EP0446134A1 - High flow monodispersed aerosol generating device - Google Patents

Abstract

The aerosol-generating device, of the sprayer type having a rotating disc, comprising a plane disc (1), dish or axisymmetric member rotating at high speed, a tube (4) for supplying the liquid to be dispersed or a liquid reservoir fitted at its base with an orifice which emerges at the centre of the axisymmetric member or in its vicinity, being able to comprise concentric obstacles disposed on the surface of the axisymmetric member, and of the type in which the rotational speed, the dimensions of the rotated member and the liquid flow rate define operating conditions such that fluid jets are formed around the periphery of the rotating member, the device being characterised in that it comprises means for disturbing the flow of the liquid to be dispersed which generate a capillary wave propagating towards the periphery of the rotating member. Application to experimental research, to agriculture. <IMAGE>

Description

The present invention relates to a device generating with a high flow rate a monodispersed aerosol, that is to say of which the droplets have a low statistical dispersion of their diameter around a defined value.

Such devices are particularly applicable to experimental research, a field in which the criterion of monodispersion is important. Monodispersed aerosol generators are used in porosity studies, in the medical field, in the pharmaceutical field. One can also consider their application to agriculture, since studies have shown that there is an optimal size for which the droplets of an aerosol are small enough to reach all parts of a plant but large enough to not be blown away by the wind. A device generating a high flow rate a monodispersed aerosol has an increased efficiency compared to known sprayers.

Fluid mechanics, and more particularly the theory of jets, shows that by subjecting a jet of fluid to a capillary wave of adequate amplitude and frequency, propagating along the jet of fluid, the jet is fragmented into droplets whose the diameter is of the order of magnitude of the half wavelength of the capillary wave.

Monodispersed aerosol generating devices are already known using this principle, in particular of the vibrating orifice type and of the electrostatic excitation type.

Monodispersed aerosol generating devices of the vibrating orifice type subject the jet of liquid coming from the orifice to a mechanical vibration of the acoustic type, which generates a capillary wave which fragments it into a monodispersed aerosol. However, the flow of liquid thus sprayed by a single jet is low. Monodispersed aerosol generating devices of the electrostatic excitation type are used only with a dielectric fluid, distributed at the periphery of a disc by the slow rotation of the latter, and ejected in the form of jets from the edge of the disc by a powerful continuous electrostatic field, while an alternating field is superimposed on this continuous field which will cause the jets to vibrate as desired and fragment them into a monodispersed aerosol.

Furthermore, aerosol generating devices have been known for a long time using a rotating disc. It is known that, if a stream of liquid is sent perpendicular to a rotating disc and at its center, this liquid spreads out on the disc under the action of centrifugal force, then resolves at the periphery according to three physical processes, depending on the operating conditions of the device: Formation of droplets, formation of jets, or formation of films.

In rotating disc devices, the conditions for forming jets, apart from the conditions for forming droplets or films, are reached for values of surface tension, flow rate, radius of the disc, density, viscosity, and the speed of rotation given in the LEUTEURTROIS formulas. Under the conditions of jet formation, the surface tension forces are weak compared to the centrifugal forces. Filaments or jets are formed in regular distribution around the disc. Their diameter decreases as one moves away from the center of the disc, until the jet resolves in drops. When the jet breaks, there is usually a main drop and one or more smaller drops called sattelites. In theory, at constant operating parameters, all the jets should fragment at the same distance from the disc; gold, mechanics never being perfect, there are poorly controlled instabilities and the length of the jets is not stationary but random, the result is that the size of the droplets and sattelites is heterogeneous. The primary aerosol collected (main droplets) is therefore not monodispersed.

Improvements have long been made to rotating disc sprayers in order to increase the flow rate. For example, patent FR 687,144 describes a disc provided with concentric grooves or ribs. However, in such a device, the concentric obstacles are disposed between them with any spacing on the disc, with no concern other than to oppose the direct flow of the fluid on the disc in order to better distribute it on the periphery and increasing the flow rate of dispersed fluid.

In conclusion on this state of the art, it can be said that in the context of monodispersed aerosol generators with supermicronic particles (20 to 1000 micrometers), rotating disc devices have the advantage of spraying more liquid than the generators of aerosols of the vibrating orifice type, but with a large heterogeneity in particle size of the aerosol formed. Although the hydraulics of rotating discs or cups seems to have been known for a long time, there is currently no technology known which makes it possible, from a device of the rotating disc sprayer type, to produce primary monodispersed aerosols free secondary aerosols.

The object of the invention is to propose a device for generating a monodispersed aerosol having a much higher flow rate than that of devices with a vibrating orifice and usable for any liquid, dielectric or not.

The invention achieves its object thanks to improvements made to devices of the rotary disc sprayer type, comprising a flat disc, cup or object with symmetry of axial revolution set at high speed of rotation, a tube for feeding the liquid to be dispersed or a liquid reservoir provided at its base with an orifice, opening into the center of the object with symmetry of axial revolution or which may include concentric obstacles placed on the surface of the object with symmetry of axial revolution, of the type in which the speed of rotation, the dimensions of the object in rotation and the flow of liquid define operating conditions such that jets of fluid form around the periphery of the object in rotation, the device being characterized in that it includes means for disturbing the flow of the liquid to be dispersed generating a capillary wave propagating towards the periphery of the rotating object.

The means for disturbing the liquid to be dispersed generate capillary waves propagating towards the periphery of the rotating disc to reach the jets and fragment them.

According to one embodiment of the device according to the invention, the means for the disturbance of the liquid to be dispersed generating capillary waves can put in longitudinal vibration either the supply tube, or the reservoir, or the liquid in the reservoir, or make vibrate the tube and the disc, either excite the surface of the liquid flowing on the disc by means of a ring centered on the disc and vibrating in contact with the film of liquid flowing from a central region, or else use a combination of these different excitations.

According to another embodiment of the device according to the invention, the means for disturbing the liquid to be dispersed generating capillary waves comprise one or more several concentric obstacles periodically arranged on the object rotated with a spacing of the order of magnitude of the value of the diameter of the droplets of the monodispersed aerosol.

• La figure 1 montre le schéma général du dispositif et de l'ensemble de l'instrumentation permettant d'apprécier la qualité de l'aérosol généré,
• la figure 2 montre des variantes de réalisation d'un obstacle à la surface du disque,
• la figure 3 présente des courbes granulométriques mettant en évidence la monodispersion de l'aérosol généré par le dispositif selon un mode de réalisation de l'invention,
• la figure 4 montre la répartition des gouttelettes générées par le dispositif selon l'invention dans une même classe granulométrique,
• la figure 5 illustre le fonctionnement d'un dispositif pulvérisateur à disque tournant dans des conditions de formation de jets au voisinage de la périphérie du disque,
• la figure 6 illustre le fonctionnement d'un dispositif pulvérisateur à disque tournant dans des conditions de formation de gouttes au voisinage de la périphérie du disque,
• la figure 7 illustre le fonctionnement d'un dispositif pulvérisateur à disque tournant dans des conditions de fonctionnement formant un film au voisinage de la périphérie du disque,
• la figure 8 est une vue d'un dispositif selon un mode de réalisation de l'invention comportant un anneau centré au dessus du disque, accouplé à la partie mobile d'un vibreur électrodynamique et vibrant au contact du film de liquide s'écoulant à la surface du disque depuis une région centrale.

Other characteristics and advantages of the present invention will appear on reading the following description of embodiments of the invention, of a non-limiting nature, with reference to the figures among which:

• FIG. 1 shows the general diagram of the device and of the entire instrumentation making it possible to assess the quality of the aerosol generated,
• FIG. 2 shows alternative embodiments of an obstacle on the surface of the disc,
• FIG. 3 presents particle size curves showing the monodispersion of the aerosol generated by the device according to an embodiment of the invention,
• FIG. 4 shows the distribution of the droplets generated by the device according to the invention in the same particle size class,
• FIG. 5 illustrates the operation of a disc sprayer device rotating under conditions of jet formation in the vicinity of the periphery of the disc,
• FIG. 6 illustrates the operation of a disc sprayer device rotating under conditions of drop formation in the vicinity of the periphery of the disc,
• FIG. 7 illustrates the operation of a disc sprayer device rotating under operating conditions forming a film in the vicinity of the periphery of the disc,
• Figure 8 is a view of a device according to an embodiment of the invention comprising a ring centered above the disc, coupled to the movable part an electrodynamic vibrator vibrating in contact with the film of liquid flowing on the surface of the disc from a central region.

In FIG. 1, it can be seen that the high-flow monodispersed aerosol generator according to the invention comprises an object with axial symmetry of revolution driven at high speed in rotation by an electric motor or an air turbine 2. In a mode of preferential embodiment, the object with symmetry of axial revolution is a flat disc 1, of interchangeable diameter with a sharp edge.

A feed tube 4 of small cross section relative to the diameter of the disc 1 or a reservoir provided at its base with an orifice, perpendicular to the latter, and the end of which is close to the surface of the disc projects the liquid to be dispersed . The distance to the center of the disc is preferably less than the diameter of the tube or the orifice of the reservoir. The other end is connected to a tank of liquid in charge 5 or to a device making it possible to provide a constant flow. A vibration generator of adjustable frequency and amplitude 6 can be linked to the supply tube, or to the reservoir provided at its base with an orifice, or to a ring 10 coaxial with the tube and placed in contact with the film of liquid s 'flowing from a central region of the disc, which it vibrates longitudinally to induce capillary waves propagating in the liquid flowing on the disc. The liquid to be dispersed is injected with a constant flow rate perpendicular to the surface of the rapidly rotating disc (1,000 to 120,000 rpm) from a reservoir and a capillary tube or from a reservoir provided at its base with an orifice. FIGS. 5, 6 and 7 illustrate the importance of the operating conditions on the way in which the liquid dispersed on the disc resolves at its periphery: FIG. 5 shows the formation of jets, FIG. 6 shows the formation of drops, the Figure 7 shows the formation of a film.

According to one embodiment of the invention, either the supply tube or the reservoir is vibrated, or the liquid in the tank, or the supply tube and the disc. One can also excite the liquid surface by a vibrating ring in contact with the liquid film spread on the disc, or else use a combination of these different excitations.

In order to verify experimentally the operation of the device according to this embodiment of the invention, the size of the drops is measured using a particle sizer 7 and the formation of these droplets is visualized using a camera 8, as illustrated in FIG. 1. The particle size analyzer 7 is positioned as shown in FIG. 1 vertically so that a laser beam 9 cuts perpendicularly the plane formed by the droplets when they leave the edge of the disc . It is thus possible to determine the spatial distribution of the size of the droplets as a function of the distance to the center of the disc and of the angle of measurement. We then obtain cumulative particle size curves in volume as presented in Figure 3 (particle size as a function of the% of droplets present in the same class) highlighting the beneficial effect of vibrations on the dispersion.

avec
   V=vitesse normale à la périphérie du disque,
   D=diamètre du jet non perturbé
   F=fréquence de vibration.To determine the order of magnitude of the vibration frequency to apply to the device, we use the RAYLEIGH formula (first order approximation). $${\text{V = PI. (2)}}^{\text{1/2}}\text{.DF}$$

with
V = normal speed at the periphery of the disc,
D = undisturbed jet diameter
F = vibration frequency.

An indication is obtained, for a flow rate of 0.2 cm3 / s, a frequency of 10,000 Hz, a drop diameter of 70 micrometers with a disc diameter of 0.5 cm.

In a particular embodiment where it is desired to obtain large diameter drops, the vibrator selected is an electrodynamic vibrator 11 operating in the frequency range 0-12000 Hz. It is associated with a conventional electric chain comprising a signal generator , an amplifier, a vibrator. The electrical parameters of voltage, frequency and intensity are then checked in the vibrator. The amplitude of the movement of the vibrator can be controlled using a displacement sensor. The feed tube 4 is placed for reasons of symmetry, perpendicular to the disc and at its center. The excitation frequency adopted is such that it is equal to the natural frequency of fragmentation of the liquid to be dispersed.

Some results are given concerning the conditions of jet formation. These were obtained with a 15 mm diameter stainless steel disc and distilled water. Large diameter drops are obtained which are easier to observe and visualize, as shown in FIG. 4. In this specific case, for a fluctuation of 1% in speed and 0.5% in flow, a fluctuation is obtained small 1.5% of the diameter of the droplets measured. As regards the evolution of the volume radius of the droplets, this is an inverse function of the speed of rotation of the disc.

For other variant embodiments, in particular with regard to the diameter of the disc, similar results are obtained in terms of dispersion. Note that for a 0.25 cm radius disc, the droplets obtained are of the order of 50 micrometers. For a disc 1.5 cm in diameter, droplets are obtained which can vary according to the speed and frequency of vibration from 150 to 300 micrometers with a low dispersion for a given speed and frequency.

More generally, the smaller the diameter of the disc, the smaller the diameter of the droplets and the smaller the flow rate, with a given fluid characteristic and under the operating conditions forming jets. Similarly, the more one wishes to obtain droplets of small diameter, the more it is necessary to increase the frequency of vibration.

In theory, a monodispersion can be obtained without an external vibrator, provided that capillary waves of adequate amplitude and wavelength are created by flow-disrupting means. This is achieved by means of an embodiment of the invention in which one or more concentric obstacles are periodically placed on the surface of the disc, of spacing of the order of magnitude of the diameter of the droplets of the monodispersed aerosol. that one wishes to obtain, the end of the disc then forming an acute angle. If this particular distribution of disturbances on the surface of the disc is not respected, the fragmentation takes place at a variable distance from the disc and there is no monodispersion of the aerosol. A concentric obstacle can take the form of singularities of the type 2a, 2b, 2c, 2d as shown in FIG. 2.

Claims (9)

Device generating with high flow rate a monodispersed aerosol, of the rotating disc sprayer type, comprising a flat disc (1), cup or object with symmetry of axial revolution set at high speed of rotation, a liquid supply tube (4) to be dispersed or a liquid reservoir provided at its base with an orifice, opening into the center of the object with axial symmetry of revolution or in its vicinity, which may include concentric obstacles placed on the surface of the object with symmetry of revolution axial, and of the type in which the speed of rotation, the dimensions of the object in rotation and the flow of liquid define operating conditions such that jets of fluid form around the periphery of the object in rotation , the device being characterized in that it comprises means disrupting the flow of the liquid to be dispersed generating a capillary wave propagating towards the periphery of the object t in rotation. Device generating at high flow rate a monodispersed aerosol according to claim 1, characterized in that the disturbing means comprise one or more concentric obstacles periodically placed on the object rotated, with a spacing of the order of magnitude of the value of diameter of the droplets of the monodispersed aerosol. Device generating with high flow rate a monodispersed aerosol according to claim 2, characterized in that the concentric obstacles consist of one or more steps (elevation of the central part of the disc) with cylindrical symmetry formed on the surface of the object put in rotation. Device generating with high flow rate a monodispersed aerosol according to claim 2, characterized in that the concentric obstacles are constituted by one or more steps (elevation of the external part of the disc) with cylindrical symmetry formed on the surface of the object put in rotation. Device generating at high flow rate a monodispersed aerosol according to any one of Claims 1 to 4, characterized in the disturbing means subjecting the feed tube (4) to a longitudinal vibratory movement. Device generating with high flow rate a monodispersed aerosol according to any one of claims 1 to 4, characterized in that the disturbing means subject the reservoir provided at its base with an orifice to a longitudinal vibratory movement. Device generating at high flow rate a monodispersed aerosol according to any one of claims 1 to 4, characterized in that the disturbing means subject the liquid contained in the reservoir to a longitudinal vibratory movement. Device generating at high flow rate a monodispersed aerosol according to any one of claims 1 to 4, characterized in that the disturbing means comprise a ring (10) centered above the object rotated and vibrating in contact with the film of liquid flowing from a central region of the rotating object. Device generating at high flow rate a monodispersed aerosol according to any one of claims 1 to 4, characterized in that the disturbing means subject the feed tube and the surface of the disc to a vibratory movement.

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