Abstract:
The present invention is a device ( 100 ) for at least partially stabilizing an unstable fluid flow within a flow channel ( 103 ) by capturing at least a portion of the unstable fluid within a vaneless diffuser having a diffuser slot ( 104 ). The present invention also includes maintaining and harnessing a substantial portion of the energy contained in the fluid as it flows through the diffuser in order to utilize the fluid to improve the condition of the flow field. An example of a beneficial use includes discharging the diffuser effluent into the flow at other points critical to instability, hence reducing the overall instability of the flow channel.

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/298,843, filed Jun. 15, 2001, which is incorporated by reference as if included herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a device for at least partially stabilizing vortex or other unstable flow in a flow channel, and in particular to a substantially radial, vaneless diffuser defined by an annular slot in the sidewalls of the flow channel. 
     BACKGROUND OF THE INVENTION 
     The use of a diffuser to reduce the velocity and increase the static pressure of a fluid passing through a system is well known. As a fluid flow enters a diffuser, kinetic energy in the fluid is converted to a static pressure rise due to conservation of angular momentum when swirl is present and conservation of linear momentum. Diffusers are often used in combination with a bladed impeller or combined inducer/impeller within a particular system. 
     Bladed impellers or combined inducer/impellers are the key component of centrifugal, mixed flow, and axial pumps, compressors, blowers, and fans to move various fluids (i.e., air, water, vapor, or combination thereof) through a system. Depending on the condition of the fluid flow as it approaches the inlet to the equipment, the design of bladed impellers or combined inducer/impellers may be critical to control instability in the fluid flow and prevent instability in the equipment overall and to control other fluid problems such as non-collateral boundary layers. Examples of instabilities in fluid flow include vortices (in any type of fluid) often created from the impeller/inducer design itself, cavitating flow in liquids caused by vortices in the fluid, or a combination thereof, and boundary layer flows which are not collateral with the main flow direction. 
     In the case of conventional pumps, bladed impellers or combined inducer/impellers are typically used to deal with very low inlet pressure conditions. As the fluid passes through the bladed section, it experiences a rise in pressure. In the case of a cavitating liquid/vapor flow, the increase in pressure may cause the vapor bubbles in the flow to collapse and/or condense thereby causing the fluid to transfer from a vapor phase back to a liquid phase. For certain applications, this is extremely critical. Turbopumps, aircraft fuel pumps, and many industrial pumps are concerned with very low inlet pressure conditions. 
     An unfortunate aspect of inducer performance is that the cavitating flow cannot be completely prevented under various operating conditions. Performance remains constant down to a very low inlet pressure, but with sufficient reduction in the inlet pressure a complete breakdown in head results. This typically occurs when cavitated (two-phase) flow, originating principally from a part-span or tip vortex, substantially fills the impeller passages. These instabilities result from the development of cavitating flow in the inducer. If this developing flow is unable to maintain a consistent, uniform, and steady flow pattern within the inducer, oscillations result. These oscillations can be serious, leading to auto-oscillation where a dynamic instability exists in the impeller and begins to propagate instabilities into the entire pumping network and possibly into downstream elements. As a result, a diffuser may be used in the inducer region to help remove a portion of either the cavitating flow or the vortices that can lead to cavitating flow in the fluid. 
     In addition to applying a diffuser to the field of pumps, the same application can be made for centrifugal, mixed flow, and axial compressors, blowers, and fans. The fundamental difference is that the cavitation that was suppressed or removed in the case of the pumps does not apply at all in the case of compressors, fans, and blowers which handle various gases. Cavitation only occurs in liquids. Nonetheless, it is possible to set up a leading edge vortex and other forms of inlet instability, which accompanies appropriate shaping of a vane leading edge. Such a vortex or other unstable zone may contain substantial energy that can negatively impact the operation of the respective equipment if not controlled. 
     As mentioned above, the use of a diffuser to reduce the velocity and increase the static pressure of a fluid passing through a system is well known when dealing with common inlet flows, but has not been previously used to swallow a tip vortex. Prior patented devices utilize various means in an attempt to address the problems related to inlet cavitation and the development of other flow instabilities within the inlet region. Allowing the flow to be pulled off through a cover slot or set of holes has been achieved in early patented work by Chapman and others (See Model 250-C301C28B Compressor Development by Dennis C. Chapman, General Motors Corporation). Allowing flow to be pulled off and then reentered upstream has also been accomplished through earlier patents by Jackson (U.S. Pat. No. 3,504,986, issued on Apr. 7, 1970), Cooper (U.S. Pat. No. 4,375,937, issued on Mar. 8, 1983), Meng (U.S. Pat. No. 4,708,584, issued on Nov. 24, 1987), and Edwards (U.S. Pat. No. 2,832,292, issued on Apr. 29, 1958). 
     Prior attempts at designing an effective diffuser for dealing with highly compromised flows such as a tip vortex have failed for various reasons. Previous diffuser designs are often focused on re-circulating flow rather than effectively diffusing flow. For example, flow is often bled off and routed through a tortuous flow path that dissipates the energy contained in the flow. By dissipating the energy in the fluid flow, the pressure contained in the fluid is reduced thereby reducing the effectiveness of any diffusing device present. In addition, diffusers of prior inventions often include vanes. Vaned diffusers have been known to cause additional instability in the flow field by causing distortion. In addition, vanes increase the difficulty of fabrication and installation of a diffuser. Still other diffuser designs fail to consider the particular characteristics of the flow field. For example, the length of other diffuser slots is often too short to cause enough static pressure to collapse and/or condense the vapor bubbles within a particular cavitating flow. 
     SUMMARY OF THE INVENTION 
     The present invention is a device for at least partially stabilizing an unstable fluid flow within a flow channel by capturing at least a portion of the unstable fluid within a vaneless diffuser. An additional aspect of the invention includes maintaining and harnessing a substantial portion of the energy contained in the fluid as it flows through the diffuser in order to take additional advantage of the fluid. An example of additional advantage includes discharging the diffuser effluent into the flow channel to help reduce instability in the flow channel. An additional aspect of the present invention is a diffuser design that is directly related to the particular fluid flow characteristics in which it will operate. 
     In one embodiment of the present invention, a device for at least partially stabilizing an unstable fluid flow within a flow channel includes an inducer or impeller residing at least partially within the flow channel, the inducer or impeller having rotatable blades for drawing flow into, or being driven by the flow in, the flow channel, the inducer or impeller rotatable about an axis, the flow channel defined by interior sidewalls of a housing, the housing at least partially surrounded by an inlet plenum, and the housing including an exit. The device also includes at least one diffuser slot having an inlet and an exit, the inlet in fluid communication with the flow channel, the diffuser slot(s) being substantially radial with respect to the axis. The device also includes at least one passage in fluid communication with the exit of the diffuser slot(s). The passage(s) may be in fluid communication with the inlet plenum, the housing exit exit, an area downstream of the housing exit, the flow channel, or a combination thereof. Finally, the diffuser slot(s) of the device generally have a radius ratio greater than or equal to 1.03 and are free of vanes. 
     In another embodiment of the present invention, a device includes multiple diffuser slots located along the flow channel. The flow is bled off of the flow channel at various points into the multiple diffuser slots. The flow in the slots is then treated similarly to that in the embodiment described above. It is contemplated within the present invention that any combination of diffuser slots may be utilized depending on the application. 
     In still another embodiment of the present invention, a device includes at least one diffuser slot located on either side of the housing exit vane and housing exit. Vortex or unstable flow is captured within the diffuser slot(s) and either discharged to the inlet plenum, back into the housing exit vane, or downstream of the housing exit. 
     In yet another embodiment of the present invention, any one of the devices having a diffuser slot as described above also includes a particle capture slot and particle trap. The particle capture slot is in fluid communication with the diffuser slot to capture any particles contained in the fluid as the fluid passes radially through the diffuser slot. The particles flow from the particle capture slot into a particle trap where they are contained. 
     Other features, utilities and advantages of various embodiments of the invention will be apparent from the following more particular description of embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, the drawings show a form of the invention that is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
     FIG. 1 is a side section view of one embodiment of the present invention; 
     FIG. 2 is a side section view of another embodiment of the present invention; 
     FIGS. 3 a - 3   d  are side section views of various embodiments of the present invention; and 
     FIG. 4 is a side section view of yet another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a device including a vaneless diffuser for reducing the velocity and increasing the static pressure of a fluid flowing through a system and for generally increasing the overall flow stability of a system. An example of the disclosed invention is depicted schematically in FIGS. 1-4, although it should be understood that the present invention is not limited to this (or any other) particular embodiment, but rather is intended to cover all devices that fairly fall within the broad scope of the appended claims. 
     The device of the present invention includes a vaneless diffuser for reducing the velocity and increasing the static pressure of a fluid flowing through a system. The vaneless diffuser of the present invention can be retrofitted to many open or closed impeller inducer pump configurations (i.e., with or without a shroud) or other equipment including bladed inducers or impellers (e.g., air-handling equipment). A substantially radial slot diffuser is placed around the inducer at a suitable position along the internal flow channel of the pump housing and provides an alternate path for the cavitated flow resulting from an unstable part-span (also called tip) vortex, which causes the instability of the impeller flow path. The inlet to the diffuser slot forms a substantially contiguous ring around the inducer and is followed by a channel of substantially radial design that provides a diffuser for the part-span vortex which naturally migrates radially away from the inducer axis due to its angular momentum. The substantially radial slot has a length that is selected to provide effective diffusion and to appropriately raise the static pressure. 
     In the case of a cavitating flow, which is trapped at the core of the vortex, the rise in static pressure causes the cavitating flow to be substantially collapsed and/or condensed from vapor back to liquid phase. Sufficient pressure recovery is achieved in the diffuser slot to return the fully condensed flow back into the inlet flow path via re-entry slots/holes and/or to the inlet plenum or downstream via return slots/holes. In the case of an unstable air flow, the diffuser slot helps to stabilize the flow by drawing at least a portion of the vortex or other unstable flow away from the inlet area thereby improving the upstream flow channel conditions. 
     In FIG. 1, diffuser  100  of the present invention generally includes an inlet  102 , a diffuser slot  104 , and one or more passages (passages include one or more re-entry slots  108  and/or one or more return slots  110 ). Inlet  102  is formed in the internal sidewalls  112  of a housing  113  and leads into diffuser slot  104 . Diffuser slot  104  is typically vaneless and substantially radial with respect to a centerline axis  107  of a flow channel  103  and generally forms an annular ring that encircles flow channel  103 . Diffuser slot  104  leads to at least one re-entry slot  108  and/or at least one return slot  110  which are also formed in sidewalls  112  of housing  113 . Note that the term “channel” as contained herein may mean any conduit for fluid flow and includes any cross-sectional shape. In addition, the term “housing” generally refers to the body of any type of equipment that may contain a fluid channel. Finally, the term “fluid” may refer to any gas including air, liquid, vapor, or any combination thereof. 
     While diffuser slot  104  in particular preferably extends substantially radially relative to axis  107  of flow channel  103 , the present invention encompasses divergence of up to about 65 degrees from a perfectly radial relationship with axis  107 . Thus, the term “substantially radial” encompasses such divergence from a perfectly radial relationship. The degree of divergence from a perfectly radial relationship that is encompassed by the present invention is influenced, as those skilled in the art will appreciate, by factors such as orientation of slot inlet flow velocity vector and diffuser/plenum space constraints. 
     The edges  116  of inlet  102  to diffuser slot  104  are typically rounded to facilitate flow into the slot. However, inlets  102  having squared edges are also contemplated in the present invention. The walls  105  that define diffuser slot  104  are typically parallel as in FIG.  1 . However, in other embodiments it is conceivable that the walls defining a diffuser slot may not be parallel (e.g., may include one or more pinch points along the slot). 
     Diffuser  100  of the present invention and more specifically the centerline of inlet  102  and diffuser slot  104  are located in flow channel  103  along housing sidewall  112  in relation to a leading edge  120  of an inducer blade  122  joined with an impeller  124 . The one or more re-entry slots  108  typically form a pathway from diffuser slot  104  to an area of flow channel  103  immediately upstream of an inducer region  126  (i.e., the region formed by leading edge  120  of inducer blade  122  and a hub  128  of impeller  124 ). 
     Typically, any rotating, swirling, vortex, cavitating, or other unstable flow conditions are found adjacent leading edge  120  of inducer  122  within inducer region  126 . Consequently, re-injection of diffused flow from re-entry slot  108  in the region of flow channel  103  immediately upstream of inducer region  126  will help reduce the amount of rotation in the area of re-injection thereby reducing upstream flow corruption from the unstable flow within inducer region  126 . 
     The one or more return slots  110  typically form a pathway that leads from diffuser slot  104  to an area within an inlet plenum  130  outside of flow channel  103  and/or a pathway that leads from diffuser slot  104  to an exit  134  of flow channel  103  or to an area downstream of exit  134 . Inlet plenum  130  is generally the area surrounding flow channel  103  and housing  113  from which fluid flow is drawn. 
     As illustrated in FIG. 1, diffuser slot  104  typically has a rectangular cross-section. In addition, one or more re-entry slots  108  and one or more return slots  110  also have substantially rectangular cross-sections. Although the term “slot” generally refers to a narrow passage, in embodiments of the present invention it is conceivable that the term slots may include passages with varying dimensions depending on the specific application. Accordingly, as used herein, the term “slot” may refer to passages of any size or cross-section. 
     As one skilled in the art will recognize, the specific dimensions and location of diffuser  100  of the present invention are selected based on the characteristics of the flow and the vortex within the flow (often influenced by inducer design) and the specific requirements for the diffuser (e.g., controlling or stabilizing unstable flow, and/or extending the cavitation performance of the pump, etc.). Other variables that impact the specific dimensions of diffuser  100  include the dimensions of flow channel  103 , impeller  124 , and inducer  122  as well as the flow rate parameters. 
     Although many variables may impact the location and specific dimensions of diffuser  100 , some general rules for determining 1) the width (W) of diffuser slot  104  and 2) the location of the centerline of diffuser slot  104  with respect to leading edge  120  of inducer  122  for embodiments of the present invention do exist. The width (W) is related to the vane or blade height of inducer  122  (or other bladed/vaned mechanism) at inlet  102  of diffuser slot  104 . 
     Specifically, W=(0.05 to 0.50)×(blade or vane height of inducer  122  at inlet  102 ). In one embodiment, W=(0.03 to 0.20)×(blade or vane height of inducer  122  at inlet  102 ). In general, the width should be small enough so as not to bleed an excessive amount of the flow from flow channel  103 . In the embodiments of the present invention contained herein, the loss in efficiency due to bleeding the flow is generally negligible due to the increase in overall equipment performance. The blade or vane height is the length of the blade or vane as measured from the surface of the impeller radially outward to the edge of the blade adjacent the sidewall of the housing. 
     The location of the centerline of diffuser slot  104  is also related to the size of the vane or blade of diffuser. The centerline of inlet  102  should typically be located along sidewalls  112  of housing  113  with respect to the span length of leading edge blade  122  and the location of leading edge  120  itself within flow channel  103 . More specifically, inlet  102  should be located a distance of up to ±70% of the blade or vane height of inducer  122  downstream or upstream of leading edge  120 , as measured parallel to axis  107 . A positive number means inlet  102  is located downstream of leading edge  120  and a negative number means inlet  102  is located upstream of leading edge  120 . Again, the blade or vane height is the length of the blade or vane as measured from the surface of the impeller radially outward to the edge of the blade adjacent the sidewall of the housing. 
     In addition to the design parameters delineated above, additional design parameters have also been developed in the course of refining diffuser  100  and other embodiments herein. First, in at least one embodiment of the present invention, it has been determined that diffuser slot  104  should typically have a radius ratio of greater than or equal to 1.03. The radius ratio is the radial extent at the exit of diffuser slot  104 , divided by the radius to inlet  102 . The radial extent at the exit of diffuser slot  104  is typically the distance from axis  107  to the termination of diffuser slot  104 . The radius to inlet  102  is typically the distance from axis  107  to inlet  102 . In another embodiment, the radius ratio ranged from about 1.03 to about 10. Substantially all slots included in the present invention will have a radius ratio according to the above. 
     Second, in at least one embodiment of the present invention, it has been determined that the flow entering diffuser slot  104  from flow channel  103  should typically range from about ½-2% to about 5-15% of the overall flow in flow channel  103  at the principal operating or design conditions. Inlet  102  and diffuser slot  104  are sized to achieve fluid flow within this range. 
     Finally, it is preferable that no vanes be incorporated within diffuser slot  104 . Diffusers having vanes have been found to increase difficulty of fabrication, increase difficulty of installation, increase inlet blockage and noise, and if poorly done, may increase distortion. Additionally, diffuser vanes would serve to break up the tip vortex rather than allow its full energy to be recovered through the unobstructed flow process of a vaneless diffuser. Likewise, other objects near inlet  102  such as labyrinth seals, other seals, bends, or other distortions to the passage would have the same adverse impact. 
     As mentioned above, the specific parameters related to the application requirements impact the specific dimensions and placement of diffuser  100 . In one embodiment of the present invention, designed for use in turbo pump applications with very high suction specific speed requirements, the dimensions of the inlet control aspects of diffuser  100  are as follows: a radial extent to the exit of diffuser slot  104  of 2.2″, a distance from the diffuser slot  104  centerline to leading edge  120  of inducer  122  of 0.3″, a diffuser slot  104  width of 0.2″, and an inlet  102  radius of 1.4″. Again, one skilled in the art will recognize that these dimensions will vary depending on the specific pumping application and the changes in the related parameters. However, the design parameters related to the sizing and location of the diffuser slot generally apply regardless of the specific application and for all embodiments described herein. 
     With reference to the arrows in FIG. 1, the operation of diffuser  100  will now be discussed. Flow from inlet plenum  130  enters flow channel  103  and flows toward hub  128  of impeller  124 . The flow enters inlet  102  of diffuser  100  and flows radially outward within diffuser slot  104 . In the embodiment illustrated in FIG. 1, diffuser  100  includes both one or more re-entry slots  108  and one or more return slots  110 . Flow from diffuser slot  104  next flows toward both re-entry slot  108  and return slot  110 . A portion of the flow from diffuser slot  104  flows into return slot  110  and radially outward to inlet plenum  130 . The remaining portion of flow from diffuser slot  104  flows into re-entry slot  108 . The flow exits re-entry slot  108  at an area within flow channel  103  directly upstream of inducer region  126  defined by inducer  122 , impeller hub  128 , and inducer leading edge  120 . The flow exiting re-entry slot  108  mixes with the flow entering flow channel  103  from inlet plenum  130  and continues onward toward hub  128  of impeller  124 . A substantial portion of the flow in flow channel  103  flows past inlet  102  of diffuser  100  and into inducer region  126 . This flow continues along the blades or vanes of inducer  120  and toward exit  134  of housing  113 . The flow exiting housing  113  typically passes through a vane  132  within housing exit  134 . Of course, in other embodiments, device  100  may include one or more re-entry slots  108  and no plenum return and/or exit slots  110  or vice versa. 
     As mentioned above, inlet  102  to diffuser slot  104  forms a substantially contiguous ring around inducer region  126  of channel  103  and is followed by a slot or channel of substantially radial design (diffuser slot  104 ) that provides a diffuser for the part-span vortex or other unstable flow which naturally migrates radially away from axis  107  due to its angular momentum. Substantially radial diffuser slot  104  has a length that is selected to provide effective diffusion and to appropriately raise the static pressure. By raising the static pressure, two-phase fluids at least partially containing vapor are collapsed and/or condensed back into single-phase fluids containing liquid. The higher static pressure causes the vapor bubbles in the vapor to compress. By including a substantially radial design and a clean inlet design (i.e., not tortuous path), the energy in the fluid drawn into diffuser slot  104  is conserved thereby increasing the efficiency of diffusion. Such a design allows for efficient diffusion and the ability take additional advantage of the fluid. An example of additional advantage includes discharging the diffuser effluent into the flow channel to help reduce instability in the flow channel. 
     FIGS. 2-4 illustrate alternative embodiments of the diffuser. The embodiment in FIG. 2 includes aspects that are identical to the embodiment in FIG.  1 . Accordingly, some of the element numbers in FIG. 2 are identical to the element numbers in FIG. 1 for identical elements. However, in FIG. 2 multiple diffuser slots  104 ,  136 , and  138  are present within sidewalls  112  of housing  113 . Diffuser slot  104  is located adjacent leading edge  120  of inducer  122 , diffuser slot  136  is located within impeller or inducer region  126  between leading edge  120  and housing exit  134 , and diffuser slot  138  is located adjacent housing exit  134 . Although not discussed with respect to FIGS. 2-4 below, the embodiments illustrated in FIGS. 2-4 generally include radius ratios as in FIG.  1  and are free of vanes as in FIG.  1 . 
     Multiple diffuser slots may be used to bleed portions of flow channel  103  along various points within the channel. In addition to the reasons for bleeding flow adjacent leading edge  120  of diffuser  122  in the case of diffuser slot  104 , it may also be desirable to bleed the flow at other points downstream from leading edge  120  of inducer  122 . In FIG. 2, additional diffuser slots  136 ,  138  are located downstream of diffuser slot  104  and leading edge  120 . In the case of diffuser slot  136 , where a shrouded impeller is used, diffuser slot  136  may be used to capture any shroud leakage flow. As for the diffuser slot  138 , it may be desirable to attempt to bleed off any remaining unstable flow such as impeller shroud leakage or system backflow prior to discharging the flow through housing exit  134 . It is contemplated that diffuser slots  136 ,  138  will be configured in a manner similar to that of diffuser slot  104  and diffuser  100 . Although FIG. 2 illustrates the presence of three diffuser slots  104 ,  136 ,  138 , in at least one embodiment, there are only two diffuser slots. Other embodiments may include four or more diffuser slots. Embodiments including multiple diffuser slots may include any combination of slots or single slots in any locations illustrated in FIG.  2 . 
     The flow through the embodiment illustrated in FIG. 2 is very similar to that in the embodiment illustrated in FIG.  1 . However, as the flow continues within flow channel  103  past diffuser slot  104 , a portion of the flow may also be bled off into diffuser slot  136 . As with diffuser slot  104 , the flow entering diffuser slot  136  may be returned to flow channel  103  in an area of the flow channel upstream of diffuser slot  136 . The flow in diffuser slot  136  may also be returned to inlet plenum  130  or discharged to an area downstream of housing exit  134 . As in the case of both diffuser slot  104  and diffuser slot  136 , a portion of the flow will bypass both diffuser slots  104  and  136  and flow toward exit  134  of flow channel  103 . Prior to exiting flow channel  103  through exit  134 , an additional portion of the flow may be bled off into diffuser slot  138 . The flow entering diffuser slot  138  may be treated similarly to the flow bled off in diffuser slots  104  and  136 . 
     FIGS. 3 a - 3   d  illustrate alternative embodiments of the diffuser slot of the present invention. In particular, FIGS. 3 a - 3   d  are related to embodiments where at least one diffuser slot is located adjacent the exit of the housing. Because the housing exit configuration illustrated in FIGS. 3 a - 3   d  is similar to those illustrated in FIGS. 1-2, any elements in FIGS. 3 a - 3   d  that are similar to elements in FIGS. 1-2 will be noted by the use of a similar element number having a prime symbol. 
     In FIG. 3 a , a portion of the flow exiting the housing is bled off into diffuser slot  138 ′thereby by-passing exit  134 ′. By locating diffuser slot  138 ′ on the outside of housing exit vane  132 ′, at least a portion of any vortex or other unstable flow will be captured by diffuser slot  138 ′. Vortex or other unstable flows are generally flows that are not collateral with the direction of the flow channel and the bulk of the flow field. The unstable flow captured in diffuser slot  138 ′ is then discharged into a diffuser configuration similar to any one previously mentioned herein, directly to the inlet plenum, or into an area downstream of housing exit  134 ′. 
     In FIG. 3 b , diffuser slot  138 ′ resides to the side of the housing exit vane  132 ′. However, unlike FIG. 3 a , the unstable flow captured in diffuser slot  138 ′ is returned to housing exit vane  132 ′ through exit return slot  140 . The flow mixes with the flow exiting the housing through housing exit vane  132 ′ and housing exit  134 ′. The by-pass flow from slot  138 ′ may also be injected into any corners of an exit channel to suppress corner stall. 
     The embodiment illustrated in FIG. 3 c  is almost identical to that in FIG. 3 b  with the exception that a diffuser slot  138 ″ is located on both sides of housing exit  132 ′ through exit return slots  140 . At least a portion of any unstable flow in the area to the sides of the housing exit vane will be captured in diffuser slots  138 ″ and returned downstream within housing exit vane  132 ′. 
     Structurally, the embodiment illustrated in FIG. 3 d  is similar to that illustrated in FIG. 3 b . However, the sidewall of diffuser slot  138 ′″ that is in common with sidewall of exit housing vane  132 ′ includes exit return holes  142 . Any unstable flow captured within diffuser slot  138 ′″ may return to housing exit vane  132 ′ through exit return holes  142  and/or through exit return slot  140 . In one embodiment, the configuration in FIG. 3 d  allows flow to be introduced into a hollow vane and exit through a cascade exit to achieve a blown flap control device. 
     FIG. 4 illustrates another alternative embodiment of the present invention. As in FIG. 3, any elements in FIG. 4 that are similar to elements in other embodiments contained herein will be noted with a prime next to the element number. In FIG. 4, diffuser  100 ′ is virtually identical to diffuser  100  illustrated in FIG.  1 . However, diffuser  100 ′ includes an additional slot. Particle capture slot  144  is used to capture particles (either solid or, in the pump case, entraining air or other non-condensing gases) from the flow exiting diffuser slot  104 ′ and lead them to a particle trap  146 . Particle capture slot  144  is typically an elongation of diffuser slot  104 ′. Particle slot  144  terminates in a generally rectangular cross-sectional area groove also known as particle trap  146 . Although not illustrated herein, additional passages or conduits that are in fluid communication with particle trap  146  may be included to allow the trap to be emptied as necessary. The remainder of diffuser  100 ′ is again virtually identical to diffuser  100  in FIG.  1 . 
     In FIG. 4, flow enters flow channel  103  from inlet plenum  130  and is drawn toward impeller hub  128  by rotating impeller  124  and inducer  122 . At least a portion of the unstable flow enters inlet  102 ′ and flows radially outward from axis  107 ′ within diffuser slot  104 ′. Due to centrifugal forces, any particles within the flow will continue radially outward from diffuser slot  104 ′ into particle capture slot  144  and finally into particle trap  146 . The remainder of the flow will flow from diffuser slot  104 ′ into at least one of one or more reentry slots  108 ′ and one or more return slots  110 ′. The flow exiting slots  108 ′ and  110 ′ will continue in a manner similar to the flow in diffuser  100 , as illustrated in FIG.  1  and described in detail above. 
     Although the components that make up diffuser  100  of the present invention are generally described as slots herein, it is foreseeable that in other embodiments of the present invention various slots may be replaced by a plurality of holes or other orifices, a plurality of corresponding chambers, and/or a plurality of any other type of conduit (i.e., pipes, channels, grooves, etc.). 
     Although the illustrations contained herein are of an open inducer/impeller, it is contemplated that embodiments of the present invention may be used with either closed or open (i.e., shrouded or unshrouded) impeller/inducer configurations. 
     In another embodiment, an active diffuser slot is included instead of a passive diffuser slot. In the embodiments described above, the diffuser slot is passive in that it remains open at all times. An active diffuser slot may be configured to remain in a default closed position and only open when the pressure in the inducer region drops to a prescribed level. 
     In still another embodiment, a diffuser slot of the present invention may also be incorporated into the design of a hydroturbine. Hydroturbines work similar to pumps and compressors. However, the flow usually passes through the impeller in the reverse direction and work is extracted from the flow as opposed to work being done on the flow as in the case of a pump or impeller. For hydroturbines, all types of vortices are possible. By using a diffuser of the present invention to allow shroud bleed at the exit (or exducer) of the turbine, analogous to the inlet of radial pumps, it is likely that the overall performance of a hydroturbine will be improved. 
     The flow stabilizing device of the present invention including a novel diffuser slot offers advantages over prior art devices. By creating a diffuser slot having a clean inlet, a non-tortuous path, and a design related to the specific flow conditions, the device of the present invention maximizes the amount of energy in the fluid that is captured/recovered. In turn, this allows for a maximum pressure recovery (the change of kinetic energy to a static pressure rise). Maximizing pressure recovery offers at least two benefits to the overall operation of a system. First, for a cavitating flow, a greater pressure recovery helps ensure that substantially all two-phase fluid is converted back to single-fluid by collapsing and/or condensing any vapor bubbles in the fluid as it flows through the diffuser slot. Second, in a non-cavitating flow or in the case of vapor flow, maximizing the energy recovered in the fluid helps to ensure that a sufficient static pressure will exist to do gain additional benefits from the fluid. Additional benefits include re-injecting the fluid upstream or elsewhere in the system to help moderate the flow condition in the area of the re-injection. Moderation is achieved by either removing vortices in the flow to prevent corruption of upstream or downstream flow or by re-injecting to help reduce fluid rotation in the area of re-injection. Improving the upstream conditions of the fluid flow may allow the equipment and the system overall to operate more efficiently. 
     While the present invention has been described in connection with a preferred embodiment, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.