Abstract:
A valveless micropump includes a hollow pump chamber having a driving element coupled thereto, an inlet channel coupled to the hollow pump chamber, and an outlet channel coupled to the hollow pump chamber. The inlet channel, the hollow pump chamber, and the outlet channel define a fluid flow path through the inlet channel, the hollow pump chamber, and the outlet channel. At least one direction-sensitive element disposed in the flow path within one of the inlet and outlet channels and comprising a direction-sensitive element, is installed at an angle which produces a drag ratio greater than unity on fluid in the flow path. The driving element may comprise an electrostatic/piezoelectric member. Various embodiments of the valveless pump include one or more of the airfoil elements mounted in one, the other or both of the inlet and outlet channels, including embodiments in which one or more cascades of the airfoil elements are mounted in the inlet channel and the outlet channel.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates generally to apparatus and methods for controlling the flow of fluids. More particularly, the invention provides a valveless pump of simple construction, and which may be made quite small using micromachining techniques. A pump according to the invention may use internal elements such as airfoil-shaped structures as direction-sensitive elements for producing different drag forces as fluid flows through the micropump in different directions. 
   Conventional pump designs typically use valves as flow directing elements. These valves allow fluid to flow from the low pressure end to the high pressure end of the pump, and to prohibit flow of the fluid back from the high pressure end to the low pressure end. Several types of valves are used in practice. Passive valves may employ an object such as a movable plate as a direction-checking component. The plate opens due to a pressure difference when fluid is pumped forward, and then closes to prevent fluid flowing backward when the pressure is reversed. Such passive valves are popular in many engineering applications. 
   Certain drawbacks limit the application of such valves in micropump designs. To begin with, it is not easy to micromachine the micro-dimensioned moving parts that such valves require. Secondly, the actions of the moving parts, such as the opening and closing of the plate, may damage cells within bio-fluids or other fragile substances. Thirdly, when the working fluid includes particles, the valve may become blocked by a collection of those particles between the moving elements. Finally, the continuous opening and closing action may lead to fatigue in the valves and failure of the micropump. 
   Active valves have similar drawbacks, but provide greater freedom for control of the fluid delivery, and less backflow. Active valves are even more difficult to fabricate, though, because of the greater complexity of the moving parts and other related structures. 
   Valveless micropumps or fixed valve micropumps have been devised and are finding increasing application, especially in bio-engineering applications. There are several advantages in valveless micropumps. Firstly, the valveless micropumps are much easier to fabricate using standard micro-machining techniques. Secondly, valveless micropumps are more reliable because there are no moving elements in the inlet and outlet channels. Thirdly, the valveless micropumps, unlike other pump designs, do not have any moving components in the inlet and outlet channels, and therefore will not cause much damage to bio-molecules. Also malfunctions due to blockages are minimized. 
   It is known in the art to provide a fixed valve conduit in which the design of the conduit is flow-direction sensitive. A lower drag force is produced when fluid flows in a forward direction than when the fluid is flowing in a backward direction. Such designs may be based on the concept of non-unit drag ratio of the backward flow to the forward flow. The efficiency of the one-directional flow conduit can be measured by such ratio. The larger the ratio, the more effective the valving action of the conduit. 
   It is also known in the art to provide a micropump having fixed valves fabricated using micromachining techniques. Again, the design thereof can be based on the concept of differentiated drag between the forward and backward flows. 
   Other work has been directed toward the aerodynamic characteristics of airfoils. Lift and drag forces have been measured for different angles of attack of airfoils from zero to 180 degrees. Airfoils have been shown to have different drag values for fluid flows arriving from different directions. The following table lists measured drag coefficients Cd for various angles of attack a: 
                                                                                                 TABLE 1               a   0   5   10   15   . . .   165   170   175   180                                C   0.010   0.014   0.018   0.190   . . .   0.230   0.140   0.055   0.025       d   3   0   8   0       0   0   0   0                    
Cd is defined by: 
             Cd   =     Drag     1   ⁢     /     ⁢   2   ⁢   ρ   ⁢           ⁢     gV   2                 Eq   .           ⁢   1               
where Drag is the drag force caused by the flow; ρ is the density of the working fluid; g is the gravitational force and V is the flow velocity.
 
   From Table 1, the drag ratios between the forward and backward flow may be obtained (from opposite directions). This ratio, η, is also known as the drag efficiency and is defined by: 
             η   =       Cd     180   -   a         Cd   a               Eq   .           ⁢   2             
 
Table 2 gives the η ratios for a ranging from 0 degrees to 15 degrees, based on Table 1 and Equation 2.
 
                                                                               TABLE 2                   Drag efficiency at Reynolds number 160,000                a   0   5   10   15   . . .                            n   2.4272   3.9286   7.4468   1.6842   . . .                        
From Table 2, it can be clearly observed that airfoils can generate very high drag efficiency. This becomes obvious when it is noted that the airfoil exhibits its streamline-body characteristic property when the flow direction is from its leading edge to its trailing edge. In the reverse flow direction when the flow is from the trailing edge to the leading edge, the airflow no longer presents itself as a “streamline body” and shows non-streamline characteristics.
 
   It would be desirable if an improved micropump could be devised to take advantage of advances in knowledge regarding the behavior of airfoils in moving fluids. Such a micropump should be reliable, efficient, of simple construction, and feasible to fabricate using known micromachining techniques. These and other advantages are provided by the novel apparatus and methods described herein. 
   SUMMARY OF THE INVENTION 
   The present invention provides a valveless micropump which includes a hollow pump chamber having a driving element coupled thereto, an inlet channel coupled to the hollow pump chamber and an opposite outlet channel coupled to the hollow pump chamber. The inlet channel, the hollow chamber and the outlet channel define a fluid flow path through the inlet channel, the hollow pump chamber, and the outlet channel. At least one direction-sensitive element is disposed in the flow path within one of the inlet and outlet channels. The direction-sensitive element may comprise an airfoil installed in the fluid flow path at an angle which produces a drag ratio greater than unity on the fluid in the flow path. The driving element may comprise an electrostatic/piezoelectric member. The airfoil element preferably has an angle of attack of 0 degrees-10 degrees. Satisfactory results may be produced at an angle of 0 degrees or 10 degrees or at some value therebetween. 
   In accordance with various embodiments of the invention, a second airfoil element may be mounted in one of the inlet and outlet channels together with the first airfoil element. The first and second airfoil elements may both be mounted in the inlet channel, or they may both be mounted in the outlet channel. As a further alternative, the first airfoil element may be mounted in the inlet channel and the second airfoil element may be mounted in the outlet channel. Still further, a first plurality of airfoil elements may be mounted in the inlet channel and a second plurality of airfoil elements may be mounted in the outlet channel. Each of the first and second pluralities of airfoil elements may comprise a single cascade of such elements or each may comprise a plurality of cascades of such elements. 
   In accordance with the invention, the airfoil elements are arranged so that they produce different drag forces on the fluid as it flows in different directions. The airfoil elements function as flow rectifying elements, allowing the fluid to flow more easily in one direction as compared with the opposite direction. The drag ratio of the backward flow against the forward flow of the micropump is therefore larger than unity. A principal feature in accordance with the invention is the ability of the valveless micropumps in accordance therewith to produce lower forward flow drag and higher backward flow drag, so that a high flow rate is produced when compared with other designs. The micropump structure is an integrated structure and can be fabricated using standard micromachining techniques. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a top view of a first embodiment of a micropump in accordance with the invention, which has a plurality of airfoil elements. 
       FIG. 1B  is a front view of the micropump of FIG.  1 A. 
       FIG. 2  is a diagrammatic plot of drag coefficient of an airfoil element for various angles of attack. 
       FIG. 3  is a diagrammatic plot showing the relationship between the angle of attack and the drag efficiency. 
       FIG. 4A  is the top view of another embodiment of a micropump in accordance with the invention having a single airfoil element at an angle of attack of 10 degrees in each of the inlet and outlet channels. 
       FIG. 4B  is a front view of the micropump of FIG.  4 A. 
       FIG. 5A  is a top view of a still further embodiment of a micropump in accordance with the invention, having multiple airfoil elements mounted only in the inlet channel thereof. 
       FIG. 5B  is a top view of a still further embodiment of a micropump in accordance with the invention having multiple airfoil elements mounted only in the outlet channel thereof. 
       FIG. 6A  is a top view of a still further embodiment of a micropump in accordance with the invention in which each of the inlet and outlet channels contains a single airflow element at an angle of attack of 0 degrees. 
       FIG. 6B  is a front view of the micropump pump of FIG.  6 A. 
       FIG. 7A  is a top view of yet another embodiment of a micropump in accordance with the invention having multiple cascades of airfoil elements in each of the inlet and outlet channels. 
       FIG. 7B  is a front view of the micropump of FIG.  7 A. 
       FIG. 8A  is a top view of yet another embodiment of a micropump in accordance with the invention in which a single cascade of airfoil elements is located in each of the inlet and outlet channels. 
       FIG. 8B  is a front view of the micropump of FIG.  8 A. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A and 1B  are top and front views, respectively, of a first embodiment of a valveless micropump  10 . The micropump  10  includes a micropump chamber  12  with an electrostatic or piezoelectric membrane  14  mounted thereon. Opposite inlet and outlet channels  16  and  18  are coupled to the micropump chamber  12 . 
   As shown in  FIG. 1A , the valveless micropump  10  has two airfoil-shaped elements  20  mounted in each of the inlet and outlet channels  16  and  18 . The airfoil-shaped elements  20  present a predetermined angle of attack relative to a central axis or axis of elongation  22  extending through the micropump  10 . One such angle  24  is shown in FIG.  1 A. Though these elements are described herein mainly as “airfoils,” it should be appreciated that the pumped fluid may be either a gas or a liquid, and is by no means limited to air. 
   The micropump chamber  12  is of generally cylindrical configuration so as to have a circular top  26 , an opposite circular bottom  28 , and a wall  30  of circular configuration extending between the top  26  and the bottom  28 . An electrostatic or piezoelectric membrane  14  serves as the driving member for the micropump  10 . The membrane  14  is of generally circular configuration and is mounted on the top  26  of the chamber  12 . Opposed inlet and outlet channels  16  and  18  are coupled to the micropump chamber  12  through openings  32  and  34  respectively in the circular wall  30  of the micropump chamber  12 . The inlet and outlet channels  16  and  18  and the micropump chamber  12  are arranged so that the central axis or axis of elongation  22  extends through each of the inlet and outlet channels  16  and  18  and through the center of the micropump chamber  12 . 
   The inlet channel  16  has opposed, generally parallel sidewalls  38  and  40  extending between a top  42  and a bottom  44 . The top  42  and bottom  44  are generally planar and continuous with the top  26  and bottom  28 , respectively, of the micropump chamber  12 . Similarly, the outlet channel  18  includes opposed, generally parallel sidewalls  46  and  48  extending between a top  50  and a bottom  52 . The top  50  and the bottom,  52  are generally planar and continuous with the top  26  and bottom  28 , respectively, of the micropump chamber  12 . 
   As shown in  FIG. 1B , the airfoil-shaped elements  20  within the inlet channel  16  extend upwardly from the bottom  44  to the top  42  thereof. Similarly, the airfoil-shaped elements  20  within the outlet channel  18  extend upwardly from the bottom  52  to the top  50  of the outlet channel  18 . 
   In the valveless micropump  10  of  FIGS. 1A and 1B , each of the inlet and outlet channels  16  and  18  has two of the airfoil-shaped elements  20  mounted therein. As will become apparent from the discussion to follow, however, other arrangements of airfoil-shaped elements  20  are possible. It is possible, for example, to mount a single one of the elements  20  or a plurality of the elements  20  in one or the other but not both of the inlet and outlet channels  16  and  18 . It is also possible to provide each of the inlet and outlet channels  16  and  18  with a single cascade or a plurality of cascades of airfoil elements  20 . 
   As shown in  FIG. 1A , each of the airfoil-shaped elements  20  has a leading edge  54  and a trailing edge  56 . Fluid flows through the inlet channel  16 , the micropump chamber  12  and the outlet channel  18  in a direction shown by arrows  58  and  60  at the inlet channel  16  and the outlet channel  18  respectively. The airfoil-shaped elements  20  are mounted so that the leading edge  54  of each faces in an upstream direction relative to the flow. As previously noted, each airfoil-shaped element  20  is mounted so as to be at a desired angle of attack relative to the central axis  36 . As previously noted, one such angle  24  is shown in FIG.  1 A. 
     FIG. 2  is a diagrammatic plot of drag coefficient as a function of angle of attack for an airfoil element of particular configuration. In the particular example shown, when the angle of attack is less than 11 degrees, the drag is very small. 
   As described above, a drag ratio can be defined as the ratio between the drag generated when the flow is from the leading edge to the trailing edge of the airfoil, and the drag generated when the flow is from the trailing edge to the leading edge. This ratio provides a relative measure of flow resistance through the micropump from the two opposing flow directions and is useful to define or quantify the efficiencies of valveless pumps. If the ratio is larger than unity, the drag generated when the working fluid flows from the leading edge to the trailing edge is lower than that generated when the flow is in the opposite direction. In other words, if the airfoil element is mounted in a channel of a micropump, and an alternating-flow fluid passes through, fluid will flow more easily and thus preferentially in a direction from the leading edge to the trailing edge of the airfoil element. Over time, a net flow of fluid will occur in this direction. If the ratio is less than unity, a net flow from the trailing edge to the leading edge results, and if the ratio is equal to unity, there will be no net flow. The higher the ratio, the higher will be the net flow, and thus the higher the efficiency of the valveless micropump. 
   Equations 1 and 2, above, can be used to calculate drag coefficients and drag ratios for a given airfoil configurations.  FIG. 3  is a diagrammatic plot of the relationship between angle of attack and drag efficiency as calculated using equations 1 and 2. The drag ratio increases steadily from 2.4272 at α=0.0 to a maximum of 7.4468 at α=10.0. This maximum is several times unity, which suggests that airfoil elements of this type can find effective use as direction sensitive flow control elements in valveless pump configurations of the type described in this document. 
   As previously noted, there is no special limitation on the number of airfoil-shaped elements  20  that can be mounted in the inlet and outlet channels  16  and  18 .  FIGS. 4A and 4B  are top and front views of a second embodiment of a valveless micropump  70 . In this embodiment, a single airfoil-element  20  is mounted in each of the inlet and outlet channels  16  and  18 . Like reference numerals are used to identify parts of the valveless micropump  70  similar to those of the valveless micropump  10  of  FIGS. 1A  and  1 B. Again, each of the airfoil-shaped elements  20  is mounted at a desired angle of attack relative to the central axis  22 . 
   In designing micropumps according to the invention, careful consideration should be given to the number of airfoil elements used, the flow-rate, and the power consumption. Additional airfoil elements increase the drag ratio and thus the directional efficiency and flow-rate, but this also results in higher power consumption. 
   It is not necessary to mount the airfoil-shaped elements  20  in both the inlet channel  16  and the outlet channel  18 . Alternative arrangements are shown in  FIGS. 5A and 5B . In the embodiment of  FIG. 5A , a valveless micropump  72  has two of the airfoil-shaped elements  20  mounted in the inlet channel  16  and no airfoil-shaped elements mounted in the outlet channel  18 . Conversely, the embodiment of  FIG. 5B  shows a valveless micropump  74  in which two of the airfoil-shaped elements  20  are mounted in the outlet channel  18 , with none in the inlet channel  16 . Again, like or similar components in  FIGS. 5A and 5B  are identified by the same reference numerals as those used in the embodiment of  FIGS. 1A and 1B . 
   The angle of attack of the airfoil-shaped elements  20  can be of any value as long as the airfoil produces a drag ratio larger than unity. It has been found, however, that an angle of attack between zero and 10 degrees provides superior results. 
     FIGS. 6A and 6B  are top and front views, respectively, of a further embodiment of a valveless micropump  76 , in which each of the inlet and outlet channels  16  and  18  contains a single one of the airfoil-shaped elements  20  mounted at an angle of attack of 0 degrees. This differs from the approximately 10 degrees angle of attack shown in the embodiment of  FIGS. 1A and 1B , but still provides a reasonable flow-rate. 
   To increase the flow rate, cascades of airfoil elements  20  can be used. This is illustrated in  FIGS. 7A and 7B , which are top and front views of yet another embodiment of a valveless micropump  78 . Unlike the valveless micropumps of the prior embodiments, the micropump  78  of  FIGS. 7A and 7B  includes a micropump chamber  80  of generally rectangular configuration, with a rectangular electrostatic/piezoelectric membrane  82  mounted on a top  84  of the micropump chamber  80 . The top  84  and an opposite bottom  86  of the micropump chamber  80  are of rectangular configuration and are generally continuous with an opposite top  88  and bottom  90  of an inlet channel  92 , respectively, and an opposite top  94  and bottom  96  of an outlet channel  98 . A central axis  100  extends through the inlet channel  92 , the micropump chamber  80  and the outlet channel  98 , and fluid flows in directions shown by arrows  102  and  104  at the inlet to the inlet channel  92  and the outlet of the outlet channel  98  respectively. 
   In the valveless micropump  78  of  FIGS. 7A and 7B , each of the inlet and outlet channels  92  and  98  is provided with cascades of the airfoil-shaped elements  20  arranged in multiple rows or cascades  106 ,  108  and  110 . The cascades  106 ,  108  and  110  of the airfoil-shaped elements  20  within each of the inlet and outlet channels  92  and  98  increase the directional efficiency of the valveless micropump  78 . 
   In valveless micropumps utilizing cascades of airfoil-shaped elements  20 , such as the valveless micropump  78  of  FIGS. 7A and 7B , there need not be any particular number of cascades.  FIGS. 8A and 8B , for example, are top and front views of yet another embodiment of a valveless micropump  112  in which a single cascade  114  of the airfoil-shaped elements  20  is used in each of inlet and outlet channels  116  and  118 . Like the valveless micropump  78  of  FIGS. 7A and 7B , the micropump  112  of  FIGS. 8A and 8B  has a rectangular micropump chamber  80  and a rectangular electrostatic/piezoelectric membrane  82  in the manner of the embodiment of  FIGS. 7A and 7B . The inlet and outlet channels  116  and  118  of  FIGS. 8A and 8B  are similar to the inlet and outlet channels  92  and  98  of the embodiment of  FIGS. 7A and 7B , but are shorter in length. A central axis  120  extends through the inlet channel  116 , the micropump chamber  80  and the outlet channel  118 . Fluid flows in a direction illustrated by an arrow  122  at the inlet end of the inlet channel  116  and an arrow  124  at the outlet of the outlet channel  118 . 
   The various embodiments of valveless micropumps in accordance with the invention are shown and described herein in terms of direction-sensitive drag-producing elements which are airfoil-shaped elements such as the elements  20 . However, the invention is not limited to airfoils. The drag-producing elements can assume any appropriate shape as long as the resulting drag ratio is larger than unity.