GB1599908A - Centrifugal pumps - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO CENTRIFUGAL PUMPS (71) We, ROLLS-ROYCE LIMITED, a British Company of 65 Buckingham Gate, London, SWIPE 6AT, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to rotary pumps of the centrifugal type for pumping liquids or a liquid and gas mixture. Typically such pumps can be used as aircraft engine fuel pumps.

Known centrifugal pumps essentially comprise a housing having inlet and outlet passages and a bladed impeller mounted for rotation in the housing, the blades being substantially radially arranged. The blades are relatively thin and between them, adjacent flow passages are defined which increase in circumferential width towards the rim of the impeller whilst in the axial sense, the depth of the passages decreases towards the impeller rim.

In these types of known pumps, there is a region of flow separation on the low pressure (low energy) side of the passages. This is associated with a high energy flow on the other side of the passages which combine to produce "jet-wake" flow at the impeller outlet. The nett effect can be a gross recirculation of flow within the blade passage.

Normally, this type of flow is not detrimental in terms of stability but does cause, in the cases of rotors having low blade numbers, low impeller head factors.

The extent of the recirculations depend on the degree of fluid guidance in the rotor passages. Thus impellers with few blades (say 3-5) are almost "filled" with recirculation vortices, whereas by going to large blade numbers (say 2040) impellers have much improved "guidance" and usually have only small vortices just in the tip regions.

The presence of these vortices usually only affects head factor and efficiency, but in the case of aircraft pumps operating on gasliquid mixtures, the effects are far more significant.

When a conventional low specific speed pump is run partially aerated the flow pattern is seen to change as follows: a) At the inner end of each blade, gas starts to collect in the low pressure. zones, giving the effect cf cavities which stay relatively fixed to each blade. Low pressure zones are always present at the inner radius of the blades, either due to the presence of trapped vortices or, more usually, to the large mis-match of inlet angle to fluid angle.

This is particularly bad in low specific speed pumps as used for aircraft fuel system duties, where the radial flow velocities, relative to the speeds of rotation, are very low giving relative inlet angles of the order 5". When the fuel flow is reduced as a function of altitude, this angle becomes even smaller, possibly down to 10. As the gas to liquid ratio increases, these cavities become generally larger and move outwards, occupying more volume both radially and circumferentially.

Eventually they spread into the zone influenced by the recirculations previously described. This carries some of the gas even further outwards, but most of it remains in these vortices.

b) Whilst all the above is going on, and because of the fact that large "holes" are formed in the impeller passages, there is an associated gradual reduction of delivery pressure from the pump. This may be a genuine reduction of generated head, but is possibly related to the general increase of fluid losses caused by the changes in flow patterns within the blade passages. This pro patterns within the blade passages. This process also reduces the average pressure in the outlet passage, usually a volute surrounding the impeller rim.

c) Except at one flow condition and not even then in many cases, there is a cir cumferential variation in static pressure around the volute casing, it being lowest in the region associated with the delivery section.

When this effect is combined with the conditions described above in the impeller, the result is to shift the centre of rotation of the orbiting gas vortices towards the delivery area of the volute. Eventually, with only slight increase of gas-to-liquid ratio, this eccentric flow field allows one or more of the recirculation vortices to traverse the line between impeller outlet and the volute.

d) Up to this point there had been a relatively steady transfer of energy from shaft to impeller blade and from the blades to the pumped fluid. This also meant that since no further energy was added downstream of the impeller, all parts of the flow downstream of this point were "downhill" in an energy sense. This is stable and, except for small regions in the recirculation zones, the flow is predominantly unidirectional across the impeller outlet plane. However, when the eccentric orbit system reaches the final state described, there are "gaps" in the impeller energy outflow field and into these, fluid from higher energy regions in the volute can and does flow. The result is to cause large delivery pressure oscillations and eventually the static pressures fall so far that the previously entrapped air expands and collects in the impeller.This completely stalls the pumping process and throughflow ceases for all practical purposes.

It may thus be concluded that the developing stages of pressure collapse in an aerated impeller are exaggerated by the eccentric flow/pressure field in the volute and that much of the instability found in the later stages of the process is associated with the joining of a gas core from a position within the blading to a zone in the volute and the reversal of flow accompanying this.

The result of the flow and pressure instability described above is that in a known type of impeller pump operating at a constant speed, as the amount of gas in the gas and liquid mixture being pumped is increased, the pressure rise through the pump will firstly decrease relatively smoothly until the volume ratio of gas to liquid reaches a value of about 0.2 when the instability will set in and the pressure rise value will fluctuate rapidly as more gas is added. The instability causes considerable vibration which imposes a practical limit on the ratio of gas and liquid that the pump can handle.

The present invention seeks to provide a centrifugal pump which can pump a wide range of gas and liquid mixtures without producing flow and pressure instability and the associated mechanical vibrations caused thereby.

The present invention provides a centrifugal pump comprising an impeller mounted for rotation in a housing, fluid inlet and discharge means into and from the impeller housing, the impeller having a plurality of substantially radially arranged blade members which between them define a plurality of fluid passages extending in a generally radial direction, each of said fluid passages com prising a radially inner circumferentially relatively wide portion and a radially outer circumferentially relatively narrow portion, each said outer portion comprising a fluid nozzle arranged to discharge fluid in a generally radial direction, the fluid nozzles being spaced around the rim of the impeller and separated from each other by the blade members.

The inner and outer passage portions may each be of constant width in the circumferential sense and may be shaped as arcs for ease of manufacture.

The outer passage portion can have a length to circumferential width ratio of the order 2:1 and a cricumferential width to axial depth ratio in the range 0.5:1 to 2:1.

The ratio between the inner passage portion and the outer passage portion circumferential width is of the order 3 :1.

The centre-lines of the inner and outer passage portions may be coincident or the centre-line of the outer passage portion may lie behind that of the inner passage portion with respect to the direction of rotation of the impeller.

The present invention will now be more particularly described with reference to the accompanying drawings in which: Fig. 1 shows a part plan view in a diagram matic form of one form of centrifugal pump according to the present invention, Fig. 2 is an elevation in a diagrammatic form of the pump shown in Fig. 1, Fig. 3 is a view in the direction of arrow "A" in Fig. 1, Fig. 4 is an enlarged view of one of the fluid passages of the pump shown in Fig.

1, Fig. 5 is an enlarged view of an alternative form of fluid passage to that shown in Fig. 4, Fig. 6 is a part plan view of a further form of centrifugal pump according to the present invention, Fig. 7 is an elevation of the centrifugal pump shown in Fig. 6, Fig. 8 is a view in the direction of arrow 'B' in Fig. 6, Fig. 9 is a view to a larger scale of part of the centrifugal pump shown in Figs. 6 to 8, Fig. 10 is a part plan view of an amended form of the centrifugal pump shown in Fig.

6 and, Fig. 11 is a view of the inlet to the centrigufal pump shown in Fig. 7 with an inlet valve.

Referring to the Figs. a centrifugal pump 10 comprises a housing 12, a shrouded impeller 14 having a shroud 1S, the impeller 14 being rotatably mounted on a shaft 16 within the housing, a fluid inlet 18 and a discharge means 20 in the form of an outlet volute chamber which surrounds the rim of the impeller 14.

The impeller comprises a plurality of blade members 22 (only some of which are shown) which extend from the centre to the rim of the impeller in a radial manner and which between them define a plurality of fluid passages 24.

Each fluid passage 24 comprises a radially inner relatively wide, in the circumferential sense, portion 24a and a radially outer relatively narrow, in the circumferential sense, nozzle-like portion 24b, the latter terminating in a discharge 26, the centre-lines of the inner and outer portions being coincident.

Between each discharge 26, the rim of the impeller has scallops 28 to give each discharge a sharp edged perimeter.

In operation the impeller 14 is rotated and the fluid to be pumped which may be wholly liquid or comprise a gas and liquid mixture enters the fluid passages 24 in the impeller ma the inlet 18 and is discharged into the volute 20 ma the nozzle-like passage portions.

Referring to Figs. 6 to 9, the centrifugal pump shown in these Figs., is analogous to the pump shown in Figs. 1 to 5 and similar constructional features have been given the same reference numbers.

The impeller 14 has a central conical spinner 50 and the passages 24 are curvilinear both in elevation (Fig. 7) and plan (Fig. 6). The blade members 22 which define the passages 24 have a slight chamfer 52 and the shroud 15 is attached to the impeller by screws 54. Fluid leakage from the gap 56 between the shroud and the housing 12 is prevented or minimised by a ring 58 which is let into the shroud 15 and which co-operates with three sealing rings 60 let into the housing 12. Preferably, the blade passages are curved as shown in Fig. 6 but the pump may work if the passages 24 are radially arranged as shown in Fig. 10.

Also, it is preferable to arrange an inlet valve 62 (see Fig. 11) as close to the inlet to the pump as convenient so that when the valve 62 is opened fluid is available immediately rather than having to wait for fluid to be supplied along a length of ducting upstream of the pump.

The inner and outer passage portions may each be of constant width in the circumferential sense and may be shaped as arcs for ease of manufacture.

The outer passage portion can have a length to circumferential width ratio of the order 2:1 and a circumferential width to axial depth ratio in the range 0.5:1 to 2:1.

The ratio between the inner passage portion and the outer passage portion circumferential widths is of the order 3:1 As has already been outlined, in known centrifugal pumps under certain conditions, when gas and liquid mixtures are being pumped, the gas and liquid phases tend to separate out with liquid (jet) being on the rearward side of the passage with reference to the direction of rotation of the impeller and the gas (wake) being on the forward side. Considerable recirculation of flow takes place in the blade passages and there is sometimes a flow between adjacent blade passages, both of these phenomena adversely affecting the pump performance.

In the arrangement of the present invention, the provision of the nozzle-like passage portion 24b takes advantage of the jet, i.e. liquid, part of the 'jet-wake' flow field and effectively suppresses the "wake", i.e. the gas. This means that the rim of the impeller 14 then appears as a series of rectangular holes of low aspect ratio as opposed to the conventional impeller, which looks like two plates separated by short thin bridges, which are the ends of the respective blades.

It can be argued that the impeller energy is being offered to the volute 20 in patches, corresponding to the nozzle-like discharges but equally, between these patches, the worst that can occur is zero added energy as opposed to the local energy field reversals which have been previously described.

The flow from these discharges 26 is guaranteeably unidirectional whilst there is enough liquid per passage 24b to form into a jet. This is because in the discharges, some static pressure is deliberately converted into kinetic energy. Thus at impeller outlet there exists an energy gradient which effec tively prevents pressure variations in the volute 20 from influencing the impeller flow.

Thus the stability of operation of the present pump should be superior to conventional passages.

Before reaching the terminal operating point when ingesting aerated fluid, a pump is by definition pumping out nearly as much gas as it swallows. There are thought to be a variety of entrainment/solution processes in impellers but the confused nature of the flow has so far prevented much in the way of analysis. So far as can be judged the present pump has a more regularised entrainment process. This is because of the geometry the passages are nearer to parallel in width and thus in the inner sections, there is a tendency for liquid to stay in one side of the passage and gas separate above it.

This is a sort of channel flow in a radial field, the flow being held onto the blade by the inertia of the fluid. Now this flow field, which sometimes occurs in conventional impellers but not usually stably, would not be consistent with the "elimination of wakes" concept. This is where the arrangement of the outlet contraction becomes effective. Certainly in the symmetrical nozzle designs but possibly in others, the liquid appears to undergo either a hydraulic jump or some closely related sudden enlargement diffusion process, which results in the liquid filling the passage prior to passing through the nozzle.

Associated with the jump or enlargement flow is a rolling vortex system which could act as a powerful entrainment/dissolving process site, being stable also because of the nature of the passage static pressure drops caused by the nozzles.

Thus the limiting gas-to-liquid ratio of the present invention should be at least as good as conventional pumps, but the processes leading up to this should be more stable.

The invention may also be applied to the pumping of non-aerated liquids when the pump inlet pressure level is reduced to such a point that the cavities forming on the blades consist of vapour zones and a flow field similar to that originally described can occur.

If the inlet pressure is still further reduced (and all this in a de-aerated liquid or weathered fuel, where problems of air release are virtually non-existent), these cavities grow larger, eventually spanning most of the inlet passage to give a picture similar to the well aerated one previously described. Again the liquid flow becomes a thin layer on the driving side of the blade and some form of sudden enlargement/cavity collapse process occurs before the liquid fills the passages. There is an accompanying reduction in delivery pressure (as before) and this is both controllable and exploitable. Devices which do this are termed vapour-core" pumps and are being used in parts of aircraft fuel systems. When large, intentional reductions of delivery pressure are caused by inlet pressure reduc tion, large vapour cores are formed.These eventually become unstable exactly as in the air/liquid case. One remedy is to use a volute of very small cross-sectional area, which tends to reduce blades to blade pressure differences but also results in poor volute efficiency.

The improved design has been observed to operate most stably up and down a delivery pressure excursion occasioned by reduction in inlet pressure, the vapour core remaining central. Again this is thought to be due to the pressure drop in the nozzles.

The nett result of operating this impeller as a vapour-core pump is therefore two fold.

a) The delivery pressure is more stable as a function of core size.

b) High efficiency volutes can be used without loss of stability, which leads to reduced fuel heating in the aircraft system applications, which occurs as the fuel is pumped.

An associated advantage of reducing re- - circulation in the tip regions is to reduce the unwanted extra torque impressed onto the rotor by flow which leaves the periphery, flows back into the volute and slows down by friction, then re-enters the rotor in a low energy region. The result of this torque increment is inevitably a higher energy absorption into the fluid, which appears as heat.

Thus by reducing this recirculation, the temperature rise across the pump should be usefully reduced. In some machines it is known that this is achieved by using close fitting volutes or physically changing the volute inlet area and hence the nozzle rotor concept, being simple and passive, offers advantages.

The preferred geometry of each blade passage is that both portions 24a, 24b are paralllel-sided in plan-form and whilst the inner portion 24a can reduce in depth in axial sense towards the rim of the impeller, the axial depth of the outer portion can remain constant. This outer passage portion should have a section at the discharge end where the passage length/width ratio is of the order 2:1, this being associated with a ratio of circumferential width to axial depth in the range 0.5:1 to 2:1.

The ratio between the circumferential widths of the inner and outer passage portions should be of the order 3 : 1 and this ratio is approximately the same as the area ratio change between the two passage portions, which will in general be slightly greater due to the taper of the housing.

The blades can with advantage be raked forwards on their inner portions to give the effect of a row of inlet blades set at a favourable angle to the inlet flow field. Circular arcs have been found to be entirely satisfactory for this purpose.

The relative positon of the outer nozzlelike passage portion 24b with respect to the centre-line of the inner passage portion is important.

Preferably the two centre-lines should be coincident but as shown in Fig. 5, the outer passage portion centre-line can lie behind the inner passage portion centre-line with respect to the direction of rotation of the impeller.

The scallops 28 between adjacent discharges 26 can be omitted and the impeller 14 can be unshrouded or partially shrouded with a shroud extending over the passage portions 24b and only partly over the passage portions 24a.

WHAT WE CLAIM IS: 1. A centrifugal pump comprising an impeller mounted for rotation in a housing, fluid inlet and discharge means into and from the impeller housing respectively, the impeller having a plurality of blade members extending to the periphery of the impeller from -the central portion of the impeller, the blade members defining between them a plurality of fluid passages extending in a generally radial direction, each of said

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. a hydraulic jump or some closely related sudden enlargement diffusion process, which results in the liquid filling the passage prior to passing through the nozzle. Associated with the jump or enlargement flow is a rolling vortex system which could act as a powerful entrainment/dissolving process site, being stable also because of the nature of the passage static pressure drops caused by the nozzles. Thus the limiting gas-to-liquid ratio of the present invention should be at least as good as conventional pumps, but the processes leading up to this should be more stable. The invention may also be applied to the pumping of non-aerated liquids when the pump inlet pressure level is reduced to such a point that the cavities forming on the blades consist of vapour zones and a flow field similar to that originally described can occur. If the inlet pressure is still further reduced (and all this in a de-aerated liquid or weathered fuel, where problems of air release are virtually non-existent), these cavities grow larger, eventually spanning most of the inlet passage to give a picture similar to the well aerated one previously described. Again the liquid flow becomes a thin layer on the driving side of the blade and some form of sudden enlargement/cavity collapse process occurs before the liquid fills the passages. There is an accompanying reduction in delivery pressure (as before) and this is both controllable and exploitable. Devices which do this are termed vapour-core" pumps and are being used in parts of aircraft fuel systems. When large, intentional reductions of delivery pressure are caused by inlet pressure reduc tion, large vapour cores are formed.These eventually become unstable exactly as in the air/liquid case. One remedy is to use a volute of very small cross-sectional area, which tends to reduce blades to blade pressure differences but also results in poor volute efficiency. The improved design has been observed to operate most stably up and down a delivery pressure excursion occasioned by reduction in inlet pressure, the vapour core remaining central. Again this is thought to be due to the pressure drop in the nozzles. The nett result of operating this impeller as a vapour-core pump is therefore two fold. a) The delivery pressure is more stable as a function of core size. b) High efficiency volutes can be used without loss of stability, which leads to reduced fuel heating in the aircraft system applications, which occurs as the fuel is pumped. An associated advantage of reducing re- - circulation in the tip regions is to reduce the unwanted extra torque impressed onto the rotor by flow which leaves the periphery, flows back into the volute and slows down by friction, then re-enters the rotor in a low energy region. The result of this torque increment is inevitably a higher energy absorption into the fluid, which appears as heat. Thus by reducing this recirculation, the temperature rise across the pump should be usefully reduced. In some machines it is known that this is achieved by using close fitting volutes or physically changing the volute inlet area and hence the nozzle rotor concept, being simple and passive, offers advantages. The preferred geometry of each blade passage is that both portions 24a, 24b are paralllel-sided in plan-form and whilst the inner portion 24a can reduce in depth in axial sense towards the rim of the impeller, the axial depth of the outer portion can remain constant. This outer passage portion should have a section at the discharge end where the passage length/width ratio is of the order 2:1, this being associated with a ratio of circumferential width to axial depth in the range 0.5:1 to 2:1. The ratio between the circumferential widths of the inner and outer passage portions should be of the order 3 : 1 and this ratio is approximately the same as the area ratio change between the two passage portions, which will in general be slightly greater due to the taper of the housing. The blades can with advantage be raked forwards on their inner portions to give the effect of a row of inlet blades set at a favourable angle to the inlet flow field. Circular arcs have been found to be entirely satisfactory for this purpose. The relative positon of the outer nozzlelike passage portion 24b with respect to the centre-line of the inner passage portion is important. Preferably the two centre-lines should be coincident but as shown in Fig. 5, the outer passage portion centre-line can lie behind the inner passage portion centre-line with respect to the direction of rotation of the impeller. The scallops 28 between adjacent discharges 26 can be omitted and the impeller 14 can be unshrouded or partially shrouded with a shroud extending over the passage portions 24b and only partly over the passage portions 24a. WHAT WE CLAIM IS:

1. A centrifugal pump comprising an impeller mounted for rotation in a housing, fluid inlet and discharge means into and from the impeller housing respectively, the impeller having a plurality of blade members extending to the periphery of the impeller from -the central portion of the impeller, the blade members defining between them a plurality of fluid passages extending in a generally radial direction, each of said

fluid passages comprising a radially inner circumferentially relatively wide portion and a radially outer circumferentially relatively narrow portion, each said outer portion comprising a fluid nozzle arranged to discharge fluid in a generally radial direction, the fluid nozzles being spaced apart around the rim of the impeller and separated from each other by the blade members.

2. A pump as claimed in claim 1 in which the inner and outer passage portions are each of a different constant width respectively in the circumferential sense.

3. A pump as claimed in claim 1 in which the centre-lines of the inner and outer passage portions are coincident and the centre-lines form an arc of a circle.

4. A pump as claimed in claim 1 in which in the centre-line of the outer passage portion lies behind that of the inner passage with respect to the direction of rotation of the impeller.

5. A pump as claimed in claim 1 in which rhe ratio between the length and the circumferential width of the outer passage portion is of the order 2:1, and the ratio between the circumferential widths of the inner and outer passage portions is of the order 3 :1.

6. A centrifugal pump constructed and arranged for use and operation substantially as herein described with reference to and as shown in Figs. 1 to 5, Figs. 6 to 9 and as modified by the arrangements shown in Figs. 10 and 11.