A microscopic test and identification method for supercritical phase transition

Abstract

The present invention discloses a supercritical phase change microscopic testing and discrimination method, which belongs to the field of fluid supercritical phase change discrimination. Firstly, an ultra-high-speed near-field microscopic optical testing system is used to collect a number of original images during the injection process under different ambient densities and ambient temperatures, and the images with consistent working conditions and clear spray development processes are selected for image processing; secondly, at the end of the injection, droplet images with consistent initial sizes and development process images are selected, and a digital image processing method is used to obtain the spatiotemporal transformation process of single droplets with different initial sizes and initial velocities; then, the spraying moment, actual droplet size, and true movement speed of the single droplet transformation process image are calculated according to the injection parameters and image calibration parameters set in the experiment; finally, the differences in the single droplet development processes under different ambient densities and ambient temperatures are analyzed based on a single variable method, their common characteristics are summarized, and the evolution characteristics of the droplet phase change process under subcritical and supercritical conditions are determined.

Description

Technical Field

[0001] The present invention belongs to the field of fluid supercriticality identification, and in particular relates to a supercritical phase change microscopic testing and identification method. Background Art

[0002] High-performance engines require continuous improvement in power density, which is achieved by boosting in-cylinder pressure. However, as boost pressure increases, the engine explosion pressure also rises, and the ambient density within the cylinder gradually increases. This can cause imbalances in the traditional fuel-air chamber, leading to poor mixing, deteriorating combustion, and reduced thermal efficiency. Improving the mixing quality of the fuel spray is one solution, but high-boost solutions increase the ambient density and temperature within the cylinder, making supercritical phase transitions more likely. Due to limitations in environmental conditions and test equipment, the conditions under which supercritical phase transitions occur and the criteria for determining their occurrence remain uncertain.

[0003] Current methods for determining supercriticality include the fuzzy determination of the mixing layer at the spray jet edge and the disappearance of the gas-liquid interface. The former relies on digitally magnified observation of spray images obtained from macroscopic spray tests, while the latter relies on the thermodynamic state of the supercritical fluid. However, these methods are inadequate for determining the supercritical phase transition occurring during spraying under real engine conditions. Summary of the Invention

[0004] The purpose of the present invention is to provide a supercritical phase transition microscopic testing and discrimination method. Based on an ultra-high-speed near-field microscopic optical testing device, the droplet development process data during and after the injection process is captured, the differences in droplet development morphology under subcritical and supercritical conditions are summarized, and the criteria for supercritical phase transition under microscopic conditions are established.

[0005] To achieve the above object, the present invention provides a supercritical phase transition microscopic testing and identification method, comprising the following steps:

[0006] Step 1: Under different ambient density and temperature conditions, an ultra-high-speed near-field microscopic optical test system is used to collect raw images of the fuel injection process under different working conditions, and raw images with clear development processes are selected for processing. Specific operations include: RGB image conversion, output of frame rate, resolution, start trigger and end time, time interval, and position coordinate information;

[0007] Step 2: Capture and mark the image of the droplets with the same initial size after the injection is completed and the image of the development process. Based on the digital image processing method, obtain the dynamic process image of the size and velocity parameters and the change of the droplets over time and space;

[0008] Step 3: Calculate the spraying time, actual droplet size, and true motion speed of the droplet development process image in step 2 according to the spray parameters and image calibration parameters set in the experiment;

[0009] Step 4: Analyze the differences in the droplet development process under different ambient densities and ambient temperatures based on the single variable method, and determine the evolution characteristics of the droplet phase change process under subcritical and supercritical conditions.

[0010] Preferably, the original image of the spray in step 1 and the initial droplet image after the spraying in step 2 are both obtained by magnifying the observation target area through an ultra-high-speed camera combined with a microscope lens, with a magnification range of 12X to 125X, a resolution of 640×800, a sampling rate of 20,000 fps, and a shooting interval of 20 μs.

[0011] Preferably, the digital image processing method in step 2 is specifically as follows: using PCC software to perform RGB pixel positioning on the collected original image, obtaining droplet images at different times by fixed droplet tracking screenshot method; and using the software ruler function to measure the pixel size, displacement and time interval occupied by the droplet in the image.

[0012] Preferably, the ambient density in step 1 is 8 to 22.8 kg / m 3 , the ambient temperature is 600K~1600K.

[0013] Preferably, the specific expressions for calculating the spraying time T, the actual droplet size S, and the true motion speed v in step 3 are as follows:

[0014] T=1 / F

[0015] S=π(R 2 +h2)

[0016] v=Δx / Δt

[0017] Where F is the frame rate of the camera, h is the initial droplet height, Δx is the droplet displacement length, Δt is the time required for the droplet displacement, and R is the circle radius. The spherical radius of the droplet is r = (R 2 +h2) / (2h).

[0018] Preferably, the evolution characteristics of the droplet phase change process under subcritical and supercritical conditions in step 4 are specifically as follows: under subcritical conditions, the droplets are initially spherical, and as time goes by, the droplet size gradually decreases and finally evaporates and disappears; under supercritical conditions, the droplets are initially spherical, and as time goes by, tails, oscillations and deformation appear, and finally diffuse and disappear.

[0019] Preferably, the single variable analysis method in step 4 is specifically as follows: first, the ambient density is fixed, the ambient temperature is increased, and the operating point that meets the supercritical phase transition characteristics is found as the critical point of the n-dodecane-nitrogen binary system; secondly, the ambient temperature is fixed, the ambient density is increased, and the operating point that meets the supercritical phase transition characteristics is found as the critical point of the n-dodecane-nitrogen binary system.

[0020] Preferably, the evolution characteristics of the droplet phase change process under supercritical conditions in step 4 include different initial sizes and different initial speed differences, wherein the initial size ranges from 40 μm to 240 μm, and the initial speed ranges from 0.3 m / s to 3.6 m / s.

[0021] Therefore, the present invention adopts the above-mentioned supercritical phase change microscopic testing and discrimination method, which has the following characteristics:

[0022] Beneficial effects:

[0023] (1) It can quickly and accurately calculate the droplet motion parameters under different working conditions;

[0024] (2) It can more accurately determine whether the droplet is in the subcritical state or the supercritical state, and provide the morphological characteristics of the supercritical phase transition process.

[0025] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is an overall flow chart of a supercritical phase change microscopic testing and discrimination method of the present invention;

[0027] Figure 2 1 is a diagram showing the calculation principle for calculating the velocity of a droplet in space in an embodiment of the present invention;

[0028] Figure 3 Schematic diagram of the macroscopic spray process under subcritical and supercritical conditions in the embodiment of the present invention, wherein (a) is a diagram of the macroscopic spray process under subcritical conditions, and (b) is a diagram of the macroscopic spray process under supercritical conditions;

[0029] Figure 4 1 is a diagram of the droplet development process under subcritical and supercritical conditions in an embodiment of the present invention, wherein (a) is a diagram of the droplet development process under subcritical conditions, and (b) is a diagram of the droplet development process under supercritical conditions;

[0030] Figure 5 These are characteristic diagrams of the droplet morphology evolution under subcritical and supercritical conditions in an embodiment of the present invention, where (a) is a characteristic diagram of the droplet morphology evolution under subcritical conditions, (b) is a characteristic diagram of the morphology evolution of conventional-sized droplets under supercritical conditions, and (c) is a characteristic diagram of the morphology evolution of large-sized droplets under supercritical conditions. DETAILED DESCRIPTION

[0031] The following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are intended to fall within the scope of protection of the present invention.

[0032] See also Figure 1-5 , a supercritical phase transition microscopic testing and discrimination method, comprising the following steps:

[0033] Step 1: Under different ambient density and ambient temperature conditions, the ultra-high-speed near-field microscopic optical test system is used to collect original images of the fuel injection process under different working conditions, and the original images with clear development process are selected for processing. The specific operations include: RGB image conversion, output frame rate, resolution, start trigger and end time, time interval, and position coordinate information. The ambient density is 8 to 22.8 kg / m 3 , the ambient temperature is 600K~1600K;

[0034] Step 2: Capture and mark the image of the droplet with the same initial size after the injection and the image of the development process, which needs to meet the following requirements: B ×cosφ=v A × cosθ; where v B is the velocity of the droplet at the initial point B and v A is the velocity of the droplet at the end point A, φ and θ are v B and v A The angle between the middle and AB is obtained. The original image and the initial droplet image after the injection are obtained by magnifying the observation target area with an ultra-high-speed camera combined with a microscope lens. The magnification range is 12X to 125X. The camera shooting parameters have a resolution of 640×800, a sampling rate of 20,000 fps, and a shooting interval of 20μs. Based on the digital image processing method, the time, size, speed and data of the droplet's movement over time and space and the corresponding single droplet image are obtained; wherein, the digital image processing method is specifically as follows: using PCC software to locate the RGB pixels of the collected original image, and obtaining the droplet images at different times by the fixed droplet tracking screenshot method; using the software ruler function to measure the pixel size, displacement and time interval occupied by the droplet in the image;

[0035] Step 3: Calculate the spraying time, actual droplet size, and true motion speed of the droplet development process image in step 2 based on the spray parameters and image calibration parameters set in the experiment. The specific expressions are as follows:

[0036] T=1 / F

[0037] S=π(R 2 +h2)

[0038] v=Δx / Δt

[0039] Where F is the frame rate of the camera, h is the initial droplet height, Δx is the droplet displacement length, Δt is the time required for the droplet displacement, and R is the circle radius. The spherical radius of the droplet is r = (R 2+h2) / (2h);

[0040] The specific image calibration process and calculations are as follows: Before the test, the image pixels are calibrated to determine the actual size corresponding to each pixel. The actual distance between each pair of holes in the calibration plate is 5mm. The actual size ratio corresponding to each unit pixel is calculated by calculating the pixel difference in the image.

[0041] Step 4: Analyze the differences in the single droplet development process under different ambient densities and ambient temperatures based on the single variable method, and determine the evolution characteristics of the droplet phase change process under subcritical and supercritical conditions. The single variable analysis method is specifically as follows: first, fix the ambient density, increase the ambient temperature, and find the operating point that meets the supercritical phase change characteristics as the critical point of the n-dodecane-nitrogen binary system; secondly, fix the ambient temperature, increase the ambient density, and find the operating point that meets the supercritical phase change characteristics as the critical point of the n-dodecane-nitrogen binary system. The evolution characteristics of the droplet phase change process under supercritical conditions include differences in different initial sizes and initial velocities, among which the initial size range is: 40μm~240μm, and the initial velocity range is: 0.3m / s~3.6m / s; the specific evolution characteristics are: under subcritical conditions, the droplet is initially spherical, and with time, the droplet size gradually decreases, and finally evaporates and disappears; under supercritical conditions, the droplet is initially spherical, and with time, a tail, oscillation and deformation appear, and finally diffuses and disappears; in particular, when the initial droplet is large in size, the supercritical phase change process is that the droplet is spherical at the initial moment, and with time, a tail is generated with the development of time and the movement of the droplet, the droplet body splits, oscillates and deforms, and finally diffuses and disappears.

[0042] Example

[0043] Select the environmental density as 22.8kg / m 3 The n-dodecane / nitrogen system at ambient temperatures of 600K and 1100K was used as the test object for the test and discrimination method. Figure 3 As shown in (a), the environmental conditions are 600K-22.8kg / m 3 The initial jet is subcritical, and the pattern is consistent with that of a conventional jet. A large number of droplets gather along the axial direction to form a liquid core, and the droplets separate from the spray periphery. A large number of liquid filaments and droplets generated by primary and secondary atomization appear on the jet surface. (b) Medium ambient conditions 1100K-22.8kg / m 3The liquid core is supercritical and the same liquid core is observed at the beginning of the spray, and droplets are generated at the periphery of the spray. At an ASOI of 2ms, the liquid core disappears and a large number of droplets are generated, most of which are deformed due to turbulence. At an ASOI value of 2.25ms, the droplets near the nozzle will gather into liquid filaments and large droplets. As the droplets move, oscillations, deformations, and disappearance of diffusion can be observed. In addition, at the end of the spray, the penetration ability of the liquid phase is significantly reduced, and a large amount of gas-like phase is formed. Figure 4 As shown in (a), at 22.8 kg / m 3 Under the ambient conditions of -600 K, the droplet maintains a spherical shape due to surface tension. As the ASOI value increases, the droplet evaporation satisfies d 2 The law, that is, the square diameter decreases linearly with time. (b) At 22.8 kg / m 3 Under the ambient conditions of -1100 K, the droplet is initially spherical. As the ASOI increases, irregular tails appear on the droplet surface, accompanied by droplet shedding, oscillation deformation, and size increase. Figure 5 As shown in (a), under subcritical conditions, the droplet initially appears spherical due to surface tension. Over time, the droplet gradually shrinks and eventually disappears. In (b) and (c), under supercritical conditions, the droplet initially appears spherical during evaporation. Over time, a tail forms, oscillates, and deforms, before eventually diffusing and disappearing. However, when the initial droplet is large, the supercritical phase transition process begins with a spherical droplet. Over time, a tail forms, and the droplet's main body splits, oscillates, and deforms, before finally diffusing and disappearing.

[0044] Therefore, the present invention adopts the above-mentioned supercritical phase change microscopic testing and discrimination method, based on the ultra-high-speed near-field microscopic optical testing device to capture the droplet development process data during and after the injection process, summarizes the differences in droplet development morphology under subcritical and supercritical conditions, and establishes the criteria for supercritical phase change under microscopic conditions.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A supercritical phase transition microscopic testing and identification method, characterized in that: The following steps are involved: Step 1: Under different ambient density and temperature conditions, an ultra-high-speed near-field microscopic optical test system is used to collect raw images of the fuel injection process under different working conditions, and raw images with clear development processes are selected for processing. Specific operations include: RGB image conversion, output of frame rate, resolution, start trigger and end time, time interval, and position coordinate information; Step 2: Capture and mark the image of the droplets with the same initial size after the injection is completed and the image of the development process. Based on the digital image processing method, obtain the dynamic process image of the size and velocity parameters and the change of the droplets over time and space; Step 3: Calculate the spraying time, actual droplet size, and true motion speed of the droplet development process image in step 2 according to the spray parameters and image calibration parameters set in the experiment; Step 4: Analyze the differences in the droplet development process under different ambient densities and ambient temperatures based on the single variable method to determine the evolution characteristics of the droplet phase transition process under subcritical and supercritical conditions; The evolution characteristics of the droplet phase transition process under subcritical and supercritical conditions in step 4 are as follows: under subcritical conditions, the droplet is initially spherical, and as time goes by, the droplet size gradually decreases and finally evaporates and disappears; under supercritical conditions, the droplet is initially spherical, and as time goes by, a tail, oscillation and deformation appear, and finally diffuses and disappears; The single variable analysis method in step 4 is specifically as follows: first, fix the ambient density, increase the ambient temperature, and find the operating point that meets the supercritical phase transition characteristics as the critical point of the n-dodecane-nitrogen binary system; second, fix the ambient temperature, increase the ambient density, and find the operating point that meets the supercritical phase transition characteristics as the critical point of the n-dodecane-nitrogen binary system.

2. A supercritical phase transition microscopic testing and identification method according to claim 1, characterized in that: The original spray image in step 1 and the initial droplet image after the spraying in step 2 were both obtained by magnifying the observation target area using an ultra-high-speed camera combined with a microscope lens. The magnification range was 12X to 125X, the camera shooting parameters had a resolution of 640×800, a sampling rate of 20,000 fps, and a shooting interval of 20 μs.

3. A supercritical phase transition microscopic testing and identification method according to claim 1, characterized in that: The digital image processing method in step 2 is as follows: use PCC software to locate the RGB pixels of the collected original image, obtain droplet images at different times through the fixed droplet tracking screenshot method; and use the software ruler function to measure the pixel size, displacement, and time interval occupied by the droplet in the image.

4. A supercritical phase transition microscopic testing and identification method according to claim 1, characterized in that: The ambient density in step 1 is 8~22.8 kg / m 3 , the ambient temperature is 600 K~1600 K.

5. A supercritical phase transition microscopic testing and identification method according to claim 1, characterized in that: The specific expressions for calculating the spraying time T, the actual droplet size S, and the true motion speed v in step 3 are as follows: T=1 / F S=π(R²+h²) v=Δx / Δt Where F is the frame rate of the camera, h is the initial droplet height, Δx is the droplet displacement length, Δt is the time required for the droplet displacement, and R is the circle radius. The spherical radius of the droplet is r = (R² + h²) / (2h).

6. The supercritical phase transition microscopic testing and identification method according to claim 1, characterized in that: The evolution characteristics of the droplet phase change process under supercritical conditions in step 4 include differences in different initial sizes and initial velocities, where the initial size ranges from 40 μm to 240 μm, and the initial velocity ranges from 0.3 m / s to 3.6 m / s.

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