GB2469489A

GB2469489A - Impeller with circumferential thickness variation

Info

Publication number: GB2469489A
Application number: GB0906477A
Authority: GB
Inventors: Alexander Viacheslavovich Lavrenko
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2009-04-16
Filing date: 2009-04-16
Publication date: 2010-10-20
Also published as: GB0906477D0

Abstract

A radial flow impeller comprises an impeller body 34 and a plurality of vanes 32 depending there from, wherein the impeller body 34 varies in thickness in a circumferential direction. The impeller body 34 may have a hub portion and a radially outer circumferential rim arranged about a common axis, the vanes 32 extending from the hub portion towards the rim, and the sectional thickness profile of the impeller body 34 varying in a cyclic manner. Alternatively, a method of manufacturing the impeller may comprise determining or predicting a stress distribution within the impeller body 34, determining a desired sectional thickness profile for the impeller body 34 based upon the stress distribution, and producing the impeller according to the sectional thickness profile. The number of cycles of variation in the thickness of the impeller body 34 may be dependent upon, and preferably equal to, the number and spacing of the vanes 32. The impeller body 34 may comprise one or more circumferential bands 46 in which the thickness of the impeller body 34 varies. The impeller may be part of a compressor or turbine, preferably located in an engine.

Description

IMPELLER

The present invention relates to an impeller and more particularly to a radial or circumferential flow impeller.

Impellers are used in a variety of industries to drive machinery or fluid flow. Radial or circumferential flow impellers differ from axial flow impellers in that they allow for a change in direction of the driving or driven fluid from a substantially radial to substantially axial direction or vice-versa. Radial or circumferential flow impellers are referred to hereinafter as radial flow impellers which may comprise compressors or turbines.

Combustion engines in aerospace, power generation and automotive applications make use of radial flow impellers, for example as compressors in gas turbine engines or else as compressors or turbines in turbocharged or supercharged internal combustion (piston) engines. Such examples represent application in which impellers may be required to operate close to their material limits. The following description proceeds in relation to such applications in particular. However it will be appreciated that a plethora of further applications exist in which impellers are used, such as, by way of example, in Heating, Ventilation and Air Conditioning (HVAC) systems and industrial processes such as chemical plants, water treatment and in oil and gas industries. This invention encompasses any such applications in which radial flow impeller efficiency is a concern.

Figures 1A and lB show an example of a conventional radial flow impeller 2 having a plurality of vanes 4 depending from a body 6. The body 6 is a body of rotation and has a cross-sectional profile which is substantially constant about the impeller axis.

A half cross-section of a conventional impeller is shown in figure lB. The impeller body 6 is typically formed as a single piece, although specific terms are typically used to refer to different regions of the impeller 2. The cob 8 is the radially innermost region of the impeller and typically the thickest part of the impeller body 6 in section. The cob is typically shaped to define an opening 10 or other mounting formation about the impeller axis for mounting the impeller 6 to a shaft (not shown), although in certain examples, such as in turbocharger arrangements, the opening may or may not be present.

In this example, the cob has a lip formation 12 which defines the radially inner limit of the root of the respective vanes 4 on the washed side of the impeller. The lip formation may be used in positioning of a nose piece at the impeller hub and may or may not be present dependent on the specific impeller design.

The sectional thickness of the impeller body 6 generally decreases from the cob 8 towards the radially outer rim 14. The surface of the impeller body is generally curved between the cob and the rim and has a greatest angle of curvature towards the cob, which angle of curvature generally diminishes towards the rim 14. The vicinity of relatively large curvature adjacent the cob on the rear surface of the impeller body is referred to as the diaphragm 16. In this example, the diaphragm is concave in shape.

As the body section tapers from the diaphragm 16 towards the rim 14, there is a region between the diaphragm and the rim over which the thickness of the impeller body section is relatively constant. This region between the diaphragm and the rim is referred to as the back plate 18.

Dependent on the impeller purpose and design, the back plate 18 may extend over a greater area of the impeller body and, in an embodiment in which the impeller body is substantially annular in shape, the impeller body itself may be referred to as a back plate. Thus it will be appreciated that the sectional thickness of the impeller body may vary significantly between the cob and the rim or else may be substantially constant in a conventional design.

The vanes 4 depend from the front-facing or washed surface 20, which tapers in a curved manner to towards the rim. As the sectional thickness of the impeller body decreases with radial distance from the cob, the depth of the vanes generally increases in a corresponding manner.

It will be appreciated that the features of the cob, diaphragm, back plate and rim are circumferential in form and may be described as hoop-continuous' or circumferentially constant.

The stiffness, radial strength and hoop strength of the impeller are important considerations in impeller design. However such considerations are not considered independently and so, if material is added to, for example, the back plate in order to increase the thickness and hence the hoop strength of the impeller in that region, a corresponding increase in thickness must be applied to the cob in the vicinity of the bore 10 and also to the diaphragm 16. Such material is added about the entire circumference of the impeller body to ensure dynamic stability is maintained and also that a hoop-continuous stress profile is achieved.

The addition of material in this manner to the back plate results in a larger, heavier impeller which has greater inertia and can reduce efficiency for a range of operating conditions at least. However such additional weight is typically considered acceptable in order to ensure that the structural integrity of the impeller is maintained. This is especially the case for applications in which the impeller operates close to its engineering limits.

It is an aim of the present invention to provide a radial impeller which meets its strength and stiffness requirements in an efficient manner.

According to one aspect of the present invention there is provided an impeller, for which the sectional thickness of the impeller body increases and decreases in a circumferential direction in a cyclic manner.

According to a second aspect of the present invention, there is provided an impeller, for which the sectional thickness of the impeller body is not constant in a circumferential direction.

According to a preferred embodiment, the variation in sectional thickness of the impeller body may be cyclic in a manner dependent upon the number and/or spacing of vanes about the impeller. The number of cycles of variation in thickness of the impeller body may be equal to the number of vanes or may be a multiple, sub-multiple or fraction of the number of vanes.

The impeller body may comprise one or more circumferential bands in which the sectional thickness of the impeller body varies. The or each circumferential band may follow a circumferential path of substantially fixed radius about the impeller body. The or each band may deviate from a path of fixed radius in a cyclic fashion.

The sectional thickness within each band may vary in a continuous or discontinuous manner.

In one embodiment, the impeller comprises a plurality of discrete bands provided at spaced radial locations about the impeller body.

The impeller may comprise a first or fluid-washed surface on which vanes are located and a second or opposing surface. The varying sectional thickness of the impeller body may result in surface perturbations in either or both of the first and/or second sides. In one embodiment, the perturbations take the form of a plurality of rises on the first or second surface. The perturbations may take the form of a plurality of inclined portions on the first or second surface. In the embodiment of inclined portions on the first surface, the inclined portions may be provided immediately adjacent a vane such that the thickness of the impeller body increases with proximity to the vane location.

Each increase in sectional thickness of the impeller body may be located and profiled to accommodate regions of greatest stress or tension within the impeller body. The locations of maximum thickness may be aligned with the locations of maximum stress or centrifugal load within the impeller body. The thickness may vary as a function of stress or centrifugal load in the impeller during operation. The distribution of impeller body material may be aligned at least in part with stress distribution over the impeller during use. This correlation may be made only within one or more circumferential bands about the impeller.

In one embodiment, the increased regions of sectional thickness may comprise a plurality of radially aligned strips. Such strips may be divided into sections falling within one or more circumferential bands.

The impeller may be arranged for use within an engine structure, such as a gas turbine engine or else an internal combustion engine.

The impeller body may comprise integrally formed hub and rim portions. The impeller body and vanes may be integrally formed. The impeller body may generally taper in sectional thickness towards the rim. Alternatively, the sectional thickness of the impeller body may be generally planar in form save for the variation in thickness according to the present invention.

According to a third aspect of the present invention, there is provided a compressor or turbine arrangement comprising an impeller according to the first or second aspect.

According to a fourth aspect of the invention there is provided a method of manufacture of an impeller comprising: determining or predicting a stress distribution within an impeller body during use; determining a sectional thickness profile for the impeller body; and producing an impeller having an impeller body which varies in thickness in a circumferential direction according the sectional thickness profile.

Any preferable features described in relation to the first, second or third aspects may also be applied to the fourth aspect of the invention.

A radial flow impeller may be considered to comprise any impeller which causes a fluid to change direction from a substantially axial flow direction to a substantially radial flow direction or vice-versa upon passing through the impeller.

Working embodiments of the present invention are described in further detail below by way of example with reference to the accompanying drawings, of which: Figure 1A shows a three dimensional view of a conventional impeller cut away through an axially-aligned plane; Figure lB shows a half section view of a conventional impel 1 e r; Figure 2 shows a part-cut-away view of an impeller mounted in a gas turbine engine; Figure 3A shows a radial view of a portion of a predicted stress profile for an impeller according to the method of the present invention; Figure 3B shows a radial view of a portion of an impeller according to an embodiment of the present invention; Figure 4A shows a three dimensional view of a segment of an impeller according to one embodiment of the present invention from the rear; Figure 4B shows a plan view of a segment of an impeller according to one embodiment of the present invention from the rear; and, Figure 4C shows a half section view of an impeller according to one embodiment of the present invention.

In figure 2, there is shown an impeller 22 mounted for use within a gas turbine engine 24. The impeller 22 is mounted within a gas path 26 within the core engine on shaft 28. The shaft provides a mechanical linkage between the impeller 22 and a turbine (not shown) such that combustion products produced within the gas turbine combustion equipment drive the turbine, which in turn drives the impeller 22 SO as to compress gas passing into the combustion equipment along gas path 26.

Gas impinging on the impeller 22 is flowing in a generally axial direction A. The impeller turns the gas such that gas is forced radially outward by the impeller in the direction of arrow B. An impeller according to the present invention is described below with reference to figures 3 and 4. The impeller may take the general form shown in figures 1A and lB and the above description of the impeller of figures 1A and lB may be applied to the embodiment of figures 3 and 4 in conjunction with the additional features described below.

In one embodiment, an impeller according to the present invention may be mounted within a gas turbine engine as described in relation to figure 2. Additionally, an impeller according to other embodiments of the present invention may be substituted for a conventional impeller in any applications in which a radial flow impeller is required as described in the introduction of the present application.

Turning now to figure 3A, there is shown a schematic of a radial view of an impeller 30 showing the vanes 32 and impeller body 34, in the vicinity of the back plate and rim, in a deflected condition during use. The at-rest condition of the impeller is shown in dotted lines by way of comparison. It will be appreciated that the deflected condition by virtue of the fluid pressure acting on the impeller and has been exaggerated in figure 3A for clarity.

Accurate predictions of the stresses within the impeller and the resulting impeller deflection can be generated using finite element analysis (FEA) techniques.

The action of the fluid on the impeller may be predicted using computational fluid dynamics (CFD) software. The skilled person will appreciate that any -or any combination of -FEA and CFD tools and other related techniques may be used to generate a stress profile for the impeller in use.

The inventor has determined that the stress distribution is localised and not in fact hoop-continuous in the radially outer portions of the impeller (rim and back plate) . In addition, the inventor has determined that the stress distribution is localised in the radially outer portions of the impeller because the sectional thickness of the impeller body material in those areas is thin enough to be affected by loads from the vanes. The centrifugal forces acting on the impeller and the moment in the vanes created by the fluid acting thereon impact on the level of stress applied to the impeller body and, therefore, the stress distribution is dependent on the vane distribution.

Relative deflection between the vanes and the radially-outer part of the impeller back plate between the vanes under centrifugal loading leads to additional bending stress in the back plate.

Accordingly the regions 36 and 38 marked on figure 3A indicate where maximum stress and thus deflection of the impeller body 34 may occur. In figure 3B, there is shown one example of a modified profile of the impeller body 34 for which the thickness of the impeller body varies in the circumferential direction. That is to say that the thickness of the impeller body, particularly in the region of the back plate and rim, increases and decreases if a line of substantially constant radius is followed about the impeller body.

Two discrete regions of increased thickness 40 and 42 are marked on figure 3B. It can be seen that the point of maximum thickness is located on the rear surface of the impeller body 34 at a location substantially beneath or slightly downstream of the vane location when viewed in light of the direction of rotation C. The provision of additional material to increase the thickness in isolated or discontinuous sections in this manner substantially avoids unwanted deflection of the vanes and impeller body during use.

In one definition of the present invention, the region of increased thickness may commence at a location slightly upstream of a vane location and may increase to a maximal value beneath or slightly downstream of the vane. The thickness of the impeller body section may diminish with distance downstream of the maximum value to a base thickness value in a mid region between adjacent vanes.

The thickness may subsequently increase in respect of the adjacent vane in a cyclic manner for each vane on the impeller.

This solution increases the impeller body's second moment of area in places with high stress concentration, particularly in the vicinity of the rim and back plate.

The application of a shaped contour at the rear of the rim provides only one specific solution and alternative modification to the sectional profile of the impeller body may be made, for example, by addition of material to the gas washed surface of impeller 44 in the vicinity of the vanes 32. Such an arrangement will typically require an increase in thickness in the impeller body immediately in advance or upstream of the vane, in the region indicated at 38 in figure 3A.

The provision of material thickness to the gas washed face or inner annulus of the impeller may carry additional benefits in improving the aerodynamic performance of the impeller. Accordingly the increased thickness may be biased towards either the gas-washed surface 44 or else the rear surface of the impeller body dependent on specific design requirements for the impeller.

Figures 4A to 4C show a further embodiment in which a plurality of circumferential bands are provided on the impeller body. In this example, the bands are provided on the rear surface of the impeller but may equally be provided on the gas-washed surface on which the vanes are mounted.

In this embodiment a total of four discrete, radially spaced bands 46 are provided in the vicinity of the back plate and towards the rim. The bands follow a substantially circumferential path although the thickness of each band in a radial direction may vary along its length as can be seen in figure 4B.

The thickness of the impeller body may vary for example between a base thickness, representative of an unmodified impeller body thickness value at a given radius, and a maximum thickness which may be as much as double the base thickness or more.

The use of defined bands 46 in which the thickness are used primarily for ease of definition, modelling and manufacture. However the design of the impeller in reality is a balance between life requirements, weight, efficiency and stiffness.

For the purpose of accurately achieving the stiffness and strength requirements of the impeller, it would be preferable to provide a greater number of bands, tending to a situation in which the bands are effectively joined up such that a smooth thickness profile is achieved in the radial direction (e.g. an infinite number of bands) However for ease of manufacture it would be preferable to retain fewer bands and accordingly a balance may be struck in which anywhere between, for example, one and ten bands are provided. Since the stress profile on the impeller varies in a radial direction as well as in a circumferential direction, it would be preferable to provide relatively thin bands.

The circumferential variations in thickness are provided in the radially outermost half of the impeller body (as defined by the radius dimension rather than by surface area) for example in the vicinity if the back plate and rim. The circumferential variations in thickness may be limited to the region bounded by a circumferential line at two-thirds of the impeller radius and the outer rim of the impeller.

An impeller manufactured according to the present invention may allow for a reduction in weight and thickness. Aside from the efficiency savings made available by reduced weight, the potential for reduced dimensions may have a beneficial impact on engine design in which the impeller is to be mounted. It is at least in part the engine length which affects engine weight and consequently fuel burn. In particular, the reduce weight towards the rim of the impeller may beneficially impact on bore stress levels which can allow for a reduction of the cob size, in particular its width, which impacts on the engine length.

In addition the potential for reduction in bore stress and cob size may allow the impeller to be operated at higher speed for a given operational life. This will increase pressure ratio and can increase efficiency for example by allowing a reduced fuel burn. Alternatively, if operated at similar speeds to a conventional impeller, the reduction in the bore stress levels can lead to a longer component life.

The invention as described in relation to figure 4 may be enhanced by using shaped contouring such that the second moment of area is increased in a gradual continuous manner rather than in discrete bands. This may result in discrete bulges being formed in the impeller body which gradually increase and decrease in height.

The invention can be applied to any impeller, for example, a conventional turbocharger. The shaping process can reduce impeller's weight, which would reduce its inertia and spool up time thus making it a more responsive system.

Claims

Claims: 1. A radial flow impeller comprising: an impeller body having a hub portion and a radially outer circumferential rim arranged about a common axis of rotation; and, a plurality of vanes depending from the impeller body and extending from the hub portion towards the rim, wherein the impeller body has a sectional thickness profile which varies in a circumferential direction in a cyclic manner.
2. A radial flow impeller according to claim 1, in which the variation in sectional thickness of the impeller body is cyclic in a manner dependent upon the number and spacing of vanes about the impeller body.
3. A radial flow impeller according to claim 1 or claim 2, wherein the number of cycles of variation in thickness of the impeller body is a multiple, sub-multiple or fraction of the number of vanes.
4. A radial flow impeller according to any preceding claim, wherein the number of cycles of variation in thickness of the impeller body is equal to the number of vanes.
5. A radial flow impeller according to any preceding claim, wherein the impeller body comprises one or more circumferential bands in which the sectional thickness of the impeller body varies.
6. A radial flow impeller according to claim 5, wherein each circumferential band follows a path of substantially fixed radius about the impeller body.
7. A radial flow impeller according to claim 6 or 7, comprising a plurality of discrete bands at spaced radial locations about the impeller body.
8. A radial flow impeller according to any preceding claim, wherein the varying thickness is accounted for by perturbations in the impeller body surface on which vanes are located.
9. A radial flow impeller according to any preceding claim, wherein the varying thickness is accounted for by perturbations in an impeller body surface which opposes the surface on which vanes are located.
10. A radial flow impeller according to claim 8 or 9, wherein each region of increased thickness commences at a location slightly upstream of a vane location and increases to a maximal value at or slightly downstream of that vane location.
11. A radial flow impeller according to any preceding claim arranged to be mounted within an engine structure.
12. A compressor or turbine arrangement comprising an impeller according to any one of claims 1 to 11.
13. A method of manufacture of a radial flow impeller comprising: determining or predicting a stress distribution within an impeller body during use; determining a desired sectional thickness profile for the impeller body based upon said stress distribution; and, producing a radial flow impeller having an impeller body and a plurality of vanes depending there-from, wherein the impeller body varies in thickness in a circumferential direction according the sectional thickness profile.
14. An impeller substantially as hereinbefore described with reference to the accompanying drawings