SU1516985A1 - Method of measuring radial component of velocity vector of rotating flow in cylindrical duct - Google Patents

Abstract

The invention relates to a measurement technique and can be used for laser doppler anemometry studies of the dynamics of a rotating flow in pipes, cyclones, dryers and other technological devices with a cylindrical wall shape. The purpose of the invention is to improve the accuracy of measuring the radial component of the velocity vector under the conditions of refraction of probing beams on the cylindrical wall of the channel by ensuring control of the radial position of the measurement point and ensuring the radial orientation of the sensitivity vector of the laser Doppler anemometer / LDA /. Two probe LDA beams lying in the plane of the cross section of the cylindrical channel direct the following auxiliary operations to measure the speed to a predetermined area inside the channel and perform the tuning process in the presence of a rotating flow. A third auxiliary beam, coherent to two probe beams, is formed, directing it into a cylindrical channel along the radius of the section under study. By relative movement of the optical system, the LDA and the channel combine the region of intersection of the probe beams with the axis of the third beam, fixing the alignment position of the appearance of additional spectral components in the spectrum of the LDA signal, closing the third beam, turning the optical system of the LDA relative to the fixed position of the measuring point, achieve a position where the Doppler frequency shift is minimal. By fixing thus the radial orientation of the sensitivity vector, the radial component of the velocity vector is measured using an LDA. Il 3

Description

The invention relates to a measurement technique and can be used for studies of hydro and aerodynamics, flows in pipes, cyclones, dryers in the chemical and other sectors of the industry, where optional.

It is necessary to carry out measurements of the velocity vector of a rotating flow in a cylindrical channel, in which measurements of its radial component are a considerable difficulty.

The aim of the invention is to increase the accuracy of measuring the radial component of the velocity vector D of refraction conditions of the LDA probe beams on the cylindrical wall of the channel by ensuring the control of the radial position of the measuring point and ensuring the radial orientation of the sensitivity vector.

Fig. 1 shows the displacement} Ile of the intersection point of the probing beams of a laser Doppler anemometer (LDA) during radial scanning of the cylindrical channel under study (OL - trajectory of the beam intersection point in the absence of refraction on the model walls, OA - in the presence of refraction, K - sensitivity vector LDA, whose direction corresponds to the measured velocity projection); FIG. 2 is an example of a device that implements the proposed method, side view} in FIG. 3 - the same, top view.

The device contains a laser 1, an LDA 2 optical unit with an integrated photodetector (LP) 3 for operation in the backscattering mode, rigidly fixed on an optical bench 4 fixed by a coordinate device including a guide 5 that can be moved relative to the response guide 6. The swivel device attached to the guide 6 includes four precision rollers 7, two of which are pressed against the surface formed. the inner radius R. part of the ring bench 8, and the other two - to the surface formed by the outer radius R of the bench 8; the latter is mounted on an optical table 9 equipped with a two-coordinate device, the degrees of freedom of which are marked in FIG. The two probe beams 10 intersect in the cylindrical channel 11 under study. The divider 12 and the mirror 13 serve to form the third laser beam. For LDA operation in direct scattering mode, the FP 14 serves, the Electronic Processing Unit 15 (processor) of the Doppler signal is connected to the FP output, and the output to the spectrum analyzer 16.

The method is carried out as follows.

15, the examined cross section of the cylindrical channel is directed by two probe beams 1 LDA 2, located in the section plane (Fig. 2). To study the radial profile of the radial component of the velocity vector Vj, (d), the starting point of the measurement is set at the center of the cylindrical channel (point P), LDA in relation to the channel (or vice versa) in the radial direction OA, new measurement point positions are established. However, due to the refraction of the nucci on the cylindrical walls, the measurement point is shifted by the OA curve, and at each new point as a result of the LDA K sensitivity vector reversal (orthogonal to the bisector of the beam convergence angle and lying in their plane) the angle y to the radius direction at this point not the true radial component v is measured, but the displaced velocity vector component

V

VM3 v. cos -I- v sin, (1)

where is the tangential component of the velocity vector,

In swirling threads v. v, which introduces a significant error in the measurement of radial velocity.

According to the measurement method, in order to control the radial position of the measurement point and the radial orientation of the sensitivity vector K, a third laser beam, coherent probing, is formed and guided in the section plane along the OA radius, the peridicular main optical axis, operating in the forward HJIH backscatter mode, and the signal processing equipment 15 register the spectrum of the Doppler signal on the spectrum analyzer 16. The point of intersection of the axes of the probe beams is set at the measurement point by moving the optical part of the LDA in two directions: along the straight, 1: parallel axis radius, and normal to it in the section plane of the channel. In the presence of a rotating stream, changes in the recorded spectrum are monitored. Appearance in it

spectral components (for example, primary beams 10 symmetrical with respect to the Doppler signal frequency or identified by alternately closing each probe beam, caused by the fact that each of the probe beams interferes with the third beam, indicates that the measurement point is on the radius under study To set the correct orientation K rotates the optical part of the LDA relative to the measurement point in the plane of the beams until the minimum Doppler frequency shift is reached - a result interference of probe beams. The third beam is preliminarily intersected; according to (1), the measured velocity v ,, is minimal

and 3 ll

when the value of the error angle is Y 0, since vo- 7 v. Accordingly, the Doppler frequency shift, proportional to the value of Vj, also takes the minimum value. The fixer is the position of the LDA and overlaps the auxiliary third beam; the radial velocity component is measured by any of the known Doppler signal conversion methods, such as spectral.

During the formation of the third beam, it is necessary to observe the conditions of the temporal coherence of the radiation of the LDA beams and the third auxiliary beam, which is achieved by choosing the permissible optical path difference. You can also use the third auxiliary beam to form

9856

the second laser, the radiation of which is coherent with the radiation of the first laser.

Claims (1)

Invention Formula A method for measuring the radial component of the velocity vector of a rotating flow in a cylindrical channel d consisting in the fact that two intersecting probe beams of the laser Doppler anemometer optical system are directed into the flow under test, combine the intersection point 5 axis probe beams with a dot measurements of the radial component of the velocity vector, determine the Doppler frequency shift of the laser Doppler anemometer signal, which determines the value of the radial component of the velocity vector, characterized in that, in order to improve the measurement accuracy, a third laser beam is formed, coherent 25 to two probe beams, directing it into a cylindrical channel along the radius of the cross section under study, aligning the point of intersection of the axes of the probe beams with the axis of the third beam, with 30, the alignment position is fixed by the presence of additional spectral components of the Doppler signal, the optical system is rotated in the plane of the probe beams around the axis passing through the measurement point of the radial velocity component to the position corresponding to the minimum value of the Doppler signal frequency, .Q is left to measure. 35 I t, 1t ft A "r it FIG. 2 Degree of freedom . New Noah Degree of freedom onmuvecKOto backgrounds