Abstract:
A flow conditioning device for incrementally stepping down pressure within a piping system is presented. The invention includes an outer annular housing, a center element, and at least one intermediate annular element. The outer annular housing includes an inlet end attachable to an inlet pipe and an outlet end attachable to an outlet pipe. The outer annular housing and the intermediate annular element(s) are concentrically disposed about the center element. The intermediate annular element(s) separates an axial flow within the outer annular housing into at least two axial flow paths. Each axial flow path includes at least two annular extensions that alternately and locally direct the axial flow radially outward and inward or radially inward and outward thereby inducing a pressure loss or a pressure gradient within the axial flow. The pressure within the axial flow paths is lower than the pressure at the inlet end and greater than the vapor pressure for the axial flow. The invention minimizes fluidic instabilities, pressure pulses, vortex formation and shedding, and/or cavitation during pressure step down to yield a stabilized flow within a piping system.

Description:
FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     One or more of the inventions disclosed herein were supported, at least in part, by a grant from the National Aeronautics and Space Administration (NASA) under Contract No. NNX12CB10C awarded by NASA, Stennis Space Center. The United States Government may have certain limited rights to at least one form of the invention(s). 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims priority from Patent Cooperation Treaty Application No. PCT/US2014/066314 filed Nov. 19, 2014 entitled Axial Flow Conditioning Device for Mitigating Instabilities. The subject matter of the prior application is incorporated in its entirety herein by reference thereto. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to a flow conditioning device and more particularly is concerned, for example, with an axial-orifice type element that steps down the pressure within an axially flowing liquid regime. Specifically, the invention suppresses noise and instabilities caused by pressure fluctuations, vortex shedding, and cavitation that typically result when the pressure within an axial flow is reduced. The invention employs viscous dissipation to step down pressure within an axial flow, straightens an axial flow, and damps out free-stream instabilities within an axial flow. The invention produces a stable axial flow devoid of pressure and velocity fluctuations. 
     2. Background 
     A single-hole orifice  1 , as shown in  FIG. 1 , is commonly used to meter flow, to damp acoustic resonances, or to reduce pressure over a small axial distance in a piping system. A single-hole orifice  1  typically includes a cylindrical outer wall  2 . A plate  3  is attached to the interior surface of the outer wall  2  so as to completely traverse the flow path formed by the outer wall  2  thereby defining an upstream side  6  and a downstream side  7 . The plate  3  includes a hole  4  that restricts flow of a fluid  5  from the upstream side  6  to the downstream side  7 . 
     The performance of a single-hole orifice  1  is typically characterized by the average discharge coefficient representative of the ratio of the measured and theoretical volumetric or mass flow rates. The device is sized for use within a system based on the Reynolds Number of the flow, mass or volumetric flow rate, and the required pressure drop. 
       FIGS. 2 a  and 2 b    illustrate the pressure and velocity across a single-hole orifice  1 , respectively. The pressure of the fluid  5  drops significantly from P 1  to P throat  and the velocity of the fluid  5  increases from V 1  to V throat  as a result of flow acceleration within the constriction of the orifice. The pressure recovers from P throat  to P 2  after the fluid  5  exits the constriction; however, P 2  is lower than P 1  because of losses due to flow resistance, frictional losses, and flow turning. 
     As seen in  FIG. 1 , hydrodynamic instabilities are formed when the fluid  5  traverses the hole  4  creating a vortex  8  attached to the plate  3  at the downstream side  7 . The vortex  8  periodically detaches from the plate  3  in a process referred to as vortex shedding. The plate  3  introduces large shearing stresses into the fluid  5  during flow acceleration as the fluid  5  negotiates the hole  4 . Shear is responsible at least in part for the periodic shedding of vortices. The flow acceleration results in a lower pressure within the fluid  5  through the plate  3 . In some cases, the pressure can fall below the vapor pressure resulting in a vapor cavity or cavitation  9 , as represented in  FIG. 2 a    with the corresponding velocity increase in  FIG. 2 b   . In other cases, the pressure in the vortex cores falls below the vapor pressure resulting in the generation of bubbles. 
     Pressure fluctuations from both the cavitation and the vortex shedding are amplified if the frequency is sufficiently similar to the natural frequency within the piping system. Sometimes it is possible for a single-hole orifice  1  to operate in a “choked flow” condition during high mass flow rates so that the pressure drop causes large vapor cavities to form. Flow transients in a piping system can disrupt the vapor cavities causing shedding and convection of vapor clouds in the downstream section of the piping system. The local temperature gradients and pressure recovery downstream can cause the clouds to condense producing violent high amplitude pressure spikes that damage the piping system. For some single-hole orifices  1 , an unstable feedback loop can form between vortex shedding and acoustic perturbations originating from upstream components resulting in an amplification of the modes that convect downstream. 
     A single-hole orifice  1  is usually one component within a multiple component piping system that could include, but is not limited to, valves, pumps, turning ducts, and diffusers. As such, a single-hole orifice  1  rarely operates under nominally “steady” conditions and therefore is subject to pressure and/or velocity fluctuations. Even during non-cavitation conditions, small perturbations in the upstream side  6  elicit a highly complex non-linear dynamic response from a single-hole orifice  1  resulting in large scale fluctuations that are convected downstream. 
     At least one source postulates that a mode conversion takes place in acoustically-modulated, confined jets through an orifice resulting in a feedback instability. Another source reports that the overall response of an orifice is bounded by the response predicted by the one-dimensional linearized theory, the exception being a local resonance condition when the driving frequency is close to the natural frequency of the Kelvin Helmholtz instability in the orifice. Regardless, instability modes, either initiated or amplified, can have a profound effect on the operation of components downstream from a single-hole orifice. Various attempts have been made to inhibit and to control the hydrodynamic instabilities associated with a single-hole orifice. 
     In one example, a globe-style control valve with anti-cavitation trims was substituted for an orifice within a piping system including an orifice and a control valve. The globe-style control valve provided the functionality of an orifice and operated in choked flow much like an orifice. This approach reduced the extent of cavitation within the piping system; however, the globe-style control valve did little to reduce vibrations within the system. Furthermore, cavitation in the globe-style control valve eroded the valve trims increasing the risk of significant damage to the piping system over time. 
     In another example, a two-stage orifice was implemented to operate in conjunction with throttle valves within an emergency cooling and containment system applicable to the downstream section of a pressurized water reactor. A first stage caused fluid to flow through tangential slots while a second stage caused fluid to flow both axially and tangentially. The two-stage orifice achieved the resistance of a single-stage orifice without the cavitation of the latter. However, the tangential flow inherent to the two-stage orifice caused significant swirl in the downstream fluid that accelerated erosion of the piping system. Furthermore, the additional stage increased the overall length of the orifice such that the device was incompatible with many applications. 
     As is readily apparent from the discussions above, the related arts do not provide a device that minimizes the instability modes associated with pressure step-down functionality. In particular, the related arts do not describe a device that avoids vortex shedding and cavitation. As such, the related arts are prone to vibrational responses that compromise the structural integrity of a piping system and to flow conditions that erode downstream components within a piping system. 
     Accordingly, what is required is a flow conditioning device that achieves the resistance required for a particular pressure drop within an axially efficient design envelope. 
     Accordingly, what is also required is a flow conditioning device that suppresses the instabilities, namely, vortex shedding and cavitation, associated with the reduction of pressure within an axial flow. 
     Accordingly, what is also required is a flow conditioning device that minimizes pressure fluctuations in a downstream flow. 
     Accordingly, what is also required is a flow conditioning device that minimizes vibrations communicable to a piping system. 
     Accordingly, what is also required is a flow conditioning device that provides flow resistance or pressure drop while minimizing the risk of cavitation in cryogenic and volatile liquids with vapor pressures higher than conventional liquids, one non-limiting example of the latter being water. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a flow conditioning device that achieves the resistance required for a specified pressure drop within an axially efficient design envelope. 
     An object of the invention is to provide a flow conditioning device that suppresses the instabilities, namely, vortex shedding and cavitation, associated with the reduction of pressure within an axial flow. 
     An object of the invention is to provide a flow conditioning device that minimizes pressure fluctuations in a downstream flow. 
     An object of the invention is to provide a flow conditioning device that minimizes vibrations communicable to a piping system. 
     An object of the invention is to provide a flow conditioning device that provides flow resistance or pressure drop while minimizing the risk of cavitation in cryogenic and volatile liquids with vapor pressures higher than conventional liquids, one non-limiting example of the latter being water. 
     In accordance with embodiments of the invention, a flow conditioning device for stepping down a pressure within an axial flow within a piping system while minimizing pressure fluctuations, vortex formation, and cavitation includes an outer annular housing, a center element, and at least one intermediate annular element. The outer annular housing includes an inlet end attachable to an inlet pipe and an outlet end attachable to an outlet pipe. The outer annular housing and intermediate annular element(s) are concentrically disposed about the center element. The intermediate annular element(s) separates the axial flow within the outer annular housing into at least two axial flow paths. Each axial flow path has an annular cross section. Each axial flow path includes at least two annular extensions that alternately direct the axial flow radially outward and inward or radially inward and outward thereby reducing the pressure as the axial flow traverses the axial flow paths so that the pressure at the outlet end is lower than the pressure at the inlet end. Also, the pressure within the axial flow along each axial flow path is greater than a vapor pressure for the axial flow. 
     In accordance with other embodiments of the invention, each axial flow path is defined by an inner annular surface and an outer annular surface. Each inner annular surface includes at least one annular extension directed toward the center element. Each outer annular surface includes at least one annular extension directed toward the outer annular housing. The annular extensions are arranged so that one annular extension disposed outward is immediately adjacent to another annular extension disposed inward. 
     In accordance with other embodiments of the invention, the annular extensions are defined by an undulated surface along each of the inner annular surface and the outer annular surface. The undulated surfaces minimize viscous drag and resistance encountered by the axial flow along each axial flow path. 
     In accordance with other embodiments of the invention, the height of the axial flow path is constant along the length of the axial flow path. 
     In accordance with other embodiments of the invention, the height of the axial flow path varies along the length of the axial flow path. 
     In accordance with other embodiments of the invention, a pair of extension tubes are separately attached to the inlet end and the outlet end. Each extension tube includes threads facilitating attachment to one of the inlet pipe and the outlet pipe. 
     In accordance with other embodiments of the invention, a pair of extension tubes are separately attached to the inlet end and the outlet end. Each extension tube includes a flange facilitating attachment to one of the inlet pipe and the outlet pipe. 
     In accordance with other embodiments of the invention, an end cap is attached to the inlet end. The end cap includes an outer ring that contacts the outer annular housing, at least one inner ring that separately contacts the intermediate annular element(s), and a hub that contacts the center element. The end cap directs the axial flow into the axial flow paths. 
     In accordance with other embodiments of the invention, a first portion and a second portion of the annular extension are disposed about at least one other annular extension. 
     In accordance with other embodiments of the invention, the flow condition device defines a module. At least two modules are attached in an end-to-end arrangement. 
     In accordance with embodiments of the invention, the flow conditioning method for stepping down a pressure within an axial flow via a flow conditioning device within a piping system while minimizing pressure fluctuations, vortex formation, and cavitation includes the steps of receiving, separating, directing, and communicating an axial flow. In the receiving step, the axial flow from the piping system is received into the flow conditioning device that includes an outer annular housing with a center element therein. In the separating step, the axial flow is separated into at least two axial flow paths via at least one intermediate annular element disposed between the outer annular housing and the center element. In the directing step, the axial flow is directed radially outward and inward or radially inward and outward within each axial flow path so as to reduce the pressure as the axial flow traverses the flow conditioning device. In the communicating step, the axial flow from the flow conditioning device is communicated into the piping system after the directing step. The pressure within the axial flow after the directing step is lower than the pressure before the directing step. The pressure within the axial flow during and after the directing step is greater than a vapor pressure for the axial flow. 
     In accordance with other embodiments of the invention, the directing step is implemented by at least two annular extensions along each axial flow path. 
     In accordance with other embodiments of the invention, the annular extensions are defined by an undulated surface. The undulated surfaces minimize viscous drag and resistance encountered by the axial flow along each axial flow path. 
     In accordance with other embodiments of the invention, a first portion and a second portion of one annular extension are disposed about at least one other annular extension. 
     In accordance with other embodiments of the invention, the separating step is further implemented by an end cap. The end cap includes an outer ring that contacts the outer annular housing, at least one inner ring that separately contacts the intermediate annular element(s), and a hub that contacts the center element. 
     In accordance with other embodiments of the invention, the directing step is implemented via at least two modules attached in an end-to-end configuration. At least one module directs the axial flow radially outward and inward or at least one module directs the axial flow radially inward and outward. 
     In accordance with other embodiments of the invention, the pressure at the inlet end and the pressure at the outlet end of one module define a pressure drop for the module whereby the pressure drops are identical between two adjacent modules. 
     In accordance with other embodiments of the invention, the pressure at the inlet end and the pressure at the outlet end of one module define a pressure drop for the module whereby the pressure drops differ between two adjacent modules. 
     A single-hole orifice element reduces pressure by increasing velocity. In contrast, the invention nearly linearly steps down the pressure within a flow by dissipating the energy within the flow, straightening the flow, and damping out free-stream instabilities within the flow. These features enable the invention to suppress the instability modes common to single-hole orifices while providing the requisite resistance to effectively reduce the pressure within a flow. 
     The invention suppresses both the hydrodynamic instability mode and the cavitation instability mode that are commonly observed during operation with a conventional single-hole orifice. The invention includes two or more concentrically disposed elements, each having annular extensions that create a flow path resistant to the flow. The shape of the annular extensions, number of concentric channels, and length of the invention alter the resistance to flow and consequently, the pressure gradient achievable by the invention. 
     The orientation of the annular extensions is critical to performance. Annular extensions oriented perpendicular to the inlet flow form a path with high resistance and large pressure losses Annular extensions oriented at an angle to the inlet flow create a more aerodynamic flow path with less viscous drag and less resistance to flow. 
     The number of concentric channels influences the resistance provided by the invention. In general, the total interior surface area of the invention in contact with a fluid is directly related to the number of concentric channels. It was observed in some designs that a reduction in the number of channels often resulted in larger but less grooves when the device length was fixed. It was also observed in other designs that a reduction in the number of concentric channels from four to three caused a one-half reduction in the pressure gradient by the invention. 
     The length of the invention also influences the resistance achievable thereby. The total interior surface area of the invention in contact with a fluid is directly related to the axial length of the invention. In general, the length of the invention and the resistance or pressure drop are linearly related when the grooves are of fixed design and disposed along each channel in a repeating arrangement. This feature allows the invention to be constructed in a modular, scalable form permitting the user to assemble two or more modules to form a single flow control device. 
     Several advantages are offered by the invention. The invention mitigates development of instabilities such as vortex shedding and cavitation. The invention minimizes pressure fluctuation at the outlet end thereof over a wide range of inlet pressure conditions including pressures representative of a saturation condition. The invention avoids pressures below the vapor pressure of the fluid thereby preventing cavitation or phase change within a downstream flow. The invention minimizes vibrations communicable to piping systems. The invention facilitates modular construction for customizable solutions based on the pressure drop required. The invention employs axial viscous mechanisms to dissipate energy, thus avoiding erosion-prone swirl flow within a downstream section of a piping system. The design is applicable to a variety of fluids including, but not limited to, liquids, one non-limiting example being water, and cryogenically-cooled liquids, one non-limiting example being liquid nitrogen. 
     The above and other objectives, features, and advantages of the preferred embodiments of the invention will become apparent from the following description read in connection with the accompanying drawings, in which like reference numerals designate the same or similar elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional aspects, features, and advantages of the invention will be understood and will become more readily apparent when the invention is considered in the light of the following description made in conjunction with the accompanying drawings. 
         FIG. 1  is a cross section view illustrating upstream and downstream flow within an exemplary single-hole orifice. 
         FIG. 2 a    is a graph illustrating pressure versus axial position along an exemplary single-hole orifice. 
         FIG. 2 b    is a graph illustrating velocity versus axial position along an exemplary single-hole orifice. 
         FIG. 3  is a side view of a piping system illustrating location of a flow conditioning device in accordance with an embodiment of the invention. 
         FIG. 4  is an exploded perspective view illustrating center element, intermediate annular element, and outer annular housing in accordance with an embodiment of the invention. 
         FIG. 5  is a cross-section perspective view illustrating an assembly including center element, intermediate annular element, and outer annular housing in accordance with an embodiment of the invention. 
         FIG. 6  is a partial-section perspective view illustrating an optional modular form of a flow conditioning device with optional end cap in accordance with an embodiment of the invention. 
         FIG. 7  is a cross-section perspective view illustrating optional spacer elements separately attached to an inlet end and an outlet end of a flow conditioning device allowing mechanical engagement via threads to an inlet pipe and an outlet pipe (pipes not shown), respectively, within a piping system in accordance with an embodiment of the invention. 
         FIG. 8  is a cross-section perspective view illustrating optional spacer elements separately attached to an inlet end and an outlet end of a flow conditioning device allowing mechanical engagement via a flange to an inlet pipe and an outlet pipe (pipes not shown), respectively, within a piping system in accordance with an embodiment of the invention. 
         FIG. 9  is a graph illustrating exemplary pressure and velocity versus axial position along a flow conditioning device in accordance with an embodiment of the invention. 
         FIG. 10  is a graph illustrating pressure versus time data downstream from an exemplary single-hole orifice. 
         FIG. 11  is a graph illustrating pressure versus time data downstream from a flow conditioning device in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to several embodiments of the invention that are illustrated in the accompanying drawings. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts. The drawings are in simplified form and are not to precise scale. 
     While features of various embodiments are separately described herein, it is understood that such features may be combinable to form other additional embodiments. 
     Components described herein are manufactured via methods, processes, and techniques understood in the art, including, but not limited to, machining, molding, forming, and three-dimensional printing. 
     Referring now to  FIG. 3 , the invention, referred to as a flow conditioning device  80 , is shown in one exemplary application within a piping system  81 . The piping system  81  could include an inlet pipe  82  mechanically coupled to a first end of the flow conditioning device  80  and an outlet pipe  83  mechanically coupled to a second end of the flow conditioning device  80 . The term pipe is understood to include devices that direct the flow of a fluid, examples including, but not limited to, conduits, pipes and tubes. A fluid is communicated into one end of the flow conditioning device  80  by the inlet pipe  82 . The fluid flows axially through the flow conditioning device  80  and is then communicated at a second end to an outlet pipe  83 . The inlet pipe  82  could include a reduction section  89  and the outlet pipe  83  could include an expansion section  90  facilitating mechanical coupling when the diameter of the flow conditioning device  80  is less than the diameter of the inlet and outlet pipes  82 ,  83 . However, it is also possible for the flow conditioning device  80  to have a diameter equal to or larger than the inlet and outlet pipes  82 ,  83  requiring the inlet pipe  82  coupled to an expansion section  90  and the outlet pipe  83  coupled to a reduction section  89 . It is likewise possible for the inlet and outlet pipes  82 ,  83  to have the same outer diameter as the flow conditioning device  80  thereby avoiding reduction and expansion sections  89 ,  90 . 
     Referring now to  FIG. 4 , one possible embodiment of the flow conditioning device  10  is shown including an outer annular housing  11 , an intermediate annular element  13 , and a center element  12 . Although one intermediate annular element  13  is illustrated, it is understood that the invention could include one or more intermediate annular element(s)  13 . 
     The outer annular housing  11 , intermediate annular element(s)  13 , and center element  12  are composed of one or more materials including, but not limited to, metals, plastics, and composites. Material selection is application dependent based on such criteria, by way of example only, as the fluid type, flow rate, pressures, temperatures, and operating environment. For example, high-strength, temperature resistant metal is preferred when the fluid is liquid oxygen and the operating environment includes feed lines to injectors in a rocket engine combustion chamber. 
     The outer annular housing  11  is an element of generally cylindrical construction with an opening that traverses the axial length thereof. The radial cross section of the outer annular housing  11  is substantially circular. The axial cross section of the outer annular housing  11  is defined by an outer annular surface  86  and an inner annular surface  14 . The outer annular surface  86  could include a cylindrical profile; although other designs are possible. The inner annular surface  14  defines the axial profile of the opening. The inner annular surface  14  could include a radial maximum  16  disposed between a pair of radial minimums  15 ,  17 . The radial minimums  15 ,  17  correspond to a circular opening smaller than the circular opening at the radial maximum  16 . The transition between each radial minimum  15 ,  17  and the radial maximum  16  could be arcuate, curved, or otherwise shaped so that the resultant structure provides a continuously variable or otherwise smooth transition from the radial minimum  15  to the radial maximum  16  and from the radial maximum  16  to the radial minimum  17 . The radial minimums  15 ,  17  and radial maximum  16  could repeat along the length of the inner annular surface  14  to form a wavy structure referred to as an undulated surface  79 . In some embodiments, an optional spacer ring  38  could be attached to at least one end of the outer annular housing  11 , as shown in  FIG. 5 . The spacer ring  38  could facilitate assembly of the flow conditioning device  10  within a particular application. 
     The intermediate annular element(s)  13  is also of cylindrical construction with a substantially circular opening that traverses the length thereof. The axial cross section of each intermediate annular element(s)  13  is defined by the inner annular surface  22  and an outer annular surface  23 . The inner annular surface  22  could include a radial maximum  26  disposed between a pair of radial minimums  24 ,  28 . The radial minimums  24 ,  28  correspond to a circular opening smaller than the circular opening at the radial maximum  26 . The transition between the each radial minimum  24 ,  28  and the radial maximum  26  could be arcuate, curved, or otherwise shaped so that the resultant structure provides a continuously variable or otherwise smooth transition from the radial minimum  24  to the radial maximum  26  and from the radial maximum  26  to the radial minimum  28 . The radial minimums  24 ,  28  and radial maximum  26  could repeat along the length of the inner annular surface  22  forming a wavy structure also referred to as an undulated surface  79 . Likewise, the outer annular surface  23  could include a radial maximum  27  disposed between a pair of radial minimums  25 ,  29 . The radial minimums  25 ,  29  correspond to an outer diameter smaller than the outer diameter at the radial maximum  27 . The transition between each radial minimum  25 ,  29  and the radial maximum  27  could be arcuate, curved, or otherwise shaped so that the resultant structure provides a continuously variable or otherwise smooth transition from the radial minimum  25  to the radial maximum  27  and from the radial maximum  27  to the radial minimum  29 . The radial minimums  25 ,  29  and radial maximum  27  could repeat along the length of the outer annular surface  23  forming a wavy structure also referred to as an undulated surface  79 . 
     The center element  12  is a rod-like or cylinder-like element with a substantially circular cross section. The axial profile of the center element  12  is defined by an outer annular surface  30 . The outer annular surface  30  could likewise include a radial maximum  32  disposed between a pair of radial minimums  31 ,  33 . The radial minimums  31 ,  33  correspond to an outer diameter smaller than the outer diameter at the radial maximum  32 . The transition between each radial minimum  31 ,  33  and the radial maximum  32  could be arcuate, curved, or otherwise shaped so that the resultant structure provides a continuously variable or otherwise smooth transition from the radial minimum  31  to the radial maximum  32  and from the radial maximum  32  to the radial minimum  33 . The radial minimums  31 ,  33  and radial maximum  32  could repeat along the length of the outer annular surface  30  forming a wavy structure also referred to as an undulated surface  79 . 
     Referring now to  FIGS. 4 and 5 , the intermediate annular element(s)  13  and center element  12  are fixed to the outer annular housing  11 , preferably via pins  18 - 21  or other fasteners. Although four sets of three pins  18 - 21  are shown, the number and arrangement of pins are design dependent and a function of the pressure loads imposed by the flow over the pins  18 - 21 . In one exemplary embodiment, two pins could be oppositely disposed about the circumference of the flow conditioning device  10  at one or more axial locations. In another exemplary design, three pins could be position about the circumference of the flow conditioning device  10  every 120-degrees at one or more axial locations. In yet another exemplary design, four pins  18 - 21  could be position about the circumference of the flow conditioning device  10  every 90-degrees at one or more locations, the latter illustrated in  FIG. 4 . 
     The outer annular housing  11 , intermediate annular element(s)  13 , and center element  12  are provided with two or more sets of holes  34 ,  35 , and  36 , respectively. Each first hole  34  should completely traverse the thickness of the outer annular housing  11  and allow for a clearance fit. Each second hole  35  should completely traverse the thickness of the intermediate annular element  13  and allow for an interference fit. Each third hole  36  could partially traverse the width of the center element  12  and allow for an interference fit. The holes  34 - 36  should align when the intermediate annular element(s)  13  and center element  12  are properly positioned within the outer annular housing  11 . 
     The outer annular housing  11 , intermediate annular element(s)  13 , and center element  12  could be positioned and held in place for assembly purposes by a fixture. A pin  18 - 21  is inserted into each outermost hole  34 . A force is then applied to the end of the pin  18 - 21  in the direction of the outer annular housing  11  so that the pin  18 - 21  traverses the hole  35  through the intermediate annular element  13  and then the hole  36  along the center element  12 . A weld plug  37  or adhesive may be applied along the outer annular surface  86  above each pin  18 - 21  to fix the pins  18 - 21  to the flow conditioning device  10 . In other embodiments, a weld or adhesive could be applied to each pin  18 - 21  at the intersection with the intermediate annular element(s)  13  and center element  12 . In the latter embodiments, the interference fit could be optional. 
     The diameter profiles of the outer annular housing  11 , intermediate annular element(s)  13 , and center element  12  should permit assembly of the various components so that the intermediate annular element(s)  13  and outer annular housing  11  are disposed about the center element  12  in a concentric arrangement. The diameter and thickness profiles of the intermediate annular element(s)  13  and center element  12  should ensure a gap between the outer annular housing  11  and outermost intermediate annular element  13 , and between each pair of immediately adjacent intermediate annular elements  13  (only one shown), and between the innermost intermediate annular element  13  and the center element  12 . The gap, referred to herein as an axial flow path  39 , should completely traverse the axially length of the flow conditioning device  10  so as to allow a fluid to pass from the inlet end  53  to the outlet end  54 , as represented in  FIG. 5 . The height  84  along each axial flow path  39  could be constant or vary along the axial length of the flow conditioning device  10 . 
     In one aspect of the invention, the number of axial flow paths  39  are determined by the number of intermediate annular elements  13 . The number of axial flow paths  39  correlates to one more than the total number of intermediate annular elements  13  within the flow conditioning device  10 . For example, one intermediate annular element  13  provides two axial flow paths  39 . In another aspect of the invention, the performance of a flow conditioning device  10  is influenced by the total number of axial flow paths  39 . As a general rule, resistance to flow increases as the number of axial flow paths  39  is increased. Furthermore, it is preferred that the flow along one axial flow path  39  not enter or mix with the flow along another axial flow path  39 . 
     The undulated surfaces  79  described herein define annular extensions  65 ,  66  along the axial flow paths  39 . The undulated surfaces  79  provide an aerodynamic pathway that minimizes viscous drag and resistance encountered by the axial flow along each axial flow path  39 . Referring now to  FIGS. 4 and 6 , each pair of radial maximums  26 ,  27  corresponds to an outward annular extension  66 , each radial maximum  32  corresponds to an outward annular extension  66 , each radial minimum  15 ,  17  corresponds to an inward annular extension  65 , and each pair of radial minimums  24 ,  25 ,  28 ,  29  corresponds to an inward annular extension  65 . In preferred embodiments, the inward annular extensions  65  are positioned so as to substantially align and the outward annular extensions  66  are positioned so as to substantially align, as represented in  FIG. 6 . Each inward annular extension  65  should substantially align with the corresponding radial minimums along the intermediate annular element(s)  13  and center element  12 . Each outward annular extension  66  should substantially align with the corresponding radial maximums along the outer annular housing  11  and intermediate annular element(s)  13 , as represented in  FIG. 6 . In some embodiments, the flow may be directed outward and then inward. In other embodiments, the flow may be direct inward and then outward. 
     Each inward annular extension  65  and each outward radial extension  66  locally controls the direction of flow along an axial flow path  39 . The inward annular extensions  65  direct the fluid to locally move radially inward in the direction of the center element  12 . The outward annular extensions  66  direct the fluid to locally move radially outward in the direction of the outer annular housing  11 . The resistance to flow is determined, in part, by the annular extensions  65 ,  66 . In general, flow resistance increases with an increase in the size and/or number of annular extensions  65 ,  66 . The dimensional properties, namely, width and height, and/or shape of the annular extensions  65 ,  66  may be the same or vary along the flow conditioning device  10  and/or between surfaces defining the axial flow paths  39 . 
     Referring now to  FIG. 6 , the flow conditioning device  10  could be constructed from an outer annular housing  11 , an intermediate annular element(s)  13 , and a center element  12  that include one inward annular extension  65  and one outward annular extension  66 . In some embodiments, the annular extensions  65 ,  66  could be continuous structures. In other embodiments, one annular extension  65  could be divided into two portions  87 ,  88  whereby a first portion  87  is disposed along one side and a second portion  88  is disposed along another side of another annular extension  66 , as illustrated in the module  47 . While the inward annular extension  65  is shown in two parts in  FIG. 6 , it is also possible for the outward annular extension  66  to be divided and disposed about an inward annular extension  65 . 
     The flow conditioning device  10  could be constructed as a unitary element with two or more annular extensions  65 ,  66  and two or more axial flow paths  39 , as represented in  FIG. 5 . It is likewise possible for the flow conditioning device  10  to be assembled from two or more modules  47 ,  48 , as represented in  FIG. 6 , facilitating stackable, scalable solutions. For example, if a single module provides a 2-psi pressure drop, then a user could stack five modules  47  in an end-to-end configuration when the required pressure drop is 10-psi. It is also possible for the modules to provide different pressure drops, thus enabling a module with a higher pressure drop to be coupled to a module with a lower pressure drop. 
     Referring again to  FIG. 6 , modules  47 ,  48  could be mechanically coupled and secured to form a flow conditioning device  10 . A first module  47  could mechanically interlock with a second module  48  via an annular slot  50  disposed along the vertical face of a first module  47  and an annular tab  49  extending from the vertical face of a second module  48 . The interlock could also provide a fluid tight seal between the modules  47 ,  48 . Module  47 ,  48  could be fixed via a circumferential weld, a mechanical device, examples including, but not limited to, fasteners or a compression band, or adhesively bonded to ensure the structural integrity for a specific application. 
     Referring again to  FIG. 6 , the flow conditioning device  10  could include an optional end cap  41  attached to the inlet end  53 . The end cap  41  could include an outer ring  42 , at least one inner ring  43 , and a hub  44 . The outer ring  42  and inner ring(s)  43  are concentrically disposed about the hub  44 . One or more spokes  45 ,  46  could extend outward from the hub  44  and secure the outer ring  42  and inner ring(s)  43  thereto. The end cap  41  could be fabricated as a single element or assembled from separate interlocking components. 
     An outer ring  42  could align with and contact the vertical end of the outer annular housing  11 . An inner ring  43  could radially align with and contact the vertical end of each intermediate annular element  13 . The hub  44  could align with and contact the vertical end of the center element  12 . The vertical end of the outer annular housing  11 , intermediate annular element(s)  13 , and/or center element  12  could include an annular tab  51  that engages an annular slot  52  disposed along the corresponding element along the end cap  41 . The various components comprising the end cap  41  could include features that improve the flow characteristics or aerodynamic properties at the inlet end  53  ensuring separation and channelization of flow into the axial flow paths  39 . 
     Referring now to  FIG. 7 , the flow conditioning device  10  could include a shoulder  91  at each of the inlet and outlet ends  53 ,  54 . A first extension tube  55  could engage the shoulder  91  at the inlet end  53 . A second extension tube  56  could engage the shoulder  91  at the outlet end  54 . The term tube is understood to include devices that direct the flow of a fluid, examples including, but not limited to, conduits, pipes and tubes. The shoulder  91  could be sized to form an interference fit with each extension tube  55 ,  56  so as to secure one end of each extension pipe  55 ,  56  to the flow conditioning device  10 . It is likewise possible for the extension tubes  55 ,  56  to be secured to the flow conditioning device  10  via other means understood in the art. A second end of each extension tube  55 ,  56  could include threads  58 ,  60 , respectively. The threads  58  along the extension tube  55  could engage complementary threading at the open end of an inlet pipe  57 . The threads  60  along the extension tube  56  could engage complementary threading at the open end of an outlet pipe  59 . 
     Referring now to  FIG. 8 , the flow conditioning device  10  could include a shoulder  91  at each of the inlet and outlet ends  53 ,  54 . A first extension tube  55  could abut the shoulder  91  at the inlet end  53 . A second extension tube  56  could abut the shoulder  91  at the outlet end  54 . A circumferential weld could fix each extension tube  55 ,  56  to the flow conditioning device  10 . It is likewise possible for the extension tubes  55 ,  56  to be secured to the flow conditioning device  10  via other means understood in the art. A second end of each extension tube  55 ,  56  could include a flange  61 ,  62 , respectively, secured thereto via a circumferential weld. The flange  61  could engage a similar structure (not shown) at the open end of the inlet pipe  57 . The flange  62  could likewise engage a similar structure (not shown) at the open end of the outlet pipe  59 . Each flange  61 ,  62  could include holes  63 ,  64 , respectively, allowing mechanical coupling via bolts or other fasteners understood in the art. 
     The coupling configurations described in  FIGS. 7 and 8  are for illustrative purposes only. Therefore, the extension tubes  55 ,  56  are attachable to inlet and outlet pipes  57 ,  59  via other means understood in the art. Furthermore, it is possible in other embodiments for the inlet and outlet pipes  57 ,  59  to be directly coupled to the flow conditioning device  10 . 
     Referring again to  FIGS. 3-8 , the flow conditioning device  10  receives a fluid from a piping system  81 . The fluid is communicated into the inlet end  53  at a first end of the flow conditioning device  10 . The fluid traverse the axial length of the flow conditioning device  10  from the inlet end  53  to the outlet end  54 , the latter at a second end of the device. The fluid is separated into two or more axial flow paths  39  upon entering the flow conditioning device  10 . The axial flow paths  39  are defined by one or more intermediate annular element(s)  13 . Each intermediate annular element(s)  13  divides the fluid flow into separate flow streams between an outer annular housing  11  and a center element  12 . An end cap  41  could be attached to the flow conditioning device  10  to improve the flow dynamics at the inlet end  53 . The fluid flow is locally directed radially outward and inward or radially inward and outward within each axial flow path  39 . The process is repeated so as to incrementally reduce the pressure within the fluid. The fluid is locally directed outward by one or more annular extensions  66  oriented in the direction of the outer annular housing  11 . The fluid is locally directed inward by one or more inward annular extensions  65  oriented in the direction of the center element  12 . In some embodiments, the annular extensions  65 ,  66  are defined by undulated surfaces  79  that less abruptly alter the local direction of flow. The undulated surfaces  79  are shaped or otherwise formed to minimize viscous drag and resistance encountered by the axial flow along each axial flow path  39 . In other embodiments, the annular extensions  65 ,  66  may be defined by discontinuities that more abruptly alter the local direction of flow. The fluid is then communicated back into the piping system  81 . The process of alternately directing fluid flow locally outward and locally inward could be performed by two or more modular units permitting an end-to-end assembly. One annular extension  65 ,  66  could be divided into a first portion  87  and a second portion  88  separately disposed at the inlet end  53  and the outlet end  54  of a flow conditioning device  10 . The pressure of the fluid reentering the piping system  81  is less than the pressure of the fluid received from the piping system  81 . Furthermore, the pressure within and exiting the flow conditioning device  80  is above a vapor pressure threshold of the fluid. 
     Referring now to  FIG. 9 , the graph illustrates exemplary pressure and velocity profiles for a fluid as it traverses an exemplary embodiment of the invention. The fluid enters the flow conditioning device  10  at the inlet end  53  with a pressure P 1  and a velocity V 1 . Next, the fluid passes along and interacts with the annular extensions  65 ,  66  resulting in a decrease  77  in pressure and an increase  92  in velocity. Both pressure and velocity decrease with each successive interaction after the first interaction, although at different rates. This process is repeated for each successive pair of annular extensions  65 ,  66 , to incrementally decrease the pressure within the fluid. The pressure within the flow conditioning device  10  is greater than the vapor pressure P v . The fluid then exits the flow conditioning device  10  at the outlet end  54  with a pressure P 2  and a velocity V 2 . The pressure P 2  at the outlet end  54  is less than the pressure P 1  at the inlet end  53 , yet greater than the vapor pressure P v . The velocity V 2  at the outlet end  54  is approximately equal to the velocity V 1  at the inlet end  53 , although in many cases a small velocity drop is possible. The flow conditioning device  10  is designed so that pressure and velocity conditions within the flow conditioning device  10  and at the outlet end  54  avoid the instabilities described herein. 
     Referring now to  FIG. 10 , an exemplary pressure versus time plot is shown for a fluid after exiting an exemplary single-hole orifice. The pressure oscillates rapidly and widely between negative and positive valves. This behavior is representative of the instabilities described herein. 
     Referring now to  FIG. 11 , an exemplary pressure versus time plot is shown for a fluid after exiting an exemplary embodiment of the invention. The pressure is substantially uniform over time within minor positive and negative excursions. This behavior is representative of instability-free fluid flow. 
     As is evident from the explanation herein, the described invention is a flow conditioning device which facilitates pressure reduction within a system that moves fluid within a controlled fashion. The invention is applicable to a variety of flow regimes, exemplary applications including, but not limited to, piping systems for conventional liquids, cryogenic liquids, and volatile liquids. Accordingly, the described invention is expected to be used, by way of example only, in propellant lines to regulate flow rate, propellant condition systems, propellant feed systems, coolant systems for rocket test stands and launch pads, coolant systems for power generating equipment, refineries, and pharmaceutical manufacturing equipment. 
     The description above indicates that a great degree of flexibility is offered in terms of the present invention. Although various embodiments have been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.