GB2048602A - Measuring flow velocity - Google Patents

Abstract

The rate of flow of a strata 19 of a liquid seeded with extremely small gold particles is measured by illuminating the strata by a laser 1, by imaging reflections from the particles within the strata onto a diffraction grating 11, and by autocorrelating the photoelectrically 13 sensed output from the grating, and the system is characterised by the presence of high pass filters 21, and in that the optical imaging arrangement has a very small depth of field just deep enough to encompass the strata 19, whereby light originating from particles outside the strata 19 and from other reflectors outside the strata 19, such as boundary surfaces, is prevented reaching the grating. <IMAGE>

Description

SPECIFICATION Method and apparatus for measuring particle flow in a fluid The present invention relates to a method and an apparatus for the measurement of the flow rate of particles in a path in liquid or gas flows, such measurements being effected relative to a reference position in a contact-less manner. The particles are illuminated by means of some form of radiation and the radiation reflected or scattered thereby evaluated.

The device for carrying out the method operates with a high lateral and axial resolution. This makes possible micro-analyses in one, two orthree coordinate directions of velocity distribution and guide motion. The invention particularly relates to a method and apparatus suitable for the microanalysis of flows of liquids and gases. One type of flow in which the present invention may be useful is the establishment of hydrodynamic causes of microthromboses on the walls of plastics material arteries or prostheses. More generally, the present invention provides the possibility of studying interaction mechanisms at the boundary surface between a solid wall and a fluid flowing therepast.

Optical velocity-measuring apparatuses are already known in which the movement of an object relative to a photoelectric receiver produces measuring values which are dependent upon the relative speed of movement of the object. In such apparatus, the object whose velocity is to be measured is radiated with coherent monochromatic radiation, and the light scattered by the object is trapped. The frequency of the light is affected by Doppler displacement and the frequency of the scattered light is measured by interference with the primary radiation or with the frequency shifted light in the opposite direction. In view ofthe number of lasers and interferometers required, such apparatus is usually extremely costly.

Alternatively, the moving object may be depicted in an image plane, in which at least two photoelectric scanners offset with respect to the direction of movement are located. The signal produced by the first scanner, that is to say, the scanner lying first in the direction of movement of the object, is stored for a pre-determined time T in a short-term storage system and is then compared with the signal supplied by the second scanner spaced from the first scanner by a distance c in a correlator. Thus, the storage time T is controlled so that the signal from the second scanner coincides in time with the delayed signal from the first scanner.This gives the velocity of the object Vo relative to the scanners, utilising the image scale, as v'O= Tc This known method necessitates the use of a controllable storage system and a correlator which are breakdown-prone and only supplies an average speed ascertained over the storage time T. Finally, the displaced object may be depicted on a grating with the line numberk per mm. Behind this, a photoreceiver absorbs the light arriving from the object, and preferably in the presence of a particular local frequency in the intensity distribution of the object, generates an alternating voltage, the frequencyfof which is proportional to the speed of the object image v relative to the grating and to k.Utilising f=v k This signal is superimposed with local frequencies which do not correspond to K and with longer period signals (constant light) corresponding to the integral over the image proportions. To suppress this constant light proportion, an apparatus is known which has a photo receiver pair having telescoped strip-like electrodes which supplies a fixed contact signal with local frequency corresponding to k only from image proportions and in which the in phase proportions of other signals are separated by a difference-forming bridge circuit. These photo receivers, due to their difficult geometry, are costly and since they cannot be produced with sufficient precision, the number of strips is limited. However, the accuracy of measurement of the system is dependent upon the number of strips.

Finally, there is also known an apparatus for compensation of the apparent movement of an object during the exposure time in aerial photography in which, by means of a roof edge or pyramid screen and a photoelectric receiver connected in series therewith control signals are produced which either reset the film in the camera or reset the optical system such that an object to be depicted during the exposure time is depicted constantly at the same point on the film. The prior art device does not provide any directional information, so that its use is limited to a predetermined direction of movement.

A method is also known in which the object is illuminated stroboscopically and the movements of existing and added diffusion particles are simultaneously recorded by means of a two-dimensional detector in the image plane. The trace length in one illumination interval is proportional to the velocity components in the image plane.

None of these known methods permits the analysis of a velocity field having a lateral and axial resolution smaller than 50,am.

The present invention seeks to produce an image in such a manner that the velocity in the image plane is, in fact, measurable with known methods,this velocity and direction of movement in the image plane, however, being linearly linked with the velocity and direction of movement in the object plane and the resolution region is detected from 1 ,um to 50 ,um.

According to the present invention, there is provided a method for the contactless measurement of the velocity or the path of a layer of particles in liquid or gas flows relative to a reference position, wherein the particles are illuminated by means of a radiation, which radiation is reflected by the particles and the diffused or scattered radiation is evaluated, wherein the images ofthe radiation source superposed on the beam reflected or diffused by the particles are filtered out, the layer to be measured being located in the focal plane of an objective lens, depicting the layer to be measured in an evaluation having a min imal depth of focus, and providing diffusing or scat tering particles in the liquid or gas flow to act as a tracer.

Also according to the present invention, there is provided an apparatus for carrying out such a method comprising a source of radiation, an objective lens which focuses the radiation beam in the focal plane of a liquid or gas flow so as to illuminate the particles therein, which particles have tracer particles added thereto, said objective lens producing an image from said particles through reflected or diffused radiation in an intermediate image plane, a further lens providing a light source image of each light source reflection, high-pass filters for filtering such light source images, a third lens for depicting the intermediate image plane on a grating and a field lens for transmitting the radiation passing through the grating plane to a detector.

By utilising the method of the present invention, the constant light background can be reduced to a minimum to keep the requirements of the dynamics ofthe detecting system as low as possible. The invention thus provides an illumination technique with a highly resolving image producing method and the use of sub-microscopic highly reflecting particles.

The illumination technique has effect of masking outthe light source images in the measuring light by spatial filtering. This makes it possible to display submicroscopic particles having a size of less than 0.1 ,am, which may be added to the fluid as highly diffusing or scattering tracers. This measure, moreover, leads to a drastic suppression of the constant light proportion, so that the evaluation of the modulated light signal is simplified.

By using an objective lens of high aperture and accordingly intensive subsequent magnification, the depth of focus of the system is considerably reduced. By this means, there may be attained an axial resolution up to 0.5 ,um with a minimum measurement area of 5 ,tzm by 20 ,am.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a schematic view of an apparatus in accordance with the present invention and Figs. 2 to 4 show different results obtained utilising the method of the present invention, which results will be detailed hereinafter.

As can be seen from Fig. 1, an illuminating beam 1 is imaged into an optical axis 16. In this particular embodiment, a weakly convergent beam of a HeNe laser is used. On one side of the mirror, an objective lens 3 is coupled to a cover glass 4 by means of oil immersion. Behind the glass 4, that is to say, remote from the mirror 2, a cell 5 for measuring the liquid or gas flow 22 is provided. The objective lens 3 produces a virtual image point source between the object plane and the objective front lens.The object, for example, a liquid layer 19 which is located in the focal plane, is therefore illuminated with a divergent light beam 1 The liquid 22 contains highly reflective, preferably spherical, particles 20, the dimensions of which are small compared with the vertical and axial resolution required ofthetechnique.

For use in a region of high resolution (of an order of magnitude of ym) gold particles are particularly suitable for use because they have a high reflectance and they can be produced in an extremely uniform size. From these particles 20, by means of a light 16' diffused or scattered in the direction of the objective 3, an enlarged image in the intermediate image plane 6 is formed by the objective lens 3.

The light distribution in this plane 6 comprises: 1) small intensive light spots originating from the scattering or diffusion particles 20 in the liquid layer 19 being illuminated; 2) larger and less intensive light spots which originate from the diffusion or scattering particles outside the focal plane 19 and 3) more or less homogeneous light distribution which originate from lens and other boundary surface reflections as well as reflections from dust particles.

Alens7 located in the reflected beam 16' behind the intermediate image plane 6, that is to say, on the side ofthe plane 6 remote from the cell 5, produces a light source image for each reflected beam. These images may be formed in any one of a number of different planes, 8', 8", 8"', generally referenced 8.

These are blocked off by suitable high pass filters 21.

Adjustment and observation of the system are effected by providing a further beam splitter 9 located on the side of the planes 8 remote from the lens 7. The beam splitter 9 produces a lateral beam 16" . A displaceable lens 14 is located in the beam path 16" so that the intermediate image plane 6 or the filter planes 8 can be seen on an observation screen 15. The main branch 16' of the beam passing through the splitter 9 is focused by means of a lens 10 so that the intermediate image plane 6 is depicted in a plane 11. The degree of magnification is thus so selected that the depth of focus in this plane is small compared with the required axial resolution.

In this plane 11, light originating from reflections are substantially completely elimated by the high pass filter 21. A grating is located in the plane 11. The grating constant is at least twice as large as the particle images in this plane. The light 18" passing through the plane 11 is transmitted, by means of a field or condensing lens 12, to a photodetector 13. Because of the conditions of image forming, each particle of the liquid layer 19, through the grid 11, produces an intensity modulated signal, the frequency (f) of which is linearly linked to the velocity component V5 parallel to the layer 19 and perpendicularly to the grid lines in accordance with the equation f ~ M V5 d in which M represents the overall magnification and d the grating constant.

Particles 20 located outside the focal plane 19 only produced a weakly modulated or unmodulated signal. The background is not modulated. The axial resolution is thus equal to the depth of focus of the image and is therefore optionally adjustable. The calculation is effected using, for example, an autocorrelator (not shown). The first maximum ofthe autocorrelation function, other than the zero point, gives the medial duration T of the modulation for a number of measured particles 20.

The velocity component V5 mentioned hereinbefore can thus be calculated as d V = T wherein d and M have the same significance as detailed hereinbefore.

An average taken over a number of particles 20 is absolutely necessary in the case of layers 19, located close to the wall of the cell, since the velocities of individual particles may deviate considerably from the median value in this region. The velocity vector parallel to the liquid layer 19 is determined bytwo measurements. For the second measurement the grid 11 is rotated through 900 about the optical axis 16'. A displacement apparatus 18 (dovetail guide) for displacing the measuring cell 5 in the axial direction 16 permits the determination of velocity components in liquid layers 19 spaced at different distances from the cover glass wall 4. By using the continuity equation, the three dimensional velocity profile in a liquid may be determined.

The output of the HeNe Laser 1 is 15 mW. The image is produced by a Leitz objective 3 having an aperture of 1.3 at 100-fold magnification in the intermediate image plane 6 and 6fold secondary magnification with the achromatic objective (F= 30 cm). The image field has a diameter of 20 ,u m, and the depth of focus is less than 0.4 ,tzm. Colloidal gold is admixed with the flowing fluid 22 as a tracer. An extremely uniform particle size was utilised. The diameter of the particles, on average, was less than 1 ,am, which lies below the resolution limit. For fading out the reflections, a high pass filter 21 having a diameter of 0.5 mm is suitable.The light 16' reflected from a particle 20 into the focal plane 19 is only slightly modulated and a good intelligence signalbackground ratio is obtained thereby. The signal originating from the background is weakened by a factor of 50 by the high pass filter 21. The ratio of the intelligence signal to the background is then 2:1 The evaluation of the photoelectric signals 16" by the detector 13 is effected utilising a Malvern K7023 correlator. At a vertical wall flow and in capillaries velocity profiles are measured with water and diluted gelatine as the liquid 22 up to a wall spacing of 1 ,am. The axial resolution thus attained was below 0.5 ,am. For a solid wall spacing, the meas urementtime is of the order of a few seconds to minutes.

Figure 2 shows the typical signal, a multiplier signal, produced during the movement of a particle 20 in the focal plane 19. Fig. 3 shows the autocorrelation function with an average of more than twenty particles 20. Fig. 4 shows the result of a measurement (wall spacing d to velocity V) on a capillary 5 of rectangular cross-section having a thickness of 58 ,lem. The capillary 5 had water 22 passed therethrough at a volume flow of 3.9 x 104 ml/s. Fig. 4 shows the good agreement of the results obtained with the expected parabolic profile.

Claims (6)

1. A method for the contactless measurement of the velocity or the path of a layer of particles in liquid or gas flows relative to a reference position, wherein the particles are illuminated by means of a radiation, which radiation is reflected by the particles and the diffused or scattered radiation is evaluated, wherein the images of the radiation source superposed on the beam reflected or diffused by the particles are filtered out, the layer to be measured being located in the focal plane of an objective lens, depicting the layer to be measured in an evaluation plane having a minimal depth of focus, and providing diffusing or scattering particles in the liquid or gas flow to act as a tracer.

2. A method as claimed in claim 1, wherein the tracer particles are particles of gold.

3. A method of contactless measurement as claimed in claim 1 substantially as hereinbefore described.

4. An apparatus for carrying out the method as claimed in any one of claims 1 to 3 comprising a source of radiation, an objective lens which focuses the radiation beam in the focal plane of a liquid or gas flow so as to illuminate the particles therein, which particles have tracer particles added thereto, said objective lens producing an image from said particles through reflected or diffused radiation in an intermediate image plane, a further lens providing a light source image of each light source reflection, high-pass filters for filtering such light source images, a third lens for depicting the intermediate image plane on a grating and a field lens for transmitting the radiation passing through the grating plane to a detector.

5. An apparatus as claimed in claim 4further including a beam splitter to produce a laterally masked branch a displaceable lens being provided in said masked branch, displacement of said lens causing the intermediate image plane or the filter plane to be depicted on an observation screen.

6. An apparatus for contactless measurement as claimed in claim 4 constructed and arranged to operate substantially as herinbefore described with reference to and as illustrated in Fig. 1 of the accompanying drawings.