U.S. patent application number 12/629699 was filed with the patent office on 2011-06-02 for fan stall inhibitor.
This patent application is currently assigned to Minebea Co., Ltd.. Invention is credited to Yousef Jarrah, Hirofumi Shoji, Nigel Strike.
Application Number | 20110129346 12/629699 |
Document ID | / |
Family ID | 44069048 |
Filed Date | 2011-06-02 |
United States Patent
Application |
20110129346 |
Kind Code |
A1 |
Jarrah; Yousef ; et
al. |
June 2, 2011 |
Fan Stall Inhibitor
Abstract
An impeller blade hub profile which inhibits stall is described.
In an embodiment, the hub profile includes a frontal rising contour
followed by a rear constant contour. In another embodiment, the hub
profile includes a frontal rapidly rising contour followed by a
substantially rear constant contour. It was discovered that this
combination is much more effective than conventionally known
profiles. Also, the present invention provides a unique way of
turning the flow within the passages of the rotating impeller
blades. In an embodiment, the blade does not turn much over either
the front portion of the hub or the rear portion of the hub, but
instead turns near the boundary between the front portion and the
rear portion.
Inventors: |
Jarrah; Yousef; (Tucson,
AZ) ; Strike; Nigel; (Phoenix, AZ) ; Shoji;
Hirofumi; (Ishikawa, JP) |
Assignee: |
Minebea Co., Ltd.
Nagano
JP
|
Family ID: |
44069048 |
Appl. No.: |
12/629699 |
Filed: |
December 2, 2009 |
Current U.S.
Class: |
416/214R |
Current CPC
Class: |
F04D 29/541 20130101;
F04D 29/384 20130101; F04D 29/329 20130101 |
Class at
Publication: |
416/214.R |
International
Class: |
F04D 29/34 20060101
F04D029/34 |
Claims
1. An impeller comprising: a hub; a plurality of fan blades
connected to the hub; a first part of the hub having a rising hub
contour along an axial length thereof; and a second part of the hub
having a substantially constant hub contour along an axial length
thereof, wherein one or more of the fan blades, each, is connected
to both the first part of the hub and to the second part of the
hub.
2. The impeller of claim 1 wherein a length of the first part of
the hub is at least one-quarter of a total length of the hub.
3. The impeller of claim 1 wherein a length of the first part of
the hub is about 30% to 60% of a total length of the hub.
4. The impeller of claim 1 wherein a leading edge portion of each
of said one or more of the fan blades is connected to the first
part of the hub and a trailing edge portion of said each of said
one or more of the fan blades is connected to the second part of
the hub.
5. The impeller of claim 1 wherein for each fan blade, a first
portion of said each fan blade is attached to the first part of the
hub and a second portion of said each fan blade is attached to the
second part of the hub, wherein about 10% of a length of said each
fan blade measured from a leading edge thereof has a first blade
angle, wherein about 10% of a length of said each fan blade
measured from a trailing edge thereof has a second blade angle.
6. An impeller comprising a hub and a plurality of fan blades
attached to the hub, wherein a radius of the hub increases in the
axial direction from a first location on the hub proximate a hub
leading edge to a second location on the hub that is distal a hub
trailing edge, wherein the radius does not substantially vary in
the axial direction from the second location on the hub to a third
location on the hub proximate the hub trailing edge.
7. The impeller of claim 6 wherein a distance between the first
location and the second location is about 30% to 60% of a distance
between the first location and the third location.
8. The impeller of claim 6 wherein each fan blade is attached to
the hub between the first location and the third location.
9. The impeller of claim 6 wherein a leading edge portion of each
fan blade is attached between the first location and the second
location and has a first blade angle, wherein a trailing edge
portion of each fan blade is attached between the second location
and the third location and has a second blade angle different from
the first blade angle.
10. The impeller of claim 9 wherein blade turning in said each fan
blade occurs proximate the second location on the hub.
11. The impeller of claim 6 wherein the radius of the hub increases
between the first location and the second location in linear
fashion.
12. An impeller comprising a hub having a first segment and a
second segment, the impeller further comprising a plurality of fan
blades attached symmetrically about the hub, each fan blade being
attached to a part of the first segment of the hub and a part of
the second segment of the hub, the first segment of the hub having
a rising hub profile, the second segment of the hub having a
substantially constant hub profile.
13. The impeller of claim 12 wherein a radius of the first segment
of the hub varies along the axial direction thereof, wherein a
radius of the second segment of the hub remains substantially the
same along the axial direction thereof.
14. The impeller of claim 13 wherein the radius of the first
segment of the hub varies in linear fashion.
15. The impeller of claim 12 wherein a length of the first segment
is about 30% to 60% of the sum of the length of the first segment
and a length of the second segment.
16. The impeller of claim 12 wherein for each fan blade, a blade
inlet angle is .beta..sub.1 for a portion of said each fan blade
equal in length to about 10% of a length of said each blade
measured from its leading edge, and a blade exit angle is
.beta..sub.2 for a portion of said each fan blade equal in length
to about 10% of the length of said each blade measured from its
trailing edge.
17. The impeller of claim 16 wherein a length of a camber line of
said each blade constitutes the length of said each blade.
18. The impeller of claim 16 wherein for said each fan blade, blade
turning occurs in a middle portion of said hub between a first
portion of said hub and a second portion of said hub.
19. A fan comprising an impeller of claim 12 disposed in a fan
housing and connected to a fan motor.
Description
BACKGROUND OF THE INVENTION
[0001] Embodiments of the present invention relate to fans, and in
particular to a hub design configured to retard or otherwise
inhibit fan stall.
[0002] FIG. 8 shows a conventional fan 800. A conventional fan
typically includes a housing 802, a rotating impeller 804
comprising a plurality of blades 806 disposed on a hub 808.
Typically, a cavity inside the impeller hub 808 houses the motor,
motor stator, drive, and bearings (not shown). FIG. 9 shows plan
view and a top view of a typical fan impeller 900 with blades 902
attached to the hub 904. The exterior side-surface of the hub 904
to which the blades are attached can be referred to as the
"impeller hub contour"; it is the contour of the outer surface of
the hub. The perimeter defined by rotation of the blades 902 can be
referred to as the "impeller tip perimeter."
[0003] The function of the fan is to capture, pressurize (by action
of the rotating blades), and deliver air. This task is accomplished
via the impeller blades rotating about an axis. Rotation may be
provided by a motor, for example. To pressurize air, two elements
must be present: the first is blades (whose shape/geometry is
design-dependent), and the second is frequency (or rotational
speed, radian/sec; externally induced).
[0004] While flowing across the blades (from inlet to outlet),
pressure increases due to two basic mechanisms: first, the flow is
forced to continuously turn along the curved surfaces of the
blades; and second, the flow streamlines tend to naturally migrate
into higher radii (and thus higher impeller speeds). But sometimes
these two flow processes, (1) turning due to blade camber and (2)
centrifuging effect, do not always produce coherent flow over all
blade surfaces.
[0005] While the impeller is rotating, the flow may, under certain
operating conditions, separate off the suction surface of some of
the blades. Referring to FIG. 9, the "suction" surface of a blade
902 is the upper surface of the blade viewed from the direction
from the inlet side; the bottom surface of the blade is referred to
as the "pressure" surface. The separation of flow off the suction
surface of the blade leads to a reduction in pressure; this
condition is called stall. Under normal operating conditions, the
flow remains attached to all the blades all the time, and the
blades are doing work efficiently. However, stall will occur when
at least some of the flow, somewhere between the impeller hub
contour and the impeller tip perimeter, is no longer able to remain
attached to all the blades; in this case, the stalled portions of
the blade surfaces become ineffective.
BRIEF SUMMARY OF THE INVENTION
[0006] Embodiments of the invention provide an impeller comprising
a hub having a unique hub contour. In an embodiment, the hub has a
hub contour (hub profile) that rises very quickly along the length
of the frontal portion of the hub and does not substantially change
over length of the rear portion of the hub. In another embodiment,
the hub contour results in two cross-sectional-area zones along the
entire flow path: a frontal rapidly shrinking zone; and a rear
constant area zone. In an embodiment, an impeller includes a hub
comprising a first part that has a varying hub contour and a second
part that has a substantially non-varying hub contour. One or more
fan blades of the impeller are attached to the hub in a manner that
each such fan blade attaches to the first part of the hub and to
the second part of the hub. In an embodiment, the length of the
first part of the hub constitutes at least a quarter of the total
length of the hub. These and other embodiments are described in
further detail below with reference to the figures which will now
be briefly described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The figures are merely diagrammatic, illustrative
representations of embodiments of the present invention, and as
such the illustrated structures are not necessarily to scale. The
figures should not be construed as design specifications for the
construction of embodiments of the present invention.
[0008] FIG. 1 illustrates an embodiment of a fan assembly in
accordance with present invention.
[0009] FIG. 2 illustrate a cross-sectional view of an embodiment of
a fan assembly in accordance with the present invention.
[0010] FIG. 3 shows a top view of an embodiment of an impeller
according to the present invention.
[0011] FIGS. 3A and 3B are schematic profiles of an embodiment of a
hub according to the present invention.
[0012] FIG. 4 shows a schematic cross-sectional view of a hub
according to the present invention.
[0013] FIG. 5A is a schematic illustration of an impeller according
to the present invention.
[0014] FIG. 5B is a top view of the rendering of FIG. 5A.
[0015] FIG. 6 is a cutaway view of a portion of the rendering of
FIG. 5A.
[0016] FIG. 7 is a schematic representation of an alternate profile
of a hub in accordance with the present invention.
[0017] FIG. 8 illustrates a conventional fan assembly.
[0018] FIG. 9 illustrate two views of a conventional impeller.
DETAILED DESCRIPTION OF THE INVENTION
[0019] FIG. 1 depicts a fan 100 embodied in accordance with the
present invention. The figure is merely a diagrammatic,
illustrative representation of an embodiment of the present
invention, and as such the illustrated structures are not
necessarily to scale. The fan 100 may include a fan housing 102.
The fan housing 102 can be provided with mounting holes 103 for
mounting the fan 100 to a device to be cooled by the fan. An
impeller 104 according to the present invention is disposed within
the fan housing 102. The impeller 104 comprises a hub 106 embodied
in accordance with the present invention, and fan blades 108
disposed on the hub in accordance with the present invention. The
figure shows arrows indicating the direction of airflow, entering
the fan 100 at an air inlet side and exiting the fan at an air
outlet side.
[0020] FIG. 2 shows a cross-sectional view of the fan 100 depicted
in FIG. 1. The figure is merely a diagrammatic, illustrative
representation of an embodiment of the present invention, and as
such the illustrated structures are not necessarily to scale. The
cross-sectional view shows that the hub 106 can incorporate a fan
motor 202. The motor 202 may include a canister-like containment
referred to as the yoke 204. The yoke 204 is connected to the
interior volume of the hub 106. A shaft 212 is attached to the yoke
204 allowing the impeller 104 to rotate about an axis of rotation
established by the shaft. A magnetic rotor element 206 may be
connected to the inside wall of the yoke 204. A magnetic stator
element 208 is fixedly mounted within the volume of space of hub
106 along the axis of rotation. In an embodiment of the present
invention, the motor 202 may be a DC brushless motor.
[0021] FIGS. 3, 3A, and 3B illustrate an embodiment of an impeller
302 according to the present invention. The figures are merely
diagrammatic, illustrative representations of an embodiment of the
present invention, and as such the illustrated structures are not
necessarily to scale. The impeller 302 is viewed from the front
(inlet side), looking toward the outlet side. In an embodiment, the
hub 304 provides two successive flow zones: First, is a frontal
zone 304a, where the flow area decreases with distance from the
inlet side of the hub toward the outlet side of the hub up to a
distance D. Second, is a rear zone 304b where the cross-sectional
flow area is substantially constant from distance D on the hub
toward the outlet side for the remaining length of the hub.
Reference letter P represents the perimeter of the area swept by
rotation of the fan blades 306. The flow area is the area between a
surface of the hub 304 and the perimeter P.
[0022] FIG. 3B illustrates the decreasing flow area characteristic
of the frontal zone portion of the hub 304. The hub 304 is
displayed in four panels, each shown in cross-section with its
profile exaggerated to facilitate illustration of the decreasing
flow area of the frontal zone 304a. The airflow direction is
indicated and the inlet side and the outlet side of the hub 304 are
shown. The first panel (1) shows a cross-section of flow area
A.sub.1 at the front end of the hub 304, designated as location
L.sub.1 on the hub. The flow area "seen" head-on by the airstream
is also shown. The perimeter P, as explained in FIG. 3B, represents
the area swept by rotation of the blades. The portion of the hub
304 "seen" by the air stream is illustrated by the hatched circle.
The area A.sub.1 is greatest at the front end of the hub 304. The
second panel (2) shows a cross-section of flow area A.sub.2 along
the axial location of the hub 304, designated as L.sub.2, that is
toward the outlet side of the hub. The flow area A.sub.2 is less
than the flow area A.sub.1, due to the increase in the area of the
hub "seen" by the airflow. The third (3) panel shows a
cross-section of flow area A.sub.3 along the axial location of the
hub 304, designated L.sub.3, that is further still toward the
outlet side of the hub. The flow area A.sub.3 is less than the flow
area A.sub.2. This progressing decrease in flow area continues
until the airflow reaches axial location D on the hub 304. Thus, in
panel (4), at axial location D, the flow area A.sub.4 remains
substantially constant along the remaining portion of the hub 304
beginning at location D in the direction toward the outlet side of
the hub.
[0023] Due to the continuously rising hub, the frontal area A
decreases along the entire axial-length of the frontal zone 304a of
the hub 304. This causes pressure to be produced via the
centrifugal effect (streamlines migrate into higher radii). Over
the axial-length of the rear zone of the hub 304, pressure is
produced via flow turning due to blade camber (i.e. curvature). The
total pressure-rise is the sum due to the two distinct mechanisms
(centrifugal effect plus flow turning).
[0024] Referring to FIG. 4, a cross-sectional view of a hub 404 in
accordance with the present invention is shown. The figure is a
diagrammatic, illustrative representation of an embodiment of the
present invention, and as such the illustrated structures are not
necessarily to scale. The cross-sectional view can be referred to
as the "hub profile" or the "hub contour" (outer surface of the hub
to which the fan blades are attached). Physical features of the hub
profile illustrated in the figure are exaggerated to facilitate
illustrating aspects of the present invention. In an embodiment of
the hub 404, the front of the hub can extend further than is
illustrated in the figure; this is indicated by the dashed outline
404a.
[0025] FIG. 4 shows an axis of rotation; a counterclockwise
rotation is shown as an example. In later discussions, the axis of
rotation lies on the Z-axis of a cylindrical coordinate system. The
direction of airflow is indicated in the figure, where a flow of
air enters at the inlet side and exit from the outlet side. The
inlet (upstream) side of the hub 404 can also be referred to as the
hub leading edge (hub LE). The outlet (downstream) side of the hub
404 can be referred to as the hub trailing edge (hub TE).
[0026] In an embodiment, the hub 404 comprises a first portion 406a
(corresponds to frontal zone 304a) and a second portion 406b
(corresponds to rear zone 304b). The first portion 406a can be
characterized as having a rising hub contour (RHC) in that the
radius, r, of the hub 404 varies along the axial length of the
first portion. The radius is the distance measured from the axis of
rotation to the outer surface (hub contour) of the hub 404. In FIG.
4, radii r.sub.1-r.sub.5 are examples of radius measurements of the
hub contour along the length of the axis of rotation, measured from
the axis of rotation to the outer surface of the hub 404. In an
embodiment, the radius of the first portion 406a of the hub 404
increases in the axial direction from the hub leading edge toward
the hub trailing edge. FIG. 4 shows two examples of radii r.sub.1
and r.sub.2 of the first portion 406a taken along the axis, where
r.sub.2>r.sub.1. The second portion 406b of the hub 404 can be
characterized as having a constant hub contour (CHC) in that the
radius of the hub does not substantially vary along the axial
length of the second portion. FIG. 4 shows three examples of radii
r.sub.3, r.sub.4 and r.sub.5 of the second portion 406b taken along
the axis, where r.sub.3 is substantially equal to r.sub.4 which is
substantially equal to r.sub.5.
[0027] In an embodiment, the hub 404 can be further characterized
by a total axial length, L. The axial length of the first portion
406a can be represented by L.sub.1 and the axial length of the
second portion 406b can be represented by L.sub.2, where
L=L.sub.1+L.sub.2. The figure also shows a leading edge portion
416a of the hub 404, a trailing edge 416b of the hub, and a middle
portion 416c of the hub. The leading edge portion 416a is a "front
part" of the first portion 406a of the hub 404. The trailing edge
portion 416b is a "rearward part" of the second portion 406b of the
hub 404. These portions of the hub are discussed further below.
[0028] FIG. 4 shows the RHC-CHC boundary disposed between the hub
leading edge end of the hub and the hub trailing edge end of the
hub. The RHC-CHC boundary need not be sharp, angled, transition as
shown in the figure. In embodiments of the hub, the transition at
the RHC-CHC boundary can be a curved, smooth, or continuous
transition.
[0029] A more formalistic description of embodiments of the present
invention will now be discussed with reference to FIGS. 5A-6. FIGS.
5A-6 are merely diagrammatic, illustrative representations of an
embodiment of the present invention, and as such the illustrated
structures are not necessarily to scale. FIG. 5A is a schematic
representation showing in profile an impeller 502 embodied in
accordance with the present invention.
[0030] In FIG. 5A, the direction of the airflow is shown, where the
flow enters on the inlet side then exits at the outlet side of the
impeller 502. The axis of rotation is shown and a direction of
rotation of the impeller 502 is indicated. The camber line is the
mean line of the blade profile. The camber line extends from the
leading edge to the trailing edge, halfway between the pressure
side and the suction side. The impeller 502 comprise the hub
(impeller hub) 504 and fan blades 506. One of the fan blades is
shown in cross-section.
[0031] The hub 504 comprises a first portion 504a that is
characterized with a rising hub contour and a second portion 504b
that is characterized by a constant hub contour, as defined and
explained above. One of ordinary skill will appreciate that the hub
504 can be manufactured as single piece, for example, by an
injection mold process where the first and second portions 504a,
504b are formed in the same step. The hub 504 can be manufactured
as two pieces in separate manufacturing steps and then connected
together. For example, the first piece can be the first portion
504a and the second piece can be the second portion 504b, which can
then be connected together to form the hub 504. These and other
manufacturing steps can be used.
[0032] Referring for a moment to FIG. 5B, a top view of the
impeller 502 looking in the downstream direction is shown. Each fan
blade (e.g., 506a) has a portion thereof (referred to herein as the
leading edge portion) that is connected to, attached, or otherwise
formed onto at least part of the first (RHC) portion 504a of the
hub 504. The figure represents this connection as 516. In addition,
fan blade 506a has another portion (referred to herein as the
trailing edge portion) thereof that is connected to, attached to,
or otherwise formed onto at least part of the second (CHC) portion
504b of the hub 504. The figure represents this connection as
518.
[0033] Referring now to FIGS. 4, 5A, and 5B, a discussion about
"blade turning" in accordance with the present invention will now
be presented. For each blade 506, the leading edge portion of the
blade has a blade angle of .beta..sub.1 (also referred to as the
blade inlet angle), which is generally illustrated in FIGS. 5A, 5B.
Likewise, the trailing edge portion of the blade 506 has a blade
angle of .beta..sub.2 (also referred to as the blade exit angle),
which also generally illustrated in FIGS. 5A, 5B The transition of
the blade angles from .beta..sub.1 to .beta..sub.2 is sometimes
referred to by those of ordinary skill in the aerodynamics arts as
"blade turning." Blade 506a illustrates an example of blade
turning. The dashed line roughly indicates the region in the blade
506a where the "turning" occurs.
[0034] Referring to FIG. 4, the RHC-CHC boundary occurs in the
middle portion 416c of the hub 404. Generally in accordance with
the present invention, blade turning occurs in the region of the
middle portion 416c of the hub 404. In an embodiment, blade turning
occurs near the RHC-CHC boundary. This embodiment is illustrated in
FIG. 5A where blade 506a shows the blade turning occurring near the
RHC-CHC boundary. However, blade turning can occur relatively
distal the RHC-CHC boundary, but within the region of the middle
portion 416c of the hub 404.
[0035] FIG. 6 shows a portion of the schematic rendering of the
impeller 502 shown in FIG. 5A. A portion of the hub 504 is shown in
cross-section, as indicated by hatched lines. FIG. 6 also shows a
portion of one of the fan blades 506, and another fan blade 506a
(illustrated in cross-section) oriented relative to the hub 504 in
accordance with the present invention.
[0036] Utilizing a cylindrical coordinate system, let Z be the
axial coordinate along the axis of rotation. Hub length L is
measured as the distance downstream from a reference plane which is
normal to the axis of rotation and located at the impeller hub LE
(leading edge). Let R be the radial coordinate from the axis of
rotation to the point of interest on the hub contour. To specify
the geometric parameters or blade shape factors, shown in FIGS. 5A,
5B, and 6, the following terms are defined: [0037] L=total axial
length (i.e. length along the axis of rotation) of impeller hub
[0038] L.sub.1=axial-length of the RHC portion 504a [0039]
L.sub.2=axial-length of the CHC portion 504b [0040] R.sub.1=hub
radius at leading edge of the RHC [0041] R.sub.2=hub radius at
trailing edge of the RHC [0042] R.sub.3=hub radius at trailing edge
of the CHC [0043] .beta.=blade angle profile (overall
turning=.beta..sub.1-.beta..sub.2) [0044] .beta..sub.1=blade angle
at the leading edge [0045] .beta..sub.2=blade angle at the trailing
edge
[0046] In an embodiment of the present invention, a hub 504 can be
characterized by the following geometric relationships:
L=L.sub.1+L.sub.2
R.sub.2=R.sub.3
30%<[L.sub.1/L]<60%
20%<[(R.sub.2-R.sub.1)/R.sub.1]<50%
10.degree.<[.beta..sub.1-.beta..sub.2]<45.degree.
[0047] Thus, R.sub.2=R.sub.3 expresses the idea that in an
embodiment, the hub radius of the second portion 504b of the hub
504 has a substantially constant radial measurement along its axial
length. In an embodiment, the axial length L.sub.1 of the first
portion 504a of the hub 504 can be about 30%-60% of the total
length of the hub, as expressed by the relation
(30%<[L.sub.1/L]<60%). Accordingly, in an embodiment, the hub
of an impeller according to the present invention has a first
portion (an RHC portion) which length is at least one-quarter of
the total length of the hub. In another embodiment, the hub of an
impeller according to the present invention has an RHC portion
which length is at least one-third of the total length of the hub.
In another embodiment, the hub of an impeller according to the
present invention has an RHC portion having a length that is at
about one-half of the total length of the hub.
[0048] In an embodiment, the contour of the first portion 504a of
the hub 504 can have a profile that is characterized by the
relation (20%<[(R.sub.2-R.sub.1)/R.sub.1]<50%). In another
embodiment, the contour of the first portion 504a can have a rising
arcuate or curved profile along the axial length of the first
portion. FIG. 7, for example, shows in schematic fashion an
embodiment in accordance with the present invention of an impeller
702 having a hub 704 and fan blades 706 (of which only one is
depicted) connected to the hub. The hub 704 has an RHC portion 704a
that has an arcuate or curved profile and a CHC portion 704b.
[0049] As discussed above, in accordance with the present
invention, blade turning occurs in the proximity of the RHC-CHC
boundary. In an embodiment, blade turning occurs near the RHC-CHC
boundary. This embodiment is illustrated in FIGS. 5A and 5B where
blade 506a. However, in general blade turning is not restricted to
the RHC-CHC boundary, but can occur within the region of the middle
portion 416c (FIG. 4) of the hub 404. In an embodiment, the leading
edge portion 416a of the hub 404 is at least 10% of the length of
the hub; i.e., the length of portion 416a is .gtoreq.10% of L.
Likewise, in an embodiment, the trailing edge portion 416b of the
hub 404 is at least 10% of the length of the hub; i.e., the length
of portion 416b is .gtoreq.10% of L. Blade turning can therefore
occur along about 80% of the length (L) of the hub 404 in the
middle portion 416c between the leading edge portion 416a and the
trailing edge portion 416b.
[0050] In an embodiment, the leading edge portion 416a of the hub
404 is about 10% to 20% of the length L of the hub. Likewise, in an
embodiment, the trailing edge portion 416b of the hub 404 is about
10% to 20% of the length L of the hub. Blade turning can therefore
occur along about 60% to 80% of the length (L) of the hub 404 in
the middle portion 416c between the leading edge portion 416a and
the trailing edge portion 416b.
[0051] In an embodiment, the blade inlet angle at the leading edge
of the blade 506a is .beta..sub.1 for at least 10% of the length of
the blade's camber line measured from the blade leading edge which
is disposed on the leading edge portion 416a of the hub. In an
embodiment, the blade exit angle at the trailing edge of the blade
506a is .beta..sub.2 for at least 10% of the length of the blade's
camber line measured from the blade trailing edge which is disposed
on the trailing edge portion 416b of the hub.
[0052] The foregoing characterizations insure in an embodiment that
(a) the RHC portion 504a is followed by the CHC portion 504b in the
downstream direction, (b) blade angle along the RHC is
substantially constant, (c) blade angle along the CHC is
substantially constant, and (d) most of the flow turning (or blade
camber/curvature) occurs near the RHC-CHC boundary. Thus, in an
embodiment, both the flow area and the blade angle (.beta.) profile
experience rapid (but continuous) changes in the vicinity of the
RHC-CHC boundary. Most of the transition/reduction from .beta.1 to
.beta.2 happens near R=R2. The cross-sectional area is shrinking
along L1 and becomes substantially constant along L2.
[0053] This unique way, in which the flow is
centrifuged-then-turned, renders the fan efficient while at the
same time inhibits stall. Along the RHC portion of the hub the flow
is centrifuged without much turning, but along the CHC portion the
flow turns without being centrifuged. Due to this balance between
the two mechanisms, the flow is forced to remain attached to the
fan blades throughout; the would-be otherwise separated flow has
nowhere to go and thus remains attached to all blade surfaces.
[0054] RHC followed by a CHC--This combination provides an
effective mechanism for inhibiting stall and for increasing the
aerodynamic efficiency.
[0055] Aerodynamics--This unique profile forces the flow to
experience a rapid area reduction near the fan inlet followed by a
constant area near the fan exit, resulting in cessation of
instabilities associated with localized separated flow zones. In
other words; once a weak localized stall is born it can grow and
gain strength in the absence of a counter inhibiting mechanism, and
when that happens the function of the fan is compromised; but our
unique hub profile was able to limit the growth of flow
instabilities and render the fan function normal.
[0056] Flow separation creates stall cells within the interior of
the rotating impeller blades and, due to rotation, these cells also
rotate in the circumferential direction but at a lower speed than
the fan rotational speed, usually the cells rotate at about 1/2 the
speed (RPM) of the fan.
[0057] Without stall 100% of the impeller flow volume performs
useful work as intended; namely, capturing, pressurizing, and
delivering air. But stall causes some of the flow volume to be
blocked, and the blocked volume does not perform useful work.
[0058] But when stall occurs; a stall cell structure (i.e. pockets
of separated low momentum flow) forms within the impeller volume
and then it develops (or grows) until it occupies a portion of the
total volume. Stall cell blockage is the term used to define the
percentage of the volume occupied by the stall cells, weak stall
may result in less than 20% blockage, and strong stall may result
in 50% blockage. Blockage values range from 0% to 50% with 20%
considered the threshold value (above which severe stall
persists).
[0059] All fans experience flow instabilities which create some
level of initial blockage. In some fan designs blockage remains low
(below 20%) and the fan will function normally, in this case the
LOCAL instability will not be felt and the primary performance
metric, namely the P-Q (pressure-flow) curve, will not even exhibit
stall. But in some designs the initial instability grows causing
blockage values to be in the 20% to 50% range, in this case the P-Q
curve will show stall.
[0060] The present invention provides a fan blade configuration
that can (a) inhibit the growth of blockage and (b) yield high
performance. In the literature blockage values are in the 0% to 50%
range; but this invention prevents blockage from exceeding the
threshold value, the configuration of this invention holds blockage
at low levels (0% to 20%). Also, our configuration (under similar
operating conditions such as speed and pressure) yields aerodynamic
efficiencies that are much better than any other design (20% to 40%
improvements).
[0061] In an embodiment, the blade angle .beta..sub.1 of the
portion of the blade attached to the RHC portion of the hub (see
FIG. 5B) is substantially constant along the axial length L1 of the
hub and the blade angle .beta..sub.2 of the portion of the blade
attached to the CHC portion of the hub is substantially constant
along the axial length L2. Most of the flow turning, measured as
the reduction in .beta., occurs near the point where L1 and L2
meet. In an embodiment the transition occurs over the middle 1/3 of
the length L of the hub.
[0062] As the fan incoming flow is reduced, some of the flow may
separate somewhere within the impeller blades; near the hub, tip,
or in between, depending on aerodynamic design parameters such as
blade solidity and camber angle profiles. In other words, the flow
simply can not remain attached to the entire blade surface (i.e.,
all the way from the LE to the TE) all the time.
[0063] This "localized detachment process" is caused by flow
separation of the suction side of the blades, producing pockets of
separated (or low momentum) flow called "stall cells." This type of
instability, which occurs when some streamlines are unable to flow
orderly about the blade's pressure and suction sides, may be weak
or strong depending on the fan's aerodynamic design and operating
conditions such as flow and speed. In general, the instability is
weak at low speeds and becomes stronger and stronger with
increasing speeds.
[0064] The stall cells may establish a coherent circulatory
structure of their own, and when they do the stall cells rotate at
about 1/2 the fan rotational speed and in the same direction as the
fan (CW or CCW). The effects are (a) reduction in pressure because
flow separation renders portions of the blades ineffective and (b)
increased noise levels due to the birth of additional spinning
modes (i.e., the stall cells).
[0065] In the remainder of this section a generalized
principle/criteria is presented and sets forth aerodynamic
conditions for preventing fan stall. By "aerodynamic conditions" is
meant the selection of the correct functional relationships between
the fan geometry and changes in thermodynamic properties (pressure,
temperature, etc.) occurring along the flow pathway.
[0066] First, some remarks are made relating to what happens when
the fan is functioning. Basically, the selected geometric
parameters respond to an imposed rotation to produce changes in
thermal properties such as pressure. Different geometric factors
produce different thermal changes when the fan is rotating, but
when three (3) relationships are satisfied the fan performance
becomes excellent. First, there are two (2) mechanisms for
producing pressure, the centrifugal effect created by the RHC
(acting along L1 only) and flow turning due to blade camber,
balancing the two is critical. Second, there are two (2) successive
cross-sectional area zones, frontal zone with shrinking area and
rear zone with constant area. Third, there are two (2) frequencies,
an external frequency (f1, radian/sec) due to rotation and an
internal frequency (f2, radian/sec) felt by the flow while it is
being pressurized.
[0067] To inhibit fan stall, three aerodynamic conditions must be
considered. First, 50% of the pressure should be produced via the
centrifugal effect while the other 50% should be produced via flow
turning. Second, the fan should have a frontal shrinking
cross-sectional-area zone followed by a rear constant
cross-sectional-area zone, the centrifugal effect should be
accomplished within the frontal zone. Third, the external frequency
due to rotation should be equal to the internal frequency
associated with pressure produced within the shrinking zone.
[0068] Quantitatively, the external frequency
f1=[2.times.Pi.times.RPM]/[60] and the internal frequency f2=[SQRT
(Delta-P/2)]/[L1]. The total pressure-rise is Delta-P, and (per the
criteria above) 1/2 of Delta-P must be performed over L1 (i.e.
within the frontal zone).
[0069] An advantage of embodiments of the present invention is that
when the frontal rapidly rising hub contour is combined with the
rear constant hub contour a greater response is demonstrated. The
effect of the combination, namely RHC followed by CHC, is very
effective reduction of stall and may prevent stall altogether.
* * * * *