WO2011055030A1

WO2011055030A1 - Method and apparatus for characterizing a flow

Info

Publication number: WO2011055030A1
Application number: PCT/FR2010/000713
Authority: WO
Inventors: Philippe Jacques Pernod; Vladimir Preobrazhensky; Yuri Pyl'nov; Nikolay Smagin
Original assignee: Centre National De La Recherche Scientifique
Priority date: 2009-11-05
Filing date: 2010-10-29
Publication date: 2011-05-12
Also published as: FR2952185A1; FR2952185B1

Abstract

The invention relates to a method for characterizing a flow of a fluid medium (M) of unknown acoustic velocity, comprising: a first phase-shift measurement of an acoustic pulse having passed through said flow along a first path, optionally comprising multiple reflections, and then along a reverse path after acoustic phase conjugation; a second phase-shift measurement of an ultrasound pulse having passed through said flow at least once, without acoustic phase conjugation; and determination of at least one velocity of said flow, from at least of two of said phase-shift measurements. The invention also relates to an apparatus for implementing such a method.

Description

METHOD AND APPARATUS FOR CHARACTERIZING A FLOW

The invention relates to a method and an apparatus for the characterization of a flow, and more particularly to a flow of a fluid medium, homogeneous or heterogeneous, whose acoustic properties are imperfectly unknown. The characterization of such a flow essentially comprises the determination of its speed, and preferably also of the speed of ultrasound in the fluid medium ("acoustic celerity"), a property which is related in particular to its composition.

Ultrasonic velocimetry is a family of techniques for determining the speed of a flow. Virtually all ultrasonic velocimeters use either the Doppler method or the "transit time difference measurement" method. The Doppler method can only be used in the presence of diffusers in the medium (such as red blood cells in the blood). The method of "measurement by transit time difference" has largely been imposed in other cases since it allows a simple calculation of the speed from the measurement, as shown for example in the document FR 2.781.048.

However, this method requires transducers and broadband electronics, which is not favorable from the point of view of the signal-to-noise ratio. One possible solution for improving the signal-to-noise ratio is to work in a narrow band and to measure the flow velocities by measuring the phase shifts experienced by an ultrasonic pulse propagating in a moving medium. Such a method is described in document FR 2,370,961.

More recent narrowband ultrasonic velocimetry techniques exploit an effect known as "phase conjugation" (or "acoustic phase conjugation" to distinguish it from its optical equivalent).

Phase conjugation is the transformation of a wave field by which the wave propagation direction becomes inverse while retaining the initial spatial distribution of phases and amplitudes. Such a transformation is possible because the equations of the field acoustic in a non-dissipative and stationary medium are invariant by time inversion.

An acoustic wave propagates from a transducer to a "phase conjugator" through a stationary medium. This wave undergoes a phase shift depending on the path followed, as well as a deformation of its wavefront if the medium is not homogeneous. After entering the phase conjugator, the wave undergoes a "reversal over time" and propagates backward, traveling exactly the same path as the one going, but in the opposite direction. In doing so, it undergoes a phase shift (and a possible deformation of its wavefront) opposite to that suffered in the "go" path. When the phase conjugate wave reaches the transducer again, the phase shift and deformation of the wavefront accumulated at the "go", especially when the medium is complex and inhomogeneous, have been compensated for during the "return".

When the propagation medium is no longer stationary because it is the seat of a flow, the invariance of the propagation equations relative to the time inversion is broken. The acoustic wave that passes through such a medium therefore undergoes a phase shift which depends on the flow velocity of the medium, and which is not compensated by the phase conjugation. The measurement of the phase of the incident wave on the transducer after phase conjugation thus contains the information relating to said flow velocity.

Compared to conventional ultrasonic velocimetry techniques, this method based on acoustic phase conjugation has the advantage of being insensitive to all stationary phase shift sources. The application of acoustic phase conjugation to ultrasonic velocimetry, under the conditions where the speed of acoustic waves in the medium is known, is described in particular by the following documents:

- Nikolay Smagin, "Characterization of liquid flows by parametric phase conjugation method of ultrasonic waves", PhD Thesis, Ecole Centrale de Lille - Institut de Radiotechnic, Electronics and Automation Moscow, November 20, 2008;

Yu V. Pyl'nov et al. "Detection of Moving Objects and Flows in Liquids by Ultrasonic Phase Conjugation", Acoustical Physics, Vol. 51, No. 1, pages 105 - 109 (2005);

V. Preobrazhensky et al. "Nonlinear Acoustic Imaging of Isoechogenic Objects and Flows Using Ultrasound Wave Phase Conjugation," Acta Acoustica united with Acoustica, Vol. 95 (2009);

V. Preobrazhensky et al. "New applications of magneto- acoustic phase conjugation", 19 ^th French Mechanics Congress, Marseille 24 - 28 August 2009.

The theory of acoustic phase conjugation and the techniques for its implementation are described by:

A. P. Brysev, L. M. Krutyanskioe, and V. L. Preobrazhenskioe, "Wave phase conjugation of ultrasonic beams", Usp. Fiz. Nauk 168, 877 (1998) [Phys. Usp. 41, 793 (1998)]; and

Qi Zhang, "Theory and Simulation of Magneto-Acoustic Phase Conjugation" PhD Thesis, Lille University of Science and Technology, June 19, 2008; see especially chapters 1 and 2.

Both the conventional techniques for determining the velocity of a flow by measurement of the acoustic phase shift and those using the acoustic phase conjugation presuppose that the acoustic celerity (ie the velocity of propagation of ultrasound) of the medium is known precisely. However, this is not the case, especially when the composition of the medium is unknown, or imperfectly known. A separate measure of acoustic celerity is not always possible, because it would be disturbed by the flow, whose velocity is not precisely known.

The present invention aims to allow the characterization of flows of this type, which can not be easily studied using techniques known from the prior art. Such a situation is frequently encountered in industry. By "characterization" is meant here primarily the measurement of the velocity of the flow, and optionally the simultaneous determination of the acoustic velocity. This magnitude may be of interest in itself, or in that it provides information on the composition of the medium, its temperature, its physical state, etc.

According to the invention, this problem is solved by a method for characterizing a flow of a fluid medium of unknown acoustic celerity, comprising:

a first phase-shifting measurement of an acoustic pulse having passed through said flow along a first path, possibly comprising multiple reflections, then following an inverse path after acoustic phase conjugation;

a second phase shift measurement of an acoustic pulse having passed through said flow at least once, without acoustic phase conjugation; and

determining, from said two phase shift measurements, at least the speed of said flow.

Such a method exploits the fact that a phase-conjugate acoustic wave propagating through a flowing medium undergoes a phase shift which depends on the flow velocity and the acoustic celerity of the medium according to a law different from that which applies. to an unconjugated wave in phase. The use of two acoustic phase measurements thus makes it possible to separate the two unknowns from the problem: flow velocity and acoustic celerity.

Sometimes only the flow velocity is of interest.

However, the method also makes it possible to determine the acoustic speed of the medium.

In a particularly advantageous embodiment of the invention, the flowing medium may be a fluid mixture, homogeneous or heterogeneous, comprising a component whose concentration is unknown. In this case, the acoustic celerity depends directly on this concentration, and the method may also include the determination, from the two so-called phase shift measurements, the acoustic celerity of the medium.

The second phase shift measurement may be performed on a pulse having passed through said flow a first time following a first path, and a second time following a second inverse path after specular reflection. In this case, it is very advantageous that the method comprises the transmission of an ultrasonic pulse through said flow and to a phase acoustic conjugator, said first measurement being performed on a component of said conjugate impulse in phase by the conjugator and said second measurement being performed on a component of said pulse reflected by a surface of said conjugator. Thus, the implementation of the method requires only a transducer and a phase conjugator: it is hardly more complex (except as regards the data processing) than a measurement of velocimetry by known acoustic phase conjugation. of the prior art.

Advantageously, the acoustic phase conjugation can be performed according to the magneto-acoustic method, but any other technique can also be used. By way of example, mention may be made of time reversal implemented by purely electronic means.

Advantageously, said or each acoustic pulse is an ultrasonic pulse (frequency at least equal to 20 kHz, preferably at least 1 MHz).

Another object of the invention is an apparatus for characterizing a flow of a fluid mixture, homogeneous or heterogeneous, comprising a component whose concentration is unknown, comprising:

an ultrasonic transducer operating in transmission and reception;

an acoustic phase conjugator acoustically coupled to said transducer via a space allowing the flow of said fluid mixture being formed between these two elements; means for measuring the phase of two pulses detected by said transducer with a time shift; and

calculating means for calculating at least the speed of said flow from the phases thus measured.

Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the accompanying drawings given by way of example and which represent, respectively:

FIG. 1, a diagram of an apparatus for simultaneously measuring the flow velocity and the acoustic velocity of a medium according to one embodiment of the invention;

Figure 2, another diagram of the same apparatus, highlighting the structure of the acoustic phase conjugator;

Figure 3, an experimental facility for simultaneous measurement of flow velocity and relative concentration of a suspension;

Figures 4 and 5, graphs illustrating the results of a measurement made using the installation of Figure 3.

The apparatus of Figure 1 comprises:

a main duct 1, of diameter D, in which flows a fluid medium M; this medium may be liquid or gaseous, homogeneous or heterogeneous (emulsion of a liquid in a liquid, suspension of a solid in a liquid, aerosol of a liquid or solid in a gas, liquid containing gas bubbles, etc. );

a secondary measurement duct 2, rectilinear and forming an angle α with respect to the main duct; this duct may contain the same fluid medium M as the main duct 1, but at rest; it may also contain a different medium, with known acoustic properties (and preferably close to that of M to achieve a good acoustic coupling), in which case the wall of the duct 1 may not be interrupted;

an acoustic transducer 3 disposed at a first end of the secondary measurement duct; and a phase acoustic conjugator 4 disposed at a second end of the secondary measurement duct, facing the acoustic transducer 3.

In the figure, L represents the distance between an emitter / receiver surface 30 of the transducer and a front surface 40 of the acoustic phase conjugator; d = D / sina is the length of the ultrasound path from the transducer to the conjugator inside the main conduit, and therefore the flow to be characterized. It should be noted that the conjugator 4 need not be placed directly opposite the transducer 3: the two elements can be connected to each other by an acoustic path comprising one or more reflections.

FIG. 2 shows, schematically, the structure of the conjugator 4, as well as the set of electronic equipment coupled to said conjugator and to the transducer 3.

The conjugator 4 is of the magneto-acoustic type. It is an active material 41 of nickel ferrite (NiO-Fe ₂ O ₃ ), of cylindrical shape with a diameter of 28 mm and a length of 150 mm. This element is subjected to a static magnetic field oriented along the axis and an optimal value of the operating point for the

conjugator used.

This static field is delivered by a first coil 42 surrounding the conjugator, powered by a direct current generator 51.

A generator 52 (channel A) emits a pulse signal of frequency f = 10 MHz and duration τ1 = 20 μs. This signal is transformed into an ultrasonic pulse by the transducer 3. The ultrasonic wave passes through the liquid medium M containing the flow to be characterized. Part of this pulse is reflected on the front face 40 of the conjugator 4 due to the acoustic impedance break between the medium M and the active material (nickel ferrite). This signal, called "reflected", crosses the flow in the opposite direction. The non-reflected portion of the incident ultrasonic pulse enters the conjugator. At this moment, the generator 52 (channel B) emits a signal called pumping ", of frequency 2f = 20 MHz and duration τ2 = 70 με. To increase the pumping intensity and reach the supercritical mode of amplification, a power amplifier is used. The interaction of the electromagnetic wave with the ultrasonic wave via the modulation of the elastic parameters of the active material results in the creation of the conjugate wave in phase, which returns to the transducer 3 by compensating for the phase shifts incurred during the first course (principle of the time reversal), except the phase shifts related to the flow (as explained above, the flow causes the rupture of the conservation law by time reversal). The signal of the conjugate wave arrives on the transducer 3 with a time offset sufficient to be decoupled from the reflected signal. The phases of the two reflected and conjugated signals are measured by analyzing the spectra of the pulses received, by means of an oscilloscope or a computer 53 to which the transducer 3 is connected.

We know from the laws of acoustics that the phase difference acquired by the reflected wave because of the flow is:

or :

it is the celerity - unknown - of the medium;

k is the wave vector of the ultrasonic signal in the medium M; it is worth k = co / c, where ω is the angular frequency of the signal (10 ⁷ π rad / sec) v _z is the projection of the flow velocity of the medium M along the z axis, propagation direction of the ultrasound .

Δφ _r is not obtained directly from the measurement of the phase of the reflected pulse, because the measured phase includes contributions related to the propagation in the secondary measurement duct, outside the flow to be characterized. Δφ _Γ is therefore deduced from the total phase shift by subtraction of a buffer phase shift. The latter is constant if the secondary duct is isolated from the channel where the flow takes place by a thin membrane acoustically transparent, and if this secondary conduit is filled with a medium in which the celerity of the waves is known and constant.

The phase difference acquired by the conjugate wave is:

and depends only on the characteristics of the flow: d (known), v _z (unknown), c (unknown), as well as the angular frequency ω.

By comparing equations [1] and [2] we obtain:

which, substituted in [1] gives:

The acoustic celerity in the middle M is therefore worth

Substituting in [3] we find that the flow velocity (or rather its projection along the z axis) is:.

The flow velocity v is: v = v _z / cos.

The measuring apparatus of Figures 1 and 2 is particularly advantageous because the same components (transducer 3, conjugator 4) are used to perform both measurements. But it would be possible to use a transducer and a reflector dedicated solely to the phase measurement of the reflected wave. It would also be possible to replace this reflector with a second transducer for measuring the phase of an ultrasonic pulse after a simple passage through the flow without reflection or phase conjugation.

A particular case consists in characterizing the flow of a medium consisting of a homogeneous or heterogeneous mixture, comprising a component whose concentration is unknown. In particular, a two-component mixture is considered. These components may in their turn be mixtures whose composition or, in any case, the acoustic celerity is known; the only unknown (apart from the flow rate) is the ratio between their concentrations.

Let c ₀ and Ci be the acoustic (known) celerities of a first and a second component of the mixture, and c the acoustic (unknown) celerity of the mixture. The concentration - unknown - of the second component is defined by a quantity n such that c = c ₀ + n (c - c ₀ ) Qn easily verifies that when n = 1, c = ci (the "mixture" contains only the second component) and when n = 0, c = c ₀ (the "mixture" contains only the first component).

It is possible to demonstrate that, under these conditions, the flow velocity and the concentration n are given, respectively, by

When the concentration is low, we have

If, moreover, the flow velocity is low relative to the velocity of the ultrasonic waves, it is also necessary to

In particular, it can be shown that equations [7] and [8] are simplified as follows:

Indeed, under these conditions, the initial equations ([1] and [2]) can be written:

From these relationships, we observe that:

From the physical point of view, this means that in this particular case, the phase of conjugate waves in phase is almost insensitive to the concentration of the variable component in the mixture, but is essentially modified by the speed of the flow, thus allowing its measured. On the other hand, the unconjugated ultrasonic signal passing through the liquid is practically insensitive to the flow velocity, and is instead sensitive to the concentration of the variable component in the mixture. An experimental demonstration of the process of the invention has been successfully performed in the case of a flow of water having suspensions of technical oil drops of varying concentration. This demonstration was carried out in the regime "low concentration - low speed" described above which allows the use of simplified equations [7] and [8].

The experimental device used for the demonstration is shown in FIG. 3. The measuring apparatus of FIGS. 1 and 2 has been adapted to a duct 1 in which a pump 61 creates a flow of water, whereas an injector 62 introduces low amounts of oil. The main conduit 1 opens into a tank 60 filled with water, in which the pump 61 draws, so as to close the hydraulic circuit.

The repetition frequency of the emission and measurement sequences was chosen at 10 Hz.

Figure 4 shows the dependence of the phase of the reflected wave (unconjugated phase) as a function of the oil concentration in the water. The linear dependence predicted by equation [9] is found and approximated by the following empirical formula:

Figure 5 shows the dependence of the conjugate wave phase (upper curve) and the reflected wave phase (lower curve) as a function of time, for a variable concentration of the suspension.

The times t ^ t ₇ correspond to the start and stop of the flow of the liquid mixture. It is observed that Δφ ₀ goes from 0 to 100 ° approximately, at startup, to return to 0 ° at the stop. Between these two extreme moments the flow velocity, and therefore Δφ ₀ , is kept constant.

The moment t ₂ corresponds to the beginning of the arrival of the oil mixed with water under the pressure of the oil column (slow injection); t ₃ corresponds to the stop of arrival of the oil due to the equalization of oil levels (in the injector 62) and water (in the outer tank 60) - the phase remains approximately constant until 44. Between the instants tj and t _5l an additional significant amount of oil was injected rapidly applying manual pressure in the injector; the time interval ts-t ₆ corresponds to the progressive decrease of the concentration of the oil by mixing with the water of the tank; at once the rest of the oil was manually injected brutally into the pipe; the time interval t6-t ₇ corresponds to the withdrawal of the oil from the pipe and its gradual distribution in the entire volume of water.

The results of the experiments confirm that, in the experimental considerations used, the phase of the conjugated ultrasonic wave in phase undergoes considerable modifications when the flow velocity changes, whereas the phase of the reflected wave changes very little (below noise level). On the contrary, the phase of the reflected wave has a strong sensitivity to the boosts of the relative concentration of the liquid whereas the sensitivity of the conjugate wave does not exceed the noise level. Therefore, the realized flowmeter allows in addition to the speed measuring function, to measure the variable concentration of a component of the mixture with a high precision (the measurable concentration variations are of a few tenths of a percent).

We have considered here only the case where the acoustic pulses used for the measurement are ultrasound, that is waves of frequency higher than 20 kHz. This is not a fundamental limitation, and sound impulses - even infrasound - can be used, at least in principle. From a technological point of view, however, the use of ultrasound is generally preferred; in particular, it is difficult to make magneto-acoustic type converters operating at frequencies below 1 MHz.

Claims

1. A method of characterizing a flow of a fluid medium (M) of unknown acoustic velocity, comprising:

2. The method of claim 1 also comprising determining, from both said phase shift measurements, the acoustic speed of the medium.

3. Method according to one of the preceding claims wherein said medium (M) is a fluid mixture, homogeneous or heterogeneous, comprising a component whose concentration is unknown, the method also comprising the determination, from both said phase shift measurements. of said unknown concentration.

4. Method according to one of the preceding claims wherein said second phase shift measurement is performed on a pulse having passed through said flow a first time following a first path, and a second time following a second inverse path after specular reflection.

The method of claim 4 including transmitting an acoustic pulse through said flow and to a phase acoustic conjugator (4), said first measurement being performed on a component of said conjugate impulse in phase by the conjugator and said second measuring being performed on a component of said pulse reflected by a surface of said conjugator.

6. Method according to one of the preceding claims wherein said acoustic phase conjugation is performed according to the magneto-acoustic method.

7. Method according to one of the preceding claims wherein said or each acoustic pulse is an ultrasonic pulse.

Apparatus for characterizing a flow of a fluid mixture, homogeneous or heterogeneous, comprising a component whose concentration is unknown, comprising:

an ultrasonic transducer (3) operating in transmission and reception;

- an acoustic phase conjugator (4) acoustically coupled to said transducer via a space allowing the flow of said fluid mixture being formed between these two elements;

means for measuring (53) the phase of two pulses detected by said transducer with a time offset; and

calculating means (53) for calculating at least the speed of said flow from the phases thus measured.