Bubble fraction sensor, flowmeter employing same, and cryogenic liquid transfer tube

Abstract

A bubble fraction sensor of this disclosure comprises: an insulating inner tube having a through-hole for passing a low temperature liquid therethrough; at least a pair of electrodes fitted to an outer peripheral surface of the inner tube; a conductive pin connected to each of the pair of electrodes; a case that surrounds the inner tube and has first insertion holes into which the conductive pins are inserted; and a filling member that is disposed between an inner wall surface of the case and a rear surface opposite to mutually-facing front surfaces of at least the pair of electrodes. The absolute value of a temperature coefficient τε1 of the relative permittivity of the filling member at -196°C-20°C is less than or equal to the absolute value of a temperature coefficient τε2 of the relative permittivity of the inner tube at -196°C-20°C.

Description

Bubble rate sensor, flow meter using same, and cryogenic liquid transfer pipe

[0001] The present disclosure relates to a bubble rate sensor for measuring the bubble rate of a cryogenic liquid such as liquid hydrogen, a flow meter using the same, and a cryogenic liquid transfer pipe.

[0002] Recently, the use of hydrogen as a potential energy storage medium has been attracting attention in line with efforts to reduce greenhouse gas emissions. Liquid hydrogen, in particular, has high volumetric efficiency and can be stored for long periods, leading to the development of various technologies for its use. However, an accurate method for measuring the flow rate required for handling large amounts of liquid hydrogen has not yet been established industrially. The main reason for this is that liquid hydrogen is a fluid that vaporizes very easily and exhibits large changes in the gas-to-liquid ratio.

[0003] Liquid hydrogen is a cryogenic liquid (boiling point -253°C), with extremely high thermal conductivity and little latent heat, which means that bubbles (voids) quickly form. As a result, liquid hydrogen flows in a gas-liquid two-phase flow, a mixture of gas and liquid, inside the pipes used for transporting it. As the bubble content varies greatly, it is not possible to accurately measure the flow rate of liquid hydrogen flowing through the pipes by simply measuring the flow velocity, as is done with normal liquids.

[0004] Therefore, development of a bubble rate meter that measures the bubble rate, which indicates the volume fraction of the gas phase in a gas-liquid two-phase flow, is underway. As such a bubble rate meter, Patent Document 1 proposes a capacitance-type void rate meter that measures capacitance using a pair of electrodes.

[0005] Patent Document 1 proposes a capacitance-type void fraction meter that includes an instrument tube body interposed via an electrically insulating joint in the middle of a piping system through which a fluid consisting of a gas phase and a liquid phase flows, a pair of electrodes arranged on the outer surface of the instrument tube body facing each other across the tube axis, an electrostatic shielding tube fitted around and surrounding the outside of the pair of electrodes, a sleeve fitted around the outside of the electrostatic shielding tube with a sealed space for enclosing an inert gas or a moisture-absorbing material, and a measuring instrument connected to the pair of electrodes and measuring the electrostatic capacitance induced between the pair of electrodes. It also describes that the space between the electrodes of this capacitance-type void fraction meter and the electrostatic shielding tube is filled with an electrically insulating plastic material to mold the electrostatic shielding tube (see Figure 1 of Patent Document 1).

[0006] Japanese Utility Model Application Laid-Open Publication No. 1-93559

[0007] The air bubble rate sensor of the present disclosure comprises an inner tube having a through hole for flowing a cryogenic liquid, at least a pair of electrodes attached to the outer surface of the inner tube, conductive pins connected to each of the pair of electrodes, a housing surrounding the inner tube and having a first insertion hole into which the conductive pin is inserted, and a filler member arranged between the inner wall surface of the housing and the back surface opposite to the opposing surfaces of at least the pair of electrodes, wherein the absolute value of the temperature coefficient τε1 of the relative dielectric constant of the filler member at −196° C. to 20° C. is equal to or less than the absolute value of the temperature coefficient τε2 of the relative dielectric constant of the inner tube at −196° C. to 20° C.

[0008] The flow meter of the present disclosure measures the flow rate of a cryogenic liquid flowing through a through-hole of an inner pipe, and includes the above-described bubble rate sensor and a flow meter that measures the flow velocity of the cryogenic liquid flowing through the through-hole. Also, the cryogenic liquid transfer pipe of the present disclosure includes the above-described flow meter.

[0009] FIG. 7B is a schematic vertical cross-sectional view showing a bubble rate sensor according to an embodiment of the present disclosure. FIG. 7C is a graph for explaining the temperature characteristics of the dielectric constant. FIG. 7D is a perspective view showing a bubble rate sensor according to another embodiment of the present disclosure. FIG. 7E is an exploded perspective view of the bubble rate sensor shown in FIG. 3. FIG. 7F is a schematic vertical cross-sectional view of the bubble rate sensor shown in FIG. 3. FIG. 7G is a schematic vertical cross-sectional view showing a bubble rate sensor according to yet another embodiment of the present disclosure. FIG. 7H is an exploded cross-sectional view showing a modified example of a portion of the bubble rate sensor shown in FIG. 1. FIG. 7H is a perspective view showing portion A in FIG. 7A. FIG. 7I is a schematic horizontal cross-sectional view showing a bubble rate sensor according to another embodiment of the present disclosure.

[0010] In the void fraction meter described in Patent Document 1, a fluid such as a cryogenic liquid flows through a meter tube. When a large temperature change occurs on the outer periphery of the meter tube, plastic materials such as PET resin have a large temperature coefficient of relative dielectric constant (i.e., high temperature dependency). This causes a large change in capacitance between the electrode and the electrostatic shield tube, making it impossible to accurately measure the bubble fraction. Therefore, an object of the present disclosure is to provide a bubble fraction sensor that can accurately measure the bubble fraction in a pipe through which a cryogenic liquid flows, as well as a flowmeter and a cryogenic liquid transfer pipe using the same.

[0011] An air bubble rate sensor according to an embodiment of the present disclosure will be described below with reference to the drawings. In the following description, an air bubble rate sensor for measuring the air bubble rate when liquid hydrogen is used as the cryogenic liquid will be described. However, for the sake of convenience, the drawings referred to below show simplified embodiments of the present disclosure. Therefore, the air bubble rate sensor disclosed below may include optional components not shown in the drawings referred to. Furthermore, the dimensions of the components in the drawings do not faithfully represent the actual dimensions of the components, the dimensional ratios of the components, etc.

[0012] 1, the air bubble rate sensor 1 of this embodiment comprises an insulating inner tube 21 having a through hole 31 for flowing liquid hydrogen, at least one pair of electrodes 4, 4 attached to the outer surface of the inner tube 21, a conductive pin 91 connected to each electrode 4, a housing 22 surrounding the inner tube 21 and having a connecting hole 24 communicating with the through hole 31 and a first insertion hole 23 for inserting the conductive pin 91, a first airtight terminal 81 for fixing the conductive pin 91, and a filler member 29 housed between the inner wall surface of the housing 22 and the back surface opposite to the opposing surfaces of at least each electrode 4. The pair of electrodes 4, 4 are arranged to face each other across the axis of the inner tube 21. The insulating inner tube 21 is made of a material having a volume resistivity of 10 １０ At both ends of the housing 22, metal pipes 25 are arranged which communicate with the through holes 31 of the inner pipe 21 via the connecting holes 24 of the housing 22.

[0013] The inner tube 21 has a pair of recesses 28 that open to the outside, and electrodes 4 are attached to the bottom surfaces of the pair of recesses 28. A filling member 29 is housed in the recesses 28. In the embodiment shown in FIG. 1 , the filling member 29 abuts against the inner wall surface of the housing 22 and further extends into the first insertion hole 23, contacting the first airtight terminal 81. Therefore, the extending filling member 29 reduces changes in impedance, thereby improving the accuracy of measuring the air void ratio. Furthermore, since the filling member 29 fills at least a portion of the first insertion hole 23, the airtightness inside the housing 22 is also improved. Note that the filling member 29 does not necessarily have to contact the first airtight terminal 81.

[0014] The housing 22 includes a frame 22a that houses the inner pipe 21 and a lid 22b that seals the opening of the frame 22a. After the inner pipe 21 is housed in the frame 22a, the frame 22a and the lid 22b are joined by welding or brazing. The frame 22a and the lid 22b each have an opening, i.e., a connecting hole 24, that communicates with the through hole 31 of the inner pipe 21. A metal pipe 25 is welded or brazed to the frame 22a and the lid 22b, respectively, so as to communicate with the through hole 31 via the opening.

[0015] Two annular portions 51a, 51b are located axially outside the housing 22, sandwiching the housing 22. The annular portions 51a, 51b have axial holes that are coaxial with the inner pipe 21 and are welded or brazed to the outer circumferential surface of a metal pipe 25 that is inserted through this axial hole. An outer pipe 26 is disposed between the annular portions 51a, 51b, and both ends of the outer pipe 26 are joined to the annular portions 51a, 51b, respectively. The outer pipe 26 has a second insertion hole 27 that opens radially. In FIG. 1, the welded or brazed joint points are indicated by the symbol W.

[0016] A first airtight terminal 81 for fixing the conductive pin 91 is provided on the outside of the first insertion hole 23 provided in the housing 22. Similarly, a second airtight terminal 82 for fixing the conductive pin 91 that is individually connected to each electrode 4 is provided on the outside of the second insertion hole 27. The first airtight terminal 81 may be provided inside or on the inside of the first insertion hole 23. Similarly, the second airtight terminal 82 may be provided inside or on the inside of the second insertion hole 27.

[0017] The outer pipe 26 is provided with a vacuum exhaust valve 15 (e.g., a vacuum exhaust needle valve), which forms a vacuum space 100 (thermal insulation layer) between the housing 22 and the outer pipe 26. In this way, the vacuum space 100 is located between the housing 22 and the outer pipe 26, which reduces evaporation of liquid hydrogen due to the outside air temperature, improves the thermal insulation performance for the inner pipe 21, reduces the generation of bubbles, and improves the accuracy of measuring the bubble rate.

[0018] The inner tube 21 preferably contains ceramics, for example, ceramics containing aluminum oxide as the main component. When aluminum oxide is the main component, the inner tube 21 can be made to have excellent mechanical properties while the raw material price and manufacturing cost are relatively low. When the inner tube 21 contains ceramics containing aluminum oxide as the main component, it may also contain, for example, silicon, magnesium, and calcium. When these elements are converted into oxides of the total 100 mass% of the components constituting the ceramic, they can be, for example, SiO ２ is 0.3 mass % to 1 mass %, MgO is 0.1 mass % to 0.4 mass %, and CaO is 0.04 mass % to 0.08 mass %.

[0019] The inner tube 21 is made of anorthite (CaAl２ Si ２ O ８ ) may be contained. Anorthite has a smaller linear expansion coefficient than aluminum oxide, so containing anorthite can improve thermal shock resistance. In particular, it is preferable that the inner tube 21 contains low thermal expansion ceramics. Low thermal expansion ceramics refer to ceramics with a linear expansion coefficient of 0±20 ppb / K or less at 22°C. Because low thermal expansion ceramics have a low linear expansion coefficient, the risk of breakage is reduced even when subjected to thermal shock from cryogenic liquids, including liquid hydrogen.

[0020] Specifically, the low-thermal expansion ceramic preferably has a main crystalline phase of cordierite, alumina, mullite, and sapphirine as secondary crystalline phases, and an amorphous phase containing Ca in the grain boundary phase. The crystalline phase ratio of the main crystalline phase is preferably 95% by mass to 97.5% by mass, and the crystalline phase ratio of the secondary crystalline phase is preferably 2.5% by mass to 5% by mass. The Ca content relative to the total amount is preferably 0.4% by mass to 0.6% by mass, calculated as CaO, and the zirconia content relative to the total amount is preferably 0.1% by mass to 1.0% by mass. The relative dielectric constant of the ceramic forming the inner tube 21 becomes closer to that of the cryogenic liquid, improving high-frequency characteristics and further improving the accuracy of measuring the porosity. The crystalline phases and their ratios in the low-thermal expansion ceramic can be analyzed using the Rietveld method with an X-ray diffractometer using CuKα radiation, with the diffraction angle 2θ range of 8 to 100° as the analysis target.

[0021] The inner tube 21 may also contain ceramics whose main component is, for example, silicon nitride or sialon. These ceramics have high mechanical strength and thermal shock resistance, so the risk of breakage even when subjected to thermal shock is reduced. Specifically, the ceramics contain calcium oxide, aluminum oxide, and oxides of rare earth elements. With respect to a total of 100 mass% of calcium oxide, aluminum oxide, and oxides of rare earth elements, the calcium oxide content is 0.3 mass% to 1.5 mass%, the aluminum oxide content is 14.2 mass% to 48.8 mass%, and the balance is oxides of the rare earth elements. The silicon nitride has a composition formula of Si ６－ＺAl Ｚ O Ｚ N ８－Ｚ (z = 0.1 to 1) and has an average crystal grain size of 20 μm or less (excluding 0 μm). The main component in a ceramic refers to a component that accounts for 60 mass% or more of the total 100 mass% of the components that make up the ceramic. In particular, the main component is preferably a component that accounts for 95 mass% or more of the total 100 mass% of the components that make up the ceramic. The components that make up the ceramic can be determined using an X-ray diffraction device (XRD). The content of each component can be determined by identifying the component, then using an X-ray fluorescence analyzer (XRF) or an ICP optical emission spectrometer to determine the content of the elements that make up the component, and converting it to the identified component. The relative density of the ceramic is, for example, 92% or more and 99.9% or less. The relative density is expressed as a percentage (ratio) of the apparent density of the ceramic determined in accordance with JIS R 1634-1998 to the theoretical density of the ceramic.

[0022] The ceramic has closed pores, and the value obtained by subtracting the average circle-equivalent diameter of the closed pores from the average distance between the centers of gravity of adjacent closed pores (hereinafter, this value is referred to as the spacing between closed pores) may be 8 μm or more and 18 μm or less. The closed pores are independent of each other. When the spacing between closed pores is 8 μm or more, the closed pores are relatively dispersed, resulting in high mechanical strength. On the other hand, when the spacing between closed pores is 18 μm or less, even if microcracks originating from the contours of the closed pores are generated due to repeated thermal shocks, the surrounding closed pores are likely to block their expansion. Therefore, when the spacing between closed pores is 8 μm or more and 18 μm or less, the inner tube 21 containing this ceramic can be used for a long period of time. The skewness of the circle-equivalent diameter of the closed pores may be greater than the skewness of the distance between the centers of gravity of the closed pores. Here, skewness is an index (statistic) that indicates how much a distribution is distorted from a normal distribution, that is, the left-right symmetry of the distribution. When the skewness is greater than 0, the tail of the distribution points to the right; when the skewness is 0, the distribution is symmetric; and when the skewness is less than 0, the tail of the distribution points to the left.

[0023] When the histograms of the equivalent circle diameter of closed pores and the distance between the centers of gravity of closed pores are superimposed, if the skewness of the equivalent circle diameter of closed pores is greater than the skewness of the distance between the centers of gravity of closed pores, the mode of the equivalent circle diameter is located to the left (zero side) of the mode of the distance between the centers of gravity of closed pores. In other words, there are many closed pores with small equivalent circle diameters, and these closed pores are more sparsely distributed, resulting in an inner tube 21 that combines mechanical strength and thermal shock resistance. For example, the skewness of the equivalent circle diameter of closed pores is 1 or more, and the skewness of the distance between the centers of gravity of closed pores is 0.6 or less. The difference between the skewness of the equivalent circle diameter of closed pores and the skewness of the distance between the centers of gravity of closed pores is 0.4 or more.

[0024] To determine the distance between the centers of gravity of the closed pores and the equivalent circle diameter, first, measure the average particle diameter D ５０ The sample is polished on a copper plate using diamond abrasive grains with an average grain size D ５０ By polishing with a tin plate using diamond abrasive grains with a roughness of 0.5 μm, a polished surface having an arithmetic mean roughness Ra of 0.2 μm or less in the roughness curve is obtained. The arithmetic mean roughness Ra of the polished surface is measured in the same manner as described above. The polished surface is observed at a magnification of 200 times, and an average range is selected. For example, an area of ​​7.2 × 10 ４ μm ２A CCD camera is used to photograph an area of ​​the closed pores (310 μm horizontal length, 233 μm vertical length) to obtain an observation image. The distance between the centers of gravity of the closed pores and the equivalent circle diameter can be determined using image analysis software for this observation image. Examples of image analysis software that can be used include Image-Pro (manufactured by Hakuto Co., Ltd.), WinROOF2023 (manufactured by Mitani Shoji Co., Ltd.), and "A-zo-kun (ver. 2.52)" (registered trademark, manufactured by Asahi Kasei Engineering Co., Ltd.). When Image-Pro is used, the closed pores on the image are considered to be particles, and the equivalent circle diameter of these particles is measured. When Image-Pro is used, the center-to-center distance between particles is measured, and this center-to-center distance can be used as the distance between the centers of gravity of the closed pores. When WinROOF2023 is used as the image analysis software, the closed pores on the image are considered to be particles, and the equivalent circle diameter of these particles is measured. When WinROOF2023 is used, the distance between the centers of gravity of closed pores can be determined by a technique called interparticle distance measurement. When "A-zo-kun (ver. 2.52)" is used, the distance between the centers of gravity of closed pores can be determined by a technique called the distance between centers of gravity method for dispersity measurement. Hereinafter, when the image analysis software "A-zo-kun" is mentioned, it refers to the image analysis software manufactured by Asahi Kasei Engineering Co., Ltd.

[0025] The setting conditions for this method are, for example, a threshold value, which is an index showing the brightness of an image, of 165, brightness is dark, and the area for removing small figures is 1 μm ２ The threshold value can be adjusted according to the brightness of the observed image, and the brightness can be set to dark, the binarization method can be set to manual, and the small figure removal area can be set to 1 μm ２ With the noise reduction filter and the noise reduction filter enabled, the threshold value can be adjusted so that the markers appearing in the observation image match the shape of the closed pores. The equivalent circle diameter of the closed pores can be determined by using a technique called particle analysis on the observation image. The setting conditions can be the same as those used to determine the distance between the centers of gravity of the closed pores. The skewness of the equivalent circle diameter and the distance between the centers of gravity of the closed pores can be determined using the function Skew provided in Excel (registered trademark, Microsoft Corporation).

[0026] An example of a method for manufacturing the inner tube 21 made of such ceramics will be described below. The case where the main component of the ceramics forming the inner tube 21 is aluminum oxide will be described. Aluminum oxide powder (purity of 99.9% by mass or more), which is the main component, and powders of magnesium hydroxide, silicon oxide, and calcium carbonate are charged into a grinding mill together with a solvent (ion-exchanged water), and the average particle size (D ５０ The powder is then pulverized until its particle size is 1.5 μm or less. An organic binder and a dispersant for dispersing the aluminum oxide powder are then added and mixed to obtain a slurry. The magnesium hydroxide powder content is 0.3 to 0.42 mass%, the silicon oxide powder content is 0.5 to 0.8 mass%, the calcium carbonate powder content is 0.06 to 0.1 mass%, and the balance is the aluminum oxide powder and unavoidable impurities, based on a total of 100 mass% of the powders. Examples of organic binders include acrylic emulsion, polyvinyl alcohol, polyethylene glycol, and polyethylene oxide.

[0027] Next, the slurry is spray-granulated to obtain granules, and then pressurized using a uniaxial press or cold isostatic press at a molding pressure of 78 MPa to 118 MPa to obtain a columnar molded body. If necessary, the molded body may be machined to form recesses 28 after firing. The molded body is fired at a firing temperature of 1580°C to 1780°C for a holding time of 2 hours to 4 hours to obtain an inner tube 21 containing ceramic. To obtain ceramics with a closed pore spacing of 8 μm to 18 μm, the molded body may be fired at a firing temperature of 1600°C to 1760°C for a holding time of 2 hours to 4 hours. The surface of the ceramic member facing the metal tube 25 may be ground to form a ground surface. The surface of the recess 28 where the electrode 4 is to be provided may also be ground to form a bottom surface. The inner tube 21 preferably has an inner diameter of 50 mm or more.

[0028] The inner peripheral wall 210 of the inner pipe 21 that forms the through hole 31 is preferably a fired surface that is not ground or polished. This makes the inner peripheral wall 210 more crystalline than the inside of the inner pipe 21, and reduces the occurrence of cracks in the inner peripheral wall 210 even when the supply and stop of the cryogenic liquid is repeated.

[0029] The frame portion 22a and the lid portion 22b that constitute the housing 22 are preferably made of, for example, austenitic stainless steel (for example, SUS316L, SUS316LN, SUS304) or the like.

[0030] The annular portions 51a, 51b are preferably formed from, for example, a Fernico alloy, an Fe—Ni alloy, an Fe—Ni—Cr—Ti—Al alloy, an Fe—Cr—Al alloy, an Fe—Co—Cr alloy, an Fe—Co alloy, an Fe—Co—C alloy, or an austenitic stainless steel with a nickel content of 10.4 mass% or more. To obtain sufficient thermal insulation performance, the outer diameters of the annular portions 51a, 51b may be 1 mm or more relative to the outer diameter of the inner pipe 21, or 10 mm or more relative to the outer diameter of the inner pipe 21, or 200 mm or less relative to the outer diameter of the inner pipe 21, or 100 mm or less. The annular portions 51a, 51b are hermetically joined to the outer peripheral surface of the metal pipe 25 by brazing. The outer tube 26 is preferably made of a metal such as austenitic stainless steel (for example, SUS316L) containing 10.4 mass % or more of nickel, or a ceramic such as silicon nitride or sialon.

[0031] The first hermetic terminal 81 constitutes a so-called hermetic connector and includes a conductive pin 91, a cylindrical first ceramic substrate 50a having a first pinhole extending in the thickness direction for inserting the conductive pin 91, and a first annular body 52a surrounding the outer circumferential surface of the first ceramic substrate 50a. The first annular body 52a functions as a sleeve for holding the first ceramic substrate 50a and preferably includes, for example, a Fernico alloy, an Fe—Ni alloy, an Fe—Ni—Cr—Ti—Al alloy, an Fe—Cr—Al alloy, an Fe—Co—Cr alloy, an Fe—Co alloy, an Fe—Co—C alloy, or an austenitic stainless steel with a nickel content of 10.4 mass% or more. This reduces the risk of embrittlement due to liquid hydrogen, thereby maintaining the accuracy of the porosity measurement over a long period of time. Examples of austenitic stainless steels having a nickel content of 10.4 mass % or more include SUS310S, SUS316L, SUS316LN, SUS316J1L, and SUS317L.

[0032] The second airtight terminal 82 is composed of a conductive pin 91, a cylindrical second ceramic substrate 50b having a second pinhole in the thickness direction for inserting the conductive pin 91, and a second annular body 52b surrounding the outer circumferential surface of the second ceramic substrate 50b. The second annular body 52b functions as a sleeve for holding the second ceramic substrate 50b and can be made of the same material as the first annular body 52a.

[0033] The electrode 4 can be made of, for example, copper foil, aluminum foil, or the like. The electrode 4 can be formed on the outer peripheral surface of the inner tube 21 by, for example, vacuum deposition, metallization, or active metal methods. A metal plate that will serve as the electrode 4 may be bonded to the bottom surface of the recess 28, which will be described later. The thickness of the electrode 4 may be 10 μm or more, or 20 μm or more, or may be 2 mm or less, and is preferably 1 mm or less.

[0034] The metal pipe 25 is disposed on both ends of the inner pipe 21 via the housing 22, and annular portions 51 a, 51 b are welded or brazed to the outer circumferential surface. By connecting the metal pipe 25 to the inner pipe 21 via the housing 22 in this manner, the inner pipe 21 becomes less susceptible to damage due to external impact. Furthermore, by welding or brazing the metal pipe 25 to the annular portions 51 a, 51 b, leakage of liquid hydrogen from the inner pipe 21 to the outside is reduced, improving the accuracy of measuring the gas void fraction. The metal pipe 25 may be a liquid hydrogen transfer pipe for transferring liquid hydrogen.

[0035] It is preferable that the absolute value of the temperature coefficient τε1 of the dielectric constant of the filler 29 at temperatures between -196°C and 20°C is equal to or less than the absolute value of the temperature coefficient τε2 of the dielectric constant of the inner tube 21 at temperatures between -196°C and 20°C. This allows the influence of changes in capacitance between the electrode 4 and the housing 22 to be reduced, enabling accurate measurement of the porosity, even when liquid hydrogen flows through the through-hole 31 and the temperature change (temperature difference) increases on the outer periphery of the inner tube 21. Here, the temperature coefficient of the dielectric constant can be defined by the following formula: Temperature coefficient of the dielectric constant = {(dielectric constant at 20°C) - (dielectric constant at -196°C)} / {(20°C) - (-196°C)}. Note that "at -196°C to 20°C" refers to the range from the liquefaction temperature of liquid nitrogen to room temperature. The range of −196° C. to 20° C. is set because, if the absolute value of the temperature coefficient τε1 is equal to or less than the absolute value of the temperature coefficient τε2 within this range, it is also applicable to liquid hydrogen (liquefaction temperature −253° C.).

[0036] The absolute value of the temperature coefficient τε1 of the relative dielectric constant of the filler member 29 is preferably 100 ppm / K or less. This reduces large changes in capacitance between the electrode 4 and the housing 22, and improves the accuracy of the porosity measurement. Furthermore, the absolute value of the temperature coefficient τε2 of the relative dielectric constant of the inner tube 21 is preferably 1000 ppm / K or less. This reduces the difference between the absolute values ​​of the temperature coefficients τε1 and τε2, allowing the porosity to be measured with high accuracy. The temperature coefficient of the relative dielectric constant can be measured, for example, by a method similar to any of the following methods. JIS R1627-1996 (Testing method for dielectric properties of fine ceramics at microwave frequencies), ASTM D-150 (Standard Test Methods for AC Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulation), and JIS C2141-1992 (Testing methods of ceramic insulators for electrical and electronic applications). Of these measurement methods, it is preferable to use the measurement method described in ASTM D150 or JIS C2141-1992. If the temperature coefficient of the dielectric constant of the filler member 29 and the inner tube 21 cannot be measured in their original shapes, the filler member 29 and the inner tube 21 may be processed, and the processed products may be used to measure the temperature coefficient of the dielectric constant.

[0037] The positive and negative signs of the temperature coefficient τε1 are preferably different from the positive and negative signs of the temperature coefficient τε2. By making the positive and negative signs of the temperature coefficient τε1 different from the positive and negative signs of the temperature coefficient τε2, the influence of the change in capacitance between the electrode 4 and the housing 22 is partially offset and reduced, thereby improving the accuracy of measuring the bubble rate.

[0038] FIG. 2 is a graph illustrating the temperature coefficient of the dielectric constant of ceramics. Specifically, the temperature coefficient of the dielectric constant is negative for ceramic A, zero for ceramic B, and positive for ceramic C. The temperature coefficient of the dielectric constant represents the slope of the graph for ceramics A, B, and C in FIG. 2. The closer the temperature coefficient is to zero, the smaller the change in capacitance due to temperature changes. Therefore, ceramic B has no temperature dependence of the dielectric constant and is the most preferable material for the filler 29 and other components. However, even if the absolute value of the temperature coefficient τε1 of the dielectric constant of the filler 29 is not zero (0), by setting it to be equal to or less than the absolute value of the temperature coefficient τε2 of the dielectric constant of the inner tube 21, the effect of changes in capacitance between the electrode 4 and the housing 22 can be reduced. Specifically, as mentioned above, the absolute value of the temperature coefficient τε1 of the inner tube 21 should be 100 ppm / K or less.

[0039] Using ceramics as the filler member 29, which has a small temperature coefficient of dielectric constant τε1, makes it less susceptible to deterioration or deformation even when subjected to large temperature changes due to the presence of cryogenic liquid, allowing for accurate measurement of the porosity over a long period of time. Examples of such ceramics include ceramics primarily composed of aluminum oxide. Alternatively, ceramics primarily composed of oxides containing at least rare earth elements (Ln), Al, M (where M is at least one of Ca and Sr), and Ti as metal elements may also be used. The rare earth elements (Ln) include at least one of scandium, yttrium, and lanthanoid elements. Here, the term "major component" refers to a component that accounts for 60% by mass or more, particularly 95% by mass or more, of the total 100% by mass of the components constituting the ceramic. An example of a ceramic that can be used as the filler member 29 is n·LnAlO, which has a molar composition formula of n·LnAlO. ３ +m・Sr ｘ Ca １－ｘ TiO ３(wherein 0≦x≦1, n+m=1, n is 0.3 to 0.7, and m is 0.3 to 0.7) is an example of a ceramic that has a temperature coefficient τε1 of the relative dielectric constant of this ceramic, which is about −100 to 40 ppm / K. An example of a ceramic that is suitable for the filler member 29 is n LaAlO, which has a composition formula based on a molar ratio: ３ +m CaTiO ３ (where n+m=1, n is 0.5 to 0.6, and m is 0.4 to 0.6) is an example of a ceramic. The temperature coefficient τε1 of the relative dielectric constant of this ceramic is about 4.3 ppm / K. On the other hand, ceramics containing high-purity alumina with a purity of 99.5 mass % or more that can be used as the inner tube 21 have a temperature coefficient τε2 of the relative dielectric constant of about 90 ppm / K. Other examples of ceramics that can be used as the inner tube 21 and have a |τε2| of 1000 ppm / K or less include the following ceramics: An oxide containing Al and Ti as metal elements, and having a composition formula based on a mass ratio of yAl ２ O ３ (1-y)TiO ２ (0.01<y<0.9). Ceramics having this composition formula have τε2 = -270 ppm / K when y = 0.79, and τε2 = -726 ppm / K when y = 0.48. The temperature coefficient of the relative dielectric constant of polymer organic compounds that can be used for the inner tube 21 is described in detail in Ishii Itaru et al., "Cryogenic Electrical Insulating Materials," Journal of the Society of Cryogenic Engineering, Vol. 10, No. 5 (1975), pp. 161-168.

[0040] The filling member 29 can be manufactured as follows. ２ O ３ Powders of rare earth oxides such as aluminum oxide, calcium carbonate and / or strontium carbonate, and titanium oxide are used. These powders are mixed together in a composition formula based on a molar ratio of n.LnAlO ３ +m・Sr ｘ Ca １－ｘ TiO ３(wherein 0≦x≦1, n+m=1, n is 0.3 to 0.7, and m is 0.3 to 0.7) are weighed out to obtain a mixed raw material, and then pure water is added to obtain a mixed raw material. Then, the average particle size (D ５０ The mixture is pulverized in a ball mill until the average particle size (D) is 2.0 μm or less to obtain a primary slurry. After drying the primary slurry, it is calcined at 1150 to 1250°C for 1 to 10 hours to obtain a calcined product. Pure water is added to the calcined product, and the average particle size (D ５０ The resulting mixture is wet-mixed and pulverized in a ball mill until the particle size is 2.0 μm or less to prepare a secondary slurry. 3 to 10 wt % of a water-soluble binder is added to and mixed with the secondary slurry to prepare a tertiary slurry. The tertiary slurry is granulated by a spray-drying method or the like to prepare granules. The resulting granules are molded by a known molding method, such as a mold press method, to prepare a molded body. The resulting molded body is sintered by holding it at 1500°C to 1600°C for 5 to 10 hours to prepare a sintered body, which is a filler member 29. The resulting sintered body may be processed into the shape of the filler member 29 as needed. Here, the filler member 29 may be molded and sintered to obtain the shape shown in FIG. 1, or multiple components containing the ceramics may be combined and molded and sintered to obtain the shape shown in FIG. 1.

[0041] Next, another embodiment of the present disclosure will be described with reference to Figures 3 to 5. Note that the same members as those shown in Figure 1 are denoted by the same reference numerals and their description will be omitted. Figure 3 is a perspective view showing the air bubble rate sensor of this embodiment, Figure 4 is an exploded perspective view thereof, and Figure 5 is a cross-sectional view thereof.

[0042] As shown in FIG. 3 , the air bubble rate sensor 11 includes a housing 221 including a frame portion 221 a and a lid portion 221 b, an inner pipe 211 housed inside the housing 221, and a metal pipe 25 inserted through the housing 221 and connected to the inner pipe 211. The housing 221 is provided with a vacuum exhaust valve 15. The frame portion 221 a and the lid portion 221 b that constitute the housing 221 are hermetically joined by brazing or welding, and the insertion portion of the vacuum exhaust valve 15 is also hermetically joined. Evacuation from the vacuum exhaust valve 15 creates a vacuum space between the housing 221 and the inner pipe 211, forming a heat insulating layer that covers the outer periphery of the inner pipe 211. As shown in FIGS. 4 and 5 , the metal pipe 25 has a concave flange portion 251 at its tip, which is fitted into the opening of the through-hole 31 of the inner pipe 211 (see FIG. 5 ). The metal tube 25 is hermetically joined to the housing 221 by brazing or welding, and the flange portion 251 is similarly hermetically joined to the inner tube 211. A first insertion hole 23 for attaching the first hermetic terminal 81 is formed on the outer circumferential surface of the housing 221.

[0043] The inner tube 211 is a tubular body having a through hole 31, and is configured as a roughly rectangular parallelepiped at its center, with the edges of this roughly rectangular parallelepiped being chamfered. Recesses 28 are formed on each of the four faces, and an electrode 4 is attached to each recess 28. The electrodes 4 are a pair of electrodes 4, 4 that face each other across the axis of the inner tube 211, and are provided in mutually orthogonal directions of the inner tube 211, for example, the vertical and horizontal directions of the inner tube 211, as shown in FIG.

[0044] A filling member 29 is housed in the recess 28 in which the electrode 4 is attached. The filling member 29 preferably fits tightly against the inner wall surface of the recess 28 without any gaps, and even if a gap exists, it is preferably arranged so as to cover the back surface of the electrode 4. The height of the filling member 29 is greater than the depth of the recess 28, so that the filling member 29 extends from the surface of the inner tube 211 to the housing 221. Therefore, even if a gap exists between the inner tube 211 and the housing 221, the filling member 29 will fill at least a portion of this gap. Therefore, the filling member 29 filling the gap reduces changes in impedance, and as a result, the accuracy of measuring the bubble rate is improved.

[0045] Next, an air bubble rate sensor according to yet another embodiment of the present disclosure will be described with reference to Figures 6, 7A, and 7B. As shown in Figure 6, in this embodiment, the filler member 291 is housed in a recess 281 on the back surface of each electrode 4 and extends from between the outer circumferential surface 212a of the inner tube 212 and the inner wall surface 222a of the casing 222 facing the outer circumferential surface 212a to between both end surfaces 212b of the inner tube 212 and the inner wall surfaces 222b of the casing 222 facing the end surfaces 212b. This eliminates the need for joining, such as brazing or welding, between the end surfaces 212b of the inner tube 212 and the inner wall surfaces 222b of the casing 222 facing the end surfaces 212b. This improves the stress relaxation effect of the inner tube 212, and allows the inner tube 212 to be made larger. Such a filling member 291 can be produced by integrally molding the above-mentioned ceramics, but it may also be possible to produce ceramics of a predetermined shape for each required portion and place them in each space. Since the rest is the same as in the above-mentioned embodiment, the same reference numerals are used and detailed description will be omitted.

[0046] FIG. 7A shows a partial modification of the air bubble rate sensor 1 shown in FIG. 1 . As shown in FIG. 7A , an insulating tube 30 having a conductor layer on its outer circumferential surface is attached to the conductive pin 91 between the electrode 4 and the first insertion hole 23. The conductor layer surrounds the vicinity of the conductive pin 91, thereby reducing the effect of changes in the relative dielectric constant between the electrode 4 and the housing 22 and reducing changes in impedance, thereby improving the accuracy of measuring the air bubble rate. Specifically, as shown in FIG. 7B , the insulating tube 30 is inserted into the through hole 290 of the filling member 29, with a portion extending into the housing 22. The insulating tube 30 may be made of, for example, the same material as the filling member 29. The insulating tube 30 may also have a conductor layer on its inner circumferential surface. This reduces changes in impedance, thereby improving the accuracy of measuring the air bubble rate. The conductor layer may be, for example, a metal layer primarily composed of Ag or Cu, or a metal layer containing a brazing material such as an Ag-Cu-Ti system. 1, the same components are denoted by the same reference numerals and detailed description thereof will be omitted. Note that the insulating tube 30 can also be applied to the embodiments shown in FIGS. 3 to 5 and 6.

[0047] Next, an air bubble rate sensor according to another embodiment of the present disclosure will be described with reference to FIG. 8 . FIG. 8 is a schematic cross-sectional view showing an air bubble rate sensor 111 according to this embodiment, i.e., a schematic cross-sectional view perpendicular to the axial direction (direction perpendicular to the plane of FIG. 8 ) of an inner pipe 213 through which a cryogenic liquid flows. The air bubble rate sensor 111 of this embodiment has a pair of first and second electrodes 4A and 4B attached to the outer surface of an inner pipe 213 having a through-hole 311 for flowing liquid hydrogen, and an intermediate electrode 4C disposed within the through-hole 311 of the inner pipe 213. The intermediate electrode 4C is disposed between the first and second electrodes 4A and 4B and faces the first and second electrodes 4A and 4B along the axial direction of the through-hole 311 of the inner pipe 213. The cross section of the through-hole 311 perpendicular to the axial direction is divided into two by a portion 7 having the intermediate electrode 4C.

[0048] In this way, since the intermediate electrode 4C is disposed within the through-hole 311 of the inner tube 213, even if the inner diameter of the through-hole 311 is increased, the capacitance is measured between the first electrode 4A and the intermediate electrode 4C and between the second electrode 4B and the intermediate electrode 4C, thereby reducing the distance between the electrodes and increasing the capacitance. Furthermore, by opposing the intermediate electrode 4C to the first electrode 4A and the second electrode 4B, the area of ​​the intermediate electrode 4C can be set large, which increases the capacitance accumulated between the electrodes and improves the accuracy of measuring the bubble fraction of liquid hydrogen. The first electrode 4A, the second electrode 4B, and the intermediate electrode 4C are all electrically connected to a capacitance measuring device 8, and the measured capacitance value is displayed on the capacitance measuring device 8.

[0049] In this embodiment as well, recesses 28A, 28B on the back surfaces of the first electrode 4A and the second electrode 4B each contain a filler 29. A filler 292 is also provided so that the conductive pin 92 connected to the intermediate electrode 4C can be inserted therethrough. Both fillers 29, 292 extend toward the first insertion hole 23 provided in the housing 223, and therefore toward the first airtight terminal 81. Since the rest of the components are the same as those in the previously described embodiment, the same reference numerals are used and detailed description thereof will be omitted.

[0050] In the present disclosure, the cross section of the inner tubes 21, 211, 212, and 213 is not limited to a substantially rectangular or circular shape, and may be another polygonal shape. The inner tubes 21, 211, 212, and 213 may be formed by arranging multiple ceramic members in the circumferential direction of the through holes 31 and 311 and joining these ceramic members together. In this case, the number of ceramic members constituting the inner tubes 21, 211, 212, and 213 is preferably an even number, such as 2, 4, 6, or 8. This is because at least one pair of opposing electrodes 4, 4 is required to measure the capacitance, and each electrode 4 is attached to a ceramic member.

[0051] Furthermore, since the capacitance is measured between opposing electrodes 4, 4, it is not necessary for electrodes 4 to be provided on all of the even number of ceramic members; for example, electrodes 4 may be provided on at least one pair of ceramic members that face each other.

[0052] As described above, the bubble rate sensors 1, 11, 111 of the present disclosure can accurately measure the bubble rate by reducing the effect of changes in capacitance between the electrode 4 and the housing 22, 221, 222, 223, even when cryogenic liquid flows through the through holes 31, 311 of the inner tubes 21, 211, 212, 213, causing large temperature changes on the outer periphery of the inner tubes 21, 211, 212, 213.

[0053] Next, a flow meter according to an embodiment of the present disclosure will be described. This flow meter measures the flow rate of liquid hydrogen flowing through inner pipes 21, 211, 212, 213, and includes the above-described bubble rate sensors 1, 11, 111 and a flow meter that measures the flow rate of the cryogenic liquid flowing through through-holes 31, 311. The bubble rate sensors 1, 11, 111 and the flow meter are attached to a liquid hydrogen transfer pipe (not shown).

[0054] The liquid hydrogen flowing through the through-holes 31, 311 is a two-phase flow of gas-liquid mixture, so the capacitance of the liquid hydrogen is measured by the bubble rate sensors 1, 11, 111, and the density d (kg / m ３ ) can be calculated. Then, the flow velocity of the liquid hydrogen (m / sec) calculated by the flow meter is calculated as v, and the cross-sectional area of ​​the through holes 31, 311 (m ２) is a, the flow rate F (kg / sec) can be calculated by the following formula: F = d × v × a In order to perform the above calculation, the flow meter further includes a calculation unit to which the bubble rate sensors 1, 11, 111 and the flow rate meter are connected. This makes it easy to measure the flow rate of liquid hydrogen, facilitating management when transferring large amounts of liquid hydrogen industrially.

[0055] The above explanation has been made regarding the liquid hydrogen bubble rate sensors 1, 11, 111 and the flowmeter using the same, but the invention can be similarly applied to other cryogenic liquids, such as liquid nitrogen (-196°C), liquid helium (-269°C), liquefied natural gas (-162°C), liquid argon (-186°C), etc. (The liquefaction temperature is indicated in parentheses.) Therefore, the cryogenic liquid in this disclosure refers to a liquid that liquefies at an extremely low temperature of -162°C or lower.

[0056] Although the embodiments of the present disclosure have been described above, the air bubble rate sensor of the present disclosure is not limited to the above-described embodiments, and various modifications and improvements are possible within the scope of the present disclosure.

[0057] DESCRIPTION OF SYMBOLS 1, 11, 111 Air bubble rate sensor 4 Electrode 4A First electrode 4B Second electrode 4C Intermediate electrode 7 Portion having intermediate electrode 4C 8 Capacitance measuring device 100 Vacuum space (heat insulating layer) 15 Vacuum exhaust valve 21, 211, 212, 213 Inner pipe 210 Inner peripheral wall 212a Outer peripheral surface 212b End surface 22, 221, 222, 223 Housing 22a, 221a Frame body portion 22b, 221b Lid portion 222a, 222b Inner wall surface 23 First insertion hole 24 Connecting hole 25 Metal pipe 251 Flange portion 26 Outer pipe 27 Second insertion hole 28, 28A, 28B Recess 29, 291, 292 Filler member 290 Through hole 30 Insulating tube 31, 311 Through hole 50a First ceramic substrate 50b Second ceramic substrate 51a, 51b Annular portion 52a First annular body 52b Second annular body 81 First airtight terminal 82 Second airtight terminal 91, 92 Conduction pin

Claims

1. an inner tube having a through-hole for flowing a cryogenic liquid; at least a pair of electrodes attached to the outer surface of the inner tube; conductive pins connected to each of the pair of electrodes; a housing surrounding the inner tube and having a first insertion hole into which the conductive pin is inserted; a filling member disposed between at least the back surfaces of the pair of electrodes opposite to the surfaces facing each other and the inner wall surface of the housing; and a bubble rate sensor in which an absolute value of a temperature coefficient τε1 of a relative permittivity of the filling member at -196°C to 20°C is equal to or less than an absolute value of a temperature coefficient τε2 of a relative permittivity of the inner tube at -196°C to 20°C.

2. The bubble rate sensor according to claim 1, wherein an absolute value of the temperature coefficient τε1 is 100 ppm / K or less.

3. The bubble rate sensor according to claim 2, wherein an absolute value of the temperature coefficient τε2 is 1000 ppm / K or less.

4. The bubble rate sensor according to claim 1, wherein a positive / negative sign of the temperature coefficient τε1 is different from a positive / negative sign of the temperature coefficient τε2.

5. The bubble rate sensor according to claim 1, wherein the filling member contains ceramics.

6. The bubble rate sensor according to claim 5, wherein the ceramics is mainly composed of aluminum oxide or an oxide containing at least a rare earth element (Ln), Al, M (M is at least one of Ca and Sr), and Ti as metal elements.

7. The bubble rate sensor according to claim 1, wherein the filling member extends into the first insertion hole of the housing.

8. The bubble rate sensor according to claim 1, wherein the filling member extends between an end face of the inner tube and the inner wall surface of the housing facing the end face.

9. The bubble rate sensor according to claim 1, wherein an insulating tube having a conductor layer on an outer peripheral surface is attached to the conductive pin and is interposed between the electrode and the first insertion hole.

10. The bubble rate sensor according to claim 9, wherein the insulating tube also has the conductor layer on an inner peripheral surface.

11. The bubble rate sensor according to claim 1, wherein an outer peripheral side of the housing is covered with a heat insulating layer.

12. an outer tube having a second insertion hole; and a second airtight terminal provided in the second insertion hole for fixing the conductive pin individually connected to the electrode in the second insertion hole, wherein the heat insulating layer is a vacuum space located at least between the housing and the outer tube. The bubble rate sensor according to claim 11.

13. The bubble rate sensor according to claim 1, wherein an inner peripheral wall of the inner tube forming the through hole is a fired surface.

14. The bubble rate sensor according to claim 1, wherein an intermediate electrode for measuring capacitance is provided between the pair of opposing electrodes.

15. A flow meter for measuring the flow rate of the cryogenic liquid flowing through the through hole, the flow meter having the bubble rate sensor according to any one of claims 1 to 14 and a flow velocity meter for measuring the flow velocity of the cryogenic liquid flowing through the through hole.

16. A cryogenic liquid transfer pipe provided with the flow meter according to claim 15.