VOID FRACTION SENSOR, FLOWMETER USING THE SAME, AND CRYOGENIC LIQUID TRANSFER PIPE

A void fraction sensor for measuring a void fraction of a cryogenic liquid includes a pipe having a flow channel in which a cryogenic liquid flows, a first electrode and a second electrode disposed outside the flow channel, and at least one intermediate electrode disposed in the flow channel and between the first electrode and the second electrode, the at least one intermediate electrode measuring capacitance with the first electrode and/or the second electrode.

TECHNICAL FIELD

The present disclosure relates to a void fraction sensor for measuring a void fraction of a cryogenic liquid such as liquid hydrogen, a flowmeter using the same, and a cryogenic liquid transfer pipe.

BACKGROUND OF INVENTION

With the recent trend of reducing greenhouse gas emissions, the use of hydrogen as a potent energy storage medium has been attracting attention. In particular, liquid hydrogen has a high volumetric efficiency and can be stored for a long period of time, and various techniques for utilizing liquid hydrogen have been developed. However, a method for accurately measuring the flow rate which is required in handling a large volume of liquid hydrogen for industrial use has not been established. A major reason for this is that liquid hydrogen is a fluid which is very easily vaporized and has a large fluctuation of gas-to-liquid ratio that fluctuates largely.

That is, liquid hydrogen is a liquid having an extremely low temperature (boiling point −253° C.) and having very high thermal conductivity and low latent heat, which causes immediate generation of voids. Therefore, in a transfer pipe, liquid hydrogen is in a so-called two-phase flow in which gas and liquid are mixed.

Because of the large fluctuation of the void content percentage, the flow rate of the liquid hydrogen cannot be accurately determined by only measuring the flow velocity in the pipe, as in ordinary liquids, when measuring the flow rate of the liquid hydrogen flowing in the pipe.

In view of the above, a void fraction sensor that measures a void fraction indicating a gas phase volume percentage of the gas-liquid two phase flow is under development. As such a void fraction sensor, Non-Patent Document 1 has proposed a capacitance type void fraction sensor that measures capacitance using a pair of electrodes. Non-Patent Document 1 has reported measuring the void fraction of liquid nitrogen using this void fraction sensor. A pipe used in this capacitance type void fraction sensor has a relatively small inner diameter of 10.2 mm.

CITATION LIST

SUMMARY

A void fraction sensor according to the present disclosure measures a void fraction of a cryogenic liquid, and includes a pipe having a flow channel in which a cryogenic liquid flows, a first electrode and a second electrode disposed outside the flow channel, and at least one intermediate electrode disposed in the flow channel and between the first electrode and the second electrode, the at least one intermediate electrode configured to measure capacitance with the first electrode and/or the second electrode.

Another void fraction sensor according to the present disclosure includes a pipe having a flow channel in which a cryogenic liquid flows, and at least one pair of electrodes configured to measure capacitance, in which the at least one pair of electrodes includes an electrode disposed outside the flow channel and an electrode disposed in the flow channel.

Still another void fraction sensor according to the present disclosure includes a pipe having a flow channel in which a cryogenic liquid flows, and at least one pair of electrodes that measures capacitance, in which the at least one pair of electrodes is disposed in the flow channel.

A flowmeter according to the present disclosure measures a flow rate of a cryogenic liquid flowing in a flow channel of a pipe, and includes the void fraction sensor described above, and a flow velocity meter configured to measure a flow velocity of the cryogenic liquid flowing in the flow channel.

The present disclosure also provides a cryogenic liquid transfer pipe provided with the flowmeter described above.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a void fraction sensor according to an embodiment of the present disclosure will be described. As an example, a void fraction sensor that measures a void fraction when liquid hydrogen is used as a cryogenic liquid will be described.FIG.1illustrates a void fraction sensor1according to an embodiment of the present disclosure. As illustrated inFIG.1, the void fraction sensor1according to the present embodiment includes a first electrode3A and a second electrode3B disposed outside a flow channel5of a pipe2in which liquid hydrogen flows through the flow channel5, and an intermediate electrode4disposed in the flow channel5of the pipe2. The intermediate electrode4is disposed between the first electrode3A and the second electrode3B so as to face the first electrode3A and the second electrode3B along the axial direction of the flow channel5of the pipe2(a direction perpendicular to the surface of the paper ofFIG.1). The flow channel5has a circular cross section perpendicular to the axial direction across the intermediate electrode4.

The first electrode3A and the second electrode3B are disposed outside the flow channel5. The first electrode3A and the second electrode3B being disposed outside the flow channel5means that the first electrode3A and the second electrode3B may be located on the outer periphery of the pipe2, as illustrated inFIG.1, or may be located inside the pipe2surrounding the flow channel5. In particular, the first electrode3A and the second electrode3B are disposed on the outer periphery of the pipe2, as illustrated inFIG.1. The first electrode3A and the second electrode3B disposed on the outer periphery of the pipe2facilitates fabrication of the void fraction sensor1.

When there are a plurality of flow channels in one pipe2, the plurality of flow channels are regarded as one flow channel, and the first electrode3A and the second electrode3B are disposed outside this group of flow channels and sandwiching this group of flow channels. When the plurality of flow channels are present in one pipe2as described above, the intermediate electrode4is located between the first electrode3A and the second electrode3B and between the adjacent flow channels.

With the intermediate electrode4disposed in the flow channel5of the pipe2, the capacitance can be measured between the first electrode3A and the intermediate electrode4and between the second electrode3B and the intermediate electrode4even when the inner diameter of the flow channel5increases, thus reducing the distance between the electrodes and increasing the capacitance.

By disposing the first electrode3A and the second electrode3B to face each other, the area of the intermediate electrode4can increase, leading to an increase in the capacitance accumulated between the electrodes and improving the measurement accuracy of the void fraction of liquid hydrogen.

The first electrode3A and the second electrode3B and the intermediate electrode4are all electrically connected to the capacitance measuring device8, and the measured capacitance values are displayed on the capacitance measuring device8.

The pipe2has a tubular body in which the flow channel5through which liquid hydrogen flows is provided and is made of an insulating ceramic. Examples of such a ceramic include ceramics containing zirconia, alumina, sapphire, aluminum nitride, silicon nitride, sialon, cordierite, mullite, yttria, silicon carbide, cermet, or β-eucryptite as a main constituent.

The insulating ceramic refers to a ceramic having a volume resistance value of at least 1010Ω·m at 20° C.

The main constituent of a ceramic refers to a constituent accounting for at least 60 mass % out of 100 mass % of all constituents constituting the ceramic. In particular, the main constituent may preferably be a constituent that accounts for at least 95 mass % out of 100 mass % of the constituents constituting the ceramic. The constituents constituting the ceramic may be obtained by using an X-ray diffractometer (XRD). For the content of each constituent, after the constituent is identified, the content of elements constituting the constituent is determined using a fluorescence X-ray analyzer (XRF) or an ICP emission spectrophotometer, and may be converted into the identified constituent.

The relative density of a ceramic is, for example, from 92% to 99.9%. The relative density, relative to the theoretical density of a ceramic, is expressed as a percentage (ratio) of the apparent density of a ceramic which is determined in accordance with JIS R 1634-1998.

The ceramic includes closed pores, and a value obtained by subtracting an average equivalent circle diameter of the closed pores from an average distance between the centers of gravity of adjacent closed pores may be from 8 μm to 18 μm (this value will hereinafter be referred to as the distance between the closed pores). The closed pores are independent of each other.

When the interval between the closed pores is 8 μm or greater, the closed pores are present in a relatively dispersed manner which increases mechanical strength. When the interval between the closed pores is 18 μm or less, even if a microcrack originating from the contour of a closed pore occurs due to repeated cold thermal shocks, the likelihood of the extension of the microcrack being blocked is high due to the surrounding closed pores. This means that the pipe2composed of this ceramic having an interval between closed pores of from 8 μm to 18 μm can be used over a long period of time.

The skewness of the equivalent circle diameter of the closed pores may be larger than the skewness of the distance between the centers of gravity of the closed pores. The skewness is an index (a statistic) indicating how much a distribution is distorted from the normal distribution. That is, the skewness indicates the bilateral symmetry of the distribution. When the skewness is greater than 0, the tail of the distribution extends to the right. When the skewness is 0, the distribution is bilaterally symmetrical. When the skewness is less than 0, the tail of the distribution extends to the left.

Overlapping histograms of the equivalent circle diameter and the distance between the centers of gravity of the closed pores indicates that the mode value of the equivalent circle diameter is located on the left side (zero side) of the mode value of the distance between the centers of gravity of the closed pores, when the skewness of the equivalent circle diameter is larger than the skewness of the distance between the centers of gravity. This means that many closed pores with small equivalent circle diameters are present and such closed pores are present sparsely, such that the inner pipe2having both mechanical strength and thermal shock resistance can be obtained.

For example, the skewness of the equivalent circle diameter of the closed pores is 1 or greater, and the skewness of the distance between the centers of gravity of the closed pores is 0.6 or less. The difference between the skewness of the equivalent circle diameter of the closed pores and the skewness of the distance between the centers of gravity of the closed pores is 0.4 or greater.

To determine the distance between the centers of gravity and the equivalent circle diameter of the closed pores, the ceramic composing the pipe is polished on a copper disc using diamond abrasive grains having an average grain diameter D50of 3 μm from one end surface of the pipe along the axial direction. Subsequently, polishing is then performed on a tin disc using diamond abrasive grains having an average grain diameter D50of 0.5 μm to obtain a polished surface having an arithmetic mean roughness Ra of 0.2 μm or less in the roughness curve. The arithmetic mean roughness Ra of the polished surface is the same as that in the method described above.

The polished surface is observed at 200× magnification and, with an average area selected, an area of, for example, 7.2×104μm2(horizontal length 310 μm by vertical length 233 μm) is captured with a CCD camera to obtain an observation image.

The distance between the centers of gravity of the closed pores can be determined for this observation image, for example, with the image analysis software “A zou-kun (ver 2.52)” (trade name of Asahi Kasei Engineering Corporation), using the method called a distance between centers of gravity method for dispersion measurement. Hereinafter, the term image analysis software “A zou-kun” refers to the image analysis software manufactured by Asahi Kasei Engineering Corporation throughout the description.

For example, the setting conditions for this method can be as follows: the threshold is 165 which is used as a measure of image brightness/darkness, the brightness level is set to dark, the small figure removal area is 1 μm2, and no noise reduction filter is set. The threshold can be adjusted according to the brightness of the observation image. The brightness level is set to dark, the binarization method is set to manual, the small figure removal area is set to 1 μm2, and the noise removal filter is set. Then, the threshold can be adjusted so that a marker appearing in the observation image matches the shape of the closed pore. For the equivalent circle diameter of the closed pores, a particle analysis method is used to determine the equivalent circle diameter of the open pores by using the observation image as a target. The setting conditions for this method may be the same as the setting conditions for calculating 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 calculated using the Skew function provided in Excel (trade name of Microsoft Corporation).

An example of a method for manufacturing a pipe made of such a ceramic is described. A pipe made of a ceramic containing aluminum oxide as the main constituent is described.

The main constituent of aluminum oxide powder (purity of at least 99.9 mass %) is put into a pulverizing mill with powders of magnesium hydroxide, silicon oxide, and calcium carbonate, and a solvent (for example, ion-exchanged water). The mixture is pulverized until an average grain diameter D50of the powders is 1.5 μm or less. Subsequently, an organic binder and a dispersing agent for dispersing the aluminum oxide powder are added and mixed to obtain a slurry.

Of the total of 100 mass % of the powders described above, the content of magnesium hydroxide powder is from 0.3 to 0.42 mass %, the content of silicon oxide powder is from 0.5 to 0.8 mass %, the content of calcium carbonate powder is from 0.06 to 0.1 mass %, and the remainder includes aluminum oxide powder and incidental impurities. The organic binder is, for example, an acrylic emulsion, polyvinyl alcohol, polyethylene glycol, polyethylene oxide, or the like.

Subsequently, the slurry is spray-granulated to obtain granules which are then pressurized at a molding pressure from 78 MPa to 118 MPa using a uniaxial press molding device or a cold isostatic press molding device to obtain a columnar powder compact.

The powder compact is cut, if necessary, to form a recess which becomes a recessed portion after firing.

A ceramic pipe composed of a ceramic is obtained by firing the powder compact at a firing temperature of from 1580° C. to 1780° C. and a retention time of 2 hours to 4 hours.

To obtain a ceramic having an interval between the closed pores of from 8 μm to 18 μm, the firing temperature is set to 1600° C. to 1760° C. and the retention time is set to 2 hours to 4 hours, for example.

The surface of the ceramic facing the flow channel may be ground to form a ground surface. A surface of the recessed portion on which the electrode is provided may be ground to form a bottom surface.

The inner diameter of the flow channel5is preferably at least 50 mm. In the void fraction sensor with the pair of electrodes provided on the outer circumferential surface of the pipe, the increase in the diameter of the pipe leads to an increase in the distance between the electrodes, causing a chance of a decrease in capacitance. However, as in the present embodiment, with the intermediate electrode4, the distance between the electrodes decreases, causing an increase in capacitance and sensitivity. By providing the intermediate electrode4, the inner diameter of the flow channel5can be increased, thus increasing the flow rate of the liquid hydrogen.

The inner diameter of the flow channel5is the maximum diameter of the flow channel5in a direction perpendicular to the intermediate electrode4. That is, the inner diameter of the flow channel5includes the thickness of the intermediate electrode4and the thickness of the support7supporting the intermediate electrode4.

As illustrated inFIG.1, the pipe2includes recessed portions6A and6B formed at portions facing each other across the axial center of the flow channel5, and the first electrode3A and the second electrode3B are disposed on the bottom surfaces of the recessed portions6A and6B, respectively. The recessed portions6A and6B and the first electrode3A and the second electrode3B may be provided over the entire length of the pipe2in the axial direction, or may be provided only in part thereof. The bottom surfaces of the recessed portions6A and6B are flat surfaces inFIG.1, but may have an arc-shaped cross section corresponding to the flow channel5.

The first electrode3A and the second electrode3B and the intermediate electrode4can be made of, for example, copper foil, aluminum foil, or the like. The first electrode3A and the second electrode3B can be provided on the bottom surfaces of the recessed portions6A and6B, respectively, by, for example, vacuum evaporation, metallization, or using an active metal method. Alternatively, the metal plates serving as the first electrode3A and the second electrode3B may be bonded to the bottom surfaces of the recessed portions6A and6B, respectively.

The intermediate electrode4is preferably disposed so as to connect two points on the inner peripheral surface, the two points facing each other in the radial direction of the flow channel5. This allows division of the flow channel5for the liquid hydrogen, reducing the distance between the electrodes and increasing the capacitance. As a result, the sensitivity of the void fraction sensor1increases, thus improving the measurement accuracy of the void fraction of liquid hydrogen.

The pipe2includes a plate-like support7that supports the intermediate electrode4in the flow channel5, and the intermediate electrode4is preferably incorporated in the support7. Since the intermediate electrode4is supported by the support7, the intermediate electrode4can be protected. In particular, the intermediate electrode4is less susceptible to damage, as it is not exposed in the flow channel5, and can be used for a long period of time. The intermediate electrode4is arranged in parallel with the first electrode3A and/or the second electrode3B, for example.

An insulating ceramic similar to that of the pipe2can be used as the support7. For this reason, the support7and the pipe2can be integrally formed by, for example, extrusion molding or cold isostatic pressing (CIP) molding. To incorporate the intermediate electrode4in the support7, a film of the intermediate electrode4, for example, can be inserted into a portion where the support7is formed during molding.

Instead of integral molding, the support7incorporating the intermediate electrode4may be formed in advance and then inserted into the flow channel5in a direction perpendicular to the axial direction.

Alternatively, the intermediate electrode4may be mounted (layered) on one surface or both surfaces of the support7so as to face one or both of the first electrode3A and the second electrode3B without incorporating the intermediate electrode4. In that case, the intermediate electrode4can also be fabricated integrally, or may be bonded after integral molding.

The thickness of each of the first electrode3A, the second electrode,3B and the intermediate electrode4is at least 10 μm, preferably at least 20 μm, and 2 mm or less, and more preferably 1 mm or less.

The distance between the first electrode3A and the intermediate electrode4is preferably electrically equal to the distance between the second electrode3B and the intermediate electrode4. By providing the electrically equal inter-electrode distances, the potential difference generated in accordance with the average thickness t22of a measurement space A is equal to the potential difference generated in accordance with the thickness t2of a measurement space B, which will be described later. Accordingly, the electrical evaluations of the void fraction for divided flow channels5aand5bcan be treated equally, thus simplifying control. The meaning of “electrically equal inter-electrode distances” will be described later.

As illustrated inFIG.1, the first electrode3A and the second electrode3B are electrically connected to the capacitance measuring device8to which the intermediate electrode4is also electrically connected, thus constituting the void fraction sensor1.

Another embodiment of the present disclosure will be described with reference toFIG.2. The same constituent members as those inFIG.1are denoted by the same reference signs, and the detailed description thereof will be omitted.

As illustrated inFIG.2, the void fraction sensor11according to the present embodiment includes a plurality of intermediate electrodes41,42, and43, and the distances between each of the plurality of intermediate electrodes41,42, and43are electrically equal to each other. By providing the plurality of intermediate electrodes41,42, and43, the distance between each of the plurality of intermediate electrodes41,42, and43can be decreased. This increases the capacitance accumulated between each of the plurality of intermediate electrodes41,42, and43and improves the measurement accuracy of the void fraction of the liquid hydrogen.

At this time, as long as the distances between each of the plurality of intermediate electrodes41,42, and43are electrically equal to each other, the distances can be appropriately changed to vary the sensitivity.

The intermediate electrodes41,42, and43are incorporated in and supported by supports71,72, and73, respectively, as in the above-described embodiment. For example, the intermediate electrodes41,42, and43are disposed parallel to the first electrode3A and/or the second electrode3A3B.

The first electrode3A and the second electrode3B and the intermediate electrodes41,42, and43are all electrically connected to the capacitance measuring device8, and the capacitance measuring device8displays measured capacitance values.

When the gaseous hydrogen becomes a gas-liquid two-phase flow in which the gaseous hydrogen gathers in the vertically upper portion of the flow channel5of the pipe2depending on the conditions of use, the measurement accuracy of the whole measurement system can be improved by changing the weight of evaluation between the sensitivity in the vertically upper portion and the sensitivity in the vertically lower portion which mainly includes liquid.

To improve the measurement accuracy, the distance between the first electrode3A and the intermediate electrode41closest to the first electrode3A is preferably electrically equal to the distance between the second electrode3B and the intermediate electrode43closest to the second electrode3B.

Similarly, the distances between each of the plurality of intermediate electrodes41,42and43are preferably electrically equal to the distance between the first electrode3A and the intermediate electrode41closest to the first electrode3A and/or the distance between the second electrode3B and the intermediate electrode43closest to the second electrode3B.

Here, the meaning of “electrically equal inter-electrode distances” is described by referring to the void fraction sensor11illustrated inFIG.2.FIGS.3A and3Bare schematic views illustrating the “electrically equal inter-electrode distances”.FIG.3Aschematically illustrates a thick insulating layer that constitutes the pipe2between the first electrode3A and the intermediate electrode41.FIG.3Bschematically illustrates a thin insulating layer between the intermediate electrodes41and42.

As illustrated inFIG.3A, assume that a potential difference generated according to a total thickness t11of the average thickness of the pipe2sandwiched between the first electrode3A and the intermediate electrode41and the thickness of the support71is defined as EH, and a potential difference generated according to the average thickness t22of the measurement space A sandwiched between the first electrode3A and the intermediate electrode41is defined as E22. On the other hand, as illustrated inFIG.3B, assume that a potential difference generated due to a total thickness t1of the thicknesses of the supporting portions71and72sandwiched between the intermediate electrodes41and42is E1, and a potential difference generated due to a thickness t2of the measurement space B sandwiched between the first electrode3A and the intermediate electrode41is E2. Then, t11, t22, t1and t2are adjusted to achieve E2=E22. This state is referred to as a state of electrically equal inter-electrode distances.

In the example illustrated inFIG.2, since the total thickness t11of the insulating ceramic, which has a dielectric constant larger than that of the cryogenic liquid, is greater than the thickness t1, the average thickness t22of the measurement space A is smaller than the thickness t2of the measurement space B.

The potential differences E1, E22, E1, and E2can be measured by the capacitance measuring device8.

The average thickness of the pipe2sandwiched between the first electrode3A and the intermediate electrode41can be determined using the mean value theorem of integration. The average thickness t22of the measurement space A sandwiched between the first electrode3A and the intermediate electrode41is a value obtained by subtracting the total thickness t11, which is the sum of the average thickness of the pipe2sandwiched between the first electrode3A and the intermediate electrode41and the thickness of the support71, from the distance between the first electrode3A and the intermediate electrode41.

In addition to the void fraction sensors1and11of the above-described embodiments, the present disclosure may provide, as the void fraction sensor, a pair of electrodes for measuring capacitance composed of the first electrode3A or the second electrode3B disposed on the outer periphery of the pipe2and the intermediate electrode4disposed in the flow channel5. That is, only one electrode3A or3B need be disposed on the outer periphery of the pipe2. Such a pair of electrodes can also provide a short distance between the electrodes, increasing the capacitance accumulated between the electrodes and improving the measurement accuracy of the void fraction. Alternatively, two or more pairs of electrodes may be provided.

Alternatively, another void fraction sensor of the present disclosure may be the void fraction sensor including a pair of electrodes disposed in the flow channel5without using the first electrode3A or the second electrode3B disposed outside the flow channel5. That is, the void fraction sensor may include, for example, the intermediate electrodes41and43, the intermediate electrodes41and42, or the intermediate electrodes42and43among the intermediate electrodes41,42and43illustrated inFIG.2. As illustrated inFIG.2, each of the intermediate electrodes41,42, and43may partially be located inside the inner peripheral surface surrounding the flow channel5.

The flowmeter according to the embodiments of the present disclosure is described. The flowmeter measures the flow rate of the liquid hydrogen flowing in the flow channel5, and includes the void fraction sensor1or11and a flow velocity meter which is not illustrated for measuring the flow velocity of the cryogenic liquid flowing in the flow channel5. The void fraction sensor1or11and the flow velocity meter are attached to a liquid hydrogen transfer pipe which is not illustrated (hereinafter may be referred to as a transfer pipe).

Since the liquid hydrogen flowing in the flow channel5is a gas-liquid mixed two-phase flow, the void fraction sensor1or11measures the capacitance of the liquid hydrogen, from which a density d (kg/m3) of the liquid hydrogen is obtained.

Accordingly, a flow rate F (kg/s) is determined by the following equation, where v is the flow velocity (m/s) of the liquid hydrogen determined by the flow velocity meter, and a is the cross-sectional area (m2) of the flow channel5.

To calculate this equation, the flowmeter further includes a calculator to which the void fraction sensor1or11and the flow velocity meter are connected. This facilitates the measurement of the flow rate of the liquid hydrogen, leading to easier control when transferring a large amount of liquid hydrogen for industrial use.

The void fraction sensors1and11for liquid hydrogen and the flowmeter using the same have been described above, but the present disclosure 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.) and the like (where the values in parentheses indicate the liquefaction temperature). Therefore, the cryogenic liquid in the present disclosure is a liquid that is liquefied at a cryogenic temperature of −162° C. or lower.

Although the preferred embodiments of the present disclosure have been described above, the void fraction sensor of the present disclosure is not limited thereto, and various changes and improvements can be made within the range set forth in the present disclosure.

REFERENCE SIGNS