Centrifugal turbomachinery

Details Centrifugal turbomachinery Centrifugal turbomachinery

2000-07-13

2002-01-15

Kwon, John (Department: 3745)

Fluid reaction surfaces (i.e., impellers)

Rotor having flow confining or deflecting web, shroud or...

Radially extending web or end plate

C416S22300B

Reexamination Certificate

active

06338610

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an improvement in an impeller incorporated in a machine generally called turbomachinery such as a centrifugal pump for pumping liquid, or a blower or a compressor for pressurizing and delivering gas.
BACKGROUND ART
FIGS. 9A through 10B
show a typical turbomachinery which is constructed by accommodating an impeller
6
having a hub
2
, a shroud
4
, and a plurality of blades
3
between the hub
2
and the shroud
4
in a casing (not shown in the drawings) having pipes and by coupling a rotating shaft
1
connected to a driving source to the impeller
6
. In such an impeller, the blade tips
3
a
of the blades
3
are covered with a shroud surface
4
a,
and a flow passage is defined by two blades
3
in confrontation with each other, a hub surface
2
a
and the shroud surface
4
a.
When the impeller
6
is rotated about an axis of the rotating shaft
1
at an angular velocity &ohgr;, fluid flowing into the flow passage from an impeller inlet
6
a
through a suction pipe is delivered toward an impeller exit
6
b,
and then discharged to the outside of the turbomachinery through a discharge pipe or the like. In this case, the surface facing the rotational direction of the blade
3
is the pressure surface
3
b,
and the opposite side of the pressure surface
3
b
is the suction surface
3
c.
The three-dimensional geometry of a closed type impeller as an example of impellers is schematically shown in
FIGS. 9A through 10B
in such a state that most part of the shroud surface is removed. In the case of an open type impeller, there is no independent part for forming the shroud surface
4
, but a casing (not shown in the drawings) for enclosing the impeller
6
serves mechanically as the shroud surface
4
. Therefore, there is no basic fluid dynamical difference between the open type impeller and the closed type impeller. Thus, only an example of the closed type impeller will be described below.
In the flow passages of such an impeller in a centrifugal turbomachinery, besides main flow flowing along the flow passages, secondary flows (flow having a velocity component perpendicular to that of the main flow) are generated by movement of low energy fluid in boundary layers on wall surfaces due to pressure gradients in the flow passages. The secondary flow affects the main flow intricately to form vortices or flow having non-uniform velocity in the flow passage, which in turn results in substantial fluid energy loss not only in the impeller but also in the diffuser or guide vanes downstream of the impeller. The total energy loss caused by the secondary flows is referred to as secondary flow loss. It is known that the low energy fluid in the boundary layers accumulated at a certain region in the flow passage due to the secondary flows causes a flow separation in a large scale, thus producing positively sloped characteristic curve and hence preventing the stable operation of the turbomachinery.
The secondary flow in the impeller is broadly classified into the blade-to-blade secondary flow generated along the shroud surface or the hub surface, and the meridional component of the secondary flow generated along the pressure surface or the suction surface of the blades. It is known that the blade-to-blade secondary flow can be minimized by making the blade profile to be backswept. Regarding the other type of the secondary flow, that is, the meridional component of the secondary flow, it is necessary to optimize the three-dimensional geometry of the flow passage, otherwise the meridional component of the secondary flow cannot be weakened or eliminated easily.
The mechanism of generation of the meridional component of the secondary flow is explained as follows: As shown in
FIG. 9B
, with regard to the relative flow in the flow passage, the reduced static pressure distribution, defined as p* (=p−0.5&rgr;u
2
), is formed by the action of a centrifugal force W
2
/R based on the streamline curvature of the main flow and by the action of Coriolis force 2&ohgr;W&thgr; based on the rotation of the impeller, where W is the relative velocity of the flow, R is the radius of streamline curvature, &ohgr; is the angular velocity of the impeller, W&thgr; is the component in the circumferential direction of W relative to the rotating shaft
1
, p is the static pressure, &rgr; is the density of fluid, u is the peripheral velocity at a certain radius from the rotating shaft
1
.
The reduced static pressure p* has a distribution in which the pressure is high at the hub side and low at the shroud side, so that the pressure gradient balances the centrifugal force W
2
/R and the Coriolis force 2&ohgr;W&thgr; which are directed toward the hub side shown in FIG.
9
B. In the boundary layer along the blade surface, since the relative velocity W is reduced by the influence of the wall surface, the centrifugal force W
2
/R and the Coriolis force 2&ohgr;W&thgr; which act on the fluid in the boundary layer become small. Accordingly, the centrifugal force and the Coriolis force cannot balance the reduced static pressure distribution p* of the main flow. As a result, the low energy fluid in the boundary layer flows towards an area of the low reduced static pressure p*, thus generating the meridional component of the secondary flow along the blade surface from the hub side toward the shroud side, on the pressure surface
3
b
or the suction surface
3
c
of the blade
3
. In
FIG. 9A
, the meridional component of the secondary flow is shown by the dashed arrows on the pressure surface
3
b
of the blade
3
and the continuous arrows on the suction surface
3
c
of the blade
3
.
The meridional component of the secondary flow is generated on both surfaces of the suction surface
3
c
and the pressure surface
3
b
of the blade
3
. In general, since the boundary layer on the suction surface
3
c
is thicker than that on the pressure surface
3
b,
the secondary flow on the suction surface
3
c
has a greater influence on performance characteristics of a turbomachinery.
When the low energy fluid in the boundary layer moves from the hub side to the shroud side, fluid flow flowing from the shroud side toward the hub side is formed at the midpoint location between two blades to compensate for fluid flow rate which has moved. As a result, as shown schematically in
FIG. 10A
, a pair of vortices having a different swirl direction from each other is formed in the flow passage between two blades. These vortices are referred to as secondary vortices. Low energy fluid in the flow passage is accumulated due to these vortices at a certain location of the impeller where the reduced static pressure p* is low, and mixed with fluid which flows steadily in the flow passage, resulting in generation of great flow loss.
Furthermore, if the non-uniform flow generated by insufficient mixing of low energy fluid having a low relative velocity and high energy fluid having a high relative velocity is discharged to the downstream flow passage of the blades, then great flow loss is generated. Such a non-uniform flow leaving the impeller makes the velocity triangle unfavorable at the inlet of the diffuser and causes a separated flow on diffuser vanes or a reverse flow within a vaneless diffuser, resulting in substantial decrease of the overall performance of the turbomachinery.
Therefore, as shown in
FIGS. 11A and 11B
, in order to optimize the distribution of the reduced static pressure p* in the impeller, it is considered to design the impeller as follows: The blade is leaned toward a circumferential direction, between the location of non-dimensional meridional distance m=0 (impeller inlet) and the location of non-dimensional meridional distance m=1.0 (impeller exit), so that the blade at the hub side precedes the blade at the shroud side in a rotational direction of the impeller. Further, the blade lean angle, defined as an angle between a surface perpendicular to the hub surface and the blade centerline on the cross-sectional view of the flow passage in the impeller, s

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