GB2168764A - Centrifugal pump impellers - Google Patents

Abstract

A rotary impeller (200) for a centrifugal pump is capable of efficiently pumping fluids comprising mixed gaseous, vapour and liquid components, e.g. aviation fuel (204) with air and vapour bubbles (202) in it caused by low pressures in the fuel line. Approximately aerofoil-shaped impeller blades (208) define unconventionally shaped passages (206) between them, the passages being provided at their inlets with guide blades (210) which intercept the fluid flow coming from the eye of the impeller, separate out the gaseous or vapour components, and form the liquid component into a high-velocity jet sheet (214) which is projected down the passage through the separated gaseous component towards the outlet of the passage, where it collides with a liquid/gas interface (215) at the point where conventional centrifugal pumping takes over in the divergent outlet portions of the passages. The liquid jet sheet (214) entrains the collected gaseous and vapour components and the turbulence caused by the jet sheet's collision with the interface (215) causes rapid remixing of the gaseous and liquid components, whereupon the gaseous component is redissolved in the liquid component due to the pressure rise across the pump. In an alternative embodiment (Figure 3, not shown) the guide blades become suitably oriented inlet guide faces of the impeller blades. <IMAGE>

Description

SPECIFICATION Centrifugal Pump Impellers The present invention relates to centrifugal pump impellers, and in particularto an impeller capable of maintaining satisfactory operation when pumping fluids comprising a mixture of gaseous and liquid components, for example, a fuel/air mixture.

Low pressure fuel pumps, as used in the fuel systems of gas turbine aeroengine, should be capable of supplying fuel to the engine under emergency conditions, when free air and fuel vapour are present with the fuel as it enters the impeller. Such conditions will occur when the aircraft fuel tank booster pumps either cease to function or have to be switched off. The twocomponent fluid flow is caused by the resulting low pump inlet pressure from the supply line, which results in the release of dissolved air and fuel vapour from the fuel. Conventional centrifugal fuel pumps have satisfactory performance when pumping gas/liquid mixtures with a modest proportion of gas, but when required to pump mixtures with a high proportion of gas, are liable to suffer a sudden collapse of pumping capacity.

An object of the present invention is to provide a centrifugal pump impeller design which is capable of pumping a larger amount of gas and vapour per unit volume of liquid than converitional impeller designs of the same overall dimensions. For the purpose of the following statements of invention and the claims, the terms "gas" and "gaseous" should be interpreted to include "vapour" and "vaporous" within their meanings.

Accordingly, the present invention provides a rotary impellerfora centrifugal pump, the pump being capable of pumping fluids comprising mixed gaseous and liquid components, the impeller comprising a central inlet eye and a plurality of impeller blades defining passages therebetween, wherein at least the outlet portion of each passage is constructed to pump liquid in a conventional manner but the inlet portion of each passage is provided with guide blade means shaped and arranged to intercept fluid flowing out of the eye, at least partially separate the gaseous component from the liquid component so that the gaseous component collects in the inlet of the passage and the liquid component flows over the guide blade means as a liquid layer, and project the liquid layer as liquid jet sheet through the collected gaseous component towards the outlet portion of the passage, a liquid/gas interface being formed in the outlet portion of the passage between liquid being pumped conventionally and the collected gaseous component, the collected gaseous componet being entrained by the liquid jet sheet and the liquid sheet impacting the interface at a velocity sufficient to remix the entrained gas with the liquid at and beyond the interface.

Note that the concept of creating a discrete jet of liquid, in order to produce a pumping action in a centrifugal pump impeller, is contrary to the conventional approach to centrifugal pumping, which is to rely on the divergent passages between the blades to convert the kinetic energy at the passage inlets to pressure at the passage outlets whilst avoiding cavitation as far as possible.

In one embodiment of the invention, the guide blade means are structurally integral with the impeller blades. In this case the guide blade means preferably comprise an inlet face at the leading edge of the leading flank of each impeller blade, the inlet faces having an acute inlet angle and making an acute angle with the trailing flanks of the impeller blades, the inlet faces being unconformable with the adjacent parts of the leading flanks of the impeller blades such that the liquid layer flowing over the inlet face separates from the impeller blade surface and forms the liquid jet sheet.

In an alternative embodiment, the guide blade means comprise discrete guide blades spaced apart from the impeller blades and having an acute inlet angle.

The preferred range of inlet angles of the guide blade means is in the range 10-500.

In order to attain the correct passage shape, the impeller blades are approximately aerofoil-shaped in chordwise cross-section.

The passages between the impeller blades preferably comprise an inlet section incorporating the guide blade means, a middle section whose longitudinal centre-line follows an approximately spiral path to accommodate the path of the liquid jet sheet, and a divergent outlet section.

Further aspects of the invention will be apparent from a reading of the following description and claims.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure la is a plan view of a conventional fuel pump centrifugal impeller for a gas turbine aeroengine; Figure 1 b is a side elevation of the impeller of Figure 1a; Figure 2 is a view similar to Figure 1 a showing a fuel pump impeller according to the present invention; Figure 3 is a view similar to Figure 2 showing a simplified embodiment of the invention which was subjected to testing to ascertain its performance characteristics; Figure 4 is a section, taken on line A-A of Figure 3, of the simplified impeller and its surrounding casing; Figure 5 is a photograph of the pump of Figure 3 and 4 under test; and Figure 6 is a graph showing sample results of tests involving the pump of Figures 3 to 5.

Referring first to Figure 1, a conventional low pressure fuel pump impeller 100 for the fuel supply system of a gas turbine aeroengine comprises a solid disc 102 attached to a drive shaft 104 and bearing eight thin blades 106 which define passages 108 between them. These passages 108are divergent towards the periphery of the disc 102. The impeller rotates in the direction of the arrow Rand is of course housed in a casing, with shaft 104 running in bearings. The housing and bearings are not shown in Figure 1, but suitable ones are shown diagrammatically in Figures 3 and 4.

The blades 106 are in the form of thin "walls" of metal or other material, which stand up at right angles to the disc 102 and for ease of manufacture are usually cast integrally with the rest of the disc in a suitable mould. The blades 106 extend from their leading edges 110 to their trailing edges 112 in a generally spiral form, the leading and trailing edges meeting the tangents to the disc radii at those points at certain blade inlet and delivery angles 'i' and 'd' respectively. Angle i is usually in the range Q25" and angle 'd' is usually in the range 30-60'.

In operation, the fuel to be pumped enters the eye of the impeller 100 from the direction of arrow I via an intake duct (not shown, but compare Figure 4).

The fuel moves towards the outside of the central eye where it is scooped up by the leading edges 110 of the blades 106 and forced into the relatively narrow inlets of passages 108 between the blades.

The pumping action relies upon the centrifugal effect of the rotating impeller 100 on the fuel to accelerate it and upon the divergent passages 108 between the blades 106 to convert the kinetic energy of the fuel into a pressure head, the latter effect being further helped by the collector volute in the pump casing (Figure 4) into which the fuel flows from the outlets of the passages 108.

During normal operation at high altitudes of a fuel supply system incorporating a prior art low pressure fuel pump impeller as just described, the fuel system is full of fuel due to the action of the aircraft fuel tank booster pumps, and there is no free gas or vapour present at the inlet to the low pressure fuel pump. However, should the booster pumps be switched off, or fail, then the lowered pressure in the supply line from the booster pumps will cause dissolved gas plus fuel vapourto be released from the fuel. Unfortunately the presence of much gas and vapour as bubbles in the fuel affects ths performance of a low pressure fuel pump incorporating an impeller 100 as described above, and can lead to a collapse of its pumping ability and consequent fuel starvation of the engine.To ensure that the engine continues to produce power, a low pressure fuel pump impeller is required which can still produce an adequate pressure head in the collector volute even when fed with fuel containing the maximum proportion of gas and vapour likely to be encountered.

Referring now to Figure 2, there is shown an impeller design according to the invention which is capable of pumping a greater amount of gas and vapour together with the fuel than the impeller shown in Figure 1. It will be seen that the difference between impeller 100 of Figure 1 and impeller 200 of Figure 2 lies in the design of the blades. To show the mode of operation of impeller 200, the fluid flow in one section of the impeller is illustrated.

Again, the impeller receives the mixed component fluid at its eye region, bubbles of air and fuel vapour 202 being present throughout the liquid fuel 204. As in the conventional impeller 100, the centrifugal effect throws the mixed fluid to the outside of the eye, where it approaches the inlets of the passages 206 between the large impeller blades 208, which are further described below. As it moves into the inlets of the passages 206, the fluid mixture encountered the leading edges of a set of small guide blades 210 which are located in between adjacent ones of the impeller blades 208. In the present case the leading edges of guide blades 210 are located halfway between the leading edges of adjacent impeller blades 208.

When the fluid mixture encounters a guide blade 210, it is scooped up by the guide blade's leading edge and is forced to flow along the side of the blade as a relatively thin sheet 211 of fuel, a lot of the gas and vapour bubbles thereby being separated from the fuel an.d collecting as a gas and vapour-filled space 212 in the inner parts of each passage 206. The fuel sheet 211, upon leading the trailing edge of each guide blade 210, forms a jet sheet 214 in each passage 206, and this jet sheet follows an approximately spiral path along or near the longitudinal centre-line of passage 206 due to the combination of radial and rotational velocity components. The jet sheets 214 are the primary means by which fuel is pumped in the inner parts of the passages 206.However, some fuel/gas mixture is also intercepted by the leading edges of the large impeller blades 208 and flows along their convex sides as fuel sheets 216. Again, gas and vapour components are forced out of fuel sheets 216 and collect in space 212. The fuel thus pumped in fuel jet sheet 214 and fuel sheet 216 fills the outlet of the pump and a front or interface 215 is formed between the liquid fuel and the collected gas and vapour, the fuel jet sheet 214 and the fuel sheet 216 impacting the interface 215 and producing much turbulence.

The air and vapour are also pumped effectively by the process. Pumping of the air and vapour components along with the liquid component occurs when the relative velocity between the jet sheet 214 and the interface 215 is sufficient to remix the incoming gas volume into the liquid by entraining the gas in space 212 with the jet sheet and then entrapping the entrained gas in the liquid fuel by means of the turbulence generated as the jet sheet 214, and otherfuel sheet 216, impinges on the interface 215. Interface 215 may be termed the "mixing front".The gas pumping process is continued as the air and vapour, having been remixed by the turbulence into the liquid behind the mixing front as small bubbles which are affected more by centrifugal forces than by buoyancy forces, are also redissolved in the liquid due to increased pressure caused by the pumping action of the outer portions of passages 206.

In operation. the impeller 200 runs at a speed related to the speed of the engine, since the drive for it is taken from an accessory gearbox driven from the engine. Under normal conditions the fuel entering the impeller 200 from the inlet is free of air or vapour bubbles and in this case the impeller functions in the same way as a conventional impeller would under the same conditions. When sufficient bubbles are present, the impeller 200 changes it mode of pumping to that shown in Figure 2 so that the outlet of the pump remains filled with liquid and the interface 215 between gas and liquid moves inwards or outwards in passages 206 until the rate at which the air and vapour is remixed into the fuel equals the rate at which it accumulates in the space 212.

The configuration of the large blades 208 will now be considered and is determined by the following design considerations: (i) The concave flanks of the impeller blades have a shape and an orientation similar to those of the blades 106 in Figure la, with a similar outlet angle at the periphery of the impeller disc and a similar inlet angle at the central area of the impeller disc, the major difference being that it may be preferable, as shown, for the concave sides of blades 208 to be shorter than blades 106 of Figure 1a for the same size of impeller disc, in order to ensure that the inlets to passages 206 are wide enough to accommodate the guide blades 210 satisfactorily.

(ii) The convex flanks of the impeller blades have a shape and orientation dependant upon the requirement that in the inner parts of the passages 206, they preferably extend approximately parallel to the guide blades 210 and to the jet sheet 214, but in the outer parts of the passages their curvature is such as to gradually bring them closer to the concave sides and cause them to have an outlet angle compatible with that of the concave sides. An approximate distinction can be made between inlet portions of the passages 206, which house the guide blades 210, middle portions of the passages, which contain the jet sheets 214 (the inlet and middle portions together being classed as "inner parts" of the passages), and outlet portions of the passages, which have a more divergent form to give a rapid pressure rise through the impeller.The approximate division between the middle and outlet portions of the passages is indicated on Figure 2 by the dotted line D. The combined effect of design considerations (i) and (ii) is to render the impeller blades approximately aerofoil-shaped.

(iii) The leading edges of the impeller blades may perhaps be located on a slightly larger radius than those of the guide blades 210 in order to ensure that they do not scoop up so much of the fluid mixture as to interfere with the efficient functioning of the jet sheet 214, their leading edges being somewhat shielded from contact with the fluid mixture by the leading edges of the guide blades.

Other design considerations are the inlet angles of the leading edges of guide blades 210. These are selected to maximise the gas pumping rate by obtaining the highest velocity for the jet sheets, and will probably be in the range 10 to 500. It is important that the jet sheets have high velocity because this maximises both entrainment of air by the jet sheets, and also turbulence at the gas/liquid boundary 215 due to the impact of the jet sheets. This turbulence helps to entrap air in the liquid and also breaks up entrapped and entrained air into a large number of small bubbles which dissolve in the liquid more quickly than a smaller number of large bubbles and are less affected by buoyancy forces.

In order to test the efficacy of the present invention, a simple pump impeller was constructed and evaluated with respect to its performance and to the behaviour of aerated fuel flow through it. The construction and dimensions of the test impeller 300 and its casing 302 are shown diagrammatically in Figure 3 and 4. The casing 302 comprises an inlet duct 303, an outlet volute 305, and bearings 307 which journal the impeller shaft 309. It will be seen that the impeller 300 has eight simple wedgeshaped impeller blades 304. Each blade 304 has a straight side 306, equivalent to the concave sides of blades 208 in Figure 2, and an opposing side 308 comprising three facets or faces which create flow conditions which approximate to those created by the two sets of blades in Figure 2. The impeller was fed with a mixture of air and fuel.

The leading edge of each blade 304 comprises a flat inlet guide face 310 which gives the blade an inlet angle approximately 30". This face scoops up and guides the mixed fluid flow whilst separating the liquid fuel from the gaseous and vapour phases, as the guide blades 210 did in Figure 2. In order to ensure separation of the flow of fuel over the inlet face 310 from the subsequent face 312, which is transitional between the inlet face 310 and the outlet face 314, inlet face 310 is unconformable with transitional face 312, i.e. there is a sharp corner at the intersection of the two faces, and therefore the fuel cannot follow the abrupt chann direction.

Hence, as in Figure 2, a jet sheet is produced which entrains air and vapour and impacts "- e slower moving fuel which fills the outlet volute 305.

The inlet side of the pump casing 302 was constructed of transparent plastic and the photograph of Figures 5 was taken during one of the tests. It will be seen that in spite of the crude shape and unitary nature of the blades 304, the jet sheet (arrowed) produced by the inlet guide face 310 is clearly visible, as is the mixing front or interface.

The photograph was taken with a 0.076 mm clearance between the transparent casing and the tops of the rotor blades 304 because flow patterns in the rotor were obscured by the fuel film on the casing if larger clearances were used. However, better performance was achieved with larger axial clearances.

In one series of tests, the aerated flow was produced by throttling the inlet to the pump, which caused air to come out of solution in the fuel due to the pump suction in the same way as would occur in an aircraft fuel system as described previously. In another series of tests; the aerated flow was produced by injecting air into the fuel flow upstream of the pump inlet. In the tests it was found that the pump operated very stabily down to very low pressure differentials across the impeller for a wide range of gas/liquid ratios. Whereas at high gas/ liquid ratios (greater than about 0.15) conventional pumps suffer a sudden collapse of pumping ability at approximately 80% of the normal pressure increment across the impeller for the particular speed considered, the present pump did not. This characteristic can be seen in Figure 6, which is a plot of pressure rise through the pump against the throttled inlet pressure to the pump for progressively increasing values of gas/liquid ratio (G/L). The part of the curve between the dotted lines if the approximate extra operational range of the present pump over conventional pumps. The tests with injected air indicated that at a simulated altitude of 6096 metres (20,000 feet) with Avturfuel at +20"C and a pump speed of 6500 rpm, the pump of Figures 3 and 4 could apply a pressure rise of 450 KPa (65 psi) to a fuel flow of 4546 litres/hour (1000 gallons per hour) if the fuel was unmixed with air (i.e. zero gas/liquid ratio) and a pressure rise of 331 KPa (48 psi) at the same flow rate with a gas/liquid ratio of 0.25.

It should be noted that because gas entrainment occurs on both sides of the jet sheet, a ventilation passage 316 in each blade, an example of which is shown by dashed lines in Figure 3, would have improved the performance of the impeller by allowing more gas and vapour to enter the space behind the jet sheet. In Figure 2 ventilation of the space behind the jet sheet is allowed for by the fact that the jet sheet is produced by a guide blade spaced apartfromthe main blade.

Although discussed in terms of its applicablility for pumping fuel in gas turbine engine fuel supply systems, the invention could also be utilised for pumps which have to pump other type of liquid/gas/ vapour mixtures, such as mixture of water and steam in power plant or process plant.

Claims (10)

CLAIMS 1. A rotary impeller for a centrifugal pump capable of pumping fluids which comprise mixed gaseous and liquid components, the impeller comprising a central inlet eye and a plurality of impeller blades defining passages therebetween, wherein at least the outlet portion of each passage is constructed to pump liquid in a conventional manner but the inlet portion of each passage is provided with guide blade means shaped and arranged to intercept fluid flowing out of the eye, at least partiaily separate the gaseous component from the liquid component so that the gaseous component collects in the inlet of the passage and the liquid component flows over the guide blade means as a liquid layer, and project the liquid layer as a liquid jet sheet through the collected gaseous component towards the outlet portion of the passage, a liquid/gas interface being formed in the outlet portion of the passage between liquid being pumped conventionally and the collected gaseous component, the collected gaseous component being entrained by the liquid jet sheet and the liquid jet sheet impacting the interface at a velocity sufficient to remix the entrained gas with the liquid at and beyond the interface. 2. A rotary impeller according to claim 1 in which the guide blade means are structurally integral with the impeller blades. 3. A rotary impeller according to claim 2 in which the guide blade means comprise an inlet face at the leading edge of the leading flank of each impeller blade, the inletfaces having an acute inlet angle and making an acute angle with the trailing flanks of the impeller blades such that the liquid layer flowing over the inlet face separatexfrom the impeller blade surface and forms the liquid jet sheet. 4. A rotary impeller according to claim 1 in which the guide blade means comprise discrete guide blades spaced apart from the impeller blades and having an acute inlet angle. 5. A rotary impeller according to any one of claims 1 to 4 in which the inlet angle of the guide blade means is in the range 10-50'. 6. A rotary impeller according to any one of claims 1 to 5 in which the impeller blades are approximately aerofoil-shaped in chordwise crosssection. 7. A rotary impeller according to anyone of claims 1 to 6 in which the passages between the impeller blades comprise an inlet s.ection incorporating the guide blade means, a middle section whose longitudinal centre-line follows an approximately spiral path to accommodate the path of the liquid jet sheet, and a divergent outlet section. 8. A rotary impeller substantially as described in this specification with reference to and as illustrated by Figure 2 of the accompanying drawings. 9. A rotary impeller substantially as described in this specification with reference to and as illustrated by Figures 3 and 4 of the accompanying drawings. Amendments to the Claims have been filed, and have the following effect: *(b) New or textually amended claims have been filed as follows:

1. A rotary impeller for a centrifugal pump, the impeller comprising a central fluid inlet eye and a plurality of angularly spaced impeller blades defining between themselves a plurality of divergent fluid flow passages, wherein the outlet portions of the passages are substantially more divergent then the inlet portions thereof, and the inlet portions of the passages are provided with guide blade means positioned to intercept fluid flowing out from the eye of the impeller, whereby when the fluid includes a substantial gaseous component mixed with a liquid component, the liquid component is concentrated by the guide blade means to produce liquid sheets flowing thereover and the gaseous component collects in the inlet portions of the passages, the guide blade means being adapted to project the liquid sheets as jet sheets through the collected gaseous component into the outlet portions of the passages for collection therein and subsequent normal pumping by contact of the collected liquid with the impeller blades, the jet sheets not being in contact with the impeller blades and thereby being effective to entrain the collected gaseous component and remix it into the liquid component in the outlet portions of the passages as bubbles small enough to be more affected by centrifugal pumping forces than by buoyancy forces.

2. A rotary impeller according to claim 1 in which the guide blade means are structurally united with the impeller blades.

3. A rotary impeller according to claim 1 or claim 2 in which the guide blade means comprise inlet faces of the impeller blades, there being an unconformity between each inlet face and the subsequent flank portion of each impeller blade to ensure break away of the liquid sheets from the impeller blades to form the jet sheets.

4. A rotary impeller according to claim 3 in which the unconformity comprises a corner where the inlet face and the subsequent flank portion meet each other.

5. A rotary impeller according to claim 1 in which the guide blade means comprise discrete guide blades spaced apart from the impeller blades and having an acute inlet angle.

6. A rotary impeller according to any one of claims 1 to 5 in which the impeller blades are approximately aerofoil-shaped in chordwise crosssection.

7. A rotary impeller according to any one of claims 1 to 6 in which the passages between the impeller blades incorporate middle portions transitional between the inlet and outlet portions, the middle portions having longitudinal centre-lines of approximately spiral form whereby the jet sheets are accommodated without impingement on the impeller blades.

8. A rotary impeller according to any one of claims 1 to 7 in which the guide blade means have an inlet angle of between 10 and 50 degrees.

9. A rotary impeller substantially as described in this specification with reference to and as illustrated by Figure 2 of the accompanying drawings.

10. A rotary impeller substantially as described in this specification with reference to and as illustrated by Figures 3 and 4 of the accompanying drawing.