A holographic background schlieren method for simultaneous measurement of continuous fluid and discrete particles

Abstract

The application discloses a holographic background schlieren method for simultaneously measuring continuous fluid and discrete particles, which records a reference background not disturbed by a flow field and a background holographic image disturbed by the flow field by using holographic coherent light, obtains focused images of the discrete particles and the background texture in the multiphase flow by a reconstruction algorithm, represents the accumulation of the refractive index gradient along the line of sight in the continuous fluid by the offset of the texture on the focused background image, and obtains the morphology, particle size and three-dimensional spatial position of the discrete particles by reconstruction. The method simultaneously records the information of the background pattern plate and the flow field, realizes the simultaneous measurement and visualization of the discrete particle parameters and the continuous fluid in the multiphase flow, and is suitable for the simultaneous measurement of the continuous fluid and the discrete particles in the multiphase flow, thereby providing data and technical support for deeply revealing a complex multiphase flow field with strong coupling.

Description

Technical Field

[0001] This invention relates to the field of three-dimensional visualization and measurement of multiphase flow, specifically to a method for simultaneously measuring holographic background schlieren of continuous fluid and discrete particles. Background Technology

[0002] In scientific research and engineering applications, multiphase flow is widely present in processes such as combustion, cavitation, atomization, and gas-liquid transport. Examples include fuel atomization in aero-engine combustion chambers, bubble flow in power plant water-cooled wall pipes, and cavitation at propeller tips. Characterizing and measuring the interactions between different phases in multiphase flow is crucial for revealing multiphase flow mechanisms and optimizing engineering design. Multiphase flow is characterized by strong multiphase coupling and complex structures. To reveal the internal interactions of multiphase flow, it is usually necessary to simultaneously obtain relevant parameters of both the discrete and continuous phases. The discrete phase typically manifests as particles, droplets, or bubbles, with key parameters including three-dimensional spatial position, size, and kinematic characteristics. The continuous phase typically manifests as gas or liquid in the flow field, with important parameters including refractive index distribution, density distribution, and flow structure. However, due to the phase mixing and complex flow structures in multiphase flow, simultaneously measuring the three-dimensional information of the discrete phase and the flow field information of the continuous phase remains a challenge.

[0003] Existing optical measurement techniques are well-established for measuring a single phase in multiphase flows. For example, techniques such as schlieren, shadowing, and background schlieren can measure refractive index changes in a flow field, thereby obtaining information on flow density distribution or flow structure. Particle image velocimetry (PIV) can obtain flow velocity information. On the other hand, holographic, multi-view imaging, and tomographic imaging techniques can measure the three-dimensional geometry and motion characteristics of discrete phases, obtaining information such as particle morphology, particle size, three-dimensional position, and trajectory.

[0004] Several combined measurement methods have been proposed for the simultaneous measurement of two-phase parameters in multiphase flow. For example, combining particle image velocimetry (PIV) with laser-induced fluorescence (PLIF) can simultaneously measure flow velocity and concentration distribution to a certain extent, and distinguish different particles in the discrete and continuous phases through dye fluorescence, allowing for simultaneous measurement using two separate cameras to record different phase states. Combining PIV with shadow imaging or scattering imaging can also be used for the joint measurement of particle motion and flow field structure. Additionally, some studies have used image processing algorithms to segment images of coexisting two phases, obtaining regions corresponding to different phase states, thereby extracting parameter information for different phase states from the same image. Overall, the related technologies for the simultaneous measurement of two-phase information in multiphase flow are still under continuous development.

[0005] For example, Chinese Patent Publication No. CN121762171A discloses a multiphase jet characterization system and method. This system includes a control unit and connected to the control unit are a particle image velocimetry unit, a phase Doppler particle analysis unit, a laser Doppler velocimetry unit, and a planar laser-induced fluorescence unit. The particle image velocimetry unit measures the macroscopic velocity field; the phase Doppler particle analysis unit measures microscopic dynamic information; the laser Doppler velocimetry unit measures the velocity profile; the planar laser-induced fluorescence unit measures the concentration field of the target component in the multiphase jet; and the control unit receives the measurement results from each unit, fuses the results, and outputs the characterization results. This system can comprehensively characterize multiphase jets, simultaneously acquire multi-dimensional physical information, and accurately reveal the solid-liquid-gas phase coupling and phase transition mechanism of multiphase jets. Chinese patent CN117249969A discloses a gas-liquid two-phase flow measurement system and method based on PIV and shadow imaging technology coupling, which can measure the characteristics of gas phase size, position, shape, velocity and other features, as well as the liquid phase flow field structure, and accurately measure the interface between the gas and liquid phases.

[0006] In summary, most existing multiphase flow measurement techniques primarily target one phase from either the continuous or discrete phases. While some combined measurement methods can acquire continuous flow field information and discrete phase structure information within the same measurement system, limitations remain in terms of system complexity, measurement accuracy, and measurement efficiency. Therefore, achieving simultaneous measurement of parameters of continuous multiphase fluids and discrete particles remains a pressing technical challenge in the field of multiphase flow diagnostics. Summary of the Invention

[0007] The purpose of this invention is to provide a method for simultaneously measuring the holographic background schlieren of continuous fluid and discrete particles: this method can quantitatively obtain parameters such as the three-dimensional spatial position, morphology, and particle size of discrete particles in multiphase flow, and can simultaneously obtain the visualization results of the refractive index change in continuous fluid.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following specific technical solution.

[0009] A method for simultaneously measuring holographic background schlieren of continuous fluid and discrete particles includes the following steps: Step 1: Perform holographic imaging on the background board with the background pattern to obtain a reference hologram of the background board; Step 2: Set up a multiphase flow field of continuous fluid and discrete particles to be measured between the background plate and the recording device, and perform holographic imaging recording on the background plate and the multiphase flow field to be measured to obtain a hologram of the multiphase flow field to be measured. Step 3: Reconstruct the reference hologram mentioned in Step 1 and the hologram of the multiphase flow field to be measured mentioned in Step 2 to obtain a focused reference background image and a background image affected by the multiphase flow field to be measured, and obtain the cumulative amount of refractive index gradient along the line of sight in the continuous fluid based on the displacement information of the texture between the background images. Step 4: Reconstruct the reference hologram from Step 1 and the hologram of the multiphase flow field to be measured from Step 2 within the multiphase flow region to be measured, and subtract the reconstructed light field of the reference hologram from the light field of the reconstructed particle image to obtain a discrete particle focused reconstructed image, and obtain discrete particle parameters based on the discrete particle focused reconstructed image.

[0010] In step 1, the background plate with the background pattern is a flat transparent plate with an opaque background pattern, and the transparent part has a uniform phase influence on the laser. The background pattern is used for subsequent processing to calculate the background image offset information caused by the multiphase flow field under test.

[0011] In step 2, the multiphase flow field to be measured is positioned between the background plate and the recording device, and the distance between the background plate and the multiphase flow field is configured such that when the recording device focuses on the discrete particles in the multiphase flow field, the background plate pattern forms a defocused image in the recording device. That is, the background plate and the multiphase flow field have a certain distance, so that when the recording device reconstructs and focuses on the discrete particles in the multiphase flow field, the background plate pattern is a defocused image and does not affect the focused particle image.

[0012] In step 3, reconstructing the reference hologram and the hologram to be tested involves first reconstructing the reference background image and determining the focus z-axis position of the background plate under interference-free conditions using a focusing criterion. Then, the hologram to be tested is reconstructed using the same focus position z, resulting in a focused reference background image and a background image affected by the multiphase flow field to be tested.

[0013] In step 3, the method for obtaining the cumulative amount of refractive index gradient along the line of sight in a continuous fluid based on the displacement information of textures between the background images includes: Step 3.1: Use image processing methods to obtain the texture of the background board along the image. Displacement vectors in two directions .

[0014] Step 3.2: Obtain the cumulative amount of refractive index gradient along the line of sight from the displacement vector. .

[0015] In step 3.2, the cumulative amount of refractive index gradient The mathematical relationship is: ; In the formula, The gradient of the refractive index is represented in the plane perpendicular to the z-axis, and L represents the distance between the flow field and the background plate. It indicates the magnitude of the refractive index of the surrounding environment.

[0016] In step 4, the discrete particle parameters typically include the three-dimensional spatial position, morphology, and equivalent circle diameter of the discrete particles.

[0017] In step 4, the hologram to be tested is reconstructed within the tested area to obtain a discrete particle focused reconstruction image. Based on this image, discrete particle parameters are obtained, namely the three-dimensional spatial position, morphology, and equivalent circle diameter of the discrete particles. The specific steps are as follows: Step 4.1: Within the set range of discrete particle appearance, reconstruct the cross-sectional image light field of the corresponding z-axis position cross-section by cross-section according to the step size, and subtract the reconstructed light field of the corresponding position of the reference hologram from the reconstructed particle image light field of the cross-section at the corresponding z-axis position. Step 4.2: In the reconstruction results, select and locate the positions of the discrete particles on the image to obtain the bounding boxes of the discrete particles. In the formula The coordinates of the top left corner of the bounding box. The bounding box is defined by its length and width. Images of particles at different z-axis positions are cropped from the cross-sectional images to obtain a sequence of cross-sectional images of particles along the z-axis. Step 4.3: Calculate the focusing criterion index of the cross-sectional image sequence to determine the most focused cross-sectional position of the particles in the cross-sectional image, thereby obtaining the discrete particle focused image and the depth position of the particles in three dimensions. .

[0018] Step 4.4: Perform image segmentation on the discrete particle focused image obtained at the particle focusing position to obtain the particle binarization mask and thus the particle morphology, and calculate its centroid position to obtain the pixel position of the discrete particle on the image. Then the three-dimensional spatial position of the particle can be obtained. ,in The specific mathematical form is: ; In the formula This represents the equivalent pixel size of the holographic system.

[0019] Step 4.5: The area of ​​the particle-binarized mask can be further converted into the equivalent circle diameter. The specific calculation formula is as follows: ; In the formula, The pixel area is statistically calculated in the binarized mask. This represents the equivalent pixel size of the holographic system.

[0020] The beneficial effects of this invention are that it provides a method for simultaneously measuring holographic background schlieren of continuous fluid and discrete particles. Specifically, it can realize the measurement of parameters such as the three-dimensional position, morphology, and particle size of discrete particles, as well as the quantitative visualization of the integral of the refractive index gradient of the continuous fluid along the line of sight. The proposed method realizes the method of simultaneously obtaining parameters of continuous fluid and discrete particles from a set of imaging equipment, simplifies the three-dimensional measurement method and device of multiphase flow, and has the advantages of simple setup, non-contact, and in-situ measurement. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the principle of holographic background schlieren imaging. Figure 2 This is a schematic diagram of a holographic background schlieren imaging system. Figure 3 Visualization of the morphology of focused particles in solid propellant combustion agglomerates; Figure 4 Figure 1 shows the measurement results of the three-dimensional particle field and combustion flow field of solid propellant combustion. The components include: 1. Background plate; 2. Flow field to be measured; 3. Discrete particles; 4. Camera; 5. Pulsed laser; 6. Spatial filter; 7. Beam splitter cube; 8. Mirror; 9. Synchronization signal generator; 10. Coherent modulator; 11. Data acquisition computer. Detailed Implementation

[0022] The specific implementation of the technical solution of the present invention will be further described below through examples and in conjunction with the accompanying drawings.

[0023] This embodiment provides a system and method for simultaneously measuring and visualizing the three-dimensional particle field and combustion heat flow field in the complex multiphase flow of solid propellant combustion, based on the holographic background schlieren method. The holographic background schlieren method can record holograms using digital coaxial holography or digital off-axis holography. Preferably, in this embodiment, an off-axis holographic system is used to establish a holographic background schlieren imaging system, thereby applying the holographic background schlieren method to simultaneously measure the continuous heat flow field and discrete burning metal particles during propellant combustion.

[0024] The holographic background schlieren system components include a laser source, a beam splitter, an object beam path, a reference beam path, and an image recording component. It is used to holographically record the background pattern and the multiphase flow field under test, obtaining a reference hologram and a hologram under test. For example... Figure 2As shown, the recording process involves a pulsed laser 5 emitting laser light, which is filtered and expanded by a spatial filter 6 to form a uniform laser surface. Then, a beam splitter 7 splits the beam to form an object beam carrying information about the measured area and a reference beam for interference. Background plates 1 are sequentially placed in the area through which the object beam passes, allowing it to carry information from the background plates before passing through the measured area, recording the measured flow field 2 and discrete particles 3. The object beam then combines with the reference beam, adjusted by a reflector 8, through the beam splitter 7 and interferes. Finally, an off-axis holographic image is recorded on the recording plane of the camera 4 and transmitted to the acquisition computer 11, completing one recording cycle. The object beam needs to pass through a coherent modulator 10 to adjust its optical path to ensure coherence with the reference beam. A synchronization signal is emitted by a synchronization signal generator 9 to achieve timing synchronization between the pulsed laser 5 and the camera 4.

[0025] In this embodiment, the multiphase flow field of solid propellant combustion is used as the flow field to be measured 2, and the burning aluminum agglomerates are used as discrete particles 3 for holographic background schlieren measurement. The propellant used is cut into strips with square bottoms. Preferably, the solid propellant strips are 5 mm × 5 mm × 8 mm in size and are positioned 285 mm in front of the camera 4. Preferably, the pulsed laser 5 used has a wavelength of 532 nm and can emit laser pulses with a width within nanoseconds. Preferably, the camera 4 has a pixel size of 3.2 μm and a resolution of 9000 × 7000. The exposure time of the camera 4 is set to 20 μs. The synchronization signal generator used has a synchronization accuracy of nanoseconds. The timing signal frequency is set to 5 Hz, the pulse width is 50 μs, and the signal delay to the pulsed laser side is set to 10 μs to ensure that the camera can capture the laser pulse within the exposure time. The synchronization timing signal is connected to the camera 4 and the pulsed laser 5 via a BNC cable to achieve timing synchronization between the two. The images captured by camera 4 are transmitted to acquisition computer 11 via a data transmission cable to save the holographic images. Preferably, the acquisition computer 11 has 16 GB of memory and at least 512 GB of storage.

[0026] Background plate 1 is a uniform quartz glass light-transmitting plate with an opaque background pattern. The background pattern is used to characterize the background image shift information caused by the multiphase flow field under test. For example... Figure 2 As shown, the background plate 1 is arranged on the object light path, allowing the object light to pass through the background plate first and then through the area to be tested during the test. Preferably, the designed background texture image consists of randomly scattered opaque dots with a diameter of 50 μm. Preferably, the distance between the background plate 1 and the camera 4 is set to 539 mm.

[0027] Simultaneous measurement of continuous thermal flow field and discrete burning metal particles in propellant combustion is achieved using the holographic background schlieren method, including the following steps: Step 1: Perform holographic imaging on the background board with the background pattern to obtain a reference hologram of the background board; like Figure 1 As shown, a uniform laser passes through the background plate 1, and the diffraction light field of its texture is recorded on the camera 4 to obtain a background plate reference hologram.

[0028] Step 2: Set up a multiphase flow field of continuous fluid and discrete particles to be measured between the background plate and the recording device, and perform holographic imaging recording on the background plate and the multiphase flow field to be measured to obtain a hologram of the multiphase flow field to be measured. like Figure 1 As shown, a uniform laser passes through a background plate 1, then sequentially through the background plate texture, the flow field to be measured 2, and discrete particles 3, and then records the light field on a camera 4 to obtain the hologram to be measured. Due to the interference of the flow field, the laser will be deflected in the flow field region, producing a light deflection angle caused by the flow field, which will cause the appearance of the object light recorded on the camera to be shifted.

[0029] Step 3: Reconstruct the reference hologram mentioned in Step 1 and the hologram to be tested mentioned in Step 2 to obtain a focused reference background image and a background image affected by the multiphase flow field to be tested, and obtain the cumulative amount of refractive index gradient along the line of sight in the continuous fluid based on the displacement information of the texture between the background images. like Figure 1 As shown, during reference image reconstruction, since the object light is not distorted by the flow field, the reconstruction process is equivalent to the free propagation of parallel light in space, thus reconstructing the background texture result, which is then the reference background image. The reconstruction algorithm can employ holographic reconstruction calculation methods based on scalar diffraction theory, such as Fresnel approximation reconstruction, angular spectrum method, and wavelet reconstruction. Preferably, in this embodiment, the reference hologram and the hologram to be measured use the same off-axis angle at the same background plate focusing position and are reconstructed using the angular spectrum method.

[0030] When reconstructing the holographic image to be tested, the same off-axis angle parameter as that used in the reconstruction of the reference hologram needs to be used. The specific steps to obtain the off-axis angle are as follows: First, the frequency domain result of the image is obtained through Fourier transform. Then, a bandpass filter is applied to the frequency domain result to retain the spectral information of the +1 term, while filtering out the spectral information of twin phases and zero-order terms. The off-axis angle is then identified from the spectrum. Specifically, the method for identifying the off-axis angle first identifies the pixel position with the highest energy in the +1 term spectrum. The specific mathematical form of the off-axis angle of the reference hologram is then obtained as follows: , In the formula, The coordinates of the center position in the image frequency domain. For the image in Number of pixels in the direction This refers to the equivalent pixel size of the holographic system.

[0031] Based on the obtained reference background texture image and the background texture image deformed by the flow field, the displacement vectors of the texture along the x and y directions on the image are quantitatively analyzed. ,like Figure 1 As shown. Preferably, in texture displacement vector analysis, a cross-correlation algorithm is used to analyze the relationship between the reference texture image and the background texture image deformed by the flow field, thereby obtaining the displacement vector of the local texture. The discrete particle size and 3D position are obtained by matching the texture displacement vector obtained from the 3D scatter plot. Draw on the same graph, such as Figure 4 As shown, simultaneous measurement of discrete particles and continuous flow fields is achieved. The cumulative amount of refractive index gradient along the line of sight can be calculated using a formula. In this embodiment, the distance from the flow field to the background plate is... It is 254 mm.

[0032] In step 4, the reference hologram from step 1 and the hologram to be tested from step 2 are reconstructed within the multiphase flow region to be tested. The reconstructed light field of the reference hologram is subtracted from the light field of the reconstructed particle image from step 2 to obtain a discrete particle focused reconstruction image. Discrete particle parameters are obtained based on the discrete particle focused reconstruction image, typically including the three-dimensional position, morphology parameters, and particle size of the discrete particles.

[0033] Step 4.1 In this embodiment, preferably, in the solid propellant combustion experiment, the range of combustion agglomerates is within 15 mm before and after the propellant strip. Therefore, the discrete particle holographic reconstruction range is set to cover the range from 270 mm to 300 mm on the z-axis, and the reconstruction step size is 0.1 mm.

[0034] Step 4.3, in this embodiment, preferably, the gradient variance of the cross-sectional image sequence along the z-axis of the particle image is used as the focusing criterion. Specifically, the gradient variance of the image is calculated as follows: first, the gradient variance of the cross-section along the z-axis is calculated using the Sobel operator. The gradient information in the direction is used to calculate the magnitude of this gradient vector, obtaining the gradient intensity of the cross-sectional image. Finally, the variance of the image gradient intensity is calculated to obtain the gradient variance result, which is used as the focusing criterion. The reconstructed cross-sectional image of the identified focused particles is shown below. Figure 3 As shown.

[0035] In step 4.4, in this embodiment, preferably, a thresholding method is used to segment the image to obtain a binarized mask for particle image segmentation.

[0036] This embodiment addresses the multiphase flow field of solid propellant combustion. By employing the holographic background schlieren method, it successfully achieved the measurement and visualization of the three-dimensional position, morphology, and particle size of discrete particles within the propellant combustion thermal plume, as well as the cumulative amount of refractive index gradient along the line of sight in the continuous thermal flow field.

[0037] The above is a detailed description of the present invention in conjunction with the embodiments. However, the implementation of the present invention is not limited to the above embodiments. Any changes, substitutions, combinations and simplifications made under the core guiding idea of ​​the present invention are included within the protection scope of the present invention.

Claims

1. A method for simultaneously measuring holographic background schlieren of continuous fluid and discrete particles, characterized in that, The method includes the following steps: Step 1: Perform holographic imaging on the background board with the background pattern to obtain a reference hologram of the background board; Step 2: Set up a multiphase flow field of continuous fluid and discrete particles to be measured between the background plate and the recording device, and perform holographic imaging recording on the background plate and the multiphase flow field to be measured to obtain a hologram of the multiphase flow field to be measured. Step 3: Reconstruct the reference hologram mentioned in Step 1 and the hologram of the multiphase flow field to be measured mentioned in Step 2 to obtain a focused reference background image and a background image affected by the multiphase flow field to be measured, and obtain the cumulative amount of refractive index gradient along the line of sight in the continuous fluid based on the displacement information of the texture between the background images. Step 4: Reconstruct the reference hologram from Step 1 and the hologram of the multiphase flow field to be measured from Step 2 within the multiphase flow region to be measured, and subtract the reconstructed light field of the reference hologram from the light field of the reconstructed particle image to obtain a discrete particle focused reconstructed image, and obtain discrete particle parameters based on the discrete particle focused reconstructed image.

2. The method according to claim 1, characterized in that, In step 1, the background plate with the background pattern is a flat transparent plate with an opaque background pattern, and the transparent part has a uniform phase influence on the incident laser.

3. The method according to claim 1, characterized in that, In step 2, the incident laser passes through the background plate and the multiphase flow field to be measured in sequence before reaching the recording device; and the distance between the background plate and the multiphase flow field to be measured is configured such that when the recording device focuses on the discrete particles in the multiphase flow field to be measured, the pattern on the background plate forms a defocused image.

4. The method according to claim 1, characterized in that, The reconstruction of the reference hologram and the hologram to be tested in step 3 includes: first, reconstructing the reference background image and determining the focus z-axis position of the background plate under interference-free conditions using the focus criterion; then, reconstructing the hologram to be tested using the same focus position z, to obtain the focused reference background image and the background image affected by the multiphase flow field to be tested.

5. The method according to claim 1, characterized in that, In step 3, the method for obtaining the cumulative amount of refractive index gradient along the line of sight in a continuous fluid based on the displacement information of textures between the background images includes: Step 3.1: Use image processing methods to obtain the texture of the background board along the image. Displacement vectors in two directions ; Step 3.2: Obtain the cumulative amount of refractive index gradient along the line of sight from the displacement vector. .

6. The method according to claim 1, characterized in that, The cumulative amount of refractive index gradient The mathematical relationship is: ； In the formula, The gradient of the refractive index is represented in the plane perpendicular to the z-axis, and L represents the distance between the flow field and the background plate. It indicates the magnitude of the refractive index of the surrounding environment.

7. The method according to claim 1, characterized in that, The discrete particle parameters in step 4 include three-dimensional spatial position, morphology, and equivalent circle diameter.

8. The method according to claim 1, characterized in that, Step 4 includes: Step 4.1: Within the set range of discrete particle appearance, reconstruct the cross-sectional image light field of the corresponding z-axis position cross-section by cross-section according to the step size, and subtract the reconstructed light field of the corresponding position of the reference hologram from the reconstructed particle image light field of the cross-section at the corresponding z-axis position. Step 4.2: In the reconstruction results, select and locate the position of the discrete particles on the image to obtain the bounding box of the discrete particles; crop the images of the particles at different z-axis positions from the cross-sectional image to obtain the cross-sectional image sequence of the particle images along the z-axis; Step 4.3: Calculate the focusing criterion index of the cross-sectional image sequence to determine the cross-sectional position where the particles are most focused in the cross-sectional image, thereby obtaining the discrete particle focusing image and the depth position in the three-dimensional position of the particles; Step 4.4: Perform image segmentation on the discrete particle focusing image obtained from the particle focusing position to obtain the particle binarization mask and thus obtain the particle morphology, and calculate its centroid position to obtain the pixel position of the discrete particle on the image, thereby obtaining the three-dimensional spatial position of the particle. Step 4.5: Binarize the area of ​​the mask using particles and further convert it to the equivalent circle diameter.

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