GB2472621A

GB2472621A - Impeller hub

Info

Publication number: GB2472621A
Application number: GB0914114A
Authority: GB
Inventors: John Hoptroff
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2009-08-13
Filing date: 2009-08-13
Publication date: 2011-02-16
Also published as: GB0914114D0

Abstract

A turbo machine comprises a centrifugal or mixed-flow impeller 10 carried on a turbo shaft 20, the impeller 10 comprising a row of impeller blades 30 extending around a rim 40a of a centrifugal or mixed-flow impeller hub 40. The hub 40 incorporates a plurality of axially-spaced support discs 42, 44 extending between the rim 40a and the turbo shaft 20, wherein at least one of the support discs 42, 44 is located in the region of the leading edge of the rim 40a for limiting radial displacement of the leading edge during normal operation of the impeller 10. Two support discs 42, 44 may be provided, one in the region of the leading edge and one in the region of the trailing edge of the impeller rim 40a. The support discs 42, 44 may comprise a cob portion 42a, 44a adjacent the turbo shaft 20, and a diaphragm portion 42b, 44b between the cob portion 42a, 44a and the rim 40a. The turbo machine may be a compressor or turbine of a gas turbine engine.

Description

DEVELOPMENTS IN OR RELATING TO TURBO MACHINERY

The present invention relates generally to turbo machinery comprising a centrifugal or mixed-flow impeller carried on a turbo shaft.

A typical arrangement for such turbo-machinery is illustrated in Figure 1, which shows an impeller 1 carried on a turbo shaft 2 for rotation about a centreline (CL) The impeller 1 comprises a series of impeller blades 3 (only one of which is visible in Figure 1) extending around the rim 4a of an annular impeller hub 4.

The hub 4 incorporates a relatively thick stress-bearing ring or "cob" 4b extending around the rotor shaft 2.

A problem with this sort of arrangement is that the "cob" 4b must be made relatively large in order to limit movement of the rim portion 4a to within blade tip clearance tolerances during normal running of the impeller 1.

A relatively large cob carries with it certain disadvantages: the resulting inertial mass of the cob is relatively high, which may adversely impact upon operation and performance of the turbo-machinery; in addition, the cob will tend to have a relatively large heat capacity or thermal mass, which may cause high thermal gradients and stresses during operation of the turbo-machinery and which may result in sub-optimal cooling rates during initial forging of the impeller hub, particularly in the quenching phase. The overall forging cost will in general also increase in relation with the size of the cob.

It is an object of the present invention to seek to overcome one or more of the disadvantages associated with the sort of turbo-machine arrangement illustrated in Figure 1.

According to the present invention, there is provided a turbo machine comprising a centrifugal or mixed-flow impeller carried on a turbo shaft, the impeller comprising a row of impeller blades extending around the rim of a centrifugal or mixed-flow impeller hub, and the hub incorporating a plurality of axially-spaced support discs extending between the rim and the turbo shaft, wherein at least one of the support discs is located in the region of the leading edge of the rim for limiting radial displacement of the leading edge during normal operation of the impeller.

The hub may in particular incorporate two support discs: a rear support disc located towards the trailing edge of the rim and a front support disc located towards the leading edge of the rim. One or both of the support discs may incorporate a cob portion adjacent the turbo shaft, and a diaphragm portion extending radially between the respective cob portion and the rim.

In a particular embodiment of the present invention, the rear support disc is the primary support disc, principally for transmitting torque from the impeller to the turbo shaft, and the front support disc is a secondary support disc, principally for limiting radial displacement of the leading edge.

The impeller may form part of a centrifugal or mixed-flow compressor or turbine, for example in a gas turbine engine.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view through part of a conventional impeller; and Figure 2 is a corresponding cross-sectional view through part of an impeller according to the present invention.

The impeller 10 shown in Figure 2 has certain features which are similar to the conventional impeller 1 shown in Figure 1. Thus, the impeller 10 likewise comprises a series of impeller blades 30 (again, only one of which is visible in Figure 1) extending around the rim 40a of an annular impeller hub 40. The hub 40 is carried on a turbo shaft 20 for rotation about the nominal centreline (CL) of a respective turbo-machine.

However, in contrast to the impeller hub 4 (Figure 1), the impeller hub 40 in Figure 2 is not supported on the rotor shaft 20 by a single, relatively large cob. Rather, the impeller hub 40 incorporates two support discs spaced axially along the nominal centreline (CL) : a rear support disc 42, located towards the trailing edge (TE) of the rim 40a, and a front support disc 44, located towards the leading edge (LE) of the rim 40a.

The rear support disc 42 comprises a cob portion 42a and a respective diaphragm portion 42b extending between the cob portion 42a and the rim 40a (the diaphragm portion 42b in this particular embodiment is radially short, but it may be longer depending upon the size of the cob portion 42a and the radial depth of the rim 40a) . Similarly, the front support disc 44 comprises a cob portion 44a and a respective diaphragm portion 44b extending between the cob portion 44a and the rim 40a.

The support disc 42 is intended to act as a primary support disc, principally for transmitting torque between the impeller and the turbo shaft, whereas the support disc 44 is intended to act as a secondary support disc, principally for limiting radial displacement of the leading edge (LE) Provision of the secondary support disc 42 allows for a relative reduction in the size of the cob portion 42a as compared to the cob 4b (Figure 1), without compromising blade tip clearance control. The inertial and thermal mass of the cob portion 42a can be correspondingly reduced.

Indeed, given practical limits on the size of the cob 4b, it is envisaged that provision of the secondary support disc 42 may actually allow for improved control of blade tip clearance as compared to use of a single, large cob axially off-set from the leading edge (LE) as in the case of Figure 1. The secondary support disc 42 also provides a ready way of increasing the vibrational "stiffness", and consequently the natural resonant frequencies, of the impeller 40.

The diaphragm portion 42b may additionally provide a convenient balancing plane for addition or removal of mass as part of a balancing correction. Suitable sacrificial balancing lands may be provided as part of the diaphragm 42b, as appropriate. The diaphragm portion 42b may also be used to mount bolted flanges for attachment to other components, as required.

Although, in the embodiments described, the impeller hub incorporates two support discs, any number of support discs may in principal be employed (provided that at least one of the support discs is located in the region of the leading edge of the rim for limiting radial displacement of the leading edge during normal operation of the impeller) The invention is particularly suited for use in centrifugal or mixed-flow compressors in gas turbine engines, but is generally suitable for use in any turbo machine comprising a centrifugal or mixed-flow impeller carried on a turbo shaft eg steam turbines, turbo chargers or hydro-electric plant.

Claims

CLAIMS1. A turbo machine comprising a centrifugal or mixed-flow impeller carried on a turbo shaft, the impeller comprising a row of impeller blades extending around the rim of a centrifugal or mixed-flow impeller hub, and the hub incorporating a plurality of axially-spaced support discs extending between the rim and the turbo shaft, wherein at least one of the support discs is located in the region of the leading edge of the rim for limiting radial displacement of the leading edge during normal operation of the impeller.
2. A turbo machine according to claim 1, wherein the hub incorporates two support discs: a rear support disc located towards the trailing edge of the rim and a front support disc located towards the leading edge of the rim.
3. A turbo machine according to claim 1, wherein one or both of the support discs incorporate a cob portion adjacent the turbo shaft, and a diaphragm portion extending radially between the respective cob portion and the rim.
4. A turbo machine according to claim 3, wherein the rear support disc is the primary support disc, principally for transmitting torque from the impeller to the turbo shaft, and the front support disc is a secondary support disc, principally for limiting radial displacement of the leading edge.
5. A turbo machine according to claim 4, wherein the impeller forms part of a centrifugal or mixed-flow compressor or turbine.
6. A turbo machine according to claim 5, in the form of a gas turbine engine.