WO2015075217A1

WO2015075217A1 - Method and optical system for detecting particles in a flow

Info

Publication number: WO2015075217A1
Application number: PCT/EP2014/075366
Authority: WO
Inventors: Raphael BAUDOIN; Laurent ZIMMER
Original assignee: Areva; Centre National De La Recherche Scientifique (C.N.R.S)
Priority date: 2013-11-25
Filing date: 2014-11-24
Publication date: 2015-05-28
Also published as: FR3013840B1; FR3013840A1

Abstract

The invention relates to an optical system for determining, in three dimensions, the velocity profile and minimum concentration of particles in a flow, the system being characterised in that it comprises a first acquisition chain (100) comprising a first acquisition lens (130), a first photosensitive sensor (110), and a first shutter (120) formed by an opaque disk having at least two openings (121) in an arc, which can allow the transmission of light beams towards said first photosensitive sensor (110), said openings (121) being inscribed in the same circle.

Description

METHOD AND OPTICAL SYSTEM OF

DETECTION OF PARTICLES IN A FLOW.

TECHNICAL AREA

The field of the invention is that of three-dimensional determination and real-time monitoring of a concentration of particles leading to the determination of the evolution of the velocity profile in a fluid flow.

The invention is particularly well suited for determining a concentration of particles in flows near obstacles.

The invention finds a particularly advantageous application in the nuclear field for the determination of particle concentration in flows in the vicinity of fuel cladding of a nuclear reactor.

STATE OF THE ART

Various methods for determining the three-dimensional concentration of particles in a flow are known.

One known method is to detect by means of several cameras visually focused particles to locate them in space. This first method, called PIV tomography (for Particle Image Velocimetry), requires many cameras (at least four) and large resources for data processing. Such a detection principle is described in particular in the document "PIV Tomography: principles and practice. 2013, F.Scarano, Measurement Science and Technology 24: 012001. However, this technique has the disadvantage of detecting many ghost particles that can be removed only by means of a large number of cameras. Moreover this phenomenon is accentuated when an obstacle is present in the flow. As an illustration, the "V3V" system of TSI consists of 3 slightly offset cameras to achieve a triangulation of focused particle positions. This allows speed field measurements in three dimensions and three components but not around obstacles.

Other types of techniques are based on the principle of defocusing. Indeed it specifies that the apparent size of the particle on the image can be linearly connected to its position relative to the focal plane of the optical system. This apparent size information then provides the third component necessary for three-dimensional positioning of the particle in space, leading to a determination of a three-dimensional concentration of particles in a volume. This principle can only apply to small particles. Indeed, beyond a critical size, the apparent size is no longer a function of the position relative to the focal plane, but also of the actual size of the particle. This critical size depends on the complete optical system. This approach therefore makes it possible to carry out a three-dimensional particle tracking with a single camera and is described in the documents: "Particle depth measurement based on depth-from-defocus 1999, S.Murata and M.Kawamur." Optics & Laser Technology, 31 (1), pp.95-102 ". and "Three-dimensional fluorescent particle tracking at micron-scale using a single camera." 2005, M.Wu, JWRoberts and M.Buckley., Experiments in Fluids, 38 (4), pp.461-465.

However, since the apparent size of the defocused particles is clearly greater than that of focused particles, these techniques do not make it possible to measure high levels of particle concentration because of the phenomenon of overlap. To limit this effect, the following techniques have been developed.

The first, available in the document "Novel interferometric measurement of size and velocity distributions of spherical particles in fluid flows, 2000, M.Maeda, T.Kawaguchi and K.Hishida, Measurement. Science and. Technology 1 1 L13-L18 "describes a method of detecting particles and tracking the evolution of the velocity profile based on the principle of optical compression by means of an optical system behavior a rectangular aperture, an objective, a pair of cylindrical lenses and an acquisition camera. The pair of lenses is positioned between the lens and the camera so as to generate a blurred image on the focal plane. The rectangular opening is positioned upstream of the lens so as to adjust the angle of incidence and improve the depth of field. The determination of the flow velocity of the fluid is obtained by a combination of known PIV (Particle Image Velocimetry) and PTV (Particle Tracking Velocimetry) techniques.

There is also known a second method described in the document "Three-dimensional particle imaging by defocusing method with an annular aperture, 2008, D. Lin, Optics Letters 33 (9): 905-7." Still based on the principle of optical defocusing, this technique makes it possible to determine the number of particles in a given volume by means of an optical system composed of an opaque disc, an objective lens and a camera, the opaque disc being positioned upstream of the lens and having an annular opening. This visualization method makes it possible to increase the quantity of detectable particles and its combination with the detection methods described in the aforementioned document is particularly suitable for very simple, slow and low turbulent microscopic flows of the sedimentation type.

However, this method does not make it possible to precisely determine the concentration of the particles and does not make it possible to detect large densities of particles, especially in a flow that is faster or more turbulent than a flow of sedimentation-type particles.

Therefore, by their inability to filter ghost particles through obstacles or by their low level of detection none of the known and aforementioned detection methods is satisfactory for accurate three-dimensional detection of a high density of particles. particles in a flow around obstacles.

SUMMARY OF THE INVENTION

In this context, the aim of the invention is to propose a method and a system based on the principle of optical defocusing making it possible to improve the detection accuracy, by avoiding the detection of so-called ghost particles, as well as the detectable particle density relative to known methods.

To this end, the subject of the invention is an optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow, the system being characterized in that it comprises a first acquisition chain. having a first acquisition lens, a first photosensitive sensor and a first shutter formed by an opaque disk having at least two arcuate apertures adapted to allow transmission of light rays to said first photosensitive sensor, said apertures being inscribed on a same circle.

The invention is particularly suitable for determining the velocity fields around obstacles making it possible to obtain the pressure fields around them. The defocusing system according to the invention makes it possible to improve the detection accuracy of the superimposed particles, in particular with respect to the optical system described in the document "Three-dimensional particle imaging by defocusing method with an annular aperture", 2008, D. Lin , Optics Letters 33 (9): 905-7. Thus, thanks to the implementation of discontinuities in the shutter contour, digital imaging simulations show an increase of about 50% in the accuracy of the particle size measurement and a decrease decreases in size. the order of 50% of the detection errors with respect to a shutter having an annular opening at its periphery, identical to the optical system described in the document mentioned above.

This significant increase of the measurement accuracy on the particle size and the decrease in detection error thus makes it possible to increase the detection rate of particles in a flow and thus makes it possible to be able to use such a device for detection in a flow. faster or turbulent than a displacement of sedimentation-type particles.

The optical system according to the invention may also have one or more of the following characteristics taken individually or in any technically possible combination:

the shutter is positioned between the acquisition lens and the photosensitive sensor;

the shutter is positioned at the center of the acquisition lens;

the system comprises a second acquisition chain comprising a second acquisition lens, a second photosensitive sensor and a second shutter formed by an opaque disk having at least two openings in an arc of a circle capable of transmitting light rays towards the second photosensitive sensor, said openings being inscribed on the same circle; the first acquisition chain and the second acquisition chain being positioned according to two distinct measurement angles with respect to the flow;

the system comprises a chain of post-processing of the images acquired by the acquisition chain or chains;

the post-processing chain comprises at least: a storage unit, a calculator and a display unit.

The subject of the invention is also a method for the three-dimensional determination of the minimum concentration of particles in a flow at means of an optical system according to the invention comprising at least one acquisition chain and a post-processing chain, the method comprising:

an image acquisition step by means of said at least one acquisition chain;

a step of correlating each image with virtual patterns stored in the storage unit of the post-processing chain, so as to determine the position and the apparent size of the particles in the flow.

Advantageously, the three-dimensional determination method of the concentration of particles in a flow further comprises a step of displaying the volume distribution of the particles in the flow on display means.

The subject of the invention is also a method for determining the particle velocity profile in a flow by means of an optical system according to the invention comprising at least one acquisition and one aftertreatment chain, the process being characterized in that what it includes:

an image acquisition step by means of said at least one acquisition chain;

a step of correlating each image with virtual patterns stored in the storage unit of the post-processing chain, so as to measure the apparent size and determine its three-dimensional position in the flow;

a step of spatio-temporal association of the positions of the particles at time t and at time t + dt on two consecutive images;

a step of determining by means of a calculator of the fields of instantaneous displacements of the particles by determining the displacement of the centers of the particles between the two consecutive images at the instant t and at the instant t + dt by cross-correlation of the respective images of the particles under the two instants.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will emerge more clearly from the description which is given below, by way of indication and in no way limitative, with reference to the following figures: FIG. 1 schematically illustrates an acquisition chain of the optical system according to the invention.

FIG. 2 schematically illustrates the principle of optical defocusing on the acquisition system illustrated in FIG.

FIG. 3 is a front view of a shutter of the acquisition chain illustrated in FIG.

FIG. 4 is an image captured by the optical system according to the invention illustrating all the particles present in a flow.

FIGS. 5a and 5b illustrate two images captured by the optical system according to the invention of a particle positioned upstream of the focal plane of the acquisition lens and of a particle positioned downstream of the focal plane of the acquisition lens. .

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

FIG. 1 schematically illustrates an acquisition chain of the optical system according to the invention. FIG. 2 diagrammatically illustrates the optical defocusing principle obtained thanks to the acquisition chain of the optical system according to the invention.

The acquisition chain 100 of the optical system according to the invention comprises: an acquisition lens 130;

a photosensitive sensor 1 10;

a shutter 120 positioned between the acquisition lens 130 and the photosensitive sensor 1 10 of the acquisition chain 100.

The specific form of the shutter 120 is illustrated more precisely with reference to FIG.

The shutter 120 is formed by an opaque disc having at its periphery at least two openings 121 in the form of a circular arc. The openings 121 allow the passage of the light rays towards the photosensitive sensor as illustrated in the block diagram of FIG. 2. Advantageously, the openings 121 are inscribed on the same circle and are three in number as represented by way of example at Figure 3.

The disk-shaped shutter 120 is positioned at the center of the acquisition lens 130. Thanks to the optical system and in particular to the acquisition chain 100 described previously, as well as to the positioning of the shutter, a particle P 0 lying in the flow to be characterized appears at the sensor photosensitive 1 10 in the form of a discontinuous ring and whose central portion is transparent.

The representation in the form of a discontinuous ring advantageously makes it possible to detect a larger number of particles in a volume, the superimposed particles being now visible as shown in the detail of FIG. 4. Thus, in the case where several particle sizes are present simultaneously, it is now possible thanks to the invention to detect a smaller particle that would be "hidden" behind a larger particle, or different particles superimposed.

The shutter 120 is positioned and held in the center of the acquisition lens 130 by three supports (not shown) so as to allow the positioning of the shutter precisely in the middle of the lens 130. The positioning accuracy in the center of the lens 130 also improves the measurement accuracy.

Thus, the discontinuous openings 121 of the shutter 120 make it possible to further improve the detection accuracy of the superimposed particles with respect to the method described in the document "three-dimensional particle imaging by defocusing method with an annular aperture", 2008, D Lin, Optics Letters 33 (9): 905-7Letters. Thanks to the implementation of discontinuities in the contour of the shutter 120, one increases of the order of 50% the precision on the measurement of the size of the particle and decreases of the order of 50% of the errors of detection relative to a shutter having an annular opening at its periphery, as described in the document cited above.

The detection accuracy is also increased by the presence of a second acquisition chain (not shown) similar to the first acquisition chain 100 described above. Advantageously, the two acquisition chains positioned at 90 ° to one another are positioned according to two different measurement angles. The two acquisition chains thus make it possible to increase the detectable particle density, the correlation measurement accuracy. crossed images of the two acquisition chains.

The acquisition chains 100 are connected to a data processing chain. The data processing chain includes:

a storage unit for storing images from acquisition strings 100; a calculator allowing:

Associating with each image recorded by the acquisition chains 100 virtual templates stored in the storage unit of the data processing chain so as to determine the position and the size of the particles in the flow with respect to canvas references;

Associating spatio-temporal positions with the particles at a time t and at a time t + dt on several consecutive images;

To determine instantaneous displacement fields of the particles in the flow by determining the displacement of the centers of the particles between the two consecutive images at time t and at time t + dt.

A display means, in particular for displaying the volume distribution of the particles detected in the flow as well as instantaneous displacement fields of the particles in the flow making it possible to determine the evolution of the velocity profile of the particles in the flow.

The optical system according to the invention is therefore suitable for determining the concentration of particles in a flow but also for determining the instantaneous speed of the particles in this flow.

Finally, compared to the optical compression method, the device according to the invention has the advantage of improving the accuracy of measuring the particle size by measuring in two directions by means of two acquisition chains. Thanks to the invention, the background noise related to the superposition of the particles is greatly reduced and densification of the noise in the context of optical compression is avoided.

The discontinuous openings 121 of the shutter 120 also have the advantage of allowing the acquisition of images of particles located on either side of the object focal plane of the acquisition lens 130. Indeed, thanks to the presence of discontinuous openings 121 on the periphery of the shutter 120, the particles appear at the level of the photosensitive sensor 1 10 in the form of a discontinuous light ring. Thus, the position of the particles on either side of the object focal plane of the acquisition lens is detectable thanks to the discontinuous position. Indeed, thanks to the device according to the invention, it appears that the positions of the discontinuities are symmetrical with respect to the focal plane. So two particles located at equal distance from the focal plane will be detected by the device with the same apparent size but with a position of the different discontinuities, as shown in Figures 5a and 5b, where the captured image illustrated in Figure 5a corresponds for example to a particle located upstream from the object focal plane of the acquisition lens 130 and the captured image illustrated in FIG. 5b corresponds to a particle situated downstream from the object focal plane of the acquisition lens 130.

The acquisition lens 1 10, or acquisition objective may include a plurality of acquisition lenses, is preferably chosen so as to obtain images of identical quality of particles located on either side of the focal plane object of the lens.

Thanks to this particularly advantageous property of the device according to the invention:

the size range of the detected particles is minimized to the equivalent studied volume; this makes it possible to detect a larger quantity of particles homogeneously in the volume, the performance of the detection algorithm decreasing with the increase in the apparent size of the particles;

the acquisition system is advantageously focused in the middle of the volume to be studied, which limits the phenomena of optical distortion and the loss of light associated with the defocusing principle;

- the detection time is decreased;

The invention also has the advantage:

to allow the realization of a simple and robust assembly (that is to say repeatable) requiring only two chains of acquisition;

to allow the realization of an inexpensive assembly with a high measurement accuracy;

to improve the detection of superimposed particles in relation to known methods;

to allow the measurement of the apparent size of the particles in two dimensions in order to improve the three-dimensional positioning of the particle;

to measure the movement of particles around obstacles where tomographic techniques fail because of the presence of ghost particles.

Claims

Optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow, the system being characterized in that it comprises a first acquisition chain (100) having a first acquisition lens ( 130), a first photosensitive sensor (1 10) and a first shutter (120) formed by an opaque disc having at least two openings (121) in an arc of a circle capable of transmitting light rays to said first photosensitive sensor (1). 10), said openings (121) being inscribed on the same circle.

Optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow according to the preceding claim characterized in that the shutter (120) is positioned between the acquisition lens (130) and the sensor photosensitive (1 10).

Optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow according to one of the preceding claims, characterized in that the shutter

(120) is positioned at the center of the acquisition lens.

Optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow according to one of the preceding claims, characterized in that it comprises a second acquisition chain (100) comprising a second lens of acquisition (130), a second photosensitive sensor (1 10) and a second shutter (120) formed by an opaque disc having at least two openings (121) in an arc of a circle capable of transmitting light rays to said second sensor photosensitive (1 10), said openings

(121) being inscribed on the same circle; the first acquisition chain (100) and the second acquisition chain (100) being positioned at two different measurement angles with respect to the flow. Optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow according to one of the preceding claims, characterized in that it comprises a chain of post-processing of the images acquired by the channel or chains acquisition (100).

Optical system for the three-dimensional determination of the velocity profile and the minimum concentration of particles in a flow according to the preceding claim characterized in that the aftertreatment chain comprises at least: a storage unit, a calculator and a display.

Method for the three-dimensional determination of the minimum concentration of particles in a flow by means of an optical system according to one of Claims 1 to 6, comprising at least one acquisition and one after-treatment chain, the method comprising :

an image acquisition step by means of said at least one acquisition chain;

A method for the three-dimensional determination of the minimum concentration of particles in a flow according to claim 7 characterized in that it comprises a step of displaying the volume distribution of the particles in the flow on display means.

A method of determining the particle velocity profile in a flow by means of an optical system according to one of claims 1 to 6, comprising at least one acquisition and an aftertreatment chain, the method being characterized in that it comprises :

an image acquisition step by means of said at least one acquisition chain;

a step of correlating each image with virtual patterns stored in the storage unit of the post-processing chain, to determine the position and the apparent size of the particles in the flow;

a step of determining, by means of a calculator, the fields of instantaneous displacements of the particles by determining the displacement of the centers of the particles between the two consecutive images at time t and at time t + dt.