The ultrasonic Doppler flow meter measuring principle Principle
Published on：2015-1-7 9:24:27 Hits：
1. The basic operating principle
Measurement of ultrasonic Doppler flow meter is not original physics-based Doppler effect. According to the acoustic Doppler effect, when there is relative motion between the sound source and the observer, the observer felt different from the frequency of the sound frequency emitted from the sound source. The frequency change due to relative motion of the two objects is proportional to the relative velocity.
In the ultrasonic Doppler flow measurement methods, ultrasonic transmitter for a fixed sound source, the solid particles together with the fluid movement of the sound source from the relative motion of the "observer" role, of course, it is only the incident to the solid ultrasonic wave reflected back to the receiver on the particles according to. Transmitting acoustic sound waves and the frequency difference between the received, it is because the movement of solid particles in the fluid produced little acoustic Doppler shift. Due to this frequency difference is proportional to the fluid flow, so measuring the flow rate of the frequency difference can be obtained. Then you can get the flow of the fluid.
Thus, a necessary condition is that the ultrasonic Doppler flow measurement: measured fluid medium should contain a certain amount of solid particles capable of reflecting sound waves or bubbles of two-phase medium. The working conditions actually it's a big advantage that this method is suitable for measuring the flow measurement of two-phase flow, it is difficult to solve the problem of other flowmeters. Therefore, as a very promising method for two-phase flow measurement and flow meter, ultrasonic Doppler flow measurement methods currently being increasingly applied.
2. Flow equation
Suppose, the angle between the ultrasonic beam and the fluid velocity is q, the ultrasonic propagation velocity is c, the fluid velocity of the fluid flow rate of suspended particulates same, are u. In the ultrasonic beams are reflected on a solid particle, for example, the difference between the derived acoustic Doppler velocity relationship.
As shown in Figure 3-39, when the ultrasonic beam encounters a grain of solid particles on the tube axis, the particle velocity u to camp along the axis of movement. Ultrasonic transmitters, the order of the particle velocity u cos a departure, so the particles should be received by the ultrasonic frequency f2 lower than the frequency f1 of the transmitted ultrasonic wave, a reduced value of
f1-- emit ultrasonic frequency;
a-- ultrasonic beam and the tube axis angle;
c-- fluid speed of sound.
Solids turn ultrasonic beam scattering to the receiver, since it u cos a speed leaving the receiver, so the receiver receives the ultrasonic frequency f3 and again lowered, the calculation similar to f2, f3 can be expressed as
The frequency difference between the received ultrasonic waves and receives ultrasonic waves frequency, i.e., the Doppler shift q f1, can be calculated:
Formula, A is the cross-sectional area of the flow test pipeline. The above flow equation shows that, when after the flowmeter, condition of the pipeline and determining the measured medium, the Doppler frequency shift is proportional to the volume flow rate, measuring the frequency shift can be obtained fluid flow qf qv.
5. Discussions about the flow equation
(1) Effect of temperature on the measurement of a fluid medium
Seen by the flow rate equation, the fluid flow though the impact of the measurement results by the sound velocity c. In general, the speed of sound in the fluid and the medium temperature, and other relevant components, is difficult to keep constant. To avoid the influence of the medium temperature measurements, component changes by the ultrasonic Doppler flow meter commonly used tube sound wedge structure, so that the ultrasonic beam and the wall first and then by the sound of the wedge into the fluid. Set the speed of sound in the sound of the wedge material is c1; fluid sound velocity is c; sound wave into the fluid by the acoustic wedge angle of incidence q; angle of refraction in the fluid as q; the angle between the ultrasound beam and the fluid flow rate for a; Figure 3 -40, according refraction theorem, whereby type visible, after the use of sound wedge structure, the relationship between flow rate and the frequency shift of formula contains only the acoustic sound velocity c1 wedge material regardless of the speed of sound in the fluid medium, c. Rather than the speed of sound in the fluid temperature changes c1 c speed of sound varies with the temperature of a number of extremely small, and independent of the fluid component. Therefore, the use of appropriate acoustic wedge material, can greatly improve the accuracy of flow measurement.
(2) the message window and the average Doppler shift
To effectively receive the Doppler shift signal, an ultrasonic Doppler flowmeter transducers usually transceiver structure, as shown in Figure 3-41. Seen from the figure, the transducer receives the reflected wave reflected signal only particles two directional beams overlap region transmit and receive the wafer within the wafer, the overlap region is called the Doppler signal information window
Figure 3-40 wedge and acoustic sound refraction
Flowmeter receiving transducer signal is received by the information window all mobile particulates reflected wave superposition, that is, its information window Doppler frequency shift is superimposed average. Average more
From the foregoing discussion, the flow meter measured Doppler shift signal reflects only information on fluid velocity within the window area, the window should be located so the request for information on the tube close to the average velocity of the region, in order to make measurements that reflect the inner tube fluid
The average flow rate.
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