Abstract:
A method for generating an aerosol includes the step of guiding a gas which flows at supersonic velocity and which has input particles suspended therein in such a way that a compression shock occurs. The input particles are broken down into smaller output particles upon crossing the compression shock. A device for generating an aerosol is also provided.

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The present invention relates to a method and device for generating an aerosol. 
     For a variety of technical and medical applications it is necessary to have liquid or solid particles uniformly distributed in a finely divided state through a gas. Such aerosol particles may have various diameters and for specific applications it is desired to have aerosol particles of a given diameter. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method and a device for generating an aerosol which allows to break up previously generated liquid particles and/or loosely linked solid particles (input particles) into substantially smaller output particles in the form of an aerosol. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method for generating an aerosol, which includes the steps of: 
     guiding a gas having input particles suspended therein and flowing at a supersonic velocity such that a compression shock occurs in the gas; and 
     breaking the input particles into output particles being smaller than the input particles by passing the input particles through the compression shock. 
     According to another mode of the invention, the gas is guided in an enclosure having a cross-section widening in a direction of flow in order to achieve the supersonic velocity. 
     According to yet another mode of the invention, the enclosure is provided such that, as seen in the direction of flow, the cross-section of the enclosure narrows prior to widening in order to achieve a sonic velocity. 
     According to another mode of the invention, the gas is guided such that the compression shock occurs, as seen in the direction of flow, before an end of the enclosure and thus inside the enclosure. 
     According to a further mode of the invention, the gas is guided such that the compression shock occurs at a point located substantially ⅔ of a distance along a length of a widening portion of the enclosure following a narrowest cross-section of the enclosure in the flow direction. 
     According to another mode of the invention, the gas is guided such that the compression shock occurs, as seen in the direction of flow, behind an end of the enclosure and thus outside the enclosure. 
     According to another mode of the invention, the input particles are fed to the gas while the gas is at rest or at subsonic velocity. 
     With the objects of the invention in view there is also provided, a device for generating an aerosol, including: 
     a gas guiding device configured to guide a gas having input particles suspended therein and flowing at a supersonic velocity; and 
     the gas guiding device being configured to generate a compression shock in the gas such that the input particles, upon crossing the compression shock, are broken down into output particles smaller than the input particles. 
     According to another feature of the invention, the gas guiding device includes an enclosure defining a flow direction, the enclosure guides the gas along the flow direction, the enclosure has a first portion with a narrowest cross-section and a second portion disposed after the first portion as seen in the flow direction, the second portion has a cross-section expanding along the flow direction. 
     According to yet another feature of the invention, the enclosure includes a third portion disposed upstream of the first portion as seen in the flow direction, the third portion has a cross-section narrowing along the flow direction. 
     According to another feature of the invention, the gas guiding device is a Laval nozzle. 
     According to yet another feature of the invention, the gas guiding device is an unmatched Laval nozzle. 
     According to another feature of the invention, a supply device is connected to the gas guiding device, the supply device supplying the input particles. The supply device may for example be an atomizer. 
     According to another feature of the invention, a supply device for supplying the input particles is disposed upstream of the narrowest cross-section of the first portion of the enclosure. 
     According to yet another feature of the invention, a supply device for supplying the input particles is disposed upstream of the cross-section of the third portion narrowing along the flow direction. 
     According to another feature of the invention, a gas supply device is connected to the gas guiding device for providing pressurized gas. The gas supply device may be a storage tank or a pump. 
     According to a further feature of the invention, the gas has a pressure between 1·10 5  Pa and 2.5·10 7  Pa, preferably between 2·10 5  Pa and 2·10 6  Pa, even more preferably between 3·10 5  Pa and 1·10 5  Pa, or substantially a pressure of 5·10 5  Pa in a resting state upstream of the cross-section of the third portion of the gas guiding device narrowing along the flow direction. 
     According to a further feature of the invention, the gas has a temperature between −20° C. and 400° C., preferably between 0° C. and 50° C., even more preferably between 10° C. and 30° C. or between 20° C. and 25° C. in a resting state upstream of the cross-section of the third portion of the gas guiding device narrowing along the flow direction. 
     According to yet a further feature of the invention, the gas is air, N 2 , O 2 , or CO 2  or a combination of these gases. 
     According to another feature of the invention, the input particles have an average size between 20 μm and 200 μm, preferably between 40 μm and 100 μm, and even more preferably between 45 μm and 60 μm. 
     According to another feature of the invention, the output particles have an average size between 1 μm and 10 μm, preferably between 2 μm and 5 μm, and also preferably of substantially 3 μm. 
     According to another feature of the invention, droplets of a liquid are supplied as the input particles. 
     According to yet another feature of the invention, water is provided as the liquid. 
     According to another feature of the invention, the liquid is used as a carrier liquid for an agent, such as a pharmacologically active agent, in particular a pharmacologically active inhalation therapy agent. 
     According to another feature of the invention, a solvent such as alcohol is provided as the liquid. 
     According to yet another feature of the invention, a combustible liquid such as a fuel is provided as the liquid. 
     According to another feature of the invention, at least some of the input particles are loosely linked particles including solid particles and/or semi-solid particles. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a method and a device for generating an aerosol, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The single FIGURE is a diagrammatic side view of a gas flow region for illustrating the method and the device according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the single FIGURE in detail, there is shown a schematic side view (i.e. sectional view) of an inner contour of a part of a nozzle  1  in which a gas flows in a flow direction indicated by arrow  2 . The nozzle  1  expands in the flow direction. In other words, the cross-section of the nozzle—that is to say, its inner cross-sectional area—increases in the flow direction. 
     Located in front of, i.e. upstream of the widening part of the (planar or round) nozzle  1  is a converging portion and a narrowest portion or throat at the transition to the diverging portion. In the operation of this type of nozzle (also known as a Laval nozzle), a flow with sonic velocity builds in the narrowest portion of the nozzle beginning at a defined pressure ratio (ratio of the pressure in front of the converging portion to the pressure in the environment behind the diverging portion), while supersonic flow prevails in the diverging portion of the nozzle. In the present example, the gas which is fed to the nozzle at its converging portion is supplied having a static pressure of approx. 5·10 5  Pa, the gas being supplied by a gas supply  5 . The gas may for example be drawn from a pressure vessel or may be provided by a compressor. The temperature of the pressure gas prior to being discharged into the nozzle is approximately room temperature, i.e. 20° C. to 30° C. 
     A supply device  6  for feeding in input particles, with the aid of which the particles that are to be broken up or split into pieces are fed in and suspended in the gas, is disposed at a suitable location, namely in front of the narrowest portion of the nozzle. The supply device  6  can be formed of a pump atomizer with which a relatively coarse drop spectrum is suspended in the gas stream. An alternative or additional technique is to feed into the gas flowing at supersonic velocity. Depending on the field of application of the generated aerosol, the input particles can be droplets of liquid such as water with or without added agents, or a solvent such as alcohol. Alternatively, it can be provided that the input particles are fuel droplets, for instance for a combustion engine or a firing plant. Finally, possibly in addition to droplets, the input particles can be loosely linked solid or semi-solid particles which will be broken down into (substantially) smaller particles. 
     The nozzle  1  is constructed in known fashion taking into account the pressure relation in which it will be operated, so that in the course of its diverging portion an underpressure relative to the environment results, i.e. relative to the space adjacent the end of the nozzle  1  (“unmatched nozzle”), as a result of which a compression shock  3  arises in the nozzle as represented in the figure. 
     Surprisingly, it has been found that the input particles carried by the gas flowing through the nozzle are broken down into a spectrum of substantially smaller particles or droplets upon passing through the compression shock, which contains a very large pressure gradient (pressure rise in a narrow space). For instance, when the core region of the compression shock, i.e. the region with the largest pressure gradient, has had a thickness of 40 μm to 50 μm in the flow direction, a resulting mean droplet diameter (logarithmic normal distribution) of between 3 μm and 10 μm has been observed, whereas the input particles have been droplets with a significantly larger diameter, such as 50 μm. 
     Given an input pressure of approximately 5·10 5  Pa and an input temperature of approximately 300 K, a Laval nozzle whose narrowest cross-section is approximately 0.03 cm 2  yields a pressure of approx. 2.5·10 5  Pa and a temperature of approximately 250 K at the narrowest portion or throat of the nozzle. Given widening of the cross-section to approximately 0.16 cm 2 , the flow velocity increases to 3.4 times the speed of sound (Mach 3.4), while the pressure drops to approx. 1·10 4  Pa and the temperature drops to less than 100 K. A compression shock effectuates a sudden pressure rise approximately to the ambient pressure (1·10 5  Pa), while the temperature rises approximately the same way to the ambient temperature. 
     It is assumed that the extremely large pressure gradient within the compression shock leads to a crushing or ripping apart of the incoming input particles, whose diameter is on the order of magnitude of the thickness of the compression shock. 
     Whereas the figure represents a situation in which the compression shock is located in front of the end of the nozzle facing in the flow direction, i.e. inside the nozzle, situations in which one or more compression shocks lie outside the nozzle are also possible. 
     The wall friction of the gas in the region of the inner wall surface of the nozzle gives rise to slanted (i.e. angled) compression shocks, which facilitates the desired crushing effect in that the particles dwell in the compression shocks for longer periods.