A liquid metal bubble detection sensor based on optical fiber reflection

Abstract

The present application belongs to the field of gas-liquid two-phase flow measurement and optical fiber sensing technology, and particularly relates to a liquid metal bubble detection sensor based on optical fiber reflection. A light signal is emitted from a laser light source, transmitted to an optical fiber probe module through a spatial light beam splitter or a fiber coupler, and reflected at the end face of the optical fiber probe module; the reflected light enters a photoelectric detector again through the spatial light beam splitter or the fiber coupler, is converted into an electric signal through a photoelectric conversion circuit, transmitted to a computer through a data acquisition card, and outputted after signal processing to obtain the void fraction and bubble frequency of the detected opaque liquid metal; the optical fiber sensor is based on the obvious difference in reflectivity at the interfaces between the opaque liquid metal, the bubbles and the optical fiber material, and the speed and distribution information of the bubbles are determined by analyzing the echo signal received by the photoelectric detector. The present application can stably and accurately complete bubble detection in a high-temperature liquid metal environment.

Description

Technical Field

[0001] This invention belongs to the field of gas-liquid two-phase flow measurement and fiber optic sensing technology, specifically relating to a liquid metal bubble detection sensor based on fiber optic reflection. Background Technology

[0002] In many industrial, chemical, and nuclear energy applications, liquid metals can serve as isolation layers or play other roles. However, in practical applications, gas leaks and the generation of bubbles within liquid metals can significantly impact system safety. Due to the extremely high specific conductivity of liquid metals, electrical impedance methods struggle to establish sufficiently large electric fields within the fluid, making it difficult to detect bubble movement. While magnetic resonance imaging (MRI) is commonly used in studies of common two-phase flows, in conductive liquid metal environments, the Lorentz force generated by the interaction of the applied magnetic field and the flow reshapes the flow pattern and alters the void distribution, thus affecting bubble structure and distribution. This severely limits the interpretability and comparability of the results. In contrast, the chemical inertness, intrinsic safety, and immunity to electromagnetic environments of fiber optic probes allow for long-term stable operation over a wider operating window. Crucially, fiber optic probes do not require light to penetrate the liquid metal; instead, they utilize changes in reflected light intensity at the gas-liquid interface to determine the phase state. Therefore, the opacity of the liquid metal no longer poses a bottleneck.

[0003] In the pool-type atmospheric pressure structure design of lead-cooled fast reactors, internal components such as steam generators are typically immersed in a main vessel coolant, such as a lead-bismuth alloy. Due to significant differences in operating conditions on both sides of the heat transfer tubes, a steam generator heat transfer tube rupture accident may occur, causing high-pressure water inside the heat transfer tubes to rush into the high-temperature lead pool, forming a gas-liquid metal two-phase flow phenomenon. The impact force generated at the moment of heat transfer tube rupture poses a serious threat to the structural integrity of the reactor and the shielding of radioactive materials. Simultaneously, the generated bubbles may migrate towards the reactor core with the primary coolant and introduce positive reactions, leading to a surge in reactor power. Research using fiber optic probes to acquire parameters of liquid metal bubble flow has direct extrapolation value for the diagnosis and handling of steam generator heat transfer tube rupture accidents. Especially in systems such as lead-cooled fast reactors, when a steam generator heat transfer tube rupture accident occurs, high-pressure steam enters the high-temperature liquid metal pool, forming a gas-liquid metal two-phase flow and inducing a violent transient process. Rapid and reliable monitoring of bubble behavior (such as bubble appearance, frequency changes, and local porosity increases) in the early stages of such accidents is crucial for accident identification and risk assessment. However, accident conditions are characterized by high temperature, strong flow, unsteady bubble swarms, rapid changes in interface conditions, and extremely short maintenance windows, which place significantly higher demands on the reliability of sensor structures and the robustness of signal processing than on conventional two-phase flows.

[0004] Therefore, this invention provides a liquid metal bubble detection sensor based on optical fiber reflection, which judges bubble behavior by the change of light reflectivity in different media, enabling it to stably and accurately complete bubble detection in a high-temperature liquid metal environment. Summary of the Invention

[0005] The purpose of this invention is to provide a liquid metal bubble detection sensor based on fiber optic reflection. Unlike conventional two-phase flow detection in transparent or semi-transparent media such as water-gas and oil-gas, liquid metals (such as lead and lead-bismuth alloys) have characteristics such as opacity, high liquefaction temperature, strong conductivity, easy oxidation, and sensitivity to interface wetting state. These characteristics make it difficult to apply traditional optical imaging, laser transmission, and other measurement methods that rely on the light transmission path of the medium. At the same time, methods such as electrical probes, differential pressure, and ultrasound are easily limited by electromagnetic interference, corrosion and insulation failure, insufficient sensor life and calibration drift in high-temperature and highly conductive media, making it difficult to achieve long-term, online, and repeatable bubble detection.

[0006] The specific technical solution adopted by this invention is as follows: A liquid metal bubble detection sensor based on fiber optic reflection enables stable bubble event identification and parameter output even under conditions of high temperature, corrosion / wetting sensitivity, strong electromagnetic fields, and difficult maintenance. This invention employs a technical approach of "insertion-type fiber optic probe + end-reflection sensitive structure + photoelectric detection and online processing." It does not rely on the light transmittance of liquid metal and does not require the construction of a cross-medium transmission optical path. Bubble detection is achieved solely by utilizing the local reflection difference generated when the sensitive end of the fiber optic probe contacts the surrounding medium, thus overcoming the difficulties in optical observation and imaging caused by the opacity of liquid metal.

[0007] The liquid metal bubble detection sensor specifically includes a laser light source, a spatial light beam splitter or fiber optic coupler, a fiber optic probe module, a photodetector, and a data acquisition card, with the data acquisition card communicating with a computer.

[0008] The light signal emitted from the laser source passes through a spatial beam splitter or fiber coupler and is transmitted to the fiber optic probe module, where it is reflected at the end face of the fiber optic probe module. The reflected light then passes through the spatial beam splitter or fiber coupler and enters the photodetector, where it is converted into an electrical signal by a photoelectric conversion circuit. The electrical signal is transmitted to a PC via a data acquisition card and, after signal processing, outputs the porosity and bubble frequency of the detected opaque liquid metal.

[0009] The fiber optic sensor is based on the significant difference in reflectivity at the interface between the opaque liquid metal and the bubble and the fiber optic material. By analyzing the patterns of the echo signals received by the photodetector, it determines the bubble velocity and distribution information.

[0010] The fiber optic probe module has a reflection-sensitive end at its front end, which includes a wedge-shaped reflective surface, a single-sloping surface, or an equivalent tilted reflective structure tilted at a certain angle relative to the fiber axis. The reflected light then passes through a beam splitter or fiber coupler before entering the photodetector. When the fiber optic probe is in the liquid metal phase, the quartz-liquid metal interface has high equivalent reflection and absorption coupling characteristics, resulting in a relatively high echo power. When a bubble passes over or covers the probe's sensitive end, causing it to partially enter the gas phase, the reflection characteristics of the quartz-gas interface change, leading to a distinguishable change in the echo power. This change is then reflected as a significantly contrasting event waveform in the voltage signal output by the photodetector module. In this way, the invention transforms the "invisible" behavior of bubbles in opaque liquid metal into collectable and identifiable changes in reflected echo electrical signals, enabling bubble event detection and statistical analysis.

[0011] Specifically, the fiber optic probe module adopts an insertable fiber optic probe, which includes a high-temperature resistant outer protective sleeve, a thermal stress isolation layer, and a fiber fixing structure. The thermal stress isolation layer is located between the fiber optic probe and the outer protective sleeve, and is used to reduce the risk of micro-bending loss, end attitude drift, or even breakage of the fiber optic cable caused by thermal cycling.

[0012] Preferably, the fiber optic probe is a replaceable fiber optic probe, which is detachably connected to the high-temperature resistant outer protective sleeve via threads, snaps, or welded seals to enable rapid replacement and consistent reproduction of contaminated or worn fiber optic probes.

[0013] Furthermore, considering the impact of the extreme environment of liquid metal on sensor reliability, this invention incorporates a high-temperature resistant fiber optic fixing structure and a thermal stress isolation structure in the insertable fiber optic probe module. Due to significant differences in thermal expansion coefficients and elastic moduli between the metal jacket and the silica fiber, and because liquid metal conditions often involve thermal cycling processes such as heating, melting, temperature stabilization, and cooling solidification, the lack of thermal stress isolation and a suitable fixing method can easily lead to micro-bending loss, end-position drift, or even breakage of the fiber, resulting in signal baseline instability and measurement failure. This invention introduces a thermal stress isolation structure, such as a ceramic liner, a compliant buffer layer, or a combination thereof, between the fiber and the outer protective jacket, allowing stress to be released during thermal cycling, thereby improving the mechanical stability and signal repeatability of the probe in high-temperature environments.

[0014] Furthermore, liquid metals have a tendency to wet and adhere, and the end face may become contaminated and worn due to oxide films, impurities, or bubble clusters, leading to baseline drift or decreased sensitivity. To reduce maintenance difficulty and improve reproducibility, this invention designs the reflection-sensitive end as a replaceable fiber optic probe. A threaded snap-fit ​​structure allows for quick replacement of the probe tip after contamination or wear, avoiding the scrapping of the entire probe and thus improving the engineering availability for long-term online monitoring.

[0015] Furthermore, the fiber optic probes can be configured into fiber optic probe arrays according to actual detection needs to detect information such as the distribution and velocity of bubbles in opaque liquid metal.

[0016] Preferably, the fiber optic probe is made of multimode high-temperature resistant silica fiber and is used to detect a variety of low-melting-point opaque metals; for metals that are liquid at room temperature, multimode plastic fiber can also be used to make the fiber optic probe.

[0017] A signal processing method for liquid metal bubble detection sensors involves acquiring the output signals of all liquid metal bubble detection sensors simultaneously, converting the original signals into corresponding square wave signals using a single threshold method, and then calculating the porosity and bubble data information within the liquid metal detection area using the square wave signals.

[0018] Preferably, the threshold in the single threshold method is determined by the maximum inter-class variance method. Specifically, a histogram is established for the amplitude of the preprocessed echo signal within a statistical window, and the threshold that maximizes the inter-class variance is selected as the single threshold. The echo signal is then binarized based on the single threshold to distinguish whether the medium contacted by the fiber optic probe is liquid or gaseous. If the medium is liquid, it corresponds to opaque liquid metal; if the medium is gaseous, it corresponds to bubbles.

[0019] Specifically, in terms of signal processing, this invention provides a bubble identification method suitable for online processing, addressing the characteristics of "noise superposition + baseline drift + irregular events" under liquid metal conditions. The method includes: filtering and preprocessing the acquired echo voltage signal to suppress pulse spikes and high-frequency noise; adaptively determining the threshold of the preprocessed signal within a preset statistical window, and segmenting the signal into a gas phase segment and a liquid phase segment to obtain the start and end times and duration of bubble events; and calculating parameters such as bubble frequency and porosity based on the proportion of the gas phase segment and the event count. In a preferred embodiment, the adaptive threshold determination can be achieved using the maximum inter-class variance method to calculate a single threshold. That is, a signal amplitude histogram is constructed within the statistical window, and the threshold that maximizes the inter-class variance is automatically determined, thereby achieving gas-liquid segmentation without the need for repeated manual parameter adjustments. This processing method can adapt to signal amplitude variations caused by factors such as light source power fluctuations, interface state changes, and circuit gain drift in a liquid metal environment.

[0020] The technical effects achieved by this invention are as follows: Firstly, for opaque liquid metal environments, this invention does not rely on transmission light paths and visual observation, but only utilizes the local reflection difference at the probe tip to achieve bubble detection, making it suitable for extreme working conditions where optical imaging is unavailable; Secondly, through the structural design of the wedge-shaped reflective sensitive end, more sensitive echo changes to bubble coverage / passage can be obtained, improving the detectability of bubble events; Third, through engineering designs such as high-temperature resistant outer protective sleeve, thermal stress isolation structure and replaceable fiber optic probe, the reliability and reproducibility of the sensor under high-temperature thermal cycling, wetting contamination and difficult maintenance conditions are improved. Fourth, through an online threshold adaptive signal processing method, the bubble frequency and porosity can be automatically calculated, which can be used for early warning of accidents, operation status assessment and long-term monitoring applications, especially suitable for strong transient gas-liquid metal two-phase flow scenarios such as steam generator heat transfer tube rupture accidents.

[0021] In this invention, the core of the sensor is a specially designed wedge-shaped fiber optic probe and single-threshold rectangular signal processing. The end face of the fiber optic probe is ground at a specific angle to form a wedge-shaped reflective structure, which enhances the reflection efficiency of the quartz-liquid metal interface, enabling the sensor to detect bubble behavior more accurately in the extreme environment of opaque liquid metal. Attached Figure Description

[0022] Figure 1 A schematic diagram illustrating the principle of a liquid metal bubble detection sensor proposed in this invention; Figure 2 This is a schematic diagram illustrating the principle of signal change in the optical fiber probe. Figure 3 This is a three-dimensional structural diagram of the tank simulating a heat transfer tube rupture accident in a steam generator, as an application case of the present invention. Figure 4 This is a schematic cross-sectional view of the tank body simulating a heat transfer tube rupture accident in a steam generator, as an application case of the present invention. Figure 5 This is a schematic diagram of the fiber optic probe structure of the liquid metal bubble detection sensor proposed in this invention; Figure 6 This is a schematic diagram of the overall connection and assembly of the liquid metal bubble detection sensor proposed in this invention.

[0023] Figure 7 This is a schematic diagram of the initial acquisition signal of the liquid metal bubble detection sensor proposed in this invention.

[0024] Figure 8 This is a comparison diagram of the square wave signal obtained after single threshold processing of the liquid metal bubble detection sensor proposed in this invention and the original signal.

[0025] The attached diagram lists the components represented by each number as follows: 1. Tank body; 2. Cover; 3. Sealing bolts; 4. Safety valve; 5. Exhaust valve; 6. Temperature control RTD; 7. Stainless steel capillary tube; 8. Fiber optic probe; 9. Air inlet pipe; 10. Fixing thread; 11. Fiber optic fixing structure. Detailed Implementation

[0026] To make the objectives and advantages of this invention clearer, the invention will be specifically described below with reference to embodiments. It should be understood that the following text is merely used to describe one or more specific embodiments of the invention and does not strictly limit the scope of protection specifically claimed by the invention.

[0027] like Figure 1 As shown, a liquid metal bubble detection sensor based on fiber optic reflection enables stable bubble event identification and parameter output even under conditions of high temperature, corrosion / wetting sensitivity, strong electromagnetic environment, and difficult maintenance. This invention employs a technical approach of "inserted fiber optic probe 8 end reflection-sensitive structure + photoelectric detection and online processing," which does not rely on the light transmittance of liquid metal and does not require the construction of a cross-medium transmission optical path. It utilizes only the local reflection difference generated when the sensitive end of the fiber optic probe 8 contacts the surrounding medium to achieve bubble detection, thus overcoming the difficulties in optical observation and imaging caused by the opacity of liquid metal.

[0028] The liquid metal bubble detection sensor specifically includes a laser light source, a spatial light beam splitter or fiber optic coupler, a fiber optic probe module, a photodetector, and a data acquisition card, with the data acquisition card communicating with a computer. The light signal emitted from the laser source passes through a spatial beam splitter or fiber coupler and is transmitted to the fiber probe module, where it is reflected at the end face of the fiber probe module. The reflected light then passes through the spatial beam splitter or fiber coupler and enters the photodetector. It is converted into an electrical signal by the photoelectric conversion circuit. The electrical signal is transmitted to the PC through the data acquisition card, and after signal processing, the porosity and bubble frequency of the detected opaque liquid metal are output. Fiber optic sensors are based on the significant difference in reflectivity at the interfaces between opaque liquid metal and bubbles and optical fiber materials. By analyzing the patterns of echo signals received by photodetectors, the velocity and distribution information of bubbles can be determined.

[0029] The fiber optic probe 8 of the fiber optic probe module has a reflection-sensitive end at its front end. This reflection-sensitive end includes a wedge-shaped reflective surface, a single-sloping surface, or an equivalent tilted reflective structure tilted at a certain angle relative to the fiber axis. The reflected light then passes through a beam splitter or fiber coupler before entering the photodetector. When the fiber optic probe 8 is in the liquid metal phase, the quartz-liquid metal interface has high equivalent reflection and absorption coupling characteristics, resulting in a relatively high echo power. When a bubble passes over or covers the probe's sensitive end, causing it to partially enter the gas phase, the reflection characteristics of the quartz-gas interface change, resulting in a distinguishable change in the echo power. This change is then reflected as a significantly contrasting event waveform in the voltage signal output by the photodetector module. In this way, the present invention transforms the "invisible" behavior of bubbles in opaque liquid metal into collectable and identifiable changes in reflected echo electrical signals, enabling bubble event detection and statistical analysis.

[0030] Specifically, the fiber optic probe module adopts an insertion-type fiber optic probe 8, which includes a high-temperature resistant outer protective sleeve, a thermal stress isolation layer, and a fiber fixing structure 11. The thermal stress isolation layer is located between the fiber optic probe 8 and the outer protective sleeve, and is used to reduce the risk of micro-bending loss, end attitude drift, or even breakage of the fiber optic cable caused by thermal cycling.

[0031] Preferably, the fiber optic probe 8 is a replaceable fiber optic probe 8, which is detachably connected to the high-temperature resistant outer protective sleeve via threads, snaps, or welded seals to achieve rapid replacement and consistent reproduction of contaminated or worn fiber optic probe 8. Figure 5 It is detachable via the fixing thread 10.

[0032] Furthermore, considering the impact of the extreme environment of liquid metal on sensor reliability, this invention incorporates a high-temperature resistant fiber optic fixing structure 11 and a thermal stress isolation structure in the insertable fiber optic probe module. Due to significant differences in thermal expansion coefficients and elastic moduli between the metal jacket and the silica fiber, and because liquid metal conditions often involve thermal cycling processes such as heating, melting, temperature stabilization, and cooling solidification, the lack of thermal stress isolation and a reasonable fixing method can easily lead to micro-bending loss, end-position drift, or even breakage of the fiber, resulting in unstable signal baselines and measurement failures. This invention introduces a thermal stress isolation structure, such as a ceramic liner, a compliant buffer layer, or a combination thereof, between the fiber and the outer protective jacket, allowing stress to be released during thermal cycling, thereby improving the mechanical stability and signal repeatability of the probe in high-temperature environments.

[0033] Furthermore, liquid metals have a tendency to wet and adhere, and the end face may become contaminated and worn due to oxide films, impurities, or bubble clusters, leading to baseline drift or decreased sensitivity. To reduce maintenance difficulty and improve reproducibility, this invention designs the reflective sensitive end as a replaceable fiber optic probe 8. A threaded snap-fit ​​structure allows for quick replacement of the probe tip after contamination or wear, avoiding the scrapping of the entire probe and thus improving the engineering availability for long-term online monitoring.

[0034] Furthermore, the fiber optic probe 8 can be configured into a fiber optic probe array according to actual detection needs to detect information such as the distribution and velocity of bubbles in opaque liquid metal.

[0035] Preferably, the fiber optic probe 8 is made of multimode high-temperature resistant silica fiber and is used to detect a variety of low-melting-point opaque metals; for metals that are liquid at room temperature, multimode plastic fiber can also be used to make the fiber optic probe 8.

[0036] The signal processing method for liquid metal bubble detection sensors involves acquiring the output signals of all liquid metal bubble detection sensors simultaneously, converting the original signals into corresponding square wave signals using a single threshold method, and then calculating the porosity and bubble data information within the liquid metal detection area using the square wave signals.

[0037] Preferably, the threshold in the single threshold method is determined by the maximum inter-class variance method. Specifically, a histogram of the preprocessed echo signal amplitude is established within a statistical window, the threshold that maximizes the inter-class variance is selected as the single threshold, and the echo signal is binarized based on the single threshold to distinguish whether the medium contacted by the fiber optic probe 8 is liquid or gaseous. If the medium is liquid, it corresponds to opaque liquid metal, and if the medium is gaseous, it corresponds to bubbles.

[0038] Specifically, in terms of signal processing, this invention provides a bubble identification method suitable for online processing, addressing the characteristics of "noise superposition + baseline drift + irregular events" under liquid metal conditions. The method includes: filtering and preprocessing the acquired echo voltage signal to suppress pulse spikes and high-frequency noise; adaptively determining the threshold of the preprocessed signal within a preset statistical window, and dividing the signal into gas phase and liquid phase segments to obtain the start and end times and duration of bubble events; calculating parameters such as bubble frequency and porosity based on the proportion of the gas phase segment and the event count. In a preferred embodiment, the adaptive threshold determination can be achieved using the maximum inter-class variance method to calculate a single threshold. That is, a signal amplitude histogram is constructed within the statistical window, and the threshold that maximizes the inter-class variance is automatically determined, thereby achieving gas-liquid segmentation measurement without the need for repeated manual parameter adjustments. This processing method can adapt to signal amplitude variations caused by factors such as light source power fluctuations, interface state changes, and circuit gain drift in a liquid metal environment.

[0039] The working principle of this invention is as follows: Figure 1The diagram illustrates the working principle of the liquid metal bubble detection sensor proposed in this invention. Laser light generated by a laser source is transmitted into the gas-liquid environment via an optical fiber coupler. The laser light is reflected at the end face of the optical fiber probe 8, and the reflected light is received by a photodetector via the optical fiber coupler. The reflected light signal received by the photodetector is converted into an electrical signal by a photoelectric conversion circuit, and then transmitted to a PC computer via a data acquisition card.

[0040] like Figure 2 The diagram shows the principle of light reflection variation in a gas-metal liquid two-phase flow using fiber optic probe 8. The refractive index of the metal is expressed as: ; According to Fresnel's formula, the ideal perpendicular incident reflectivity R is... ; The refractive index of quartz, the core material of optical fiber, is approximately Based on the refractive index values ​​of common liquid metals and common gas phases, the reflected light from a quartz optical fiber is greater in a liquid metal than in a gas phase. Therefore, when the fiber probe 8 is in the liquid metal phase, more light is reflected back from the fiber, resulting in a stronger optical signal received by the detector. When a bubble passes through, the fiber probe 8 enters the gas phase, and more light is refracted into the bubble, weakening the reflected light and reducing the received signal. This causes a sudden drop in the received signal curve. Therefore, a concave signal waveform is generated when the bubble passes through, such as... Figure 7 As shown in the figure above, after obtaining the bubble signal, a threshold is calculated using the maximum inter-class variance algorithm, and single-threshold 0 and 1 signal processing is performed to transform the original bubble signal into an ideal rectangular signal. This allows for better determination of the gas phase ratio and event count, and the calculation of parameters such as bubble frequency and porosity. The processed signal is shown below. Figure 8 As shown.

[0041] This invention primarily addresses bubble detection in cases of heat transfer tube rupture in steam generators. In this embodiment, a sealed container filled with liquid lead-bismuth alloy is used, and gas is introduced to simulate a heat transfer tube rupture in a steam generator. Figure 3 - Figure 5As shown, a cover 2 is provided on the top of the tank 1. Four sets of sealing bolts 3 are connected to the top of the cover 2 in a ring. A safety valve 4 is connected to the surface of the cover 2. An exhaust valve 5 is connected to the side of the cover 2 near the safety valve 4. A temperature control resistor 6 is connected to the side of the cover 2 near the exhaust valve 5. A stainless steel capillary tube 7 is horizontally inserted into the inside of the tank 1 as a fixing structure. The stainless steel capillary tube is fixed to the tank wall by a threaded structure. An air inlet pipe 9 is provided at the lower end of the capillary tube 7. There are three sets of capillary tubes. A quartz fiber probe 8 is connected through the inside of each set of capillary tubes. A ceramic liner is placed between the high-temperature resistant quartz fiber and the external fixed capillary tube. A wedge surface is opened at the detection end of each set of fiber. According to the working environment of the embodiment, if the liquid phase in which the fiber probe 8 is located is a liquid lead-bismuth alloy phase, the preferred opening angle of the wedge surface is 45°.

[0042] The effect achieved by the embodiments is that, for device operation, specific links are as follows: Figure 6 As shown, this embodiment uses a 650nm red light source. The laser generated by the light source is transmitted into the gas-liquid environment through an optical fiber coupler. The laser is reflected at the end face of the optical fiber probe 8, and the reflected light is received by a photodetector through the optical fiber coupler. The reflected light signal received by the photodetector is converted into an electrical signal by a photoelectric conversion circuit, and then sent to a computer for data analysis via a data acquisition card. The bubble supply section consists of an air compressor and a flow controller, which provides a stable bubble input by adjusting the gas flow rate. A lead-bismuth alloy with a melting point of 125℃ is injected into the tank 1, filling it to two-thirds of the tank volume, ensuring that the liquid level after melting is higher than the top optical fiber probe 8. After confirming the airtightness, the exhaust valve 5 is opened and the argon flow rate is adjusted to 0.1L / min, continuously flowing for 20 minutes to completely remove residual oxygen and prevent oxidation of the lead-bismuth alloy during subsequent heating. After reopening the exhaust valve 5, the heating device is started. During heating, argon gas is continuously input at 0.1L / min to simulate a leak caused by a rupture in the heat transfer tube of the steam generator. The internal temperature of tank 1 is monitored in real time by a temperature-controlled thermal resistor 6, and raised to 150℃, which is higher than the melting point of lead-bismuth alloy (125℃) to ensure complete melting. When a continuous bubbling sound is heard in tank 1, it indicates that the metal has completely melted. At this point, the data acquisition system is activated to record the laser signal reflected back by the fiber optic probe 8. Figure 7 As shown. By adjusting the argon flow meter range from 0.1 to 0.5 L / min, dynamic bubble characteristic data under different gas flow conditions were obtained. During this period, the computer continuously converted the original signal into a corresponding 01 square wave signal using a self-adjusting single threshold method, such as... Figure 8 As shown, the behavior of the bubbles is determined by the obtained square wave signal to determine whether an accident has occurred. After data acquisition is completed, the laser source is turned off and the inlet valve and exhaust valve 5 are closed in sequence. The insulation layer is removed and the tank is allowed to cool naturally to room temperature for 24 hours.

[0043] The calculation steps for a specific case are as follows: The collected data is from the minimum value I min To the maximum value I max Make a profit N Calculate the probability of falling within each interval, where the i-th interval is... i The probability of each interval is p i , i =1,2,…, N .

[0044] The midpoint of each interval is ; A pre-selected threshold can be found. t ,make This allows us to obtain the total probability values ​​on both sides of the given value. and .

[0045] ; Calculate the average strength on both sides. and .

[0046] ; Then, the inter-class variance of the signal is calculated. ; The extreme value algorithm can be used to obtain the variance between classes. Maximum final threshold .

[0047] ; Its corresponding threshold signal strength for ; Voltage signals collected from the same working environment I Below this threshold The signal is the bubble behavior signal generated when the probe enters the bubble; otherwise, it is considered that no bubble has passed through.

[0048] ; This is used to obtain a standard binary signal of 0 and 1. Determine bubble information. This involves obtaining the corresponding bubble frequency based on the number of rectangular signal events occurring per unit time, and determining the porosity of the measurement area based on the proportion of rectangular signal events to the total number of signal events.

[0049] The formula for calculating porosity is: ; in, It is the first k The dwell time of a bubble event. N for T The total number of bubble events identified within a given time period.

[0050] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained in this invention are implemented according to conventional methods in the art unless otherwise specified or limited.

Claims

1. A liquid metal bubble detection sensor based on fiber optic reflection, characterized in that: The optical signal emitted from the laser source passes through a spatial beam splitter or fiber coupler and is transmitted to the fiber optic probe module, where it is reflected at the end face of the fiber optic probe module. The reflected light then passes through a spatial beam splitter or fiber optic coupler and enters a photodetector. It is then converted into an electrical signal by a photoelectric conversion circuit. The electrical signal is transmitted to a PC via a data acquisition card, where it is processed and output to determine the porosity and bubble frequency of the detected opaque liquid metal. The fiber optic sensor is based on the significant difference in reflectivity at the interface between the opaque liquid metal and the bubble and the fiber optic material. By analyzing the patterns of the echo signals received by the photodetector, it determines the bubble velocity and distribution information.

2. The liquid metal bubble detection sensor based on fiber optic reflection according to claim 1, characterized in that: The light signal emitted by the laser source is coupled into the optical fiber, passes through the optical fiber coupler, and enters the optical fiber probe module. The reflected light from the optical fiber probe then passes through the optical fiber coupler and is guided into the photodetector.

3. The liquid metal bubble detection sensor based on fiber optic reflection according to claim 1, characterized in that: The optical signal emitted by the laser source is coupled into the fiber optic probe module after passing through the spatial beam splitter, and the reflected light then passes through the spatial beam splitter into the photodetector.

4. A liquid metal bubble detection sensor based on fiber optic reflection according to claim 2 or 3, characterized in that: The fiber optic probe module adopts an insertion-type fiber optic probe, which includes a high-temperature resistant outer protective sleeve, a thermal stress isolation layer, and a fiber fixing structure. The thermal stress isolation layer is located between the fiber optic probe and the outer protective sleeve.

5. A liquid metal bubble detection sensor based on fiber optic reflection according to claim 2 or 3, characterized in that: The fiber optic probe is a replaceable fiber optic probe, which is detachably connected to the high-temperature resistant outer protective sleeve via threads, snaps, or welded seals.

6. A liquid metal bubble detection sensor based on fiber optic reflection according to claim 2 or 3, characterized in that: The fiber optic probes can be configured into fiber optic probe arrays according to actual testing needs.

7. A liquid metal bubble detection sensor based on fiber optic reflection according to claim 2 or 3, characterized in that: The fiber optic probe is made of multimode high-temperature resistant quartz fiber and is used to detect a variety of low-melting-point opaque metals. For metals that are liquid at room temperature, a multimode plastic fiber is used to make the fiber optic probe. The front end of the fiber optic probe is provided with a reflection-sensitive end, which includes a wedge-shaped reflective surface that is tilted at a certain angle relative to the fiber axis.

8. A liquid metal bubble detection sensor based on fiber optic reflection according to claim 2 or 3, characterized in that: The signal processing method of the liquid metal bubble detection sensor is as follows: After acquiring the output signals of all liquid metal bubble detection sensors at the same time, the original signal is converted into a corresponding square wave signal using a single threshold method, and the porosity and bubble data information in the liquid metal detection area are calculated through the square wave signal.

9. A liquid metal bubble detection sensor based on fiber optic reflection according to claim 8, characterized in that: The threshold in the single threshold method is determined by the maximum inter-class variance method. Specifically, a histogram of the preprocessed echo signal amplitude is established within a statistical window, and the threshold that maximizes the inter-class variance is selected as the single threshold. The echo signal is then binarized based on the single threshold to distinguish whether the medium contacted by the fiber optic probe is liquid or gaseous. If the medium is liquid, it corresponds to opaque liquid metal; if the medium is gaseous, it corresponds to bubbles.

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