Abstract:
An aerosol transport system. The aerosol transport system comprising a tube and having a plurality of windings that permit the flow of aerosols, including particles, there-through so as to compensate for gravity and/or centrifugal forces and reduce the settling of particles within said tube.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and/or used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to transporting aerosol over horizontal, or near horizontal distances, in a tube with reduced particle settling in such tube. 
     BACKGROUND OF THE INVENTION 
     In many situations, it is desirable to transport aerosol over long distances. For example, aerosol collectors may be mounted in one location and aerosol detectors may be mounted elsewhere. Or it can be desirable to multiplex one detector to many collectors. Current means for transporting aerosol particles horizontally are limited. Generally, particles with diameters larger than 5 microns can only be transported for limited distances or with very low transport efficiency. Typical transport means are a straight tube. Straight tubes transport small particle sizes efficiently only over limited distances. Losses in straight tubes are high due to competing mechanisms. At low speeds, gravitational sedimentation dominates. At high speeds, turbulent inertial losses dominate. If there are bends in the tube path, there will be losses at the bends at high speeds. 
     Aerosol particles traveling with the flow in a straight, horizontal tube are subject to deposition on the tube inner surfaces due to gravitational settling. In laminar flow, significant particle deposition can occur. Particle deposition is a technical challenge, especially for the transport of larger particles. Meaning that, at low speeds and in laminar flow, the only important deposition mechanism is that of gravity. This mechanism is called gravitational settling. 
     In conventional aerosol transport systems considerable settling of particles is inevitable and, in long distances, can exceed 50% of the conveyed particles, impairing the effectiveness of such transport. 
     Accordingly there is need and market for an aerosol transport system having a tube that reduces gravitational settling of particles and otherwise overcomes the above prior art shortcomings. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention provides a method of reducing the settling of particles during transport of an aerosol in a tube over long distances and includes configuring the tube in a coil about a least one axis that is substantially horizontal and flowing the aerosol along the tube in a winding path so as to compensate for gravity to reduce the settling of particles on the interior walls of the tube. 
     The invention further provides a method for reducing such settlement of particles by flowing the aerosol through a tube configured in countervailing helical paths so as compensate for gravity and centrifugal forces and reduce settlement of the particles within the tube. 
     The invention further provides an aerosol transport system employing a tube having a plurality of coils or windings that permit the flow of aerosols, including particles, there-through so as to compensate for gravity and/or centrifugal forces and reduce the settling of particles within said tube. 
     DEFINITIONS 
     By long distances as used herein, is meant a length of tube at least 10 times its diameter. 
     By substantially horizontal as used herein, is meant a tube that is wound about an axis that is predominately horizontal, or nearly so, over its length but which axis can have up and down gradients along the way. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will become more apparent from the following detailed specification and drawings in which; 
         FIG. 1  is a schematic view of an aerosol unit having a particle therein affected by certain vectors in aerosol flow; 
         FIG. 2  is a schematic, cross-sectional view of an aerosol unit according to the method of the present invention; 
         FIG. 3  is a fragmentary, side elevational view of a tube winding embodying the present invention; 
         FIG. 4  is a fragmentary, side elevational view of another tube winding embodying the present invention; 
         FIG. 5  is a schematic, cross-sectional view of yet another tube winding embodying the present invention; and 
         FIG. 6  is a schematic view of another aerosol unit embodying the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the present invention in detail, one can consider a particle  8  in a small volume of fluid slug  10 . The slug  10  is large compared to the size of the particle  8 , but small compared the length scale of the tube, per  FIG. 1 . In this figure, g is the gravitational force, n is the orientation of a chosen surface on the fluid slug  10 , and v is the fluid velocity. Because the fluid flow is laminar, the particle  8  moves with the fluid slug  10 , except as affected by external forces, such as gravity, acting upon it. 
     In practical scenarios, the gravitational force on the particle  8  is always present. However, the time average gravitational force in the reference frame of the slug  10  can be reduced to zero. This is done by rotating the slug  10 , with its axis of rotation, substantially horizontal, per  FIG. 2 .  FIG. 2  shows flow around a cylinder  11 . When the slug  10  is at the top of the cylinder  11 , gravity accelerates the particle  8  downward in the reference frame of the slug  10  (in a direction opposing the vector n). When the slug  10  is at the bottom of the cylinder  11 , gravity accelerates the particle  8  upwards in the reference frame of the slug  10  (in a direction parallel to the vector n). 
     One can apply the above method of a rotating fluid slug to minimize settling of particles in laminar flow through a substantially horizontal tube. For example, in  FIG. 3 , the tube  12  is wound in a coil around a guide cylinder  14 . The total fluid path is only slightly larger than the helical circumference of the cylinder  14 , when the coil of the tube  12  is so wound. 
     The winding path of the fluid introduces a centrifugal force on the particle  8  ( FIG. 1 ). This force is directed at all times towards an outer wall within the tube  12 , where the outer wall is considered the wall farthest from a coil axis  9 . Although deposition due to gravity has been removed from the system, the newly introduced centrifugal force will cause particles  8  ( FIG. 1 ) to deposit on an interior of the outer wall of the tube  12 . 
     One removes the centrifugal deposition by periodically reversing the sense of the coil rotation. This reverses the position of the “inner” and “outer” wall of the tube and reverses the direction (in the slug reference frame) of the centrifugal force on the particle  8  ( FIG. 1 ). One method is to use two guide wires  16 ,  18 . A tube  13  winds around one guide  16  in one direction, for example clockwise. Then after several rotations, the tube  13  crosses to the other guide  18  and winds around that guide  18  in the opposite sense, in this case counterclockwise, per  FIG. 4 . 
     In another embodiment (not shown), a single guide wire can be used if one wraps the tube loosely around the guide wire or even around an imaginary axis, several turns, straightens the tube, and bends it to wrap several coils around the wire or axis in the opposite sense. 
     In another embodiment, higher flows can be achieved by coiling several tubes  20  in parallel around the guide cylinder  22  or other axis, per  FIG. 5 . 
     In yet another embodiment, per  FIG. 6 , the invention employs a rectangular tube  24  and no guide wire or cylinder. This implementation relies on fluid streamlines following the inner walls of the tube  24 . In this version, the rectangular tube  24  bends gradually along the length of its axis  25  so that what was the bottom face  26  at the start of the tube  24  becomes the top face after some distance. The faces  26  are covered by outside walls  28 . The tube  24  is thus twisted, in this manner this manner through one or more rotations, causing the zero averaging of the gravitational force on fluid slugs  10  ( FIG. 1 ) herein following the tube streamlines. Zero averaging of the centrifugal force occurs by periodic reversing of the twist in the rectangular tube  24 . 
     Looking at relevant equations, consider a tube of diameter d wrapped around a horizontal guide cylinder of radius R. Let the horizontal distance along the guide cylinder of a full rotation of the tube be L. The distance along the tube path to travel L along the guide is D=((2πR) 2 +L 2 ) 1/2 . Gravitational acceleration is g and the gravitational settling velocity is V TS . Fluid flows through the tube at speed V. The component of the velocity along the guide axis is V L =V(L/D)≈V if L&gt;&gt;R. The component of the velocity azimuthal to the guide is Vφ=2π(R/D)V≈2π(R/L)V. The centrifugal acceleration of a particle in the flow is a c =V φ   2 /R≈(2πV/L) 2 R. The terminal centrifugal velocity is V TC =V TS (a c /a g )≈(V TS /a g )(2πV/L) 2 R. The distance within the fluid slug frame a particle moves experiencing a constant gravitational force for a time t is δ g =V TS t. Similarly, for a constant centrifugal force δ c =V TC t. The time for the fluid to flow through one rotation is t g =D/V≈L/V and so the maximum distance a particle is displaced by gravity is less than δ g1 =V TS t g /2≈V TS (L/V)/2. Let N be the number of coil turns before the tube switches guides. The time for the fluid to cycle between rods (and therefore rotation sense) is t c =2ND/V≈2NL/V and so the maximum distance a particle is displaced by centrifugal force is less than δ c1 =V TS t c /2≈N(V TS /a g )(2π) 2 (V/L)R. 
     The following example is intended to illustrate the present invention and should not be construed in limitation thereof. 
     EXAMPLE I 
     In operation, an aerosol transport is employed for aerosol transport through a tube across a building. The 10 μm size particles employed have a sedimentation velocity of about 0.3 cmfs. The transport distance is 100 meters, the tube diameter is 1 cm, and the guide radius is 1 cm. Let the length L (of full particle rotation in the tube) be 20 cm. Finally, let the velocity be 100 cmfs. In this case, the transit time is about 20 seconds and the Reynolds number is about 600. Let N be 2. Then the maximum displacement downward due to gravity is 0.06 cm and the maximum displacement radially due to centrifugal force is 0.12 cm. 
     Thus it can be seen how novel and useful the aerosol transport system of the invention is. That is, it is a means for transporting relatively large aerosol particles over long distances inside a tube to arrive at a destination with greatly reduced particle fall-out or losses due to gravity or centrifugal forces. 
     Commercial Applications of the aerosol transport system of the Invention are seen as follows. 
     Multiplexing an aerosol detector to several aerosol collectors. An example of this is the U.S. Postal Service using one aerosol detector to identify aerosols from several letter-handling machines. 
     Facility Protection. An example of this is placing aerosol collectors throughout a building and transporting the aerosol back to a single aerosol analysis room. 
     Sampling wand for bioaerosol collection in difficult to reach locations such as detecting aerosols located behind walls and within ducts.