Patent Application
20110129346
Embodiments of the present invention relate to fans, and in particular to a hub design configured to retard or otherwise inhibit fan stall.
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).
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.
While the impeller is rotating, the flow may, under certain operating conditions, separate off the suction surface of some of the blades. Referring to
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.
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.
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).
Referring to
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
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 L1 and the axial length of the second portion 406b can be represented by L2, where L=L1+L2. 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.
A more formalistic description of embodiments of the present invention will now be discussed with reference to
In
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.
Referring for a moment to
Referring now to
Referring to
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
In an embodiment of the present invention, a hub 504 can be characterized by the following geometric relationships:
L=L
1
+L
2
R2=R3
30%<[L1/L]<60%
20%<[(R2−R1)/R1]<50%
10°<[β1−β2]<45°
Thus, R2=R3 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 L1 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%<[L1/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.
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%<[(R2−R1)/R1]<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.
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
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.
In an embodiment, the blade inlet angle at the leading edge of the blade 506a is β1 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 β2 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.
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 (β) profile experience rapid (but continuous) changes in the vicinity of the RHC-CHC boundary. Most of the transition/reduction from β1 to β2 happens near R=R2. The cross-sectional area is shrinking along L1 and becomes substantially constant along L2.
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.
RHC followed by a CHC—This combination provides an effective mechanism for inhibiting stall and for increasing the aerodynamic efficiency.
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.
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 ½ the speed (RPM) of the fan.
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.
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).
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.
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).
In an embodiment, the blade angle β1 of the portion of the blade attached to the RHC portion of the hub (see
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.
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.
The stall cells may establish a coherent circulatory structure of their own, and when they do the stall cells rotate at about ½ 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).
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.
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.
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.
Quantitatively, the external frequency f1=[2×Pi×RPM]/[60] and the internal frequency f2=[SQRT (Delta-P/2)]/[L1]. The total pressure-rise is Delta-P, and (per the criteria above) ½ of Delta-P must be performed over L1 (i.e. within the frontal zone).
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.
