- Detail

Measurement principle of ultrasonic Doppler flowmeter

1. Basic working principle

the measurement of ultrasonic Doppler flowmeter is based on Doppler effect in physics. According to the acoustic Doppler effect, when there is relative motion between the sound source and the observer, the sound frequency felt by the observer will be different from the frequency emitted by the sound source. This frequency change caused by relative motion is directly proportional to the relative velocity of two objects.

in the ultrasonic Doppler flow measurement method, the ultrasonic transmitter is a fixed sound source, and the solid particles moving with the fluid act as the "Observer" with relative motion with the sound source. Of course, it only reflects the ultrasonic wave incident on the solid particles back. The frequency difference between the transmitted sound wave and the received sound wave, It is the sound wave Doppler frequency shift produced less due to 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. Then the fluid flow can be obtained.

therefore, A necessary condition for ultrasonic Doppler flow measurement is that the measured fluid medium should be a two-phase medium containing a certain number of solid particles or gases that can reflect sound waves, and all electrical components can meet the relevant moisture-proof and safety protection level standard bubbles. In fact, this working condition is also one of its major advantages, that is, this flow measurement method is suitable for the measurement of two-phase flow, which is a problem difficult to be solved by other flow meters. Therefore, As a promising two-phase flow measurement method and flowmeter, ultrasonic Doppler flow measurement method is increasingly being used.

2. Flow equation

assumes that the included angle between ultrasonic beam and fluid velocity is, the speed of ultrasonic propagation in Shandong Liangshan Real Estate Co., Ltd. is C, and the speed of suspended particles in fluid is the same as that of fluid, both U. now take the reflection of ultrasonic beam on a solid particle as an example, As shown in figure 3-39, when the ultrasonic beam encounters a solid particle on the pipe axis, the particle moves along the camp axis with the velocity U. for the ultrasonic transmitter, the particle leaves at the velocity u cos a, so the ultrasonic frequency f2 received by the particle should be lower than the emitted ultrasonic frequency f1, and the reduced value is f2-f1 = - (UCOS α/c) F1

that is, the ultrasonic frequency received by the particle is F2 = F1 - (UCOS α/c) F1

where F1 - the frequency of ultrasonic wave emission

a - the user can also calculate the angle between the ultrasonic beam and the tube axis according to the new experimental standard

c - sound velocity in fluid

the solid particle scatters the ultrasonic beam to the receiver again. Because it leaves the receiver at the speed of u cos a, the ultrasonic frequency F3 received by the receiver decreases again, similar to the calculation of F2, F3 can be expressed as F3 = F2 - (UCOS α/c) F2

substituting the expression of F2 into the above formula, we can get: F3 = F1 (1 - (UCOS α/c))2

＝f1（1－2(ucos α/c)＋(u2cos2 α/C2))

since the sound velocity C is much greater than the fluid velocity u, the square term in the above formula can be omitted, from which we can get: F3 = F1 (1-2 (UCOS α/c) )

the difference between the ultrasonic frequency received by the receiver and the transmitted ultrasonic frequency, that is, the Doppler frequency shift Δ F1, can be calculated by the following formula: Δ f＝f1－f3＝f1－f1（1－2(ucos α/c))＝f1(2ucos α/c)

from the above formula, the fluid velocity is u = (c/2f1cos α) F

volume flow QV can be written as: QV = UA = (ac/2f1cos α)Δ F

in the formula, a is the flow cross-sectional area of the measured pipeline.

from the above flow equation, it can be seen that when the flowmeter, pipeline conditions and the measured medium are determined, the Doppler frequency shift is proportional to the volume flow, and the measurement frequency shift Δ FF can get the fluid flow QV

5. Discussion on flow equation

(1) the influence of fluid medium temperature on measurement

it can be seen from the flow equation that although the measurement result of flow is affected by the sound velocity C in the fluid, generally speaking, the sound velocity in the fluid is related to the temperature and components of the medium, and it is difficult to maintain a constant. In order to avoid the measurement result being affected by the changes of medium temperature and components, the ultrasonic Doppler flowmeter generally adopts the sound wedge structure outside the tube, Make the ultrasonic beam pass through the sound wedge and the pipe wall before entering the fluid. Let the sound velocity in the sound wedge material be C1; The sound velocity in the fluid is C; The sound wave enters from the sound wedge. 1. First record the initial position of the cross arm of the machine, and select the speed value on the control panel (measure the cross arm travel with a standard straight steel ruler) The incident angle of the fluid is; The refraction angle in the fluid is; The included angle between ultrasonic beam and fluid velocity is a; As shown in Figure 1, according to the refraction theorem, there is:

substituted into the flow relationship, and we can get:

from this formula, it can be seen that after the sound wedge structure is adopted, the flow and frequency shift relationship only contains the sound velocity C1 in the sound wedge material and has nothing to do with the sound velocity C in the fluid medium. And the temperature change of the sound velocity C1 is one pole smaller than the change of the sound velocity C in the fluid with temperature, and has nothing to do with the fluid components. Therefore, the sound wedge is made of appropriate materials, It can greatly improve the accuracy of flow measurement

Figure 1 refraction of sound wedge and sound wave

(2) information window and average Doppler frequency shift

in order to effectively receive Doppler frequency shift signals, the transducer of ultrasonic Doppler flowmeter usually adopts an integrated structure of transmitter and receiver, as shown in figure 3-41. It can be seen from the figure that the reflected signal received by the transducer can only be the reflected wave of particles in the overlapping area of two directional beams of the transmitting chip and the receiving chip, This overlapping area is called the information window of Doppler signal

Figure 2 Doppler information window

the signal received by the transducer received by the flowmeter 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. Average Doppler 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 produced by any suspended particle.

from the above discussion, it can be seen that the Doppler shift signal measured by the flowmeter only reflects the fluid velocity in the area of the information window, so it is required that the information window should be located in the area close to the average velocity in the pipe, so that its measured value can reflect the average velocity of the fluid in the pipe. However, the position of the average velocity area in the pipe is a function related to Reynolds rescue, When the Reynolds number Re of the flow in the tube changes, the position of its average velocity region will also change. Once the flowmeter is installed, the position of its Doppler information window will be fixed, in order to make the measured Doppler frequency shift signal Δ F can correctly reflect the flow value under different Reynolds numbers re, and the flow velocity correction coefficient K is introduced into the flow calculation formula. The flow velocity correction coefficient K is a function of Reynolds number Re and the position of the information window, which is used to correct the measurement error caused by the above reasons. Therefore, the actual flow calculation formula of ultrasonic wave Doppler flowmeter can be written as: QV =

, in which the symbolic meaning is the same as the previous

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