Visual test platform for temperature field of low-temperature phase-change material based on schlieren method

Abstract

The invention discloses a low-temperature phase change material temperature field visualization test platform based on a schlieren method. The platform comprises a schlieren optical imaging system, a low-temperature Dewar and an image acquisition and processing system. In the schlieren optical imaging system, light emitted by a light source forms parallel test light beams through a beam expanding collimator, and the parallel test light beams penetrate through a low-temperature phase change material area in a sample cavity in a low-temperature Dewar system, then enter a convergent lens and form focusing light spots on a focal plane of the convergent lens; the blade is arranged at the focal plane and is used for partially cutting off a focusing light spot, so that light deflection caused by the refractive index gradient in the phase change material is converted into gray scale contrast change, and a schlieren image is recorded by the high-speed camera; and the image acquisition and processing system performs background correction, deflection angle calculation, refractive index gradient inversion, temperature gradient reconstruction and temperature field reconstruction on the schlieren image, and performs dynamic visual display. The invention provides a non-contact full-field visualization test means for researching low-temperature PCM phase change temperature field distribution and phase interface evolution.

Description

Technical Field

[0001] This invention relates to the fields of cryogenic engineering, phase change energy storage and optical visualization measurement technology, and in particular to a temperature field visualization testing platform for cryogenic phase change materials based on schlieren method. Background Technology

[0002] Phase change materials (PCMs) can absorb or release a large amount of latent heat during melting and solidification, thus possessing significant application value in fields such as cryogenic energy storage, spacecraft thermal control, superconducting magnet cooling, cryogenic refrigeration buffering, and cryogenic biological preservation. The phase change process of cryogenic PCMs is accompanied by significant temperature gradients, phase interface migration, and possible natural convection and density stratification. Accurate acquisition of the spatiotemporal distribution of the temperature field (or temperature gradient field) and the evolution of the phase interface during the phase change process is a crucial foundation for revealing the heat transfer mechanism of cryogenic phase change, optimizing material and heat exchange structure design, and improving system efficiency and reliability.

[0003] Current methods for measuring temperature during low-temperature PCM phase transitions still have significant limitations. Traditional point-contact temperature measurement methods, such as thermocouples and platinum resistance thermometers, can only obtain localized, discrete temperature data, making it difficult to characterize the rapidly changing temperature gradient distribution and dynamic evolution near the phase interface. Furthermore, sensor insertion can disturb heat transfer and flow during phase transitions. While infrared thermography can achieve non-contact surface measurement, the surface emissivity of materials changes significantly with temperature and phase state at low temperatures, making accurate calibration difficult. In addition, infrared measurements cannot penetrate transparent cavities to obtain information about the interior of the PCM, failing to meet the needs of studying the internal temperature field of low-temperature phase transitions.

[0004] To obtain full-field information, researchers have developed various full-field optical measurement and visualization methods. These methods can be divided into two technical routes: interferometric full-field measurement and non-interferometric refractive index gradient visualization measurement. Interferometric methods (such as Mach-Zehnder interferometry) form interference fringes through the coherent superposition of reference and test light, and obtain the temperature field based on phase changes, making them suitable for highly sensitive absolute temperature measurements inside transparent or semi-transparent media. However, in low-temperature PCM testing scenarios, interferometry is extremely sensitive to optical path phase stability, optical path matching, and mechanical vibration. The introduction of low-temperature Dewars and their windows leads to an increased test optical path and generates additional phase noise, requiring the configuration of precise optical path compensation, vibration isolation, and phase calculation systems to maintain fringe quality. This results in complex platform structures and difficult debugging. When the sample has a strong refractive index gradient or local scattering, the interference fringes are prone to breakage, leading to a decrease in the reliability of phase inversion.

[0005] In contrast, the Schlieren method is a typical non-interferometric full-field visualization method for refractive index gradients. It utilizes the minute deflections that occur when a parallel beam passes through a transparent medium with a refractive index gradient. By placing a blade or spatial cutoff component on the focal plane, the deflection is converted into grayscale contrast on the imaging plane, thus directly displaying the spatial distribution of the refractive index gradient (i.e., temperature gradient). The Schlieren method excels at displaying strong temperature gradient regions near phase interfaces and phase transition-induced convection structures. It does not rely on coherent superposition or phase stability, is relatively robust to environmental disturbances, and has a relatively simple system setup and experimental debugging, making it more suitable for dynamic visualization of low-temperature phase transition processes.

[0006] Despite the advantages of schlieren imaging, its application in low-temperature PCM phase transition research still faces key challenges: First, low-temperature Dewar, cold screens, and multi-layer optical windows introduce background refraction and distortion effects, requiring structural design matching the schlieren optical path and effective background correction; otherwise, non-physical grayscale distributions are easily generated. Second, low-temperature phase transition research demands high temperature control accuracy and stability, and requires the ability to flexibly simulate different cooling boundary forms (such as different cold source surface geometries and positions) to study the impact of boundary conditions on the phase transition process. Third, radiation heat transfer accounts for a significant proportion in low-temperature environments; without a cold screen shield at the same temperature as the cold stage, external radiation heat leakage will cause additional temperature gradients, interfering with the characterization of real phase transition heat transfer characteristics by schlieren images. Current technology lacks a dedicated experimental platform that can integrate "full-field visualization measurement of schlieren gradients—adjustable cooling boundaries—radiation interference suppression—high-precision temperature control" in low-temperature environments, thus limiting the in-depth application of schlieren imaging in low-temperature PCM phase transition mechanism research and engineering performance evaluation. Summary of the Invention

[0007] To overcome the shortcomings of existing technologies, this invention provides a low-temperature phase change material temperature field visualization testing platform based on schlieren method. It can not only realize full-field, dynamic, non-contact visualization observation of the temperature gradient field / temperature field and phase interface evolution during the PCM phase change process at low temperature, but also solve key technical problems in low-temperature visualization measurement such as flexible simulation of boundary conditions, suppression of radiation heat interference, and ensuring the contrast and stability of schlieren images.

[0008] A low-temperature phase change material temperature field visualization testing platform based on schlieren method, including a schlieren optical imaging system, a low-temperature Dewar, and an image acquisition and processing system; The schlieren optical imaging system includes a light source, a beam expander and collimator, a first plane mirror, a second plane mirror, a converging lens, a blade, and a high-speed camera. The light emitted from the light source is collimated by a beam expander to form a parallel test beam. After being deflected by the first plane mirror, the parallel test beam passes through the optical window on one side of the low-temperature Dewar system and enters the low-temperature phase change material region inside the sample cavity. It then exits through the optical window on the other side, is deflected by the second plane mirror, and enters the converging lens to form a focused spot on its focal plane. The blade is positioned at the focal plane to partially cut off the focused spot, causing the light deflection caused by the refractive index gradient in the phase change material to be converted into a grayscale contrast change, which is then recorded by a high-speed camera to obtain a schlieren image. The image acquisition and processing system is connected to a high-speed camera and is used to perform background correction, deflection angle calculation, refractive index gradient inversion, temperature gradient reconstruction, and temperature field reconstruction on schlieren images, and to perform dynamic visualization display.

[0009] Furthermore, the light source is a laser point source or an LED point source. Preferably, an approximate point source is formed through a pinhole or the end face of an optical fiber to ensure the focus sharpness of the schlieren imaging.

[0010] Furthermore, the beam expander collimator is a beam expander collimating lens group, used to expand and collimate the diverging beam of the light source into a uniform parallel beam, and the diameter of the collimated beam is adjustable.

[0011] Furthermore, the converging lens is a convex lens or an equivalent converging imaging component, used to focus the parallel beam of light passing through the sample in the sample cavity onto the focal plane, and to record the grayscale contrast change after the blade is cut off by a high-speed camera.

[0012] Furthermore, the blade is an adjustable spatial cutoff plate, whose position and occlusion ratio can be finely adjusted in the horizontal or vertical direction to change the sensitivity and contrast of schlieren measurement.

[0013] Furthermore, the high-speed camera and the temperature control system of the low-temperature Dewar system are triggered synchronously to achieve a one-to-one correspondence between the schlieren image acquisition sequence and the melting / solidification temperature program of the phase change material.

[0014] Furthermore, the low-temperature Dewar system includes a Dewar canister and a cold source, a cold stage, a heater, a replaceable cold head, a sample chamber, and a cold screen disposed within the Dewar canister. The system comprises a cold source, a cold stage, and a heater. The cold source is thermally connected to the cold stage to provide low-temperature cooling. The replaceable cold head is detachably connected to the cold stage, with its lower end thermally connected to the sample chamber. The heater is thermally connected to the cold stage or the replaceable cold head and achieves constant or programmed temperature control via a PID controller. The cold screen surrounds the sample chamber and is at or near the same temperature as the cold stage. Optical windows are positioned relative to each other on the Dewar flask and the cold screen to ensure low-loss transmission of the schlieren test optical path.

[0015] Furthermore, the geometry of the cold source surface of the replaceable cold head is one or more of the following: a plane, a cylindrical surface, a spherical cap surface, a wedge-shaped surface, or a stepped surface, used to change the cooling boundary conditions of the sample chamber.

[0016] In the image acquisition and processing system, the image processing software is used to perform background correction, grayscale change extraction, and deflection angle / refractive index gradient inversion on the schlieren image sequence. Combined with the refractive index-temperature calibration relationship or thermo-optic coefficient (dn / dT), it realizes the reconstruction and dynamic visualization of the temperature gradient field and temperature field.

[0017] The image acquisition and processing system performs background correction, deflection angle calculation, refractive index gradient inversion, temperature gradient reconstruction, and temperature field reconstruction on the schlieren image. The specific process is as follows: S1. Schlieren Image Acquisition and Preprocessing: During the melting / solidification experiment of low-temperature phase change materials, a high-speed camera simultaneously acquired schlieren image sequences to obtain the grayscale field. For grayscale fields Dark field correction and filtering noise reduction are performed, and geometric correction is applied to the fixed distortion introduced by the optical window. Represents spatial coordinates on the imaging plane; Indicates the time of image acquisition during the experiment; S2, Background Correction: The grayscale values ​​of the background schlieren image at the initial moment were collected under a stable, isothermal state without phase change. The difference is used to obtain the schlieren image of the pure refractive index gradient response. : ; S3, Deflection Angle Calculation: According to the principle of schlieren imaging, the normalized grayscale change caused by the cutoff at the focal plane corresponds approximately linearly to the deflection angle of the test light: ; in, The system sensitivity coefficient is determined by the blade obstruction ratio, imaging focal length, and light source size, and can be obtained through calibration experiments with a known refractive index gradient or a known temperature difference field; thus, the deflection angle field is obtained. ; S4, Refractive Index Gradient Field Inversion: The optical path integral of the deflection angle and the refractive index gradient satisfies: ; ; By combining the sample thickness with the optical path geometry, the refractive index gradient field is obtained. , ; Indicates the refractive index of the sample. Represents the coordinates of the direction of light propagation; S5, Temperature gradient field / temperature field reconstruction: The refractive index gradient is converted into a temperature gradient using the thermo-optic coefficient or calibration curve of the low-temperature phase change material: ; ; When the boundary temperature of the cold head and the geometric symmetry constraint of the sample are known, the temperature gradient field can be further integrated or the Poisson equation can be solved to reconstruct the temperature field. , The thermo-optic coefficient, representing the phase change material within a temperature range, can be obtained through independent calibration experiments.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. Realize full-field visualization of low-temperature PCM phase transition process: Utilizing the sensitivity of schlieren imaging to refractive index gradient, it can non-contactly display the internal temperature gradient field and dynamic evolution of the phase interface during the phase transition process, breaking through the limitations of point temperature measurement.

[0019] 2. Strong adaptability to low temperature environment and high system robustness: Schlieren method does not rely on coherent interference and phase stability, avoids phase noise and optical path matching difficulties introduced by low temperature Dewar, and has good measurement stability and repeatability.

[0020] 3. Flexible and adjustable boundary conditions: The replaceable cold head supports multiple cooling geometry / contact methods, which facilitates systematic research on the influence of different boundary structures on low-temperature phase change heat transfer and phase interface evolution.

[0021] 4. Significant suppression of radiative heat interference: The cold screen, which is at the same temperature as the cold stage, effectively reduces radiative heat leakage, making the temperature gradient reflected by the schlieren image closer to the real phase change heat transfer process.

[0022] 5. Wide and stable temperature control range and high system integration: The cold source and heater form a wide temperature range active control system, which can cover approximately 77K to room temperature and above; the synergistic integration of optics, cryogenics and image processing simplifies experimental setup and increases efficiency. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1This is a schematic diagram of the overall structure of a low-temperature phase change material temperature field visualization testing platform based on schlieren method according to an embodiment of the present invention.

[0025] Figure 2 This is a schematic diagram of the low-temperature Dewar structure in an embodiment of the present invention.

[0026] In the figure: 1-Light source, 2-Beam expander and collimator, 3-First plane mirror, 4-Second plane mirror, 5-Converging lens, 6-Blade, 7-High-speed camera, 8-Cold source, 9-Cold stage, 10-Heater, 11-Replaceable cold head, 12-Sample chamber, 13-Cold screen, 14-Visualization window. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] It should be noted that, unless otherwise specified, the features in the following embodiments and implementation methods can be combined with each other.

[0029] A schlieren-based low-temperature phase change material temperature field visualization testing platform includes a schlieren optical imaging system, a low-temperature Dewar, and an image acquisition and processing system.

[0030] like Figure 1 As shown, the schlieren optical imaging system includes a light source 1, a beam expander and collimator 2, a first plane mirror 3, a second plane mirror 4, a converging lens 5, a blade 6, and a high-speed camera 7. The light source 1 generates a beam of light that is approximately a point source. After being expanded and collimated by the beam expander and collimator 2, the beam of light becomes a parallel test beam. The parallel test beam is deflected by the first plane mirror 3 and passes through the optical window 14 on the side wall of the low-temperature Dewar and the cold screen. After passing through the PCM sample area in the sample cavity 12, it exits from the optical window 14 on the other side. After being deflected by the second plane mirror 4, it enters the converging lens 5 and forms a focused spot on its focal plane. The blade 6 is set at the focal plane and partially cuts off the focused spot, so that the small light deflection caused by the refractive index gradient (temperature gradient) in the PCM is converted into gray-scale contrast changes on the imaging plane. The high-speed camera 7 records the schlieren image sequence.

[0031] Unlike interferometry, this invention does not require beam splitting, beam combining, or reference arm structures, thus avoiding the stringent requirements for phase stability and optical path matching under low-temperature conditions, thereby improving the robustness and operability of the system under low-temperature Dewar conditions.

[0032] The cryogenic Dewar system provides stable and controllable cryogenic boundary conditions for PCM phase transition, including the Dewar container and the cold source 8, cold stage 9, heater 10, replaceable cold head 11, sample chamber 12, cold screen 13 and optical window 14 disposed inside the Dewar container. The cold source 8 is a refrigerator cold head or a cryogenic liquid cold source (liquid nitrogen / liquid helium), which generates cooling and transfers it to the sample chamber 12 through a high thermal conductivity cold stage 9; the heater 10 is arranged on the cold stage 9 or the replaceable cold head 11, and realizes constant temperature, linear heating and cooling or stepped temperature program through a PID controller; the replaceable cold head 11 is detachably connected to the cold stage 9 through a standardized interface, and the lower end of the cold head is in contact with or thermally connected to the sample chamber 12. Different cooling boundary conditions can be simulated by replacing cold heads with different geometries; the cold screen 13 surrounds the sample chamber 12 and is at the same temperature or nearly at the same temperature as the cold stage 9, which is used to block the influence of high temperature radiation from the Dewar wall on the PCM sample and reduce radiation heat leakage; optical windows 14 are opened at relative positions on the Dewar tank and the cold screen 13 to ensure low-loss transmission of the schlieren test optical path.

[0033] The sample cavity 12 is made of a transparent, low-temperature heat-resistant material, allowing parallel test beams to pass through and housing the low-temperature phase change material. The inner surface of the cold screen 13 is a low-emissivity coating or a multi-layered heat-insulating structure, used to suppress interference from external radiative heat leakage on schlieren measurements.

[0034] The image acquisition and processing system includes a synchronization trigger module, a computer workstation, and image processing software. The high-speed camera 7 is synchronized with the temperature control system to achieve a one-to-one correspondence between the temperature program and the acquisition time of the schlieren image. The image processing software is used to perform background correction, grayscale change extraction, and deflection angle / refractive index gradient inversion on the schlieren image sequence. It also combines the refractive index-temperature calibration relationship or thermo-optic coefficient (dn / dT) to reconstruct and dynamically visualize the temperature gradient field and temperature field.

[0035] When the parallel test beam passes through the PCM region in the sample cavity 12, the temperature gradient caused by the phase transition results in a non-uniform refractive index distribution, causing a slight deflection of the test beam. The deflected beam is focused onto the focal plane by the converging lens 5 and partially cut off at the blade 6, making the deflection manifest as a grayscale change on the imaging plane of the high-speed camera 7. By analyzing the changes in grayscale with space and time, the refractive index gradient field can be obtained, and the temperature gradient field or temperature field can be further inverted. The inversion process in this embodiment includes the following steps: 1. Schlieren Image Acquisition and Preprocessing During the low-temperature PCM melting / solidification experiment, a high-speed camera 7 simultaneously acquired schlieren image sequences to obtain the grayscale field. The acquired images are subjected to dark field correction and filtering for noise reduction, and the fixed distortion introduced by the optical window 14 is geometrically corrected.

[0036] 2. Background Removal Background schlieren images were acquired under a stable, isothermal state without phase change. The schlieren image obtained by subtracting the experimental image from the background image is the pure refractive index gradient response image. ; 3. Grayscale-Deflection Angle Calibration According to the principle of schlieren imaging, the normalized grayscale change caused by the cutoff at the focal plane corresponds approximately linearly to the deflection angle of the test light: ; in, The system sensitivity coefficient is determined by the blade obstruction ratio, imaging focal length, and light source size, and can be obtained through calibration experiments with a known refractive index gradient (or a known temperature difference field). The deflection angle field is then obtained. .

[0037] 4. Refractive index gradient field inversion The optical path integral of the deflection angle and the refractive index gradient satisfies: ; ; By combining the sample thickness with the optical path geometry, the refractive index gradient field can be obtained. , .

[0038] 5. Temperature gradient field / temperature field reconstruction Using the thermo-optic coefficient or calibration curve of a low-temperature PCM, the refractive index gradient is converted into a temperature gradient: ; ; When the boundary temperature of the cold head and the geometric symmetry constraint of the sample are known, the temperature gradient field can be further integrated or the Poisson equation can be solved to reconstruct the temperature field. The final output includes a temperature gradient cloud map, isotherms (optional), and dynamic evolution results of the phase interface.

[0039] The phase transition test steps are as follows: 1. Inject low-temperature PCM into sample chamber 12 and seal it to ensure that the test beam can pass through the PCM area; 2. Select a replaceable cold head 11 with a specific geometry to establish thermal contact with the sample chamber 12 in order to set the required cooling boundary; 3. Start the cold source 8 to cool the cold table 9, and use the heater 10 to achieve constant temperature or programmable temperature rise and fall under PID control, so that the PCM undergoes melting or solidification phase change. 4. The high-speed camera 7 is synchronously triggered with the temperature control system to acquire schlieren image sequences in real time. ; 5. By performing background subtraction, deflection angle inversion, and temperature gradient / temperature field reconstruction using image processing software, the evolution of the phase interface position and temperature field (or temperature gradient field) over time can be obtained, which can be used to analyze the low-temperature phase change heat transfer characteristics under different boundary conditions.

[0040] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A visualization testing platform for the temperature field of low-temperature phase change materials based on schlieren method, characterized in that, This includes schlieren optical imaging systems, cryogenic Dewars, and image acquisition and processing systems; The schlieren optical imaging system includes a light source (1), a beam expander collimator (2), a first plane mirror (3), a second plane mirror (4), a converging lens (5), a blade (6), and a high-speed camera (7). The light emitted by the light source (1) is collimated by the beam expander (2) to form a parallel test beam. The parallel test beam is deflected by the first plane mirror (3) and passes through the optical window (14) on one side of the low temperature Dewar system into the low temperature phase change material region in the sample cavity (12). It is then emitted from the optical window (14) on the other side, deflected by the second plane mirror (4) and incident on the converging lens (5) to form a focused spot on its focal plane. The blade (6) is set at the focal plane to partially cut off the focused spot, so that the light deflection caused by the refractive index gradient in the phase change material is converted into a gray-scale contrast change, and the schlieren image is recorded by the high-speed camera (7). The image acquisition and processing system is connected to a high-speed camera (7) and is used to perform background correction, deflection angle calculation, refractive index gradient inversion, temperature gradient reconstruction and temperature field reconstruction on the schlieren image, and to perform dynamic visualization display.

2. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The light source (1) is a laser point light source or an LED point light source.

3. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The beam expander collimator (2) is a beam expander collimator lens group used to expand and collimate the diverging beam of the light source (1) into a uniform parallel beam, and the diameter of its collimated beam is adjustable.

4. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The converging lens (5) is a convex lens or an equivalent converging imaging component, used to focus the parallel beam after passing through the sample in the sample cavity (12) onto the focal plane, and to record the grayscale contrast change after the blade (6) is cut off by the high-speed camera (7).

5. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The blade (6) is an adjustable spatial cutoff plate, whose position and occlusion ratio can be finely adjusted in the horizontal or vertical direction to change the sensitivity and contrast of schlieren measurement.

6. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The high-speed camera (7) is synchronously triggered with the temperature control system of the low-temperature Dewar system to achieve a one-to-one correspondence between the schlieren image acquisition sequence and the melting / solidification temperature program of the phase change material.

7. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The low-temperature Dewar system includes a Dewar tank and a cold source (8), a cold stage (9), a heater (10), a replaceable cold head (11), a sample chamber (12), and a cold screen (13) installed inside the Dewar tank. The cold source (8), the cold stage (9) and the heater (10) constitute a temperature control system. The cold source (8) is thermally connected to the cold stage (9) to provide low-temperature cooling to the cold stage (9). The replaceable cold head (11) is detachably connected to the cold stage (9). The lower end of the replaceable cold head (11) is thermally connected to the sample chamber (12). The heater (10) is thermally connected to the cold stage (9) or the replaceable cold head (11) and achieves constant temperature or program temperature control through a PID controller. The cold screen (13) surrounds the sample chamber (12) and is at the same or nearly the same temperature as the cold stage (9). Optical windows (14) are opened at relative positions on the Dewar jar and the cold screen (13) to ensure low-loss transmission of the schlieren test optical path.

8. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The geometry of the cold source surface of the replaceable cold head (11) is one or more of the following: plane, cylindrical surface, spherical cap surface, wedge surface or stepped surface, used to change the cooling boundary conditions of the sample cavity (12).

9. The low-temperature phase change material temperature field visualization testing platform based on schlieren method according to claim 1, characterized in that, The image acquisition and processing system performs background correction, deflection angle calculation, refractive index gradient inversion, temperature gradient reconstruction, and temperature field reconstruction on the schlieren image. The specific process is as follows: S1. Schlieren Image Acquisition and Preprocessing: During the melting / solidification experiment of low-temperature phase change materials, a high-speed camera 7 simultaneously acquired schlieren image sequences to obtain the grayscale field. For grayscale fields Dark field correction and filtering noise reduction are performed, and geometric correction is performed on the fixed distortion introduced by the optical window 14. Represents spatial coordinates on the imaging plane; Indicates the time of image acquisition during the experiment; S2, Background Correction: The grayscale values ​​of the background schlieren image at the initial moment were collected under a stable, isothermal state without phase change. The difference is used to obtain the schlieren image of the pure refractive index gradient response. : ； S3, Deflection Angle Calculation: According to the principle of schlieren imaging, the normalized grayscale change caused by the cutoff at the focal plane corresponds approximately linearly to the deflection angle of the test light: ； in, The system sensitivity coefficient is determined by the blade obstruction ratio, imaging focal length, and light source size, and can be obtained through calibration experiments with a known refractive index gradient or a known temperature difference field; thus, the deflection angle field is obtained. ; S4, Refractive Index Gradient Field Inversion: The optical path integral of the deflection angle and the refractive index gradient satisfies: ； ； By combining the sample thickness with the optical path geometry, the refractive index gradient field is obtained. , , Indicates the refractive index of the sample. Represents the coordinates of the direction of light propagation; S5, Temperature gradient field / temperature field reconstruction: The refractive index gradient is converted into a temperature gradient using the thermo-optic coefficient or calibration curve of the low-temperature phase change material: ； ； When the boundary temperature of the cold head and the geometric symmetry constraint of the sample are known, the temperature gradient field can be further integrated or the Poisson equation can be solved to reconstruct the temperature field. , The thermo-optic coefficient of the phase change material is obtained through independent calibration experiments within a temperature range.

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