Test method for start-up flow velocity of stagnant air mass in long distance pressurized water pipeline

Abstract

The application discloses a test method for starting flow velocity of a long-distance water pressure pipeline air mass, which comprises the following steps: acquiring corresponding geometric parameters of air masses with various geometric sizes and actual starting velocities for driving the air masses to move by tests; calculating characteristic lengths of the air masses by using various characteristic length calculation methods of the air masses according to the geometric parameters, and calculating drag coefficients based on the characteristic lengths; calculating theoretical starting flow velocities by using a starting flow velocity model of the air masses according to the drag coefficients of the air masses with various sizes and various forces in water; calculating relative errors between the actual starting velocities corresponding to each size and the theoretical starting flow velocities corresponding to different characteristic length calculation methods, and drawing a relative error curve corresponding to the same characteristic length calculation method; constructing a characteristic length calculation model of the air mass according to a change trend of the relative error curve, updating the drag coefficients by using the characteristic length calculation model, and then updating the starting flow velocity model.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of water network engineering safety water delivery, and particularly relates to a test method for starting flow velocity of a long-distance pressurized water pipeline. BACKGROUND

[0002] Long-distance pressurized water engineering, as a key infrastructure for guaranteeing regional water resource balanced allocation, plays a core role in the fields of urban water supply and cross-basin water transfer. However, due to the influence of pipeline terrain undulation, flow velocity change and gas dissolution-outgassing mechanism, the phenomenon of trapped air mass is prone to occur at local high points of the pipeline, which not only affects the water delivery efficiency, but also brings safety hazards to the engineering operation.

[0003] In the long-distance pressurized water pipeline system, to realize the release of trapped air mass, two key problems need to be solved: one is how to drive the trapped air mass to move along the pipeline by water flow, and the other is to install an exhaust valve at a suitable position downstream in coordination with the movement of the air mass. Therefore, it is necessary to determine the starting flow velocity of the trapped air mass, so as to accurately control the water flow and ensure that the water flow can drive the air mass to move downstream along the pipeline at a proper flow velocity.

[0004] At present, in the research on the starting flow velocity of trapped air mass, some scholars conduct research on the starting flow velocity by constructing a calculation model. In the construction process of the calculation model, the accuracy of the calculation is mainly evaluated by numerical simulation. Although numerical simulation has the advantages of low cost and strong repeatability, in the simulation process, a large number of simplifications and assumptions need to be made for complex actual working conditions, which makes the simulation results deviate from the actual situation to a certain extent, and it is difficult to evaluate the precision of the constructed calculation model. When the calculation model is applied to actual engineering to calculate the starting velocity, there will also be a certain deviation between the calculated value and the actual value. Therefore, a method for accurately evaluating the constructed starting velocity model is urgently needed. SUMMARY

[0005] In view of the above problems in the prior art, the test method for starting flow velocity of trapped air mass in long-distance pressurized water pipeline provided by the present application solves the problem that it is difficult to accurately evaluate the precision of the constructed calculation model in the existing simulation process due to a large number of simplifications and assumptions for complex actual working conditions.

[0006] In order to achieve the above-mentioned application purposes, the technical scheme adopted by the present application is as follows:

[0007] A test method for starting flow velocity of trapped air mass in long-distance pressurized water pipeline is provided, which comprises the following steps:

[0008] S1, obtaining the geometric parameters corresponding to trapped air masses of various geometric sizes and the actual starting velocity driving the movement of the trapped air masses by test;

[0009] S2, according to the geometric parameters of the trapped gas mass of each size, the characteristic length of the trapped gas mass is calculated by using a plurality of characteristic length calculation methods of the trapped gas mass, and the drag coefficient is calculated based on the characteristic length;

[0010] S3, according to the drag coefficient of the trapped gas mass of each size and the plurality of forces in water, the theoretical starting flow rate is calculated by using a starting flow rate model of the trapped gas mass, and the expression of the starting flow rate model is:

[0011]

[0012] Wherein, v w is the starting flow rate; μ is the dynamic viscosity of the liquid; C D is the drag coefficient; ρ l is the density of the liquid phase; C l is the shear lift coefficient; A d and A s are the projection area of the trapped gas mass in the direction perpendicular to the fluid flow direction and the projection area in the direction parallel to the fluid flow direction respectively; F B is the buoyancy of the trapped gas mass; F G is the gravity of the trapped gas mass; F Sx and F Sy are the components of the surface tension force in the x direction and the y direction respectively;

[0013] S4, the relative error between the actual starting speed corresponding to each size and the theoretical starting flow rate corresponding to different characteristic length calculation methods is calculated, and the relative error curve corresponding to the same characteristic length calculation method is drawn;

[0014] S5, according to the change trend of the relative error curve, the characteristic length calculation model of the trapped gas mass is constructed, the drag coefficient is updated by using the characteristic length calculation model, and then the starting flow rate model is updated.

[0015] Further, the plurality of characteristic length calculation methods are equivalent volume method, equivalent projection method, equivalent surface area method, maximum length method, maximum width method and maximum thickness method;

[0016] According to the change trend of the relative error curve, the equivalent projection method, the maximum width method and the equivalent surface area method are selected to construct the characteristic length calculation model of the trapped gas mass:

[0017]

[0018] Wherein, d b is the characteristic length of the trapped gas mass; d 投影 is the characteristic length corresponding to the equivalent projection method; d 表面 is the characteristic length corresponding to the equivalent surface area method; S ais the total surface area of the trapped bubble in contact with the liquid; b is the maximum width of the trapped bubble.

[0019] Further, the buoyancy F of the trapped bubble is calculated B , the gravity F of the trapped bubble is calculated G , and the expression of the components F Sx and F Sy are:

[0020] F B = p l gV a , F G = p a gV a

[0021]

[0022] wherein g is the acceleration of gravity; p a is the density of the gas phase; p l is the density of the gas phase; V a is the volume of the trapped bubble; a is the maximum length of the trapped bubble; and a and b are the front and back inclination angles of the trapped bubble, respectively.

[0023] Further, the drag coefficient is calculated by using the Tomiyama model, in which the drag coefficient is calculated by arbitrarily giving a starting flow rate in the calculation process, and then iteratively updated until the starting flow rate converges, so as to obtain the drag coefficient; the Tomiyama model is:

[0024]

[0025] wherein p a is the density of the gas phase; m is the dynamic viscosity of the liquid; d b is the characteristic length of the trapped bubble; g is the acceleration of gravity; s is the surface tension coefficient of the gas-liquid interface; Re b and Eo are the Reynolds number and the Bond number of the trapped bubble, respectively.

[0026] Further, the step S1 further comprises:

[0027] S11, injecting the trapped bubble into the trapped bubble test device, collecting image information of the trapped bubble when the trapped bubble is stable, and extracting geometric parameters of the trapped bubble by using image processing software;

[0028] S12, starting the power mechanism of the trapped bubble test device, and gradually adjusting the power size until the moving distance of the trapped bubble in adjacent time periods is greater than a preset threshold;

[0029] S13, calculating the actual starting flow rate of the trapped bubble according to the current monitoring flow rate and the cross-sectional area of the pipeline, and then discharging the trapped bubble in the trapped bubble test device.

[0030] S14, judging whether the number of selected different size stagnant air masses in the current test group is equal to the preset number, if yes, entering step S15, otherwise selecting the size not selected in the current test group and returning to step S11;

[0031] S15, judging whether all test groups have been tested, if yes, entering step S16, otherwise selecting the next test group and returning to step S11;

[0032] S16, using the starting flow rate corresponding to each air mass size as the actual starting speed for driving the movement thereof.

[0033] Further, the stagnant air mass test device comprises a plurality of test pipes connected into a loop through flanges, at least one of the test pipes being a transparent pipe, and a camera device is arranged outside the transparent pipe to collect images of the fluid and the stagnant air mass in the pipe; a power mechanism is sealingly mounted on the test pipe and extends into the pipe to drive the flow of the water in the test pipe, and a valve for injecting and discharging water, a gas injection valve for injecting gas, and an electromagnetic flowmeter for flow monitoring are arranged on the test pipe.

[0034] Further, step S12 further comprises:

[0035] S121, setting the initial rotating speed of the frequency converter connected to the motor of the power mechanism, and then starting the motor to stabilize the circulation of the water flow in the test pipe;

[0036] S122, judging whether the stagnant air mass is started through the images collected by the camera device, if yes, entering step S13, otherwise entering step S123;

[0037] S123, adjusting the initial rotating speed of the frequency converter = initial rotating speed + preset rotating speed, and then returning to step S122;

[0038] The standard for judging whether the stagnant air mass is started is that the moving distance Δs of the center of mass of the stagnant air mass detected at adjacent times is greater than a preset distance, and the calculation formula of Δs is:

[0039]

[0040] wherein, x t and y t are the x and y coordinates of the center of mass of the stagnant air mass at time t; x t+1 and y t+1 are the x and y coordinates of the center of mass of the stagnant air mass at time t+1.

[0041] Further, before step S11, there is further included calibrating the electromagnetic flowmeter and detecting the air tightness of the test pipe.

[0042] Further, the image processing software is ImageJ image processing software.

[0043] The beneficial effects of the present application are: the actual starting speed can be determined through the test, and then the relative error between the theoretical starting flow rate corresponding to the different characteristic length calculation methods and the actual starting speed is calculated, and then a new characteristic length calculation model is constructed based on the transformation trend of the relative error curve, so as to update the starting flow rate model; in the process of evaluating and updating the starting flow rate model through the above-mentioned manner, the geometric parameters of the stagnant gas mass corresponding to the actual starting flow rate and the theoretical starting speed are completely the same, so that the accuracy of the evaluation is ensured, and then the characteristic length calculation model of the stagnant gas mass is constructed combined with the change trend of the relative error curve, which integrates the advantages of multiple methods, thereby ensuring the high reliability of the data calculated by the updated starting flow rate model. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 It is a flow chart of the test method for the starting flow rate of the stagnant gas mass in the long-distance pressurized water pipeline.

[0045] Figure 2 It is a schematic diagram of the kinetic analysis of the stagnant gas mass.

[0046] Figure 3 It is a comparison diagram of the relative error curves of the stagnant gas mass obtained under multiple characteristic length calculation methods.

[0047] Figure 4 It is a comparative analysis diagram of the theoretical starting flow rate and the actual flow rate corresponding to the characteristic length calculation of the stagnant gas mass by using the equivalent projection method.

[0048] Figure 5 It is a comparative analysis diagram of the theoretical starting flow rate and the actual starting flow rate calculated by using the updated starting flow rate model.

[0049] Figure 6 It is a structural schematic diagram of the test device for the transport characteristics of the stagnant gas mass in the pressurized water pipeline.

[0050] Figure 7 It is a variety of structural schematic diagrams of transparent pipelines, wherein (a) is a inverted siphon pipeline, (b) is a “j” shaped pipeline, (c) is a horizontal pipeline, and (d) is a gravity flow pressure pipeline.

[0051] Figure 8 It is a sectional schematic diagram of a power shaft.

[0052] Wherein, 1, test pipeline; 2, flange; 3, camera equipment; 4, power mechanism; 41, motor; 42, frequency converter; 43, power shaft; 431, connecting rod; 432, shaft sleeve; 433, lubricating cavity; 44, propeller; 5, valve; 6, gas injection valve; 7, electromagnetic flowmeter; 8, rectifier grid; 9, exhaust hole. DETAILED DESCRIPTION

[0053] The specific embodiments of the present application are described below to facilitate the understanding of the present application for those skilled in the art, but it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the present application defined and determined by the appended claims, and all the inventions utilizing the concept of the present application are within the scope of protection.

[0054] Reference Figure 1 , Figure 1 A flow chart of a test method for the starting flow velocity of a stagnant air mass in a long-distance pressurized water pipeline is shown; as shown in Figure 1 , the method S includes steps S1-S5.

[0055] In step S1, the actual starting velocity corresponding to the geometric parameters of the stagnant air mass of various geometric sizes is obtained by test;

[0056] In an embodiment of the present application, step S1 further includes steps S11-S16:

[0057] S11, inject the stagnant air mass into the stagnant air mass test device, and collect the image information when the stagnant air mass is stable, and extract the geometric parameters of the stagnant air mass by using image processing software; the image processing software of the present scheme is preferably ImageJ image processing software. ImageJ is a free and open source image processing software developed by National Institutes of Health (NIH), which is originally designed for processing medical and biological images, and with the continuous development, its application field has been expanded to material science, physics, chemistry and other disciplines. ImageJ can not only perform conventional editing operations such as cropping, rotating, scaling, etc. on images, but also has powerful analysis functions, which can accurately measure the length, area, perimeter and other geometric parameters of the stagnant air mass. The specific operation process is as follows:

[0058] (1) Image acquisition

[0059] Top-view photography: To measure the geometric characteristics of the trapped air mass in the plane parallel to the flow direction, a high-definition industrial camera was used for photography. The camera was fixed on a specially designed tripod, ensuring that the lens was vertically downward aimed at the center position of the air mass, and the distance between the camera and the air mass was kept at 30 cm to avoid the influence of perspective distortion on the measurement results. The shooting site used a set of soft light boxes to create a diffuse reflection lighting environment, which uniformly lit from different angles to ensure that the light could fully and evenly cover the surface of the air mass, effectively eliminating reflections and shadows, and then obtaining clear and accurate top-view images.

[0060] Front-view photography: The camera lens was strictly perpendicular to the plane where the trapped air mass was located by fine tuning, while ensuring that the distance between the camera and the air mass was stable at 25 cm. Other parameter settings were the same as top-view photography. Figure 1

[0061] (2) Preliminary image processing

[0062] The images obtained by photography were imported into the professional image processing software ImageJ in RAW format. RAW format preserves the original data information of the image, providing greater flexibility for subsequent processing. During the import process, the software automatically identifies the color mode and resolution of the image and other parameters.

[0063] Crop and rotation correction: Since the photographed image may contain extra background area, the crop tool of ImageJ was used to accurately frame the area where the trapped air mass was located, and unnecessary interference information was removed. For images with inclination, the rotation function of the software was used to correct the image to the standard horizontal and vertical orientation according to the horizontal or vertical reference line in the image (such as the pipe wall, ruler edge, etc.), laying a foundation for subsequent accurate measurement.

[0064] (3) Define the ruler

[0065] After preliminary image processing, the straight ruler in the image was used as the actual size reference for calibration. The scale accuracy of this ruler was 0.1 cm, and during photography, the ruler and the trapped air mass were ensured to be in the same plane, and the scale was clear and visible. A 5.00 cm long scale interval on the ruler was selected as the reference for subsequent ruler setting.

[0066] Software ruler setting: In ImageJ, the "Set Scale" function was used to define the ruler. First, the straight line measurement tool of the software was used to accurately measure the pixel length corresponding to the selected 5.00 cm scale interval in the image. Assuming that the measured pixel length is x c , then the actual size of the trapped air mass x = λx c , where x c is the actual size of the trapped air mass; λ is the scale.

[0067] ​After the completion of the ruler definition setting, the software will automatically convert the pixel value to the actual physical size during the measurement process, ensuring the accuracy of the measurement results.

[0068] (4) Geometric parameter measurement

[0069] Plan view parameter measurement: In the plan view, the maximum length, maximum width, and projected area of the stagnation air mass are measured. In the software, the straight line measurement tool is used to measure the length a of the stagnation air mass in the flow direction (i.e., the maximum extension distance of the air mass along the flow path) and the width b perpendicular to the flow direction (i.e., the maximum dimension of the air mass perpendicular to the flow path direction). When measuring, the outermost boundary points of the air mass are taken as the starting and ending points to ensure that the measurement results accurately reflect the geometric characteristics of the air mass at this viewing angle.

[0070] Area measurement: Using the region selection tool of ImageJ, the selection area is drawn along the accurate boundary of the stagnation air mass. When drawing, try to follow the true contour of the air mass as closely as possible to ensure the accuracy of the measurement results. After completing the drawing of the selection area, the area of the selection area is automatically calculated.

[0071] Front view measurement: In the software, the angle measurement tool is used to measure the included angle formed by the front end and the rear end of the air mass with the wall surface, i.e., the front inclination angle and the rear inclination angle, taking the wall surface as the reference line. When measuring, carefully select the characteristic points at the junction of the air mass and the wall surface to ensure the accuracy of the measurement angle. For the measurement of the thickness of the air mass, the straight line measurement function of the software is used to draw a line segment perpendicular to the wall at the thickest part of the air mass. The software will convert the pixel length of the line segment to the actual physical length according to the previously set ruler, thereby obtaining the thickness of the air mass.

[0072] S12, start the power mechanism of the stagnation air mass test device, and gradually adjust the power size until the stagnation air mass moves a distance greater than a preset threshold in adjacent time periods; step S12 further comprises:

[0073] S121, set the initial speed of the frequency converter connected to the motor of the power mechanism, and then start the motor to stabilize the circulation of the water flow in the test pipeline;

[0074] S122, determine whether the stagnation air mass is started by the image collected by the camera equipment, if started, proceed to step S13, otherwise proceed to step S123;

[0075] S123, adjust the initial speed of the frequency converter = initial speed + preset speed, and then return to step S122;

[0076] The standard for determining whether the stagnation air mass is started is that the center of mass of the stagnation air mass moves a distance Δs greater than a preset distance in adjacent time, and the calculation formula of Δs is:

[0077]

[0078] wherein x t and y t are the x, y coordinates of the center of mass of the trapped air mass at time t; x t+1 and y t+1 are the x, y coordinates of the center of mass of the trapped air mass at time t+1.

[0079] S13, calculating the actual start-up flow rate of the trapped air mass according to the current monitored flow rate and the pipe cross-sectional area, and then discharging the trapped air mass in the trapped air mass test device;

[0080] S14, judging whether the number of trapped air masses of different sizes selected in the current test group is equal to the preset number of times, if yes, entering step S15, otherwise selecting a size not selected in the current test group and returning to step S11;

[0081] S15, judging whether all experimental groups have been tested, if yes, entering step S16, otherwise selecting the next experimental group and returning to step S11;

[0082] S16, using the start-up flow rate corresponding to each air mass size as the actual start-up speed for driving the movement thereof.

[0083] The size grouping of the trapped air mass is 0-2 cm, 2-4 cm, 4-6 cm, 6-8 cm, 8-10 cm, 10-12 cm, and 12-14 cm, and the same group of working conditions is repeatedly tested for three times, for example, the 10-12 cm group, and the air mass length can be selected as 10.5 cm, 11 cm, and 11.5 cm for testing.

[0084] As shown in Figures 6 to 8 The trapped air mass test device includes a plurality of test pipes 1 connected into a loop by flanges 2, in the present scheme, the test pipes 1 include straight pipes and arc pipes, and the loop is formed by the combination of the two types of pipes and the flanges 2 to simulate a long-distance pressurized water conveying pipe.

[0085] Meanwhile, according to actual engineering requirements, test pipe sections of different types such as horizontal, down slope, and up slope can be designed to simulate water conveying pipes under various complex terrain conditions, thereby providing convenience for studying the transport characteristics of air masses in different pipe environments.

[0086] In the present scheme, at least one test pipe 1 is a transparent pipe, and a camera device 3 for collecting images of the fluid and trapped air mass in the pipe is arranged outside the transparent pipe, and the camera device 3 preferably adopts a high-definition camera, and its collection range covers the entire transparent pipe as much as possible.

[0087] The power mechanism 4 extending into the test pipeline 1 is sealingly installed on the test pipeline 1 to drive the water flow in the test pipeline 1; in the implementation process, the power mechanism 4 preferably comprises a motor 41 and a frequency converter 42 connected with the motor 41 through a cable, the output shaft of the motor 41 is connected with a power shaft 43 sealingly and rotatably connected with the test pipeline 1 or the flange 2, and the end of the power shaft 43 located in the test pipeline 1 is provided with a propeller 44.

[0088] The motor 41 is accurately controlled by the frequency converter 42, and the propeller 44 rotates at high speed under the drive of the motor 41 to provide stable power for the water body in the test pipeline 1. By changing the power of the motor 41 through the frequency converter 42, the rotating speed of the propeller 44 is adjusted to achieve the purpose of obtaining different water flow speeds and meet the demand of various flow speeds in the test.

[0089] The test pipeline 1 is provided with a valve 5 for injecting and discharging water, a gas injection valve 6 for injecting gas, and an electromagnetic flowmeter 7 for flow monitoring. The gas injection valve 6 is preferably a one-way valve to enable it to inject a stagnant gas mass and prevent water flow from flowing backward, thereby ensuring the stability and reliability of the test process. The electromagnetic flowmeter 7 can monitor the water flow in real time, and the average flow speed of the pipeline cross section can be calculated according to the cross-sectional size of the test pipeline 1.

[0090] The electromagnetic flowmeter 7 preferably has high-precision flow measurement capability and can monitor the water flow in real time, with a measurement accuracy of ±0.5%, which can provide accurate data support for studying the relationship between gas mass transport and water flow and help researchers accurately grasp the gas mass movement characteristics under different flow conditions.

[0091] As shown in FIG. Figure 7 In the implementation process, the transparent pipeline is preferably a plurality of transparent pipe segments connected through the flange 2 to form an inverted siphon pipeline (as shown in FIG. Figure 7 ) or a "J" shaped pipeline (as shown in FIG. Figure 7 ), or a horizontal pipe (as shown in FIG. Figure 7 ) or a gravity flow pressure pipe (as shown in FIG. Figure 7 ). When performing simulation tests, different styles of transparent pipelines can be selected according to the simulated terrain conditions to ensure accurate simulation under different terrain conditions.

[0092] The inverted siphon is usually in the form of a downward concave inverted V-shaped structure, which is suitable for working conditions where the water conveying line needs to cross deep valleys, rivers, roads and other obstacles. The water flow is realized through the downward concave pipeline, which is especially suitable for scenes where the channel intersects with obstacles and the water level difference is suitable.

[0093] The "J" shaped pipeline is usually a structure with multiple ups and downs, which is suitable for working conditions where the terrain is complex, there are multiple changes in height, or multiple obstacles (such as multiple hills and multiple valleys) need to be continuously crossed. Through the combination of the rising and falling of the pipeline, it can flexibly adapt to the undulating terrain.

[0094] Horizontal pipe is suitable for flat terrain, natural elevation is very small, due to lack of natural head difference, need to rely on pump station pressure driving water flow, often in the city long distance water, horizontal crossing building or dense pipeline scene.

[0095] Gravity flow pressure pipe is usually a single slope inclined structure, suitable for the condition of significant natural elevation, that is, the water source is located at a high place (such as a reservoir, a mountain top water source), and the water destination is located at a low place. The static water pressure formed by the terrain elevation drives the water flow without additional power, which is energy-saving and efficient, and is suitable for long distance gravity flow water supply.

[0096] As shown in Figure 8 , the power shaft 43 includes a connecting rod 431 fixedly connected with the output shaft and a shaft sleeve 432 sleeved outside the connecting rod 431; one end of the shaft sleeve 432 is fixed on the motor 41 housing at the output shaft, the other end penetrates into the test pipeline 1, and is sealingly and fixedly connected with the test pipeline 1; the propeller 44 is installed at the end of the connecting rod 431 located in the test pipeline 1. A lubricating cavity 433 is formed between the shaft sleeve 432 and the connecting rod 431, and the lubricating cavity 433 is filled with lubricating oil. The arrangement of the lubricating oil can effectively reduce the rotating friction and ensure the stable and efficient operation of the propeller 44.

[0097] Again participate Figure 6 , the test pipeline 1 is provided with a flow straightener 8 adjacent to the output side of the power mechanism 4 and / or the inlet end of the transparent pipeline. The flow straightener 8 can effectively eliminate water flow turbulence and make the water flow form a stable and uniform flow state.

[0098] In implementation, the scheme is preferably that when the test pipeline 1 is provided with the flow straightener 8 adjacent to the output side of the power mechanism 4 and the inlet end of the transparent pipeline, the grid length of the flow straightener 8 at the output side of the power mechanism 4 is smaller than the grid length of the flow straightener 8 at the inlet end of the transparent pipeline. In this way, the water flow entering the transparent pipeline can form a more stable and uniform flow state, so as to facilitate the collection of images of the trapped air mass.

[0099] In implementation, the scheme is preferably that the test pipeline 1 is provided with an exhaust hole 9 adjacent to the output side of the power mechanism 4, and the air injection valve 6 is arranged at the inlet end of the transparent pipeline. The test pipeline 1 at the installation position of the power mechanism 4 is a conical pipe, both ends of the conical pipe are connected with the test pipeline 1 through flanges 2, and the power mechanism 4 penetrates into the inside of the conical pipe through the flange 2 at the large end of the conical pipe.

[0100] The operation process of the trapped air mass test device provided by the scheme is as follows:

[0101] Before the experiment, according to the purpose of the experiment, the test pipe 1 of different shape is selected, and the connection mode of each test pipe 1 and the position of the exhaust hole 9 are adjusted. At the beginning of the experiment, water is injected through the valve 5 to ensure that there is no stagnant gas in the pipe, and then the motor 41 and the frequency converter 42 are started, and the water flow is adjusted to the predetermined speed and flow to push the water flow in the test pipe 1. The frequency converter 42 adjusts the speed of the motor 41 according to the preset test conditions to accurately control the water flow speed. The water flow enters the transparent pipe after passing through the rectifier grid 8.

[0102] The gas is injected into the transparent pipe through the one-way gas injection valve 6, and the stagnant gas formed by the gas will start to move under the action of the water flow. The electromagnetic flowmeter 7 monitors the water flow in real time, and the average flow velocity of the pipe section can be calculated according to the cross-sectional size of the test pipe 1.

[0103] The movement process of the stagnant gas to the exhaust port direction is observed through the high-definition camera equipment 3, and the dynamic characteristic changes such as the movement trajectory and speed change of the gas under different water flow velocities are recorded; when the gas moves to the position of the exhaust hole 9, the gas can be discharged through the exhaust hole 9.

[0104] After the experiment, the motor 41 and the related equipment are turned off, and the researchers observe and record the movement trajectory, speed change and other parameters of the gas under different water flow velocities to analyze the gas transport characteristics.

[0105] In implementation, the scheme preferably further comprises calibrating the electromagnetic flowmeter and detecting the air tightness of the test pipe before step S11. The two operation methods are described in detail as follows:

[0106] When calibrating the electromagnetic flowmeter, the calibration steps need to be strictly followed to ensure that the measurement accuracy meets the requirements of the gas transport characteristic test, and when the error exceeds ±0.5%, the calibration needs to be re-calibrated. The specific calibration process is as follows:

[0107] A triangular water weir is used as a standard flow source for calibration, and the electromagnetic flowmeter is installed according to the "first ten and last five" principle to ensure one-way water flow. In addition, the water temperature needs to be stabilized at 20±2℃, and the water flow needs to be ensured to fill the pipe.

[0108] The first step is zero calibration, close the upstream valve, let the water in the pipeline flow still for 5 minutes, then read the display value of the electromagnetic flowmeter, if not zero, then carry out zero correction through the meter head, etc. The second step is to measure, first adjust the valve to control the water flow, make the weir head H stable in the range of 0.05-0.3m, use high-precision water level gauge (accuracy ±0.1mm) to measure H value. According to the straight triangle weir flow formula to calculate the standard flow. Repeat the measurement for 3 times to take the average value, if the error of any point exceeds ±0.5%, then recalibrate the water level gauge zero point, check whether the weir is blocked by sundries, if necessary, adjust the gain coefficient K of the flowmeter to correct. After correction, the full range error needs to be verified again to ensure that it is within ±0.5%. Finally, through long-term stability test (24h drift ≤±0.1%) and multi-condition retest, the calibration work of the flowmeter is completed.

[0109] Sealing test of test device: after the electromagnetic flowmeter is installed on the test platform, water is injected through the valve to ensure that there is no air mass remaining in the pipeline, and then the sealing test of the pipeline is carried out; then the visual observation method and paper towel detection method are used to detect whether the flange, valve and other connections are wet.

[0110] In step S2, according to the geometric parameters of the air mass of each size, the characteristic length of the air mass is calculated by using a plurality of characteristic length calculation methods of the air mass, and the drag coefficient is calculated based on the characteristic length; the plurality of characteristic length calculation methods are preferably equivalent volume method, equivalent projection method, equivalent surface area method, maximum length method, maximum width method and maximum thickness method.

[0111] In step S3, according to the drag coefficient of the air mass of each size and a plurality of forces in water (the force analysis of the air mass in the water conveying pipeline can refer to the figure), the theoretical starting flow rate is calculated by using the starting flow rate model of the air mass, and the expression of the starting flow rate model is:

[0112]

[0113] Wherein, v w is the starting flow rate, m / s; μ is the dynamic viscosity of the liquid, the dynamic viscosity at water temperature of 20℃ is 1.0016×10 -3 pa.s; C D is the drag coefficient; ρ l is the density of the liquid phase, the density of water is 1000kg / m 3 ; C l is the shear lift coefficient, which is 0.5 in this scheme; A d and A s are the projection area of the air mass in the direction perpendicular to the fluid flow and the projection area in the direction parallel to the fluid flow respectively; F B is the buoyancy of the air mass; FG For the gravity of the trapped air mass; F Sx and F Sy These are the components of the surface tension force in the x and y directions, respectively.

[0114] Calculate the buoyancy F of the trapped air mass B Gravity F of the stagnant air mass G and component F Sx and F Sy The expression is:

[0115] F B =ρ l gV a F G =ρ a gV a

[0116]

[0117] Where g is the acceleration due to gravity; ρ a The density of the gas phase is taken as the air density at 20℃, which is 1.205 kg / m³. 3 ;ρ l V is the density of the liquid phase; a denoted as denoted as , where 'a' is the volume of the trapped air mass; 'a' is the maximum length of the trapped air mass; and 'α' and 'β' are the forward and backward tilt angles of the trapped air mass, respectively.

[0118] The drag coefficient in this scheme is calculated using the Tomiyama model. During the calculation, the drag coefficient is calculated based on an arbitrary starting flow velocity, and then iteratively updated until the starting flow velocity converges to obtain the drag coefficient. The Tomiyama model is as follows:

[0119]

[0120] Where, ρ a ρ is the density of the gas phase; μ is the dynamic viscosity of the liquid, which is taken as the dynamic viscosity of water at a temperature of 20℃ in this patent; d b denoted as the characteristic length of the trapped air mass; is the gravitational acceleration; σ is the surface tension coefficient of the gas-liquid interface, which is taken as 0.072 N / m in this patent; Re b Eo and Eo are the Reynolds number and Bond number of the stagnant air mass, respectively.

[0121] In step S4, the relative error between the actual starting speed for each dimension and the theoretical starting flow rate corresponding to different characteristic length calculation methods is calculated, and a relative error curve corresponding to the same characteristic length calculation method is plotted. Multiple relative error curves are referenced. Figure 3 , Figure 3 The curves abc represent the maximum length method, the maximum width method, and the maximum thickness method, respectively.

[0122] By Figure 3 Comprehensive analysis shows that the error of the equivalent projection method is the smallest, but as the length of the gas group increases, the error shows a growing trend. In order to accurately predict the starting flow rate of the stagnant gas group and provide help for engineering practice, the theoretical starting flow rate needs to be corrected to reduce the error.

[0123] Figure 4 The theoretical starting flow rate and the actual flow rate of the stagnant gas group corresponding to the characteristic length calculated by the equivalent projection method are shown in the comparative analysis graph. By Figure 4 It can be seen that as the length of the gas group increases, the theoretical value gradually decreases compared with the actual value. The fundamental reason is that the drag coefficient of the stagnant gas group is too large, and since the drag coefficient is closely related to the characteristic length, the characteristic length needs to be corrected.

[0124] In step S5, according to the change trend of the relative error curve, a characteristic length calculation model of the stagnant gas group is constructed, and the drag coefficient is updated by using the characteristic length calculation model, and then the starting flow rate model is updated.

[0125] In this scheme, according to the change trend of the relative error curve, the equivalent projection method, the maximum width method and the equivalent surface area method are selected to construct a characteristic length calculation model of the stagnant gas group:

[0126]

[0127] Wherein, d b is the characteristic length of the stagnant gas group; d 投影 is the characteristic length corresponding to the equivalent projection method; d 表面 is the characteristic length corresponding to the equivalent surface area method; S a is the total surface area of the stagnant gas group in contact with the liquid; and b is the maximum width of the stagnant gas group.

[0128] The characteristic length calculation model of the stagnant gas group constructed in this scheme can more comprehensively describe the geometric and dynamic characteristics of the gas group. For irregularly shaped gas groups, this correction method can consider the size information in different directions, so that the calculated characteristic length can more accurately represent the actual size of the gas group in the fluid. The error results after correction are shown in Figure 5 .

[0129] By Figure 5 The results after correction show that at the initial stage when the long axis of the bubble is close to 0cm, the actual flow rate curve and the theoretical flow rate model curve are almost completely coincident, both starting from about 5cm / s. This shows that the improved model can accurately capture the initial state of the starting flow rate when dealing with very small size bubbles, and the correction method of the characteristic length effectively avoids the large deviation of the model at the initial stage, so that the calculation results are highly consistent with the actual situation.

[0130] With the increase of the long axis from 0 cm to 14 cm, the two curves always keep similar rising trend. The actual flow rate gradually increases from about 5 cm / s to about 14 cm / s, and the theoretical flow rate curve also increases at a similar rate. This shows that the improved model can closely follow the growth trend of the actual flow rate in the whole range of bubble size variation, dynamically reflecting the influence of the bubble long axis size on the starting flow rate. The synchronization of this speed change further proves the effectiveness of the improved method, which enables the model of the starting flow rate of the stagnant gas mass to accurately simulate the variation law of the starting flow rate of the bubble under different sizes.

Claims

1. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline, characterized in that, The method comprises the steps of: S1, obtaining the geometric parameters of the trapped air mass of various geometric sizes and the actual starting speed driving the movement of the trapped air mass through experiments; S2, calculating the characteristic length of the trapped air mass according to the geometric parameters of the trapped air mass of each size, calculating the drag coefficient based on the characteristic length by using a plurality of characteristic length calculation methods of the trapped air mass; S3, calculating the theoretical starting flow rate of the trapped air mass according to the drag coefficient of the trapped air mass of each size and a plurality of forces in water, and using a starting flow rate model of the trapped air mass, the expression of the starting flow rate model being: where v w is the start-up flow rate; μ is the dynamic viscosity of the liquid; C D is the drag coefficient; p l is the density of the liquid; C l is the lift coefficient; A d and A s are the projected area of the air pocket in the direction perpendicular to the fluid flow and in the direction parallel to the fluid flow, respectively; F B is the buoyancy force of the air pocket; and F G is the gravitational force of the air pocket. F Sx and F Sy respectively the components of the surface tension force in the x and y directions; S4, calculating the relative error between the actual starting speed corresponding to each size and the theoretical starting flow rate corresponding to different characteristic length calculation methods, and drawing a relative error curve corresponding to the same characteristic length calculation method; S5, constructing a characteristic length calculation model of the trapped air mass according to the trend of the relative error curve, updating the drag coefficient by using the characteristic length calculation model, and then updating the starting flow rate model.

2. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline according to claim 1, characterized in that, The plurality of characteristic length calculation methods are equivalent volume method, equivalent projection method, equivalent surface area method, maximum length method, maximum width method and maximum thickness method; According to the trend of the relative error curve, the equivalent projection method, the maximum width method and the equivalent surface area method are selected to construct the characteristic length calculation model of the trapped air mass: where d b is the characteristic length of the stagnant gas volume; d 投影 is the characteristic length corresponding to the equivalent projection method; d 表面 is the characteristic length corresponding to the equivalent surface area method; S a is the total surface area of the stagnant gas volume in contact with the liquid; and b is the maximum width of the stagnant gas volume.

3. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline according to claim 1, characterized in that, F = (ρ - ρ0) g V B F = (ρ - ρ0) g V G F = (ρ - ρ0) g V Sx F = (ρ - ρ0) g V Sy F = (ρ - ρ0) g V F B = p l gV a , F G = p a gV a where g is the acceleration due to gravity; p a is the density of the gas phase; p l is the density of the liquid phase; V a is the volume of the trapped air mass; a is the maximum length of the trapped air mass; and a and b are the front and back rake angles of the trapped air mass, respectively.

4. The test method for start-up flow velocity of stagnant air pocket in long distance pressurized water pipeline according to claim 1, characterized in that, The drag coefficient is calculated by using the Tomiyama model, in which the drag coefficient is calculated by arbitrarily giving a starting flow rate, and then the starting flow rate is iteratively updated until the starting flow rate converges, so as to obtain the drag coefficient; the Tomiyama model is: where p a is the density of the gas phase; m is the dynamic viscosity of the liquid; d b is the characteristic length of the gas slug; g is the gravitational acceleration; s is the surface tension coefficient at the gas-liquid interface; Re b and Eo are the Reynolds and Bond numbers of the gas slug, respectively.

5. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline according to any one of claims 1 to 4, characterized in that, Step S1 further comprises: S11, injecting the trapped air mass into the trapped air mass test device, collecting image information of the trapped air mass when the trapped air mass is stable, and extracting geometric parameters of the trapped air mass by using image processing software; S12, starting the power mechanism of the trapped air mass test device, and gradually adjusting the power size until the trapped air mass moves a distance greater than a preset threshold in adjacent time periods; S13, calculating the actual starting flow rate of the trapped air mass according to the current monitoring flow rate and the pipe cross-sectional area, and then discharging the trapped air mass in the trapped air mass test device; S14, determining whether the number of trapped air masses of different sizes selected in the current test group is equal to a preset number, if yes, proceeding to step S15, otherwise selecting a size not selected in the current test group and returning to step S11; S15, determining whether all experimental groups have been tested, if yes, proceeding to step S16, otherwise selecting the next experimental group and returning to step S11; S16, using the starting flow rate corresponding to each air mass size as the actual starting speed driving the movement of the trapped air mass.

6. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline according to claim 5, characterized in that, The trapped air mass test device comprises a plurality of test pipes connected into a ring circuit through flanges, at least one test pipe is a transparent pipe, and a camera device for collecting images of fluid and trapped air mass in the transparent pipe is arranged outside the transparent pipe; a power mechanism extending into the pipe to drive the flow of water in the test pipe is sealingly installed on the test pipe, and a valve for injecting and discharging water, a gas injection valve for injecting gas, and an electromagnetic flowmeter for flow monitoring are arranged on the test pipe.

7. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline according to claim 6, characterized in that, Step S12 further comprises: S121, set the initial rotating speed of the frequency converter connected with the motor of the power mechanism, and then start the motor to make the water in the test pipeline circulate stably; S122, judge whether the stagnant air mass is started or not through the image collected by the camera, if yes, go to step S13, otherwise go to step S123; S123, adjust the initial rotating speed of the frequency converter = initial rotating speed + preset rotating speed, and then return to step S122; The standard for judging whether the stagnant air mass is started or not is that the moving distance Δs of the centroid of the stagnant air mass detected at adjacent times is greater than a preset distance, and the calculation formula of Δs is: where x t and y t are the x, y coordinates of the center of mass of the stagnant air mass at time t; x t+1 and y t+1 are the x, y coordinates of the center of mass of the stagnant air mass at time t+1.

8. A test method for determining the start-up flow velocity of a stagnant air pocket in a long distance pressurized water pipeline according to claim 6, characterized in that, Before step S11, calibration of the electromagnetic flowmeter and detection of the air tightness of the test pipeline are further included.

9. The test method for start-up flow velocity of stagnant air pocket in long distance pressurized water pipeline according to claim 5, characterized in that, The image processing software is ImageJ image processing software.

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