Internal flow field peeping type PIV (particle image velocimetry) measuring system and method for backflow combustion chamber

Abstract

The invention discloses a flow field peeping type PIV measuring system and method in a backflow combustion chamber, and belongs to the technical field of flow field measurement in the backflow combustion chamber of an aero-engine, the flow field peeping type PIV measuring system in the backflow combustion chamber comprises a cooling purging device and a laser endoscope and / or a camera endoscope arranged in the cooling purging device; a cooling liquid channel, a purging gas channel and an endoscope channel which are not communicated with one another are arranged in the cooling purging device; the cooling liquid channel comprises a cooling liquid inlet channel from the connecting end to the detection end and a cooling liquid outlet channel from the detection end to the connecting end; the purging gas channel comprises a gas inlet located at the connecting end and a gas outlet located at the detecting end. The detection end is provided with the airflow guide structure, so that the purging gas flows to the end face of the endoscope channel along the end face of the detection end in a wall-attached mode, a stable wall-attached airflow barrier is formed, high-temperature fuel gas and soot are prevented from making direct contact with the mirror face of the endoscope, and mirror face pollution is effectively prevented.

Description

Technical Field

[0001] The present invention relates to the technical field of internal flow field measurement of aero-engine reverse flow combustors, and particularly to an endoscopic PIV measurement system and method for the internal flow field of a reverse flow combustor. Background Art

[0002] Due to the complex structure of aero-engine combustors and the narrow internal space, it is extremely difficult to measure the internal flow field. In the early days, people used pitot tubes, hot wire anemometers, etc. to measure the flow field, but they are contact-type single-point measurements, which have great interference on the flow field and low measurement accuracy. With the rapid development of computer technology, laser technology, and image processing technology, Particle Image Velocimetry (PIV) has emerged. It can achieve non-contact, transient, and full-field flow field measurements and has gradually become the main means for measuring the internal flow field of aero-engine combustors. Considering the complexity of the structure of aero-engine combustors, there are still some technical problems in directly applying PIV technology to measure its internal flow field, and it is impossible to achieve a good optical path layout. In order to ensure the smoothness of the measurement optical path, the existing methods have simplified the structure of the object to be measured and made large-area window openings to ensure the smoothness and reasonable layout of the optical path during measurement. However, this simplification of the structure and large-area window opening will damage the original structure of the test piece, and obviously this method will greatly change its original internal flow characteristics and it is impossible to obtain the true flow characteristics.

[0003] Since the endoscopic technology can break through the optical path limitation of the traditional PIV measurement technology in the restricted space of aero-engine combustors, it provides the possibility for measuring the flow field in the restricted space and has great development potential. It can reduce the optical path requirements during the measurement of the internal flow field and does not require the simplification and large-area window opening of the object to be measured. However, when directly applied to the restricted space of aero-engine combustors, there are many technical problems, especially the high-temperature carbon soot pollution and difficult cooling of the front-end optical glass surface of the endoscope, which seriously affect the measurement effect. Summary of the Invention

[0004] The present invention provides an endoscopic PIV measurement system and method for the internal flow field of a reverse flow combustor to solve the technical problems that the endoscopic technology is polluted by high-temperature carbon soot and difficult to cool when applied to the PIV measurement of the internal combustion chamber of an engine, resulting in the authenticity of the measurement results being affected.

[0005] According to one aspect of the present invention, there is provided an in-situ endoscopic PIV measurement system for the internal flow field of a recirculation combustion chamber, comprising a cooling and purging device, and a laser endoscope and / or a camera endoscope disposed within the cooling and purging device; the purging and cooling device includes a detection end for extending into the recirculation combustion chamber and a connection end located outside the combustion chamber, and a coolant channel, a purging gas channel, and an endoscope channel that are not interconnected are provided within the cooling and purging device; the endoscope channel is used to accommodate the laser endoscope and / or the camera endoscope; the coolant channel includes a coolant inlet channel from the connection end to the detection end and a coolant outlet channel from the detection end to the connection end; the purging gas channel includes an air inlet at the connection end and an air outlet at the detection end, and an air flow guiding structure is provided at the detection end of the purging and cooling device for guiding the gas at the air outlet to flow along the wall of the detection end face to the end face of the endoscope channel.

[0006] Optionally, the endoscope channel is located between the coolant inlet channel and the coolant outlet channel.

[0007] Optionally, the purging and cooling device includes an inner sleeve, an outer sleeve, and a plurality of partition plates connecting the inner sleeve and the outer sleeve. The endoscope channel is formed within the inner sleeve, and the coolant inlet channel, the coolant outlet channel, and the purging gas channel are located between the inner sleeve and the outer sleeve and are formed by being separated by the plurality of partition plates. Through holes are provided on the partition plates between the coolant inlet channel and the coolant outlet channel.

[0008] Optionally, a Powell prism is provided at the end of the laser endoscope. The Powell prism is located within the endoscope channel, and the end face of the Powell prism is flush with the end face of the detection end.

[0009] Optionally, it further includes a camera and a laser. The camera and the camera endoscope are connected by a telescopic adapter ring to adjust the imaging distance between the camera and the endoscope, and the laser and the laser endoscope are connected by a light guide arm.

[0010] Optionally, an optical interface with a focal length of not less than 75 mm is provided between the telescopic adapter ring and the camera endoscope.

[0011] Optionally, the laser endoscope and the camera endoscope are arranged vertically, and the optical paths intersect within the internal flow field of the combustion chamber.

[0012] According to another aspect of the present invention, there is also provided an in-situ endoscopic PIV measurement method for the internal flow field of a recirculation combustion chamber, which includes the following steps: Step S1: Assembling and fixing the combustion chamber test piece; Step S2: Assembling the in-situ endoscopic PIV measurement system for the internal flow field of the recirculation combustion chamber; Step S3: Under open conditions, calibrate the PIV measurement system and correct the imaging distortion. When calibrating, the positional relationship between the camera endoscope and the laser endoscope is determined according to their positional relationship during subsequent PIV measurement. At the same time, adjust and ensure that the two laser beams coincide; Step S4: Install and fix the laser endoscope and the camera endoscope on the combustion chamber test piece respectively, ensure that the laser endoscope and the camera endoscope are arranged vertically, and at the same time make the ends of the laser endoscope probe and the camera endoscope probe flush with the wall surface, and make the purge gas direction at the end of the endoscope consistent with the combustion chamber gas flow direction at its position; Step S5: Adjust the test conditions to reach the specified technical requirement state, and at the same time adjust the tracer particle generator to make the tracer particle concentration 6 - 8 tracer particles per interrogation area; Step S6: The computer issues a collection command, and the laser and the camera work synchronously under the control of the synchronous controller, and collect the original images of the tracer particles; Step S7: The computer performs cross - correlation algorithm calculation on the tracer particle pictures to obtain the velocity field within the entire measurement section.

[0013] Optionally, step S2 includes the following steps: Connect the PIV measurement system. Connect the computer to the synchronous controller, the synchronous controller is respectively connected to the laser and the camera, and the laser outlet is connected to the light guide arm; The outlet of the light guide arm is connected to the laser endoscope. According to the size of the measurement area, select a Powell prism with an appropriate divergence angle, and install the Powell prism at the end of the laser endoscope through threads to form a sheet - shaped laser; Connect the camera endoscope probe to the optical interface, and the optical interface is then connected to the camera; Connect the cooling and purging device to the outlets of the coolant cooling device and the purge gas device.

[0014] Optionally, calibrating the PIV measurement system and correcting the imaging distortion include the following steps: Set a ground glass behind the calibration plate, and then set a light source behind the ground glass. The illumination light of the light source first irradiates on the ground glass, and then irradiates on the calibration plate in the form of diffuse reflection light after passing through the ground glass. At the same time, cooperate with the long - time exposure of the camera to obtain a calibration original picture with high brightness and uniform light intensity distribution.

[0015] In summary, the present application includes at least one of the following beneficial technical effects: By constructing a cooling and purging device and arranging independent coolant channels, purging gas channels and endoscope channels inside it, efficient cooling and effective anti-fouling of the laser endoscope and / or camera endoscope are achieved, thus solving the technical problem of measurement result distortion in endoscopic PIV measurement caused by high-temperature soot pollution and difficult cooling. Specifically, the coolant channel includes a liquid inlet channel leading from the connection end to the detection end and a liquid outlet channel returning from the detection end to the connection end, which can realize the circulation cooling of the coolant and effectively reduce the working temperature of the endoscope probe in the high-temperature combustion environment. Similarly, this device can also be applied in extremely cold temperatures. At the same time, the purging gas channel guides the cold air flow to enter from the connection end. At the detection end, through the provided air flow guiding structure, the purging gas flows along the wall of the detection end face to the end face of the endoscope channel, forming a stable wall-attached air flow barrier to block the direct contact between the high-temperature gas and soot and the endoscope mirror surface, effectively preventing mirror surface pollution. The above cooling and purging designs ensure that the optical elements at the front end of the endoscope still have good imaging clarity and reliability in high-temperature and highly polluted environments, guaranteeing the data authenticity and stability during the PIV measurement process.

[0016] In addition to the purposes, features and advantages described above, the present invention has other purposes, features and advantages. The present invention will be further described in detail below with reference to the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings constituting a part of this application are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings: Figure 1 It is a schematic diagram of an endoscopic PIV measurement system in the internal flow field of the recirculation combustion chamber of the present invention; Figure 2 It is a schematic structural diagram of the cooling and purging device of the present invention; Figure 3 It is a side view of the cooling and purging device of the present invention; Figure 4 It is a cross-sectional schematic diagram of the cooling and purging device of the present invention; Figure 5 It is an A-A cross-sectional view of the cooling and purging device for the camera endoscope of the present invention; Figure 6 It is a B-B cross-sectional view of the cooling and purging device for the camera endoscope of the present invention; Figure 7 It is an A-A cross-sectional view of the cooling and purging device for the laser endoscope of the present invention; Figure 8 It is a B-B cross-sectional view of the cooling and purging device for the laser endoscope of the present invention; Figure 9 It is a schematic installation diagram of the laser endoscope of the present invention in the combustion chamber; Figure 10 Schematic diagram of the purging direction of the end face of the endoscope of the present invention; Figure 11 Schematic diagram of calibrating the PIV measurement system and correcting imaging distortion of the present invention; Figure 12 Schematic diagram of the coolant circulation system; Figure 13 Schematic diagram of the gas supply system of the purging gas device. Detailed implementation manners

[0018] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways defined and covered by the following.

[0019] The following will be further described in detail with reference to the attached Figures 1-13 This application will be further described in detail.

[0020] The embodiments of this application disclose an endoscopic PIV measurement system and a test method for the internal flow field of a recirculation combustion chamber.

[0021] Referring to Figure 1 , this embodiment provides an endoscopic PIV measurement system for the internal flow field of a recirculation combustion chamber. This system is applicable to complex environments with high temperature and limited space, and can realize non-contact, transient, and full-field measurement of the real flow state inside the combustion chamber without damaging the combustion chamber structure. The system includes: Computer: Used to collect and process image data and control the working process of the system; Synchronization controller: Used to realize the synchronous control of the laser and the camera; Laser: Emits a double-pulse laser beam for irradiating tracer particles in the flow field; Light guide arm: A flexible optical fiber component for guiding the laser from the laser to the laser endoscope; Laser endoscope: Used to introduce the laser output by the light guide arm into the combustion chamber. A Powell prism is installed at its end and is flush with the end face of the detection end of the cooling and purging device to form a sheet laser with uniform energy distribution; Camera: Used to capture two frames of images formed by tracer particles; Camera endoscope: Used to transmit the internal image of the combustion chamber to the photosensitive element of the camera; Tracer particle generator: Used to generate particles with a particle size of 1-2 μm and an appropriate concentration, which enter the measurement area with the air flow; Test piece: That is, the structure of the recirculation combustion chamber to be measured.

[0022] Based on the problems that the endoscopic PIV measurement system is prone to problems such as endoscopic mirror surface contamination, overheating and damage of optical components, image distortion, and decline in measurement accuracy in high-temperature, strongly polluted, and confined space environments such as aeroengine recirculation combustors, and the reason that it is difficult to achieve continuous and stable operation through a single cooling or cleaning method in traditional solutions, a cooling and purging device is set up in this embodiment.

[0023] Referring to Figures 2-4 , the cooling and purging device includes two parts: a connection end and a detection end. Among them, the detection end is used to be inserted into the internal part of the measured recirculation combustor, and the connection end is located in the external platform area. There are three types of non-communicating functional channels inside the cooling and purging device, namely a coolant channel, a purging gas channel, and an endoscope channel. The functions of each channel are clear and the structure is compact. The specific structure is as follows: The coolant channel includes a coolant inlet channel and a coolant outlet channel. The coolant inlet channel introduces low-temperature coolant from the connection end and transports it to the detection end through the channel; near the detection end, the coolant is introduced into the coolant outlet channel through a through hole provided on the partition plate and flows back to the connection end for discharge, forming a closed-loop cooling path. This structure is used to reduce the working temperature of the end of the endoscope probe in the high-temperature combustion environment, avoid problems such as thermal expansion, defocusing, or ablation of the optical lens, and ensure the stable operation of the optical system.

[0024] The purging gas channel includes an air inlet provided at the connection end and an air outlet provided at the detection end. The detection end is provided with an air flow guiding structure for guiding the purging gas (such as compressed air or nitrogen) to flow along the device end face close to the wall to the opening end of the endoscope channel. In this embodiment, the guiding structure is a bent air groove, which makes the air flow at the air outlet turn 90 degrees and flow along the end face of the detection end. Through the above structural design, a stable air curtain is formed on the lens surface by the purging gas, effectively blocking the forward impact and adhesion of high-temperature combustion gas and soot particles, preventing mirror surface contamination, and improving imaging clarity and system reliability.

[0025] The endoscope channel runs through the central axis of the cooling and purging device and is used to install a laser endoscope or a camera endoscope. The through hole size of this channel is closely matched with the outer diameter of the inserted endoscope. When necessary, heat-conducting materials (such as heat-conducting silicone grease, graphite sheets, etc.) can be filled in the gap to enhance heat conduction and heat dissipation effects. When installing the endoscope, its end face should be flush with the end face of the detection end to ensure continuous optical path and facilitate the formation of a uniform coverage by the purging air flow.

[0026] The endoscope channel is located between the coolant inlet channel and the coolant outlet channel. The coolant forms a symmetric circulation or an enclosed cooling path when flowing through both sides of the endoscope channel, so that heat can be conducted more evenly from the endoscope probe to the outside and taken away by the coolant, significantly improving the cooling effect.

[0027] In a specific embodiment, the cooling and purging device adopts an overall coaxial structure design, consisting of an inner sleeve and an outer sleeve to form a double-layer cylinder, and a plurality of radial partitions arranged along the axial direction are provided between the inner and outer cylinders. Through the partitioning effect of the partitions, functional cavities isolated from each other are formed between the inner and outer sleeves, including a coolant inlet cavity, a coolant outlet cavity, and a purging gas channel cavity. The partitions can be made by machining, welding, or modular assembly, or can also be processed by additive manufacturing. Through the fine separation of the space between the inner and outer cylinders, each channel can operate independently within a limited volume, avoiding the occurrence of unstable factors such as gas-liquid mixing. Secondly, the axially arranged partitions help to guide the coolant and purging gas to form a stable and directional flow path in the channels, thereby improving the heat transfer efficiency and the accuracy of air flow control. In addition, this structure is conducive to processing and assembly, the internal cavities are regular and clear, facilitating the realization of standardized manufacturing and later maintenance, and helping to improve the engineering reliability and service life of the overall system.

[0028] In practical applications, the cooling and purging device can be customized according to the specific endoscope size and installation method. A standard interface is provided on the outside of the connection end for quickly connecting the coolant cooling pipeline and the purging gas source pipeline. The whole device is fixed to the outer shell of the combustion chamber test piece through a positioning flange or a clamping mechanism, and the end face of the detection end is flush with the inner wall surface, which not only meets the requirements of the optical path layout but also can minimize the interference with the original flow field to the greatest extent. Through the composite layout of the channel structure, dual cooling of gas and liquid, and the end-face wall-attached purging design, the instability of the endoscope system operating in extreme environments is solved, ensuring the long-term and high-quality data acquisition ability of the PIV system.

[0029] For the laser endoscope and the camera endoscope, there is one cooling and purging device respectively, and there are slight differences in the structures of the two cooling and purging devices. Referring to Figure 5 and Figure 6 , the endoscope channel of the cooling and purging device for the camera endoscope is a through cylindrical shape; referring to Figure 7 and Figure 8 , the endoscope channel of the cooling and purging device for the laser endoscope is provided with a step near the detection end, and a threaded hole is provided in the step for installing a Powell prism.

[0030] Referring to Figure 12 and 13 , to achieve the continuous and stable supply of the purging gas and the coolant, in this embodiment, a supporting gas supply system and a coolant cooling system are further provided, and the two are respectively connected to the purging gas channel and the coolant channel, jointly constituting the basic support system of the cooling and anti-fouling control module of the present invention. The adjustable purging gas volume of the endoscope can be achieved through a regulating valve, and the temperature of the coolant can be adjusted through a constant temperature device.

[0031] To achieve high-quality PIV (Particle Image Velocimetry) measurement of the internal flow field of an aeroengine's recirculation combustor, the embodiments of the present invention use a laser endoscope to introduce a laser beam into the combustor interior, and a Powell prism is arranged at the outlet end of the laser endoscope to convert the laser beam into a sheet laser beam with uniform energy distribution. This structure can effectively provide a laser measurement light source with illumination uniformity and direction accuracy under the conditions of no window opening in the combustor and extremely limited optical path space, which is the key to achieving high-precision PIV measurement.

[0032] The laser endoscope is connected to an external laser through a light guiding structure. Specifically, a light guiding arm is used to guide the double-pulse laser beam output by the laser to the inlet end of the laser endoscope. The light guiding arm can be a flexible optical fiber light guiding component or a mirror reflection light guiding pipe, which is used to achieve long-distance laser transmission in an environment with limited space. The light guiding arm has the characteristics of strong bending resistance, high thermal stability, and high transmission efficiency, and is suitable for long-term use in the high-temperature and vibration-complex structural environment around the combustor.

[0033] The laser endoscope is a slender rod-shaped structure, and a collimating lens and an optical channel are arranged inside it to achieve the directional transmission of the laser. To ensure the formation of a high-quality sheet light surface in the measurement area, a Powell prism is arranged at the outlet end of the laser endoscope in this embodiment. The Powell prism is an aspherical optical element, and its working principle is to reconstruct the incident Gaussian collimated laser beam into a fan-shaped laser light sheet with uniform transverse energy distribution. Compared with the traditional cylindrical lens, the Powell prism can effectively solve the problem of uneven energy distribution with bright center and dark edges of the sheet laser, and significantly improve the illumination uniformity and image contrast.

[0034] The Powell prism is installed in the front-end channel of the laser endoscope through a threaded connection method, and its end face is flush with the end face of the detection end of the cooling and purging device. This design, on the one hand, ensures the unobstructed divergence of the sheet laser beam, and on the other hand, the wall-attached flow formed by the purging gas at the end face can cool and prevent pollution of the Powell prism, avoiding thermal damage and soot pollution during the laser transmission process, and ensuring the cleanliness of the illumination surface and the stability of the divergence angle. In practical applications, according to the size of the measurement area and the laser energy requirements, the expansion angle of the Powell prism can be selected as 10°, 20°, 30°, 45°, etc., and the specific parameters can be selected according to the internal optical path layout of the combustor.

[0035] To achieve clear and low-distortion image acquisition of the internal flow field of an aeroengine reverse flow combustor, an extendable adapter ring is provided between the camera and the camera endoscope in the embodiments of the present invention. The extendable adapter ring is a mechanical structure located between the camera and the optical component, and its length is adjustable. It is used to compensate for the distance error between different camera flange distances (i.e., the distance from the lens interface surface to the imaging sensor) and the exit pupil position of the endoscope, so as to achieve coplanar matching of the imaging plane of the optical system. When replacing cameras or endoscopes of different models, only the length of the adapter ring needs to be adjusted to achieve quick re-matching, without replacing the main structural components, significantly improving the versatility of the system hardware and the test debugging efficiency.

[0036] To solve the problems of small imaging circle size, large vignetting at the edge, and serious image distortion of traditional endoscopes, an optical interface lens with a focal length of not less than 75 mm is provided between the extendable adapter ring and the camera endoscope in this embodiment. By extending the imaging path and optimizing the beam convergence angle, this optical interface can significantly expand the effective imaging circle formed on the camera photosensitive chip, reduce the vignetting phenomenon in the edge area of the image, and effectively control imaging distortion, improving image quality and particle image clarity.

[0037] The optical interface is preferably a fixed-focus lens with a focal length set to be not less than 75 mm, which can not only provide better spatial resolution but also avoid the loss of depth of field caused by excessive magnification. In addition, the lens should have good chromatic aberration correction performance and low distortion characteristics to meet the strict requirements of PIV image processing for image geometric accuracy.

[0038] This embodiment also discloses an endoscopic PIV measurement method for the internal flow field of a reverse flow combustor, including the following steps: Step S1: Assembly and fixation of the combustor test piece.

[0039] Prepare the test section of the aeroengine reverse flow combustor according to the test requirements, and appropriately process its structure. Preset the installation hole positions of the laser endoscope and the camera endoscope at the specified positions. Fix the combustor test piece on the support frame or the test platform to ensure its stable position during operation without vibration or displacement, providing a good installation basis for subsequent laser illumination and image acquisition. The function of this step is to ensure the mechanical stability of the test environment and avoid measurement errors or data distortion caused by the loosening of the test piece.

[0040] Step S2: Assembly of the endoscopic PIV measurement system for the internal flow field of the reverse flow combustor.

[0041] Step S2 includes the following steps: Step S21: Connection of the PIV measurement system. Connect the computer to the synchronization controller, connect the synchronization controller to the laser and the camera respectively, and connect the laser outlet to the light guide arm; First, complete the master control and trigger connection of the PIV system. Connect the host computer (computer) to the synchronization controller for data, and connect the synchronization controller to the laser and the camera respectively to achieve synchronous control and time triggering of the two. This connection method ensures strict timing between the double-pulse laser emitted by the laser and the double-frame exposure of the camera, providing guarantee for the accuracy of subsequent particle image acquisition. At the same time, the outlet end of the laser is connected to the laser endoscope through an optical light guide component (light guide arm) to establish a laser transmission path. The function of this step is to construct a control closed-loop between laser illumination and image acquisition, ensuring precise synchronization of the laser and the camera within the microsecond level, so as to ensure the certainty of the particle imaging interval and the accurate solution of the velocity vector.

[0042] Step S22: Connect the outlet of the light guide arm to the laser endoscope. According to the size of the measurement area, select a Powell prism with an appropriate divergence angle, and install the Powell prism at the end of the laser endoscope through threads to form a sheet-like laser. Accurately couple the laser beam output by the light guide arm to the inlet port of the laser endoscope, so that the laser is transmitted along the optical axis of the endoscope to the outlet end. According to the planar size and angular requirements of the measurement area inside the combustion chamber, select a Powell prism with a specific divergence angle (such as 10°, 20°, 30°, etc.), and install it at the outlet end of the laser endoscope through threads. The Powell prism can convert the Gaussian laser beam into a fan-shaped sheet beam with uniform energy distribution to achieve planar illumination. The function of this step is to construct a high-quality measurement laser light sheet to meet the strict requirements of endoscopic PIV for the illumination uniformity, imaging clarity and particle excitation consistency of the measurement area.

[0043] Step S23: Connect the camera endoscope probe to the optical interface, and then connect the optical interface to the camera. Connect the rear end of the camera endoscope to the optical interface with a preselected focal length (≥75 mm), and connect it to the camera body through the optical interface. The camera endoscope is used to guide the internal image of the combustion chamber to the camera sensor chip; the optical interface can perform secondary collimation or imaging quality optimization processing on the optical path entering the camera. By reasonably matching the optical path systems of the camera, the interface lens and the endoscope, a larger imaging circle can be obtained on the camera imaging chip, reducing image vignetting and edge distortion, and improving the particle image resolution and recognition rate. The function of this step is to construct a stable and high-quality imaging optical path system, providing a reliable image basis for subsequent particle image acquisition and velocity vector calculation.

[0044] Step S24: Connect the cooling and purging device to the outlets of the coolant cooling device and the purge gas device. The endoscope channel is closely fitted with the outer diameter of the endoscope, and the end is flush with the detection end face of the device; subsequently, the cooling and purging device is connected to the coolant circulation system and the air supply system, which are respectively used to provide coolant and purging gas. The coolant channel is connected to a constant-temperature coolant circulation system to achieve efficient cooling of the front-end lens of the endoscope; the purging gas channel is connected to a high-pressure clean air source or a nitrogen cylinder to prevent soot particles from adhering to the mirror surface.

[0045] Step S3: Under open conditions, calibrate the PIV measurement system and correct the imaging distortion. When calibrating, the positional relationship between the camera endoscope and the laser endoscope is determined according to their positional relationship during subsequent PIV measurement, and at the same time, adjust and ensure that the two laser beams coincide.

[0046] Refer to Figure 11 , calibrating the PIV measurement system and correcting the imaging distortion include the following steps: Set a ground glass on the back of the calibration plate, and arrange a surface light source (such as a point light source, a surface light source, a volume light source, etc.) behind the ground glass. The illumination light emitted by the light source first irradiates on the surface of the ground glass and is converted into uniform diffuse reflection light under the action of its rough structure, and then irradiates on the calibration plate in a soft and diffused form, so as to form an image with high brightness, uniform distribution and no strong contrast at the edge in the camera's field of view. In order to further improve the imaging quality, cooperate with the camera to set the long exposure mode to fully receive the low-intensity reflected light and avoid low image gray level or large noise caused by insufficient illumination. The technical effect of this method is to effectively reduce the image non-uniformity caused by problems such as local reflection, backlight, and vignetting, improve the recognizability and geometric stability of the calibration image, thereby enhancing the accuracy of image distortion correction, and providing an accurate calibration basis for pixel-physical mapping and velocity field calculation in the subsequent flow field measurement of the PIV system. During the calibration process, the relative spatial relationship between the laser endoscope and the camera endoscope needs to be kept consistent with their subsequent installation positions, and the double-pulse laser is ensured to completely coincide on the same plane by adjusting the laser path. The function of this step is to obtain accurate pixel-physical coordinate conversion parameters and distortion correction coefficients, providing a geometric basis for velocity calculation.

[0047] Step S4: Install and fix the laser endoscope and the camera endoscope on the combustion chamber test piece respectively, ensure that the laser endoscope and the camera endoscope are arranged vertically, and at the same time make the ends of the laser endoscope probe and the camera endoscope probe flush with the wall surface, and make the purging gas direction of the endoscope end consistent with the combustion chamber gas flow direction at its position.

[0048] Refer to Figure 10, after calibration, insert the laser endoscope and the camera endoscope into the reserved installation holes of the test piece respectively, and fix them integrally with the cooling and purging device. During the installation process, it should be ensured that the laser endoscope and the camera endoscope are arranged vertically to form a measurement plane; and the end faces of the two endoscopes need to be flush with the inner wall of the combustion chamber to prevent interference with the mainstream structure. At the same time, adjust the cooling and purging structure so that the outlet direction of the purging gas is consistent with the internal gas flow direction of the combustion chamber to achieve wall attachment flow and reduce the interference with the mainstream flow field. This step ensures the stability of the laser illumination and imaging path, and at the same time realizes the cooling and anti-pollution effects.

[0049] Step S5: Adjust the test conditions to reach the specified technical requirements state, and at the same time adjust the tracer particle generator so that the tracer particle concentration is 6 - 8 tracer particles per interrogation domain; Start the test system, adjust the working condition parameters such as the intake air, fuel supply, and ignition of the combustion chamber, increase the fuel supply, and make the internal flow field reach the target operating state. Synchronously adjust the tracer particle generator to control the particle concentration so that there are an average of 6 - 8 particles in each interrogation area (i.e., the cross-correlation calculation grid) in the image. This concentration range can ensure the clarity of the image while improving the correlation of particle matching between image frames, thereby enhancing the stability and accuracy of velocity calculation.

[0050] Step S6: The computer issues a collection command, and the laser and the camera work synchronously under the control of the synchronization controller, and collect the original images of the tracer particles; After the test conditions are stable, the computer sends a collection command to the synchronization controller. The synchronization controller simultaneously triggers the laser to emit double-pulse laser and controls the camera to complete double-exposure imaging. The laser irradiates the particles in the sheet area and scatters them instantaneously twice for imaging, and the camera captures two frames of particle images and saves them to the computer. This step ensures time synchronization and data integrity, and is the key to obtaining the velocity field.

[0051] Step S7: The computer performs cross-correlation algorithm calculation on the tracer particle pictures to obtain the velocity field within the entire measurement section.

[0052] After the collection is completed, use the cross-correlation image processing algorithm to process the two frames of particle images, calculate the particle displacement and convert it into fluid velocity information. The function of this step is to convert the original image data into physical quantity results, constituting the final output data of PIV measurement.

[0053] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. Endoscopic PIV measurement system for flow field in recirculation combustion chamber, characterized by: It includes a cooling and purge device and a laser endoscope and / or a camera endoscope arranged in the cooling and purge device; The purge cooling device includes a detection end for extending into the reflow combustion chamber and a connection end located outside the combustion chamber, and the cooling purge device is provided with a coolant channel, a purge gas channel and an endoscope channel which are not connected to each other; The endoscope channel is used to accommodate a laser endoscope and / or a camera endoscope; The coolant channel includes a coolant inlet channel from the connecting end to the detection end and a coolant outlet channel from the detection end to the connecting end; The purge gas channel includes an air inlet at the connecting end and an air outlet at the detecting end, and the detecting end of the purge cooling device is provided with an air flow guiding structure for guiding the gas from the air outlet to flow along the end surface of the detecting end to the end surface of the endoscope channel.

2. The endoscopic PIV measurement system for flow field in a reflow combustion chamber according to claim 1, characterized in that: The endoscope channel is located between the cooling liquid inlet channel and the cooling liquid outlet channel.

3. The endoscopic PIV measurement system for flow field in a reflow combustion chamber according to claim 2, characterized in that: The purge cooling device includes an inner sleeve, an outer sleeve and a plurality of partitions connecting the inner sleeve and the outer sleeve. An endoscope channel is formed in the inner sleeve. The coolant inlet channel, the coolant outlet channel and the purge gas channel are located between the inner sleeve and the outer sleeve and are separated by a plurality of partitions. A through hole is provided on the partition between the coolant inlet channel and the coolant outlet channel.

4. The endoscopic PIV measurement system for flow field in a reflow combustion chamber according to claim 1, characterized in that: A Powell prism is provided at the end of the laser endoscope. The Powell prism is located in the endoscope channel, and the end face of the Powell prism is flush with the end face of the detection end.

5. The endoscopic PIV measurement system for flow field in a reflow combustion chamber according to claim 4, characterized in that: It also includes a camera and a laser. The camera and the camera endoscope are connected through a retractable adapter ring to adjust the imaging distance between the camera and the endoscope. The laser and the laser endoscope are connected through a light guide arm.

6. The endoscopic PIV measurement system for flow field in a reflow combustion chamber according to claim 5, characterized in that: An optical interface with a focal length of not less than 75 mm is arranged between the telescopic adapter ring and the camera endoscope.

7. The endoscopic PIV measurement system for flow field in a reflow combustion chamber according to claim 5, characterized in that: The laser endoscope and the camera endoscope are arranged vertically, and their optical paths intersect in the flow field inside the combustion chamber.

8. A method for measuring the flow field in a recirculating combustion chamber by endoscopic PIV, using the recirculating combustion chamber flow field endoscopic PIV measurement system according to any one of claims 1 to 7, characterized in that: The steps include: Step S1: assembling and fixing the combustion chamber test piece; Step S2: assembling the endoscopic PIV measurement system for the flow field in the reflow combustion chamber; Step S3: Under open conditions, the PIV measurement system is calibrated and the imaging distortion is corrected. During calibration, the positional relationship between the camera endoscope and the laser endoscope is determined according to the positional relationship between the two during subsequent PIV measurement, and the two laser beams are adjusted to ensure overlap; Step S4: respectively install and fix the laser endoscope and the camera endoscope on the combustion chamber test piece, ensure that the laser endoscope and the camera endoscope are arranged vertically, and at the same time, the ends of the laser endoscope probe and the camera endoscope probe are flush with the wall surface, and the purge gas direction of the endoscope end is consistent with the gas flow direction of the combustion chamber where it is located; Step S5: adjusting the test conditions to meet the specified technical requirements, and adjusting the tracer particle generator so that the tracer particle concentration is 6 to 8 tracer particles for each query domain; Step S6: the computer issues an acquisition instruction, the laser and the camera work synchronously under the control of the synchronization controller, and acquire the original image of the tracer particles; Step S7: The computer performs cross-correlation calculation on the tracer particle image to obtain the velocity field in the entire measurement section.

9. The method for measuring the flow field in a recirculation combustion chamber by endoscopic PIV according to claim 8, characterized in that: Step S2 includes the following steps: S21: PIV measurement system connection, connect the computer to the synchronization controller, the synchronization controller is connected to the laser and the camera respectively, and the laser outlet is connected to the light guide arm; S22: The outlet of the light guide arm is connected to a laser endoscope. A Powell prism with a suitable divergence angle is selected according to the size of the measurement area. The Powell prism is installed at the end of the laser endoscope through a thread to form a sheet laser. S23: The camera endoscopy probe is connected to the optical interface, and the optical interface is connected to the camera; S24: Connect the cooling and purge device to the outlet of the coolant cooling device and the outlet of the purge gas device.

10. The method for measuring the flow field in a reflow combustion chamber by endoscopic PIV according to claim 8, characterized in that: The calibration and image distortion correction of the PIV measurement system includes the following steps: A frosted glass is set on the back side of the calibration plate, and a light source is set on the back side of the frosted glass. The light from the light source is first irradiated on the frosted glass, and then passes through the frosted glass and irradiates the calibration plate in the form of diffuse reflection light. At the same time, the camera is used for long exposure to obtain a calibration original image with high brightness and uniform light intensity distribution.

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