Determining a characteristic of airflow through an opening

Abstract

A system (1) is proposed for determining a characteristic of an airflow (2) through an opening (3). The system comprises: a support (10) configured to be positioned around the opening; an emission system (20), using a single laser source, of signals (21, 22) propagated in free space, mounted on the support; a disturbance detection system (30) for detecting disturbances (42, 43) in the propagated signals, also mounted on the support; and a processing unit (40) configured to: measure a time lag (41) between two detections, by the detection system, of at least one disturbance in signals emitted by the emission system and propagated through the opening along distinct propagation paths; and determine a characteristic of the airflow, taking into account the measured time lag and a spatial separation between the propagation paths. Abstract figure: Figure 2

Description

Title of the invention: Determination of a characteristic of an airflow through an opening technical field

[0001] This disclosure relates to the determination of a characteristic of airflow through an opening. More specifically, it relates to a system for determining a characteristic of airflow through an opening, as well as a corresponding method and computer program. Prior art

[0002] In the field of airflow measurements through an opening, a commonly used method is velocity sounding using anemometers. Velocity sounding is a measurement of the air velocity at several specific points in an opening in order to estimate an average velocity representative of the overall flow. There are different types of anemometers capable of measuring air velocity at specific points: propeller anemometers, thermal anemometers, or pressure-sensing anemometers (Pitot tubes) that measure air velocity at specific points.

[0003] However, this approach has limitations. First, it is strongly influenced by the non-homogeneous velocity distribution within the opening, which can vary depending on numerous parameters such as the geometry of the opening, upstream and downstream conditions, and local disturbances. This necessitates multiplying the measurement points to obtain a reliable estimate, which is often impractical and increases the measurement time. Furthermore, propeller anemometers and pressure-sensor anemometers are generally ineffective at low air velocities due to their limited sensitivity and the inaccuracy of the results obtained under these conditions.

[0004] To overcome these shortcomings, it is known to apply laboratory-adjusted correction coefficients to the measurements obtained. However, these coefficients are sensitive to the specific environmental and geometric conditions of each opening, which limits their range and introduces significant uncertainty when experimental conditions differ from those studied in the laboratory. Thus, despite the application of correction coefficients, obtaining a velocity sounding using an anemometer is metrologically unreliable and poorly suited to low flow velocities.

[0005] The use of a balometer is another commonly used method for measuring airflow through an opening. A balometer is a device that covers The balometer completely covers the opening and captures the air passing through it to directly measure its volumetric flow rate. This approach is often preferred for its apparent simplicity, as it eliminates the need for multiple point measurements, as is the case with velocity sounding. However, it is not without limitations. The balometer alters the flow conditions by introducing additional resistance, which can distort the actual air velocity and skew the results. This problem is particularly critical when air velocities are low and the pressure difference across the opening is small, as is often the case in industrial or controlled environments. To overcome this limitation, so-called "compensating" balometers have been developed, incorporating devices to reduce the impact of resistance on the flow.However, these devices remain ineffective when pressure differences are extremely small, because the variations induced by the compensation system can be on the order of the very difference they are trying to measure. Thus, even with these improvements, the use of a balometer remains unsuitable for low air velocity conditions, making it difficult to use for applications requiring high metrological accuracy.

[0006] There is therefore a need for a system capable of measuring, in a non-intrusive and metrologically controlled manner, a characteristic of an airflow through an opening for these environments, including when the flow velocity is low (e.g. less than 3 m / s, or less than 1 m / s), while overcoming the limitations of current techniques. Summary

[0007] This disclosure improves the situation.

[0008] According to one aspect, a system for determining a characteristic of an airflow through an opening is proposed, the system comprising: a support configured to be positioned around the opening, a system for emitting signals propagated in free space by a single laser source, mounted on the support, a system for detecting disturbances in the propagated signals, mounted on the support, and a processing entity configured for: to measure a time lag between two detections, by the detection system, of at least one disturbance in signals emitted by the transmission system and propagated through the aperture along distinct propagation paths, and determine a characteristic of the airflow, taking into account the measured time lag and a spatial separation between the propagation paths.

[0009] According to another aspect, a method for determining a characteristic of an airflow through an opening is proposed, the method comprising: a measurement of a time lag between two detections of at least one disturbance in signals emitted by an emission system comprising a single laser source and propagated through the opening along distinct propagation paths, and a determination of the airflow characteristic, taking into account the measured time lag and a spatial separation between the propagation paths.

[0010] In another aspect, a computer program is proposed comprising instructions which, when the program is implemented by a processor, lead to the implementation of the process as defined herein. In another aspect, a non-transient, computer-readable recording medium is proposed on which such a program is recorded.

[0011] The proposed technique relies on the use of at least two light beams propagating in free space, allowing the measurement of a time lag between similar disturbances detected on each of the beams. This time lag, combined with the known spatial separation between the beams, makes it possible to determine a characteristic of the airflow (for example, its velocity).

[0012] The proposed technique offers many advantages.

[0013] The detection of disturbances directly on beams propagated in free space allows a fine analysis of flow characteristics, including velocity, but also thermal gradients or turbulence, and this in a non-intrusive manner, that is to say without interfering with the flow conditions in the propagation medium during the measurement.

[0014] Each light beam, thanks to the optical noise characterizing the perturbation of its propagation, integrates the local variations of the considered airflow characteristic along its propagation path. Thus, the method takes into account local fluctuations along a measurement line without requiring additional soundings, reducing uncertainties related to the representativeness of the measurement and minimizing potential biases. The proposed technique therefore makes it possible to provide an accurate estimate of an average flow characteristic while capturing any gradients or turbulence affecting the medium through which it flows.

[0015] Unlike other techniques sensitive to the direction of flow, the proposed technique offers increased robustness in configurations where the flows are mainly unidirectional.

[0016] The proposed technique does not depend on calibrations specific to precise environmental or geometric conditions.

[0017] By valuing the time of flight between detections of similar disturbances, the proposed technique is particularly well-suited to low flow velocities (typically < 1 m / s), where conventional methods are less accurate. This makes it possible to obtain reliable results even in low-dynamic environments.

[0018] The support allows for stable and precise positioning of the emission and detection systems around the aperture, which can reduce undesirable variations due to external conditions and offer better reproducibility of measurements. The use of a single laser source naturally eliminates biases related to pointing noise and power fluctuations of the emitted beams, thus providing better accuracy in detecting variations in the light signals emitted by the emission system.

[0019] The features described in the following paragraphs may optionally be implemented independently of each other or in combination with each other.

[0020] In one example, the transmission system is configured to generate signals propagated along parallel propagation paths. Such a configuration facilitates the direct comparison of disturbances between beams by ensuring a constant spatial spacing, thus reducing uncertainties in time-shift calculations.

[0021] In one example, the transmission and detection systems are configured to operate with a predetermined spatial spacing between the propagation paths. Such a configuration allows for precise calibration, taking into account environmental or geometric conditions. An appropriate spacing improves the spatial and temporal resolution of the measurements.

[0022] In one example, the detection system comprises at least two detectors having a fixed spatial spacing. Such a configuration can help improve the alignment of the sensors with the beams, minimizing positioning errors and increasing detection accuracy.

[0023] In one example, the emission system includes an optical splitting device for the single laser source to generate the propagated signals. Such a device allows, in a simple manner, the emission of several beams with coherent properties, improving the reproducibility of measurements while eliminating variations related to multiple sources.

[0024] In one example, the perturbation is an angular deviation induced by a variation in refractive index caused by the airflow. The proposed technique takes advantage of the mirage effect to exploit such a perturbation.

[0025] In one example, the determined characteristic of the airflow is a velocity less than or equal to 3 meters per second. Such a range of velocities, often overlooked by traditional methods, meets the specific requirements of industrial and controlled environments.

[0026] In one example, the support includes at least one adjustable mechanism for adjusting the relative position of the emission system and the detection system. Such a mechanism makes the support adaptable to various opening configurations, facilitating its integration into diverse industrial environments. Brief description of the drawings Fig. 1

[0027] [Fig.1] illustrates a unidirectional mirage effect, according to an example of embodiment. Fig. 2

[0028] [Fig.2] illustrates a system for determining a characteristic of an airflow through an opening, according to an example of an embodiment. Description of the implementation methods

[0029] The proposed technique relates, according to one aspect, to a system for determining a characteristic of an airflow through an opening.

[0030] Some specific terms are now clarified for a better understanding of the proposed technique.

[0031] An opening refers to any form of space or passage through which a fluid, such as air, can flow. It can be permanent, like a duct or an opening in a wall or partition, or temporary, like an open or ajar door. A hopper is a specific type of opening, often rectangular or circular, installed in industrial or controlled environments to allow air circulation between two areas while limiting unwanted interactions between those areas. The use of a hopper may, for example, be required by containment or ventilation requirements. An opening may also include one or more devices that allow or modify the passage of air, such as a partially open grille or shutter, provided that these elements do not completely obstruct the airflow.Other examples include a porous membrane that allows air to pass through while filtering out certain particles, an elbow that diverts the flow without blocking it, or a blind or adjustable louvers that modify the direction and flow rate without interrupting it. Conversely, a fully encapsulated duct, that is, one closed by continuous walls and without direct contact with the external environment, does not constitute an opening, as it does not allow exchange between two areas.

[0032] An airflow or air stream through an opening refers to the movement of a volume of air, as a gaseous fluid, between two distinct environments or spaces via that opening. This flow can be caused by differences pressure, temperature variations, or mechanical forces (such as a fan) can influence airflow. The flow can include laminar (regular and structured) or turbulent (chaotic) flows, as well as exchanges involving air mixtures containing particles (aerosols, dust) or secondary gases, provided that the dominant phase remains gaseous. An airflow can be continuous or discontinuous.

[0033] A characteristic of an airflow through an opening refers to any measurable or calculable physical or dynamic property that describes the behavior of the flow through the opening. These characteristics include parameters such as velocity, volumetric flow rate, pressure, direction, temperature, the chemical composition of the moving air, the direction of flow, etc.Determining a flow characteristic can also include simply detecting the existence of a flow (verifying that it is not zero), measuring a velocity gradient (spatial variation of velocity within the opening), a temporal variation of the flow velocity, or calculating an average velocity (spatial and / or temporal) from measurements at several points and / or at several times. Conversely, a static characteristic of air in a stationary environment, unrelated to fluid flow through any opening, cannot be considered a characteristic of airflow through an opening.

[0034] Determining a characteristic of an airflow means any operation or set of operations that provides information about such a characteristic. Determination may thus refer to one or more direct measurements using one or more sensors and / or a calculation based on one or more measurements from this or these sensors. For example, measuring the velocity of a flow using an anemometer is a determination by direct measurement, while calculating a volumetric flow rate using the flow velocity and the area of ​​the opening is an indirect determination by calculation. A velocity sounding is an example of determining a characteristic (in this case, the velocity) of an airflow.

[0035] An element or device is positioned around an opening when it is placed so as to frame that opening, either by being arranged against the edges of the opening or by being placed in the immediate vicinity of the opening, without completely obstructing the airflow. For example, a rigid frame or structure fixed around a hopper to integrate sensors or measuring systems is a device positioned around the opening. This positioning may be temporary or permanent, and it must allow functional interaction with the opening (measurement, control, or adaptation). On the other hand, an element located inside or outside of the opening without directly interacting with its edges or periphery would not be considered as positioned around it.

[0036] A support is a physical structure designed to hold, position, or fix one or more devices and / or systems in a stable and precise manner. It must ensure the functionality of the device(s) and / or system(s) it supports while also ensuring their stability, even in the event of environmental variations (vibrations, external forces, etc.). A support can be rigid (for example, a metal frame), semi-rigid (an articulated arm with controlled flexibility), or flexible (an adjustable deformable rod). Its main function is to hold the device(s), system(s), or element(s) it supports in a defined configuration.

[0037] A device or system is mounted on a support when it is fixed or attached to the support in a stable manner, ensuring proper positioning and orientation. This fixing may be permanent (e.g., by screws or adhesive) or temporary (e.g., by clips, magnetic fasteners, or sliding clips), provided that the device or system remains in position during use. A device or system simply placed on the support without secure fixing would not be considered mounted on the support.

[0038] A support can be adjustable, meaning that it can allow for a change in the position or orientation of the devices it carries relative to the opening. Adjustments can be made along one or more degrees of freedom in translation and / or rotation. The adjustment means can include one or more articulated or flexible arms, one or more sliding fixings, one or more telescopic systems, and / or one or more micrometric adjustments.

[0039] A laser source refers to an optical device capable of generating a coherent, monochromatic beam of light. It can be used to produce an optical signal propagated through an aperture. For example, a semiconductor laser mounted on a support is a laser source. A single laser can be used to produce several distinct beams by beam splitting, using prisms, mirrors, or optical splitters. This simplifies alignment and eliminates power variations between beams. Such beams can retain the coherent and directional properties inherent in the use of a laser source.

[0040] Free-space signal propagation refers to the path of a light beam through an open medium, without physical guidance such as an optical fiber. For example, a laser beam passing through an opening such as a hopper is propagated in free space. This propagation can be influenced by the properties of the medium, such as temperature or air composition.

[0041] A propagation path refers to the trajectory followed by a light signal through a medium. For example, a laser beam emitted from point A to point B follows a propagation path. Propagation paths of a plurality of light rays or light beams can be parallel, convergent or divergent depending on the configuration.

[0042] The path separation is the measurable distance between two distinct paths followed by light signals. Geometrically, if the beams are parallel, the path separation is constant and corresponds to the perpendicular distance between the two paths. For example, two parallel beams separated by 10 mm along their entire length have a path separation of 10 mm. If the beams are converging, the path separation gradually decreases as the beams approach each other, until it becomes zero at the point of convergence. If the beams are diverging, the path separation gradually increases as they move away from each other, and it can be measured at a given point by taking the perpendicular distance between the paths at that point.

[0043] A signal perturbation refers to any change in its intrinsic or dynamic properties, whether intentional or accidental, resulting from internal or external factors. These properties include, for example, the signal's intensity, direction, frequency, phase, or spatial coherence. A perturbation can affect the overall behavior of the signal, altering its ability to transmit information or interact with its environment as expected. Signal perturbations can be classified into two main categories. Internal perturbations are due to variations or instabilities within the signal source itself or its generation mechanisms. For example, internal perturbations can include fluctuations in intensity, frequency, and / or phase due to disturbances at the signal source.External disturbances are induced by interactions of the signal with its environment or unforeseen external conditions, such as vibrations, obstacles, or variations in the propagation medium.

[0044] When a signal is propagated in free space, specific perturbations can occur due to interactions between the signal and its environment. These perturbations can alter various spatial, energetic, or spectral properties of the signal during its path, and they are mainly due to three factors: the medium traversed, the obstacles encountered, and the measurement environment.

[0045] Disturbances related to the medium through which the signal passes can be caused by a temperature gradient. Temperature variations in the air locally modify the refractive index of the medium, resulting in refraction of the signal, which gradually deviates from its rectilinear path. The presence of particles or aerosols can cause a scattering phenomenon, which reduces the signal intensity and alters its coherence. In addition, spectral dispersion can occur if the chemical composition of the medium varies. modifying the spectral width of the signal. Furthermore, atmospheric turbulence, which consists of chaotic fluctuations in density, pressure and / or temperature in the air, causes rapid and unpredictable deformations of the signal's trajectory and shape, resulting in spatial distortions or a loss of coherence.

[0046] Disturbances caused by physical obstacles can be due to the presence of objects with various optical properties. Obstacles such as grids or membranes can cause, at least locally, diffraction, broadening the spatial distribution of the light intensity of a signal and creating interference patterns. Partially or totally opaque obstacles can block or reduce some of the light energy transmitted by the signal, resulting in significant attenuation of the intensity. Furthermore, a reflective surface can redirect part of the signal away from its intended path, generating side beams or spurious signals that can disrupt the analysis of a primary signal.

[0047] Disturbances related to the measurement environment can be caused by the presence of air currents and / or vibrations. Unexpected or non-homogeneous flows around the signal can combine several effects such as scattering, diffraction, or local refraction, causing spatial and temporal variations in the signal properties. Mechanical vibrations of the support or optical system can also induce oscillations in the direction or position of the signal, affecting its alignment and stability.

[0048] A photodetector is a light-sensitive component that converts a light signal into an electrical signal. For example, a photodiode that captures variations in the intensity and / or position of a light beam is a photodetector. A photodetector functions as a measuring device capable of providing a usable response from an optical stimulus. A photodetector is a sensor in the sense that it translates a physical variation, for example, in intensity, position, frequency, and / or phase, into measurable data.

[0049] Detecting a disturbance in a propagated signal involves identifying and measuring any change in the propagated signal. For example, a photodetector can be configured to detect an angular deflection of a beam, a fluctuation in its intensity, a change in its frequency and / or phase. The detected disturbances can be translated into signals usable for analysis, for example in the form of electrical signals, in order to characterize the properties of the medium through which the signal passes.

[0050] A processing unit is a hardware and / or software unit configured to analyze and process signals from a detector or sensor. The processing unit may include a processor, a microcontroller, an FPGA (Field- Programmable Gate Array) or dedicated software. For example, a processing entity can calculate a flow velocity from a detected time lag between two signals, or determine a flow direction by comparing the intensity variations of several beams. It can also perform more complex operations, such as filtering data to remove noise, correlating multiple measurements, or running predictive algorithms.

[0051] Some known techniques for measuring air velocity through an opening are now presented for a better understanding of their limitations and thus the comparative advantages provided by the proposed technique.

[0052] A common approach for measuring air velocity through an opening relies on the use of anemometers to perform spatial sampling at different points in the opening. These points are chosen to be representative of the overall flow. The measured velocities are then combined to obtain an arithmetic or weighted average, intended to reflect the average velocity through the opening.

[0053] However, industrial anemometers exhibit a marked sensitivity to flow direction. For example, evaluations have shown a significant decrease in metrological performance in the presence of a longitudinal displacement (roll angle) greater than 30° or a displacement about the yaw axis (tilt angle) greater than 20°. This orientation dependence makes anemometers unsuitable for omnidirectional measurements or in complex flow conditions.

[0054] Nevertheless, the spatial sampling method remains relevant for measuring a flow velocity at the immediate outlet of a conduit, where the flows are, except in exceptional circumstances (for example in the presence of abrupt obstacles), mainly unidirectional and aligned with the axis of the orifice.

[0055] This method, although conceptually simple, is strongly influenced by the implementation conditions, which can introduce significant uncertainties. Thus, the accuracy of a measurement depends on the distance between the anemometer probe and the opening, particularly in areas of velocity decay. The orientation of the probe relative to the airflow, especially when it is swirling or gyratory, is also likely to introduce measurement biases. Finally, a suboptimal distribution of measurement points or a poorly calibrated scan can lead to data that are not representative of the actual flow. These factors make this method particularly sensitive to the geometric configuration of the opening, the local flow conditions, and the characteristics of the anemometer used.

[0056] There are three main categories of industrial anemometers: propeller anemometers, pressure probe anemometers (Pitot tubes), and thermal anemometers.

[0057] The operation of propeller anemometers is based on the principle of a pseudo-proportionality relationship between the rotational speed of the propeller driven by the airflow and the flow velocity: Vrot = k^ where 7? is the diameter of the helix expressed in meters, Vrot is the rotation speed of the propeller expressed in revolutions per second, Vair is the airflow speed expressed in meters per second, and & is a calibration function that can depend on several parameters, including, for example, the rotation speed of the propeller, the shapes and orientations of the blades, expressed without units.

[0058] Propeller anemometers are generally used in industrial applications: for mobile measurements (typically at the level of the mouths) with a substantial helix diameter (often in portable models with a handle or that can be placed behind a measuring cone). fixed measurement (models without integrated handles on a cylinder that can be inserted into a sheath, generally called precision or laboratory propeller anemometers or windmill anemometers) directly integrated into a pipe (in these cases, we generally speak of a finned wheel).

[0059] There are also cup, reel or vane anemometers, the operation of which is similar to that of propeller anemometers.

[0060] In industrial settings, propeller anemometers are not suitable for measuring airflow velocities below 3 m / s. Indeed, friction and the starting inertia of the propeller blades make measurements obtained at these low speeds very uncertain.

[0061] The operating principle of a pressure probe anemometer, or Pitot tube, consists of collecting the total pressure using an orifice placed facing the flow and the static pressure in an area where the fluid is stationary. It is a tubular device consisting of a cylindrical antenna with two sets of orifices. The first set, positioned around the circumference of the antenna, allows for the measurement of static pressure in the flow. The second set, positioned at the end of the antenna, facing the flow, consists of a single orifice and allows for the measurement of total pressure. The difference between these two pressures is measured at Using a differential pressure sensor, information about the airflow velocity r can be retrieved via the following relationship: p_, where A is a coefficient related to a calibration of the Pitot tube and to Pitot tube compressibility corrections, expressed without units and having a value close to 1, AP is the differential pressure measured by the Pitot tube, expressed in Pascals, and P is the air density, expressed in kilograms per cubic meter.

[0062] In industrial settings, Pitot tubes are not suitable for measuring airflow velocities below 3 m / s. Indeed, at such airflow velocities, differential pressure measurements are only on the order of 5 Pa. At such differential pressures, errors related to measurement noise, environmental fluctuations such as variations in temperature or air density, or calibration inaccuracies become proportionally significant compared to the measured value, resulting in significant uncertainty.

[0063] The general principle of a thermal anemometer is based on the temperature of an element, which can be a wire or a film. The wire of a thermal anemometer is very thin, 2 to 10 microns in diameter, and is generally made of platinum or tungsten. The film of a thermal anemometer is also very thin, 0.5 to 2 microns thick, and is generally made of nickel. The element (wire or film) is heated by the Joule effect and cooled by the airflow. Regulation is performed so that the current or temperature returns to its initial level; the energy required for this regulation is then proportional to the airflow.

[0064] The generally accepted simplified relationship (known as King's law) is as follows: zxn, WHERE v is the airflow velocity, and C is the resistance of the heated element at the considered temperature, expressed in Ohms. Rq is the resistance of the element at room temperature, expressed in Ohms; I is the current used to heat the element to the temperature considered, expressed in Amperes; and a, expressed in Watts, b, expressed in W.sn / min, etn, a unitless exponent, are coefficients determined by calibration and considered independent of v.

[0065] King's law is applicable only under conditions close to those defined during the calibration of the anemometer. These conditions include parameters such as ambient temperature and the physical properties of the air, which influence heat transfer between the heated element and the surrounding environment. Outside of these conditions, the accuracy of the measurement may be compromised.

[0066] To minimize the effects of thermal inertia, anemometer manufacturers generally favor a so-called constant temperature anemometer (CTA) configuration. In this configuration, the resistance Re of the heated element is regulated to maintain a constant temperature, thus providing a fast and accurate response to variations in flow velocity. The temporal resolution of measurements obtained with such a configuration is on the order of 10 to 100 kHz. Conversely, the constant current anemometer (CCA) configuration, where the current I remains fixed and fluctuations in R(i) are measured, is no longer preferred due to the delays induced by the heat capacity of the heated element.

[0067] In practice, among industrial anemometers, only hot-wire or hot-film anemometers are capable of measuring flow velocities below 1 m / s. However, despite their sensitivity, thermal anemometers have significant limitations at low flow velocities, requiring specific calibration. Indeed, the coefficients a, b, and n of King's law are considered constant and independent of the flow velocity. Yet, in reality, the thermal convection that governs heat transfer between the heated element and the surrounding air depends in a complex way on the flow velocity. At very low velocities, phenomena such as temperature differences between the air and the heated element, or local variations in convection conditions, can modify the behavior of the heat flux, making the assumption of constant coefficients imprecise.Thus, without specific calibration for these low speeds, the reliability of the measurements obtained is limited. Furthermore, at low speeds, transient or laminar flow regimes can disturb heat transfer more than flow regimes at higher speeds, which amplifies the discrepancies between physical reality and measurements obtained with constant coefficients a, b, and n.

[0068] Apart from the use of anemometers, it is known to measure the air velocity through an opening using a balometer. The direct measurement of the velocity of an airflow through an opening using a balometer is based on a simple principle: covering the opening with a measuring device that collects the entire airflow. The balometer comprises a collector positioned over the opening being tested and directed towards the flow, as well as a flow meter placed at the outlet of the collector. The collector can be a rigid cone or a fabric on a frame. The airflow through the opening is thus measured by the flow meter, and an average airflow velocity is deduced from the airflow measurement.

[0069] In contexts where air velocities are low, such as air transfers between two rooms, this method has major limitations. Indeed, the balometer collector introduces additional resistance, altering the natural flow rate. flow. This bias is particularly noticeable when the pressure drop induced by the balometer becomes significant relative to the existing pressure differences. The pressure drop is significant when the ratio between the inlet area (connected to the opening) and the outlet area (connected to the flow meter) of the manifold is greater than or equal to 2. To minimize this effect, so-called "compensating" flow meters have been developed, using a method called the "zero method," where pressure control is implemented to compensate for disturbances. However, when the pressure differences are extremely small, as is the case for airflow velocities below 3 m / s, the pressure control introduces variations on the order of this difference, making the measurement inaccurate and metrologically unsuitable.

[0070] Thus, the use of a balometer for velocities below 3 m / s, or below 1 m / s, is limited by two main factors: the bias due to the resistance introduced by the collector, which modifies the actual flow rate, and the difficulty of measuring extremely small pressure differences, which are close to the accuracy limits of industrial sensors.

[0071] In certain industrial contexts, it is necessary to verify that the air velocity through an opening exceeds a minimum threshold, set for example at 0.5 m / s, 1 m / s, 1.5 m / s, 2 m / s, 2.5 m / s or 3 m / s in order to ensure dynamic containment and prevent the spread of contaminants.

[0072] This criterion is particularly relevant in Nuclear Power Plants (NPPs), where the design of mechanical ventilation in hazardous areas is based on strict containment requirements. In NPPs, ventilation of hazardous areas is generally designed to impose a direction of airflow: air must move from an area identified as "safe" to an "at-risk" area, where contaminating gases or aerosols may be present. This airflow dynamic aims to prevent any back-diffusion of contaminants to non-hazardous areas by maintaining an active air barrier. To achieve this, a common method is to use transfer ventilation, in which fresh or mixed air is directed into the hazardous area through an opening (often a shaft), generally located in a wall or door.This system differs from conventional ventilation (supply and return in the same room) by utilizing a dedicated opening to maintain a directed airflow.

[0073] To guarantee the effectiveness of this transfer ventilation, several criteria can be evaluated, including a pressure difference (or differential pressure) between the rooms to maintain an airflow directed towards the risk zone, and a minimum air velocity through the opening, ensuring decoupling Sufficient airflow is required to limit the spread of contaminants. Historically, some nuclear power plants have referred to ISO 11933-4, which recommends a minimum air velocity of 0.5 m / s for various types of contaminants. This velocity must be verified through specific control tests, taking into account actual operating parameters. These verifications are necessary to ensure the maintenance of effective dynamic containment in accordance with applicable standards and guidelines.

[0074] Similar approaches are also implemented in contexts such as controlled area work sites or nuclear facilities undergoing decommissioning. For work in nuclear power plants, a minimum overall airflow velocity of 0.5 m / s is recommended at airlock openings (such as door slats or penetrations) with the doors closed. The recommended overall airflow velocity is 1 m / s. ISO 17873 is often used as a reference for defining these minimum velocities. In nuclear facilities undergoing decommissioning, a minimum overall airflow velocity of 1 m / s is recommended for an orifice with a diameter of 100 mm. This criterion is used to ensure effective dynamic containment in enclosures containing contaminants.

[0075] Measuring these minimum speeds is not trivial, because it must take into account several physical phenomena influencing airflows.

[0076] At an opening, the airflow is influenced by the differential pressure AP and the geometry of the opening. The infiltration velocity can be described by a power law: Vinf^Cdii^)^ where Cdi, expressed in m / s / Pa11, is a coefficient depending on the local contraction of the airflow through the opening, P is the air density, and n, varying between 0.5 and 1, is a flow rate exponent related to the geometric and dynamic characteristics of the opening.

[0077] Once in a free volume, the air velocity decreases as a function of the distance traveled. This decrease also follows a power law: where Vo, expressed in m / s, is the initial velocity of the airflow at the target point, K is a coefficient dependent on environmental conditions (such as the Reynolds number), X is a distance, expressed in meters, from the opening, and n is an exponent generally varying between 0.5 and 1 (for example, n = 0.5 for a planar jet and n = 1 for a circular jet).

[0078] To estimate a distance X below which the decrease in airflow velocity between the two beams can be considered negligible, one can rely on the typical values ​​of the coefficient^, the exponent R, and the initial velocity Vo. When^ is less than 10 mm, and for airflow velocities less than or equal to 1 m / s, the velocity decrease is less than 5% and can reasonably be ignored.

[0079] These phenomena make the measurements sensitive to the geometry of the hopper, the surrounding conditions and the measuring devices used, thus increasing the uncertainties.

[0080] The hopper typologies encountered for use cases are generally hollow rectangular orifices with dimensions between 150 and 500 mm.

[0081] The Reynolds number, Re, is defined by the formula where P is the mass volumetric volume of air, v is the air flow velocity, L is a linear dimension characteristic of the opening and is the dynamic viscosity of the air.

[0082] Flow type boundary zones are generally defined as follows: Re < 2000 corresponds to laminar flow, 2000 <Re< 4000 correspond à un écoulement en transition, et 4000 < Re correspond à un écoulement turbulent

[0083] The minimum speed sought being around 0.5 m / s, under normal operating conditions (20 to 40 °C), the numerical application gives a Reynolds number at the outlet of the hopper of at least 5000, characteristic of a turbulent flow.

[0084] Turbulent flow is subject to random disturbances or fluctuations consisting of a wide variety of scales and frequencies. Since the air distributed through the hopper comes from an area conditioned with dissipative equipment, the temperature of the air passing through the hopper is assumed to be non-uniform. Therefore, a propagation of temperature fluctuations via the flow is expected (and thus a variation in the refractive index). This effect is accentuated by the heterogeneity of the temperature of the air traversed.

[0085] However, when grids are positioned at the outlet of a hopper, the flow can then be laminar, no longer allowing, a priori, for random effects capable of producing a signature. In these scenarios, a source of perturbation of the optical index can be expected, in a way that is known per se. In particular, a known way to modify the optical index, frequently used in laser velocimetry techniques, consists of injecting an optical tracer into the flow. Tracers are frequently used in the laboratory for particle image velocimetry applications. PIV (Particle Tracking Velocimetry), PTV (Particle Tracking Velocimetry), and LDV (Laser Doppler Velocimetry) are examples of methods used with helium-filled soap bubbles, suspended oil droplets, or gas vapors, such as propylene glycol. The use of these methods also induces absorption effects, resulting in fluctuations in the transmitted optical power.

[0086] Two common approaches were tested in the laboratory to verify the criterion of compliance with a minimum speed set at a value less than or equal to 1 m / s through a hopper of 100 mm diameter.

[0087] According to a first approach, a point velocity sampling method was adopted. In this method, the surface of the hopper was divided into a grid to locally measure the airflow velocity at several points using a thermal anemometer. An average flow velocity was then calculated from the point measurements obtained. However, this method has metrological weaknesses for several reasons. The measurement interface is located at the geometric boundary between two distinct environments: a hopper of fixed dimensions, where the flow can be approximated as forced flow in a pipe, and a free volume, where the flow is considered to be free surface flow.The shape of the velocity profile and the linear decrease in velocity within this zone depend on numerous parameters, such as the size and depth of the hopper, the presence of upstream or downstream grates, and geometric obstacles located before, inside, or after the hopper. These factors result in a velocity distribution that is neither symmetrical nor homogeneous, complicating the representativeness of point measurements. To reduce uncertainties and obtain representative results, a large number of precisely positioned measurement points would be required, significantly increasing the implementation complexity.

[0088] A known variant of this method, using a hot-wire anemometer, proposes calculating the average velocity from four point measurements and then multiplying this value by a correction factor Kv. This factor depends on multiple factors, such as the type and dimensions of the hopper, the characteristics of its internal and external environment, and the measurement distance between the hot wire and the opening. However, Kv is very sensitive to these factors, making its use metrologically unreliable as soon as the conditions differ slightly from those characterized in the laboratory. Even when this factor is metrologically validated for a given configuration, a significant uncertainty remains.

[0089] Moreover, since the speeds sought are often less than 1 m / s, and industrial thermal anemometers are not suitable for speeds close to At 0.5 m / s, spot measurements at these low speeds become difficult to use.

[0090] According to a second approach, a direct method was adopted using a balometer, where the manifold covers the hopper and measures the overall airflow through the opening to deduce, from this measured flow rate, the average airflow velocity. This method showed significant deviations from the reference values, particularly in the blowing configuration. These deviations are mainly due to the additional resistance introduced by the balometer manifold, which alters the actual airflow rate through the hopper. Even with compensating devices designed to correct these biases, the results obtained are not sufficiently reliable to meet metrological requirements.

[0091] Neither of the two methods tested in the laboratory (point sounding with a thermal anemometer or the use of a balometer) produced results reliable enough for industrial application. The point sounding method suffers from high sensitivity to local and environmental parameters, particularly via the correction factor Kv, which introduces significant uncertainty. The balometer method is affected by significant biases related to the perturbation of the natural flow, even with attempts at compensation. These limitations demonstrate that neither the thermal anemometer nor the balometer can accurately measure velocities below 1 m / s or meet metrological requirements in configurations requiring high reliability. An alternative solution is therefore necessary to guarantee accurate and reliable measurements in these specific contexts.

[0092] The proposed technique aims to overcome the limitations of known techniques for determining airflow characteristics through openings.

[0093] The physical principle on which the proposed technique is based is the mirage effect. As illustrated in [Fig. 1], in geometric optics, the mirage effect is a refraction phenomenon characterized by the angular deviation of light beams (21) when they pass through a medium exhibiting a refractive index gradient. Such a gradient can be induced, for example, by a temperature gradient resulting from an airflow (2).

[0094] In a one-dimensional approximation, and for a small index gradient, assuming that the deviation of the light beam occurs in a plane, the angle of deviation can be expressed by the following relation: "dx where D is the length, expressed in meters, of the optical path traveled by the light beam through the medium, n is the optical index of the medium, and ΔI is the index gradient within the medium in the considered direction. dx

[0095] The variation of the optical index in layers of air at different temperatures can be described by the Gladstone-Dale law, which relates the optical index n of a gas (for example air) to its density P: n - 1 = k . p where k is a constant (in air, k is about 2.26 x 10⁴ m³ / kg), and P is the density, expressed in kg / m³, of the gas, which can be considered inversely proportional to its temperature (in Kelvin) and proportional to its pressure (in Pascal).

[0096] Measuring the velocity of a fluid flow, such as air, using the mirage effect relies on detecting optical disturbances caused by refractive index gradients in the medium through which a light beam passes. These disturbances, also called "optical noise," result from local variations in the density, composition, and / or temperature of the air, these local variations being induced by the fluid flow. By tracking the propagation and fluctuations of these disturbances through the light beam, it is possible to correlate their movement with the dynamics of the fluid flow.

[0097] As illustrated in [Fig. 2], according to one aspect of the proposed technique, a system (1) is presented that is designed to take advantage of the properties of the mirage effect in order to determine a characteristic of an airflow (2) through an opening (3). This determination system comprises the following elements: a support configured to be positioned around the opening, a system for emitting at least two signals (21, 22) propagated in free space, a system for detecting disturbances (42, 43) in the propagated signals, and a processing unit (40). 1. In one embodiment, the support is a U-shaped frame whose dimensions are equal to or greater than those of the opening it surrounds. This frame can be attached to the opening or positioned flush with it. The frame can, for example, be adjustable in height and width, allowing for precise adaptation to the opening. The arms of the U can be positioned to frame the lateral and lower (or upper) edges of the opening. The frame carries two systems mounted respectively on the left and right arms of the U, thus on either side of the opening when the frame is attached to the opening or positioned flush with it. These systems can, for example, be attached using sliding fasteners. These sliding fasteners can be configured to allow for adjustment The systems can be positioned vertically, independently or jointly, which can facilitate their alignment for uniform measurements or their sequential movement to scan the entire height of the opening. For example, two systems can be positioned at different heights corresponding to specific fluxes or areas where thermal gradients are expected. It is understood that the support geometry is not limited to a U-shaped frame. The support can take various forms adapted to the geometry of the opening (e.g., a closed rectangular frame, a circular ring, a polygonal frame, etc.). It is also understood that the sliding fixings can be replaced by any adjustable mechanism allowing for adjustment of the relative position of the emission and detection systems.

[0098] The support can be configured to include additional features to optimize measurements taken through the aperture. For example, the support can be equipped with motorized width and height adjustment systems, facilitating precise and rapid alignment of the mounted devices and enabling efficient sampling of several areas of the aperture at various positions. These motorized adjustments can also automate the movement of the measurement systems to perform sequential scans, thus minimizing manual intervention and improving measurement reproducibility. To limit the influence of unwanted optical contributions outside the measurement area of ​​interest, the support can be configured to incorporate occulting elements or optical screens.These elements can be positioned to restrict the measurement field to only the light propagation lines crossing the targeted airflow, thus avoiding biases caused by disturbances outside the analysis area. Alternatively, the support can simply be dimensioned appropriately to precisely frame the measurement field of interest, so that only the relevant light lines are intercepted by the detection system. The support can also be designed to offer increased mechanical rigidity, reducing vibrations or deformations that could affect measurement accuracy. For example, a rigid support can be made from materials with a low coefficient of thermal expansion (such as special metal alloys or reinforced composites) to ensure dimensional stability even under varying temperature conditions.Another optional feature of the mount is the ability to include integrated calibration devices. For example, optical targets or reflectors can be attached to the mount to periodically calibrate the position and intensity of the light beams, thus ensuring increased measurement reliability. Finally, the mount can be configured for... This allows for increased modularity by permitting the addition or removal of devices according to specific measurement needs. This modularity can, for example, include articulated or removable arms that allow elements of the transmission and / or detection system to be repositioned or reoriented without requiring complete dismantling of the support. In one embodiment, the support can be equipped with a suspended structure, such as a motorized rail, allowing the entire measurement system to be moved along a hopper or a large opening. Such a configuration can, for example, enable measurements to be taken in areas that are difficult to access or that require precise adjustments in multiple dimensions.

[0099] The signal emission system comprises a single laser source configured to emit a light beam that is split, for example by an optical splitter, into at least two distinct signals. For example, a beam splitter having a cube or blade structure may be provided to divide the beam light, for example, in a 50 / 50 ratio. One advantage of using a single laser source is that it eliminates pointing noise and power fluctuations in the source.

[0100] Optical elements, such as lenses or mirrors, can be used in the emission system to modify the propagation paths of the beam and / or the signals resulting from this splitting. For example, a converging lens can be positioned immediately after the beam split to focus the beams from the splitting at specific points downstream, thus creating converging beams. Conversely, a diverging lens can be used to create diverging beams. Mirrors can be used to redirect the emitted beams, allowing complex propagation paths to be configured. For example, parallel emitted beams can be reflected to become diverging or converging, or even to bypass an obstacle.

[0101] According to one embodiment, the beam emitted by the laser source is split optically into at least two distinct beams. Such a split can be implemented to generate parallel, converging, or diverging beams. The spatial separation between the beams, particularly when they are parallel, can be chosen according to various criteria, for example, based on the dimensions of the aperture, the expected characteristics of the airflow (e.g., areas of high turbulence or thermal gradient), and / or the results of previous measurements. This diversity of configurations allows the system to be adapted to different applications, such as the detection of the velocity, direction, or other characteristics of the airflow.

[0102] The system for detecting disturbances in the propagated signals is configured to capture the light beams emitted by the emitting system after their propagation through the medium. The detection system can be designed to adapt to different signal configurations (parallel or not) and to use one or more photodetectors. Its role is to detect disturbances affecting the captured light beams. The detection system converts these variations into electrical signals or any other usable format, which it transmits to the processing unit for further analysis.

[0103] The processing entity is configured to receive information from the detection system and to analyze this information in order to determine one or more characteristics of the airflow.

[0104] The joint operation of the detection system and the processing entity is based on: the interception, by the detection system (30), of light signals (21, 22) having distinct propagation paths, and the determination, by the processing entity (40), of a time lag (41) between fluctuations induced by the airflow at the propagation paths of these intercepted signals.

[0105] Specifically, the processing entity may, for example, analyze the information received to: detect characteristic fluctuations of disturbances in light signals propagated through the aperture (e.g., temporal variations or specific amplitudes), measure the time lag (41) between similar disturbances detected in different signals, captured successively by one or more photodetectors, and calculate airflow characteristics using the measured time lag and the known spatial separation between the propagation paths of the signals considered.

[0106] Several examples of possible configurations of the detection system are now presented.

[0107] In one example configuration, when the emitting system is configured to emit parallel beams (i.e., beams with parallel propagation paths), the detection system may include a plurality of photodetectors (31, 32) positioned to intercept each of the beams respectively. For example, if two parallel beams are emitted, two photodetectors may be spaced the same distance apart as the beams. The photodetectors may be of the same type or of different types. For example, two (or more) position detectors and / or two (or more) light intensity detectors may each be positioned along the propagation path of a corresponding beam. In this configuration, the photodetectors can, for example, be aligned at the same altitude as the laser source to allow for uniform measurements on the same horizontal plane, thus improving detection accuracy. In this case, the measurement principle relies on the successive detection of the deflection of one beam by a photodetector, followed by the corresponding deflection of another beam by a different photodetector. These detections allow for the measurement of a time lag between the observed disturbances in the beams.

[0108] In alternative configurations, the beams may not be parallel, but convergent or divergent. As in the previous configuration, the detection system may then comprise a plurality of position detectors and / or a plurality of light intensity detectors, one detector being positioned on the propagation path of a corresponding beam.

[0109] Another possible configuration involves using a single photodetector to successively detect disturbances in two beams. This configuration relies on the precise positioning of the photodetector in an area where the beam propagation paths are sufficiently close to allow their interception at different times. A suitable optical system, comprising, for example, at least one lens, at least one mirror, and / or at least one prism, can be used in the detection system to direct several beams (parallel or not) onto the same photodetector. This configuration reduces the number of photodetectors required while maintaining the ability to detect disturbances in each beam. The beam fluctuations, although focused at a single point, remain discernible to the photodetector according to their respective timing.However, this requires sufficient temporal decoupling between the disturbances to allow their differentiation.

[0110] Regardless of the configuration considered, the measurement principle remains based on the successive detection of disturbances (42, 43) in several propagated signals and the determination of a time lag (41) between these detections. Using this measured time lag and the known spatial separation between the beam propagation paths, the processing entity can determine a characteristic of the airflow (for example, a local or average airflow velocity). The calculation may include one or more correction terms, which can, for example, be determined during a calibration of the detection system, so as to improve the accuracy of the flow velocity measurement.These corrective terms can, for example, take into account the spatial configuration of the beams, in particular when the beam propagation paths are not parallel and / or are not in the same horizontal plane and / or are affected by the use of an optical system. These corrective terms can, for example, take into account the spatial configuration of the opening, particularly when this configuration induces complex temperature or density gradients in the air at low flow velocities.

[0111] In a configuration where the detection system (30) comprises several photodetectors (31, 32), the support can be configured to allow: movement of the entire detection system, for example by translation along an axis, while maintaining a fixed spatial spacing between the photodetectors; and / or an independent displacement of each photodetector relative to another, for example by translation, to adjust their relative position according to specific measurement needs or variations in airflow.

[0112] This flexibility can also be applied to the emission system (20), allowing control of the spatial separation between two beams (21, 22) or their converging, parallel, or diverging nature. For example, the support can be configured to allow coordinated movement of the emitters to maintain a constant separation between parallel beams. It can also allow independent movement of each emitter, so as to adjust the relative angle between the beams and choose a converging or diverging configuration.

[0113] In one embodiment, two parallel light beams, separated by a distance dx, are emitted in the same horizontal plane. Two photodetectors (31, 32) are positioned to capture the beams respectively, with a spacing identical to that of the beams (dx). In this example, the flow velocity can be calculated by the processing unit as follows. In the absence of air or when the medium is homogeneous (without an optical index gradient), the light beams follow straight paths. Each photodetector records a constant signal corresponding to the light intensity of the intercepted beam, assuming that noise sources in the measurement chains (e.g., vibrations or power fluctuations of the laser source) are negligible.When air with a different refractive index than the surrounding environment (for example, heated or cooled air) passes through the beams, refractive index gradients appear. These gradients cause the beams to deflect due to refraction, altering the light path. If the refractive index of the flowing air varies over time (for example, due to turbulence or fluctuating temperature gradients), the beam deflection also changes. Each photodetector then records fluctuations in the light signal, these fluctuations being representative of the evolution of the disturbances in the flow. Since the two beams are separated by a distance dx, the disturbances caused by the airflow propagate from one beam to the other with a time delay dt, related to . the air speed. This time lag dt can be measured by the processing unit by comparing the signal variations recorded by the two photodetectors.

[0114] The air flow velocity (r) can thus be determined from the following relation: r _ ^where dx is the distance, expressed in meters, between the two beams (or between the two photodetectors), and dt is the time shift, expressed in seconds, measured by the detection system between similar disturbances recorded by the two photodetectors.

[0115] In addition to determining a velocity, the processing entity can be configured to determine other characteristics of the airflow. For example, the time lag measured from an analysis by the processing entity of variations in the intensity or position of signals captured by the detection system, and the known spatial separation between the propagation paths of these signals through the aperture, can be used to identify a thermal gradient and / or turbulence in the airflow and / or to quantify such a thermal gradient and / or the intensity and / or frequency of such turbulence. The processing entity can also be configured to detect asymmetries in perturbations of non-parallel beams to infer a flow direction.

[0116] Within the general principle of detecting and processing disturbances in propagated light signals, several photodetection technologies can be used to implement the detection system. Each of these technologies offers distinct advantages depending on the intended application. The choice of one technology over another may also be influenced by the possibility of adding auxiliary elements, such as an optical tracer (for example, smoke), to enhance the optical refractive index disturbance caused by the airflow and facilitate detection.

[0117] Position-sensitive detectors (PSDs) are an example of photodetectors commonly used to measure the position of a light spot. A one-dimensional position photodiode typically consists of a PIN diode comprising a P-doped (positively charged) layer or region, an undoped (intrinsic) layer or region, and an N-doped (negatively charged) layer or region, two electrodes connected to the ends of the P-doped region, and a common cathode. The active area of ​​such a photodiode extends over at least a portion of the P-doped region. This active area may, for example, be rectangular, with length L and width l.

[0118] When a signal is captured by a position photodiode, any deviation of the signal that may be induced by a change in the refractive index of the medium propagation results in an evolution of the electrical signals generated by the photodiode.

[0119] Indeed, when a light beam is captured by a position photodiode, a one-dimensional displacement dx of the beam relative to the center of the photodiode's active surface can be deduced from the electrical signals it generates. The photodiode generates a total photocurrent It, a function of the captured light intensity, which is divided into two currents h and h, respectively connecting the two anodes to the common cathode. If the light signal is not centered on the active surface but offset by a distance x from the center, in the direction of the length L of the active surface (assumed to be rectangular), the intensities are given by the following relationships: 7t- pr(fx) et / 2 = p^x + |j, where Pp is the homogeneous resistivity of the P-doped region of the photodiode.

[0120] The one-dimensional displacement dx can then be calculated from the intensities measured I and L according to the relation dx- - ( / - / ,). This relation shows that dx is ( / ,+ / 2) directly proportional to the relative difference between the two measured electric intensities. If the displacement dx is small compared to the length L, it is also proportional to the variation of the optical index of the medium traversed, according to the relation ^x _ where D is the length of the optical path traveled by the light beam, n the average angle of refraction of the propagation medium and ® the angle of deviation of the beam (also called "angular deviation" of the beam). The variation in refractive index dn is itself related to the air density P by the relation n⁻¹ = k . p, where k is the Gladstone-Dale constant. Under certain assumptions (constant pressure and constant relative humidity), P is inversely proportional to temperature. Thus, a temperature variation causes a beam deflection, which can be expressed as the displacement ^x _ ^.^p.

[0121] The beam deviation angle can be estimated by applying Snell's law: arcsin(π) = arcsin(π)p "i and are the refractive indices of the propagation medium before and after the temperature variation, and #iest is the angle of incidence of the light beam relative to the normal to the interface where the temperature variation is located.

[0122] For a temperature variation AT of 0.1°C around 20°C, and applying the usual simplifications for small angles, the value is approximately 3.5 x 104 radians or 0.02°. The corresponding density variation dp is approximately 4.1 x 104 kg / m3 and the corresponding one-dimensional displacement dx is approximately equal to 80 pm. The resolution of the photodetector must therefore be less than, if possible much less than, this order of magnitude of displacement for accurate detection.

[0123] For two-dimensional measurements, four rectangular position photodiodes can, for example, be arranged to form a quadrant detector. Analog processing of the electrical signals between the anodes and cathode of each photodiode makes it possible to provide a position measurement (x, j) on the quadrant detector of a light signal captured at a given instant along an x-axis and a y-axis: X = ~ ----V - ( / ,+ / 2+ / 3+ / 4) L ■ y=7 -----? 2 ( / ,+ / 2+ / 3+ / 4)

[0124] A quadrant detector therefore allows a precise measurement of the position of the light beam in a plane, facilitating the identification of angular deviations and spatial fluctuations.

[0125] In certain configurations, a doublet of simple, light-sensitive photodiodes can be used. This type of detection is particularly relevant when the change in refractive index is accompanied by light absorption (for example, in the presence of an optical tracer). The signature of the change in light intensity is then directly used to characterize the disturbances.

[0126] When it comes to detecting a deviation in a light signal induced by variations in the refractive index, in a plane of propagation of the assumed unidirectional flow, the use of a position photodiode is often preferable. This type of photodiode allows for the direct detection of a signature representative of the displacement of the light beam, thus providing an accurate interpretation of spatial fluctuations. In configurations where only the displacement of the beam is relevant (for example, to measure a flow velocity or a temperature gradient), the simplicity and accuracy of detection by a position photodiode make it suitable.

[0127] In laboratory tests, position detectors demonstrated sufficient sensitivity to capture the displacements of light beams, but their performance can be limited without the use of an optical tracer. The addition of optical tracers in specific environments improves the overall efficiency of the system by increasing the contrast of light fluctuations related to variations in refractive index.

[0128] However, in configurations where light absorption or global intensity variations are also relevant indicators (for example, in the presence of an optical tracer to accentuate disturbances), the use of light intensity photodiodes can complement or replace measurements from position photodiodes.

[0129] Light intensity photodiodes are photodetectors designed to directly exploit the electrical response generated by light incident on their sensitive surface. This response is generally proportional to the light intensity captured, and their transfer function is linear for a given wavelength. The main advantage of light intensity photodiodes lies in their increased sensitivity, which allows for precise measurement of light intensity fluctuations, even for small or rapid variations in the light beam.

[0130] In the context of detecting a disturbance affecting the propagation of a light signal in free space, a segmented (multi-sided) photodiode can be used to measure the angular deviation of a light beam, for example a deviation of the order of 0.02°, caused by a mirage effect linked to a variation of optical index in the medium traversed.

[0131] A segmented photodiode is divided into several active areas, called segments, each capable of generating an electrical signal in response to incident light. For example, a two-segment photodiode is composed of two sensitive parts (designated A and B). The operating principle of a segmented photodiode is based on analyzing the differences between the electrical signals generated by adjacent segments when a light beam initially captured by the segmented photodiode is deflected laterally.

[0132] In free-space optical detection, a disturbance in the medium (for example, a thermal gradient) causes an angular deflection of the light beam. This deflection causes a displacement of the beam's point of impact on the sensitive surface of the segmented photodiode, thus modifying the distribution of light captured by segments A and B. The difference in the electrical signals generated by these segments is directly proportional to the lateral displacement of the light beam, while their sum remains proportional to the total light intensity of the captured beam. This principle makes it possible to deduce both the beam deflection and, in some cases, fluctuations in its overall intensity.

[0133] In one embodiment, if a two-segment photodiode A and B is exposed to a laterally deflected light beam, the surfaces SA and SB illuminated on segments A and B respectively can be estimated by the following relations: c R2 , j \ and c R2 { , A \, where R is the radius $A-T (Æ"4a) —(æ + 4«)' j of the light beam and a is the angle of deviation of the light beam. The difference between the illuminated surfaces can be estimated by the relation g_Σ = The electric intensity generated by each segment can be considered proportional to the corresponding illuminated area, thus Ia - K $a SB, where K is a constant related to the sensitivity of the photodiode and the luminous flux. Thus, IB-IA = K {7rR24a)et + K nR1.

[0134] It follows that JLÇ. _ By applying an approximation to small angles, to knowing A _ [an(g ), this relation becomes _ 4d .

[0135] In one embodiment, two segmented photodiodes are positioned to intercept two distinct light beams, each having passed through an airflow. These light beams, initially aligned, undergo lateral displacement caused by the mirage effect, due to refractive index perturbations generated by the airflow. This displacement affects the two beams successively, with a time lag corresponding to the flow velocity.

[0136] The first segmented photodiode has an active surface divided into two sensitive parts, called A1 and B2. The light from the first intercepted beam is distributed over these two sensitive parts according to the position of the beam on the active surface. Similarly, the second segmented photodiode is composed of two sensitive parts A2 and B2, and intercepts the second light beam.

[0137] The airflow causes a lateral displacement of the first beam on the first photodiode, followed by a displacement of the second beam on the second photodiode. This lateral displacement modifies the distribution of light between the sensitive parts of each photodiode, resulting in a variation of the generated electrical intensities. IA and Ib represent the respective electrical intensities generated by the sensitive parts A and B of the first photodiode. Ib and Ib represent the respective electrical intensities generated by the sensitive parts A and B of the second photodiode.

[0138] The difference between these intensities for a photodiode is proportional to the lateral displacement of the light beam on its active surface. Furthermore, the sum of the intensities for a photodiode is proportional to the total intensity of the captured light beam. This allows us to define a relative beam displacement for each photodiode based on the following relationships: , r , and} ib Aa2 r , where dt is the lateral displacement measured on the first photodiode, d2 is the lateral displacement measured on the second photodiode, and is the radius of the light spot intercepted by each photodiode.

[0139] The relative offset between the displacements measured on the two photodiodes, caused by the airflow, can then be calculated as follows: , , ^Is ^ ' \R ■ d2 ' « 1=

[0140] This relative shift d2-dx is directly related to the perturbation caused by the airflow. When this offset is analyzed together with the time offset dt measured by the processing entity between the disturbances captured successively by the two photodiodes, it is possible to determine precise characteristics of the airflow, such as its velocity, via the relation v = dx!dt, where dx corresponds to the spatial separation between the two beams; or information on the thermal gradients or turbulences that affect the flow.

[0141] The proposed technique also covers, according to one aspect, a method using the system for determining a characteristic of an airflow through an opening to determine such a characteristic of such an airflow through such an opening,

[0142] The proposed technique also covers, in one aspect, a computer program comprising instructions which, when the program is implemented by a processor (for example, a processor of the processing entity), lead to the implementation of such a process.

[0143] The proposed technique also covers, according to one aspect, a storage device, for example a computer memory of any known type, storing in a transient or non-transient manner all or part of the instructions of such a program.

[0144] These technical solutions can be implemented in various contexts where it is necessary to evaluate airflows accurately, particularly in industrial, commercial or controlled environments.

[0145] Possible applications include: the measurement of low air velocities through openings, such as hoppers or passages between two areas with small pressure differences (on the order of Pascals), monitoring flow rates in ventilation ducts, for example to optimize air conditioning or air treatment systems, or the control of infiltration rate in confined or pressurized enclosures, such as depressurized rooms for the containment of contaminants.

[0146] The present technical solutions make it possible to carry out measurements in a non-intrusive manner, including in demanding or sensitive industrial environments, such as nuclear power plants (for example, measurements at the level of hoppers of air transfer) or laboratories requiring rigorous airflow controls.

[0147] The proposed solution can be adapted, simplified or made more complex depending on the objectives. For example, it can be miniaturized for applications requiring integration into confined spaces, or it can be combined with additional optical elements to increase the sensitivity or range of the measurements.

[0148] This disclosure is not limited to the examples described above, which are given for illustrative purposes only, but encompasses all the variations that a person skilled in the art can consider in the context of the protection sought.

Claims

Demands

1. System (1) for determining a characteristic of an airflow (2) through an opening (3), the system comprising: a support (10) configured to be positioned around the opening, an emission system (20), by a single laser source, of signals (21, 22) propagated in free space, mounted on the support, a disturbance detection system (30) of disturbances (42, 43) in the propagated signals, mounted on the support, and a processing entity (40) configured to: measure a time lag (41) between two detections, by the detection system, of at least one disturbance in signals emitted by the emission system and propagated through the opening along distinct propagation paths, and determine a characteristic of the airflow, taking into account the measured time lag and a spatial separation between the propagation paths.

2. System according to the preceding claim, wherein the transmission system is configured to generate signals propagated along parallel propagation paths.

3. A system according to any one of the preceding claims, wherein the transmission system and the detection system are configured to operate with a predetermined spatial spacing between the propagation paths.

4. System according to any one of the preceding claims, wherein the detection system comprises at least two detectors (31, 32) having a fixed spatial spacing.

5. System according to any one of the preceding claims, wherein the emission system includes an optical splitting device for the single laser source to generate the propagated signals.

6. System according to any one of the preceding claims, wherein the perturbation is an angular deviation induced by a variation in optical index caused by the airflow.

7. System according to any one of the preceding claims, wherein the determined characteristic of the airflow is a velocity less than or equal to 3 meters per second.

8. A system according to any one of the preceding claims, wherein the support comprises at least one adjustable mechanism allowing to adjust the relative position of the emission system and the detection system.

9. Method for determining a characteristic of an airflow through an opening, the method comprising: a measurement of a time lag between two detections of at least one disturbance in signals emitted by an emission system comprising a single laser source and propagated through the opening along distinct propagation paths, and a determination of the characteristic of the airflow, taking into account the measured time lag and a spatial separation between the propagation paths.

10. A computer program comprising instructions which, when the program is implemented by a processor, lead to the implementation of the method according to the preceding claim.

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