WO1998016747A1

WO1998016747A1 - Compressor

Info

Publication number: WO1998016747A1
Application number: PCT/GB1997/002622
Authority: WO
Inventors: William Kenneth Bruffell; Alan John Parkes; Roy Saxton
Original assignee: Holset Engineering Company Limited
Priority date: 1996-10-12
Filing date: 1997-09-25
Publication date: 1998-04-23
Also published as: GB9621338D0; GB2319809A

Abstract

An enhanced map width compressor in which an impeller wheel (2) is mounted for location within a chamber defined by a housing (1) between an inlet and an outlet, the wheel including blades (4) each of which sweeps across an inner surface (8) of the housing chamber (1). The housing (1) defines a passageway (13, 20) communicating with at least one opening (12) located in the inner surface of the housing chamber. The pressure within the passageway (13, 20) is controlled, for example as a function of flow rate through the compressor. The slots (12) may be radial with respect to the wheel axis, a pressure differential being established between the compressor inlet and the passageway (13, 20) in low flow conditions to produce an improved surge line position. Alternatively, the slots (12) may be axial, the pressure in the passageway being higher than that in the compressor inlet in high flow conditions.

Description

COMPRESSOR

The present invention relates to a compressor, and in particular to a

compressor having an enhanced map width.

Map width enhanced compressors are known which comprise a

stationary housing defining an inlet and an outlet, and an impeller wheel

mounted for rotation within a chamber defined by the housing between the inlet

and outlet, the wheel including a series of blades each of which has an upstream

edge adjacent the downstream end of the inlet and a radially outer edge which is

in close proximity to and sweeps across an inner surface of the housing chamber.

A passageway as defined by the housing between a first opening in a portion of

the inlet spaced from the upstream edges of the blades and a second opening in

the inner surface of the housing chamber which is swept by the radially outer

edges of the blades.

The passageway increases the amount of gas reaching the impeller wheel

during high flow and high rpm operation and recirculates gas to the compressor

inlet during low flow operation. Such an arrangement results in improved

stability at all speeds and a shift in the characteristics of the compressor. This

shift can be represented as a widening of a standard "map" which plots the total

pressure ratio of the compressor against corrected air flow. Details of a

conventional map width enhanced compressor are described in European Patent

Specification No. 0229519. Although conventional map width enhanced compressors exhibit good

stability, it would be advantageous to be able to enhance the performance of such

compressors in low and high flow conditions. It is an object of the present

invention to provide such an improved compressor.

According to the present invention, there is provided a compressor

comprising a stationary housing defining an inlet and an outlet, and an impeller

wheel mounted for rotation within a chamber defined by the housing between

the inlet and outlet, the wheel including a plurality of blades each of which

sweeps across an inner surface of the housing chamber, and the housing defining

at least one passageway communicating with at least one opening located in the

surface of the housing chamber adjacent the wheel, wherein pressure control

means are provided to control the pressure within the passageway. The pressure

may be controlled as a function of the flow rate through the compressor.

The invention improves the compressor characteristics by modifying the

pressure which would normally be achieved in the passageway defined in a

conventional map width enhanced compressor. The change in pressure could be

produced by any convenient means, for example by providing a venturi in the

inlet, the passageway communicating with the throat of the venturi.

Alternatively, in contrast to a conventional map width enhanced compressor, the

passageway may be connected to an independent pressure source rather than to

a part of the inlet upstream of the compressor wheel. In one possible arrangement the pressure control means maintains a

pressure within the passageway less than that within the inlet at least when the

flow rate is low relative to a predetermined flow rate. The passageway may

extend between the opening adjacent the wheel and a further opening located in

the throat of a venturi within the inlet. The passageway may be blocked when

the flow rate is high relative to a predetermined flow rate. A further passageway

may be arranged to bypass the venturi, means being provided to obstruct the

bypass passageway when the flow rate is low relative to a predetermined flow

rate.

The passageway adjacent the opening may be arranged such that gas

issuing from the opening is directed in a direction with a substantial axial

component relative to the axis of rotation of the wheel, and the pressure control

means may maintain a pressure within the passageway which is higher than that

within the inlet at least when the flow rate is high relative to a predetermined

threshold. The inlet may be defined by a tubular cone received within a tubular

support, the passageway being defined between the cone and the housing, the

opening being defined between one end of the cone and the housing, and the

passageway communicating with the pressure control means through at least one

opening in the tubular support.

Alternatively, each blade may comprise an upstream edge adjacent the

inlet, a first radially outer edge which is in close proximity to and sweeps across

the inner surface of the housing, a second radially outer edge which is in close proximity to and sweeps across a surface defined by the inlet, and an

intermediate edge extending between the first and second radially outer edges,

the opening facing the intermediate edges of the vanes.

In another arrangement in accordance with the invention, the passageway

adjacent the opening is arranged such that gas issuing from the opening is

directed in a direction with a substantial radial component relative to the axis of

rotation of the wheel, and the pressure control means maintains a pressure

within the passageway which is higher than that within the inlet only when the

flow rate is low relative to a predetermined threshold.

The predetermined thresholds referred to above may be selected to suit

each particular application and the pressure within the passageway may be

modulated in a manner appropriate to a particular application. Thus for

example in a particular arrangement the pressure within the passageway may be

varied continuously with flow rate or a parameter related to flow rate, or may

change in a plurality of steps over a range of flow rates.

Embodiments of the present invention will now be described, by way of

example, with reference to the accompanying drawings, in which:

Figure 1 is a cross-section through part of a conventional map width

enhanced compressor;

Figure 2 is a graph of the total pressure ratio against air flow corrected to

15 C for the compressor of Figure 1 ; Figure 3 is a graph representing static pressure along the surface of the

compressor housing swept by the compressor wheel blades against the

meridional distance along the axis of the wheel, showing the effect of a slot in the

housing wall of the compressor of Figure 1;

Figure 4 is a graph similar to that of Figure 3 showing the effect of

reducing the pressure within the slot in accordance with the present invention;

Figure 5 is a graph similar to that of Figure 3 but showing the pressure

for conditions in which the compressor is operating at maximum efficiency and

there is no flow through the compressor housing slot;

Figure 6 is a view similar to that of Figure 3 but showing the pressure at a

flow rate corresponding to the compressor operating near choke;

Figure 7 is a view similar to that of Figure 6 but showing the effect of

increasing the pressure within the slot of the housing of Figure 1;

Figure 8 is a map of total pressure ratio against airflow corresponding to

that of Figure 2 but showing the improvement which results from applying a

partial vacuum to the slot during low flow conditions;

Figure 9 illustrates a first structure in accordance with the present

invention for applying a partial vacuum in the slot;

Figure 10 shows a second embodiment of the invention for applying a

partial vacuum to the slot except during high flow conditions;

Figure 11 illustrates a multislot structure in accordance with the

invention; Figure 12 schematically represents a further embodiment of the present

invention in which a venturi is provided with a bypass;

Figure 13 schematically represents a further embodiment of the invention

in which slots direct a flow axially relative to the compressor wheel;

Figure 14 illustrates the response achieved with the arrangements

illustrated in Figure 13;

Figure 15 schematically represents a further embodiment of the invention

in which the slots direct flow axially;

Figure 16 illustrates an alternative embodiment of the present invention

in which radial flow from the slots is used to control effective inlet size; and

Figure 17 represents the performance achieved with an arrangement as

illustrated in Figure 16.

Referring to Figure 1, the illustrated compressor comprises a housing 1

within which an impeller wheel 2 is mounted for rotation about an axis 3. The

wheel 2 supports an array of blades 4 each of which has an upstream edge 5, a

downstream edge 6, and a radially outer edge 7 which is in close proximity to

and sweeps across an inner surface 8 of the housing. The housing includes a

tubular section 9 immediately upstream of the impeller wheel which forms a

short tubular inlet to the impeller wheel. The housing also defines an outlet

which is connected to a passageway 10 to which the impeller wheel delivers

compressed air. The housing defines a tubular intake 1 1 which extends around and

projects from the end of the tubular inlet 9. An annular slot 12 is formed

through the housing wall so as to open in the face 8 which is swept by the radially

outer edges 7 of the impeller blades. The slot 12 communicates with an annular

space 13 defined between the housing sections 9 and 1 1 and that annular space

together with the slot 12 defines a passageway which extends from the upstream

end of the inlet 9 to the surface 8.

In operation, the wheel 2 is rotated for example by a turbine (not shown)

attached to a common shaft with the wheel 2. This causes air to be drawn into

the wheel 2 through intake 1 1 and inlet 9. The air is compressed and then fed to

its ultimate destination via the passageway 10.

The pressure in the annular space 13 defined between the housing

sections 9 and 11 is normally lower that atmospheric pressure. During high flow

and high rpm operation of the compressor, the pressure in the slot 12

immediately adjacent the impeller wheel blades 4 is less than the pressure in the

annular space between the housing portions 9 and 1 1, and as a result air flows

though the slot 12 towards the impeller wheel. If the flow through or the rpm of

the impeller wheel 2 then falls, so the amount of air drawn into the wheel 2

through the slot 12 decreases until equilibrium is reached. A further decrease in

impeller wheel flow results in the pressure within the slot 12 immediately

adjacent the wheel blades 4 being greater than the pressure in the annular space

between the housing sections 9 and 11 and therefore air flows through the slot 12 away from the impeller 2. This air flow is recirculated to the compressor inlet.

The flow through the slot 12 thus improves the stability of the compressor at all

speeds as compared with a compressor in which the slot 12 is not present.

Figure 2 shows a typical compressor characteristic using the conventional

map width enhanced structure as illustrated in Figure 1. Three points 14, 15 and

16 within this characteristic have been selected for analysis, the graphs shown in

Figures 3 and 4 corresponding to point 14, the graph shown in Figure 5

corresponding to point 15, and the graphs shown in Figures 6 and 7

corresponding to point 16. Point 14 represents conditions in which the

compressor is operated close to surge, point 15 represents operation at maximum

efficiency, and point 16 represents operation close to choke flow. It would be

highly desirable if the surge limit represented by line 17 could be moved to the

left so as to increase the circulation flow and thereby stabilise the compressor at

lower net flows, and it would also be advantageous to be able to move the choke

limit represented by line 18 to the right.

Figure 3 shows the pressure distribution within the compressor assuming

that the operating conditions are such that the pressure within the slot 12 of

Figure 1 is substantially equal to atmospheric. The shaded area represents the

effect of the presence of the slot 12, the broken line representing the pressure

which would apply in the absence of the slot 12. Figure 4 represents the same

operating conditions as Figure 3 except for the fact that the pressure within the

slot 12 has been reduced from 1 bar to 0.8 bars. It will be noted that the shaded area has increased considerably, representing a significantly increased flow of air

into the impeller wheel 2 from the tubular section 9 and from the wheel into the

slot 12 of Figure 1. Figure 5 shows the pressure distribution at the maximum

efficiency operating point 15 of Figure 2. There is minimal net flow into or away

from the impeller wheel. Figure 6 shows the pressure distribution assuming

operation at point 16 of Figure 2, the shaded area representing the flow of air

into the impeller wheel. Figure 7 shows the effect of increasing the pressure in

the slot 12 so as to increase the flow into the impeller wheel.

The improved flow characteristics represented in Figures 4 and 7 can be

achieved in accordance with the invention in any convenient manner, various

possibilities being described below with reference to Figures 9 to 17. The effect

of applying the pressure as indicated in Figure 4 is illustrated in Figure 8 in

which it will be noted that as compared with the surge limit line 17 as shown in

Figures 2 and 8 which is relatively close to operational point 14 a new

considerably displaced surge limit has been established as represented by line 19.

Referring to Figure 9, a first embodiment of the present invention is

illustrated. The same reference numerals are used in Figures 9 to 17 for

equivalent components shown in Figure 1. In Figure 9, it will be noted that the

slot 12 opens into annular spaces 13 and 20 defined on their radially outer sides

by the intake 11 and a tubular intake extension and on their radially inner sides

by a tubular member 21. The tubular member 21 is supported on ribs 22 and

the intake 11 and intake extension are interconnected by a hose 23. The spaces 13 and 20 define a passageway between the slot 12 and an opening 24 located in

the throat of a venturi defined within the intake. Thus air flow towards the

impeller wheel 2 results in a pressure differential between the opening 24 and the

region immediately upstream of the upstream edges 5 of the impeller wheel.

This differential pressure tends to increase the flow of air from the slot 12

towards the opening 24. This is clearly advantageous at low net flow rates as

represented by Figure 4. It would be a disadvantage however at high flow rates.

In a second embodiment of the invention illustrated in Figure 10, a flap

valve 25 controlled by an actuator 26 is arranged to selectively obstruct the

passageway between the slot 12 and the opening 24. Thus at relatively low flow

rates the flap valve 5 is open to establish a pressure differential as indicated in

Figure 4. The flow rate is sensed and used to control the actuator 26 such that

the flap valve 25 closes when the condition represented by point 15 in Figure 8 is

reached, that condition corresponding to Figure 5. Thereafter, assuming

conditions change to those represented by point 16 in Figure 8 the pressure

distributions correspond to that shown in Figure 6 except for the fact that there

would be no net flow through the slot 12.

On compressors of relatively large physical size more than one slot could

be used. Such an arrangement is illustrated in Figure 11. In Figure 11 two slots

27, 28 are provided in the surface 8 which is swept out by the radially outer

edges 7 of the impeller wheel 2. The slot 27 communicates with a passage 29 and

the slot 28 communicates with a passage 30. These two passages may in turn communicate with a common opening located adjacent a venturi upstream of the

impeller wheel or alternatively may communicate with separate openings located

at different positions in a venturi throat. Pressures would be supplied to the

openings that would be less than the pressure immediately upstream of the

impeller wheel. In such an arrangement the pressure in passageway 29 would

typically be less than ambient while the pressure in passageway 30 would be

higher than in passageway 29 but less than in the opening in wall 8 defined by

the slot 28. Other options would be to arrange for the pressure in passageways

29 and 30 to be greater than that within the blade outer edge (edge 7 in Figure 1).

The passageway 30 could receive gas from a different source as compared

with the passageway 29. For example the passageway 30 could be connected to

receive exhaust gas from the turbine inlet of a turbocharger, the exhaust gas

being delivered to passageway 30 through a cooler.

Figure 12 illustrates an alternative embodiment of the invention for use in

applications where it is desired to achieve a large displacement of the surge line

in low flow conditions without unduly restricting the gas flow in high flow

conditions. As shown in Figure 12, a venturi is defined by an annular passage 31

which extends around a circular section passage 32 within which a butterfly

valve 33 is positioned. In high flow conditions the butterfly valve is open as

shown so that the passage 31 bypasses the venturi and the pressure in the slots 12

is as a result close to atmospheric. In contrast, in low flow conditions, the

butterfly valve is closed to direct air through the venturi and thereby apply a reduced pressure to the slots 12. Thus in the arrangement of Figure 12 the

venturi differential can be matched to the required position of the surge line

without unduly restricting flow in higher flow conditions.

Figure 7 suggests that, in high flow rate conditions, increasing the

pressure in the slot 12 of a conventional arrangement such as that shown in

Figure 1 would be expected to result in increased flow that should manifest itself

in a displacement towards the right of the choke flow limit 18 of Figure 2. This

would be highly desirable. However, tests have shown that little benefit can be

gained if the slot 12 is directed towards the wheel axis. It has been discovered

however that if the slot is arranged to extend in a direction with a substantial

axial component, for example in a direction inclined by from 10 to 15 degrees to

the axis, the choke flow line can be displaced in a significant manner.

Figure 13 illustrates an embodiment of the invention with axially-directed

slots 12 communicating with an annular passageway 13. The slots 12 face edges

34 of the blades which extend radially between radially outer edges 35 and 36

such that each blade has a step shape. The edge 35 sweeps across an inner

surface of a tubular sleeve 37 which defines one side of the passageway 13. With

the arrangement of Figure 13, choke capacity may be increased when the

passageway is pressurised without adversely affecting the surge line position.

Figure 14 illustrates the performance that can be achieved with the

arrangement of Figure 13. Lines 38 and 39 respectively indicate the surge line

and choke flow conditions achieved with a conventional arrangement as shown in Figure 1. Lines 40 and 41a represent performance with a structure as shown

in Figure 13 without the passageway 13 being pressurised. Applying a pressure

of 1.2 bar abs to the passageway displaced the surge line to the position indicated

by line 38 and displaced the choke line to the position indicated by line 41b.

Application of a still higher pressure of 1.4 bar abs did move the surge line at

mid-range speeds but not at higher speeds. Too high a pressure causes a

relatively large proportion of the flow to come from the pressurising source,

resulting in increased parasitic losses, in addition to demanding a high power

pressurising source. As is apparent from Figure 7, the slot should be near to the

upstream edge of the wheel to minimise the pressure required for a given flow

through the slot. It is believed that the combination of a stepped wheel as in

Figure 13 and pressurisation of the passageway 13 should enable the use of a

larger than normal profile downstream of the step, thereby increasing the

maximum flow for a given size of wheel.

The arrangement of Figure 13 makes it possible to achieve desirable

displacements of both the surge and choke flow positions. In some circumstances

it is desirable only to displace the choke flow, and an arrangement which can

achieve this is shown in Figure 15 which shows a passageway 13 defined between

an inlet sleeve 42 and a tubular housing 43. The passageway 13 communicates

with an annular cavity 44 via openings 45 in the housing 43. The compressor

wheel (not shown) is positioned with the upstream edges of its blades adjacent

the upstream edge 46 of openings 47 positioned adjacent the wheel. Air from the passageway 13 is directed towards the wheel through these openings in a

directions with a substantial axial component. In use, in high flow conditions

pressure is applied to the cavity 44 to increase the flow through the compressor

and hence displace the choke flow to the right in the manner indicated by lines

39 and 41 of Figure 14. This arrangement also has the advantage of increasing

the pressure ratio at a given speed and flow compared with standard

arrangements.

With axially directed slots as shown in Figures 13 and 15, pressure may

be applied only at flows greater than the normal surge flow. The application of

pressure over a wide range of flow values may provide benefits however. For

example, even at the maximum efficiency point gas will flow into the wheel if the

pressure is high enough.

In the embodiments of Figures 13 and 15, it has been found that it is

important to arrange the openings such that air is directed with a substantial

axial component. This has revealed however a useful effect which can be

achieved by directing air in a direction with a substantial radial component in

low flow conditions. Figure 16 shows a further embodiment of the invention

which takes advantage of this effect.

The arrangement of Figure 16 is similar to that of Figure 1 except that the

inlet wall 9 is extended. In low flow conditions the passageway 13 is connected to

a source of pressurised air (not shown) rather than communicating with the inlet.

In low flow conditions the passageway 13 is pressurised, and as a result an air flow indicated by line 48 is established. This has the effect of blocking air flow to

radially outer portions of the upstream edges 5 of the blades, compressing the air

flow as indicated by lines 49. Thus the inlet diameter of the compressor can be

effectively reduced and as a result the surge line can be displaced as desired. The

extent of the displacement can be controlled by controlling the pressure within

the passageway 13. A side effect of pressurising the passageway 13 is that the

noise which can result from reverse flow through the slots is suppressed. Thus in

a given application a larger inlet diameter may be used than would otherwise be

the case, resulting in an increase in choke flow as compared with a standard size

inlet.

Figure 17 represents the displacement of the surge line which can be

achieved with the arrangement of Figure 16. Line 50 represents the surge line if

the passageway 13 is not pressurised, and line 51 represents the surge line when

the passageway 13 is pressurised.

Thus the invention enables desirable displacement of the surge line in low

flow conditions either by drawing air from the compressor through slots directed

at a wide range of angles relative to the wheel axis, or by delivering air to the

compressor through slots directed with a substantial radial component relative

to the wheel axis. The invention also enables a desirable displacement of the

choke flow position in high flow conditions by delivering air to the compressor

through slots directed with a substantial axial component relative to the wheel

axis. Although in the description reference is made to "slots", it will be

appreciated that the passageway opening in the surface 8 swept by the impeller

blades could be in the form of a series of spaced-apart openings.

Claims

1. A compressor comprising a stationary housing defining an inlet and an

outlet, and an impeller wheel mounted for rotation within a chamber

defined by the housing between the inlet and outlet, the wheel including a

plurality of blades each of which sweeps across an inner surface of the

housing chamber, and the housing defining at least one passageway

communicating with at least one opening located in the surface of the

housing chamber adjacent the wheel, wherein pressure control means are

provided to control the pressure within the passageway.

2. A compressor according to claim 1, wherein the pressure within the

passageway is controlled as a function of the flow rate through the

compressor.

3. A compressor according to claim 2, wherein the pressure control means

maintains a pressure within the passageway less than that within the inlet

at least when the flow rate is low relative to a predetermined flow rate.

4. A compressor according to claim 3, wherein the passageway extends

between the opening adjacent the wheel and a further opening located in

the throat of a venturi within the inlet.

5. A compressor according to claim 4, comprising means for blocking the

passageway when the flow rate is high relative to a predetermined flow

rate.

6. A compressor according to claim 4 or 5, comprising a bypass passageway

extending between opposite sides of the venturi, and means for

obstructing the by pass passageway when the flow rate is low relative to a

predetermined flow rate.

7. A compressor according to any preceding claim, wherein the passageway

adjacent the opening is arranged such that gas issuing from the opening is

directed in a direction with a substantial axial component relative to the

axis of rotation of the wheel, and the pressure control means maintains a

pressure within the passageway which is higher than that within the inlet

at least when the flow rate is high relative to a predetermined threshold.

8. A compressor according to claim 7, wherein the inlet is defined by a

tubular cone received within a tubular support, the passageway being defined between the cone and the housing, the opening being defined

between one end of the cone and the housing, and the passageway

communicating with the pressure control means through at least one

opening in the tubular support.

9. A compressor according to any preceding claim, wherein each blade

comprises an upstream edge adjacent the inlet, a first radially outer edge

which is in close proximitΛ' to and sweeps across the inner surface of the

housing, a second radially outer edge which is in close proximity to and

sweeps across a surface defined by the inlet, and an intermediate edge

extending between the first and second radially outer edges, the opening

facing the intermediate edges of the vanes.

10. A compressor according to claim 1 , wherein the passageway adjacent the

opening is arranged such that gas issuing from the opening is directed in a

direction with a substantial radial component relative to the axis of

rotation of the wheel, and the pressure control means maintains a

pressure within the passageway which is higher than that within the inlet

only when the flow rate is low relative to a predetermined threshold.

11. A compressor substantially as herein before described with reference to

Figure 9, 10, 11, 12, 13, 15 or 16 of the accompanying drawings.