Natural circulation visualization experiment device and method

Abstract

The application relates to a natural circulation visualization experiment device and method, which comprises a circulation loop formed by sequentially connecting a circulating pump, a preheater, a parallel channel visualization experiment section, a condenser and a back pressure valve, and further comprises an analysis system; the circulation loop is a natural circulation established by using the preheater as a heat source and the condenser as a cold source; the structure of the parallel channel visualization experiment section comprises at least two flow channels arranged in parallel, visual windows arranged on the side walls of the at least two flow channels respectively, and a heating device for heating the flowing working medium in the at least two flow channels; the analysis system is used for: observing the mesoscopic characteristics of the flowing working medium in the parallel channel visualization experiment section through the visual windows; collecting flow field information to obtain a natural circulation instability occurrence boundary graph, establishing a database to obtain an instability flow mathematical model, constructing a prediction model based on artificial intelligence learning, and realizing model prediction results through virtual experiments.

Description

Technical Field

[0001] This invention relates to the field of fluid measurement technology, and in particular to a natural circulation visualization experimental apparatus and method. Background Technology

[0002] Natural circulation is an energy transfer method that relies solely on the driving force generated by the density difference between cold and hot fluids to propel fluid circulation. It requires no external power source, reducing the system's dependence on external power and effectively improving operational safety. It is the primary circulating cooling method for advanced nuclear reactors and a crucial means of emergency cooling after a reactor shutdown. However, two-phase flow is easily affected by small disturbances, causing significant oscillations in flow rate, pressure drop, and cross-sectional vapor content, similar to vibration in mechanical systems. Sustained flow oscillations can lead to forced mechanical vibrations in equipment, causing resonance. Furthermore, flow oscillations can severely impact local heat transfer performance, drastically reducing the critical heat flux density and, in severe cases, jeopardizing 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 possess thermal-hydraulic characteristics such as compact structure, enhanced heat transfer, and flexible design and application. Parallel narrow rectangular channels in various arrangements can better protect the reactor. However, the small hydraulic diameter and high permissible heat flux density of narrow rectangular channels, coupled with the narrow space effect, make bubbles more susceptible to compression and deformation. Therefore, flow instability phenomena are more likely to occur in these channels, and the generation mechanisms and pulsation patterns of various flow instabilities differ from those in conventional channels. Furthermore, in small integrated nuclear reactors, due to the limited space, numerous parallel narrow rectangular channels are used as passive safety measures. These channels can influence each other during unstable flows, resulting in more intense unstable flows and severely affecting normal equipment operation.

[0004] Currently, domestic and international natural circulation experiments mainly focus on single-channel systems within the atmospheric pressure range. Yang Ruichang, Salah, and others have experimentally studied unstable natural circulation flows, and the derived criterion relationships show good agreement within lower pressure ranges. However, the channel types in these experiments are mostly circular tubes, and the pressure ranges involved are extremely limited. As the requirements for operating environment pressure and temperature of equipment gradually increase, it is necessary to explore the characteristics of natural circulation within higher pressure ranges. Existing parallel channels are mainly used in compact nuclear submarines, so experiments primarily focus on oscillating conditions in the ocean. Yan Chaoxing, Otsuji, and others have experimentally investigated the effects of channel amplitude and period on the pressure drop of two-phase flow and the location of CHF points under external oscillation conditions. Most of these experiments are based on external influences, with limited research on the impact of the channel's own internal unstable flows. Zhou Tao, Fang Xiaolu, Sheng Cheng, and others have conducted natural circulation experiments in narrow rectangular single-channel systems at atmospheric pressure, but their research mainly focuses on the bubble generation process and changes in heat transfer characteristics, lacking a classification and study of unstable flows at medium and high pressures. Meanwhile, Zhou Tao et al. from the applicant team have applied for a patent for a single-channel supercritical / subcritical water natural circulation experimental rig and have built the rig to conduct some research. However, the physical properties of supercritical fluids differ significantly from those of conventional fluids, and the experimental rig is only designed for a single vertical channel, resulting in limited applicability. With the development of small integrated nuclear reactors, the importance of narrow rectangular parallel channels is becoming increasingly apparent, and the angles of channel assembly are becoming more diverse depending on equipment requirements. However, there is currently a lack of experiments on instability related to medium and high pressures. Therefore, the applicant believes that it is essential to conduct a detailed study of the instability of natural circulation flow in narrow rectangular parallel channels and to find the stable boundary for safe operation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a natural circulation visualization experimental device and method. The purpose is to use visualization to observe the distribution and flow pattern changes of bubbles in the channel when instability occurs, and to obtain the main characteristics of instability occurrence.

[0006] The technical solution adopted in this invention is as follows:

[0007] This application provides a natural circulation visualization experimental device, including a circulation loop consisting of a circulation pump, a preheater, a parallel channel visualization experimental section, a condenser, and a back pressure valve connected in sequence, and also includes an analysis system;

[0008] The inlet end of the circulating pump is connected to the outlet end of the storage tank, and the outlet end of the storage tank is equipped with a booster pump and a control valve.

[0009] The circulation loop is a natural circulation established by the preheater as a heat source and the 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 test section are equipped with flow meters and thermometers, and pressure gauges are connected in parallel between the inlet and outlet ends of the parallel channel visualization test section.

[0010] The structure of the parallel channel visualization experimental section includes at least two flow channels arranged in parallel, viewing windows respectively disposed on the sidewalls of the at least two flow channels, and a heating device for heating the flowing working medium in the at least two flow channels; the roughness of the inner walls of the at least two flow channels is different; the cross-section of each flow channel is a narrow rectangle with a large length-to-width ratio.

[0011] The analysis system includes:

[0012] The image acquisition device is used to observe the mesoscopic characteristics of the flowing working medium in the parallel channel visualization experimental section through the viewing window, including the generation and development of bubbles and the change of flow pattern under unstable flow conditions.

[0013] The data acquisition and analysis system is used to collect flow field information in the parallel channel visualization experimental section, including temperature, pressure, pulsation frequency, period, amplitude, and vapor content after unstable flow occurs; it obtains the natural circulation instability boundary map based on the process information, establishes a database to obtain the mathematical model of unstable flow, and constructs a prediction model based on artificial intelligence learning, so as to obtain the model prediction results through virtual experiments.

[0014] The further technical solution is as follows:

[0015] At least two flow channels are arranged in a parallel, intersecting, or herringbone pattern.

[0016] The at least two flow channels are located on both sides of the cover plate, and the viewing window is sealed and installed on the side of the flow channel by locking components. An electric heating element or heat pipe for indirect heating of the working fluid in the flow channel is installed in the middle of the cover plate. The electric heating element or heat pipe is fixed by a high-temperature resistant sealing gasket; or a structure indirect heating is carried out by covering the viewing window with a transparent heating film.

[0017] The viewing window is made of quartz glass, aluminosilicate glass, or borosilicate glass; the flow channel is made of stainless steel, carbon steel, chromium-molybdenum steel, or chromium-molybdenum-vanadium steel; the cover plate is made of high-strength steel; and the high-temperature resistant sealing gasket is made of asbestos gasket, flexible graphite gasket, metal composite gasket, or polytetrafluoroethylene gasket.

[0018] Valves are provided at the inlet and outlet of the at least two flow channels to allow one or both flow channels to be connected to the circulation loop.

[0019] The working fluid in the circulation loop is deionized water, with an operating temperature of 20℃-600℃ and an operating pressure of 0.1MPa-14.2MPa.

[0020] The preheater uses indirect heating to preheat the flowing working fluid; the cold side of the condenser is cooling water.

[0021] It also includes a pressure stabilizing tank, which is connected in series in the circulation loop and located downstream of the condenser, for stabilizing the pressure of the circulation loop.

[0022] The inlet and outlet ends of the parallel channel visualization test section are equipped with arc-shaped connectors, with multiple interfaces along the arc direction. The inlet and outlet of the parallel channel visualization test section are connected to different interfaces to adjust the tilt angle of the entire parallel channel visualization test section for experiments at different tilt angles.

[0023] This application also provides a method for measuring unsteady natural circulation flow in the aforementioned natural circulation visualization experimental apparatus, comprising:

[0024] Turn on the booster pump at the outlet of the storage tank to inject deionized water into the circulation loop. When the pressure reaches the set value, turn on the circulation pump.

[0025] The preheater and condenser are turned on to form a heat source and a cold source respectively, thus creating a natural circulation.

[0026] During the circulation of deionized water, the heating temperature of the working fluid in the parallel channel visualization experimental section is adjusted to introduce thermal disturbance.

[0027] The data acquisition and analysis system collects flow field information in the loop, including the pulsation frequency, period, amplitude, vapor content, temperature, and pressure information after unstable flow occurs;

[0028] The image acquisition device captures images of the fluid flow pattern and bubble development information in the parallel channel visualization experimental section through the viewing window;

[0029] The unstable flow phenomena within the flow channel are obtained by acquiring image information;

[0030] By collecting flow field information, a boundary map of natural circulation instability is obtained, a database is constructed, and an instability flow prediction model is built based on an artificial neural network. Subsequent experiments can be conducted using virtual experiments to obtain the model prediction results.

[0031] The beneficial effects of this invention are as follows:

[0032] This invention constructs a natural circulation experimental device with narrow rectangular parallel channels under medium and high pressure conditions. It can experimentally simulate the flow instability phenomenon inside the natural circulation system. By using visualization methods, image acquisition, data acquisition and analysis, the main characteristics of the instability are obtained and the stability boundary is obtained. This solves the problem of the hazards caused by the flow instability inside the system when natural circulation is used as a passive safety system for nuclear reactors.

[0033] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of an embodiment of the present invention.

[0035] Figure 2 This is a three-dimensional structural diagram of the parallel channel visualization experimental section according to an embodiment of the present invention.

[0036] Figure 3 This is a cross-sectional view of the parallel channel visualization experimental section according to an embodiment of the present invention.

[0037] In the diagram: 1. Storage tank; 2. Circulating pump; 3. Preheater; 4. Flow meter; 5. Thermometer; 6. Differential pressure gauge; 7. Arc connector; 8. Parallel channel visualization experimental section; 9. Condenser; 10. Pressure stabilizing 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. Viewing window; 19. Locking element; 20. High-temperature resistant sealing gasket; 21. Electric heating element; 22. Class II roughness narrow rectangular flow channel; 23. Fastening pin; 24. Outer shell. Detailed Implementation

[0038] The specific embodiments of the present invention are described below with reference to the accompanying drawings.

[0039] like Figure 1 As shown, the natural circulation visualization experimental device of this embodiment includes a circulation loop consisting of a circulation pump 2, a preheater 3, a parallel channel visualization experimental section 8, a condenser 9, and a back pressure valve 11 connected in sequence, and also includes an analysis system.

[0040] The inlet end of the circulating pump 2 is connected to the outlet end of the storage tank 1, and the outlet end of the storage tank 1 is equipped with a booster pump 12 and a control valve.

[0041] The circulation loop is a natural circulation established by using the preheater 3 as a heat source and the 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 experimental section 8 are equipped with flow meters 4 and thermometers 5. The inlet and outlet ends of the parallel channel visualization experimental section 8 are connected in parallel with pressure gauges 6.

[0042] The structure of the parallel channel visualization experimental section 8 includes at least two flow channels arranged in parallel, viewing windows 18 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.

[0043] For details, see Figure 2 and Figure 3 At least two flow channels (Type I roughness narrow rectangular flow channel 17 and Type II roughness narrow rectangular flow channel 22) are located on both sides of the cover plate 16, and the viewing window 18 is sealed and installed on the side of the flow channel by fastening pin 23. At the same time, the viewing window 18 is fixed on the outside by the outer shell 24 and the locking member 19.

[0044] An electric heating element 21 or heat pipe for indirect heating of the working fluid in the flow channel is installed in the middle of the cover plate 16. The electric heating element 21 or heat pipe is fixed by a high-temperature resistant sealing gasket 20; or an indirect heating is achieved by covering the viewing window 18 with a transparent heating film.

[0045] Specifically, multiple armored thermocouple probes are installed along the length of the flow area in the cover plate.

[0046] The roughness of the inner walls of at least two flow channels is different;

[0047] At least two flow channels are arranged in a parallel, intersecting, or herringbone pattern.

[0048] At least two flow channels have a cross-section that is a narrow rectangle with a large length-to-width ratio;

[0049] The preferred dimensions for the flow channel are: 1000mm in length, 40mm in width, and 2mm in height.

[0050] The preferred diameter for other pipes in the circulation loop is DN25.

[0051] Valves are installed at the inlet and outlet of at least two flow channels to allow one or both flow channels to be connected to the circulation loop.

[0052] The viewing window is made of quartz glass, aluminosilicate glass, or borosilicate glass; the flow channel is made of stainless steel, carbon steel, chromium-molybdenum steel, or chromium-molybdenum-vanadium steel; the cover plate is made of high-strength steel; and the high-temperature resistant sealing gasket is made of asbestos gasket, flexible graphite gasket, metal composite gasket, or polytetrafluoroethylene gasket.

[0053] The analysis system includes:

[0054] The image acquisition device is used to observe the mesoscopic characteristics of the working fluid flowing in the parallel channel visualization experimental section 8 through a viewing window, including the generation and development of bubbles and the change of flow pattern under unstable flow conditions.

[0055] The data acquisition and analysis system is used to collect flow field information within the parallel channel visualization experimental section 8, including the temperature, pressure, pulsation frequency, period, amplitude, vapor content, and circulation flow rate of the working fluid after unstable flow occurs. Based on the flow field information, a natural circulation instability boundary map is obtained (a graph with dimensionless phase transition number and dimensionless subcooling converted from the instability point through dimensionless transformation as the coordinate axes), and a database is established to obtain an instability flow mathematical model. A prediction model is constructed using digital twin technology, and artificial intelligence learning is performed to obtain the model prediction results through virtual experiments. The model takes into account various influencing variables that may exist in the natural circulation system, such as the heating power of the experimental section, the loop pressure, and the inlet subcooling, and outputs information such as the location of the instability initiation point, flow pattern, instability flow type, and stage under the given conditions. The loop cloud map is visualized to more intuitively reflect the overall temperature, pressure, and flow field state of the system.

[0056] By comparing the flow-pressure drop curves at the occurrence of flow instability with images captured by a high-speed camera, and combining the obtained relevant data, the mechanism of flow instability in narrow rectangular channels can be further analyzed in depth.

[0057] 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.

[0058] In this embodiment, the working fluid in the circulating loop is deionized water, with an operating temperature of 20℃-600℃ and an operating pressure of 0.1MPa-14.2MPa.

[0059] Preheater 3 uses indirect heating to preheat the fluid.

[0060] The cold side of condenser 9 is filled with cooling water.

[0061] It also includes a pressure stabilizing tank 10, which is connected in series in the circulation loop and located downstream of the condenser 9, for stabilizing the pressure of the circulation loop.

[0062] The pressure stabilizing tank 10 can be a nitrogen tank, equipped with a regulating valve and a pressure reducing valve.

[0063] As an optimization method, the inlet and outlet ends of the parallel channel visualization experimental section 8 are respectively equipped with arc-shaped connectors 7, which have multiple interfaces along the arc direction. The inlet and outlet of the parallel channel visualization experimental section 8 are connected to different interfaces to adjust the tilt angle of the entire parallel channel visualization experimental section 8 in order to conduct experiments at different tilt angles.

[0064] Taking the storage of deionized water in a tank and the simultaneous opening of two flow channels with different roughness as an example, the working method of the natural circulation visualization experimental device in this embodiment is as follows:

[0065] Turn on the booster pump at the outlet of the storage tank to inject deionized water into the circulation loop. When the pressure reaches the set value, turn on the circulation pump.

[0066] Turn on the preheater 3 and condenser 9 to form a heat source and a cold source respectively to create a natural circulation;

[0067] During the circulation of deionized water, the heating temperature of the working fluid in the parallel channel visualization experimental section 8 is adjusted to introduce thermal disturbance.

[0068] The data acquisition instrument 15 collects flow field information in the loop, including information such as pulsation frequency, period, amplitude, vapor content, temperature, and pressure after unstable flow occurs. The sampling period is 0.1 seconds to 5 seconds, and the acquisition period can be manually set according to the needs of different stages of the experiment.

[0069] The high-speed camera 14 captures images of the fluid flow pattern and bubble development in the two channels of the parallel channel visualization experimental section 8 through the viewing window.

[0070] The data acquired by the data acquisition instrument 15 and the high-speed camera 14 is transmitted to the data processing and control terminal 13 for further processing.

[0071] Based on the collected image information, the unstable flow phenomena in the Class I roughness narrow rectangular flow channel 17 and the Class II roughness narrow rectangular flow channel 22 can be obtained;

[0072] Based on the collected flow field information, a boundary map of natural circulation instability is obtained, a database is constructed, and an instability flow prediction model is built based on artificial neural networks. Subsequent experiments can be conducted using virtual experiments to obtain the model prediction results.

[0073] Specifically, when the vapor content at the outlet of the parallel channel visualization test section 8 is greater than 0.8 and the maximum temperature rise exceeds 15℃ / s, an alarm is issued. When the alarm is issued, 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 test section 8.

[0074] Specifically, by connecting the inlet and outlet of the parallel channel visualization experimental section 8 to different interfaces on the arc connector, and adjusting the tilt angle of the parallel channel visualization experimental section 8, experimental data at different tilt angles can be obtained.

[0075] Narrow rectangular channels have small hydraulic diameters, allowable high heat flux densities, and are more prone to flow instabilities due to their narrow space, which differs from conventional pipelines. Therefore, this application designs an experimental rig for narrow rectangular parallel channels under medium- and high-pressure conditions to study the occurrence and development of unstable flows within these channels. It analyzes the mutual interference of unstable flows in parallel channels and the impact of different channel roughness and arrangement angles on unstable flows, providing a reference for improving the safety of natural circulation systems. By collecting data and constructing mathematical models, virtual experiments are conducted using digital twins and other methods to predict the results.

[0076] It will be understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A natural cycle visualization experimental device, characterized in that, It includes a circulation loop consisting of a circulating pump (2), a preheater (3), a parallel channel visualization experimental section (8), a condenser (9), and a back pressure valve (11) connected in sequence, and also includes an analysis system; The inlet end of the circulating pump (2) is connected to the outlet end of the storage tank (1), and the outlet end of the storage tank (1) is equipped with a booster pump (12) and a control valve. The circulation loop is a natural circulation established by the preheater (3) as a heat source and the 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 experimental section (8) are equipped with flow meters (4) and thermometers (5). Pressure gauges (6) are connected in parallel between the inlet and outlet ends of the parallel channel visualization experimental section (8). The structure of the parallel channel visualization experimental section (8) includes at least two flow channels arranged in parallel, viewing windows respectively arranged on the side walls of the at least two flow channels, and a heating device for heating the flowing working medium in the at least two flow channels. The roughness of the inner walls of the at least two flow channels is different; the cross-section of each flow channel is a narrow rectangle with a large length-to-width ratio; The analysis system includes: The image acquisition device is used to observe the mesoscopic characteristics of the working fluid in the parallel channel visualization experimental section (8) through the viewing window, including the generation and development of bubbles and the change of flow pattern under unstable flow conditions. The data acquisition and analysis system is used to collect flow field information in the parallel channel visualization experimental section (8), including temperature, pressure, pulsation frequency, period, amplitude, and vapor content after unstable flow occurs; the natural circulation instability boundary map is obtained based on the process information, and a database is established to obtain an unstable flow mathematical model; a prediction model is constructed based on artificial intelligence learning, and the model prediction results are obtained through virtual experiments. The instability boundary diagram is a graph with dimensionless phase transition number and dimensionless undercooling, which are converted from the instability operating point through dimensionless transformation, as the coordinate axes.

2. The natural cycle visualization experimental device according to claim 1, characterized in that, At least two flow channels are arranged in a parallel, intersecting, or herringbone pattern.

3. The natural cycle visualization experimental device according to claim 1, characterized in that, The at least two flow channels are located on both sides of the cover plate, and the viewing window is sealed and installed on the side of the flow channel by locking components. An electric heating element or heat pipe for indirect heating of the working fluid in the flow channel is installed in the middle of the cover plate. The electric heating element or heat pipe is fixed by a high-temperature resistant sealing gasket; or a structure indirect heating is carried out by covering the viewing window with a transparent heating film.

4. The natural cycle visualization experimental device according to claim 3, characterized in that, The viewing window is made of quartz glass, aluminosilicate glass, or borosilicate glass; the flow channel is made of stainless steel, carbon steel, chromium-molybdenum steel, or chromium-molybdenum-vanadium steel; the cover plate is made of high-strength steel; and the high-temperature resistant sealing gasket is made of asbestos gasket, flexible graphite gasket, metal composite gasket, or polytetrafluoroethylene gasket.

5. The natural cycle visualization experimental device according to claim 1, characterized in that, Valves are provided at the inlet and outlet of the at least two flow channels to allow one or both flow channels to be connected to the circulation loop.

6. The natural cycle visualization experimental device according to claim 1, characterized in that, The working fluid in the circulation loop is deionized water, with an operating temperature of 20℃-600℃ and an operating pressure of 0.1MPa-14.2MPa.

7. The natural cycle visualization experimental device according to claim 1, characterized in that, The preheater (3) preheats the fluid using an indirect heating method; the cold side of the condenser (9) is cooling water.

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

9. The natural cycle visualization experimental device according to claim 8, characterized in that, The inlet and outlet ends of the parallel channel visualization experimental section (8) are respectively provided with arc-shaped connectors (7), which have multiple interfaces along the arc direction. The inlet and outlet of the parallel channel visualization experimental section (8) are connected to different interfaces to adjust the tilt angle of the entire parallel channel visualization experimental section (8) for conducting experiments at different tilt angles.

10. A method for measuring unsteady natural circulation flow using the natural circulation visualization experimental apparatus according to any one of claims 1-9, characterized in that, include: Turn on the booster pump (12) at the outlet of the storage tank (1) and inject deionized water into the circulation loop. When the pressure reaches the set value, turn on the circulation pump. Turn on the preheater (3) and condenser (9) to form a heat source and a cold source respectively to form a natural circulation; During the circulation of deionized water, the heating temperature of the working fluid in the parallel channel visualization experimental section (8) is adjusted to introduce heat disturbance; The data acquisition and analysis system collects flow field information in the loop, including the pulsation frequency, period, amplitude, vapor content, temperature, and pressure information after unstable flow occurs; The image acquisition device captures images of the fluid flow pattern and bubble development information in the parallel channel visualization experimental section (8) through the viewing window; The unstable flow phenomena within the flow channel are obtained by acquiring image information; By collecting flow field information, a boundary map of natural circulation instability is obtained, a database is constructed, and an instability flow prediction model is built based on an artificial neural network. Subsequent experiments can be conducted using virtual experiments to obtain the model prediction results.

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