Natural circulation two-channel visual experimental analysis device

Abstract

The invention relates to a natural circulation double-channel visualization experiment analysis device which is composed of a circulation loop and an analysis system, wherein the circulation loop is formed by sequentially connecting a circulation pump, a preheater, a parallel channel visualization experiment section, a condenser and a back pressure valve. The circulation mode is natural circulation established by the preheater serving as a heat source and the condenser serving as a cold source; the visual experiment section comprises two flow channels which are arranged in parallel and are made of different materials, at least two communicating channels between the parallel flow channels, visual windows which are respectively arranged on the side walls of the two flow channels and are made of visual materials with different light transmittances, and a heating device for heating flowing working media in the two flow channels; the analysis system is used for observing the mesoscopic characteristics of the flowing working medium in the parallel channel visual experiment section through a visual observation window; collecting flow field information, and optimizing and screening the collected data; obtaining a natural circulation instability occurrence boundary diagram by using related software based on the screened data, and establishing a database to obtain an instability flow mathematical model; the experimental device is a visualized medium-high pressure natural circulation intelligent analysis rack with two channels, working media between the two channels can be mixed, and the inclination angle of an experimental section can swing.

Description

Technical Field

[0001] The invention relates to the field of nuclear energy technology or mechanical equipment technology, in particular to a natural circulation dual-channel visual experimental analysis device. Background Art

[0002] Natural circulation is a type of energy transmission that can drive the circulation of fluids only by the driving force generated by the density difference between cold and hot fluids. No external power source is required, which can reduce the system's dependence on external power sources and effectively improve the safety of the operating system. It is the main circulation cooling method for advanced nuclear reactors during operation and an important means of emergency cooling after a shutdown accident. However, two-phase flow can easily cause large fluctuations in flow rate, pressure drop, and cross-sectional steam content due to small disturbances, which is similar to the vibration phenomenon in mechanical systems. Continuous flow oscillations can cause forced mechanical vibrations in the equipment, causing resonance. Flow oscillations can also greatly affect local heat transfer performance, significantly reduce critical heat flux density, and seriously endanger the safety of system operation.

[0003] Miniaturization and integration are important directions for the development of two-phase systems. Narrow rectangular channels in natural circulation systems have thermal hydraulic characteristics such as compact structure, enhanced heat transfer, and flexible design and application. Narrow rectangular channels in parallel with various arrangements can better protect the reactor. However, the hydraulic diameter of narrow rectangular channels is small, the allowable heat flux is large, and the bubbles are more susceptible to extrusion and deformation due to the narrow space effect. Therefore, flow instability is more likely to occur in the channel, and the generation mechanism and pulsation change law of various flow instabilities are different from those of conventional channels. At the same time, in small integrated nuclear reactors, due to the small space, a large number of parallel narrow rectangular channels are used in the passive safety measures. These channels will affect each other when unstable flow occurs, resulting in more intense unstable flow, which seriously affects the normal operation of the equipment.

[0004] At present, natural circulation experiments at home and abroad are mainly aimed at single channels within the normal pressure range. Yang Ruichang, Salah and others have studied the natural circulation instability flow through experiments, and the obtained criterion relationship is in good agreement within the lower pressure range. However, in the above experiments, the channel types are mostly circular tubes, and the pressure range involved in the experiment is extremely limited. As the requirements of the equipment for the working environment pressure, temperature, etc. gradually increase, it is necessary to explore the natural circulation characteristics within a higher pressure range. The existing parallel channels are mainly used in compact nuclear submarines, so the experiments are mainly based on the swing conditions in the ocean. Yan Chaoxing, Otsuji and others have explored the influence of the amplitude and period of the channel under external swing on the pressure drop of the two-phase flow and the location of the CHF point through experiments. Most of the above experiments are from external influences, and there is little research on the influence of the instability flow inside the channel. Zhou Tao, Fang Xiaolu, Sheng Cheng and others have conducted natural circulation experiments on narrow rectangular single channels at normal pressure, but they mainly focus on the process of bubble generation and the changes in heat transfer characteristics, and lack the division and research of medium and high pressure instability flows. At the same time, Zhou Tao and others from the applicant team have applied for a patent for a single-channel supercritical / subcritical water natural circulation experimental bench, and built an experimental bench to conduct some research, but the physical properties of supercritical fluids are quite different from those of conventional fluids, and the experimental bench is only for a single vertical channel, so its applicability is poor. With the development of small integrated nuclear reactors, the importance of narrow rectangular parallel channels has begun to emerge, and the angles of channel assembly have become more diverse with equipment requirements, but there is currently a lack of medium and high pressure related instability experiments. Therefore, the applicant believes that it is necessary to conduct a detailed study of the natural circulation flow instability of narrow rectangular parallel channels and find the stable boundary for safe operation. Summary of the invention

[0005] The purpose of the present invention is to address the problems existing in the background technology and propose a natural circulation dual-channel visualization experimental analysis device, the purpose of which is to use visualization means to observe the distribution of bubbles and flow pattern change characteristics in the channel when instability occurs, and obtain the main characteristics of the instability.

[0006] The technical solution of the present invention is a natural circulation dual-channel visualization experiment analysis device, comprising a circulation loop and an analysis system formed by sequentially connecting a circulation pump, a preheater, a parallel channel visualization experiment section, a condenser and a back pressure valve, and the characteristic is that the circulation loop is a natural circulation loop established by using a preheater as a heat source and a condenser as a cold source; the storage tank is used to store liquid or gas, the inlet and outlet ends of the parallel channel visualization experiment section are both provided with a flow meter and a temperature meter, and a pressure gauge is connected in parallel between the inlet and outlet ends of the parallel channel visualization experiment section;

[0007] The analysis system includes an image acquisition device and a data acquisition and analysis system; the image acquisition device is used to observe the mesoscopic characteristics of the flowing medium in the parallel channel visualization test section through a visual window, including the temperature, pressure, pulsation frequency, period, amplitude, and steam content after the unstable flow occurs; the collected data is screened and optimized using an artificial intelligence algorithm; based on the screened data, the natural circulation instability occurrence boundary map is obtained using the existing software origin, and a database is established to obtain an unstable flow mathematical model, and a prediction model is constructed based on artificial intelligence learning to achieve model prediction results through virtual experiments;

[0008] The structure of the parallel channel visualization experimental section includes at least two parallel flow channels arranged in parallel, visual windows respectively arranged on the side walls of at least two flow channels, and a heating device for heating the flowing working medium in at least two flow channels; the two flow channels are arranged in parallel, cross or herringbone shape, and the cross-section of each flow channel is a narrow rectangle with a relatively large length and width.

[0009] Preferably, the inner walls of the two flow channels have different roughnesses and are both provided with protrusions that can be independently controlled to move up and down; the number, height and size of the protrusions are controlled by a controller; the protrusions form vortex generators in the flow channel to disturb the working medium in the flow channel;

[0010] Preferably, two connecting channels are provided between the two parallel flow channels, and are respectively located at the two ends of the parallel flow channels, for mixing the working fluids in the two flow channels; valves are provided on the two parallel flow channels and the two connecting channels; wherein electric valves are provided at the two ends of the parallel flow channels, respectively, and the opening and closing are electrically controlled to realize the connection of any one or both flow channels to the circulation loop; both connecting channels are provided with valves composed of β-eucryptite-silicon carbide composite thermosensitive material and photosensitive material; the opening and closing of the valves are controlled in a passive manner to control the mixing of the working fluids in the two flow channels. The aforementioned valve is a passive component, which refers to a component that performs its function without relying on external input. There are generally no active components in the passive component, and the execution of its function is completed after sensing the change of certain parameters, such as pressure, temperature, and flow rate. In the present application, the opening and closing of the valve is controlled by the change of temperature and light.

[0011] Preferably, at least two flow channels are respectively located on both sides of the cover plate, and the viewing window is sealed and installed on the side of the flow channel through a locking piece, and a heating device for indirectly heating the working medium in the flow channel is installed in the middle of the cover plate; the heating device is an electric heating plate or a heat pipe, and the electric heating plate or the heat pipe is fixed by a high-temperature resistant material gasket; or a transparent heating film is used to cover the viewing window for indirect heating.

[0012] The material of the viewing window is quartz glass, aluminosilicate glass, sapphire or high silica glass;

[0013] The flow channel material is titanium or magnesium alloy;

[0014] The cover material is high-strength steel;

[0015] The high temperature resistant sealing gasket is an asbestos gasket, a flexible graphite gasket, a metal composite gasket or a polytetrafluoroethylene gasket.

[0016] Preferably, the flowing working medium in the circulation loop is deionized water, and its working temperature is 20° C.-600° C. and its working pressure is 0.1 MPa-14.2 MPa.

[0017] The preheater uses indirect heating to preheat the flowing working medium; the cold side of the condenser is cooling water.

[0018] It also includes a pressure stabilizing tank which is connected in series in the circulation loop and is located downstream of the condenser for stabilizing the pressure of the circulation loop.

[0019] The inlet and outlet ends of the parallel channel visualization experiment section are respectively provided with arc connectors, on which a plurality of interfaces are provided along the arc direction. The inlet and outlet of the parallel channel visualization experiment section are connected to different interfaces to adjust the inclination angle of the entire parallel channel visualization experiment section so as to conduct experiments with different inclination angles.

[0020] A natural circulation unstable flow measurement method of the natural circulation dual-channel visual experimental analysis device as described above comprises:

[0021] Turn on the booster pump at the outlet of the storage tank and inject deionized water into the circulation loop. When the pressure reaches the set value, turn on the circulation pump.

[0022] Turn on the preheater and condenser to form heat source and cold source respectively to form natural circulation;

[0023] Turn on the vortex generator arranged in the flow channel, and control the number, height and size of the protrusions in the flow channel through the host computer according to the flow conditions of the working medium to disturb the working medium;

[0024] During the circulation of deionized water, the heating temperature and light of the working fluid in the parallel channel visualization experimental section are adjusted to introduce thermal disturbance; the two middle valves are both contracted to open the connecting channels in the two parallel flow channels to mix the flowing working fluids in the two flow channels;

[0025] The flow field information in the loop is collected through the data acquisition and analysis system, including the pulsation frequency, period, amplitude, steam content, temperature, pressure and other information after the unstable flow occurs;

[0026] The image acquisition device is used to acquire the flow pattern of the fluid and the bubble development information in the flow channel of the parallel channel visualization experimental section through the visual window;

[0027] The unstable flow phenomenon in the flow channel is obtained through the collected image information;

[0028] First, a learning model is constructed using the BP neural network, and then the collected data is input to train the model, so that the model tends to be perfect and the result closest to the expected output value can be obtained; then the neural network is directly used to optimize the flow field information collected in the parallel channel visualization experiment section 8 and filter out unreasonable data; the natural circulation instability occurrence boundary map is obtained by using the existing software origin through the filtered flow field information, and a database is constructed. Finally, a prediction model is constructed based on artificial neural networks, such as convolutional neural networks CNN or graph neural networks GNN learning. Subsequent experiments can use virtual experiments to obtain model prediction results.

[0029] Compared with the prior art, the present invention has the following beneficial technical effects:

[0030] The present invention constructs a natural circulation experimental device with narrow rectangular parallel channels under medium and high pressure conditions to experimentally simulate the flow instability phenomenon inside the natural circulation system. By using visualization means, through image acquisition, data acquisition and analysis, the main characteristics of the instability are obtained, and the stability boundary is obtained, thereby solving the problem of hazards caused by the flow instability inside the system when natural circulation is used as a passive safety system of a nuclear reactor.

[0031] Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic structural diagram of an embodiment of the present invention.

[0033] Figure 2 It is a schematic diagram of the three-dimensional structure of the parallel channel visualization experimental section according to an embodiment of the present invention.

[0034] Figure 3 It is a cross-sectional view of a parallel channel visualization experimental section according to an embodiment of the present invention.

[0035] Figure 4 Schematic diagram of the structure of the protrusion according to an embodiment of the present invention.

[0036] Figure numerals: 1. storage tank; 2. circulation pump; 3. preheater; 4. flow meter; 5. temperature gauge; 6. differential pressure gauge; 7. arc connector; 8. parallel channel visualization experiment section; 9. condenser; 10. pressure regulating tank; 11. back pressure valve; 12. booster pump; 13. data processing and control terminal; 14. high-speed camera; 15. data acquisition instrument; 16. cover plate; 17. Class I roughness narrow rectangular flow channel; 18. visual window; 19. locking piece; 20. high temperature resistant sealing gasket; 21. heating device; 22. Class II roughness narrow rectangular flow channel; 23. fastening pin; 24. outer shell; 25 connecting channel. DETAILED DESCRIPTION

[0037] Example 1

[0038] like Figure 1 As shown, the natural circulation dual-channel visual experimental analysis device of this embodiment includes a circulation loop and an analysis system formed by sequentially connecting a circulation pump 2, a preheater 3, a parallel channel visual experimental section 8, a condenser 9 and a back pressure valve 11;

[0039] The inlet end of the circulation pump 2 is connected to the outlet end of the storage tank 1, and the outlet end of the storage tank 1 is provided with a booster pump 12 and a control valve;

[0040] The circulation loop is a natural circulation established by using a preheater 3 as a heat source and a condenser 9 as a cold source. The storage tank 1 is used to store liquid or gas. The inlet and outlet ends of the parallel channel visualization experiment section 8 are both provided with a flow meter 4 and a temperature meter 5. A pressure gauge 6 is connected in parallel between the inlet and outlet ends of the parallel channel visualization experiment section 8.

[0041] The structure of the parallel channel visualization experiment section 8 includes two flow channels arranged in parallel, visual windows 18 respectively arranged on the side walls of the two flow channels, four connecting channels between the two flow channels, and a heating device for heating the flowing working medium in at least two flow channels;

[0042] For details, see Figure 2 and Figure 3 The two flow channels (the narrow rectangular flow channel 17 with class I roughness and the narrow rectangular flow channel 22 with class II roughness) are respectively located on both sides of the cover plate 16. The two flow channels are arranged in parallel, cross or herringbone shape. There are four connecting channels 25 between the two flow channels, and the connecting channels can promote the mixing of the working fluids in the two flow channels; the visual window 18 is sealed and installed on the side of the flow channel by the fastening pin 23, and the visual window 18 is fixed on the outside by the outer shell 24 and the locking member 19.

[0043] An electric heating plate 21 or a heat pipe is installed in the middle of the cover plate 16 for indirectly heating the working medium in the flow channel. The electric heating plate 21 or the heat pipe is fixed by a high temperature resistant material gasket 20; or a transparent heating film is used to cover the visual window 18 for indirect heating;

[0044] Specifically, a plurality of armored thermocouple probes are arranged in the cover plate 16 in an area adjacent to the flow along the length direction thereof.

[0045] The roughness of the inner walls of the two flow channels is different; both have convex parts. Figure 4 As shown, the protrusion can be set to a chimney shape; by adjusting the height, the position entering the flow channel can be adjusted; preferably, the lifting of the protrusion can be controlled by hydraulic control; two relatively sealed cavities are set on the inner wall of the flow channel, and the lifting of the entire protrusion is controlled by controlling the oil pressure of the hydraulic oil entering the upper or lower cavity; in this embodiment, the inside of the protrusion is connected to the controller, and the number, height and size of the protrusions are intelligently controlled by the host computer according to the flow conditions of the working fluid; specifically, firstly, a learning model is constructed using a BP neural network, and the original data of the fluid disturbance is collected using an image acquisition device in the analysis system, and the signal is transmitted to the data acquisition and analysis system, and the constructed BP neural network model is used to calculate the relevant parameters of the fluid flow, such as the pulsation frequency, amplitude and other parameters. If the parameter is too high or too low, an intelligent control instruction is output, and the height and number of the protrusions are increased or decreased by the controller; in this way, the model is trained to make it perfect; at this time, when the neural network receives a signal with a small pulsation frequency and a small amplitude of the working fluid, the number and height of the protrusions are intelligently increased to enhance the disturbance in the tube, and vice versa.

[0046] The two flow channels are arranged in parallel, cross or herringbone shape.

[0047] The cross-sections of the two flow channels are narrow rectangles with a large aspect ratio;

[0048] The preferred dimensions of the flow channel are: 1000 mm in length, 40 mm in width, and 2 mm in height.

[0049] The preferred diameter of other pipelines in the circulation loop is DN25.

[0050] Valves are arranged at the inlet and outlet of the two flow channels to connect either or both flow channels to the circulation loop.

[0051] The material of the viewing window is quartz glass, high-silica glass, sapphire or borosilicate glass; the flow channel material is titanium or magnesium alloy; the cover plate material is high-strength steel; the high-temperature resistant sealing gasket is asbestos gasket, flexible graphite gasket, metal composite gasket or polytetrafluoroethylene gasket.

[0052] The analysis system includes:

[0053] An image acquisition device is used to observe the mesoscopic characteristics of the flowing medium in the parallel channel visualization experimental section 8 through a visualization observation window, including the generation and development of bubbles and flow pattern changes under unstable flow conditions;

[0054] The data acquisition and analysis system uses the BP neural network to first build a learning model, and then inputs the manually collected data to train the model, so that the model tends to be perfect and can obtain the result closest to the expected output value; the neural network is then used directly to optimize the flow field information collected in the parallel channel visualization experiment section 8 and filter out unreasonable data, including the temperature, pressure, pulsation frequency, period, amplitude, steam content, and circulation flow of the flowing medium after unstable flow occurs; the unstable flow prediction model is built based on the convolutional neural network CNN or the graph neural network GNN, and the subsequent experiments can use virtual experiments to obtain the model prediction results. The model inputs various influencing variables that may exist in the natural circulation system, such as the heating power of the experimental section, the circuit pressure, and the inlet subcooling, and outputs the information such as the location of the instability starting point, flow type, unstable flow type and stage under this condition through the prediction model, and visualizes the circuit cloud map to more intuitively reflect the overall temperature, pressure, and flow field state of the system.

[0055] By comparing the flow rate-pressure drop curve when flow instability occurs with the image taken by the high-speed camera, and combining the relevant data obtained, the mechanism of flow instability in the narrow rectangular channel can be further analyzed.

[0056] The image acquisition device may specifically be a high-speed camera 14 ; the data acquisition and analysis system specifically includes a data processing and control terminal 13 and a data acquisition instrument 15 .

[0057] The flowing working medium in the circulation loop of this embodiment is deionized water, and its working temperature is 20° C.-600° C. and its working pressure is 0.1 MPa-14.2 MPa.

[0058] Preheater 3 preheats the flowing working medium by indirect heating;

[0059] The cold side of the condenser 9 is cooling water.

[0060] It also includes a pressure stabilizing tank 10, which is connected in series in the circulation loop and is located downstream of the condenser 9 for stabilizing the pressure of the circulation loop.

[0061] The pressure stabilizing tank 10 may be a nitrogen tank, which is provided with a regulating valve and a pressure reducing valve.

[0062] As an optimization method, the inlet and outlet ends of the parallel channel visualization experiment section 8 are respectively provided with arc connectors 7, on which multiple interfaces are provided along the arc direction. The inlet and outlet of the parallel channel visualization experiment section 8 are connected to different interfaces to adjust the inclination angle of the entire parallel channel visualization experiment section 8 to conduct experiments with different inclination angles.

[0063] The working method of the natural circulation visualization experimental device of this embodiment is as follows: deionized water is stored in a storage tank and two flow channels with different roughness are opened at the same time:

[0064] Turn on the booster pump at the outlet of the storage tank and inject deionized water into the circulation loop. When the pressure reaches the set value, turn on the circulation pump.

[0065] Turn on the preheater 3 and the condenser 9 to form a heat source and a cold source respectively to form a natural circulation;

[0066] The vortex generator arranged in the flow channel is turned on, and the number and size of the protrusions in the flow channel are controlled according to the flow conditions of the working medium to disturb the working medium.

[0067] During the circulation of deionized water, the heating temperature and light of the working medium in the parallel channel visualization experiment section 8 are adjusted to introduce heat disturbance, and the two middle valves are contracted to open the connecting channel 25 in the two parallel flow channels to mix the flowing working medium in the two flow channels.

[0068] The flow field information in the loop is collected through the data acquisition instrument 15, including the pulsation frequency, period, amplitude, steam content, temperature, pressure and other information after the unstable flow occurs. The sample period is 0.1 second to 5 seconds, and the collection period can be manually set according to the needs of different stages of the experiment; the locking parts are opened to manually replace the flow channel material, and the above steps are repeated to collect the flow field information; there are many kinds of flow channel materials, which can be manually replaced to obtain multiple groups of experimental data.

[0069] The high-speed camera 14 is used to collect images of the flow patterns, bubble development and other information in the two flow channels of the parallel channel visualization experimental section 8 through the visual window; the data collected by the data acquisition instrument 15 and the high-speed camera 14 are transmitted to the data processing and control terminal 13 for the next step of processing: according to the collected image information, the unstable flow phenomenon in the narrow rectangular flow channel 17 of type I roughness and the narrow rectangular flow channel 22 of type II roughness can be obtained; after the collected data is optimized and screened by the BP neural network, the natural circulation instability boundary map is obtained based on reasonable data using the existing software origin to build a database. Based on the convolutional neural network CNN or the graph neural network GNN, the unstable flow prediction model is constructed, and the model prediction results can be obtained by virtual experiment.

[0070] Specifically, when the steam content at the outlet of the parallel channel visualization experiment section 8 is greater than 0.8 and the maximum temperature rise exceeds 15°C / s, an alarm is issued, and the data processing and control terminal 13 executes corresponding protection measures, automatically starts the circulation pump, and stops the heating of the parallel channel visualization experiment section 8.

[0071] Specifically, by connecting the inlet and outlet of the parallel channel visualization experiment section 8 to different interfaces on the arc connector and adjusting the inclination of the parallel channel visualization experiment section 8, experimental data at different inclinations can be obtained.

[0072] Narrow rectangular channels have a small hydraulic diameter, allowable heat flux density is large, and the narrow space effect is more prone to flow instability and is inconsistent with conventional pipelines. Therefore, this application designs a narrow rectangular parallel channel test bench under medium and high pressure conditions to study the occurrence and development of unstable flow in narrow rectangular channels, analyze the mutual interference of unstable flow in parallel channels, and the influence of different channel roughness, layout angles, and mixing between channels on unstable flow, to provide a reference for improving the safety of natural circulation systems, and to build mathematical models by collecting data, and use digital twins and other means to conduct virtual experiments to predict results.

[0073] Those skilled in the art can understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A natural circulation dual-channel visual experimental analysis device, comprising a circulation loop and an analysis system formed by sequentially connecting a circulation pump (2), a preheater (3), a parallel channel visual experimental section (8), a condenser (9) and a back pressure valve (11), characterized in that: The circulation loop is a natural circulation loop established by using a preheater (3) as a heat source and a condenser (9) as a cold source; the storage tank (1) is used to store liquid or gas, the inlet and outlet ends of the parallel channel visualization experiment section (8) are both provided with a flow meter (4) and a temperature meter (5), and a pressure gauge (6) is connected in parallel between the inlet and outlet ends of the parallel channel visualization experiment section (8); The analysis system comprises an image acquisition device and a data acquisition and analysis system; the image acquisition device is used to observe the mesoscopic characteristics of the flowing medium in the parallel channel visualization experiment section (8) through a visual window (18), including the temperature, pressure, pulsation frequency, period, amplitude, and steam content after the occurrence of unstable flow; the collected data is screened and optimized using an artificial intelligence algorithm; based on the screened data, a natural circulation instability occurrence boundary map is obtained using the existing software origin, and a database is established to obtain an unstable flow mathematical model, and a prediction model is constructed based on artificial intelligence learning to achieve model prediction results through virtual experiments; The structure of the parallel channel visualization experiment section (8) comprises at least two parallel flow channels arranged in parallel, visual windows (18) respectively arranged on the side walls of at least two flow channels, and a heating device (21) for heating the flowing working medium in at least two flow channels; the two flow channels are arranged in a parallel, cross or herringbone shape, and the cross section of each flow channel is a narrow rectangle with a relatively large length and width ratio.

2. The natural circulation dual-channel visualization experimental analysis device according to claim 1 is characterized in that: The inner walls of the two flow channels have different roughness and are both provided with protrusions that can be independently controlled to move up and down; the number, height and size of the protrusions are controlled by a controller; the protrusions form vortex generators in the flow channel to disturb the working fluid in the flow channel.

3. The natural circulation dual-channel visualization experimental analysis device according to claim 1 is characterized in that: Two connecting channels (25) are arranged between the two parallel flow channels and are respectively located at the two ends of the parallel flow channels for mixing the working fluids in the two flow channels; valves are arranged on the two parallel flow channels and the two connecting channels (25); electric valves are arranged at the two ends of the parallel flow channels respectively, and opening and closing are controlled by electric means to connect any one or both flow channels to a circulation loop; valves composed of β-eucryptite-silicon carbide composite thermal sensitive material and photosensitive material are arranged on the two connecting channels (25), and opening and closing of the valves are controlled to control mixing of the working fluids in the two flow channels.

4. The natural circulation dual-channel visualization experimental analysis device according to claim 1, characterized in that: At least two flow channels are respectively located on both sides of the cover plate (16), and a viewing window (18) is sealed and installed on the side of the flow channel through a locking piece (19). A heating device (21) for indirectly heating the working medium in the flow channel is installed in the middle of the cover plate (16); the heating device (21) is an electric heating plate or a heat pipe, and the electric heating plate or the heat pipe is fixed by a high-temperature resistant material gasket; or a structure in which a transparent heating film is covered on the viewing window (18) for indirect heating.

5. The natural circulation dual-channel visualization experimental analysis device according to claim 1 is characterized in that: The material of the visual window (18) is quartz glass, aluminosilicate glass, sapphire or high-silicon glass; The flow channel material is titanium or magnesium alloy; The cover plate (16) is made of high-strength steel; The high temperature resistant sealing gasket is an asbestos gasket, a flexible graphite gasket, a metal composite gasket or a polytetrafluoroethylene gasket.

6. The natural circulation dual-channel visualization experimental analysis device according to claim 1 is characterized in that: The flowing working medium in the circulation loop is deionized water, with an operating temperature of 20°C-600°C and a working pressure of 0.1MPa-14.2MPa.

7. The natural circulation dual-channel visualization experimental analysis device according to claim 1 is characterized in that: The preheater (3) preheats the flowing working medium by indirect heating; the cold side of the condenser (9) is cooling water.

8. The natural circulation dual-channel visualization experimental analysis device according to claim 1 is characterized in that: It also includes a pressure stabilizing tank (10) which is connected in series in the circulation loop and is located downstream of the condenser (9) for stabilizing the pressure of the circulation loop.

9. The natural circulation dual-channel visualization experimental analysis device according to claim 1, characterized in that: The inlet and outlet ends of the parallel channel visualization experiment section (8) are respectively provided with arc-shaped connectors (7), on which a plurality of interfaces are provided along the arc direction. The inlet and outlet of the parallel channel visualization experiment section (8) are connected to different interfaces, so that the inclination angle of the entire parallel channel visualization experiment section (8) can be adjusted to conduct experiments with different inclination angles.

10. A natural circulation unstable flow measurement method of the natural circulation dual-channel visualization experimental analysis device according to any one of claims 1 to 9, characterized in that: include: Turn on the booster pump (12) at the outlet of the storage tank (1), inject deionized water into the circulation loop, and when the pressure reaches the set value, turn on the circulation pump; Turn on the preheater (3) and the condenser (9) to form a heat source and a cold source respectively to form a natural circulation; Turn on the vortex generator arranged in the flow channel, and control the number, height and size of the protrusions in the flow channel through the host computer according to the flow conditions of the working medium to disturb the working medium; During the circulation of deionized water, the heating temperature and light of the working medium in the parallel channel visualization experimental section (8) are adjusted to introduce thermal disturbance; the two middle valves are both contracted to open the connecting channel (25) in the two parallel flow channels to mix the flowing working medium in the two flow channels; The flow field information in the loop is collected through the data acquisition and analysis system, including the pulsation frequency, period, amplitude, steam content, temperature, pressure and other information after the unstable flow occurs; The image acquisition device is used to acquire images of the flow pattern and bubble development information of the fluid in the flow channel of the parallel channel visualization experimental section (8) through the visual window (18); The unstable flow phenomenon in the flow channel is obtained through the collected image information; The collected flow field information is screened and optimized using an artificial neural network, and then the natural circulation instability boundary map is obtained using the existing software origin. A database is constructed, and an unstable flow prediction model is constructed based on artificial neural networks such as convolutional neural networks (CNN) or graph neural networks (GNN). Subsequent experiments can all use virtual experiments to obtain model prediction results.

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