A spherical storage tank device for three-dimensional electrical capacitance tomography of cryogenic fluids

Abstract

The present invention discloses a spherical storage tank device for three-dimensional capacitance tomography of cryogenic fluids. The device includes a regular pentagonal electrode group, a regular hexagonal electrode group, a spherical container, and a shielding cover. The device can use a spherical container to store and transport cryogenic fluids and realize three-dimensional imaging of the fluid. It uses a structure in which regular pentagonal and regular hexagonal electrodes are staggered like the surface of a football, so that the electrodes can more completely cover the surface of the spherical container, providing a dense and evenly distributed electrode layout, thereby improving the resolution and sensitivity of the inversion imaging. The device can be used in a wide range from room temperature to deep cryogenic temperatures, and multiple large-scale temperature cycles do not affect the measurement accuracy; it uses non-invasive measurement and will not interfere with the flow of fluid in the container; the present invention can be used for the storage and transportation of gas phase, liquid phase, and gas-liquid two-phase of cryogenic fluids, while monitoring the phase distribution of cryogenic fluids inside the container through three-dimensional imaging.

Description

Technical Field

[0001] The present invention relates to the fields of cryogenic refrigeration engineering technology and sensor science and technology, and in particular to a spherical storage tank device used for deep cryogenic fluid three-dimensional electrical capacitance tomography (ECVT). Background Art

[0002] Cryogenic fluids (such as liquid nitrogen (~78K) and liquid oxygen (~90K)) are widely used in chemical, air separation, refrigeration and other industries, involving key issues of design optimization and operational safety. Fluids in low-temperature environments usually exhibit special physical properties, such as significant differences in dielectric constants and thermal expansion characteristics of materials. For the detection of two-phase flows of cryogenic fluids (such as liquid nitrogen and liquid oxygen), accurate measurement of phase distribution is of great significance. Compared with fluids at room temperature (such as water / air), the gas-liquid dielectric constant ratio of cryogenic fluids (such as liquid nitrogen and liquid oxygen) is generally less than 1.5, which makes the imaging results extremely sensitive to measurement noise. In low-temperature environments, the shrinkage rates of metals and non-metals are quite different, and traditional electrode installation methods (such as bonding and ablation) can easily lead to loosening or even pipe rupture. In addition, non-metallic materials are prone to brittle cracking at low temperatures, which places higher requirements on the design of sensors.

[0003] Electrical Capacitance Tomography (ECT) technology measures the capacitance between electrode arrays and uses an inversion algorithm to reconstruct the phase distribution image of the fluid. The capacitance sensor is the key to the ECT system, and its core component is the electrode array. Different electrode array arrangements can adapt to the monitoring of fluid phase distribution and phase content in different flow types and container shapes. The shielding electrodes and shielding covers in the sensor provide electromagnetic protection for the sensor to prevent external electric field interference. As a non-invasive measurement method, compared with other measurement methods such as capacitance probe method, radio frequency sensor, particle image velocimetry, etc., ECT has the advantages of easy assembly, fast imaging speed, low cost, no interference with the flow field, and the ability to simultaneously obtain phase content and phase distribution data. It is suitable for two-phase flow monitoring of low-temperature fluids. Based on traditional ECT, three-dimensional electrical capacitance volume tomography (ECVT) has been developed, which can reconstruct a three-dimensional fluid image in the measurement area and obtain fluid phase distribution and phase content.

[0004] However, there are few experimental studies on electrical capacitance tomography applied to low-temperature two-phase flows. In low-temperature environments, differences in material thermal expansion coefficients can cause problems such as electrode detachment and wrinkling, affecting measurement accuracy. This results in a short and disposable operating time for traditional fixed sensors when used in low-temperature environments.

[0005] At the same time, there are few existing experimental studies on monitoring the flow field and flow pattern inside spherical containers. Traditional ECT sensors are usually coated on the surface of a circular tube, and the electrodes are mostly arranged in a ring or segmented shape to form multiple capacitance measurement channels. The arrangement of the circular tube electrodes is relatively regular, but there may be blind spots and measurement errors in complex flow fields.

[0006] In addition, there are few existing experimental studies focusing on the reconstruction of three-dimensional flow pattern images. Traditional ECT sensors usually invert two-dimensional flow pattern images in a certain cross-section. However, unlike circular tubes, in spherical containers, it is impossible to obtain the fluid flow state inside the entire container through the fluid phase distribution and phase content of a single cross-section.

[0007] In addition, in the gas-liquid two-phase scenario of room-temperature fluids, when stored in containers, the static or flowing speed is low, the Reynolds number of the fluid flow is small, and it is easy to maintain a laminar state. Due to the density difference, the upper layer is the gas phase and the lower layer is the liquid phase. Therefore, the differential pressure method, the full valve method, and other liquid level measurement methods can be used to determine the laminar interface and thus determine the fluid phase distribution. However, for the gas-liquid two-phase scenario of deep-temperature fluids, due to the local disturbances caused by temperature differences and phase changes, the fluid easily enters a turbulent state. Turbulence will produce unstable flow, leading to the generation and aggregation of bubbles, forming bubbly flow or a more complex gas-liquid two-phase distribution. In this way, conventional liquid level measurement methods will not be able to accurately measure the distribution of low-temperature fluids. At the same time, conventional liquid level measurement methods are mostly invasive measurements, which require opening more pipes or adding valves and other devices to the container, which affects the fluid flow and sealing while increasing the design and manufacturing costs.

[0008] Zhang Xiaobin, Tian Zenan, Gao Xinxin, and Xia Tao from the Institute of Refrigeration and Cryogenics, Zhejiang University [1][2][3] et al. have conducted theoretical and experimental research on the octupole capacitance sensor used for the measurement of cavitation rate of low-temperature fluid two-phase flow and the capacitance tomography technology suitable for the measurement of phase distribution and cavitation rate of low-temperature fluid. The direction of these studies is mainly focused on two-dimensional inversion images, which are quite different from the present invention in terms of device structure and imaging dimension. [4] The article theoretically verified the electrical capacitance tomography technology used in cryogenic fluid measurement and found that its numerical experimental results were good. However, the article did not involve the specific structural design of the electrical capacitance tomography sensor for cryogenic fluid two-phase flow measurement, and its solution algorithm was also very different from the present invention. [5] et al. proposed a new type of structural array planar capacitance sensor, which consists of seven regular hexagonal sensing electrodes forming a honeycomb structure array. However, the electrodes of this sensor are arranged in the same plane and can only invert two-dimensional flow pattern images, which is very different from the structure and function of the present invention. [6]et al. proposed a three-dimensional electrical capacitance tomography sensor to detect and visualize the distribution of water inside cement-based materials at room temperature. The sensor's electrodes were arranged around a rectangular pipe. Simulation results showed that the sensor could be used to accurately determine the locations of cracks and reservoirs in all samples, but its horizontal resolution was poor, which was very different from the application scenario and sensor structure design of the present invention.

[0009] Chinese patent CN117871621A discloses a visual electrical capacitance tomography system. This invention is applicable to low-temperature environments and circular pipe structures. Electrodes are arranged on the outside of the circular pipes, and it can monitor the two-dimensional imaging of flow patterns in real time, which is significantly different from the structure of the present invention.

[0010] Chinese patent CN117871620A discloses an indoor detection device and method for water damage in asphalt mixture based on capacitance measurement. The invention is applicable to room temperature. The electrode sheets are arranged on the outside of the measuring circular tube, which can realize a three-dimensional stereoscopic image of water damage. The electrode sheets are in direct contact with the fluid and are fixed by bolts. There are significant differences between the device structure and applicable scenarios of the present invention.

[0011] Chinese patent CN117554439A discloses a structural design of an ECT sensor that uses vacuum coating to directly attach metal electrodes to a measuring tube. The temperature environment used in this invention is low temperature, and the electrode sheet is installed on the outside of the measuring tube. A clamping device is used to fasten the conductive nails to the electrode surface to achieve circuit conduction. This invention has significant differences in device structure from the present invention.

[0012] Chinese patent CN116519755A discloses a hollow, square-cylinder-shaped sensor composed of four planar electrical capacitance tomography plates, designed to detect internal fruit damage at room temperature. In this invention, each plate is arranged with at least two rows of electrodes, each row containing multiple electrodes. The electrodes arranged on the four plates form a sensor array capable of reconstructing a two-dimensional cross-sectional image or a three-dimensional stereoscopic image of the damaged part of the fruit. This differs significantly from the present invention in terms of applicable scenarios and electrode arrangement.

[0013] Chinese patent CN115639254A discloses a non-360° rotating scanning capacitance tomography sensor. In this invention, each capacitance detection plate is arranged around the outer circumference of an insulating rotating drum. Through the rotation of the insulating drum, the sensor can obtain more capacitance projection data while maintaining the original capacitance measurement circuit's difficulty and noise interference resistance. It is applicable to pipelines with concentric pipe structures, and differs significantly from the present invention in both device structure and measurement data acquisition mechanism.

[0014] In summary, based on the physical properties of various materials in low-temperature environments and the characteristics of the dielectric constants of the gas-liquid phases of cryogenic fluids, it is necessary to design a spherical tank device for three-dimensional electrical capacitance tomography of deep-cryogenic fluids. This technology has the characteristics of non-invasive measurement and can accurately obtain the three-dimensional phase fraction and phase distribution in the spherical container.

[0015] [1] Lü Haizhou, Wang Tao, Zhou Kai, et al. Comparative study of classic algorithms for LN_2-VN_2 two-phase flow imaging based on electrical capacitance tomography[J]. Cryogenic Engineering, 2023, (02): 24-31.

[0016] [2] Zhou Kai, Wang Jun, Lü Haizhou, et al. Experimental study on inversion imaging of low-temperature fluid phase distribution based on ECT technology [J]. Energy Engineering, 2024, 44(01): 68-72. DOI: 10.16189 / j.nygc.2024.01.009.

[0017] [3] Lü Haizhou, Wang Tao, Zhou Kai, et al. Study on spatial resolution of cryogenic fluid capacitance tomography[J]. Cryogenic Engineering, 2023, (05): 8-16.

[0018] [4] Xie Huangjun. Theoretical and experimental study on low-temperature two-phase flow inversion based on multi-electrode capacitance sensor[D]. Zhejiang University, 2021.DOI:10.27461 / d.cnki.gzjdx.2021.000043.

[0019] [5] Guo Zhiheng, Ren Jiaming. Design of a new type of capacitance array sensor for planar capacitance tomography system [J / OL]. Journal of Electronic Measurement and Instrumentation, 1-10 [2024-07-12].

[0020] [6]Wentao Wang,Kaiyue Zhao,Peng Zhang,Jiuwen Bao,Shanbin Xue,Investigation of water ingress into uncracked and cracked cement-basedmaterials using electrical capacitance volume tomography,Materials&Design,Volume220,2022,110877,ISSN

[0021] 0264-1275, https: / / doi.org / 10.1016 / j.matdes.2022.110877. Summary of the Invention

[0022] The present invention provides a spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography (ECVT). The device can be used for the storage and transportation of cryogenic fluid gas phase, liquid phase, or gas-liquid two-phase. By measuring the capacitance between two electrodes, data can be transmitted to a computer, and a corresponding inversion algorithm can be used to image and monitor the phase distribution of the cryogenic fluid two-phase flow within the spherical container. The device can store and transport cryogenic fluid; it is non-invasive and does not affect the fluid flow state within the monitoring range; it also has a simple modular structure and is easy to disassemble and assemble, suitable for monitoring flow patterns within the spherical container; it is suitable for a wide range from room temperature to cryogenic temperatures; the use of polytetrafluoroethylene as the fluid container material can ensure that multiple large temperature cycles do not affect measurement accuracy; the electrode coverage is large, and it can achieve high-precision three-dimensional flow pattern imaging.

[0023] The technical solutions of the present invention are as follows:

[0024] The present invention discloses a spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography (ECVT), the device comprising a regular pentagonal electrode sheet group, a regular hexagonal electrode sheet group, a spherical container and a shielding cover;

[0025] The regular pentagonal electrode sheet group includes 12 regular pentagonal electrode sheets, which are in the shape of regular pentagonal inward-bent thin sheets, and the bending radius is equal to the outer diameter of the spherical container; the regular hexagonal electrode sheet group includes 20 regular hexagonal electrode sheets, which are in the shape of regular hexagonal inward-bent thin sheets, and the bending radius is equal to the outer diameter of the spherical container; the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group are tightly coated on the outer surface of the spherical container to form a spherical electrode sheet array;

[0026] The spherical container is provided with a cavity inside and a circular tube to allow fluid to enter and exit;

[0027] The shielding cover wraps the spherical container. The shielding cover is made of a thin metal wall and is matched with the spherical container at the same center. The inner diameter of the shielding cover is larger than the outer diameter of the spherical container. The circular tube on one side of the spherical container passes through the shielding cover and a flat welding flange is provided at the passing end for connecting to an external pipeline. The inside of the shielding cover and the outside of the spherical container form a closed space to cover the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group. The closed space is vacuumed.

[0028] Furthermore, the matching relationship between the regular pentagonal electrode sheet and the regular hexagonal electrode sheet is as follows: the sides of each regular pentagonal electrode sheet are adjacent to the sides of the regular hexagonal electrode sheet, each regular hexagonal electrode sheet has three sides adjacent to the regular pentagonal electrode sheet, and the other three sides are adjacent to other regular hexagonal electrode sheets. In order to reduce the impact between the electrodes, there is a certain gap between adjacent electrodes. Since the inner angle of the regular pentagonal electrode sheet and the inner angle of the regular hexagonal electrode sheet can approximately form a complete circular angle when spliced, this combination can form an approximate spherical shape. The electrodes have no overlapping areas, and the total span covered is close to the entire sphere. The regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group are arranged around the surface of the spherical container. The combined spherical electrode sheet array is matched with the spherical container at the same center, and is evenly and tightly coated on the outer surface of the spherical container.

[0029] The present invention also provides a low-temperature fluid phase distribution image monitoring method based on the device, which comprises the following steps:

[0030] 1) Using the device to obtain the empty field capacitance C of the spherical container in the empty field state e And the full-field capacitance C in the full-field state f ; Use the device to obtain the actual capacitance C under the current detection scene state c , calculate the normalized capacitance value Y;

[0031] 2) Obtain the normalized sensitivity S and the three-dimensional grid coordinate matrix inside the spherical container;

[0032] 3) Obtain the normalized dielectric constant g;

[0033] 4) The normalized dielectric constant g is combined with the three-dimensional grid coordinate matrix inside the spherical container. Each element in g corresponds one-to-one to the three-dimensional tetrahedral grid divided into three-dimensional space. The relative size of the element reflects whether the fluid is in the liquid phase or gas phase in the three-dimensional grid corresponding to the element. A three-dimensional image reflecting the phase distribution of the low-temperature fluid inside the spherical container is drawn to realize phase distribution image monitoring of the low-temperature fluid.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1) The device of the present invention uses a structure in which regular pentagonal and regular hexagonal electrodes are arranged alternately, similar to the surface of a soccer ball, so that the electrodes can more completely cover the surface of the spherical container, providing a dense and evenly distributed electrode layout, thereby improving the resolution and sensitivity of the inversion imaging.

[0036] 2) This device can store and transport cryogenic fluids and achieve three-dimensional imaging of the fluids; it can be quickly assembled and disassembled; it can be used in a wide range from room temperature to deep cryogenic temperatures, and multiple large-scale temperature cycles will not affect the measurement accuracy.

[0037] 3) This device adopts non-invasive measurement and will not interfere with the flow of fluid in the container; the pole piece coverage rate is large, the measurement imaging accuracy is high, and the structural electromagnetic shielding effect is good.

[0038] 4) The present invention can be used for the storage and transportation of cryogenic fluids in gas phase, liquid phase, and gas-liquid dual phase, while monitoring the phase distribution of the cryogenic fluid inside the container through three-dimensional imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is an isometric view of a spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) according to the present invention.

[0040] Figure 2 The figure is a top view of a spherical storage tank device used for three-dimensional electrical capacitance tomography (ECVT) of cryogenic fluids according to the present invention.

[0041] Figure 3 This is a cross-sectional view along line AA and a partial view along line B of a spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) according to the present invention.

[0042] Figure 4 This is an isometric view of a spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) according to the present invention, showing the spherical container and the flat welding flange after being matched.

[0043] Figure 5 This is an isometric view of a spherical storage tank device used for deep-cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) according to the present invention, after the regular pentagonal electrode group and the regular hexagonal electrode group are matched.

[0044] Figure 6 This is a schematic diagram of the finite element mesh division of a spherical storage tank device used for deep-cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) of the present invention.

[0045] Figure 7 This is a comparison diagram of the preset phase distribution (top) and the inverted phase distribution (bottom) of laminar flow in an embodiment of a spherical storage tank device applied to deep-cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) of the present invention.

[0046] Figure 8 This is a comparison diagram of the preset phase distribution (top) and the inverted phase distribution (bottom) of bubbly flow in an embodiment of a spherical tank device applied to cryogenic fluid three-dimensional electrical capacitance tomography (ECVT) of the present invention.

[0047] In the figure: 1. regular pentagonal electrode sheet, 2. regular hexagonal electrode sheet, 3. spherical container, 4. shielding cover. DETAILED DESCRIPTION

[0048] The present invention will be further described below with reference to the accompanying drawings.

[0049] like Figure 1-4 As shown, this embodiment provides a spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography (ECVT), which includes a regular pentagonal electrode sheet group, a regular hexagonal electrode sheet group, a spherical container 3 and a shielding cover 4.

[0050] like Figure 1 and 5 As shown, in one embodiment of the present invention, the regular pentagonal electrode sheet group includes 12 regular pentagonal electrode sheets 1, which are in the shape of regular pentagonal inward-bent thin sheets, and the bending radius is equal to the outer diameter of the spherical container 3; the regular hexagonal electrode sheet group includes 20 regular hexagonal electrode sheets 2, which are in the shape of regular hexagonal inward-bent thin sheets, and the bending radius is equal to the outer diameter of the spherical container 3; the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group are tightly wrapped on the outer surface of the spherical container 3 to form a spherical electrode sheet array; the matching relationship between the regular pentagonal electrode sheet 1 and the regular hexagonal electrode sheet 2 is: each regular pentagonal electrode sheet 1 has an edge adjacent to the edge of the regular hexagonal electrode sheet 2, and each regular hexagonal electrode sheet 2 has three edges adjacent to the regular pentagonal electrode sheet 1, and the other three edges are adjacent to other regular hexagonal electrode sheets 2. In order to avoid coupling interference between the electric fields of adjacent electrodes, there is a gap between adjacent electrodes, and the gap width l is equal to the inner diameter R of the spherical container 3 in The relationship satisfies l / R in ≥1 / 50. At the same time, in order to ensure the imaging accuracy, the electrode needs to be more completely covered on the outer surface of the spherical container 3. A larger gap will affect the electrode coverage. Therefore, the relationship between the gap width and the inner diameter of the spherical container 3 satisfies 1 / 50≤l / R in ≤1 / 25; the electrodes have no overlapping area, and the assembled electrode array is aligned with the spherical container 3 and tightly covers the outer surface of the spherical container 3. In a preferred embodiment of the present invention, the regular pentagonal electrode sheet 1 and the regular hexagonal electrode sheet 2 are both made of copper plates.

[0051] like Figure 1As shown, the spherical container 3 has a cavity inside and a circular tube for fluid inlet and outlet. In this embodiment, the spherical container is made of polytetrafluoroethylene and has a circular tube on one side of the horizontal axis for fluid inlet and outlet. The circular tube on the one side of the spherical container is coaxially matched with a flat welding flange for connection to an external pipeline. One of the regular hexagonal electrode sheets has a circular through hole with a diameter equal to the outer diameter of the circular tube on the one side of the spherical container. The through hole and the circular tube are coaxially matched. The shielding cover is a spherical shell structure made of thin metal wall. The shielding cover has a diameter larger than the spherical container. The shielding cover has a circular through hole on one side of the horizontal axis with a diameter equal to the outer diameter of the circular tube on the one side of the spherical container. The shielding cover and the spherical container are coaxially matched. The interior of the shielding cover and the exterior of the spherical container form an enclosed space that encloses the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group. This enclosed space is vacuum-insulated to minimize heat leakage in the system structure, thereby preventing even small evaporation of cryogenic fluids (such as liquid nitrogen and liquid oxygen) from affecting the experimental results.

[0052] The shielding cover 4 wraps around the spherical container 3. The shielding cover 4 is made of a thin metal wall and is matched with the spherical container 3 at the same center. The inner diameter of the shielding cover 4 is larger than the outer diameter of the spherical container 3. The circular tube on one side of the spherical container 3 passes through the shielding cover 4 and a flat welding flange is provided at the passing end for connecting to an external pipeline. The interior of the shielding cover 4 and the outside of the spherical container 3 form a closed space to cover the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group. The closed space is vacuumed.

[0053] like Figures 1-4 As shown in a specific embodiment, the shielding cover 4 is divided into two parts, the upper and lower parts, and a number of screw holes are distributed along the horizontal circumference at the connection between the two parts. The screw holes are coaxially matched with the bolts to combine the upper and lower parts of the shielding cover 4; the shielding cover 4 is provided with a through hole for vacuuming, and the target vacuum degree is lower than 10 -3 pa, the through hole is coaxially matched with the circular tube on one side of the spherical container 3 for connecting a vacuum pump; the shielding cover 4 is grounded when the device is working.

[0054] In a specific embodiment of the present invention, a structure with 12 regular pentagonal pole pieces and 20 regular hexagonal pole pieces, a total of 32 electrode pole pieces, is used. The overall experimental device assembly diagram is shown in FIG. Figure 1 As shown, the cryogenic working fluid is liquid nitrogen-nitrogen gas, and two typical flow conditions, stratified flow and bubbly flow, are used as imaging examples. According to the ECVT imaging principle shown in formula (1):

[0055] Y=S·g (1)

[0056] In the above formula, Y is the normalized capacitance, S is the normalized sensitivity, and g is the normalized dielectric constant. Sensitivity is the rate of change of the electrode capacitance with the infinitesimal dielectric constant in the spherical container. The normalized capacitance Y can be obtained in practical applications by measuring the electrode capacitance value and performing normalization operations; the normalized sensitivity S can be obtained by finite element calculations; and the normalized dielectric constant g can be obtained by combining the normalized capacitance Y and the normalized sensitivity S with an inversion algorithm. In the process of using finite element calculations to obtain the normalized sensitivity S, the coordinates of each three-dimensional tetrahedral mesh of the spherical container are also obtained. The relative size of the elements in the normalized dielectric constant g matrix can reflect whether the fluid in the corresponding mesh is in the liquid or gas phase. Therefore, the three-dimensional flow pattern image in the spherical container can be simulated and drawn based on the normalized dielectric constant g.

[0057] The specific implementation method for obtaining the normalized capacitance value Y is as follows: First, set the interior of the spherical container to an empty state (in this embodiment, the spherical container is set to be filled with nitrogen), number the 32 electrodes (numbered from ① to ③2), use voltage U to stimulate any electrode ①, and keep the other electrodes at 0 potential. At this time, measure and obtain ①-②, ①-③, ... ①- The capacitance value between the electrode pairs, then use voltage U to stimulate electrode ②, measure ②-③, ②-④...②- The capacitance value between the electrodes, and so on. It should be noted that the capacitance measurement between the two electrode combinations is not repeated, that is, when the capacitance measurement between ① and ② is completed, the capacitance value between ② and ① is no longer measured. When all 32 electrodes are stimulated in sequence, 496 capacitance measurements can be obtained, which are arranged into a matrix according to the electrode number, which is the empty field capacitance C e , as shown in formula (2):

[0058]

[0059] Similarly, the inside of the container is set to a full-field state, that is, the container is filled with the working medium to be measured (in this embodiment, liquid nitrogen is filled in this step), and the electrodes can be stimulated in sequence to obtain the full-field capacitance C f .

[0060] Then set the inside of the container to the typical laminar flow condition, such as Figure 7 As shown, the interior of the spherical container is divided into two parts by a plane, which are filled with liquid nitrogen and nitrogen gas respectively, and the electrodes are stimulated in turn to obtain the laminar flow state capacitance C c .

[0061] According to formula (3), the normalized capacitance Y is calculated c :

[0062]

[0063] Similarly, assuming that the inside of the container is a typical flow condition of bubbling flow, the normalized capacitance Y of the bubbling flow state can be obtained p .

[0064] Normalized sensitivity S is used to reflect the rate of change of the electrode capacitance in the spherical container with the dielectric constant of the microelement. Its value depends on the geometric dimensions of the spherical container and the arrangement of the electrode plates. It is calculated and solved using the finite element method after the device structure and dimensions are determined. After the device is manufactured, the normalized sensitivity S is an inherent property of the device and does not change with changes in the state of the fluid to be measured. The specific implementation method of obtaining normalized sensitivity S in the present invention is as follows: the finite element method is used to divide the three-dimensional space into a fine tetrahedral grid. Figure 6 That is, the tetrahedral grid division form shown in a specific embodiment is obtained, the three-dimensional coordinate matrix of each tetrahedral grid is obtained, the electric field intensity corresponding to each tetrahedral grid of 32 pole pieces under the single excitation state of electric potential U is measured in turn, and the sensitivity field between different pole piece pairs corresponding to each grid is calculated according to formula (4), and then organized into the normalized sensitivity S inside the entire spherical container.

[0065]

[0066] In formula (4), S ij is the corresponding change in capacitance between the i-th electrode and the j-th electrode when the dielectric constant inside the spherical container changes, E i is the electric field strength generated when the ith electrode is excited by the potential U alone, E j Similarly, V is the three-dimensional space inside the entire spherical container, and dσ is the volume element of V.

[0067] The specific implementation method for obtaining the normalized dielectric constant g is as follows: Since the normalized sensitivity S is not a square matrix, it is impossible to solve the normalized dielectric constant g by matrix inversion according to formula (1). The most commonly used and basic method in the field is to use the LBP inversion algorithm to obtain the initial value of the normalized dielectric constant g(0). The calculation principle of LBP is shown in formula (5):

[0068] g(0)=S T ·Y (5)

[0069] In formula (5), S Tis the transposed matrix of the sensitivity matrix S. The LBP algorithm is essentially a simple linear projection. It does not take into account the noise and error in the capacitance measurement. Therefore, the reconstructed image is prone to artifacts and noise, resulting in low spatial resolution. Therefore, the Landweber inversion algorithm is used to iteratively update the normalized dielectric constant initial value g(0) obtained by the LBP algorithm to the normalized dielectric constant g, gradually reducing the error between the reconstruction result and the actual measurement value. The calculation principle of Landweber is shown in formula (6):

[0070] g(n+1)=g(n)+λS T (Y-Sg(n)) (6) In formula (6), g(n) represents the normalized dielectric constant in the nth iteration, and λ is the relaxation parameter used to adjust the convergence speed and stability of the Landweber inversion algorithm. The normalized dielectric constant g is obtained through cyclic iterative calculation. Each element in the matrix g corresponds one-to-one to the three-dimensional tetrahedral grid divided into three dimensions. The relative size of the element reflects whether the fluid is in the liquid phase or the gas phase in the corresponding three-dimensional grid (the closer the element value is to 0, the gas phase is indicated, and the closer it is to 1, the liquid phase is indicated). Under liquid nitrogen-nitrogen gas conditions, the fluid is in the liquid phase when g is high and in the gas phase when g is low. Combining the normalized dielectric constant g and the finite element three-dimensional grid coordinate matrix obtained in the process of solving the normalized sensitivity S, a three-dimensional image reflecting the distribution of the cryogenic fluid phase inside the spherical container is drawn. Figure 7 The preset laminar flow pattern and the corresponding inversion results are shown. Figure 8 The preset bubbly flow pattern and the corresponding inversion results are displayed. Compared with the preset flow pattern, it can be seen that the invented device can achieve three-dimensional imaging while maintaining high imaging accuracy.

[0071] The specific embodiments described above further illustrate the structure, technical solutions and operating methods of the device of the present invention in actual use. It should be understood that the above is only a specific implementation example of the present invention and is not intended to limit the present invention. However, any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A spherical storage tank device for three-dimensional electrical capacitance tomography of cryogenic fluids, characterized in that: The device comprises a regular pentagonal electrode sheet group, a regular hexagonal electrode sheet group, a spherical container (3) and a shielding cover (4); The regular pentagonal electrode sheet group includes 12 regular pentagonal electrode sheets (1), which are in the shape of regular pentagonal inward-bent thin sheets, and the bending radius is equal to the outer diameter of the spherical container (3); the regular hexagonal electrode sheet group includes 20 regular hexagonal electrode sheets (2), which are in the shape of regular hexagonal inward-bent thin sheets, and the bending radius is equal to the outer diameter of the spherical container (3); the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group are tightly coated on the outer surface of the spherical container (3) to form a spherical electrode sheet array; The spherical container (3) is provided with a cavity inside and a circular tube for fluid to enter and exit; The shielding cover (4) wraps the spherical container (3). The shielding cover (4) is made of a thin metal wall and is coaxial with the spherical container (3). The inner diameter of the shielding cover (4) is larger than the outer diameter of the spherical container (3). The round tube on one side of the spherical container (3) passes through the shielding cover (4) and a flat welding flange is provided at the passing end for connecting to an external pipeline. The inside of the shielding cover (4) and the outside of the spherical container (3) form a closed space to cover the regular pentagonal electrode sheet group and the regular hexagonal electrode sheet group. The closed space is vacuumed.

2. The spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography according to claim 1, characterized in that: The matching relationship between the regular pentagonal electrode sheet (1) and the regular hexagonal electrode sheet (2) is as follows: the sides of each regular pentagonal electrode sheet (1) are adjacent to the sides of the regular hexagonal electrode sheet (2); each regular hexagonal electrode sheet (2) has three sides adjacent to the regular pentagonal electrode sheet (1), and the other three sides are adjacent to other regular hexagonal electrode sheets (2); there is a gap between adjacent electrode sheets, and the relationship between the gap width and the inner diameter of the spherical container (3) satisfies 1 / 50≤l / R in ≤1 / 25, each electrode has no overlapping area, and the combined electrode array is matched with the spherical container (3) at the same center and tightly covers the outer surface of the spherical container (3).

3. The spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography according to claim 1, characterized in that: The spherical container (3) is made of polytetrafluoroethylene material. One of the regular hexagonal electrode sheets is provided with a circular through hole for the circular tube on one side of the spherical container (3) to pass through. The diameter of the through hole is the same as the outer diameter of the circular tube on one side of the spherical container (3), and the through hole and the circular tube are coaxially matched.

4. The spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography according to claim 1, characterized in that: The regular pentagonal electrode sheet (1) and the regular hexagonal electrode sheet (2) are both made of copper plates.

5. The spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography according to claim 1, characterized in that: The shielding cover (4) is a spherical shell structure. A circular through hole is opened on one side of the shielding cover (4) for the single-side circular tube of the spherical container (3) to pass through. The diameter of the through hole is the same as the outer diameter of the single-side circular tube of the spherical container (3).

6. The spherical storage tank device for cryogenic fluid three-dimensional electrical capacitance tomography according to claim 1, characterized in that: The shielding cover (4) is divided into two parts, an upper part and an lower part. A plurality of screw holes are distributed along the horizontal circumference at the connection between the two parts. The screw holes are coaxially matched with the bolts to combine the upper and lower parts of the shielding cover (4); The shielding cover (4) is provided with a through hole for vacuuming, and the target vacuum degree is lower than 10 -3 pa, a through hole coaxially matched with a circular tube on one side of the spherical container (3) for connecting a vacuum pump; The shielding cover (4) is grounded when the device is working.

7. A method for monitoring low-temperature fluid phase distribution images based on the device of claim 1, characterized in that: The steps include: 1) Using the device to obtain the empty field capacitance C of the spherical container in the empty field state e And the full-field capacitance C in the full-field state f ; Use the device to obtain the actual capacitance C under the current detection scene state c , calculate the normalized capacitance value Y; 2) Obtain the normalized sensitivity S and the three-dimensional grid coordinate matrix inside the spherical container; 3) Obtain the normalized dielectric constant g; 4) The normalized dielectric constant g is combined with the three-dimensional grid coordinate matrix inside the spherical container. Each element in g corresponds one-to-one to a three-dimensional tetrahedral grid divided into three dimensions. The relative size of the element reflects whether the fluid is in the liquid phase or the gas phase in the three-dimensional grid corresponding to the element. A three-dimensional image reflecting the phase distribution of the cryogenic fluid inside the spherical container is drawn to realize the phase distribution image monitoring of the cryogenic fluid.

8. The method according to claim 7, characterized in that The step 1) is specifically as follows: 1.1) Set the fluid in the measurement domain to be in gas phase, and the interior of the spherical container is in an empty state; Use voltage U to excite any electrode, and keep the other electrodes at 0 potential. At this time, measure the capacitance value between the excited electrode and the electrode pair formed by the other electrodes respectively; replace the electrode excited by voltage U, and measure the capacitance value between the excited electrode and the electrode pair formed by the other electrodes respectively. Repeat the operation until the capacitance value between the electrode pair formed by any two electrodes is obtained. Arrange the capacitance values ​​into a matrix, which is the empty field capacitance C e ; 1.2) Set the fluid in the measurement domain to be in liquid phase. At this time, the interior of the spherical container is in a full-field state. Use the capacitance value measurement method in step 1.1) to obtain the capacitance value between any two electrode pairs in the full-field state, and arrange them to obtain the full-field capacitance C f ; 1.3) Use the capacitance value measurement method of step 1.1) to obtain the capacitance value between any two electrodes in the current state, and arrange them to obtain the actual capacitance C under the current detection scene state. c ; 1.4) Calculate the normalized capacitance Y using the following formula: c :

9. The method according to claim 7, characterized in that The step 2) is specifically as follows: The finite element method is used to divide the three-dimensional space inside the spherical container into fine tetrahedral grids, and the coordinate matrix of all three-dimensional grids is obtained. The electric field intensity corresponding to each grid of 32 electrodes under the single excitation state of potential U is measured in sequence. The sensitivity field between different electrode pairs corresponding to each grid is calculated according to the following formula, and then the normalized sensitivity S inside the entire spherical container is compiled. Where S ij (k) is the sensitivity value of the kth finite element grid in the three-dimensional space inside the spherical container between the i-th pole piece and the j-th pole piece, E i 、E j are the electric field intensities generated when the i-th and j-th electrodes are excited by the potential U separately, and V is the three-dimensional space inside the entire spherical container, that is, the spherical container of the experimental device.

10. The method according to claim 7, characterized in that The step 3) is specifically as follows: The LBP inversion algorithm is used to obtain the initial value of the normalized dielectric constant g(0). The calculation formula of the LBP inversion algorithm is shown as follows: g(0)=S T ·Y In the above formula, S T is the transposed matrix of the sensitivity matrix S. The Landweber inversion algorithm is used to iteratively update the normalized dielectric constant g(n) to gradually reduce the error between the reconstructed result and the actual measurement value. The calculation principle of Landweber is shown in the following formula: g(n+1)=g(n)+λS T (Y-Sg(n)) In the above formula, g(n) represents the normalized dielectric constant in the nth iteration, and λ is the relaxation parameter used to adjust the convergence speed and convergence stability of the Landweber inversion algorithm. The normalized dielectric constant g is calculated through cyclic iterative calculation.

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