in the excited state is unstable and undergoes rapid collision transfer with surrounding molecules, achieving population in adjacent energy levels (ν'=0, 1). It is ultimately quenched or returns to the ground state, emitting fluorescence at 283 nm and 310 nm bands, releasing energy. Fluorescence emission is not a single wavelength, but rather, within a certain wavelength range, the fluorescence signals generated by the transitions of the 283 nm and 310 nm bands are collected, and a two-dimensional temperature distribution is obtained based on the ratio of the fluorescence intensities of the two bands.

[0050] The embodiment provided by the present invention is based on the demand for measuring the non-uniform temperature field at the combustion chamber outlet in the development of aircraft engines. In order to solve the problems of the dual-line OH-PLIF temperature measurement method, such as complex laser system, low signal-to-noise ratio, severe attenuation along the process, and limited measurable flame size, a single-line excitation OH plane laser induced fluorescence temperature measurement method is proposed. The OH-based A is excited by the quadrupled frequency of Nd:YAG laser. 2 Σ + (ν'=2)←X 2 The P1(10) rotation excitation line of the Π(ν''=0) AX(2, 0) spectral band is used, and the ratio of the fluorescence signals of the two spectral bands at 283 nm and 310 nm is used as a sensitive temperature indicator for temperature measurement. This can effectively reduce the complexity of the measurement system, improve the signal-to-noise ratio of the fluorescence image, provide a non-contact measurement method for two-dimensional temperature distribution under high pressure conditions, and provide support for the design and development of aircraft engine combustion chambers.

[0051] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present application should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. A single-line excitation OH plane laser-induced fluorescence temperature measurement system, characterized in that: The system uses only one solid-state laser to excite OH fluorescence and includes an Nd:YAG laser, a sheet light shaping lens assembly, two ICCD cameras, and a data acquisition device. The Nd:YAG laser outputs pulsed laser light that has undergone quadruple frequency processing. The sheet light shaping lens assembly is disposed at the output end of the Nd:YAG laser, and the central axis of the sheet light shaping lens assembly coincides with the central axis of the output end of the Nd:YAG laser. The laser sheet light output by the sheet light shaping lens assembly illuminates a test area in the flame, and reacts with OH radicals in the flame to generate OH radical fluorescence. The two ICCD cameras are disposed opposite the flame and are respectively connected to the data acquisition device for communication. A bandpass filter is disposed in each of the two ICCD cameras, and each bandpass filter has a different bandwidth. The two ICCD cameras synchronously measure the OH radical fluorescence to obtain a dual-spectrum fluorescence image. The data acquisition device acquires the dual-spectrum fluorescence image, and two-dimensional temperature distribution measurement is achieved through data processing.

2. The single-line excitation OH planar laser-induced fluorescence temperature measurement system according to claim 1, characterized in that: The wavelength of the pulsed laser after the quadruple frequency treatment is 266.1 nm, the pulse frequency is 10 Hz, the laser beam diameter is 12 mm, and the laser energy is 20 mW.

3. The single-line excitation OH planar laser-induced fluorescence temperature measurement system according to claim 1, characterized in that: The light shaping lens assembly comprises a plano-concave spherical lens and a plano-convex cylindrical lens. The plano-concave spherical lens is arranged toward the Nd:YAG laser, and the plano-convex cylindrical lens is arranged toward the flame.

4. The single-line excitation OH planar laser-induced fluorescence temperature measurement system according to claim 3, characterized in that: The thickness of the laser light sheet output by the light sheet shaping lens group is 0.2-1 mm, and the height is not less than 10 mm.

5. The single-line excitation OH planar laser-induced fluorescence temperature measurement system according to claim 4, characterized in that: The laser sheet light output by the sheet light shaping lens assembly has a thickness of 1 mm and a height of 15 mm.

6. The single-line excitation OH planar laser-induced fluorescence temperature measurement system according to claim 1, characterized in that: The bandwidths of the bandpass filters in the two ICCD cameras are 280 nm and 310 nm, respectively.

7. A single-line excited OH plane laser-induced fluorescence temperature measurement method, characterized in that: The method adopts the single-line excitation OH planar laser-induced fluorescence temperature measurement system according to any one of claims 1 to 6, and the method comprises: Control the Nd:YAG laser to output pulsed laser after quadruple frequency processing; The pulsed laser passes through the light-sheet shaping lens group to form a laser sheet light to illuminate the area to be measured in the flame, and reacts with the OH radical in the flame to produce OH radical fluorescence; Two ICCD cameras were used to synchronously measure the OH group fluorescence to obtain dual-spectrum fluorescence images. The data acquisition device acquires the dual-spectrum fluorescence image and realizes two-dimensional temperature distribution measurement through data processing.

8. The single-line excitation OH plane laser-induced fluorescence temperature measurement method according to claim 7, characterized in that: The two-dimensional temperature distribution measurement is achieved through data processing, including: Preprocessing of dual-spectrum fluorescence images; The pre-processed fluorescence image is registered in pixel space using a camera calibration plate, and grayscale interpolation is performed based on the nearest neighbor interpolation method to generate a new dual-spectrum fluorescence grayscale image. Perform pixel-by-pixel fluorescence signal intensity division on the new dual-spectrum fluorescence grayscale image to obtain the fluorescence signal intensity ratio of each pixel; The temperature calibration coefficient is calibrated, and the temperature of each pixel is obtained based on the calibrated temperature calibration coefficient and the ratio of the fluorescence signal intensity of each pixel to form a final two-dimensional temperature distribution image.

9. The single-line excitation OH plane laser-induced fluorescence temperature measurement method according to claim 7, characterized in that: The calculation formula for the temperature of each pixel is: ， Where T is temperature, C is the temperature calibration coefficient, R is the fluorescence signal intensity ratio, and E2 and E3 are the nearby states of the OH group indirectly excited by collision.

10. The single-line excitation OH plane laser-induced fluorescence temperature measurement method according to claim 7, characterized in that: The temperature calibration coefficient is calibrated by experimental calibration or simulation calibration. The experimental calibration comprises: placing the tip of a B-type platinum-rhodium thermocouple at the central axis of the combustion flame, recording the temperature change of the flame, obtaining a thermocouple experimental calibration coefficient, and using the thermocouple experimental calibration coefficient as the temperature calibration coefficient; The simulation calibration includes: using combustion dynamics simulation software CHEMKIN to perform numerical calculation on the flame adiabatic temperature, obtaining an adiabatic temperature calibration coefficient, and using the adiabatic temperature calibration coefficient as the temperature calibration coefficient.

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