WO2014079056A1

WO2014079056A1 - Cavity flow meter

Info

Publication number: WO2014079056A1
Application number: PCT/CN2012/085255
Authority: WO
Inventors: 路明
Original assignee: Lu Ming
Priority date: 2012-11-26
Filing date: 2012-11-26
Publication date: 2014-05-30

Abstract

A cavity flow meter, comprising a circulation pipe (8) provided with a front connecting flange (7) and a back connecting flange (9) thereon; a measurement cavity (2) is disposed on the wall surface of the circulation pipe (8); the cavity (2) has a sharp leading edge (3) and a trailing edge (5); a pressure sensor (12) is installed at the bottom of the measurement cavity (2) and is connected to a measurement circuit (13); a Reynolds number counter (10) is installed on the wall surface of the circulation pipe (8) at the upstream of the cavity (2) in the flow direction (1), and transmits data to the measurement circuit (13) via a data wire (11). When fluid flows by the cavity on the wall surface of the pipe of the flow meter, the fluid generates self-sustained oscillation; the vortex frequency is in proportion to the incoming flow speed, therefore the flow value can be obtained by measuring pressure oscillation frequency.

Description

Cavity flowmeter

Technical field

The invention relates to the field of fluid measurement, and is a device for measuring fluid flow, in particular to utilize hole flow.

Background technique

The flow meter is a meter that indicates the total amount of fluid being measured flowing over a selected time interval. It can be used to measure the flow of fluid in pipes and open channels. In the fields of metallurgy, electric power, coal, chemical, petroleum, transportation, construction, textile, food, medicine, agriculture, environmental protection, etc., there are a wide variety of flow meters. To date, there are as many as 60 types of flow meters available for industrial use. In the field of fluid flow measurement technology, the most widely used flowmeters are mostly differential pressure flowmeters, such as orifice flowmeters, nozzle flowmeters, venturi flowmeters, and the like. In addition, turbine flow meters and vortex flow meters are also used. Flow meters of the above type are simple in construction and low in cost, but face an intractable problem in measurement.

Since the fluid to be tested often contains more impurities or corrosive components, it may have corrosion or wear on the components of the flowmeter, or blockages may occur at the time, resulting in high maintenance cost and excessive measurement error. Even the measurement failed. In short, the current industrial field requires a flow meter that is simple in structure, accurate, and capable of eliminating the above drawbacks.

Cavity Flow is a common fluid flow phenomenon in engineering. For example, the flow in the gap between the train cars, the flow in the aircraft landing gear bay, and the like. Hole flow has always been a research hotspot in the field of fluid mechanics. Because there is a measurable, use-valued movement mechanism in this flow phenomenon. Figure 1 shows a theoretical schematic of the internal mechanism of a hole flow. As shown in Figure 1, when the fluid flows (1) through the cavity (2), at the leading edge (3) of the hole, that is, at the corner, this is theoretically a flow singularity, so the fluid The rate of change of pressure in the direction perpendicular to the flow direction (1) in the viscous boundary layer of (1) suddenly becomes large, thereby causing flow separation at the leading edge (3). Due to the separation of the fluid, a shear layer (4) of fluid moving downstream is formed at the leading edge. The shear layer (4) collides with the trailing edge of the hole (5), is reflected back to the leading edge (3) in the hole (2), and forms a reflection opposite to the incoming stream (1) in the hole (2) Come back to the flow (6). The reflected flow (6) to the leading edge (3) forms a further disturbance to the fluid at the leading edge (3), and the continuous incoming flow (1) continues to separate at the leading edge, forming a shear layer (4). This process continues with the flow of the fluid, so that in the cavity (2), a swirling flow is generated, and the pressure field in the cavity (2) changes periodically, so that the hole flows to form a Self-sustained osci l lation in a cavity ₀ This self-sustained oscillation is mainly related to the geometry of the holes and the Reynolds number of the incoming flow. When describing the motion of a fluid (such as air, water, etc.), a dimensionless number, Reynolds number, is used, which describes the ratio of the inertial force to the viscous force of the fluid. The Reynolds number Tfe of the compressible fluid can be expressed by the following formula.

Re =

^ (1) where A _∞ represents the density, velocity, and dynamic viscosity coefficient of the fluid flow, respectively; Ζ represents the characteristic length dimension of the object, and in the hole flow, it can be represented by the length of the hole opening in FIG. A large number of experiments have shown that the Reynolds number is in the range of 2χ10 ⁴ to 7χ10 ⁵ , the structure of the vortex in the hole flow is stable, and the relationship between the vortex generation frequency and the flow rate of the fluid flow is uniquely determined. In the oscillating flow field such as hole flow, a dimensionless parameter is often used, and the Strouhal number is used to describe the characteristics of the flow field. Strouhal number, S is defined as

St = ^- (2)

V _∞ where f is the frequency at which the vortex falls off. In the above Reynolds number range, the Strouhal number is generally a fixed value of 0.2. Thus, in the flow of holes, if the frequency of shedding of the vortex is obtained, the flow rate of the fluid can be obtained.

Summary of the invention

SUMMARY OF THE INVENTION An object of the present invention is to provide a cavity flowmeter that operates using the principle of hole flow, which is a flowmeter made by the principle of a self-sustained oscillation phenomenon generated when a fluid flows through a cavity on a wall surface of a flowmeter.

According to the above theory, a hole flow meter for measuring the flow rate of steady flow in a pipe which works by the principle of hole flow can be designed. 2 is a schematic structural view of a hole flow meter proposed by the present invention. The figure shows that the cavity flowmeter includes a flow conduit (8) with a front flange (7) and a rear flange (9). There is a hole (2) for measurement on the wall of the flow pipe (8), and the hole (2) has a sharp leading edge (3) and a trailing edge (5). The measuring hole (2) is equipped with a pressure sensor (12) at the bottom and connected to the measuring circuit (13). In the flow direction (1), upstream of the cavity (2), the Reynolds number calculator (10) is installed on the wall of the flow pipe (8) and transmitted to the measuring circuit (13) through the data line (11).

When the flow meter is working, the fluid enters the flow conduit (8). After the measurement circuit (13) is started, the transient pressure signal in the pressure sensor (12) is read. Since at this time, the self-sustained oscillation in the hole (2) has been formed, it is inevitable that a vortex flow pattern like (6) in Fig. 1 is generated in the hole (2). Measuring circuit (13) obtains pressure within a certain sampling time The transient signal then decomposes the frequency of this vortex. Methods for decomposing frequency signals from time domain signals are well known and will not be described here.

Since the vortex frequency is proportional to the incoming flow velocity, the Strouhal number is a fixed value of 0.2, and the opening length Z of the cavity (2) is known, the flow velocity _∞ can be obtained according to the formula (2). At this time, since the density, velocity and dynamic viscosity coefficient of the fluid flow are known, the Reynolds number of the fluid is calculated according to the formula (1) by the Reynolds number calculator (10) installed on the circulation pipe (8), and The data line (11) is sent to the measuring circuit (13). According to the foregoing theory, if the calculated Reynolds number is in the range of ² × ¹⁰ ⁴ to 7χ10 ⁵ , the measurement is effective, and the flow value of the steady flow in the pipeline is obtained; otherwise, the warning that the measurement range is exceeded is proposed.

Since the flow measurement of the hole flowmeter proposed by the present invention is carried out in the cavity, there is no component inside the meter that hinders the flow of the fluid, so the pressure loss is very small, and the structure is more firm and reliable, and is not afraid of corrosion, wear, blockage, etc. Easy to assemble and disassemble, simple and practical.

DRAWINGS

Figure 1 is a theoretical schematic of the internal mechanism of hole flow. In the figure, 1 fluid flows, 2 holes, 3 front edges, 4 shear layers, 5 trailing edges, and 6 reflected back flows (vortexes).

2 is a schematic structural view of a hole flow meter. In the figure, 1 fluid flows, 2 holes, 3 front edges, 5 trailing edges, 7 connected front flanges, 8 flow pipes, 9 connected rear flanges, 10 Reynolds number calculator, 11 data lines, 12 pressure sensors, 13 Connect to the measurement circuit.

detailed description

The principle and structure of a hole flow meter proposed by the present invention will be further described below in a specific embodiment. A schematic structural view of a specific embodiment is shown in Fig. 2. The cavimeter flowmeter includes a flow conduit (8) having a front flange (7) connected thereto and a rear flange (9) connected between the tubes to be measured. There is a cavity (2) on the bottom edge of the circulation pipe (8), the opening length L is 50 mm, and the cross-sectional shape along the depth direction (the direction perpendicular to the incoming flow) is rectangular, thus having a sharp leading edge (3) and rear. Along the (5), a pressure sensor (12) is attached to the bottom and connected to the measuring circuit (13). A Reynolds number calculator (10) is mounted on the wall of the flow conduit (8) upstream of the cavity (2) along the flow direction (1) and transmitted to the measurement circuit (13) via the data line (11).

The device is used to measure a steady flow in the pipe, ie the incoming flow rate is constant. The front flange (7) of the device, Connect the rear flange (9) between the pipes to be measured. When measuring, when the flow flows through the hole (2), the fluid boundary layer generates flow separation at the leading edge of the hole (3), and the separated shear layer collides with the trailing edge of the hole (5), in the cavity (2) The middle is reflected back to the leading edge (3), creating a disturbance to the fluid at the leading edge, where the fluid flow (1) continues to separate. This process continues as the fluid flows, so that in the cavity (2), a vortex is generated, causing periodic oscillation of the hole (2) pressure. The pressure sensor (12) for measuring the vortex frequency is connected to the measuring circuit (13), and the transient signal of the pressure is obtained within 10 seconds of the sampling time by the sampling frequency of 1 KHZ, and then the frequency of the vortex is decomposed by the FFT method. According to formula (2), /, Z and the available flow velocity _{∞ are known} . At this time, since the density of the incoming flow and the dynamic viscosity coefficient are both known, the Reynolds number calculator (10) can calculate the Reynolds number of the fluid flow (1) according to the formula (1) and transmit it through the data line (11). To the measurement circuit (13). According to the foregoing theory, if the calculated Reynolds number is in the range of ² × ¹⁰ ⁴ to 7χ10 ⁵ , the measurement is effective, and the flow value of the steady flow in the pipe is obtained; otherwise, the measuring circuit (13) proposes a warning that the measuring range is exceeded.

For effective measurements, the flow rate can be obtained by multiplying the flow velocity in the future by the cross-sectional area of the pipe. The flow value can be converted to a standard current signal output.

List of reference numerals

1 fluid flow

2 holes

3 frontiers

4 shear layer

5 trailing edge

6 reflected back flow (vortex)

7 connection front flange

8 circulation pipeline

9 connected rear flange

10 Reynolds number calculator

11 data lines

12 pressure sensor

13 measurement circuit.

Claims

Claim

A cavity flow meter characterized in that it comprises a continuous flow conduit (8) having a front flange (7) connected thereto and a rear flange (9) connected thereto; on the wall of the flow conduit (8) There is a hole (2) for measurement, the hole (2) has a sharp leading edge (3) and a trailing edge (5); the bottom of the measuring hole (2) is equipped with a pressure sensor (12) connected to the measuring circuit (13) In the flow direction (1), upstream of the cavity (2), the Reynolds number calculator (10) is installed on the wall of the circulation pipe (8), and is transmitted to the measuring circuit (13) through the data line (11). .

The hole flowmeter according to claim 1, wherein the hole (2) has a rectangular cross-sectional shape in a direction perpendicular to the incoming flow.