Endoscopic PIV measurement system and method for flow field in recirculation combustion chamber

Abstract

The present invention discloses an endoscopic PIV measurement system and method for the flow field in a recirculating combustion chamber, which belongs to the technical field of flow field measurement in a recirculating combustion chamber of an aircraft engine. The endoscopic PIV measurement system for the flow field in a recirculating combustion chamber includes a cooling and purge device and a laser endoscope and / or a camera endoscope arranged in the cooling and purge device; the cooling and purge device is provided with a coolant channel, a purge gas channel and an endoscope channel that are not connected to each other; 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 located at the connecting end and an air outlet located at the detection end. The present application provides an airflow guiding structure at the detection end, so that the purge gas flows along the end face of the detection end to the end face of the endoscope channel, forming a stable wall-adhering airflow barrier, which blocks the high-temperature combustion gas and carbon smoke from directly contacting the endoscope mirror surface, and effectively prevents the mirror surface from being contaminated.

Description

Technical Field

[0001] The present invention relates to the technical field of flow field measurement in a recirculating combustion chamber of an aero-engine, and in particular to an endoscopic PIV measurement system and a measurement method for the flow field in a recirculating combustion chamber. Background Art

[0002] Due to the complex structure and confined interior space of aircraft engine combustion chambers, measuring the flow field within them is extremely difficult. Early methods used Pitot tubes and hot-wire thermofilm instruments to measure the flow field. However, these methods are contact-based, single-point measurements that significantly interfere with the flow field and result in low measurement accuracy. With the rapid development of computer technology, laser technology, and image processing, particle image velocimetry (PIV) has emerged. It enables non-contact, transient, and full-field flow field measurements and has gradually become the primary method for measuring flow fields within aircraft engine combustion chambers. Given the structural complexity of aircraft engine combustion chambers, directly applying PIV to flow field measurements within them presents several technical challenges, including the difficulty of achieving optimal optical path arrangement. To ensure a clear optical path, existing approaches have simplified the structure of the test piece and introduced large-scale windowing to ensure smooth and optimal optical path arrangement during measurement. However, this structural simplification and large-scale windowing disrupt the original structure of the test piece, significantly altering the inherent flow characteristics within the test piece and preventing the acquisition of true flow characteristics.

[0003] Since endoscopic technology can break through the optical path limitations of traditional PIV measurement technology in the confined space of aircraft engine combustion chambers, it provides a possibility for confined space flow field measurement and has great development potential. It can reduce the optical path requirements when measuring internal flow fields and does not require simplification of the object to be measured or large-area windowing. However, when directly applied to the confined space of aircraft engine combustion chambers, there are many technical difficulties, especially the optical glass surface at the front end of the endoscope is contaminated by high-temperature carbon soot and is difficult to cool, which seriously affects the measurement effect. Summary of the Invention

[0004] The present invention provides an endoscopic PIV measurement system and method for the flow field in a recirculating combustion chamber, so as to solve the technical problem that when endoscopic technology is applied to PIV measurement in an engine internal combustion chamber, the authenticity of the measurement results is affected by high-temperature carbon soot contamination and cooling difficulties.

[0005] According to one aspect of the present invention, there is provided an endoscopic PIV measurement system for the flow field in a recirculating combustion chamber, comprising 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 comprises a detection end for extending into the recirculating combustion chamber and a connecting end located outside the combustion chamber, and the cooling and purge device is provided with a cooling liquid channel, a purge gas channel and an endoscope channel that are not connected to each other; the endoscope channel is used to accommodate the laser endoscope and / or the camera endoscope; 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 purge gas channel comprises an air inlet located at the connecting end and an air outlet located at the detection end, and the detection end of the purge cooling device is provided with an airflow guide structure for guiding the gas at the air outlet to flow along the end face of the detection end to the end face of the endoscope channel.

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

[0007] Optionally, the purge cooling device includes an inner sleeve, an outer sleeve, and multiple 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 multiple partitions. Through holes are opened on the partitions 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 in the endoscope channel, and the end face of the Powell prism is flush with the end face of the detection end.

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

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

[0011] Optionally, the laser endoscope and the camera endoscope are arranged vertically, and their optical paths intersect in the flow field inside the combustion chamber.

[0012] According to another aspect of the present invention, a method for endoscopic PIV measurement of the flow field in a recirculating combustion chamber is provided, which comprises the following steps:

[0013] Step S1: Assembling and fixing the combustion chamber test piece;

[0014] Step S2: assembling the endoscopic PIV measurement system for the flow field in the recirculation combustion chamber;

[0015] 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 based on the positional relationship between the two during subsequent PIV measurement. At the same time, adjustments are made to ensure that the two laser beams coincide.

[0016] Step S4: The laser endoscope and the camera endoscope are respectively mounted and fixed on the combustion chamber test piece, ensuring that the laser endoscope and the camera endoscope are arranged perpendicularly, and the ends of the laser endoscope probe and the camera endoscope probe are flush with the wall surface, and the direction of the purge gas at the end of the endoscope is consistent with the direction of the combustion chamber airflow at its location;

[0017] Step S5: Adjust the test conditions to meet the specified technical requirements, and at the same time adjust the tracer particle generator so that the tracer particle concentration is 6 to 8 tracer particles per query domain;

[0018] Step S6: The computer issues an acquisition instruction, and the laser and camera work synchronously under the control of the synchronization controller and acquire the original image of the tracer particles;

[0019] Step S7: The computer performs cross-correlation calculation on the tracer particle image to obtain the velocity field in the entire measurement section.

[0020] Optionally, step S2 includes the following steps:

[0021] Connect the PIV measurement system, connect the computer to the synchronization controller, connect the synchronization controller to the laser and camera respectively, and connect the laser outlet to the light guide arm;

[0022] The outlet of the light guide arm is connected to the laser endoscope. According to the size of the measurement area, a Powell prism with a suitable divergence angle is selected and installed on the end of the laser endoscope through a thread to form a sheet laser.

[0023] The camera endoscope probe is connected to the optical interface, which is then connected to the camera;

[0024] Connect the cooling and purge device to the coolant cooling device outlet and the purge gas device outlet.

[0025] Optionally, calibrating the PIV measurement system and correcting imaging distortion may include the following steps:

[0026] A frosted glass is set on the back of the calibration plate, and a light source is set on the back of the frosted glass. The light from the light source first shines on the frosted glass, and then shines on the calibration plate in the form of diffuse reflection after passing through the frosted glass. 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.

[0027] In summary, this application includes at least one of the following beneficial technical effects:

[0028] By constructing a cooling and purging device and arranging independent cooling liquid channels, purge gas channels and endoscope channels therein, efficient cooling and effective anti-fouling of the laser endoscope and / or camera endoscope are achieved, thereby solving the technical problem of distortion of measurement results due to high-temperature carbon soot pollution and cooling difficulties in endoscopic PIV measurement. Specifically, the cooling liquid channel includes a liquid inlet channel from the connecting end to the detection end and a liquid outlet channel from the detection end back to the connecting end, which can realize coolant circulation cooling and effectively reduce the operating temperature of the endoscope probe in a high-temperature combustion environment. Similarly, this device can also be used in extremely cold temperatures; at the same time, the purge gas channel guides the cold air flow to enter from the connecting end, and at the detection end, through the airflow guide structure provided, the purge gas flows along the end face of the detection end to the end face of the endoscope channel, forming a stable wall-adhering airflow barrier, blocking the high-temperature combustion gas and carbon soot from directly contacting the endoscope mirror surface, and effectively preventing mirror surface contamination. The above cooling and purge design ensures that the optical components at the front end of the endoscope still have good imaging clarity and reliability in high-temperature and high-pollution environments, and guarantees the authenticity and stability of the data during the PIV measurement process.

[0029] In addition to the above-described objects, features and advantages, the present invention has other objects, features and advantages. The present invention will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings, which constitute part of this application, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0031] Figure 1 Schematic diagram of the endoscopic PIV measurement system for the flow field in a recirculating combustion chamber according to the present invention;

[0032] Figure 2 It is a structural schematic diagram of the cooling and purge device of the present invention;

[0033] Figure 3 A side view of the cooling and purge device of the present invention;

[0034] Figure 4 It is a cross-sectional schematic diagram of the cooling and purge device of the present invention;

[0035] Figure 5 This is a cross-sectional view of the cooling and purge device for the camera endoscope according to the present invention;

[0036] Figure 6 BB is a cross-sectional view of the cooling and purge device for the camera endoscope of the present invention;

[0037] Figure 7 This is a cross-sectional view of the cooling and purge device for the laser endoscope according to the present invention;

[0038] Figure 8 BB is a cross-sectional view of the cooling and purge device for laser endoscope of the present invention;

[0039] Figure 9 This is a schematic diagram of the installation of the laser endoscope of the present invention in a combustion chamber;

[0040] Figure 10 Schematic diagram of the sweeping direction of the end face of the endoscope of the present invention;

[0041] Figure 11 A schematic diagram of the present invention for calibrating a PIV measurement system and correcting imaging distortion;

[0042] Figure 12 Schematic diagram of the coolant circulation system;

[0043] Figure 13 Schematic diagram of the air supply system for the purge gas device. DETAILED DESCRIPTION

[0044] The embodiments of the present invention are described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways as defined and covered below.

[0045] The following is combined with Figure 1-13 This application is described in further detail.

[0046] The embodiments of the present application disclose an endoscopic PIV measurement system and a testing method for the flow field in a reflow combustion chamber.

[0047] Reference Figure 1 This embodiment provides an endoscopic PIV measurement system for the flow field within a recirculating combustion chamber. This system is suitable for complex environments with high temperatures and limited space. It can achieve non-contact, transient, and full-field measurement of the actual flow state within the combustion chamber without damaging the combustion chamber structure. The system includes:

[0048] Computer: used to collect and process image data and control system workflow;

[0049] Synchronous controller: used to achieve synchronous control of laser and camera;

[0050] Laser: emits a double-pulse laser beam to irradiate tracer particles in the flow field;

[0051] Light guide arm: a flexible fiber optic component used to guide laser light from the laser to the laser endoscope;

[0052] Laser endoscope: used to guide the laser output from the light guide arm into the combustion chamber. A Powell prism is installed at its end, and it is flush with the end surface of the cooling and purge device to form a sheet laser with uniform energy distribution.

[0053] Camera: used to capture two frames of images formed by tracer particles;

[0054] Camera endoscope: used to transmit the image inside the combustion chamber to the camera photosensitive element;

[0055] Tracer particle generator: used to generate particles with a diameter of 1~2μm and appropriate concentration, which enter the measurement area with the air flow;

[0056] Test piece: the recirculation combustion chamber structure to be tested.

[0057] Because the endoscopic PIV measurement system is prone to problems such as endoscope mirror contamination, optical component overheating and damage, image distortion, and decreased measurement accuracy in high-temperature, highly polluted, and confined space environments such as the recirculation combustion chamber of an aircraft engine, and because it is difficult to achieve continuous and stable operation through a single cooling or cleaning method in traditional solutions, a cooling and purge device is provided in this embodiment.

[0058] Reference Figure 2-Figure 4 The cooling and purge device consists of a connection end and a detection end. The detection end is used to insert into the recirculation combustion chamber under test, and the connection end is located in the external platform area. The cooling and purge device is internally provided with three types of functional channels that are not interconnected, namely the coolant channel, the purge gas channel, and the endoscope channel. Each channel has a clear function and a compact structure. The specific structure is as follows:

[0059] The coolant channel consists of a coolant inlet and a coolant outlet. The coolant inlet channel draws low-temperature coolant from the connection end and delivers it to the detection end. Near the detection end, the coolant is directed into the coolant outlet channel through holes in the baffle, where it flows back to the connection end for discharge, forming a closed-loop cooling path. This structure is used to reduce the operating temperature of the endoscope probe tip in high-temperature combustion environments, preventing thermal expansion, defocusing, or ablation of the optical lens, and ensuring stable operation of the optical system.

[0060] The purge gas channel includes an air inlet at the connection end and an air outlet at the detection end. The detection end is equipped with an airflow guide structure to guide the purge gas (e.g., compressed air or nitrogen) along the end face of the device and along the wall to the open end of the endoscope channel. In this embodiment, the guide structure bends the air groove, causing the airflow at the air outlet to make a 90-degree turn and flow along the end face of the detection end. Through this structural design, the purge gas forms a stable air curtain on the lens surface, effectively blocking the direct impact and adhesion of high-temperature combustion gas and carbon soot particles, preventing mirror contamination, and improving image clarity and system reliability.

[0061] The endoscope channel runs through the center axis of the cooling and purge unit and is used to mount a laser or camera endoscope. The channel's through-hole dimensions closely match the outer diameter of the endoscope being inserted. If necessary, the gap can be filled with thermally conductive material (such as thermal grease or graphite sheets) to enhance heat conduction and dissipation. When installed, the endoscope's end face should be flush with the end face of the detection end to ensure a continuous optical path and facilitate even coverage of the purge airflow.

[0062] The endoscope channel is located between the coolant inlet and outlet channels. This allows the coolant to form a symmetrical circulation or surrounding cooling path as it flows through both sides of the endoscope channel. This allows heat to be more evenly transferred from the endoscope probe to the outside and carried away by the coolant, significantly improving the cooling effect.

[0063] In a specific embodiment, the cooling and purge device adopts a coaxial structural design as a whole, with an inner sleeve and an outer sleeve forming a double-layer cylinder, and a plurality of radial partitions arranged in the axial direction are provided between the inner and outer cylinders. Through the partitioning effect of the partition, functional cavities separated from each other are formed between the inner and outer sleeves: including a coolant inlet cavity, a coolant outlet cavity and a purge gas channel cavity. The partition can be made by machining, welding or modular assembly, and can also be processed by additive manufacturing. By finely separating the space between the inner and outer cylinders, each channel operates independently within a limited volume, avoiding the occurrence of unstable factors such as gas-liquid mixing. Secondly, the axially arranged partitions help guide the coolant and purge gas to form a stable and directional flow path in the channel, thereby improving the heat transfer efficiency and airflow control accuracy. In addition, this structure is conducive to processing and assembly, and the internal cavity has clear rules, which facilitates standardized manufacturing and subsequent maintenance, helping to improve the engineering reliability and service life of the overall system.

[0064] In actual applications, the cooling and purging device can be customized according to the specific size and installation method of the endoscope. A standard interface is provided on the outside of the connection end for quick connection of the coolant cooling pipeline and the purge gas source pipeline. The entire device is fixed to the outer shell of the combustion chamber test piece through a positioning flange or a clamping mechanism. 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 minimizes the interference with the original flow field. Through the composite arrangement of the channel structure, dual cooling of gas and liquid, and the end face wall-mounted purging design, the instability of the endoscope system in extreme environments is solved, ensuring the long-term and high-quality data acquisition capability of the PIV system.

[0065] The laser endoscope and the camera endoscope are each provided with a cooling and purge device, and the two cooling and purge devices have slight differences in structure. Figure 5 and Figure 6 , the endoscope channel of the cooling and purging device for the camera endoscope is a through cylindrical shape; Figure 7 and Figure 8The 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 the Powell prism.

[0066] Reference Figure 12 and 13 To ensure a continuous and stable supply of purge gas and coolant, this embodiment further includes a matching air supply system and coolant cooling system. These systems are connected to the purge gas and coolant channels, respectively, and together form the foundational support system for the cooling and anti-fouling control module. A regulating valve allows for adjustable purge gas flow, while a thermostat controls the coolant temperature.

[0067] To achieve high-quality PIV (particle image velocimetry) measurements of the flow field within an aircraft engine's recirculating combustion chamber, this embodiment of the present invention uses a laser endoscope to direct a laser beam into the combustion chamber. A Powell prism is placed at the laser endoscope's exit to convert the laser beam into a sheet-like laser beam with uniform energy distribution. This structure effectively provides a laser measurement light source with uniform illumination and precise directionality, even in the absence of combustion chamber windows and with extremely limited optical path space. This structure is key to achieving high-precision PIV measurements.

[0068] The laser endoscope is connected to an external laser via a light-guiding structure. Specifically, a light-guiding arm guides the dual-pulse laser beam output by the laser to the entrance of the laser endoscope. This light-guiding arm can be a flexible fiber-optic light-guiding assembly or a mirror-reflective light-guiding conduit, enabling long-distance laser transmission in spatially confined environments. The light-guiding arm offers strong bending resistance, high thermal stability, and high transmission efficiency, making it suitable for long-term use in the high-temperature and complex vibration environments surrounding combustion chambers.

[0069] The laser endoscope is a slender rod-shaped structure with a collimating lens and optical channel inside for directional transmission of the laser. To ensure that the laser forms a high-quality sheet light surface within the measurement area, this embodiment sets a Powell prism at the exit end of the laser endoscope. The Powell prism is an aspheric optical element that works by reconstructing the incident Gaussian collimated laser beam into a fan-shaped laser light sheet with uniform lateral energy distribution. Compared with traditional cylindrical lenses, the Powell prism can effectively solve the problem of uneven energy in the sheet laser, where the center is bright and the edges are dark, significantly improving illumination uniformity and image contrast.

[0070] The Powell prism is installed in the front channel of the laser endoscope via a threaded connection, and its end face is flush with the detection end face of the cooling and purge device. On the one hand, this design ensures the unobstructed divergence of the sheet laser beam. On the other hand, the wall-adhering flow formed by the purge gas on the end face can cool and prevent contamination of the Powell prism, avoiding thermal damage and carbon soot contamination during laser transmission, ensuring the cleanliness of the illumination surface and the stability of the divergence angle. In actual applications, depending on the size of the measurement area and the required laser energy, the expansion angle of the Powell prism can be selected to be 10°, 20°, 30°, 45°, etc. The specific parameters can be selected according to the optical path layout inside the combustion chamber.

[0071] To capture clear, low-distortion images of the flow field within the recirculating combustion chamber of an aircraft engine, an embodiment of the present invention incorporates a retractable adapter ring between the camera and the camera endoscope. This retractable adapter ring is a mechanical structure located between the camera and the optical assembly. Its length is adjustable, compensating for the distance error between the camera flange focal distance (i.e., the distance from the lens interface to the imaging sensor) and the endoscope's exit pupil, thereby achieving coplanar matching of the optical system's imaging plane. When replacing a different camera or endoscope model, simply adjusting the adapter ring length allows for quick re-matching without replacing the main structural components, significantly improving the versatility of the system hardware and test debugging efficiency.

[0072] To address the issues of small imaging circles, large vignetting around edges, and severe image distortion associated with traditional endoscopes, this embodiment incorporates an optical interface lens with a focal length of no less than 75mm between the retractable adapter ring and the camera endoscope. By extending the imaging path and optimizing the beam convergence angle, this optical interface significantly expands the effective imaging circle formed on the camera's photosensitive chip, reduces vignetting around the edges of the image, and effectively controls image distortion, improving image quality and particle image clarity.

[0073] The optical interface is preferably a fixed-focus lens with a focal length of at least 75mm. This provides good spatial resolution while avoiding depth of field loss due to over-magnification. Furthermore, the lens should have excellent chromatic aberration correction and low distortion to meet the stringent geometric accuracy requirements of PIV image processing.

[0074] This embodiment also discloses a method for measuring the flow field in a recirculating combustion chamber by endoscopic PIV, comprising the following steps:

[0075] Step S1: Assembling and fixing the combustion chamber test piece.

[0076] Prepare the aircraft engine reflow combustion chamber test section according to the test requirements and appropriately modify its structure. Pre-set mounting holes for the laser endoscope and camera endoscope are located in designated locations. Secure the combustion chamber test piece to a support frame or test platform to ensure stable position, no vibration or shifting during operation, and provide a good foundation for subsequent laser illumination and image acquisition. This step ensures the mechanical stability of the test environment and prevents measurement errors or data distortion caused by loose specimens.

[0077] Step S2: Assembling the endoscopic PIV measurement system for the flow field in the recirculation combustion chamber.

[0078] Step S2 includes the following steps:

[0079] Step S21: PIV measurement system connection: connect the computer to the synchronization controller, connect the synchronization controller to the laser and camera respectively, and connect the laser outlet to the light guide arm;

[0080] First, complete the master control and trigger connections for the PIV system. Connect the host computer (computer) to the synchronization controller, and connect the synchronization controller to the laser and camera, respectively, to achieve synchronized control and time triggering. This connection ensures strict timing between the dual-pulse laser emitted by the laser and the dual-frame exposure of the camera, guaranteeing the accuracy of subsequent particle image acquisition. At the same time, the laser outlet is connected to the laser endoscope via an optical light guide assembly (light guide arm) to establish a laser transmission path. This step establishes a closed-loop control loop between laser illumination and image acquisition, ensuring precise synchronization of the laser and camera within microseconds, thereby ensuring the determinism of the particle imaging interval and the accurate solution of the velocity vector.

[0081] Step 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.

[0082] Accurately couple the laser beam output from the light-guiding arm to the inlet port of the laser endoscope, transmitting the laser light along the optical axis of the endoscope to the outlet. Based on the planar dimensions and angular requirements of the measurement area within the combustion chamber, select a Powell prism with a specific divergence angle (such as 10°, 20°, 30°, etc.) and thread it onto the outlet of the laser endoscope. The Powell prism converts a Gaussian laser beam into a fan-shaped sheet beam with uniform energy distribution, achieving planar illumination. This step creates a high-quality measurement laser light sheet that meets the stringent requirements of endoscopic PIV for illumination uniformity, imaging clarity, and particle excitation consistency in the measurement area.

[0083] Step S23: the camera endoscope probe is connected to the optical interface, and the optical interface is then connected to the camera;

[0084] Connect the rear end of the camera endoscope to an optical interface with a preselected focal length (≥75mm), and then connect it to the camera body through the optical interface. The camera endoscope is used to guide the image inside the combustion chamber to the camera's photosensitive chip; the optical interface can perform secondary collimation or image quality optimization on the light path entering the camera. By properly matching the optical path system of the camera, interface lens, and endoscope, a larger imaging circle can be obtained on the camera imaging chip, reducing image vignetting and edge distortion, and improving particle image resolution and recognition rate. The purpose of this step is to build a stable, high-quality imaging optical path system, providing a reliable image foundation for subsequent particle image acquisition and velocity vector solution.

[0085] Step S24: connecting the cooling and purge device to the outlet of the coolant cooling device and the outlet of the purge gas device;

[0086] The endoscope channel fits snugly within the endoscope's outer diameter, with the end flush with the device's inspection surface. The cooling and purge device is then connected to the coolant circulation system and air supply system, which provide coolant and purge gas, respectively. The coolant channel is connected to the constant-temperature coolant circulation system to efficiently cool the endoscope's front end lens. The purge gas channel is connected to a high-pressure clean air source or nitrogen cylinder to prevent soot particles from adhering to the mirror surface.

[0087] 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 based on the positional relationship between the two during subsequent PIV measurement. At the same time, the two laser beams are adjusted to ensure overlap.

[0088] Reference Figure 11, calibration and imaging distortion correction of the PIV measurement system include the following steps: placing a piece of frosted glass on the back of the calibration plate, and arranging a surface light source (such as a point light source, a surface light source, a volume light source, etc.) on the back of the frosted glass. The illumination light emitted by the light source first shines on the surface of the frosted glass and is converted into uniform diffuse reflection light under the action of its rough structure. It then shines on the calibration plate in a soft and diffuse form, thereby forming an image with high brightness, uniform distribution, and no strong contrast at the edges in the camera's field of view. In order to further improve the imaging quality, the camera is set to a long exposure mode to fully receive low-intensity reflected light and avoid low grayscale or high noise in the image due to insufficient lighting. The technical function of this method is to effectively reduce image non-uniformity caused by local reflections, backlighting, dark corners and other problems, 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-to-physical mapping and velocity field solution in subsequent flow field measurements of the PIV system. During calibration, the relative spatial relationship between the laser endoscope and the camera endoscope must be maintained consistent with their subsequent installation positions. The laser path must be adjusted to ensure that the dual laser pulses completely overlap in the same plane. This step is used to obtain accurate pixel-to-physical coordinate conversion parameters and distortion correction coefficients, providing a geometric basis for velocity calculations.

[0089] Step S4: Install and fix the laser endoscope and 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 of the endoscope end consistent with the airflow direction of the combustion chamber where it is located.

[0090] Reference Figure 10 After calibration, insert the laser endoscope and camera endoscope into the reserved mounting holes of the test piece respectively, and fix them as a whole with the cooling and purge device. During the installation process, the laser endoscope and the camera endoscope should be arranged vertically to form a measurement plane; and the end faces of the two endoscopes should 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 purge structure so that the outlet direction of the purge gas is consistent with the direction of the airflow inside the combustion chamber, so as to achieve wall-adhering flow and reduce interference with the mainstream flow field. This step ensures the stability of the laser illumination and imaging path, while achieving cooling and anti-pollution effects.

[0091] Step S5: Adjust the test conditions to meet the specified technical requirements, and at the same time adjust the tracer particle generator so that the tracer particle concentration is 6 to 8 tracer particles per query domain;

[0092] The test system was started, and combustion chamber operating parameters such as air intake, fuel supply, and ignition were adjusted, with fuel supply increased to achieve the target internal flow field. The tracer particle generator was simultaneously adjusted to control particle concentration, ensuring an average of 6 to 8 particles within each interrogation region (i.e., the cross-correlation calculation grid) in the image. This concentration range ensures image clarity while improving the correlation of particle matching between image frames, thereby enhancing the stability and accuracy of the velocity solution.

[0093] Step S6: The computer issues an acquisition instruction, and the laser and camera work synchronously under the control of the synchronization controller and acquire the original image of the tracer particles;

[0094] After the test conditions stabilize, the computer sends acquisition instructions to the synchronization controller, which simultaneously triggers the laser to emit a double laser pulse and controls the camera to complete double-exposure imaging. The laser illuminates the particles in the sheet area, creating two scattered images. The camera captures two frames of particle images and saves them to the computer. This step ensures time synchronization and data integrity and is critical for velocity field acquisition.

[0095] Step S7: The computer performs cross-correlation calculation on the tracer particle image to obtain the velocity field in the entire measurement section.

[0096] After acquisition, the two particle images are processed using a cross-correlation image processing algorithm to calculate particle displacement and convert it into fluid velocity information. This step converts the raw image data into physical quantities, forming the final output data of the PIV measurement.

[0097] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. The endoscopic PIV measurement system for the flow field in the recirculation combustion chamber is 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. 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 cooling liquid channel includes 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 purge gas channel includes an air inlet located at the connecting end and an air outlet located at the detection end, and the detection 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 face of the detection end to the end face of the endoscope channel; the endoscope channel is located between the cooling liquid inlet channel and the cooling liquid outlet channel.

2. The endoscopic PIV measurement system for flow field in a recirculating combustion chamber according to claim 1, characterized in that: The purge cooling device includes an inner sleeve, an outer sleeve and multiple 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 multiple partitions. Through holes are opened on the partitions between the coolant inlet channel and the coolant outlet channel.

3. The endoscopic PIV measurement system for flow field in a recirculating 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.

4. The endoscopic PIV measurement system for the flow field in a recirculating combustion chamber according to claim 3, 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.

5. The endoscopic PIV measurement system for flow field in a recirculating combustion chamber according to claim 4, characterized in that: An optical interface with a focal length of not less than 75 mm is provided between the telescopic adapter ring and the camera endoscope.

6. The endoscopic PIV measurement system for flow field in a recirculating combustion chamber according to claim 4, 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.

7. A method for measuring the flow field in a recirculating combustion chamber using an endoscopic PIV measurement system for the flow field in a recirculating combustion chamber according to any one of claims 1 to 6, 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 recirculation 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 based on the positional relationship between the two during subsequent PIV measurement. At the same time, adjustments are made to ensure that the two laser beams coincide. Step S4: The laser endoscope and the camera endoscope are respectively mounted and fixed on the combustion chamber test piece, ensuring that the laser endoscope and the camera endoscope are arranged perpendicularly, and the ends of the laser endoscope probe and the camera endoscope probe are flush with the wall surface, and the direction of the purge gas at the end of the endoscope is consistent with the direction of the combustion chamber airflow at its location; Step S5: Adjust the test conditions to meet the specified technical requirements, and at the same time adjust the tracer particle generator so that the tracer particle concentration is 6 to 8 tracer particles per query domain; Step S6: The computer issues an acquisition instruction, and the laser and 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.

8. The endoscopic PIV measurement method for the flow field in a recirculating combustion chamber according to claim 7, characterized in that: Step S2 includes the following steps: S21: Connect the PIV measurement system. Connect the computer to the synchronization controller. Connect the synchronization controller to the laser and camera respectively. Connect the laser outlet to the light guide arm. S22: The outlet of the light guide arm is connected to a laser endoscope. According to the size of the measurement area, a Powell prism with a suitable divergence angle is selected and installed on the end of the laser endoscope through a thread to form a sheet laser. S23: The camera endoscope probe is connected to the optical interface, and the optical interface is then 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.

9. The endoscopic PIV measurement method for the flow field in a recirculating combustion chamber according to claim 8, characterized in that: Calibration of the PIV measurement system and correction of imaging distortion include the following steps: A frosted glass is set on the back of the calibration plate, and a light source is set on the back of the frosted glass. The light from the light source first shines on the frosted glass, and then shines on the calibration plate in the form of diffuse reflection after passing through the frosted glass. 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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