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Acoustics means of liquid flow measurement

Prof. S. Shkundin

Moscow State Mining University

Laboratory estimation has shown that on the one hand acoustical methods in flow measurements have not realised their potential possibilities, and on the other hand none of the existing means of measurement of the flow allows to create flowmeter, which should be able:

- to measure speeds of flow in the range of 0.05 ... 30 m/s;

- not to add hydrodynamic resistance into the controlled flow,

- not to disturb hydrodynamic curve;

- to measure average flow speed in the cross section;

- to have stable characteristics, allowing the decrease of error.

The suggested measurement method [1] satisfies all of the above-mentioned demands. The method of measurement based on liquid-acoustic interaction involves vibrations excitation in the cylindrical wave-guide liquid duct, their reception at some distance from the excitation point and a comparative analysis of radiated and received vibrations as a result of which an informative signal (e.g., vibration phase difference) is singled out, which serves as a flow rate measure. It differs from others known in that waves are being radiated and received by excitement of the liquid conduit elements, acoustically isolated from each other. This method gives accuracy and provides exception of liquid conduit effect on the hydrodynamic field of the flow.

The description of wave propagation process in the tubes without flow was suggested by E.Scuchic [1]. From this description we made the conclusion which waves could spread in the round channel of the given diameter. The spread rate of wave fronts of these vibrations is equal to one of sound velocity in open space with the same medium. In the measurement channel the sound speed in the direction of the waveguide axle is expressed:

C o = C = 2 p f / k x

where : k x - wave number for propagation to

the waveguide axle direction;

f - frequency of the radiated vibrations.

For the analytic description of the liquid-acoustic interaction, which is the base of acoustical flowmetry, it is necessary to solve the boundary problem for the equation with partial derivatives. For the first approach the flowmeter channel may be performed of unlimited length.

The means of the correction determination, corresponding to the reflected waves field with the help of normal modes reflection coefficient calculation is offered. The means named is based on the sewing together method.

The analytic dependencies of the velocities upon the dimensions, channel walls material and liquid medium characteristics have been got using G. Gohnson's and K. Ogimoto's work [3].

Acoustic integrating flowmeters

Acoustic integrating flowmeters based on ultrasonic wave propagation through a tunnel cross-section are free from all the deficiencies referred to earlier. The working acoustic beam accumulates information about vectors of longitudinal velocities of the flow, which forms the hydrodynamic curve, and as such is an integrating flowmeter. Such flowmeter is able to determine the average velocity of the stream in cross-section with consistancy. The principle of acoustic integrating flowmetry is based on the use of two pairs of electric-acoustic converters. One of them works streamwise and the other in the opposite direction. In other words, acoustic signals used by the first pair of converters are accelerated by the stream and signals used by the second pair are decelerated by the stream.

The value of this deceleration or acceleration is a measure of the controlled average velocity. Particular devices may consist of only two converters, each of them taking turns in working as an emitter and as a receiver.

Such a system based on two pairs of converters allows a reduction in error, concerned with variation of sound velocity.

Figure 1 shows the location of the emitter, the receiver and the acoustic ray path in moving (2,3,4) stream. Kinematics of a moving material point will be determined by two forces: pressure in moving liquid flow and the excess pressure in an acoustic wave.


Fig. 1. Acoustic rays propagation in the hydrodynamic field.

where : L the length of through sounded base

V(r) the liquid velocity in plan cross section.

1,2 electroacoustic transformers;

h cross dimensions

Fig. 2. Frequency-impulse flowmeter function scheme based on synchro-ring principle.


There is the ability to measure integral vector characteristics of currencies by measurement of three components of this vector, fig. 3.

Fig. 3. Three component vector measurement of the currency.

References.

Puckov L. Shkundin S. oth. : The method of measurement of air-gas flow velocity. Author's certificate ?16822590, 1991.

E. Scuchic. Basic acoustics. Moscow 1976, vol.2, pages 112-116.

G. Gohnoson and K. Ogimoto. Sound radiation from a finite length unfledged circular duct with uniform axial flow. Acoustic Society of America papers. 68, 1980.

 
 
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