CN112611477A - Device for reconstructing combustion flame temperature field by laser absorption spectrum scanning light path - Google Patents

Abstract

The invention relates to a device for reconstructing the temperature field of a combustion flame by a laser absorption spectrum scanning optical path. Signal processing components, mirrors, laser transmitting and receiving devices correspond to the number of beams modulated by the fiber beam splitter; the fiber beam splitter divides the laser modulated by the distributed feedback laser into multiple beams, and each beam is transmitted and received by the corresponding laser The device emits to the corresponding mirror, which is rotated by a high-speed stepping motor, distributed in the space to ensure that the optical path sweeps the entire area to be measured, returns in parallel through the hollow retroreflector, and is received by the laser transmitting and receiving device and converted into an electrical signal , to get the light intensity signal. The invention realizes the integration of signal transceiver, reduces measurement components, miniaturized integration, can realize rapid reconstruction of temperature field, obtain more comprehensive optical path information, and improve measurement accuracy.

Description

A device for reconstructing the temperature field of combustion flame by laser absorption spectrum scanning optical path

technical field

The invention relates to the technical field of temperature field measurement devices, in particular to a device for reconstructing a temperature field of a combustion flame by scanning an optical path of a laser absorption spectrum.

Background technique

Aero-engine is a complex and sophisticated thermal machinery, which directly affects the performance, reliability and economy of the aircraft. The combustion chamber is an essential and important part of the engine. It releases the chemical energy in the fuel through combustion, converts the chemical energy into heat energy, generates high temperature and high pressure gas, and improves the engine's working ability. When designing the combustion chamber, it is necessary to ensure good combustion stability, high combustion efficiency and low emission pollution.

The combustion field temperature and component concentration measurement based on TDLAS technology has non-invasion through the combustion field, fast response time, and strong anti-electromagnetic interference ability. The measuring instrument and equipment system is easy to install in a relatively small space and other advantages, it is easy to install in the combustion site and used to monitor the combustion state.

The signal receiving module used in the existing measuring instrument system adopts an independent transmitting and receiving device, which has the disadvantage that two sets of transmitting and receiving devices are built at the same time, the space utilization rate is low, and the additional transmitting and receiving device will affect the arrangement of the optical path. The optical path information in the entire temperature field cannot be fully obtained, resulting in low measurement accuracy.

SUMMARY OF THE INVENTION

The invention provides a device for reconstructing the temperature field of combustion flame by laser absorption spectrum scanning optical path, which solves the shortcomings of complicated optical path arrangement and less effective information of the extracted optical path in the existing measuring device.

The technical scheme adopted in the present invention is as follows:

A device for reconstructing the temperature field of combustion flame by laser absorption spectrum scanning optical path, including a function generator, a laser controller, a distributed feedback laser, a fiber beam splitter, a hollow retroreflector and a signal processing component, as well as a mirror and a laser emission The receiving device, the mirror, the laser transmitting and receiving device correspond to the number of beams modulated by the optical fiber beam splitter; the optical fiber beam splitter divides the laser modulated by the distributed feedback laser into multiple beams, and each beam is composed of The corresponding laser emitting and receiving device is emitted to the corresponding mirror, and the mirror is driven and rotated by the driving mechanism and distributed in the space to ensure that the optical path sweeps the entire area to be measured, returns in parallel through the hollow retroreflector, and is returned by the hollow retroreflector. The laser emitting and receiving device receives and converts into an electrical signal to obtain a light intensity signal.

As a further improvement of the above technical solution:

The structure of the laser transmitting and receiving device includes a collimator, a wedge-shaped prism and a photodetector arranged in sequence along the optical path for collimating the laser beam; the wedge-shaped prism is a semi-transparent and semi-reflecting prism, which is close to the collimator One end is mirror-coated with a semi-transparent and semi-reflective film corresponding to the wavelength band of the distributed feedback laser light, and one end is mirror-coated with an anti-reflection film facing away from the collimator to ensure that the laser reflected back by the hollow retroreflector can enter better the photodetector.

The wedge-shaped prism is a wedge-shaped structure with a trapezoidal cross-section, and one side thereof is inclined relative to the corresponding other side.

The wedge-shaped prism is a circular prism whose one side is inclined by 1° relative to the corresponding other side, and the main purpose is to eliminate the interference effect of the signal.

The bandwidth of the photodetector is 10MHz, and it is installed at the tail of the laser transmitting and receiving device through a fixed frame. The laser light returned in parallel by the hollow retroreflector is received by the photodetector and converted into an electrical signal to obtain a light intensity signal.

The driving mechanism adopts a high-speed stepping motor, and each mirror is connected to the corresponding high-speed stepping motor, and is driven by the high-speed stepping motor to rotate.

There are two laser controllers and two distributed feedback lasers, respectively, and the function generator respectively inputs modulation signals into the two distributed feedback laser controllers through two channels. The output wavelength and light intensity of the distributed feedback laser are modulated; the optical fiber beam splitter divides the laser modulated by the distributed feedback laser into four beams.

There are four laser transmitting and receiving devices in total, which are respectively arranged at the four corners of the optical path. The arrangement position of the laser transmitting and receiving devices ensures that the optical path passing through the hollow retroreflector is not blocked.

The signal processing component includes a signal acquisition system and a computer, and the light intensity signal is transmitted to the signal acquisition system by the corresponding laser transmitting and receiving device, and processed by the computer.

The beneficial effects of the present invention are as follows:

The invention optimizes the compact structure of the measuring device, realizes the integration of signal transceiver, reduces the measuring components, miniaturizes the integration, realizes rapid reconstruction of the temperature field, obtains more comprehensive optical path information, and improves the measuring accuracy.

The laser transmitting and receiving device of the present invention realizes the function of simultaneous transmission and reception. One end face of the wedge-shaped prism can prevent light from other wavelength bands from entering the detector and saturate the detector; Good entry detector. The laser emitting and receiving device of the present invention can be arranged in the four corners of the optical path according to actual needs to ensure that the optical path passing through the hollow retroreflecting mirror is not blocked. The optical path structure arrangement is more compact.

The reflecting mirror of the invention is driven and rotated by a high-speed stepping motor, and can quickly acquire the information in the optical path, thereby improving the time resolution of the reconstructed temperature field.

The parameters of each component of the device of the present invention can establish a unified temperature field detection standard, which is repeatable, and has strong adjustability and operability.

Description of drawings

FIG. 1 is a schematic structural diagram of the measuring device of the present invention.

FIG. 2 is a schematic structural diagram of a laser transmitting and receiving device of the present invention.

In the figure: 1. Function generator; 2. Laser controller; 3. Distributed feedback laser; 4. Optical fiber beam splitter; 5. Reflector; 6. High-speed stepping motor; 7. Laser transmitting and receiving device; 701, Collimator; 702, wedge prism; 703, photodetector; 704, fixed frame; 8, hollow retroreflector; 9, signal acquisition system; 10, computer.

Detailed ways

The specific embodiments of the present invention will be described below with reference to the accompanying drawings.

As shown in FIG. 1, the device for reconstructing the temperature field of combustion flame by laser absorption spectrum scanning optical path in this embodiment includes a function generator 1, a laser controller 2, a distributed feedback laser 3, a fiber beam splitter 4, and a hollow retroreflector 8 and the signal processing assembly, also including the mirror 5, the high-speed stepping motor 6 and the laser transmitting and receiving device 7, the number of beams modulated by the reflecting mirror 5, the high-speed stepping motor 6, the laser transmitting and receiving device 7 and the fiber beam splitter 4 Corresponding respectively; the optical fiber beam splitter 4 divides the laser modulated by the distributed feedback laser 3 into multiple beams, each beam is transmitted to the corresponding mirror 5 by the corresponding laser transmitting and receiving device 7, and the mirror 5 is driven by the high-speed stepping motor 6 Drive to rotate, distribute in space, ensure that the optical path sweeps the entire area to be measured, returns in parallel through the hollow retroreflector 8, is received by the laser transmitting and receiving device 7 and converted into an electrical signal to obtain a light intensity signal.

As shown in FIG. 2 , the structure of the laser transmitting and receiving device 7 includes a collimator 701 , a wedge prism 702 and a photodetector 703 arranged in sequence along the optical path for collimating the laser beam;

Specifically, the laser transmitting and receiving device 7 is the main device for realizing the function of simultaneous transmission and reception. The collimator 701 collimates the laser beam to ensure that the laser beam is not too divergent to receive light in the entire optical path structure; the center of the laser transmitting and receiving device 7 is a semi-transparent and semi-reflective coating wedge prism 702, which adopts a semi-transparent and semi-transparent coating. The reflective prism can make 50% of the laser light pass through the area to be measured by being reflected by the reflective surface. The laser beam is reflected back in parallel by the hollow retroreflector 8 and then passes through the semi-transparent and semi-reflective prism again to hit the target surface of the photodetector 703. The mirror surface of one end of the transflective prism close to the collimator 701 is coated with a transflective film corresponding to the wavelength band emitted by the distributed feedback laser 3, which can well prevent the light of other wavelength bands from entering the photodetector 703 and saturate the photodetector 703. An anti-reflection film is coated on one end of the back of the transflective prism on the collimator 701 to ensure that the laser light reflected by the hollow retroreflector 8 can better enter the photodetector 703 .

As an embodiment, the wedge prism 702 is a circular prism with one side inclined relative to the other side, and the main purpose is to eliminate the interference effect of the signal. Specifically, the cross section of the wedge prism 702 is a right-angled trapezoid, and the hypotenuse is inclined by 1° with respect to the right-angled side. The photodetector 703 at the tail of the laser transmitting and receiving device 7 adopts a photodetector 703 with a self-designed bandwidth of 10 MHz. The entire photodetector 703 is fixed by the fixing frame 704 . The laser light returned in parallel by the hollow retroreflector 8 is received by the photodetector 703 and converted into an electrical signal to obtain a light intensity signal.

As an embodiment, each mirror 5 is connected to a high-speed stepping motor 6 and is driven by the high-speed stepping motor 6 to rotate. The mirror 5 can also be driven to rotate by other driving and transmission mechanisms.

The high-speed stepping motor 6 drives the mirror 5 to rotate, which can quickly acquire information in the optical path, thereby improving the time resolution of the reconstructed temperature field.

As an embodiment, there are two laser controllers 2 and two distributed feedback lasers 3 respectively, and the function generator 1 respectively inputs the modulated signals to the two distributed feedback laser controllers 2 through two channels. 2. Modulate the output wavelength and light intensity of the corresponding distributed feedback laser 3;

As an embodiment, the optical fiber beam splitter 4 divides the laser light modulated by the distributed feedback laser 3 into four beams.

As an embodiment, there are four laser emitting and receiving devices 7 , which are respectively arranged at four corners of the optical path. The arrangement position of the laser emitting and receiving devices 7 ensures that the optical path passing through the hollow retroreflector 8 is not blocked. The arrangement of the entire optical path structure is more compact.

As an embodiment, the signal processing component includes a signal acquisition system 9 and a computer 10 , and the light intensity signals are transmitted to the signal acquisition system 9 by the corresponding laser transmitting and receiving device 7 , and processed by the computer 10 .

The measurement method for reconstructing the temperature field of combustion flame by laser absorption spectrum scanning optical path in this embodiment is based on the measurement device shown in Figure 1 and Figure 2:

Function generator 1, two laser controllers 2, two distributed feedback lasers 3, fiber beam splitter 4, four laser transmitting and receiving devices 7, four mirrors 5 and corresponding stepping motors, hollow retroreflectors 8. Signal acquisition system 9, computer 10.

The function generator 1 inputs the modulated signal into the laser controller 2, and the laser controller 2 tunes the output wavelength and light intensity of the distributed feedback laser 3; it is divided into four beams by the fiber beam splitter 4, and each beam is transmitted and received by the laser. The device 7 is emitted to the reflector 5, the reflector 5 is connected with the high-speed stepping motor 6, the stepping motor drives the reflector 5 to rotate, the optical path sweeps across the area to be measured, returns in parallel through the hollow retroreflector 8, and is sent by the laser transmitting and receiving device 7. The photodetector 703 in the device receives and converts it into an electrical signal to obtain a light intensity signal. Both the background signal and the transmission signal are transmitted to the signal acquisition system 9 by the corresponding photodetectors 703 , and processed by the computer 10 .

The measurement method specifically includes the following steps:

Step 1, the function generator 1 inputs the modulation frequency f _m1 signal into the No. 1 laser controller 2 through channel 1, and the No. 1 laser controller 2 tunes the output wavelength and light intensity of the No. 1 distributed feedback laser 3;

The function generator inputs the modulation frequency f _m2 signal into the No. 2 laser controller 2 through channel 2, and the No. 2 laser controller 2 tunes the output wavelength and light intensity of the No. 2 distributed feedback laser 3;

In step 2, the two beams of modulated light in step 1 are coupled, and divided into four beams by the fiber beam splitter 4, each beam is transmitted to the mirror 5 by the corresponding laser transmitting and receiving device 7, and the mirror 5 follows the high-speed step. When the motor 6 rotates, the optical path sweeps across the area to be measured, returns in parallel through the hollow retroreflector 8, and is received by the photodetector 703 in the laser emission receiving device 7 and converted into an electrical signal to obtain a light intensity signal;

Step 3, through the optical etalon, obtain respectively the etalon signal I _v (t) corresponding to No. 1, No. 2 distributed feedback laser 3, thereby obtain the time frequency response characteristic v (t);

Step 4: Process the measured background light intensity and transmitted light intensity through the process of digital phase locking and low-pass filtering, and extract the second harmonic after deducting the background normalization. According to the Beer-Lambert law, by normalizing the second harmonic and The time-frequency response characteristic v(t) obtained in step 3, the integral absorption area A is fitted by the least squares algorithm, the temperature is measured according to the dual-wavelength line intensity ratio method, and the concentration value is further calculated;

Firstly, the background light intensity I ₀ (i) and the actually measured transmitted light intensity I _t (i) are processed by digital phase-locking technology, and the x-component and y-component expansions of the nth harmonic of the i-th laser of the signal are obtained respectively. As shown in formula (1), where n is 1 or 2:

Secondly, the harmonic components of each signal are extracted by a low-pass filter, as shown in equation (2):

The obtained harmonic components of the i-th laser background light intensity signal and transmitted light intensity signal can be expressed as formula (3):

Finally, the obtained i-th laser harmonic signal is subjected to the first harmonic normalization process after subtracting the background to obtain formula (4):

When the medium gas distribution on the measurement path is uniform, the average temperature and concentration on the path can be calculated by the following formula:

R为线强比值，是温度的单调函数。S(T) is the absorption line intensity of the gas, A is the integral absorption area, X is the volume fraction of the gas, L is the specific length of the measured optical path, P is the total gas pressure in atm, and φ _v is a linear function , and after normalization

R is the line intensity ratio, which is a monotonic function of temperature.

The fifth step is to solve the two-dimensional transient temperature field.

Based on the measurement principle of the two-dimensional tomographic reconstruction technology, in order to obtain the parameter information in the two-dimensional space in the measured area, the reconstruction area needs to be discretized. Grid, assuming that parameters such as temperature within each discrete grid are uniformly distributed.

After the i-th laser ray passes through the measurement area, the effective optical path length in each discrete grid is l _i,j , which is the path matrix of the laser ray. The specific value can be determined by the geometric relationship (the width of the discrete grid and the The angle α) between the ray and the coordinate axis is determined.

When the distribution of the medium gas on the path is not uniform, formula (5) is transformed, the light is absorbed by the medium gas in the discrete grid, and the intensity weakens when it leaves the measurement area. Absorption equation:

Among them, A _{v1, i} and A _{v2, i} are the path absorption integral values of the i-th ray after passing through the measurement area, i is the number of the laser beam, j is the discrete grid number, α _{v1, j} and α _{v2, j} represents the gas absorption coefficient within each discrete grid.

Formula (11) and formula (12) are the specific discrete equations of the two H2O absorption lines:

Among them, α _v1,j =(PS _T X) _v1,j and α _v2,j =(PS _T X) _v2,j are the absorption coefficients of the two absorption lines in the discrete grid, m is the total The number of rays, n is the total number of grids divided.

The ART algorithm can obtain ideal results in the reconstruction process based on incomplete projection values, so it is widely used in the reconstruction of two-dimensional combustion fields. For the ART algorithm, the solution process is carried out one ray after another. After each ray is considered, the image is updated once, that is, one iteration is completed. If the image does not meet the convergence requirements, the above process will be It is repeated until the convergence condition is satisfied.

When using the iterative algorithm to solve the two-dimensional temperature and concentration distribution, it is first necessary to solve the gas absorption coefficients α _v1,j and α _v2,j in the discrete grid. The specific expression of the absorption coefficient α _j calculated by the ART algorithm is as follows:

是通过重建迭代程序计算得到单个波长下每个离散网格内的气体吸收系数，λ为松弛因子，其具体取值在0～2之间，常用于提高收敛速度和获取较高的重建质量，k代表总的迭代循环次数。由公式(6)可知，两条吸收谱线线强的比值可以通过迭代求解得到的每个离散网格内的吸收系数α_v1，j和α_v2，j的比值求得，进而利用公式(7)计算得到每个网格内的温度值T_j。in,

is the gas absorption coefficient in each discrete grid under a single wavelength calculated by the reconstruction iterative program, λ is the relaxation factor, and its specific value is between 0 and 2, which is often used to improve the convergence speed and obtain higher reconstruction quality. k represents the total number of iteration loops. It can be seen from formula (6) that the ratio of the line intensities of the two absorption lines can be obtained by the ratio of the absorption coefficients α _v1,j and α _v2,j in each discrete grid obtained by iterative solution, and then using formula (7) ) is calculated to obtain the temperature value T _j in each grid.

Claims (8)

1. A device for reconstructing a combustion flame temperature field by a laser absorption spectrum scanning light path comprises a function generator (1), a laser controller (2), a distributed feedback laser (3), an optical fiber beam splitter (4), a hollow retroreflector (8) and a signal processing assembly, and is characterized by further comprising a reflector (5) and a laser emission receiving device (7), wherein the number of light beams modulated by the optical fiber beam splitter (4) corresponds to that of the reflector (5) and the laser emission receiving device (7) respectively;

the optical fiber beam splitter (4) divides the laser modulated by the distributed feedback laser (3) into a plurality of beams, each beam is emitted to a corresponding reflector (5) by a corresponding laser emitting and receiving device (7), the reflectors (5) are driven by a driving mechanism to rotate and are distributed in space, a light path is ensured to sweep the whole area to be measured, the light path returns in parallel through the hollow retroreflector (8), and the light path is received by the laser emitting and receiving devices (7) and converted into an electric signal to obtain a light intensity signal.

2. The device for reconstructing the combustion flame temperature field through the laser absorption spectrum scanning light path according to claim 1, wherein the structure of the laser emitting and receiving device (7) comprises a collimator (701) for collimating the laser beam, a wedge prism (702) and a photodetector (703) which are sequentially arranged along the light path;

the wedge-shaped prism (702) is a semi-transparent semi-reflecting prism, a semi-transparent semi-reflecting film of a wave band corresponding to the outgoing light of the distributed feedback type laser (3) is plated on the mirror surface of one end of the wedge-shaped prism close to the collimator (701), and an anti-reflection film is plated on the mirror surface of one end of the wedge-shaped prism, which is opposite to the collimator (701), so that the laser reflected by the hollow retroreflector (8) can better enter the photoelectric detector (703).

3. The device for reconstructing the combustion flame temperature field through the laser absorption spectrum scanning light path according to claim 2, wherein the wedge-shaped prism (702) is in a wedge-shaped structure with a trapezoidal section, and one surface of the wedge-shaped prism is obliquely arranged relative to the other corresponding surface.

4. The device for reconstructing the combustion flame temperature field through the laser absorption spectrum scanning light path according to claim 2, wherein the bandwidth of the photoelectric detector (703) is 10M, the photoelectric detector is installed at the tail of the laser emission and receiving device (7) through a fixing frame (704), and the laser light which returns in parallel through the hollow retroreflector (8) is received by the photoelectric detector (703) and converted into an electrical signal to obtain a light intensity signal.

5. The device for reconstructing the combustion flame temperature field through the laser absorption spectrum scanning light path according to claim 1, wherein the driving mechanism adopts a high-speed stepping motor (6), and each reflector (5) is connected with the corresponding high-speed stepping motor (6) and driven by the high-speed stepping motor (6) to rotate.

6. The device for reconstructing the combustion flame temperature field through the laser absorption spectrum scanning light path according to claim 1, wherein the laser controller (2) and the distribution feedback type laser (3) are respectively and correspondingly arranged in two numbers, the function generator (1) respectively inputs modulation signals into the two distribution feedback type laser controllers (2) through two channels, and the laser controllers (2) modulate the output wavelength and the light intensity of the corresponding distribution feedback type laser (3); and the optical fiber beam splitter (4) splits the laser modulated by the distributed feedback laser (3) into four beams.

7. The device for reconstructing the combustion flame temperature field through the laser absorption spectrum scanning light path according to claim 6, is characterized in that the number of the laser emission receiving devices (7) is four, and the laser emission receiving devices are respectively arranged at four corners of the light path, and the arrangement position of the laser emission receiving devices (7) ensures that the light path passing through the hollow retroreflector (8) is not shielded.

8. The device for reconstructing the temperature field of the combustion flame through the laser absorption spectrum scanning light path according to claim 1, wherein the signal processing assembly comprises a signal acquisition system (9) and a computer (10), and the light intensity signals are transmitted to the signal acquisition system (9) through the corresponding laser emitting and receiving devices (7) and processed by the computer (10).

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