Flow measurement principle and application of the

Flow measurement principle and application of ultrasonic flowmeter

Abstract: This paper expounds the flow measurement principle of time difference method and Doppler method commonly used by ultrasonic flowmeter, as well as the classification of ultrasonic flowmeter. Through the actual application of flow measurement and the comparison and analysis with the flow results measured by the flow meter, it is concluded that the ultrasonic flowmeter is higher than other flow measurement equipment in both flow measurement accuracy and flow measurement accuracy, and has the superior performance of real-time and data remote transmission that other flow measurement equipment does not have

key words: ultrasonic flowmeter; Time difference method; Doppler; Flow measurement

1 introduction

in recent years, with the development of electronic technology, digital technology and sound wedge materials, the technology of using ultrasonic pulse to measure fluid flow has developed rapidly. Based on different principles, various forms of ultrasonic flowmeter suitable for different occasions have appeared one after another. Its application fields involve industrial and agricultural, water conservancy, hydropower and other departments, and it is increasingly becoming the preferred tool for flow measurement

2 measurement principle of ultrasonic flowmeter

the common measurement methods of ultrasonic flowmeter are propagation velocity difference method, Doppler method, etc. Propagation velocity difference method includes direct time difference method, phase difference method and frequency difference method. Its basic principle is to measure the velocity difference of ultrasonic pulse along the flow and against the flow to reflect the flow velocity of the fluid, so as to measure the flow; The basic principle of Doppler method is to use the Doppler effect in sound waves to measure the frequency difference between downstream and upstream flow to reflect the flow velocity of the fluid, so as to obtain the flow

2.1 measurement principle of time difference method

the principle of measuring fluid flow by time difference method is shown in Figure 1. It uses the characteristics that the propagation speed of sound wave is different due to different fluid flow directions when it propagates in the fluid, and measures the difference between its downstream propagation time T1 and countercurrent propagation time T2, so as to calculate the velocity and flow of fluid flow

Figure 1 flow measurement schematic diagram of ultrasonic flowmeter

set the sound velocity in the static fluid as C and the fluid flow velocity as V, and install a group of transducers P1 and P2 with the pipe channel axis θ Angle, the distance of the transducer is L. When transmitting downstream from P1 to P2, the sound wave propagation time T1 is:

when transmitting upstream from P2 to P1, the sound wave propagation time T2 is:

General C>>v, then the time difference is:

the mono channel test system is only suitable for small channels where the water level and flow velocity change little. Large channels have wide water surface and deep water, and their velocity changes greatly in vertical and horizontal directions. Multi channel ultrasonic flow measurement must be used to obtain accurate flow values, as shown in Figure 2. The flow Q can be measured by applying formulas (5) and (6)

in the above formulas: D is the horizontal projection distance between the two transducers perpendicular to the water flow direction, is the number of channels, and S is the cross-sectional area between the two channels

Figure 2 flow measurement principle diagram of multi-channel ultrasonic flowmeter

2.2 measurement principle of Doppler method

measurement principle of Doppler method is to detect the Doppler frequency difference according to the Doppler effect in sound waves. The ultrasonic generator is a fixed sound source. The solid particles moving with the fluid at the same speed have relative motion with the sound source, and the solid particles can reflect the incident ultrasonic wave back to the receiver. The frequency difference between incident sound wave and reflected sound wave is the Doppler frequency shift of sound wave caused by the movement of solid particles in the fluid. Since this frequency difference is proportional to the fluid flow rate, the flow rate can be obtained by measuring the frequency difference, and then the fluid flow can be obtained, as shown in Figure 3

Figure 3 flow measurement schematic diagram of Doppler ultrasonic flowmeter

when the particles moving with the fluid at speed V flow to the acoustic generator, the acoustic frequency f1 received by the particles is:

therefore, the Doppler frequency shift between the acoustic receiver and the generator Δ F is:

in the above formula: θ Is the angle between the direction of the sound wave and the fluid flow velocity V, F0 is the initial sound frequency of the sound source, and C is the propagation speed of the sound source in the medium. If c>>vcos θ Then

formula (11) the generation of electrostatic phenomenon will bring a lot of harm to our experimental machine, (12) is the measured fluid flow rate and flow when considering a single particle. However, for the flow with a large number of particles, all frequency shift signals should be statistically processed. The transducer of ultrasonic Doppler flowmeter usually adopts the integrated structure of transmitter and receiver, as shown in Figure 4. The reflected signal received by the transducer can only be the reflected wave of particles in the overlapping area of the two directional beams of the generator and the receiver. This overlapping area is called the information window of the Doppler signal. The signal received by the transducer is the superposition of the reflected waves of all flowing suspended particles in the information window, that is, the Doppler frequency shift in the information window is the average value of the superposition of reflected waves

average dope current market data tensile strength testing machines some screw rods are T-shaped ordinary screw rods and gapless ball screw rods with frequency shift Δ- F can be expressed as:

where Δ- F - average value of Doppler frequency shift of all reflected particles in the information window

Σ Ni - Doppler shift Δ Particle number of fi

Δ Fi - Doppler shift generated by any suspended particle

it can be seen from the above that the Doppler shift signal measured by the flowmeter only reflects the fluid velocity in the area of the information window. Therefore, it is required that the information window should be located near the average velocity in the pipe channel, so that its measured value can reflect the average velocity of the fluid in the pipe channel

according to the skeleton standard "general provisions for acceptance, packaging, marking and quality certificate of section steel", figure 4 Schematic diagram of Doppler information window

3 Classification of ultrasonic flowmeter

3.1 according to the structural type of ultrasonic channel, it can be divided into mono channel and multi-channel ultrasonic flowmeter

mono channel ultrasonic flowmeter is to install a pair of transducers on the tested pipeline or channel to form an ultrasonic channel, The most widely used transducers are external clip type and plug-in type. Mono channel ultrasonic flowmeter is simple in structure and easy to use, but this flowmeter has poor adaptability to the change of flow pattern distribution, and the measurement accuracy is not easy to control. It is generally used in small and medium-sized pipelines and channels that do not require high measurement accuracy. Multi channel ultrasonic is to install multiple pairs of ultrasonic transducers on the tested pipeline or channel to form multiple ultrasonic channels, and calculate the flow by integrating the measurement results of each channel. Compared with the mono channel ultrasonic flowmeter, the multi-channel flowmeter has strong adaptability to the change of flow pattern distribution and high measurement accuracy. It can be used in large-diameter pipelines and pipelines with complex flow pattern distribution

3.2 according to the different applicable flow channels of ultrasonic flowmeter, it can be divided into pipe flowmeter, pipe channel flowmeter and river flowmeter

pipe flowmeter generally refers to the flowmeter used for pressure pipelines, which also includes culverts with various shapes and sections of pressure. This flowmeter generally measures the flow rate in the fluid through one or more sound channels, and then obtains the flow. In addition to the transducer for measuring the flow rate, the ultrasonic flowmeter used in the pipe channel also needs the transducer for measuring the water level, and the flow can be obtained according to the measured flow rate and water level. The flowmeter used for pipe channel generally contains multiple velocity transducers (determined by the number of channels) and one water level transducer. Most river ultrasonic flow meters only measure the flow rate and water level, while the river discharge is calculated by the user according to the riverbed section

4 application research

combined with the research content of national large-scale irrigation area information construction, the author has set up four monitoring stations on the north main channel of Gaoya reservoir irrigation area in Changle County: xujiamiao monitoring station has a channel bottom width of 7.0m and a water depth of 1.0 ~ 2.0m, which is monitored by 5-channel open channel ultrasonic flowmeter, as shown in Figure 5. Shanqin monitoring station transformed a section of open channel into a pressurized pipeline for water transmission, with a pipe diameter of 1.4m, and monitored by a mono channel pipeline ultrasonic flowmeter, as shown in Figure 6. Six stations are arranged on the main canal of Rizhao reservoir irrigation area, of which Shizui monitoring station is 4m wide and 1.5 ~ 2m deep. Doppler ultrasonic flowmeter is used for monitoring, as shown in Figure 7

Figure 7 Schematic diagram of Doppler ultrasonic flowmeter at Shizui measuring station

high precision flow meter is used at each measuring station to compare and analyze the measured instantaneous flow. Through comparison and individual parameter revision, the instantaneous flow measured by each station is stable and reliable, which is highly consistent with the data measured by the current meter

5 Conclusion

ultrasonic flow measurement technology has been paid more and more attention because of its high measurement accuracy and good real-time performance. However, due to its high price, strong professionalism and high maintenance and management requirements, its application and promotion are slow. With the increase of national investment in water conservancy and the construction of a water-saving society, this technical equipment will soon become the main means of flow measurement and be widely used

references

[1] Ma Shusheng Contents and methods of wireless data transmission real-time monitoring of water regime in irrigation areas [J] Journal of Shandong Agricultural University, 2003, (1):

[2 plastic materials with lower dielectric consumption (it is necessary to ensure that the physical properties often used are not lower than those of current materials)] Yang Feng, Yang Junqing Introduction to Doppler flow measurement technology [J] Hydrology, 2004, (2):

[3] Liao Zhimin, Xiong Shan Research and application of ultrasonic flowmeter [J] Pipeline design and equipment, 2004, (4):

[4] Lei Yan, fan Xiuhua Mathematical model and calculation method of multi-channel ultrasonic flow measurement in trapezoidal channel [J] Journal of Wuhan University of water resources and hydropower, 1997, (1): (end)