PATENT DOCUMENT

Publication Number: US-11339793-B2
Application Number: US-201916452074-A
Country: US
Kind Code: B2

Title: Fan flow directing features, systems and methods

Abstract:
Systems and methods are provided for mitigating recirculation of backflow fluid through a fan. The fan includes a housing having a channel that extends from an inlet to an outlet of the housing. A rotor assembly is positioned within the channel and is configured to direct a fluid flow from the inlet to the outlet. The rotor assembly includes a hub, a plurality of fan blades, and a shroud disposed about a circumference of the fan blades, where a radial gap extends between the shroud and the housing. The radial gap is configured to receive a portion of the fluid flow from the outlet as backflow fluid. The rotor assembly also includes an inlet flange that is configured receive the backflow fluid from the radial gap and to direct the backflow fluid in a direction away from the inlet prior to discharge of the backflow fluid from the radial gap.

Claims:
What is claimed is: 
     
       1. A fan for directing a fluid through an enclosure of an electronic device, comprising:
 a housing having a channel defined therein, wherein the channel extends from an inlet of the housing to an outlet of the housing; 
 a rotor assembly positioned within the channel and configured to rotate about a central axis of the channel to direct a fluid flow from the inlet to the outlet, wherein the rotor assembly includes:
 a hub; 
 a plurality of fan blades extending from the hub; 
 a shroud disposed about a circumference of the plurality of fan blades and coupled to the plurality of fan blades, wherein a radial gap extends between the shroud and the housing, and the radial gap is configured to receive a portion of the fluid flow from the outlet as backflow fluid; and 
 an inlet flange extending from the shroud, wherein the inlet flange is configured to receive the backflow fluid from the radial gap and direct the backflow fluid in a direction away from the inlet prior to discharge of the backflow fluid from the radial gap, wherein the housing includes an outer wall that circumscribes the rotor assembly, wherein the inlet flange extends radially across an end portion of the outer wall to define a vertical gap between the end portion and the inlet flange, and wherein a width of the vertical gap is non-uniform about a circumference of the outer wall to non-uniformly bias discharge of the backflow fluid about the circumference of the outer wall. 
 
 
     
     
       2. The fan of  claim 1 , wherein the inlet flange extends across the outer wall of the housing such that a distal end of the inlet flange is positioned exterior to the channel, wherein the distal end of the inlet flange extends in the direction away from the inlet. 
     
     
       3. The fan of  claim 2 , wherein the direction away from the inlet extends generally orthogonal to the central axis of the channel. 
     
     
       4. The fan of  claim 1 , wherein the end portion is a first end portion of the outer wall proximate the inlet, wherein the outer wall has a second end portion proximate the outlet, and wherein the inlet flange curves around the first end portion of the outer wall to direct the backflow fluid generally along an exterior surface of the outer wall toward the outlet. 
     
     
       5. The fan of  claim 4 , wherein the inlet flange forms an additional radial gap that extends between the inlet flange and the exterior surface of the outer wall, wherein a width of the additional radial gap is substantially constant along a length of the additional radial gap. 
     
     
       6. The fan of  claim 1 , wherein the outer wall includes a height profile that defines the width of the vertical gap, wherein the height profile includes a pair of crest points forming constricted sections of the vertical gap configured to discharge the backflow fluid at a first flow rate and a pair of trough points forming expanded sections of the vertical gap configured to discharge the backflow fluid at a second flow rate that is greater than the first flow rate. 
     
     
       7. The fan of  claim 6 , wherein the pair of crest points are positioned diametrically opposite one another along the outer wall and the pair of trough points are positioned diametrically opposite one another along the outer wall, wherein a first axis extending through the pair of crest points extends generally orthogonal to a second axis extending through the pair of trough points. 
     
     
       8. A fan for directing a fluid through an enclosure of an electronic device, comprising:
 a housing having a channel defined therein, wherein the channel extends from an inlet of the housing to an outlet of the housing; 
 a rotor assembly positioned within the channel and configured to rotate about a central axis of the channel to direct a fluid flow in a downstream direction along the central axis from the inlet to the outlet, wherein the rotor assembly includes:
 a hub; 
 a plurality of fan blades extending from the hub; 
 a shroud disposed about a circumference of the plurality of fan blades and coupled to the plurality of fan blades, wherein a radial gap extends between the shroud and the housing, and the radial gap is configured to receive a portion of the fluid flow from the outlet as backflow fluid; and 
 a flow mitigation feature protruding radially from the shroud and extending into the radial gap, wherein the flow mitigation feature includes a protrusion that extends from an exterior surface of the shroud and spirals helically about the exterior surface, wherein, during rotation of the rotor assembly, the protrusion is configured to engage with the backflow fluid occupying the radial gap to force an additional portion of the backflow fluid in the downstream direction along the central axis to generate a pressure within the radial gap that reduces a flow rate of the backflow fluid entering the radial gap. 
 
 
     
     
       9. The fan of  claim 8 , wherein the protrusion spirals helically about and along the central axis from the inlet toward the outlet. 
     
     
       10. The fan of  claim 8 , wherein the rotor assembly includes an inlet flange extending from the shroud, wherein the inlet flange is configured to receive the additional portion of the backflow fluid from the radial gap and direct the additional portion of the backflow fluid in a direction away from the inlet prior to discharge of the additional portion of the backflow fluid from the radial gap. 
     
     
       11. The fan of  claim 10 , wherein the direction away from the inlet extends generally orthogonal to the central axis of the channel. 
     
     
       12. The fan of  claim 8 , wherein the hub and the plurality of fan blades collectively form a blade assembly, wherein the shroud includes a plurality of grooves formed within an interior surface of the shroud, and wherein the plurality of grooves is configured to receive and engage with the plurality of fan blades to couple the blade assembly to the shroud. 
     
     
       13. The fan of  claim 12 , wherein an aperture is formed within each groove of the plurality of grooves, wherein a blade protrusion extends radially from each fan blade of the plurality of fan blades, and wherein respective blade protrusions of the plurality of fan blades are configured to engage with corresponding apertures of the plurality of grooves upon insertion of the blade assembly into the shroud to couple the blade assembly to the shroud via a snap fit. 
     
     
       14. A flow generation unit for directing a fluid through an enclosure of an electronic device, comprising:
 a housing having a first outer wall that defines a first channel through the housing and a second outer wall that defines a second channel through the housing; 
 a first fan including a first rotor assembly positioned within the first channel and configured to direct a respective fluid flow from an inlet of the first channel to an outlet of the first channel, wherein the first rotor assembly includes a first inlet flange that extends across the first outer wall to form a first vertical gap between the first inlet flange and the first outer wall, wherein the first vertical gap is configured to receive a portion of the respective fluid flow from the outlet of the first channel as backflow fluid of the first fan, and wherein the first inlet flange is configured to discharge the backflow fluid of the first fan through the first vertical gap in a direction away from the inlet of the first channel; and 
 a second fan including a second rotor assembly positioned within the second channel and configured to direct a respective fluid flow from an inlet of the second channel to an outlet of the second channel, wherein the second rotor assembly includes a second inlet flange that extends across the second outer wall to form a second vertical gap between the second inlet flange and the second outer wall, wherein the second vertical gap is configured to receive a portion of the respective fluid flow from the outlet of the second channel as backflow fluid of the second fan, wherein the second inlet flange is configured to discharge the backflow fluid of the second fan through the second vertical gap in a direction away from the inlet of the second channel, and wherein the first vertical gap of the first fan and the second vertical gap of the second fan each include a respective constricted section having a relatively narrow width and a respective expanded section having a relatively large width to non-uniformly bias discharge of the backflow fluid of the first fan about a circumference of the first outer wall and to non-uniformly bias discharge of the backflow fluid of the second fan about a circumference of the second outer wall. 
 
     
     
       15. The flow generation unit of  claim 14 , wherein the respective constricted section of the first vertical gap and the respective constricted section of the second vertical gap are positioned substantially adjacent one another and the respective expanded section of the first vertical gap and the respective expanded section of the second vertical gap are positioned substantially opposite one another to mitigate interaction between the backflow fluid of the first fan and the backflow fluid of the second fan. 
     
     
       16. A fan for directing a fluid through an enclosure of an electronic device, comprising:
 a housing having an outer wall that defines a channel, wherein the channel extends from an inlet of the housing to an outlet of the housing; and 
 a rotor assembly positioned within the channel and configured to rotate about a central axis of the channel to direct a fluid flow in a downstream direction along the channel from the inlet to the outlet, wherein the rotor assembly includes:
 a hub; 
 a plurality of fan blades extending from the hub; 
 a shroud disposed about a circumference of the plurality of fan blades and coupled to the plurality of fan blades, wherein a radial gap extends between the shroud and the housing, and the radial gap is configured to receive a portion of the fluid flow from the outlet as backflow fluid; and 
 an inlet flange extending from the shroud, wherein the inlet flange protrudes beyond the plurality of fan blades and beyond the outer wall in an upstream direction, opposite the downstream direction, and wherein the inlet flange is configured to receive the backflow fluid from the radial gap and direct the backflow fluid in a direction away from the inlet prior to discharge of the backflow fluid from the radial gap. 
 
 
     
     
       17. The fan of  claim 16 , wherein the outer wall circumscribes the rotor assembly, wherein the inlet flange extends radially across an end portion of the outer wall to define a vertical gap between the end portion and the inlet flange, and wherein a height of the outer wall is such that the vertical gap is positioned upstream of the plurality of fan blades, with respect to the downstream direction of the fluid flow along the channel. 
     
     
       18. A fan for directing a fluid through an enclosure of an electronic device, comprising:
 a housing having a channel defined therein, wherein the channel extends from an inlet of the housing to an outlet of the housing; 
 a rotor assembly positioned within the channel and configured to rotate about a central axis of the channel to direct a fluid flow from the inlet to the outlet, wherein the rotor assembly includes:
 a hub; 
 a plurality of fan blades extending from the hub; 
 a shroud disposed about a circumference of the plurality of fan blades and coupled to the plurality of fan blades, wherein a radial gap extends between the shroud and the housing, and the radial gap is configured to receive a portion of the fluid flow from the outlet as backflow fluid; and 
 a flow mitigation feature protruding radially from the shroud and extending into the radial gap, wherein, during rotation of the rotor assembly, the flow mitigation feature is configured to engage with the backflow fluid occupying the radial gap to generate a pressure within the radial gap to reduce a flow rate of the backflow fluid entering the radial gap, wherein the flow mitigation feature includes a fan blade of the plurality of fan blades, and wherein the fan blade extends through an exterior surface of the shroud to protrude past the exterior surface of the shroud. 
 
 
     
     
       19. A flow generation unit for directing a fluid through an enclosure of an electronic device, comprising:
 a housing having a first outer wall that defines a first channel through the housing and a second outer wall that defines a second channel through the housing; 
 a first fan including a first rotor assembly positioned within the first channel and configured to direct a respective fluid flow from an inlet of the first channel to an outlet of the first channel, wherein the first rotor assembly includes a first inlet flange that extends across the first outer wall to form a first vertical gap between the first inlet flange and the first outer wall, wherein the first vertical gap is configured to receive a portion of the respective fluid flow from the outlet of the first channel as backflow fluid of the first fan, and wherein the first inlet flange is configured to discharge the backflow fluid of the first fan through the first vertical gap in a direction away from the inlet of the first channel; and 
 a second fan including a second rotor assembly positioned within the second channel and configured to direct a respective fluid flow from an inlet of the second channel to an outlet of the second channel, wherein the second rotor assembly includes a second inlet flange that extends across the second outer wall to form a second vertical gap between the second inlet flange and the second outer wall, wherein the second vertical gap is configured to receive a portion of the respective fluid flow from the outlet of the second channel as backflow fluid of the second fan, wherein the second inlet flange is configured to discharge the backflow fluid of the second fan through the second vertical gap in a direction away from the inlet of the second channel, wherein the first outer wall includes a first wall height to position the first vertical gap of the first fan at a first height and the second outer wall includes a second wall height that is less than the first wall height to position the second vertical gap of the second fan at a second height that is less than the first height, such that the first vertical gap is configured to discharge the backflow fluid of the first fan at the first height and the second vertical gap is configured to discharge the backflow fluid of the second fan at the second height to mitigate interaction between the backflow fluid of the first fan and the backflow fluid of the second fan. 
 
     
     
       20. The flow generation unit of  claim 19 , wherein the second inlet flange of the second fan includes a circumferential end face that extends toward the second outer wall in a direction generally parallel to the second outer wall, wherein the first vertical gap of the first fan is configured to discharge the backflow fluid of the first fan onto the circumferential end face of the second inlet flange of the second fan.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 62/756,859, entitled “FAN FLOW DIRECTING FEATURES, SYSTEMS AND METHODS”, filed Nov. 7, 2018, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to fans, such as those used for cooling electronics, and, more particularly, to flow directing features for mitigating a recirculation of backflow air through such fans. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     One or more fans (e.g., axial fans) are commonly included in various electronic devices such as, for example, computers (e.g., servers, desktop computers), or a variety of other stationary or portable electronic devices. The fans are typically used to direct a working fluid (e.g., air) through an enclosure of the electronic device and across certain components (e.g., a central processing unit, a power supply unit, a graphics processing unit) within the enclosure that may generate thermal energy (e.g., heat). Accordingly, the working fluid may absorb the generated thermal energy (e.g., via convective heat transfer) and transfer the thermal energy to an ambient environment (e.g., the atmosphere) surrounding the electronic device. In this manner, the fans may ensure that an operational temperature of components included in the electronic device remains below a target value or within a desired range. 
     In many cases, operation of the fan(s) may generate audible noise (e.g., acoustic energy) that propagates from the fans. Unfortunately, the generated noise may be unpleasant to a user operating the electronic device and/or other persons located in proximity to the fan(s). 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     The present disclosure relates generally to flow directing features for a fan (e.g., an axial fan) of an electronic device. In particular, the flow directing features discussed herein are configured to mitigate or substantially reduce the recirculation of backflow air within the fan and, thus, mitigate formation of flow structures that may disturb air flow upstream of fan blades of the fan. Flow disturbances of this sort may lead to the generation of broadband and tonal noise when interacting with the fan blades, as well as a reduction in net air flow through the fan. For example, typical fans generally include a rotor that is disposed within a channel of a fan housing and configured to rotate about a central axis of the channel. The rotor includes a plurality of fan blades that are configured to engage with a fluid (e.g., air) surrounding the fan and direct the air through the channel in an intended direction of air flow (e.g., a first flow direction). In certain cases, a shroud may be disposed about and coupled to the fan blades. Accordingly, the shroud may rotate with and form an outer perimeter of the rotor. A radial gap (e.g., a shroud gap) often extends between the shroud and a wall of the channel to enable unrestricted rotational motion of the rotor relative to the housing. In many cases, operation of the fan generates a pressure differential on opposing sides of the housing (e.g., a lower pressure at the inlet and a higher pressure at the outlet), which induces a backflow of air that flows through the radial gap in a direction opposite to the intended direction air flow through the housing. The backflow of air may discharge near an inlet of the housing and generate disturbances near the fan blades that may interact with the fan blades and disturb air flow through the fan. That is, a region of disturbed or non-uniform air flow may be created near and/or within the fan housing, which often generates unpleasant audible noise. 
     Accordingly, embodiments of the present disclosure are directed toward various flow directing features that may be included in the fan to mitigate (e.g., redirect) the recirculation of backflow air (e.g., high pressure air discharged from the radial gap) through the fan blades and/or block a discharge of the backflow air from the fan housing. By way of example, embodiments of the present disclosure include a rotating inlet flange (e.g., on the rotating shroud of the fan) that forms an upstream end portion of the fan (e.g., of the rotor) and guides backflow air discharging from the radial gap in a direction diverging away from an inlet of the fan. In this manner, the rotating inlet flange may reduce or substantially eliminate recirculation of backflow air through the housing of the fan. Embodiments of the present disclosure also include backflow mitigation feature(s) that extend radially from the rotating shroud of the rotor and project into the radial gap between the rotor and stationary housing. As described in detail below, these backflow mitigation features may increase an aerodynamic resistance (e.g., an aerodynamic impedance) or a static pressure within the radial gap to counter-act the pressure differential generated between the inlet and an outlet of the fan housing, thus mitigating air recirculation through the radial gap. As such, the backflow mitigation features may generate a stagnation of air within the radial gap that blocks a discharge of backflow air from the fan housing back to the inlet region of the fan. By employing the aforementioned techniques alone or in any combination, air flow disturbances resulting from the backflow of air may be inhibited from forming near and/or around the fan blades, thus reducing a magnitude of audible noise that may be generated during operation of the fan. 
     Various refinements of the features noted above may exist in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a front view an example of an electronic device having one or more fans, in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a perspective view of an example of a fan that may be included in the electronic device of  FIG. 1 , in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a partial cross-sectional view of an example of the fan of  FIG. 2 , in accordance with an embodiment of the present disclosure; 
         FIG. 4  is a perspective view of an example of a fan having a rotating inlet flange, in accordance with an embodiment of the present disclosure; 
         FIG. 5  is a partial cross-sectional view of an example of the fan of  FIG. 4 , in accordance with an embodiment of the present disclosure; 
         FIG. 6  is an example of a graph illustrating a magnitude of acoustic energy that may be generated by the fans of  FIGS. 2 and 4 ; 
         FIG. 7  is an example of a chart illustrating a prominence of acoustic energy at harmonics of the blade-passing frequency that may be generated by the fans of  FIGS. 2 and 4 ; 
         FIG. 8  is an example of a chart illustrating a prominence of acoustic energy at harmonics of the blade-passing frequency that may be generated by the fans of  FIGS. 2 and 4 ; 
         FIG. 9  is an example of a chart illustrating a prominence of acoustic energy at harmonics of the blade-passing frequency that may be generated by a shroudless fan and the fan of  FIG. 4 ; 
         FIG. 10  is a partial cross-sectional view of an example of a shroudless fan, in accordance with an embodiment of the present disclosure; 
         FIG. 11  is a perspective view of an example of the shroudless fan of  FIG. 10 , in accordance with an embodiment of the present disclosure; 
         FIG. 12  is an example of a graph illustrating a magnitude of acoustic energy that may be generated by the fan of  FIG. 10 , in accordance with an embodiment of the present disclosure; 
         FIG. 13  is an example of a graph illustrating a magnitude of acoustic energy that may be generated by the fan of  FIG. 4 , in accordance with an embodiment of the present disclosure; 
         FIG. 14  is an example of a chart illustrating a magnitude of the prominence of blade pass frequency acoustic energy that may be generated for various fans, in accordance with an embodiment of the present disclosure; 
         FIG. 15  is an example of a chart illustrating a magnitude of the prominence of blade pass frequency acoustic energy that may be generated for various fans, in accordance with an embodiment of the present disclosure; 
         FIG. 16  is an example of a graph illustrating a correlation between a produced air flow rate and a magnitude of generated acoustic energy for various fans, in accordance with an embodiment of the present disclosure; 
         FIG. 17  is an example of a chart illustrating a magnitude of the prominence of blade pass frequency acoustic energy that may be generated for various fans, in accordance with an embodiment of the present disclosure; 
         FIG. 18  is an example of a chart illustrating a magnitude of the prominence of blade pass frequency acoustic energy that may be generated for various fans, in accordance with an embodiment of the present disclosure; 
         FIG. 19  is an example of a chart illustrating a magnitude of the prominence of blade pass frequency acoustic energy that may be generated for various fans with different blade angles, in accordance with an embodiment of the present disclosure; 
         FIG. 20  is an example of a chart illustrating a magnitude of the prominence of blade pass frequency acoustic energy that may be generated for various fans with different blade angles, in accordance with an embodiment of the present disclosure; 
         FIG. 21  is an example of a graph illustrating a correlation between a produced air flow rate and a magnitude of generated acoustic energy that may be generated for various fans with different blade sweep angles, in accordance with an embodiment of the present disclosure; 
         FIG. 22  is an example of a graph illustrating a magnitude of acoustic energy that may be generated by the fan of  FIG. 4  in free air, in accordance with an embodiment of the present disclosure; 
         FIG. 23  is an example of a graph illustrating a magnitude of acoustic energy that may be generated by the fan of  FIG. 4  when placed within an enclosure, in accordance with an embodiment of the present disclosure; 
         FIG. 24  is an example of a graph illustrating a correlation between a produced air flow rate and a magnitude of generated static pressure for various fans, in accordance with an embodiment of the present disclosure; 
         FIG. 25  is an example of a chart illustrating a spectrogram of acoustic energy vs. fan speed and sound frequency generated by the fans of  FIGS. 2 and 4 , in accordance with an embodiment of the present disclosure; 
         FIG. 26  is an example of a graph illustrating results of a spectrograph analysis of air flow velocity of the fan of  FIG. 4 , in accordance with an embodiment of the present disclosure; 
         FIG. 27  is partial cross-sectional view of an example of the fan of  FIG. 4  having an extended rotating inlet flange, in accordance with an embodiment of the present disclosure; 
         FIG. 28  is close-up cross-sectional view of an example of the fan of  FIG. 27  having an extended rotating inlet flange, in accordance with an embodiment of the present disclosure; 
         FIG. 29  is a cross-sectional view of an example of the fan of  FIG. 4  having a variable axial gap, in accordance with an embodiment of the present disclosure; 
         FIG. 30  is a perspective view of an example of a flow generation unit having fans of  FIG. 29 , in accordance with an embodiment of the present disclosure; 
         FIG. 31  is planar view of an example of the flow generation unit of  FIG. 30 , in accordance with an embodiment of the present disclosure; 
         FIG. 32  is a partial cross-sectional view of an example of a pair of adjacent fans at a rotating inlet flange interface, in accordance with an embodiment of the present disclosure; 
         FIG. 33  is a partial cross-sectional view of an example of the fan of  FIG. 4  having a helical backflow mitigation feature, in accordance with an embodiment of the present disclosure; 
         FIG. 34  is a perspective view of an example of the fan of  FIG. 4  having discrete backflow mitigation features, in accordance with an embodiment of the present disclosure; 
         FIG. 35  is a perspective view of an example of an unassembled two-piece rotor assembly that may be used to manufacture the fan of  FIG. 4 , in accordance with an embodiment of the present disclosure; 
         FIG. 36  is a close-up perspective view of an embodiment of fan blades of the rotor assembly of  FIG. 35 , in accordance with an embodiment of the present disclosure; 
         FIG. 37  is a close-up perspective view of an embodiment of the shroud of the rotor assembly of  FIG. 35 , in accordance with an embodiment of the present disclosure; 
         FIG. 38  is a close-up perspective view of an embodiment of the assembled rotor assembly of  FIG. 35 , in accordance with an embodiment of the present disclosure; 
         FIG. 39  is a close-up perspective view of an embodiment of the assembled rotor assembly of  FIG. 35 , in accordance with an embodiment of the present disclosure; 
         FIG. 40  is a planar top view of an embodiment of a rotor illustrating fan blade sweep angles, in accordance with an embodiment of the present disclosure; 
         FIG. 41  is a planar top view of an embodiment of a shrouded rotor having variable blade spacing, in accordance with an embodiment of the present disclosure; 
         FIG. 42  is a partial cross-sectional view of an example of the fan of  FIG. 4  having a stationary flow impedance feature, in accordance with an embodiment of the present disclosure; 
         FIG. 43  is a partial cross-sectional view of an example of the fan of  FIG. 4  having a rotating flow impedance feature, in accordance with an embodiment of the present disclosure; and 
         FIG. 44  is a partial cross-sectional view of an example of the fan of  FIG. 4  having a stationary flow impedance feature and a rotating flow impedance feature, in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. 
     As briefly discussed above, one or more flow generating devices (e.g., fans) are typically used to direct an air flow or other working fluid across certain components of an electronic device that may generate and release thermal energy. For example, a fan may be coupled to an enclosure of an electronic device and configured to circulate a continuous flow of cooling air through the enclosure, thereby preventing an accumulation of heated air within the enclosure. The fan typically includes a rotor disposed within a housing of the fan. The housing defines a channel (e.g., a flow path) along which air may flow through the housing. The rotor is configured to rotate about a central axis of the channel. Specifically, the rotor may include an electric motor or other suitable actuator that is configured to impart a torque on the rotor, thereby inducing rotation of the rotor relative to the housing of the fan. The rotor includes a hub having a plurality of angled fan blades extending radially therefrom. A circular shroud or ring may be disposed about and coupled to the fan blades, thereby forming an outer perimeter of the rotor. The fan blades engage with air surrounding the fan when the hub rotates, thereby forcing the air through the channel from an inlet to an outlet of the fan. In fans having a shrouded rotor, a radial gap extends between the rotating shroud and the housing to enable unobstructed rotation motion of the fan relative to the housing. 
     Operation of the fan generates a pressure differential between the inlet (e.g., low/ambient pressure) and the outlet (e.g., higher pressure) of the fan. This pressure differential may generate a backflow of air that flows through the radial gap between the rotating shroud and the housing from the outlet of the fan toward the inlet. In certain cases, this backflow air may be re-drawn into the inlet of the fan and disrupt air flow (e.g., cause fluidic disturbances to the air flow) around the fan blades. As noted above, this recirculation of backflow air through the fan may significantly increase audible noise that may be generated during operation of the fan. 
     Accordingly, the fan may be equipped with a rotating inlet flange that forms an upstream end portion (e.g., an inlet portion) of the rotor and directs backflow air in a direction away from the inlet. That is, the rotating inlet flange may include a contoured profile that redirects backflow air in a direction extending radially outward from the fan inlet as the backflow air is discharged from the housing. As such, the rotating inlet flange may enable the backflow air to discharge about a circumference of the housing in a direction away from the inlet, thereby reducing or substantially eliminating a likelihood of backflow air being drawn into the inlet of the fan. In some embodiments, a width of the radial gap between a terminal interface of the housing and the rotating inlet flange, referred to hereinafter as an axial gap or a vertical gap, may vary about a circumference of the housing. Width variations of the axial gap may be used to adjust a flow rate of backflow air discharging near certain portions of the housing. That is, a flowrate of backflow air may be biased toward particular side(s) (e.g., end portions) of the fan (e.g., or a fan array). As described in detail below, this flow biasing technique may reduce an amount of backflow air that may be transferred between fans disposed in close proximity to one another. 
     In certain embodiments, the fan may include a backflow mitigation feature, or multiple backflow mitigation features, that are included in the fan in addition to, or in lieu of, the rotating inlet flange. As discussed below, the backflow mitigation feature may reduce or substantially eliminate a backflow of air through the radial gap. For example, in some embodiments, the backflow mitigation feature may include a helical protrusion that extends from an outer surface the shroud and projects into the radial gap. Similar to the fan blades, the helical protrusion may engage the air within the radial gap and force the air in a flow direction toward the outlet of the fan. In some embodiments, the backflow mitigation feature may thereby generate a pressure within the radial gap that may partially or fully counter-act the pressure differential generated between the inlet and the outlet during operation of the fan. By reducing or mitigating the pressure difference from the fan outlet to the radial gap, air backflow into the radial gap may be mitigated or substantially prevented. In this manner, the backflow mitigation feature may mitigate a likelihood of air recirculation between the radial gap and the fan, thereby reducing audible noise that may be emitted by the disturbed air flow through the fan. 
     In further embodiments, one or more of the fan blades may be configured to protrude through the shroud of the fan to form a portion of, or all of, the backflow mitigation feature. That is, the fan blades may extend radially past the shroud and protrude into the radial gap, thereby engaging with the air within the radial gap and blocking (e.g., counteracting) a flow of backflow air in a similar manner as discussed above. These and other features will be described in detail below with reference to the drawings. 
     With the foregoing in mind,  FIG. 1  is a schematic diagram of an embodiment of an electronic device  10  that may include the features of the present disclosure. The electronic device  10  may take the form of a computer (e.g., a server), a portable electronic device, or any other suitable type of electronic device. Such computers may include computers that are generally portable (e.g., laptops, notebooks, and tablet computers) as well as computers that are generally used in one place (e.g., conventional desktop computers, workstations, and/or servers). By way of example, the depicted electronic device  10  may include a housing or an enclosure  12  having certain electrical components of the electronic device  10  disposed therein. The electronic device  10  may include one or more fans  40 , which are coupled to the enclosure  12  and operable to direct a flow of working fluid (e.g., air) across certain components within the enclosure  12 . For example, the fans  40  may be configured to direct a flow of ambient atmospheric air across a central processing unit (CPU) of the electronic device  10 , such that the air may absorb thermal energy (e.g., via convective heat transfer) from the CPU. The fans  40  may discharge the heated air through one or more outlets  41  of the enclosure  12 . In this manner, the fans  40  may ensure that an operational temperature of the CPU, or a temperature of any other component within the enclosure  12 , remains below a target value or within a desired range. 
     With the preceding in mind,  FIG. 2  is a perspective view of an embodiment of one of the fans  40 A. To facilitate discussion, the fan  40 A and its components will be described with reference to a radial axis  42  and a vertical axis  44 . The fan  40 A includes a housing  48 , which includes an outer wall  50  that forms a channel  52  extending through the housing  48 . The channel  52  extends along the vertical axis  44  and defines a flow path for a fluid, such as air, which may flow through the housing  48  via the channel  52 . A rotor assembly  56  is disposed within the channel  52  and configured to force air along the flow path from an inlet  58  of the housing  48  (e.g., a first end portion of the housing  48 , an inlet of the channel  52 ) to an outlet  60  of the housing  48  (e.g., a second end portion of the housing  48 , an outlet of the channel  52 ). 
     For example, the rotor assembly  56  may include a hub  64  that is configured to rotate about the vertical axis  44  or a centerline  66  (e.g., a central axis) of the channel  52 . That is, the hub  64  may be coupled to a motor  68 , as shown in  FIG. 3 , which is configured to rotate the hub  64  relative to the housing  48 . The motor  68  is coupled to a portion  70  of the housing  48 , such that rotational motion of the motor  68  relative to the housing  48  is blocked. Accordingly, the motor  68  may apply a torque to the hub  64  and thus impart rotational motion to the rotor assembly  56 . The motor  68  may include any suitable electric motor or actuator that can be powered directly from an alternating current (AC) or direct current (DC) power source. As an example, the motor  68  may include a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or another suitable motor. In some embodiments, the motor  68  is electrically coupled to a variable speed drive (VSD)  72 , as shown in  FIG. 3 , which may be configured to supply electrical energy to the motor  68  at a particular voltage, current, and/or frequency. Accordingly, the VSD  72  may be used to dynamically adjust an operational speed (e.g., between 500 and 3500 RPM) of the motor  68 , and thus, increase or decrease a rotational speed of the hub  64 . 
     As shown in the illustrated embodiment of  FIG. 2 , the rotor assembly  56  includes a plurality of fan blades  80  that extend radially from the hub  64  and couple to a shroud  82  disposed about a circumference of the fan blades  80 . Accordingly, the fan blades  80  define a plurality of fluid passages  84  that extend between an interior surface  86  of the shroud  82  and an exterior surface  88  of the hub  64 . By way of example, in certain embodiments, the rotor assembly  56  may include an outer diameter between about 50 millimeters (mm) and about 200 mm. However, in other embodiments, the rotor assembly  56  may include an outer diameter that is less than 50 mm or greater than 200 mm. Although the hub  64  includes 7 fan blades  80  in the illustrated embodiment of FIG.  2 , it should be noted that in other embodiments, the rotor assembly  56  may include any suitable quantity of fan blades  80  extending from the hub  64 . That is, the rotor assembly  56  may include 2, 3, 4, 5, 6, 7, 8, or more fan blades  80 . 
     Each of the fan blades  80  includes a pressure surface  90 , as shown in  FIG. 3 , which is oriented toward an intended direction of air flow through the channel  52 , and a suction surface  92  disposed opposite the pressure surface  90 . The pressure surface  90  engages with air surrounding the rotor assembly  56  when the rotor assembly  56  rotates about the centerline  66 , such that the pressure surface  90  may direct the air through the channel  52  of the fan  40 A. For example, the motor  68  may be configured to rotate the rotor assembly  56  counter-clockwise direction  94  about the centerline  66 , thereby enabling the fan blades  80  to generate an air flow through the channel  52  in a first direction  96  from the inlet  58  to the outlet  60  of the housing  48 . 
     In some embodiments, the housing  48  may include a mounting flange  100  that extends from the outer wall  50  and enables the fan  40 A to couple to a suitable portion of the enclosure  12 . For example, the mounting flange  100  may include one or more apertures  102  defined therein, which enable fasteners to extend through the mounting flange  100  and facilitate coupling the fan  40 A the enclosure  12 . Accordingly, the fan  40 A may be used to circulate an air flow through the enclosure  12  (e.g., via an inlet and outlet of the enclosure  12 ) to remove thermal energy from certain components of the electronic device  10  that may generate heat, as noted above. Although the mounting flange  100  extends from the outer wall  50  near the outlet  60  of the housing  48  in the illustrated embodiment, it should be noted that the mounting flange  100  may be situated near any other portion of the housing  48  (e.g., near the inlet  58 ) in other embodiments of the fan  40 A. 
       FIG. 3  is a partial cross-sectional view of the fan  40 A taken along line  3 - 3  of  FIG. 2 . As shown in the illustrated embodiment, a radial gap  120  (e.g., a shroud gap) extends between the inner surface of the outer wall  50  and the outer surface of the shroud  82 . The radial gap  120  may preclude physical contact between the outer wall  50  and the shroud  82  to ensure that the housing  48  does not inhibit rotational motion of the rotor assembly  56 . The fan  40 A includes an inlet flange  122  that is coupled to a first end portion  124  of the outer wall  50 , proximate the inlet  58 . The inlet flange  122  may extend radially inward (e.g., toward the centerline  66 ) and span across the radial gap  120 . For example, in some embodiments, an inner diameter of the inlet flange  122  (e.g., a diameter at a tip  126  of the inlet flange  122 ) may be substantially equal to an inner diameter of the shroud  82  (e.g., a diameter extending across the interior surface  86  of the shroud  82 ). In this manner, the inlet flange  122  may facilitate guiding air into the fluid passages  84  extending between the fan blades  80 . 
     In some embodiments, the housing  48  includes an inner wall  130  that extends from a second end portion  132  of the outer wall  50  toward the rotor assembly  56 . The inner wall  130  may form an outlet ring  134  that is disposed proximate a downstream end portion  136  of the shroud  82 . Similar to the inlet flange  122  discussed above, an inner diameter of the outlet ring  134  may be substantially equal to the inner diameter of the shroud  82 . Accordingly, the inner wall  130  may guide air discharging from the fluid passages  84  toward the outlet  60  of the housing  48 . 
     It is important to note that vertical gaps extend between the inlet flange  122  and the shroud  82 , and the shroud  82  and the outlet ring  134 , respectively. That is, a first vertical gap  140  extends between the inlet flange  122  and an upstream end portion  142  of the shroud  82 , and a second vertical gap  146  extends between the downstream end portion  136  of the shroud  82  and the inner wall  130 . As with the radial gap  120 , the first and second vertical gaps  140 ,  146  may ensure that physical contact between the shroud  82 , the inlet flange  122 , and the inner wall  130  is precluded, thereby enabling the rotor assembly  56  to rotate freely within the housing  48 . As a non-limiting example, in some embodiments, a width of the first vertical gap  140 , a width of the second vertical gap  146 , or both, may be between about 0.5 mm and about 2 mm. 
     As noted above, operation of the fan  40 A may generate a region of high pressure air proximate the outlet  60  of the housing  48  and a region of low pressure air proximate the inlet  58  of the housing  48 . In other words, an air pressure near the outlet  60  may be greater than an air pressure near the inlet  58 . This pressure differential may generate a secondary air flow, or a backflow of air (e.g., as indicated by arrow  150 ), which enters the radial gap  120  via the second vertical gap  146  and flows through the radial gap  120  toward the inlet  58 . That is, the backflow of air may flow in a second direction  152 , which is generally opposite to the first direction  96  of air flow along the fluid passages  84  of the fan blades  80 . The backflow of air may discharge from the radial gap  120  via the first vertical gap  140  and re-enter the fluid passages  84 . As such, a portion of the air flowing through the channel  52  may be recirculated about a perimeter of the shroud  82 . 
     Unfortunately, this stream of air circulating through the radial gap  120  may disturb a flow of mainstream air entering the inlet  58  (e.g., increase turbulence of the air flow entering the inlet  58 ), thereby generating and/or increasing audible aero-acoustic noise (e.g., acoustic energy) that may be unpleasant to users operating the electronic device  10 . As discussed in detail below, this audible noise may be particularly prominent within certain harmonic frequency ranges of the fan  40 A. Accordingly, embodiments of the present disclosure are directed toward a rotating inlet flange that is configured to reduce or substantially eliminate a recirculation of backflow air through the radial gap  120  and the fluid passages  84  of the rotor assembly  56 . As such, the rotating inlet flange may lower a magnitude (e.g., a decibel level) of audible tonal (e.g., harmonic) noise that may be generated during operation of the fan  40 A. 
     With the preceding in mind,  FIG. 4  is a perspective view of an embodiment of the fan  40 B having a rotating inlet flange  160  (e.g., an angled inlet flange). As noted above, the rotating inlet flange  160  is configured to reduce or substantially eliminate a likelihood of air recirculation about the shroud  82  during operation of the fan  40 B. The rotating inlet flange  160  may be formed integrally with the rotor assembly  56  (e.g., via an injection molding process), or may couple to the rotor assembly  56  via suitable fasteners or adhesives (e.g., bonding glue). In the present example the rotating inlet flange  160  is formed integrally with shroud  82 , such that the rotating inlet flange  160  forms a portion (e.g., the upstream end portion  142 ) of the shroud  82 . 
     To facilitate the subsequent discussion,  FIG. 5  depicts a partial cross-sectional view of the fan  40 B taken along line  5 - 5  of  FIG. 4 . As shown in the illustrated embodiment of  FIG. 5 , the rotating inlet flange  160  extends from a generally cylindrical section of the shroud  82  and defines a gap, referred to herein as a vertical gap  170 , which extends between the first end portion  124  of the outer wall  50  and a lower circumferential edge of the rotating inlet flange  160 . 
     It is important to note that the rotating inlet flange  160  includes a profile  172  (e.g., a curved profile) that diverges radially from the generally cylindrical section of the shroud  82  to an outer edge (e.g., a distal end) of the rotating inlet flange  160 . Accordingly, air flowing through the radial gap  120  may be guided along the profile  172  of the rotating inlet flange  160  prior to discharging from the housing  48 . As such, the profile  172  may redirect air discharging from the radial gap  120  generally along the radial axis  42 , away from the centerline  66  of the channel  52 . That is, the backflow air discharges from the radial gap  120  in a direction diverging from the centerline  66 . In this manner, the rotating inlet flange  160  may mitigate a likelihood of the fan  40 B re-ingesting backflow air via the rotor assembly  56 , thereby reducing or substantially eliminating air recirculation between the radial gap  120  and the fluid passages  84 . As such, the rotating inlet flange  160  may significantly reduce audible noise that may be generated during operation of the rotor assembly  56 . 
     For example,  FIG. 6  is an embodiment of a graph  180  illustrating a magnitude of acoustic energy (e.g., in decibels) that may be generated at a particular operational speed (e.g., in revolutions per minutes) of the fans  40 A,  40 B for various harmonic frequencies of the fans  40 A,  40 B. A fundamental harmonic frequency of the fans  40 A,  40 B may be indicative of a calculable blade pass frequency (BPF) of the fans  40 A,  40 B. Equation I (EQ I) below illustrates an embodiment of the analytical relationship that may be used to determine the blade pass frequency, BPF, given the rotational speed of the fans  40 A,  40 B in revolutions per minute, N, and the quantity of fan blades  80  included in the rotor assembly  56 , k. 
     
       
         
           
             
               
                 
                   BPF 
                   = 
                   
                     Nk 
                     60 
                   
                 
               
               
                 
                   ( 
                   
                     EQ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     Accordingly, sequential harmonic frequencies of the fans  40 A,  40 B may be determined by calculating multiples of the blade pass frequency (e.g., multiples of the fundamental harmonic frequency). With the foregoing in mind, the graph  180  illustrates a magnitude of acoustic energy that may be generated by the fans  40 A,  40 B at various harmonic frequencies. In particular, line  182  illustrates acoustic energy that may generated by the fan  40 A, while line  184  illustrates acoustic energy that may be generated by the fan  40 B. 
     As shown in the graph  180  of  FIG. 6 , the fan  40 A may generate significantly more acoustic energy at the first through fourth harmonic frequencies as compared to a magnitude of acoustic energy that may be generated by the fan  40 B at these frequencies. Specifically, the fan  40 A may generate spikes in acoustic energy at multiples of the blade pass frequency of the fan  40 A (e.g., at the first four harmonic frequencies). These spikes in acoustic energy may be amplified by the recirculation of backflow air through the fan  40 A, and may be prominently audible (e.g., of a higher magnitude of acoustic energy) over remaining acoustic energy generated by the fan  40 A (e.g., acoustic energy that may be generated via operation of the motor  68 ). In other words, although many aspects of the fans  40 A,  40 B make noise, such as the air flow generated by the fans  40 A,  40 B, the noise of the motors, etc., the noise spikes generated at harmonics of the BPF due to the recirculation of backflow air in the fan  40 A are quite noticeable to users as compared to the other fan noise. Conversely, as shown in the graph  180 , the fan  40 B may not generate distinguishable spikes in acoustic energy, except at the third harmonic frequency in this example, because the rotating inlet flange  160  mitigates recirculation of backflow air into the inlet  58  of the fan  40 B. Accordingly, the BPF noise due to recirculation of the backflow air in the fan  40 B is much less prominent comparatively. 
       FIG. 7  is an embodiment of a chart  186  illustrating a prominence of acoustic energy that may be generated by at the first four BPF harmonic frequencies of the fans  40 A, 40 B due to the recirculation of backflow air through the fans  40 A,  40 B. In other words,  FIG. 7  illustrates a prominence of tonal acoustic energy (e.g., audible tonal noise) that may be generated in part by the recirculation of backflow air, which may be separately discernable (e.g., of higher magnitude) from remaining acoustic energy that may be generated by the fans  40 A,  40 B (e.g., acoustic energy that may be generated by motor  68 , airflow through the fans, etc.). As shown in the illustrated embodiment of the chart  186 , the fan  40 A may generate significant acoustic energy at the first four harmonic frequencies of the fan  40 A due to the recirculation of backflow air through the fan  40 A. In contrast, the rotating inlet flange  160  of the fan  40 B may substantially mitigate noticeable audible noise that may be generated at the first, the second, and the fourth harmonic frequencies, as the rotating inlet flange  160  may mitigate recirculation of backflow air through the fan  40 B (e.g., the prominence of acoustic energy generated due to backflow air recirculation may be substantially negligible at the first, the second, and the third harmonic frequencies of the fan  40 B). Although the fan  40 B may generate discernable acoustic energy due to backflow air recirculation at the third harmonic frequency, a magnitude of this acoustic energy is less than a magnitude of the acoustic energy that may be generated by the fan  40 A (e.g., due to backflow air recirculation). 
     As another example,  FIG. 8  is an embodiment of a chart  187  that illustrates a prominence of acoustic energy (e.g., in decibels) that may be generated by air flow through the fan  40 A and the fan  40 B at various blade pass frequency orders (e.g., at various harmonic frequencies) when the fans  40 A,  40 B operate at a different operational speed than the operational speed of the fans  40 A,  40 B in  FIG. 7 . 
       FIG. 9  is an embodiment of a chart  188  illustrating a prominence of acoustic energy that may be generated by air flow through a shroudless fan  40 C, as shown in the illustrated embodiments of  FIGS. 10 and 11 , and air flow through the fan  40 B, for the first five blade pass frequency harmonics of the fans  40 B,  40 C at a particular operational speed of the fans  40 B,  40 C. As shown in the illustrated embodiment of  FIG. 10 , a tip gap  189  extends between tips of the fan blades  80  (e.g., radially outermost points of the fan blades  80 ) and the inner wall  130  of the housing  48 , such that the rotor assembly  56  may rotate freely within the housing  48 . In shroudless fans (e.g., the shroudless fan  40 C), tip leakage vortices often form around the tips of the fan blades  80 , thereby generating undesirable aero-acoustic noise (e.g., acoustic energy) that may propagate from the shroudless fans  40 C. For example, relatively high pressure air near the pressure surface  90  of the fan blades  80  may flow (e.g., leak) through the tip gap  189  to a region of relatively low pressure air near the suction surface  92  of the fan blades  80 , thereby forming the tip leakage vortices around the blade tips of the fan blades  80 . As discussed in detail below, the shroud  82  may block air flow around the tips of the fan blades  80  from the pressure surface  90  to the suction surface  92  and, thus, substantially eliminate acoustic energy that may be generated due to the formation of tip leakage vortices within the housing  48 . 
       FIG. 12  is an embodiment of a graph  190  illustrating a magnitude of acoustic energy that may be generated by the shroudless fan  40 C at various frequencies for a particular operational speed (e.g., in revolutions per minute) of the shroudless fan  40 C. As shown in the graph  190 , the shroudless fan  40 C may generate spikes  191  of acoustic energy that occur at particular frequencies (e.g., multiples of the blade pass frequency of the shroudless fan  40 C). 
       FIG. 13  is an embodiment of a graph  192  illustrating a magnitude of acoustic energy that may be generated by the fan  40 B at various frequencies for a particular operational speed (e.g., in revolutions per minute) of the fan  40 B that is equal to the operational speed of fan  40 C of  FIG. 12 . As shown in the illustrated embodiment, the inclusion of the rotating inlet flange  160  may significantly reduce a magnitude of acoustic energy spikes  193  that may be generated by the fan  40 B at certain blade pass frequency multiples of the shroudless fan  40 C, as compared to the spikes  191  of acoustic energy generated by the shroudless fan  40 C at these frequencies. 
       FIGS. 14-21  are various embodiments of charts  194  and/or graphs  195  illustrating relationships between acoustic energy that may generated by various fans at certain blade pass frequency harmonics or output air flow rates of the fans. In particular, the charts  194  and/or the graphs  195  may compare these parameters with respect to the fan  40 A of  FIG. 2 , the fan  40 B of  FIG. 4 , and the shroudless fan  40 C of  FIG. 10 . It should be noted that  FIGS. 17-21  additionally illustrate relationships between the aforementioned parameters and fans (e.g., the fans  40 A,  40 B, and/or  40 C) having various blade sweeps or fan blade angles, which will be discussed in detail below. 
       FIG. 22  is an embodiment of a graph  196  illustrating a magnitude of acoustic energy that may generated by the fan  40 B when the fan  40 B is not coupled to an enclosure or housing (e.g., not coupled to the enclosure  12 ). Specifically, the graph  196  illustrates a magnitude of acoustic energy that may be generated at various frequencies for a particular operational speed of the fan  40 B while the fan  40 B is situated in an ambient environment (e.g., not coupled to another structure or enclosure). 
       FIG. 23  is an embodiment of a graph  197  illustrating acoustic energy that may be generated by the fan  40 B when the fan  40 B is coupled to an enclosure (e.g., the enclosure  12 ). In particular, the graph  196  illustrates a magnitude of acoustic energy that may be generated at various frequencies for a particular operational speed of the fan  40 B that may be the same as the operational speed of the fan  40 B in  FIG. 22 . 
       FIG. 24  is an embodiment of a graph  198  illustrating a correlation between air flow rates (e.g., in cubic feet per minute) that may be generated by various fans and resulting pressure rises that may be formed across a housing (e.g., between an inlet and an outlet) of these fans. In particular, the graph  198  illustrates relationships between the aforementioned parameters with respect to the fan  40 B and the shroudless fan  40 C. 
       FIG. 25  is an embodiment of a chart  199  illustrating a magnitude of acoustic energy that may be generated by the fan  40 A of  FIG. 2  and the fan  40 B of  FIG. 4  at various blade pass frequencies. As shown in the illustrated embodiment, the fan  40 A may generate a moderate to high amount of acoustic energy at certain operation speeds of the fan  40 A (e.g. at speeds between 1250 RPM and 2250 RPM). In some embodiments, a predominant portion of this acoustic energy may be generated at the harmonic frequencies of the fan  40 A, due to the recirculation of backflow air through the fan  40 A. In contrast, the fan  40 B may generate acoustic energy of a lesser magnitude (e.g., low to moderate magnitude) throughout the aforementioned operations speeds (e.g., speeds between 1250 RPM and 2250 RPM), because the rotating inlet flange  160  may mitigate an amount of acoustic energy that may be generated at the harmonic frequencies of the fan  40 B (e.g., due to the recirculation of backflow air through the fan  40 B). Practically speaking, in many implementations, the tonal noise generated by the fan  40 A in the medium high to high fan speed ranges may simply be too loud, while conversely, the noise generated by the fan  40 B in the same fan speed ranges may be acceptable. 
       FIG. 26  is an embodiment of a chart  200  illustrating spectrogram analysis results of the outlet air flow for the fan  40 B at a particular operational speed of the fan  40 B. 
     Returning to  FIG. 5 , although the profile  172  of the rotating inlet flange  160  is shown as having a curved contour in the illustrated embodiment, it should be noted that the rotating inlet flange  160  may alternatively include a linear profile, a stepped or jagged profile, or any other suitable profile or edge. In some embodiments, the rotating inlet flange  160  may protrude vertically past and at least partially across the outer wall  50 . For example, a diameter of the rotating inlet flange  160  may be substantially equal to a diameter of the outer wall  50 , such that the vertical gap  170  extends axially between the outer wall  50  and the rotating inlet flange  160 . That is, a diametric dimension at a distal end  202  of the rotating inlet flange  160  may be substantially equal to a diametric dimension of the outer wall  50  at the first end portion  124 . In some embodiments, the rotating inlet flange  160  may extend radially past the outer wall  50 . In other embodiments, the rotating inlet flange  160  may terminate at an elevation that is below a height of the first end portion  124  of the outer wall  50 , such that the rotating inlet flange  160  does not protrude past the outer wall  50 . However, even in such embodiments, the profile  172  of the rotating inlet flange  160  may guide backflow air discharging from the radial gap  120  over the outer wall  50  and in a direction extending away from the centerline  66 . 
     In certain embodiments, the rotating inlet flange  160  may be configured to discharge the backflow air in a direction substantially similar to an intended direction of air flow through the fan  40 B (e.g., along the first direction  96 ). By way of example,  FIG. 27  is a partial cross-sectional view of an embodiment of the fan  40 B in which the rotating inlet flange  160  is configured to discharge backflow air generally in the first direction  96 . As shown in the illustrated embodiment, the rotating inlet flange  160  extends about the first end portion  124  of the outer wall  50  such that the distal end  202  of the rotating inlet flange  160  may be oriented generally along the vertical axis  44 . As such, the rotating inlet flange  160  may form an additional gap  203  (e.g., an additional radial gap, a portion of the vertical gap  170 ) that extends between an exterior surface of the outer wall  50  and the rotating inlet flange  160 . That is, the distal end  202  may be positioned axially between (e.g., with respect to the centerline  66 ) the first end portion  124  of the outer wall  50  and the second end portion  132  of the outer wall  50  to form the additional gap  203 . Accordingly, the rotating inlet flange  160  may receive backflow air (e.g., as shown by the arrow  150 ) in the second direction  152 , guide the backflow air along the profile  172  to redirect a flow direction of the backflow air generally along the first direction  96 , and discharge the backflow air toward the outlet  60  of the fan  40 B. It should be appreciated that the profile  172  of the rotating inlet flange  160  may be adjusted to discharge the backflow air any other direction extending away from the centerline  66 . 
       FIG. 28  is a close-up cross-sectional view of an embodiment of the fan  40 B illustrating another embodiment of the rotating inlet flange  160  that directs backflow air downwardly in the first direction  96 . As shown in the illustrated embodiment, the rotating inlet flange  160  may extend substantially close to an exterior surface of the outer wall  50  (e.g., within 0.5 mm to 2 mm of the exterior surface of the outer wall  50 ). Accordingly, the rotating inlet flange  160  may not significantly increase a total diametric dimension of the fan  40 B, even though the rotating inlet flange  160  extends about the exterior of the outer wall  50  (e.g., protrudes radially past the outer wall  50 ). As such, multiple fans  40 B may be placed in close proximity to one another without interference between the rotating inlet flanges  160  of the fans  40 B. In some embodiments, a width (e.g., a radial dimension) of the additional gap  203  may be substantially constant along a length of the additional gap  203 . That is, a portion of the rotating inlet flange  160  extending from an intermediate portion  205  of the rotating inlet flange  160  to the distal end  202  of the rotating inlet flange  160  may extend substantially parallel to an exterior surface of the outer wall  50  (e.g., at a distance within 0.5 mm to 2 mm of the exterior surface of the outer wall  50 ). 
     In certain embodiments, the fan  40 B may be configured to discharge the backflow air non-uniformly (e.g., non-axisymmetric) about a circumference of the outer wall  50 . In other words, the fan  40 B may be configured to discharge the backflow air along a first portion of the outer wall  50  at a flow rate that is less than or greater than a flow rate of backflow air discharged along a second portion of the outer wall  50 . As discussed in detail below, this configuration may enable multiple fans  40 B to be positioned in close proximity to one another while mitigating a transfer of backflow air between adjacent fans  40 B. Accordingly, this flow biasing technique may reduce audible noise that may be generated due to the recirculation of backflow air between neighboring fans  40 B. 
     To facilitate discussion,  FIG. 29  is a cross-sectional view of an embodiment of the fan  40 B. In the illustrated embodiment, the outer wall  50  has a first height  204  (e.g., a maximum height) at a first point  206  along the outer wall  50 , and has a second height  208  (e.g., a minimum height) at a second point  210  along the outer wall  50  (e.g., a point diametrically opposite the first point  206 ). A height of the outer wall  50  may decrease uniformly or non-uniformly on either side of the fan  40 B from the first point  206  to the second point  210 . As an example, line  211  illustrates a height variation of the outer wall  50  between the first point  206  and the second point  210 . 
     It is important to note that such height variations along the outer wall  50  (e.g., variations in a circumferential height profile of the outer wall  50 ) may vary a width of the vertical gap  170  at various locations about the outer wall  50 . That is, the width of the vertical gap  170  may increase or decrease about a circumference of the outer wall  50  proportionally to a decrease or increase, respectively, in a local height of the outer wall  50 . As such, in the present example, a first width of the vertical gap  170  may be relatively small at the first point  206  (e.g., a constricted section) of the outer wall  50 , while a second width of the vertical gap  170  is relatively large at the second point  210  (e.g., an expanded section) of the outer wall  50 . 
     Adjusting a local width of the vertical gap  170  may facilitate regulating flow parameters (e.g., flow rate, dynamic pressure) of the backflow air discharging from the radial gap  120 . For example, restricting a width of the vertical gap  170  along a particular section of the outer wall  50  may decrease the flow rate of backflow air discharging near this section of the outer wall  50 . Conversely, enlarging the width of the vertical gap  170  along a section of the outer wall  50  may increase the flow rate of the backflow air discharging near this section of the outer wall  50 . Accordingly, height variations along the outer wall  50  may be used to bias a discharge of backflow air to certain portion(s) of the fan  40 . Specifically, in the present example, a flow rate of the backflow air near the first point  206  may be relatively small, as indicated by arrow  216 , while a flow rate of the backflow air is relatively large near the second point  210 , as indicated by arrow  218 . 
     As another clarifying example,  FIG. 30  is a perspective view of an embodiment of a flow generation unit  220  that includes a plurality of fans  40 B. Specifically, the illustrated embodiment of the flow generation unit  220  includes a first fan  40 B 1 , a second fan  40 B 2 , and a third fan  40 B 3 , which respectively include a first outer wall  50 B 1 , a second outer wall  50 B 2 , and a third outer wall  50 B 3  that are integrated within a common housing  240 . To facilitate the subsequent discussion, it should be noted that the first, the second, and the third outer walls  50 B 1 ,  50 B 2 ,  50 B 3  are bisected by a centerline  242  extending through diametric endpoints of the outer walls  50 B 1 ,  50 B 2 ,  50 B 3 . 
     In the exemplary embodiment of the flow generation unit  220  discussed herein, the first outer wall  50 B 1 , the second outer wall  50 B 2 , and the third outer wall  50 B 3  each include respective maximum heights at crest points  244 , which are positioned along the centerline  242 , and crest points  246 , which are position along respective axes  248  extending generally orthogonal to the centerline  242 . Respective minimum heights of the first outer wall  50 B 1 , the second outer wall  50 B 2 , and the third outer wall  50 B 3  are located at respective trough points  250 , which may be positioned between (e.g., at a midpoint of) respective crest points  244 ,  246 . 
     The respective heights (e.g., respective height profiles) of the first, the second, and the third outer walls  50 B 1 ,  50 B 2 ,  50 B 3  may vary uniformly or non-uniformly between the crest points  244 ,  246  and the respective trough points  250 . In this manner, the fans  40 B 1 ,  40 B 2 ,  40 B 3  may each include constricted sections  252  along which respective vertical gaps  170  of the fans  40 B 1 ,  40 B 2 ,  40 B 3  are relatively small at the crest points  244 ,  246  and expanded sections  254  along which the respective vertical gaps  170  are relatively large at the trough points  250 . 
     As shown in the illustrated embodiment, the constricted sections  252  may be disposed between each of the fans  40 , while the expanded sections  254  are located near portions the outer walls  50 B 1 ,  50 B 2 ,  50 B 3  that are oriented away from one another. In this manner, each of the fans  40 B 1 ,  40 B 2 ,  40 B 3  may discharge a majority of their respective backflow air in a radial direction that is oriented away from neighboring fans  40 B 1 ,  40 B 2 ,  40 B 3  of the flow generation unit  220 . This flow biasing configuration may therefore decrease a quantity of backflow air that may be discharged from one fan (e.g., the first fan  40 B 1 ) and ingested by and recirculated through an adjacent fan (e.g., the second fan  40 B 2 ). 
     For clarity,  FIG. 31  is a planar side view of an embodiment of the flow generation unit  220 , illustrating the constricted sections  252  and the expanded sections  254  of the fans  40 B 1 ,  40 B 2 ,  40 B 3 . It should be appreciated that in other embodiments, the crest points  244 ,  246  and/or the trough points  250  may be positioned along any other portion(s) of the first, the second, and the third outer walls  50 B 1 ,  50 B 2 ,  50 B 3 . 
     In some embodiments, backflows of air of adjacent fans  40 B may be discharged at different elevations, thereby reducing a likelihood of backflow air interaction between the fans  40 B. For example,  FIG. 32  is a partial cross-sectional view of an embodiment of adjacent outer walls of a pair of neighboring fans. For sake of discussion, the pair of fans are described as the first and second fans  40 B 1 ,  40 B 2 , and will reference their respective components. As shown in the illustrated embodiment, a height  260  (e.g., an axial height, a dimension along the vertical axis  44 ) of the first outer wall  50 B 1  exceeds an axial height  262  of the second outer wall  50 B 2 . A rotating inlet flange  264  of the second fan  40 B 2  includes a protrusion  266  (e.g., an axial protrusion) that extends from the intermediate portion  205  of the rotating inlet flange  264  toward the second outer wall  50 B 2 . A diametric dimension of the intermediate portion  205  may be substantially equal to a diametric dimension of the second outer wall  50 B 2  (e.g., a diametric dimension at a respective first end portion  267  of the second outer wall  50 B 2 ), and the protrusion  266  may extend from the intermediate portion  205  in a direction that may be substantially parallel to the second outer wall  50 B 2 . In some embodiments, the height  260  of the first outer wall  50 B 1  may be substantially equal to an elevation of a lower end point  268  (e.g., the distal end  202 ) of the protrusion  266 . In other embodiments, the protrusion  266  may extend axially past the first outer wall  50 B 1 , such that the protrusion  266  and the first outer wall  50 B 1  overlap with one another with respect to the vertical axis  44 . 
     In any case, the differences in height between the first outer wall  50 B 1  and the second outer wall  50 B 2  may enable a first backflow of air  270  discharging from the first fan  40 B 1  to impinge upon a circumferential end face  272  of the protrusion  266 , while a second backflow of air  274  discharging from the second fan  40 B 2  may impinge upon an exterior surface  276  of the first outer wall  50 B 1 . In this manner, the first and second backflows of air  270 ,  274  may be dispersed into an ambient environment, while a negligible amount of backflow air is directed toward and re-ingested by the first fan  40 B 1  and/or the second fan  40 B 2 . 
     In some embodiments, the fans  40 A,  40 B, and/or  40 C may include a backflow mitigation feature, or multiple backflow mitigation features, which are configured to reduce or substantially eliminate a flow of backflow air through the radial gap  120 . For example,  FIG. 33  is a partial cross-sectional view of an embodiment of a fan  40 D (e.g., any one of the fans  40 A,  40 B, and/or  40 C) having a backflow mitigation feature  300  disposed about an exterior surface  302  of the shroud  82 . As shown in the illustrated embodiment, the backflow mitigation feature  300  may include protrusions that extend radially from the shroud  82 , toward an interior surface of the outer wall  50 . In some embodiments, these protrusions spiral helically downward (e.g., along the first direction  96 ) in a clockwise direction  303  about the shroud  82 . That is, the backflow mitigation feature  300  descends from the inlet  58  toward the outlet  60  while revolving about the shroud  82  in the clockwise direction  303 . In some embodiments, the backflow mitigation features  300  may descend about the shroud  82  in the same profile as the fan blades  80  (e.g., a profile at an interface between the fan blades  80  and the shroud  82 ). 
     The backflow mitigation feature  300  may engage with air occupying the radial gap  120  when the rotor assembly  56  rotates about the centerline  66  (e.g., in the counter-clockwise direction  94 ), such that the backflow mitigation feature  300  may attempt to partially block the flow of air in the second direction  152  or force the air in the first direction  96 . In some embodiments, the backflow mitigation feature  300  may thereby generate a pressure within the radial gap  120  that is sufficient to fully or partially counteract the pressure differential generated between the vertical gap  170  and the second vertical gap  146  during operation of the fan  40 D and, thus, result in substantially reduced or eliminated air backflow in the radial gap  120 . Accordingly, the backflow mitigation feature  300  may generate a stagnation of air within the radial gap  120  that substantially blocks additional air from entering the radial gap  120  via the second vertical gap  146 , or discharging from the radial gap  120  via the vertical gap  170 . In this manner, the backflow mitigation feature  300  may reduce, or substantially eliminate a flow of backflow air through the radial gap  120 . 
     In some embodiments, the backflow mitigation feature  300  may include a single helical protrusion that extends continuously about a circumference of the shroud  82 . However, in other embodiments, the backflow mitigation feature  300  may include multiple separated features or protrusions that may be spaced equally (e.g., in an axisymmetric or uniform manner) about the circumference of the shroud  82  (e.g., as shown in the illustrated embodiment of  FIG. 34 ). Although the protrusions of the backflow mitigation feature  300  are shown as having a quadrilateral cross-sectional shape in the illustrated embodiment of  FIG. 33 , it should be noted that the backflow mitigation feature  300  may include any other suitable cross-sectional shape including, but not limited to, a semi-circular cross-sectional shape, a triangular cross-sectional shape, or a non-uniform cross-sectional shape. Moreover, it should be noted that a cross-sectional shape and/or a protrusion width (e.g., a dimension by which the backflow mitigation feature  300  extends radially from the shroud  82 ) of the backflow mitigation feature  300  may vary along a height (e.g., a dimension along the vertical axis  44 ) of the shroud  82 . 
     As an example, in some embodiments, the backflow mitigation feature  300  may include a first group of features that are positioned on the shroud  82  near the inlet  58  and have a first cross-sectional shape and a first protrusion width, while a second group of features are positioned on the shroud  82  near the outlet  60  and have a second cross-sectional shape (e.g., a different cross-sectional shape) and a second protrusion width (e.g., a different protrusion width). It should be appreciated that the geometry and/or the protrusion width of the backflow mitigation feature  300  may be tuned to minimize air backflow through the radial gap  120  at particular operational speeds of the fan  40 D. 
     In certain embodiments, the backflow mitigation feature  300  may include a portion of the fan blades  80 . For example, in some embodiments, the rotor assembly  56  may be manufactured (e.g., via an injection molding process) such that one or more of the fan blades  80  protrude radially through the shroud  82 , thereby forming the backflow mitigation feature  300 . Accordingly, during operation of the fan  40 D, a portion of the fan blades  80  protruding radially past the shroud  82 , referred to herein as a protruding portion, may engage the air within the radial gap  120  (e.g., via the pressure surface  90  of the fan blades  80 ) and thereby attempt to force the air in the first direction  96 . Similar to the discussion above, in this manner, the protruding portion of the fan blades  80  may generate a static pressure rise in the first direction  96  within the radial gap  120  that may be sufficient to counteract the pressure differential between the vertical gap  170  and the second vertical gap  146  of the fan  40 D, thus blocking a backflow of air through the radial gap  120 . 
     In some embodiments, the rotor assembly  56  may be manufactured as a single piece component via an injection molding process. For example, to form the rotor assembly  56 , a heated (e.g., liquid) polymeric material may be injected into a mold (e.g., a negative mold) having the shape of the rotor assembly  56 . Upon cooling of the polymeric material, the mold may be split (e.g., into two or more individual pieces), thereby enabling removal of the rotor assembly  56  from the mold. However, due to its shape, mold lines may form on certain portions of the rotor assembly  56  that were adjacent to seams of the mold during the injection molding process. Specifically, mold lines may be formed on the inner surface of the shroud  82  and outer surface of the hub  64 . Unfortunately, such mold lines may cause turbulent air flow during operation of the fan  40  (e.g., any of the fans  40 A,  40 B,  40 C,  40 D), which may generate acoustic energy (e.g., audible noise) during operation of the fans  40 A,  40 B,  40 C, and/or  40 D. 
     In some embodiments, to facilitate manufacture of the rotor assembly  56  and prevent the formation of mold lines on certain portions of the rotor assembly  56  (e.g., the fan blades  80 ), the hub  64  and the fan blades  80  may be formed as a single-piece component that is separate of the shroud  82  (e.g., in a two-piece design). For example, as shown in the illustrated embodiment of  FIG. 35 , the hub  64  and the fan blades  80  may be formed as a blade assembly  340 , which is separate of the shroud  82 . As discussed in detail below, in such embodiments, the shroud  82  may include a plurality of grooves  342  (e.g., helical grooves) that are configured to receive respective blade tips  344  (e.g., end faces) of the fan blades  80  and enable the blade assembly  340  to couple to the shroud  82 . 
     To facilitate discussion,  FIG. 36  is a close-up perspective view of an embodiment of the fan blades  80 . In some embodiments, each of the fan blades  80  may include one or more protrusions  346  (e.g., spherical nubs) that extend radially from the blade tips  344 . Each of the protrusions  346  may be configured engage with a recess  350  (e.g., as shown in  FIG. 37 ) disposed within a respective one of the grooves  342 . For example, to insert the blade assembly  340  into the shroud  82 , each of the fan blades  80  may first be aligned with corresponding grooves  342 . Subsequently, the blade assembly  340  may be rotated relative to the shroud  82  (e.g., in the counter-clockwise direction  94 ) such that the blade tips  344  may navigate along a length of the grooves  342  and draw the blade assembly  340  into the shroud  82 . 
     In some embodiments, radial dimensions extending between the protrusions  346  and a center of the blade assembly  340  may exceed respective radial dimensions extending between the recesses  350  and a center of the shroud  82  (e.g., by approximately 0.5 mm). Accordingly, the shroud  82 , the blade assembly  340 , or both, may temporarily deform while the blade assembly  340  is inserted into the shroud  82 . 
     For example, in some embodiments, the blade assembly  340  may be constructed of a relatively rigid material, such as glass-filled plastic, while the shroud  82  may be constructed of an elastically deformable material, such as a non-glass filled polymeric material. Accordingly, the shroud  82  may temporarily deform (e.g., flex, bend) while the blade assembly  340  is inserted into the shroud  82 . 
     The blade assembly  340  may be rotated relative to the shroud  82  until the protrusions  346  of the blade tips  344  engage with respective apertures  360  defined within the shroud  82 . Accordingly, upon proper alignment of the blade assembly  340  within the shroud, the shroud  82  may snap (e.g., lock) into place (e.g., return to its pre-deformed state, via a snap fit), and thus, couple the blade assembly  340  to the shroud  82 . 
     In some embodiments, an adhesive (e.g., an epoxy resin) may be disposed within the grooves  342  prior to the mating process of the blade assembly  340  and the shroud  82 . This adhesive may lubricate the interface between the blade tips  344  and the grooves  342  during this mating process and facilitate translating the blade tips  344  along the grooves  342 , thus facilitating insertion of the blade assembly  340  within the shroud  82 . Moreover, the adhesive will harden (e.g., cure) after installation of the blade assembly  340 , thereby bonding the blade assembly  340  to the shroud  82  and enhancing a structural rigidity of the rotor assembly  54 . 
     In some embodiments, a diametric dimension between opposing fan blades  80  may be marginally greater than (e.g., by 0.2-0.5 mm) a diametric dimension between opposing grooves  342  of the shroud  82 . In this manner, a compressive force may remain between the shroud  82  and the blade tips  344  after installation of the blade assembly  340 , which may facilitate forming an air-tight seal (e.g., a fluidic seal) at an interface  361  (e.g., as shown in  FIG. 38 ) between the fan blades  80  and the shroud  82 . In some embodiments, the shroud  82  may be heated prior to assembly of the rotor assembly  56 , thereby temporarily expanding the shroud  82  (e.g., increasing an inner diameter of the shroud  82 ). Accordingly, an amount of interference between the blade assembly  340  and the shroud  82  may be reduced to facilitate insertion of the blade assembly  340  into the shroud  82 . Upon installation of the blade assembly  340  within the shroud  82 , the shroud  82  may cool and contract (e.g., an inner diameter of the shroud  82  may return to a dimension corresponding to an unheated state of the shroud  82 ). Accordingly, the shroud  82  may apply a compressive force (e.g., radially inward) to the fan blades  80 , thereby ensuring that a fluidic seal is created and maintained between the blade tips  344  and the shroud  82 .  FIG. 39  is a perspective view of and embodiment of the rotor assembly  56  in an assembled configuration, in which the blade assembly  340  is disposed within the shroud  82 . 
     In some embodiments, certain of the grooves  342  may be slots that fully extend through a thickness of the shroud  82 . In such embodiments, certain of the fan blades  80  corresponding to these slots (e.g., referred to herein as protruding blades) may be sized to include a radial dimension that exceeds a radial dimension of the shroud  82 . Accordingly, upon complete insertion of the blade assembly  340  within the shroud  82 , the protruding blades may align with the slots and extend through the slots (e.g., radially past an exterior surface  352  of the shroud  82 ). The remaining fan blades  80  corresponding to the grooves  342  may concentrically align the blade assembly  340  within the shroud  82  to ensure that the blade assembly  340  is centered within the shroud  82 . In this manner, the protruding blades may act as the backflow mitigation feature  300  discussed above, and thereby prevent or substantially reduce a flow of backflow air through the radial gap  120 . 
       FIG. 40  is a planar top view of an embodiment of the rotor assembly  56 . As shown in the illustrated embodiment, the fan blades  80  extend radially from the hub  64  and may arc toward a direction of rotation of the rotor assembly  56  (e.g., in the counter-clockwise direction  94 , in a forward sweeping orientation). As an example, in this manner, a tip  358  of one of the fan blades  80  may be oriented along an axis  362  that is offset from a line  364  extending radially from the centerline  66  by an angle  366 . As a non-limiting example, the angle  366  may be between 45 degrees and about 80 degrees. 
     The forward sweeping design of the fan blades  80  may reduce a radial velocity component of air flowing across the fan blades  80  during operation of the fan  40 . In some embodiments, reducing radial air flow across the fan blades  80  may diminish broadband noise (e.g., audible noise) that is generated due to turbulent air flow across respective leading edges, trailing edges, and or tip regions of the fan blades  80  (e.g., generated due to separation of airflow from the suction surface  92  and/or tip leakage vortices around the fan blades  80  of shroudless fans). Accordingly, the forward sweeping blade design of the rotor assembly  54  may be used in conjunction with any one or combination of the aforementioned flow directing features to reduce an amount of acoustic energy (e.g., audible noise) generated during operation of the fan  40 . 
       FIG. 41  is a planar top view of another embodiment of the rotor assembly  56 . As shown in the illustrated embodiment, the fan blades  80  may be located in a non-uniform spacing about the centerline  66 , such that a cross-sectional area of one or more of the fluid passages  84  may be different. In some embodiments, this variable blade spacing may further reduce tonal noise (e.g., audible noise) associated with the BPF harmonics (e.g., tonal noise) that may be generated at certain frequencies during operation of the fan  40  by spreading the acoustic energy across a range of frequencies instead of a single frequency. 
     In some embodiments, the fans  40 A,  40 B,  40 C, and/or  40 D may include a flow impedance feature, or multiple flow impedance features, which are configured to impede or reduce a flow of backflow air through the radial gap  120 . For example,  FIG. 42  is a partial cross-sectional view of an embodiment of a fan  40 E (e.g., any one of the fans  40 A,  40 B,  40 C, and/or  40 D) having a flow impedance feature  380  that may be disposed about an inner surface  382  of the outer wall  50 . The flow impedance feature  380  may include one or more stationary flow impedance ribs  384  that extend radially from the inner surface  382  and protrude into the radial gap  120 . In some embodiments, the stationary flow impedance ribs  384  may each include a rib that extends circumferentially about the inner surface  382  in a symmetric or uniform manner (e.g., with respect to the centerline  66 ). For example, an axial distance (e.g., along the centerline  66 ) between the first end portion  124  of the outer wall  50  and a respective one of the stationary flow impedance ribs  384  may be substantially constant about a circumference of the outer wall  50 . 
     As shown in the illustrated embodiment, the stationary flow impedance ribs  384  may constrict several portions of the radial gap  120  to impede a backflow of air along these portions of the radial gap  120 . Indeed, by constricting portions of the radial gap  120 , the stationary flow impedance ribs  384  may generate a pressure drop along the radial gap  120  in the second direction  152 , and thus, impede the flow of backflow air through the radial gap  120  in the second direction  152 . In some embodiments, an axial distance between each of the stationary flow impedance ribs  384  (e.g., with respect to the centerline  66 ) may be substantially equal. In other embodiments, the axial distance between certain of the stationary flow impedance ribs  384  may be different. For example, in some embodiments, the stationary flow impedance ribs  384  positioned near the first end portion  124  of the outer wall  50  may be spaced closer together (e.g., with respect to an axial distance between adjacent stationary flow impedance ribs  384 ) or further apart to one another as compared to the stationary flow impedance ribs  384  positioned near the second end portion  132  of the outer wall  50 . Moreover, in certain embodiments, a radial width (e.g., with respect to the centerline  66 ) of one or more of the stationary flow impedance ribs  384  may be substantially equal to one another or different from one another. It should be appreciated that the stationary flow impedance ribs  384  may be formed integrally with the outer wall  50 . 
       FIG. 43  is a partial cross-sectional view of another embodiment of the fan  40 E. In some embodiments, the flow impedance feature  380  may be disposed about the exterior surface  352  of the shroud  82  instead of the inner surface  382  of the outer wall  50 . Particularly, the flow impedance feature  380  may include one or more rotating flow impedance ribs  390  that extend radially from the exterior surface  352  and protrude into the radial gap  120 . In some embodiments, the rotating flow impedance ribs  390  may each include a rib that extends circumferentially about the exterior surface  352  in a symmetric or uniform manner (e.g., with respect to the centerline  66 ). For example, an axial distance between the downstream end portion  136  of the shroud  82  and a respective one of the rotating flow impedance ribs  390  may be substantially constant about a circumference of the shroud  82 . 
     Similar to the stationary flow impedance ribs  384 , the rotating flow impedance ribs  390  may constrict several portions of the radial gap  120  to impede a backflow of air along these portions of the radial gap  120 . That is, by constricting portions of the radial gap  120 , the rotating flow impedance ribs  390  may generate a pressure drop along the radial gap  120  in the second direction  152 , and thus, impede the flow of backflow air through the radial gap  120  in the second direction  152 . In some embodiments, an axial distance between each of the rotating flow impedance ribs  390  (e.g., with respect to the centerline  66 ) may be substantially equal. In other embodiments, the axial distance between certain of the rotating flow impedance ribs  390  may be different. For example, in some embodiments, the rotating flow impedance ribs  390  positioned near the rotating inlet flange  160  may be spaced closer together (e.g., with respect to an axial distance between adjacent rotating flow impedance ribs  390 ) or further apart to one another as compared to the rotating flow impedance ribs  390  positioned near the downstream end portion  136  of the shroud  82 . Moreover, in certain embodiments, a radial width (e.g., with respect to the centerline  66 ) of one or more of the rotating flow impedance ribs  390  may be substantially equal to one another or different from one another. 
     It should be appreciated that the rotating flow impedance ribs  390  may be formed integrally with the shroud  82 . Accordingly, in some embodiments, the rotating flow impedance ribs  390  may stiffen the shroud  82  to reduce vibration of the shroud  82  and the rotor assembly  56  during operation of the fan  40 E. Indeed, in certain embodiments, the rotating flow impedance ribs  390  may reduce or substantially mitigate vibrations that may occur at a natural vibrational frequency of the rotor assembly  56 . 
     In some embodiments, the fan  40 E may include both the stationary flow impedance ribs  384  and the rotating flow impedance ribs  390 . To better illustrate and to facilitate the following discussion,  FIG. 44  is a partial cross-sectional view of an embodiment of the fan  40 E that includes the stationary flow impedance ribs  384  and the rotating flow impedance ribs  390 . As shown in the illustrated embodiment, the rotating flow impedance ribs  390  may be positioned axially between neighboring stationary flow impedance ribs  384  to form a serpentine flow path that extends along a length of the radial gap  120 . In this manner, the stationary flow impedance ribs  384  and the rotating flow impedance ribs  390  may cooperate to impede or to restrict the flow of backflow air along the radial gap  120  in the second direction  152 . 
     It should be appreciated that, in some embodiments, the stationary flow impedance ribs  384  may be axially aligned (e.g., with respect to the centerline  66 ) with the rotating flow impedance ribs  390 . That is, the stationary flow impedance ribs  384  may be configured to extend along the radial axis  42  toward the rotating flow impedance ribs  390 . In this manner, the stationary flow impedance ribs  384  and the rotating flow impedance ribs  390  may cooperate to constrict particular portion(s) of the radial gap  120 . 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure. 
     The techniques presented and claimed herein are referenced and applied to material objects and concrete examples of a practical nature that demonstrably improve the present technical field and, as such, are not abstract, intangible or purely theoretical. Further, if any claims appended to the end of this specification contain one or more elements designated as “means for [perform]ing [a function] . . . ” or “step for [perform]ing [a function] . . . ”, it is intended that such elements are to be interpreted under 35 U.S.C. 112(f). However, for any claims containing elements designated in any other manner, it is intended that such elements are not to be interpreted under 35 U.S.C. 112(f).

Metadata:
Filing Date: 20190625
Publication Date: 20220524
Grant Date: 20220524
Priority Date: 20181107
Inventors: AIELLO, ANTHONY J.
TAN, CHENG P.
DYBENKO, JESSE T.
NAGHIB LAHOUTI, ARASH
PRATHER, ERIC R.
HERROU, PHILIPPE P.
RASOULI, ASHKAN
DEGNER, BRETT W.
LECLERC, MICHAEL E.
Assignee: APPLE INC
CPC Classifications: [{"code": "F04D29/541", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/667", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/403", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/326", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D25/166", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/326", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/522", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/661", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D19/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "F04D29/164", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D19/002", "inventive": true, "first": true, "tree": "[]"}, {"code": "F04D1/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D27/0269", "inventive": true, "first": true, "tree": "[]"}, {"code": "F04D29/403", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D29/326", "inventive": true, "first": false, "tree": "[]"}, {"code": "F04D27/0269", "inventive": true, "first": true, "tree": "[]"}, {"code": "F04D1/10", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 70458447