Solid propellant laser ignition energy measuring method based on optical fiber transmission

Abstract

The invention discloses a solid propellant laser ignition energy measuring device and method based on optical fiber transmission. The solid propellant laser ignition energy measuring device is composed of a high-pressure transparent window combustion chamber, a laser, a light path, a line scanning camera, a computer and corresponding software. Laser emitted by the laser device is output through the optical fiber and enters the high-pressure combustion chamber through an optical fiber connector on an end cover of the combustion chamber, the optical fiber connector is provided with a collimator on an optical fiber connecting flange at the inner side end of the combustion chamber, and the laser is irradiated on an ignition end face of the solid propellant after being focused. After an ignition command is executed on the computer, the laser starts to continuously emit light, and the computer records the time as ignition starting time; the computer collects flame light intensity data generated by the line scanning camera in real time and compares the data with threshold value data when the propellant is ignited until the flame light intensity data is larger than the threshold value, the computer immediately records the time as the ignition time, and meanwhile the laser is commanded to stop emitting light. And calculating ignition energy according to the ignition power, the power correction coefficient, the ignition time and the ignition starting time.

Description

Technical Field

[0001] The invention relates to a testing device and method for the laser energy required when a solid propellant is ignited by laser, and belongs to the field of energetic material combustion parameter determination. Background Art

[0002] Solid propellant converts its chemical potential into thermal energy of high-temperature gas through the combustion process in the rocket engine, and then converts the thermal energy of high-temperature gas into kinetic energy for rocket flight through the nozzle. The combustion performance of solid propellant is one of the important research contents in the research and development process of solid propellant.

[0003] Ignition is the initial stage of combustion. Studying the ignition process is of great significance for revealing the combustion mechanism of solid propellants and regulating combustion performance. Lasers are favored by researchers because of their high output energy, controllable ignition time and energy, and no interference.

[0004] Ignition energy is an important parameter in the laser ignition process. Accurately measuring the laser ignition energy is of great significance for studying the laser ignition process.

[0005] Existing laser ignition device and Figure 1 The device shown in (Tu Chengyin, Energetic Materials, Vol.30, No.8, 2022: 811-818) is similar. The laser is composed of CO 2 The laser is emitted, reflected by a mirror, focused by a lens, and shines through the zinc selenide glass window of the high-pressure combustion chamber onto the ignition end face of the solid propellant, igniting the solid propellant. In this scheme, the laser propagates over a long distance in the air, and the energy attenuation is very large; external factors such as vibration can easily cause the displacement of optical path components such as mirrors, causing the laser beam to deviate from the ignition end face.

[0006] The laser emission power can be set through the laser control device. The energy density at the time of ignition can be calculated based on the spot size and emission power of the laser irradiated on the ignition end face of the solid propellant; the energy required for ignition can be calculated based on the emission power and the time interval from the emission of the laser to the ignition of the solid propellant (laser ignition delay time). Therefore, the core of measuring laser ignition energy is to accurately measure the ignition delay time of the laser ignition process.

[0007] C. Zanotti in Italy (Propellants, Explosives, Pyrotechnics 23, 254 - 259 (1998)) set a "shutter" on the optical path, and the preset shutter exposure time was the ignition delay time. Li Zexu (Journal of Pyrotechnics and Explosives, Vol. 47, No. 11) preset the duration of laser emission through the controller of the laser, and this time was the laser ignition delay time. Both of these methods are the "method of presetting the light output duration of the laser". The test system and experimental process are relatively simple, but it cannot be determined that the preset time is exactly the actual laser ignition delay time of the solid propellant. Even after multiple experiments, it is difficult to measure the true ignition delay time.

[0008] Yan Xiaoting (Graduate Thesis of National University of Defense Technology, November 2016) installed a light-emitting diode (LED) lamp within the shooting range of a high-speed camera to measure the laser ignition delay time. The LED lamp and the laser igniter were controlled by the same module. The LED lamp was in the luminous state when the laser igniter was off, and when the laser igniter was started, the LED lamp went out and could be observed through the high-speed camera. This moment was taken as the initial moment of the laser ignition process. The appearance of the initial flame of the solid propellant could be observed through the shooting results of the high-speed camera. The time difference could be calculated based on the frame number difference and frame rate between the two, which was the ignition delay time.

[0009] Multiple researchers (Cheng-Yin Tu. Energetic Materials. Vol. 30, No. 8, 2022: 811 - 818; Lian-bo Li. Aerospace Science and Technology 76, 2018: 394–401; AG Korotkikh (Russia) International Conference on Recent Trends in Physics 2016 (ICRTP2016); Alexander Korotkikh. MATEC Web of Conferences 110, 01042 (2017); Alexander Korotkikh. MATEC Web of Conferences 115, 03016 (2017)) used two photodiodes to collect the laser light output signal and the solid propellant flame signal respectively. Due to CO 2The light emitted by the laser is in the infrared region, and the visible light high-speed camera cannot detect the light output of the laser. Therefore, a part of the light output from the laser is separated and made to irradiate a photodiode that can sense infrared light. The light signal is converted into a voltage signal by the photodiode, amplified, and then output to an LED. The LED is installed within the shooting angle of view of the high-speed camera. When the laser starts to emit light, the LED emits light and is captured by the high-speed camera, and this moment is taken as the zero starting point of the laser ignition process. The initial signal of the photodiode that collects the flame signal of the solid propellant is used as the ignition moment of the solid propellant, and the difference between the two is the ignition delay time.

[0010] In the existing methods, the method of presetting the light output duration of the laser requires multiple experiments to measure the ignition delay time. Other methods need to judge when the solid propellant is ignited according to the shooting results of high-speed photography after the combustion ends. However, there is a process for the ignition of the solid propellant from a very small light spot to gradually grow into a complete flame, and there are great subjective factors of people in determining which moment it is ignited.

[0011] In addition, after the solid propellant is ignited, the combustion performance such as the burning rate often needs to be measured simultaneously. In the method that requires the high-speed photography result to judge the ignition end point, the laser always exists during the entire combustion process of the solid propellant, that is, the solid propellant is always burned under the condition of external additional energy, and the test result at this time obviously deviates from the true value. Summary of the Invention

[0012] The purpose of the present invention is to solve the problem of accurately measuring the minimum energy required when using a laser to ignite a solid propellant.

[0013] The laser ignition device of the present invention is shown in Figure 2 It consists of a high-pressure transparent window combustion chamber (including a combustion chamber kettle body, a combustion chamber end cover, a combustion chamber end cover compression ring, a grain rack, a transparent window), a laser and an optical path (optical fiber, optical fiber connector, optical fiber coupler and collimator), a line-scan camera and a computer, as well as corresponding software.

[0014] The laser emitted by the laser is output through an optical fiber, and is connected to the optical fiber flange of the "optical fiber connector" outside the combustion chamber end cover with an optical fiber connector. It is transmitted through the optical fiber inside the "optical fiber connector" to the optical fiber flange of the "optical fiber connector" inside the combustion chamber end cover. A laser collimator is installed on this flange (the collimator has a replaceable light-transmitting protective cover to prevent damage to the collimator by combustion products), and the ignition laser is focused and irradiated on the ignition end face of the solid propellant. After the ignition command is executed on the computer, the laser starts to continuously emit laser light, and the computer simultaneously records this time as the start ignition time; the computer real-time collects the image data generated by the line scan camera, and compares the collected data with the threshold data when the solid propellant is ignited. Until there is image data greater than the threshold value, the computer immediately records this time as the ignition time of the solid propellant, and simultaneously issues an instruction to stop the laser from emitting light.

[0015] Ignition energy = ignition power × power correction factor × (ignition time - start ignition time)

[0016] Ignition energy density = ignition energy / spot area

[0017] The features of the present invention are: 1) Outside the combustion chamber, the laser is transmitted through an optical fiber, avoiding interference from external factors on laser transmission and reducing energy attenuation during laser transmission; 2) The computer completely judges and records the light emission moment of the laser and the appearance moment of the combustion flame light of the solid propellant, eliminating errors caused by human subjective judgment; 3) Once the solid propellant is ignited, the system will immediately stop the laser from emitting light, avoiding the influence of the laser on the combustion process during subsequent experiments. Brief Description of the Drawings

[0018] Figure 1 Existing laser ignition device

[0019] Figure 2 Laser ignition device of the present invention

[0020] Figure 3 Optical fiber connector Detailed Description of the Invention

[0021] The present invention will be specifically described below through embodiments. It is necessary to point out that the following is only used to further illustrate the present invention and cannot be understood as a limitation on the protection scope of the present invention. Any equivalent changes made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0022] (I) Composition of the test device

[0023] The test device of the present invention is as Figure 2 shown.

[0024] 1) High-pressure transparent window combustion chamber

[0025] Design and manufacture combustion chambers with different pressure resistances (up to 100 MPa). The combustion chamber consists of a kettle body, an end cover, an end cover compression ring, a transparent window, an air inlet, an air outlet, and a supporting gas boosting system. There is a grain fixing rack on the inner side of the end cover, and an "optical fiber connector" hole is left on the end cover.

[0026] 2) Optical fiber connector

[0027] To transmit the laser into the high-pressure combustion chamber through the optical fiber, a special connector needs to be designed and developed.

[0028] The connector is shown in Figure 3 . It is a hollow columnar structure. There is a bare optical fiber in the hollow space, which is connected to the optical fiber flanges at both ends of the connector, and the hollow space is filled with sealant. There is a nut at the lower end of the connector, and there are threads on the outer side of the connector near the nut that match the "optical fiber connector" hole on the combustion chamber end cover. Put on the sealant ring and install the optical fiber connector on the end cover of the combustion chamber.

[0029] 3) Laser

[0030] The laser has both ignition laser in the infrared band and indicating light in the visible light band; there are matching hardware interfaces and communication protocols between the laser and the computer, and its output light power, start of light output, stop of light output and other parameters and actions can be remotely controlled by the computer; the laser outputs light energy through the optical fiber; the laser is transmitted into the high-pressure transparent window combustion chamber through the "optical fiber connector".

[0031] 4) Line-scan camera

[0032] The line-scan camera has more than 2048 pixels and a line frequency of more than 100 kHz; it includes a supporting power supply, data line, data acquisition card, and imaging lens.

[0033] 4) Computer

[0034] The computer should have a data acquisition card slot for the line-scan camera and a hardware interface required for the communication connection of the laser.

[0035] 5) Control and data acquisition and processing software

[0036] Compile control and data acquisition and processing software to complete the following tasks:

[0037] ① Control the light source switching of the laser (output ignition laser or indicating light);

[0038] ② Set the light output power of the laser;

[0039] ③ Control the start and stop of the laser's light output;

[0040] ④ Set the operating parameters of the line-scanning camera, such as the line frequency and exposure time;

[0041] ⑤ Collect the image data generated by the line-scanning camera in real time;

[0042] ⑥ Judge in real time whether the solid propellant is ignited according to the imaging data of the line-scanning camera;

[0043] ⑦ Be able to read and record the time at critical moments from the computer clock.

[0044] 6) System integration

[0045] Integrate the combustion chamber, laser, line-scanning camera, computer and corresponding software into a complete test system.

[0046] (2) Experimental process

[0047] 1) Connect the laser optical path

[0048] Connect the light output port of the laser to the fiber optic flange on the "fiber optic connector" outside the combustion chamber end cap with an optical fiber; install a collimator on the fiber optic flange of the "fiber optic connector" inside the end cap.

[0049] 2) Laser power calibration

[0050] After the laser emits laser light according to the set power, energy consumption will occur during the transmission process. Therefore, the laser energy reaching the ignition end face of the solid propellant will be less than its initial emission energy. This requires calibrating the light energy actually received by the solid propellant.

[0051] Take out the combustion chamber end cap from the combustion chamber and measure the distance L from the laser collimator to the ignition end face of the solid propellant. Remove the solid propellant grain rack on the end cap and place a light energy power meter at a distance L directly in front of the laser collimator.

[0052] Set a laser set ignition power on the computer, let the laser emit light continuously, and read the actual ignition power displayed by the power meter; change different set ignition powers to obtain the corresponding relationship and correction coefficient between the set ignition power on the laser and the actual ignition power for power correction during subsequent experiments.

[0053] In the above experimental device, the laser emits an indicator light, measures the spot diameter, and calculates the spot area for calculating the power density.

[0054] 3) Setting of the ignition threshold

[0055] The laser used for ignition is infrared light, while the combustion flame light of the solid propellant is visible light. The line-scanning camera used in the system is a visible-light camera. Therefore, the camera can only sense the flame light and cannot sense the laser used for ignition. Set a photoelectric data generated by the camera slightly higher than the background light condition as the "ignition threshold". If the output value of the camera is lower than the threshold, it indicates that the solid propellant has not been ignited. If there is data higher than the threshold in the output value of the camera, it indicates that the solid propellant has been ignited.

[0056] 4) Install the grain

[0057] Outside the combustion chamber, install the solid propellant grain on the grain rack on the combustion chamber end cover; turn on the indicating light and check whether the light spot irradiates on the ignition end face of the solid propellant. If necessary, finely adjust the position of the grain; then install the end cover into the combustion chamber and tighten the end cover retaining ring.

[0058] 5) Ignition

[0059] Pressurize nitrogen into the combustion chamber through the pressurization system to reach the required experimental pressure.

[0060] Set the laser ignition power; the computer enters the data acquisition state for the line-scanning camera.

[0061] Execute the ignition command on the computer, and the laser starts to continuously emit laser light. The computer simultaneously records this time as the start ignition time.

[0062] The computer real-time collects the image data generated by the line-scanning camera and compares the collected data with the threshold data when the solid propellant is ignited. If all the data is less than the threshold value, it indicates that the solid propellant has not been ignited; once there is data greater than the threshold value, it indicates that the solid propellant has been ignited. At this time, the computer immediately records this time as the ignition time of the solid propellant and simultaneously issues an instruction to stop the laser from emitting light.

[0063] 6) Calculate the ignition energy

[0064] Ignition energy = Set ignition power × Power correction factor × (Ignition time - Start ignition time)

[0065] Ignition energy density = Ignition energy / Spot area.

Claims

1. A solid propellant laser ignition energy measurement device and method based on optical fiber transmission, characterized in that: It consists of a high-pressure transparent window combustion chamber, a laser, an optical fiber, an optical fiber connector, a line scan camera, a computer and corresponding software.

2. The device according to claim 1, characterized in that: There is a fiber optic connector mounting hole on the combustion chamber end cover.

3. The device according to claim 1, characterized in that: The fiber optic connector is a hollow columnar structure with a bare optical fiber in the hollow space, which is connected to the fiber optic flanges at both ends of the connector. The hollow space is filled with sealant. The connector is installed in the "fiber optic connector mounting hole" of the end cap through threads. The fiber optic connector and the fiber optic connector mounting hole are sealed by a sealing rubber ring.

4. The device according to claim 1, characterized in that The laser must have both infrared ignition laser and visible light indicator light; there must be a suitable hardware interface and communication protocol between the laser and the computer.

5. The device according to claim 1, characterized in that: The laser transmits the laser light into the high-pressure transparent window combustion chamber through optical fiber and "fiber optic connector".

6. The device according to claim 1, characterized in that: A laser collimator is installed on the fiber optic connection flange at the inner end of the combustion chamber of the fiber optic connector to focus the laser and irradiate it on the ignition end face of the solid propellant.

7. The method according to claim 1, characterized in that: The laser power loss in the optical path is calibrated to obtain the power correction coefficient.

8. The method according to claim 1, characterized in that: After the ignition command is executed, the laser starts to emit light continuously, and the computer records this time as the start of ignition. The computer collects the flame intensity data generated by the line scan camera in real time, and compares the collected data with the threshold data when the propellant is ignited. When the flame intensity data is greater than the threshold value, the computer immediately records this time as the ignition time of the solid propellant, and issues a command to stop the laser from emitting light. The ignition energy of the solid propellant is calculated based on the laser's emission power, power correction coefficient, start of ignition time, and ignition time.

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