Patent Publication Number: US-2010110826-A1

Title: Fractal static mixer

Description:
TECHNICAL FIELD 
     The present invention relates to mixers for homogenizing inhomogeneous fluid mixtures; more particularly, to static mixers having no moving parts; and most particularly, to a static mixer having sequential fractal stages derived in a power progression. 
     BACKGROUND OF THE INVENTION 
     Static mixers for homogenizing inhomogeneous fluid mixtures are well known. See, for example, U.S. Pat. Nos. 7,331,705; 7,316,503; and 7,338,543. A static mixer is defined herein as a mixing device with no moving parts, as opposed to a dynamic mixer. Static mixers can be very useful in applications wherein dynamic mixing is either unnecessary or impractical, as in the inline mixing of a plurality of flowing fluid materials, whether gaseous or liquid. 
     A problem not recognized in the prior art is a need to mix in sequential stages at progressively finer levels. In general, prior art static mixers comprise a plurality of substantially identical mixing units that purport to achieve homogeneity by providing a very large number of fluid crossings or turbulences within the overall flow stream. However, if the material flow stream is highly inhomogeneous and/or striated across the cross-sectional area of the flow tube, it can be very difficult achieve homogeneity in a mixer having multiple but identical stages. Effective mixing may require a large number of stages, occupying a relatively large volume, being expensive to manufacture, and causing a large and undesirable pressure drop through the mixer. 
     What is needed in the art is a simple, short, and relatively inexpensive static mixing device. 
     It is a principal object of the present invention to provide homogeneity from disparate conjoined streams, and especially gaseous materials, which streams may differ in, for example, composition, density, temperature, and/or flow rate. 
     It is a further object of the invention to provide such homogeneity within a static mixer having relatively few stages. 
     SUMMARY OF THE INVENTION 
     Briefly described, a multiple-stage static mixer in accordance with the present invention utilizes a modular pattern and fractally progressive sub-modular patterns wherein the flow of materials is divided and rotated through a central angle about the flow axis of each modular and sub-modular pattern at each stage. The modular pattern comprises a plurality of elements spaced apart rotationally, each element being inclined to the flow axis. Each stage is mathematically related to the previous stage to have a power progression in an increased number of modular patterns. For example, a four-element mixer has four elements in the first stage, 16 elements in the second stage, and 64 elements in the third stage. Similarly, a three-element mixer has three elements in the first stage, 9 elements in the second stage, and 27 elements in the third stage. Mixing thus proceeds from relatively coarse to very fine in just a few elements which is a far more efficient methodology than is found in prior art non-progressive multiple-stage static mixers. The mixer may be adapted to both round and rectangular flow tubes and is especially suited to mixing multiple streams of gases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an isometric view of a mixing module in accordance with the present invention; 
         FIG. 2  is a elevational front view of the mixing module shown in  FIG. 1 , showing clockwise rotation of flow through the module; 
         FIG. 3  is a symbolic representation of the mixing module shown in  FIGS. 1 and 2 ; 
         FIG. 4  is a symbolic representation of a second-stage, fractal mixing module; 
         FIG. 5  is a symbolic representation of a third-stage fractal mixing module; 
         FIG. 6  is a schematic isometric view of a rectilinear three-stage fractal mixer in accordance with the present invention; 
         FIG. 7  is a schematic isometric view of a cylindrical three-stage fractal mixer in accordance with the present invention; and 
         FIG. 8  is an elevational front view of a tubular fractal mixer. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 through 3 , a module  10  is shown, defining a first stage for a multi-stage fractal mixer in accordance with the present invention. The mixer employs a series of spaced-apart stages disposed sequentially in a flow path for homogenization of an inhomogeneous fluid mixture, as described below. The sequential stages use the same mixer pattern at various scales, based on iterative affine transformations in a mathematical power progression. For the following discussion, an exemplary mixer having a square cross-section is employed, although a mixer in accordance with the present invention is not limited to any specific cross-sectional shape, including for example round (tubular) or hexagonal. 
     Module  10  employs four mixing elements  12   a,    12   b,    12   c,    12   d,  each element being secured along a first edge  14   a,    14   b,    14   c,    14   d  in a plane  16  generally transverse of the direction  18  of fluid flow through module  10 . It will be seen that module  10  may be formed conveniently from a single square of sheet stock by cutting along the bisectors of the opposite sides and then from each corner to the midpoint of each side. The resulting n number of elements  12   a,    12   b,    12   c,    12   d  may then be turned at a predetermined angle from plane  16  in axial direction  18 . Fluid flowing in axial direction  18  of the mixer upon striking each element will be diverted in respective directions  20   a,    20   b,    20   c,    20   d,  imparting, in the example, an overall clockwise spin  22  about axis  24  to the flowing material as it passes through first stage module  10 , shown symbolically in  FIG. 3 . (Of course, it will be appreciated that an enantiomorphic module, not shown, will impart a counterclockwise spin, to equal effect). 
     Module  10  may be considered to have a length L along each side that preferably is also the transverse dimension of the fluid conduit into which module  10  is to be installed. To provide fluid rotation and mixing at progressively reduced scales, in accordance with the present invention, homothetical modules of fractional lengths of L are produced and installed as follows. 
     Referring now to  FIG. 4 , a second stage module  110  having an overall side length L comprises n 1  sub-modules each having n mixing elements, in the present example n being 4 ( 10 ′ a,    10 ′ b,    10 ′ c,    10 ′ d ), each sub-module having a side length L/ 2 . In a flow conduit, module  110  is axially spaced apart from module  10  by a distance preferably of approximately L. The total fluid flow striking module  110  is thus divided into four equal flows, each of which is turned, in the example, in a clockwise direction  122  in passing through module  110 . 
     Similarly, and referring now to  FIG. 5 , a third stage module  210  having an overall side length L comprises n sub-modules,  110   a,    110   b,    110   c,    110   d,  in turn comprising n 2  sub-modules  10 , each having a side length of L/ 4 . Again, module  210  is axially spaced apart from module  110  by a distance preferably of approximately L. The total fluid flow striking module  210  is thus divided into n 2 =16 equal flows, each of which is turned in a clockwise direction  222  in passing through module  210 . 
     The multiple stages of a mixer in accordance with the present invention thus are related by the general power series L/n 0 , L/n 1 , L/n 2  . . . L/n j , where n is the number of mixing modules and may be any integer. It will be appreciated that this series may be extended to any desired value of j, although in practice for mixing gases a three-stage series wherein n=4 has been found to provide a high degree of homogeneity. It will be further appreciated that for values of n&gt;4, the number of sub-modular units  10  rapidly becomes unwieldy, e.g., n=5 (5, 25, 125), or n=6 (6, 36, 216). Thus, mixers wherein n=3 or 4 are generally preferable. 
     Referring to  FIG. 6 , a three-stage mixer  1000  comprising rectangular stages  10 ,  110 ,  210  as just described is shown for installation in a rectangular flow conduit  1002 . 
     As noted above, a mixer in accordance with the present invention may be adapted to a flow conduit of any desired cross-sectional shape, for example, and referring now to  FIG. 7 , a cylindrical tube  2002  comprising three-stage mixer  2000 . In this example, each stage may be formed, as by stamping, from a circular blank of sheet stock. Each module thus includes four portions  30  formed between the arc  32  and the chord  34  (identical with L) of each side of the rectangular mixer element. Portions  30  prevent channeling of fluid past the stages. 
     Referring to  FIG. 8 , a front elevational view is shown of another embodiment  3000  of a multiple-stage mixer in accordance with the present invention. Although n=4, it is seen that each individual element  12  is formed having a curved side  3004 , substantially elliptical, to fit the inner wall of cylindrical tube  2002 . Free edges  3006 ,  3008  may be formed as desired, although preferably entrance edges  3006  lie in plane  16  ( FIG. 1 ) transverse to the direction of flow. In the example shown, the free corners are square, but obviously any other desired angle and shape to elements  12  may be provided within the scope of the present invention. It will be seen that manufacture of a mixer  3000  is likely to be considerably more complicated and expensive than the previously-described examples, as the mixing elements of each stage must be formed and attached individually to the inner wall of cylindrical tube  2002 , rather than simply stamping each stage from sheet stock as described above for mixers  1000 ,  2000 . 
     In summary, a multi-stage fluid mixer in accordance with the present invention comprises an assemblage of modular and sub-modular stages of modular length L/n 0  and sub-modular lengths L/n 1 , L/n 2  . . . L/n j  located at various distances downstream from the initial module of unit length L/n 0 . The smallest scale and the distances between stages may be optimized for any particular application, based on process parameters such as mass flow rate, temperature, pressure, and the like. Individual flow rotations may be either clockwise or counterclockwise, and rotation orientations may be combined in any stage in any desired combination. 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.