A method and system for refractive index compensation in optical measurements of internal flow fields

Abstract

This invention discloses a refractive index compensation method and system for optical measurement of internal flow fields, relating to the field of optical measurement. The method includes: measuring the spot position of a laser beam when it does not pass through an acrylic calibration box; directing a high-frequency laser beam perpendicularly to the outer surface of the acrylic temperature calibration box and into the box, passing through a thin-walled circular tube (not a diameter chord); capturing the laser spot position using a high-speed camera, synchronizing the laser and the high-speed camera using a synchronizer, and simultaneously capturing multiple spot images at a certain temperature; batch processing the captured spot images on a server using image processing technology to obtain the center position of the spot; comparing the horizontal displacement of the laser beam with a reference position; measuring the temperature of a sodium iodide aqueous solution; establishing a linear relationship between the horizontal displacement and the temperature of the sodium iodide aqueous solution; and determining the optimal refractive index compensation temperature based on the linear relationship. This method improves the measurement accuracy of the optimal refractive index compensation temperature.

Description

Technical Field

[0001] This invention relates to the field of optical measurement, and more particularly to a refractive index compensation method and system for optical measurement of internal flow fields. Background Technology

[0002] Currently, the geometry of nuclear reactor pressure vessels and fuel assemblies is complex, making internal flow field measurement difficult. The function of a nuclear reactor pressure vessel is to contain the reactor coolant and form the coolant pressure boundary. It is typically a nuclear-grade stainless steel pressure vessel, composed of components such as inlet pipes, inlet nozzles, a hemispherical upper head, an annular cylinder, a hemispherical lower head, an annular basket, the reactor core, outlet nozzles, and outlet pipes. Inside the upper and lower heads are complex structures including lower support plates, support columns, control rod drive mechanisms, and instrument conduits, used to support, control, and monitor the reactor core and its operating status.

[0003] Currently, there is no technology to test the optimal refractive index compensation temperature for sodium iodide aqueous solution and plexiglass, yet the optimal refractive index compensation temperature is crucial for refractive index compensation technology for sodium iodide aqueous solution and plexiglass. This is because the refractive index of plexiglass is sensitive to the wavelength of the light source and temperature, and the refractive index of sodium iodide aqueous solution is also related to the mass fraction of sodium iodide.

[0004] There are generally two methods for measuring the optimal refractive index compensation temperature: the reflection method and the refractive method. The reflection method utilizes the difference in refractive index between solids and liquids. A laser emits a laser beam, which is then incident on the surface of a solid immersed in a liquid. A laser power meter is used to measure the intensity of the reflected light from the solid surface. The test temperature is adjusted, and when the intensity of the reflected light decreases to zero, this temperature is considered the optimal refractive index compensation temperature. This method requires very high sensitivity and accuracy in the instrument used to measure the reflected light; when the intensity of the reflected light is low, accurate measurement is difficult. Therefore, this method has a relatively large measurement error and is difficult to accurately determine the optimal refractive index compensation temperature. Summary of the Invention

[0005] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is to improve the measurement accuracy of the optimal refractive index compensation temperature.

[0006] To achieve the above objectives, the present invention provides a refractive index compensation method for internal flow field optical measurement, comprising:

[0007] The position of the laser spot when the laser beam does not pass through the plexiglass calibration box is measured and used as a reference position for the laser beam to propagate in a straight line;

[0008] The laser emitted by the high-frequency laser is directed perpendicularly into the outer surface of the plexiglass temperature calibration box, passing through the chord of the thin-walled circular tube (not the diameter).

[0009] The laser spot position is captured by a high-speed camera, and the laser and the high-speed camera are synchronized by a synchronizer to capture multiple spot photos at a certain temperature.

[0010] The captured light spot images are processed in batches on the server using image processing technology to obtain the center position of the light spot;

[0011] The horizontal displacement of the laser beam is obtained by comparing it with the reference position of the laser beam.

[0012] Measure the temperature of the sodium iodide aqueous solution;

[0013] Establish a linear relationship between horizontal displacement and the temperature of sodium iodide aqueous solution;

[0014] The optimal refractive index compensation temperature is determined based on the linear relationship.

[0015] Furthermore, the method further includes: adjusting the temperature of the sodium iodide aqueous solution with a mass fraction of W in the calibration box, thereby adjusting the refractive index difference between the sodium iodide aqueous solution and the plexiglass, so that the laser beam passing through the thin-walled circular tube undergoes different horizontal displacements, obtaining the horizontal displacement of the laser beam at multiple temperatures, wherein 0.63≤W≤0.64.

[0016] Furthermore, the method also includes: fitting a linear relationship between displacement and temperature to determine the temperature when the horizontal displacement is zero, and the temperature when the horizontal displacement is zero is the optimal refractive index compensation temperature.

[0017] Furthermore, the spatial resolution of the high-speed camera is 20 micrometers per pixel.

[0018] Furthermore, the multiple light spot photos include 1000 light spot photos.

[0019] Furthermore, the method also includes setting a temperature measuring point every 0.3℃ within the range of 20-50℃, increasing the number of temperature measuring points to 100.

[0020] Furthermore, the method also includes: measuring the optimal refractive index compensation temperature of the plexiglass using a 527nm wavelength laser and within a temperature range of 20-50℃; and obtaining the optimal refractive index compensation temperature by fitting a linear function using the horizontal displacement and temperature data from 100 measured experimental data points.

[0021] Furthermore, the method also includes: a spot center position measurement error of 1 pixel, a horizontal displacement range of 200 pixels relative to the spot with an error of 0.5%, a temperature error of 0.15℃ within the range of 20-50℃, a thermocouple temperature measurement error of 0.1℃, a linear function fitting error of 0.2%, a temperature error of 0.06℃ within the range of 20-50℃, and a comprehensive uncertainty of 0.19℃ for the optimal refractive index compensation temperature.

[0022] Furthermore, the method also includes: using a high-speed camera with a pixel size of 7.4 micrometers to improve the measurement accuracy of the spot center position to 0.2%, with a corresponding temperature error of 0.06℃ in the range of 20-50℃, and a comprehensive uncertainty of 0.13℃ for the optimal refractive index compensation temperature.

[0023] The present invention also provides a refractive index compensation system for optical measurement of internal flow field, including a server, a high-frequency laser, a high-speed camera, a synchronizer, an organic glass temperature calibration box with an internal organic glass thin-walled circular tube, an electromagnetic stirrer, and a light screen;

[0024] An electric heating rod and a thermocouple are installed on the flange of the acrylic calibration box. Together with an electromagnetic stirrer, they control and measure the temperature of the sodium iodide aqueous solution in the calibration box, so that the sodium iodide aqueous solution and the acrylic thin-walled circular tube reach a uniform temperature.

[0025] Technical effects:

[0026] 1. By selecting a sodium iodide aqueous solution with a mass fraction in the range of 0.63-0.64%, high-precision refractive index compensation can be achieved throughout the year in temperatures ranging from 0-50℃.

[0027] 2. By utilizing the slight difference in refractive index to generate a large horizontal displacement of the laser beam and using a high-speed camera to capture the light spot, the optimal refractive index compensation temperature is measured, improving the measurement accuracy to the range of 0.1-0.2℃ and achieving high-precision refractive index compensation.

[0028] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0029] Figure 1 This is a front view of the optimal refractive index compensation temperature measurement system according to a preferred embodiment of the present invention;

[0030] Figure 2 This is a top view of a preferred embodiment of the optimal refractive index compensation temperature measurement system of the present invention;

[0031] Figure 3 This is a schematic diagram of the optimal refractive index compensation temperature measurement results of a preferred embodiment of the present invention. Detailed Implementation

[0032] The following description, with reference to the accompanying drawings, illustrates several preferred embodiments of the present invention to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0033] In the accompanying drawings, components with the same structure are indicated by the same numerical designation, and components with similar structures or functions are indicated by similar numerical designations. The dimensions and thicknesses of each component shown in the drawings are arbitrary, and the present invention does not limit the dimensions and thicknesses of each component. To make the illustrations clearer, the thickness of some components has been appropriately exaggerated in the drawings.

[0034] In existing technologies, nuclear reactor pressure vessels and fuel assemblies are geometrically complex, making internal flow field measurements difficult. The function of a nuclear reactor pressure vessel is to contain the reactor coolant and form the coolant pressure boundary. It is typically a nuclear-grade stainless steel pressure vessel, composed of components such as inlet pipes, inlet nozzles, a hemispherical upper head, an annular cylinder, a hemispherical lower head, an annular basket, the reactor core, outlet nozzles, and outlet pipes. Inside the upper and lower heads are complex structures including lower support plates, support columns, control rod drive mechanisms, and instrument conduits, used to support, control, and monitor the reactor core and its operating status.

[0035] Typically, the pressure vessel of a megawatt-class pressurized water reactor (PWR) is approximately 4 meters in diameter and 10 meters in height. Examples include the Westinghouse AP1000 pressure vessel and the Rosatom VVER pressure vessel. Coolant enters the pressure vessel through the inlet pipe and inlet nozzle, flowing downwards along the descending section between the inner wall of the pressure vessel and the outer wall of the hopper, entering the lower head. Under the influence of internal structural components within the lower head, such as eddy current suppressors, flow distribution groups, or fairings, it enters the fuel assemblies through flow distribution holes on the lower core support plate. The coolant flows upwards within the fuel assemblies in the core, passing the upper core support plate and entering the upper chamber. After flushing the control rod guides and instrument conduits within the upper chamber, the coolant exits the pressure vessel through the outlet nozzle. During this process, the coolant undergoes multiple direction changes after entering the pressure vessel, especially when entering the lower chamber. Because the lower chamber is approximately hemispherical, the abrupt changes in flow path generate numerous vortices, leading to significant flow instability. Therefore, vortex suppressor plates and flow distribution skirts are needed to suppress these vortices and control the flow rate into each channel of the reactor core. The reactor core often incorporates components such as positioning grids and mixing wings to enhance heat transfer. Reactor pressure vessels typically have three horizontally arranged inlets and three horizontally arranged outlets, or four horizontally arranged inlets and two horizontally arranged outlets.

[0036] Liquid metal-cooled reactors include sodium-cooled fast reactors, lead-cooled fast reactors, and lead-bismuth alloy-cooled fast reactors. They all contain an internal heat exchanger, a fairing, a core, control rods, and a riser section. The coolant is naturally circulated from the heat exchanger to the core and then to the riser section through a coolant density difference. Sodium-cooled fast reactors can also be equipped with electromagnetic pumps to achieve forced circulation. The fairing is used to manage the flow distribution at the core inlet, providing good inlet boundary conditions for fuel assembly cooling and temperature flattening. For example, the Xe-100 pebble bed high-temperature gas-cooled reactor designed by X-energy in the United States uses spherical fuel elements, which are stacked inside a pressure vessel to form the core. Helium is used as the coolant, and a compressor creates forced circulation to cool the core. The reactor power is regulated by control rods. Fuel spheres can be refueled online via a refueling mechanism at the bottom of the core.

[0037] Pressure vessels, with pressurized water reactors as a typical example, have complex internal flow channel geometry, exhibiting complex flow phenomena such as abrupt channel contractions, jet impacts, multiple sharp turns in flow, curved surface flow, flow around cylinders, and orifice plate flow. These phenomena are key to influencing the core inlet flow distribution and determine heat and mass transfer within the fuel assemblies. Therefore, refined research on the flow field within the pressure vessel is urgently needed. However, the conical geometry of the pressure vessel inlet nozzle, the annular geometry of the descending section, the hemispherical shell geometry of the lower head, the complex geometry of the internal components of the lower head, the complex geometry of the internal components of the upper chamber, and the complex geometry of the fuel assemblies all present challenges to velocity field measurement.

[0038] Pressurized water reactor fuel assemblies consist of cylindrical rod bundles and positioning grids. The fuel assemblies are arranged in square (US AP1000 fuel assemblies) or regular hexagonal (Russian VVER fuel assemblies) patterns. Positioning grids are installed at different heights of the rod bundles to fix their position and promote heat and mass transfer. Liquid metal (liquid sodium, liquid lead, or liquid lead-bismuth alloy) cooled reactor fuel assemblies consist of hexagonal rod bundles and metal wires wound around the surface of the rod bundles. The circular metal wires serve to position the rod bundles and promote heat and mass transfer. The core of a pebble bed reactor is composed of spherical fuel pellets with a diameter of 60 mm, and helium gas is used to cool the core from top to bottom.

[0039] The flow state of the coolant within a rod-bundle fuel assembly is primarily determined by the rod bundle geometry and positioning devices (positioning grids or wire windings), while the coolant flow within the core of a pebble bed reactor is determined by the fuel spheres. Therefore, studying the flow field distribution within fuel assemblies is crucial for investigating fuel heat transfer performance and improving fuel economy and safety. However, the complex geometry of fuel assemblies presents challenges for flow field measurement. By employing appropriate refractive index compensation techniques, selecting suitable transparent solid materials to fabricate pressure vessel or fuel assembly experimental models, and choosing a suitable liquid working medium to simulate the coolant, the refractive indices of the solid and liquid working media in the geometrically complex pressure vessel or fuel assembly can be compensated to be the same. This allows for the use of mature optical visualization measurement techniques, such as particle image velocimeters, to conduct refined measurements of the flow field within the pressure vessel or fuel assembly.

[0040] To conduct precise flow field measurements within pressure vessels or fuel assemblies, high-precision refractive index compensation technology is required. This necessitates a transparent solid with high transmittance, high mechanical strength, good machinability, and the ability to adapt to complex geometries. The liquid working fluid must be chemically stable, non-toxic, odorless, non-volatile, and have minimal viscosity to ensure the rationalization of the measurement device and experimental model dimensions under similar Reynolds numbers. This avoids the problem of excessively large experimental system sizes due to the high viscosity of the liquid working fluid. Currently, commonly used refractive index compensation techniques in published literature mainly include fluorinated ethylene propylene copolymer with water, p-isopropyltoluene with plexiglass, and sodium iodide aqueous solution with plexiglass. Among these, the refractive index compensation accuracy of fluorinated ethylene propylene copolymer with water is not high, limiting its application to measuring the cross-sectional flow field of rod-bundle fuel assemblies. Furthermore, the poor transmittance of fluorinated ethylene propylene copolymer restricts its application to the fabrication of thin-walled short rod bundles. Therefore, its applicability is limited and it is difficult to meet the measurement needs of pressure vessels and various fuel assemblies. While the refractive index compensation accuracy of isopropyltoluene with plexiglass is slightly higher, the high viscosity of isopropyltoluene and the large size of the experimental models make it difficult to meet the measurement requirements of pressure vessels and various fuel assemblies. The refractive index compensation technology using sodium iodide aqueous solution and plexiglass has the following advantages: plexiglass has high light transmittance, high mechanical properties, excellent processing performance, and can adapt to complex geometric requirements; sodium iodide aqueous solution is chemically stable, non-toxic, odorless, non-volatile, and its kinematic viscosity is close to that of water, making it suitable for measuring pressure vessels and various fuel assemblies. Therefore, the refractive index compensation technology using sodium iodide aqueous solution and plexiglass has been selected as a refined optical refractive index compensation measurement technique for the flow field within nuclear reactor pressure vessels and fuel assemblies.

[0041] Currently, there are publicly available international publications on the use of sodium iodide aqueous solution and plexiglass refractive index compensation technology, such as the refractive index compensation experimental device at Johns Hopkins University in the United States (see: O. Uzol, YCChow, J. Katz, C. Meneveau. Unobstructed particle image velocimetry measurements within anaxial turbo-pump using liquid and blades with matched refractive indices. Experiments in Fluids 33 (2002) 909–919) and the PRIUS-I experimental device at the Korea Atomic Energy Research Institute (see: Seok Kim, Byong Gook Jeon, Hae-Seob Choi, Dong-Jin Euh, Sang-KiMoon. Experimental visualization of flow structure inside subchannels of a 4x6rod-bundle. Annals of Nuclear Energy 140 (2020) 107097). However, neither of these methods provides a technique for testing the optimal refractive index compensation temperature for sodium iodide aqueous solution and plexiglass, which is crucial for refractive index compensation technology. This is because the refractive index of plexiglass is sensitive to the wavelength and temperature of the light source, and the refractive index of sodium iodide aqueous solution is also related to the mass fraction of sodium iodide.

[0042] There are generally two methods for measuring the optimal refractive index compensation temperature: the reflection method and the refraction method (see: R. Budwig. Refractive index matching methods for liquid flow investigations. Experiments in Fluids, 17, 1994, 350-355). The reflection method utilizes the difference in refractive index between solids and liquids. A laser emits a laser beam, which is then incident on the surface of a solid immersed in the liquid. A laser power meter is used to measure the intensity of the reflected light from the solid surface. The test temperature is adjusted, and when the intensity of the reflected light decreases to zero, this temperature is considered the optimal refractive index compensation temperature. This method requires very high sensitivity and accuracy of the instrument used to measure the reflected light; when the intensity of the reflected light is low, accurate measurement is difficult. Therefore, this method has a relatively large measurement error and is difficult to accurately obtain the optimal refractive index compensation temperature.

[0043] The refraction method uses a camera to photograph the area of ​​the shadow cast by light refraction behind a sphere immersed in a liquid working medium. A magnetic stirrer is used to adjust the test temperature, and the optimal refractive index compensation temperature is considered reached when the refracted shadow area decreases to zero. However, this method has drawbacks, including the difficulty in achieving uniform sphere temperature, the susceptibility of shadow area determination to subjective observation, and the inability to guarantee the accuracy of the optimal refractive index compensation temperature measurement.

[0044] In summary, the existing technology has the following drawbacks:

[0045] 1. The reflection method for optimal refractive index compensation temperature measurement has very high requirements for the sensitivity and error of the reflected light measuring instrument. When the intensity of the reflected light is low, it is difficult to measure accurately.

[0046] 2. The refraction method for optimal refractive index compensation temperature measurement has the disadvantages of uneven temperature of the sphere and the susceptibility of the shadow area judgment to the subjective influence of the observer, making it difficult to guarantee measurement accuracy.

[0047] 3. The above methods cannot amplify the minute refractive index difference between transparent solids and liquids in refractive index compensation technology, nor can they measure this minute difference using precise measuring instruments. Furthermore, the measurement system is complex and it is difficult to guarantee measurement accuracy.

[0048] like Figure 1 and Figure 2 As shown, to solve the technical problems in the prior art, in Embodiment 1 of the present invention, a refractive index compensation method for internal flow field optical measurement is provided, which measures the optimal temperature for refractive index compensation of sodium iodide aqueous solution and plexiglass; based on the measured experimental data of the refractive index of plexiglass and sodium iodide aqueous solution, the empirical relationship between their refractive indices is fitted as follows:

[0049] n PMMA (T,λ)=1.57575856-(1.05×10 -4 ℃ -1 )T-(2.3785×10 -4 nm -1 )λ+(1.6707×10 - 7 nm -2 )λ 2 n NaI (T,c,λ)=1.20786821-(2.31429×10 -4 ℃ -1 )T+(0.43229)c+(5542nm 2 )λ -2

[0050] In the formula, the unit of temperature T is ℃, the unit of laser wavelength λ is nm, the unit of sodium iodide mass fraction c is 1, and the refractive index measurement accuracy is 0.0005.

[0051] Considering the year-round temperature range of 0-50℃ and commonly used air-cooled tower and electric heating temperature control technologies, the optimal refractive index compensation temperature is set at around 30℃ to achieve precise temperature control of the sodium iodide aqueous solution throughout the year. The high-frequency laser wavelength commonly used in time-resolved particle imaging velocimeters is 527nm; considering the refractive index of plexiglass is approximately 1.490, the mass fraction of the sodium iodide aqueous solution can be between 0.63 and 0.64, for example, 0.631. Therefore, the empirical relationship between the refractive indices of plexiglass and sodium iodide aqueous solution simplifies to:

[0052] n PMMA (T)=1.496811794-(1.05×10 -4 ℃ -1 )T

[0053] n NaI (T)=1.500597904-(2.31429×10 -4 ℃ -1 )T

[0054] The optimal refractive index compensation temperature can be estimated to be 29.95℃ using empirical formulas. Considering a refractive index measurement accuracy of 0.0005, the corresponding error of the optimal refractive index temperature is 2.16℃. This error is too large and cannot meet the requirements for precise measurement of the flow field within the complex geometry of pressure vessels and fuel assemblies.

[0055] Therefore, it is necessary to amplify the minute refractive index difference between plexiglass and sodium iodide aqueous solution, and to measure this minute difference using precise optical instruments. This patent employs the refractive method, such as... Figure 1 and Figure 2 As shown, the optimal refractive index compensated temperature measurement system includes a server, a high-frequency laser, a high-speed camera, a synchronizer, an acrylic temperature calibration box with a built-in thin-walled acrylic tube, an electromagnetic stirrer, and a light screen. An electric heating rod and thermocouple are mounted on the flange of the acrylic calibration box, which, together with the electromagnetic stirrer, control and measure the temperature of the sodium iodide aqueous solution inside the calibration box, ensuring that the sodium iodide aqueous solution and the acrylic thin-walled tube reach a uniform temperature state as quickly as possible. The acrylic thin-walled tube is designed to achieve a uniform temperature state as quickly as possible.

[0056] To measure the optimal refractive index compensation temperature of sodium iodide aqueous solution and plexiglass, the position of the laser beam spot before it passes through the plexiglass calibration box is first measured as a reference position for the laser beam's straight-line propagation. Then, a high-frequency laser beam is directed perpendicularly into the plexiglass temperature calibration box, passing through a thin-walled circular tube (not the diameter chord). Due to the slight refractive index difference between the plexiglass and the sodium iodide aqueous solution, the laser beam is refracted at a small angle after passing through the thin-walled circular tube, resulting in a horizontal displacement. A screen is mounted 3 meters away from the plexiglass calibration box to magnify this horizontal displacement. A high-speed camera is used to photograph the laser spot position. The laser and high-speed camera are synchronized using a synchronizer, and 1000 spot images are simultaneously captured at a specific temperature. The captured spot images are then batch-processed on a server using image processing technology to obtain the center position of the spot. Comparing this to the laser beam reference position yields the horizontal displacement of the laser beam.

[0057] The temperature of a sodium iodide aqueous solution with a mass fraction of W inside the calibration box is adjusted using an electric heating rod above and a magnetic stirrer below, preferably W = 0.631. This adjusts the refractive index difference between the sodium iodide aqueous solution and the acrylic glass, causing different horizontal displacements of the laser beam passing through the thin-walled circular tube. The temperature of the sodium iodide aqueous solution is measured by a thermocouple located above the calibration box. Because the refractive index change caused by temperature variation is very small, and the refractive index difference between the acrylic glass and the sodium iodide aqueous solution is small, the horizontal displacement of the laser beam maintains a linear relationship with temperature. After obtaining the horizontal displacement of the laser beam at multiple temperatures, the linear relationship between displacement and temperature is fitted to determine the temperature at which the horizontal displacement is zero, which is the optimal refractive index compensation temperature.

[0058] To improve the measurement accuracy of the optimal refractive index compensation temperature, a high-speed camera with a spatial resolution of 1 pixel (20 micrometers) was used to reduce systematic errors, and up to 1000 samples were used to reduce random errors. Temperature measurement points were set every 0.3℃ within the 20-50℃ range, increasing the number of temperature measurement points to 100, which reduced the random errors when using linear regression to fit the linear function between horizontal displacement and temperature. The thermocouples used to measure the temperature were calibrated by the local metrology bureau, achieving a temperature measurement accuracy of 0.1℃.

[0059] For a sodium iodide aqueous solution with a mass fraction of 0.631, the optimal refractive index compensation temperature of the acrylic glass was measured under a 527 nm laser wavelength and within a temperature range of 20-50 °C. The measurement results are as follows: Figure 3As shown in the figure, the optimal refractive index compensation temperature of 30.28℃ was obtained by fitting a linear function using horizontal displacement and temperature data from 100 experimental data points. Errors in the experiment included spot center position error, thermocouple temperature measurement error, and linear function fitting error. The spot center position measurement error was 1 pixel, which corresponds to a 0.5% error relative to the horizontal displacement range of 200 pixels, and a temperature error of 0.15℃ within the 20-50℃ range. The thermocouple temperature measurement error was 0.1℃. The linear function fitting error was 0.2%, corresponding to a temperature error of 0.06℃ within the 20-50℃ range. The overall uncertainty of the optimal refractive index compensation temperature was 0.19℃. Compared to the measurement results based on the refractive index of plexiglass and sodium iodide aqueous solution, the estimated optimal refractive index compensation temperature measurement error of 2.16℃ represents an order of magnitude improvement in measurement accuracy. Furthermore, to further improve measurement accuracy, a camera with a smaller pixel size can be used to capture images of the light spot, such as a camera with a pixel size of 7.4 micrometers. This can improve the measurement accuracy of the center position of the light spot to 0.2%, corresponding to a temperature error of 0.06℃ in the range of 20-50℃. Ultimately, the overall uncertainty of the optimal refractive index compensation temperature is 0.13℃.

[0060] In summary, the method steps provided in Example 1 are as follows:

[0061] (1) Measure the position of the laser spot when the laser beam does not pass through the plexiglass calibration box, and use it as a reference position;

[0062] (2) The laser emitted by the high-frequency laser is directed perpendicularly into the calibration box from the outer surface of the box, passing through the chord of the thin-walled circular tube (not the diameter).

[0063] (3) Install the screen 3 meters away from the calibration box to amplify the horizontal displacement of the laser beam spot caused by the slight difference in refractive index;

[0064] (4) Use a high-speed camera to photograph the location of the laser spot, synchronize the laser and the high-speed camera through a synchronizer, and take 1,000 photos of the spot at a certain temperature.

[0065] (5) On the server, batch process the captured spot images using image processing technology to obtain the center position of the spot and compare it with the reference position of the laser beam spot to obtain the horizontal displacement of the laser beam;

[0066] (6) Adjust the temperature of the sodium iodide aqueous solution by using the electric heating rod inside the calibration box and the magnetic stirrer outside the calibration box to make the laser beam spot passing through the thin-walled circular tube undergo different horizontal displacements;

[0067] (7) After the temperature of the sodium iodide aqueous solution and the plexiglass stabilizes, the temperature is measured by the thermocouple located in the calibration box;

[0068] (8) After obtaining the horizontal displacement of the laser beam spot at multiple temperatures, fit the linear relationship between displacement and temperature, and find the temperature at which the horizontal displacement is zero, which is the optimal refractive index compensation temperature.

[0069] (9) After the measurement is completed, analyze the measurement error, including the error of the center position of the spot, the error of the thermocouple temperature measurement and the error of the linear function fitting, and give the uncertainty of the temperature measurement result with the best refractive index compensation.

[0070] In Embodiment 2 of the present invention, a refractive index compensation system for internal flow field optical measurement is provided, which is the optimal refractive index compensation temperature measurement system described in Embodiment 1. Its structure and function are the same as those of the optimal refractive index compensation temperature measurement system in Embodiment 1, and will not be described again in this embodiment.

[0071] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method of refractive index compensation for optical measurement of internal flow fields, characterized in that, The method comprises the following steps: Measuring the position of the laser spot when the laser beam does not pass through the plexiglass calibration box, as the reference position of the laser beam along the straight line; The laser emitted by the high-frequency laser is vertically incident into the box through the outer surface of the plexiglass temperature calibration box, and passes through the chord of the thin-walled circular tube which is not the diameter; Using a high-speed camera to take pictures of the laser spot position, synchronizing the laser and high-speed camera through a synchronizer, and synchronously taking multiple photos of the laser spot at a certain temperature; Batch processing the photographed spot images on the server through image processing technology to obtain the center position of the spot; Comparing with the reference position of the laser beam to obtain the horizontal displacement of the laser beam; Measuring the temperature of the sodium iodide aqueous solution; Establishing a linear relationship between the horizontal displacement and the temperature of the sodium iodide aqueous solution; According to the linear relationship, the optimal refractive index compensation temperature is determined.

2. The method of claim 1, wherein, The method further comprises adjusting the temperature of the sodium iodide aqueous solution with a mass fraction of W in the calibration box, thereby adjusting the refractive index difference between the sodium iodide aqueous solution and the plexiglass, making the laser beam passing through the thin-walled circular tube have different horizontal displacements, and obtaining the horizontal displacement of the laser beam at multiple temperatures, wherein 0.63≤W≤0.

64.

3. The method of claim 2, wherein, The method further comprises fitting the linear relationship between the displacement and the temperature, and obtaining the temperature when the horizontal displacement is zero, which is the optimal refractive index compensation temperature.

4. The method of claim 1, wherein, The spatial resolution of the high-speed camera is 1 pixel 20 microns.

5. The method of claim 1, wherein, The multiple spot photos include 1000 spot photos.

6. The method of claim 1, wherein, The method further comprises setting a temperature measuring point every 0.3℃ in the range of 20-50℃, and increasing the number of temperature measuring points to 100 points.

7. The method of claim 1, wherein, The method further comprises measuring the optimal refractive index compensation temperature of the laser with a wavelength of 527 nm and in the range of 20-50℃, fitting a linear function to obtain the optimal refractive index compensation temperature by using the measured horizontal displacement and temperature data of the 100 experimental data points.

8. The method of claim 1, wherein, The method further comprises that the measurement error of the center position of the spot is 1 pixel, the error is 0.5% relative to the horizontal displacement range of the spot which is 200 pixels, the corresponding temperature error in the range of 20-50℃ is 0.15℃, the thermocouple temperature measurement error is 0.1℃, the linear function fitting error is 0.2%, and the corresponding temperature error in the range of 20-50℃ is 0.06℃, and the comprehensive uncertainty of the optimal refractive index compensation temperature is 0.19℃.

9. The method of claim 1, wherein, The method further comprises using a high-speed camera with a pixel size of 7.4 microns to improve the measurement accuracy of the center position of the spot to 0.2%, the corresponding temperature error in the range of 20-50℃ is 0.06℃, and the comprehensive uncertainty of the optimal refractive index compensation temperature is 0.13℃.

10. A refractive index compensation system for optical measurement of internal flow fields, characterized in that The method comprises a server, a high-frequency laser, a high-speed camera, a synchronizer, a plexiglass temperature calibration box with a built-in plexiglass thin-walled circular tube, an electromagnetic stirrer, and a light screen. The electric heating rod and the thermocouple are installed on the flange of the plexiglass calibration box, and together with the electromagnetic stirrer, they control and measure the temperature of the sodium iodide aqueous solution in the calibration box, so that the temperature of the sodium iodide aqueous solution and the plexiglass thin-walled circular tube reaches a uniform temperature state.

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