1. The basic principle

Doppler ultrasonic flow meter measuring the other is the Doppler effect in physics-based. According to acoustic Doppler effect, when the sound source and the relative motion between the observers, the observers felt by the frequency of the sound will be different from the sound source issued by the frequency. This relative movement and because of the frequency change and the two objects is directly proportional to the relative velocity.

In Doppler ultrasonic flow measurement methods, ultrasonic transmitter for a fixed sound source, together with the fluid movement of solid particles from the sound source with the relative movement of "observer" role, of course, it is only the incident to the solid Ultrasonic reflection on the particles to receive it. Launching and receiving sound frequency difference between the sound wave, is due to solid particles in the fluid movement and less production of acoustic Doppler frequency shift. As a result of this poor frequency proportional to the fluid flow rate, measured frequency Poor velocity can be obtained. Then can be fluid flow.

Therefore, the Doppler ultrasonic flow measurement of a necessary condition: the measured fluid medium should be able to contain a certain amount of reflective acoustic solid particles or bubbles, and so the two-phase medium. The working conditions in fact, it's a big advantage, This flow measurement that is a suitable method of measurement in the two-phase, which is Flowmeter other difficult issues. Therefore, as a very promising method of measuring the flow and flow meter, Doppler ultrasonic flow measurement methods Is being increasingly applied.

2. Flow equation

Assumptions, ultrasound beam and fluid velocity to the angle q, ultrasonic transmission speed c, the velocity of fluid in the RSP and the same fluid flow, are now u. ultrasonic beam in a reflection on the solid particles as an example. Export acoustic Doppler velocity and the poor relationship.

As shown in Figure 3-39, when the ultrasonic beam in the axis of having a solid particles, the particles to speed u camps along the axis of movement. Ultrasonic transmitter on the case, the particles to u cos a rate of departure, Particle received ultrasonic frequency f2 should be lower than the launch of the ultrasonic frequency f1, to reduce the numerical

f1DDThe launch of ultrasonic frequency;

aDDUltrasonic beam and the angle of the axis;

cDDFluid in the speed of sound.

Solid particles will ultrasonic beam scattering to the receiver,Because it to u cos a The speed to leave the receiver, the receiver before the ultrasonic frequency f3 lower again, f2 similar to the calculation, f3 can be expressed as

Receiver before the launch of ultrasonic frequency and the difference between the ultrasonic frequency, Doppler frequency shift q f1, may be under-calculation:

Where, A channel for the flow of the measured cross-sectional area. Flows from the above equation tells us that when the flow meter, pipeline conditions and tested medium identified, the Doppler frequency shift is proportional to the volume of traffic, measured frequency shift can be fluid qf Traffic qv.

5. Equation on the flow of the discussion points

(1) fluid medium temperature on the measurement of the impact

This shows that flow from the equation, although the flow measured by the speed of sound c fluid in the impact of general practice, the fluid medium in the speed of sound and the temperature, composition, and so on, it is very difficult to maintain constant. In order to avoid the media temperature measurements, group The impact of the changes, Doppler ultrasonic flowmeter general use of the wedge, sound structure, the first ultrasound beam through the wall of sound wedge and re-entering the fluid.Acoustic materials located in the wedge for the speed of sound is c1;Fluid in the speed of sound is c;Acoustic sound wedge into the fluid from the angle of q ;Ultrasonic beam and fluid flow for the angle is aAs shown in Figure 3-40,According to refraction theorem, which for the foreseeable, a wedge of sound, the traffic and frequency shift in the relationship only with acoustic material wedge in the speed of sound c1 with the fluid medium in the speed of sound c nothing to do.C1 velocity and temperature changes than the fluid in the speed of sound c with temperature changes in a number of very small and has nothing to do with the fluid component. Therefore, the use of appropriate materials sound wedge, can substantially improve the flow measurement accuracy.

(2) the information window and the average Doppler frequency shift

To effectively receive Doppler shift signal, Doppler ultrasonic transducer flowmeter of the commonly used transceiver integrated structure, as shown in Figure 3-41. Visible from the map, the transducer to receive signals only the reflection Is launching the chip and receive chips at the two overlapping beams of particles within the region of the reflected waves, the region known as the Doppler signal overlap of information window

Figure 3-40 wedge and the acoustic sound of refraction

Flowmeter receiving transducer signal is received by the information from all mobile suspended particulates in the window of the reflected wave stack, that is, its information within the window for the Doppler frequency shift superimposed on average. The average number

From the above discussion indicates that the flow meter measured the Doppler shift signals reflect only the information window within the region velocity, so the information required within the window should be located close to the average velocity of the region, in order to reflect the measurement of Fluid

The average flow rate.