GB2074662A - Rotary Impeller for a Centrifugal Pump - Google Patents

Abstract

The impeller 11 is particularly adapted to enable it to pump mixed flows of gas and liquid such as fuel in an aircraft fuel tank. The impeller comprises a rotor having a plurality of flow passages 20 extending from an inlet eye 21 to the rotor periphery. Each flow passage 20 comprises an inlet duct 22 of substantially constant or convergent cross-section and initially extending at an angle which may be between 40 and 75 DEG to the tangent to the inlet eye. The inlet duct opens into an exit duct 23 which diverges such that at their outlets, the passages occupy most of the peripheral circumference of the rotor. The right hand portion of the Figure shows modifications of the inlet duct. <IMAGE>

Description

SPECIFICATION Rotary Impeller for a Centrifugal Pump This invention relates to a rotary impeller for a centrifugal pump. The impeller is particularly suitable for use in a backing pump which is used in the transfer of fuel from the fuel tanks of an aircraft to the gas turbine engines which propel it.

One problem which has arisen with backing pumps of this nature lies in the necessity under some conditions for the pump to operate with very low absolute inlet pressure. Under these conditions the solubility of air and vapour in the fuel may be such as to cause the flow into the pump to be a mixed flow of fuel and air or other gas. This mixed flow tends to inhibit the operation of normal centrifugal pumps because they cannot effectively pump the gaseous component.

We have identified ways in which the gas pumping capacity of an impeller may be optimised and the present invention relates to an impeller whose capacity to pump gas is improved compared with the prior art but which still retains a good pumping efficiency.

According to the present invention a rotary impeller for a centrifugal pump comprises a rotor having formed therein a plurality of flow passages each extending from an inlet eye formed about the axis of the rotor to the outer rotor periphery, each passage comprising an entry duct of substantially constant or convergent cross section extending at a high angle to the tangent and an exit duct which diverges in the plane of the rotor from its junction with the inlet duct so that at their outlets the passages occupy most of the peripheral circumference of the rotor.

In one embodiment the inlet eye comprises a cylindrical cavity coaxial with the rotor and the inlet to the passages is formed by the intersection of the passages and the cylindrical surface of the eye. Alternatively, the inlet ducts may be arranged to intersect to form smoother inlets for the fluid into the passages. As a further alternative the lip of the inlet duct may be deformed towards the tangent so as to provide a smooth inlet which may locally constrict the duct.

We prefer to use an inlet duct extending at an angle in the range 450 to 750 and which may comprise 600 to the tangent and the overall passage shape may be formed by one wall comprising a curve of constant angle to the tangent and a second wall which cooperates with the first wall to form the inlet and exit ducts.

The invention will now be particularly described with reference to the accompanying drawings in which: Figure 1 is an axial cross-section through a centrifugal pump having an impeller in accordance with the invention and, Figure 2 is a composite section on the line 2 2 showing three dimensions.

In Figure 1 there is shown a centrifugal pump which is suitable for instance to be used as the backing pump in the fuel supply system for a gas turbine engine. The pump consists of a casing 10 within which operates a rotary impeller 11. The casing 10 consists of an inlet passage 12, an impeller chamber 13 and an outlet diffuser passage 14. The impeller chamber 13 is formed with a disc shaped cavity therein within which rotates the closely fitting and correspondingly disc shaped impeller 11. The impeller 11 is driven from a shaft 1 6 which is formed in the present case integrally with the impeller. The shaft 1 6 is carried in a bearing 1 7 from the housing 10, the bearing 1 7 also providing sealing means for the prevention of leakage from the pump.It will be appreciated that although for simplicity the embodiment described shows a disc-shaped impeller with passages lying in a plane perpendicular to the rotor axis, other shapes are possible. Thus in particular it would be possible to have the passages lying on the surface of a core, or other body of revolution.

In general operation of the pump is conventional in that the pumped fluid entering the inlet 12 is pumped in a radially outwardly direction by rotation of the impeller 11 and is diffused through the passages 14 to its location of use.

When the flow of fluid to the inlet 12 is simply liquid without any substantial free gas it is only necessary to design the impeller 11 to avoid cavitation induced by sharp accelerations or changes in pressure of the liquid. However, where the flow into the inlet 12 has a considerable gas content the gas, (which may include air and vapour) may separate from the liquid and in this case the impeller must be designed to pump the gas as well as the liquid.

Figure 2 shows how the impeller of the present invention is designed to allow gas pumping to occur. It should be noted that Figure 2 is a composite view and that only the left-hand half of the impeller shown relates to the first embodiment. The right-hand half of the drawing is split into upper and lower portions which show second and third embodiments of the invention respectively. Referring first to the embodiment shown in the left-hand half of the drawing the impeller 11 consists of a backing rotor disc 1 8 from which project the impeller blades 19.

Although in this specification we have used conventional nomenclature and referred to the impeller blades 19, it is in fact more instructive and more useful from the design point of view to consider the shape of the fluid flow passage between the impeller blades 1 9 and generally indicated at 20.

The blades 19 define first of all an inlet eye 21 at the centre of the disc 1 8. This eye comprises a cavity of cylindrical shape and is coaxial with the disc 18 and hence the impeller 11. From the eye 21 extends a plurality of entry ducts 22 each being of substantially constant cross-section and extending initially at an angle 6 to the tangent to the surface of the inlet eye at the intersection with the duct. It will be seen that the duct 22 does not extend in a straight line but lies on part of an involute curve. In the present instance the angle 6 is 600, but we believe that satisfactory operation of the pump may be achieved with any angle in the range 450--750.

Each entry duct 22 extends over approximately half of the radial extent of the backing disc 11 until it breaks into a diverging outlet duct 23. The duct 23 diverges sharply in such a way as to cause the outlets from adjacent passages to be divided from each other only by the relatively thin extremities of the blades 1 9, that is the outlet portions of the passages take up the majority of the circumferential periphery of the impeller.

Again it will be noted that the upstream face 24 of each passage 20 is formed by a continuous curve and that the variation between the entry ducts and exit ducts is produced by shaping the downstream face 25 of each passage in the form of a dog leg. This is a relatively simple way of defining the required passage shape, however, it should be appreciated that other shapes of passage faces could be used. In particular the outer parts of the blades could be shaped to provide the desired outlet blade angle for the pump. In the present instance the curve of the upstream face is one which provides a constant intersection angle with the local tangent.

Operation of the impeller described is that the relatively high angle 6 of the entry duct provides relatively high acceleration of the fluid entering the pump. Consequently a high velocity of flow is quickly reached as the fluid passes through the entry ducts 22. Such a high velocity is conducive to efficient gas entrainment and consequently any pocket of gas existing in the initial portions of the entry ducts 22 will become entrained in the liquid in a relatively short radial distance. It is in fact preferred if the final entrainment of the gas into the liquid occurs just before the transition between the entry duct 22 and the exit duct 23.

Once the gas has been entrained in the duct 22 it becomes possible to allow the impeller to operate in a manner more nearly approaching that of a conventional impeller. Therefore the divergent portion 23 is provided in which the liquid velocities may be maintained at a relatively low level and in consequence the fluid friction on the surfaces of the duct 23 is kept at a minimum.

The overall operation of the second and third embodiments illustrated in the upper and lower portions of the right-hand part of Figure 2 is the same as that described above. However, in order to maintain the normal operation of the pump as cavitation free as possible it may be necessary to provide a smoother inlet from the eye 21 into the entry ducts 22. In the second embodiment the inlets to the ducts 22 are formed by the intersections of these ducts forming lips where shape is modified by providing shaped projections 26 from the upstream face of each of the ducts 22. These projections effectively form deformed portions of the lip and are deformed toward the tangent to the eye. In this way a smoother entry is obtained but at the expense of some local reduction is cross-section area of the inlet to the ducts 22.The third embodiment simply allows the adjacent ducts 22 to intersect one another at their junction with the eye 21 thus forming a sharp lip at 27. Again this provides a smoother entry but without any substantial restriction of the duct area.

It should also be understood that the entry ducts 22 may require to be of convergent section rather than the constant cross-section described.

Although the impeller of the invention has been described above in connection with a particular type of pump housing it will be appreciated that there will be a wide variety of housing shapes with which the impeller could be used. Again, although the invention is particularly useful in a backing pump in the fuel system of a gas turbine engine it could in fact be used in any conditions where there was an expectation of having to pump a mixed flow of (;quid and gas.

Claims (8)

Claims

1. A rotary impeller for a centrifugal pump comprising a rotor having formed therein a plurality of flow passages each extending from an inlet eye formed about the axis of the rotor to the outer rotor periphery, each passage comprising an entry duct of substantial constant or convergent cross-section extending at a high angle to the tangent and an exit duct which diverges in the plane of the rotor from its junction with the inlet duct so that at their outlets the passages occupy most of the peripheral circumference of the rotor.

2. A rotary impeller as claimed in claim 1 and in which said inlet eye comprises a cylindrical cavity coaxial with the rotor and the inlet to said entry ducts comprises the intersection of the ducts with the surface of the eye.

3. A rotary impeller as claimed in claim 1 and in which the inlets to said entry ducts are formed by the intersection of two adjacent ducts.

4. A rotary impeller as claimed in claim 3 and in which the lips formed by said intersections are deformed towards the tangent to provide a smoother entry for fluid and a local reduction in a passage area.

5. A rotary impeller as claimed in any one of the preceding claims and in which said entry duct initially extends at an angle of between 450 and 750 to the tangent.

6. A rotary impeller as claimed in any one of the preceding claims and in which said entry duct initially extends at an angle of substantially 600 to the tangent.

7. A rotary impeller as claimed in any one of the preceding claims in which each said passage is formed between two surface one of which is disposed on a continuous curve and the other of which is of dog leg shape to define said entry and exit ducts.

8. A rotary impeller substantially as hereinbefore particularly described with reference to the accompanying drawings.