Fluid compressors

Abstract: An axial flow or centrifugal flow compressor includes a casing treatment for enhancing the compressor's fluid dynamic stability. In one variant, the casing treatment (Fig.1) comprises one or more circumferentially extending grooves ( 40 ) that each receive indigenous fluid from the compressor flowpath at a fluid extraction site ( 56 ) and discharge indigenous fluid into the flowpath at a fluid injection site ( 58 ), circumferentially offset from the extraction site, where the migrated fluid is better able to advance against an adverse pressure gradient in the flowpath. Each groove is oriented so that the discharged fluid enters the flowpath with a streamwise directional component that promotes efficient and reliable integration of the introduced fluid into the flowpath fluid stream ( 20 ). In a second variant of the invention (Fig.8), the casing treatment comprises a circumferentially extending compartment ( 62 ), typically comprising a voluminous pressure compensation chamber ( 64 ) and a single passage ( 66 ) circumferentially coextensive with the chamber, for establishing fluid communication between the chamber and the flowpath. The compartment attenuates the inordinate circumferential pressure difference across the tips of excessively loaded compressor blades ( 16 ), making the compressor less susceptible to tip vortex induced instabilities.

Description: The present invention relates to stability enhancing casing treatments for fluid compressors, such as the compressors and fans used in turbine engines, and particularly to casing treatments that discourage development of potentially destabilizing vortices near the tips of the compressor blades. Centrifugal and axial flow compressors include a fluid inlet, a fluid outlet and one or more arrays of compressor blades projecting outwardly from a rotatable hub or shaft. A casing, whose inner surface defines the outer boundary of a fluid flowpath, circumscribes the blade arrays. Each compressor blade spans the flowpath so that the blade tips are proximate to the outer flowpath boundary, leaving a small clearance gap to enable rotation of the shaft and blades. During operation, the compressor pressurizes a stream of working medium fluid, impelling the fluid to flow from a relatively low pressure region at the compressor inlet to a relatively high pressure region at the compressor outlet. Because compressors urge the working medium fluid to flow against an adverse pressure gradient (i.e. in a direction of increasing pressure) they are susceptible to stall, a localized fluid dynamic instability that locally impedes fluid flow through the compressor and by surge, a larger scale fluid dynamic instability characterized by fluid flow reversal and disgorgement of the working medium fluid out of the compressor inlet. Compressor stall and surge are obviously undesirable. If the compressor is a component of an aircraft gas turbine engine, a surge is especially unwelcome since it causes an abrupt loss of engine thrust and can damage critical engine components. In a turbine engine, surge or stall may be provoked by any of a number of influences, among them fluid leakage through the clearance gap separating each blade tip from the compressor case. Leakage occurs because the fluid pressure adjacent the concave, or pressure surface of each blade exceeds the pressure along the convex, or suction surface of each blade. The leaking fluid interacts with the fluid flowing through the primary flowpath to form a fluid vortex. The strength of the vortex depends in part on the size of the clearance gap and on the pressure difference or loading between the suction and pressure sides of the blade. Compressors can usually tolerate vortices of limited strength. However a locally excessive clearance gap or locally excessive loading of one or more blades can generate a vortex powerful enough to seriously disrupt the progress of fluid through the flowpath, resulting in a surge or stall. Compressor designers strive to develop compressors that are highly tolerant of potentially destabilizing influences. One way that designers enhance compressor stability is by incorporating special features, referred to as casing treatments, in the compressor case. One type of stability enhancing casing treatment is a series of circumferentially extending grooves, each substantially perpendicular to the streamwise direction (the predominant direction of fluid flow in the flowpath). U.K. Patent Application 2,158,879 depicts such a casing treatment, but does not elaborate on the physical mechanisms responsible for improving stability. It is thought that the grooves provide a means for fluid to exit the flowpath at a locale where the blade loading is severe and the local pressure is high, migrate circumferentially to a locale where the pressure is more moderate, and re-enter the flowpath. The migrated fluid is thus better positioned to contend with the adverse pressure gradient in the flowpath. Moreover, the fluid migration helps relieve the locally severe blade loading. It has also been observed that the presence of the grooves degrades compressor efficiency, presumably because fluid re-enters the flowpath in a direction substantially perpendicular to the streamwise direction, resulting in efficiency losses as the re-entering fluid collides with and mixes turbulently with the flowpath fluid stream. The re-entering fluid, lacking any appreciable streamwise directional component of its own, may also tend to recirculate unbeneficially into and out of the groove. Another type of casing treatment is shown in U.S. Patent 5,762,470 and U.K Patent Application 2,041,149. These patents disclose compressors employing a manifold to alleviate circumferential pressure nonuniformities that may be associated with destabilizing tip leakage vortices. The manifold shown in U.S. Patent 5,762,470 is an annular cavity that communicates with the flowpath by way of a series of slots separated by a gridwork of ribs. U.K. Patent Application 2,041,149 discloses a centrifugal compressor having a manifold that communicates with flowpath through a set of slotted diffuser vanes. The application also discloses an axial flow compressor with a manifold radially outboard of the compressor flowpath and a manifold chamber radially inboard of the flowpath. A spanwise slot on the suction surface of each compressor blade places the compressor flowpath in fluid communication with the inboard manifold chamber. The compressor vanes include similar slots that connect the flowpath to the outboard manifold. Notwithstanding the possible merits of the disclosed arrangements, they clearly introduce a measure of undesirable manufacturing complexity into the compressor. Still another type of casing treatment is shown in U.S. Patents 5,282,718, 5,308,225, 5,431,533 and 5,607,284, all of which are assigned to the present applicant. These patents describe variations of a turbine engine casing treatment known as vaned passage casing treatment (VPCT). The disclosed casings include a passageway occupied by a set of anti-swirl vanes. Fluid extraction and injection passages place the vaned passageway in fluid communication with the compressor flowpath. During operation, fluid with degraded axial momentum, but high tangential momentum, flows out of the flowpath by way of the extraction passage, through the vane set, and then back into the flowpath by way of the injection passage. The vane set redirects the fluid, exchanging its tangential momentum for increased axial momentum so that the injected fluid is more favorably oriented than the extracted fluid. Despite the merits of the vaned passage casing treatment, it is not without certain drawbacks. The vaned passageway consumes an appreciable amount of space, a clear disadvantage considering the space constraints typical of aerospace applications. The treatment also presents manufacturing and fabrication challenges. Moreover, debris may clog portions of the vaned passageway, compromising the effectiveness of the treatment. Finally, the treatment degrades compressor efficiency by allowing pressurized fluid to recirculate to a region of lower pressure in the compressor flowpath. The efficiency loss may be mitigated by employing a regulated system as proposed in U.S. Patent 5,431,533. However the regulated system introduces additional complexity. Finally, U.S. Patent 5,586,859, also assigned to the present applicant, discloses a "flow aligned" casing treatment in which a circumferentially extending plenum communicates with the flowpath by way of discrete extraction and injection passages. The flow aligned treatment, like VPCT, recirculates pressurized fluid to a lower pressure region, introducing the fluid into the flowpath in a prescribed direction to achieve optimum performance. However the flow aligned casing treatment suffers from many of the same disadvantages as VPCT. EP 0 719 908 discloses a baffled passage casing for compressor blades. US 4 714 406 discloses a gas turbine casing. US 5 137 419 discloses an axial flow compressor surge margine improvement. Notwithstanding the existence of the above described casing treatments, compressor designers continually seek improved ways to reliably enhance compressor stability and minimize any attendant efficiency loss without complicating manufacture of the compressor or its components. From a first aspect the present invention provides a fluid compressor as claimed in claim 1. From a second aspect the present invention provides a method of augmenting fluid from stability as claimed in claim 15. In one preferred embodiment of the present invention, a compressor casing treatment comprises one or more circumferentially extending grooves that each receive indigenous fluid from the compressor flowpath at a fluid extraction site and discharge indigenous fluid into the flowpath at a fluid injection site. Fluid extraction occurs at a site where the fluid pressure in the compressor flowpath is relatively high and the streamwise momentum of the fluid is relatively low. Fluid injection occurs at a site, circumferentially offset from the extraction site, where the flowpath fluid pressure is more modest and the streamwise momentum of the fluid is relatively high. Thus, each groove diverts fluid circumferentially to a location where the fluid is better able to advance against the flowpath adverse pressure gradient. Each groove is oriented so that the discharged fluid enters the flowpath with a streamwise directional component that promotes efficient integration of the introduced fluid into the flowpath fluid stream. The streamwise component also counteracts any tendency of the introduced fluid to recirculate locally into and out of the groove. The inventive casing treatment of the preferred embodiment at least is advantageous in many respects. It improves compressor stability without excessively penalizing compressor efficiency. The treatment is simple, and so can be incorporated without adding appreciably to the cost of the compressor or unduly complicating its manufacture. Unlike some prior art casing treatments, the inventive treatment is relatively unlikely to become clogged by foreign objects. The treatment can operate passively, avoiding the weight, bulk, cost and complexity of a control system. The grooved treatment is space efficient, making it readily applicable to the core engine compressors of a turbine engine. Some preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic, cross sectional side view typical of an axial flow compressor or fan for a turbine engine and showing a grooved casing according to one aspect of the present invention. Figure 1A is a cross-sectional view of a compressor blade taken in the direction 1A--1A of Fig. 1 . Figure 2 is a schematic, perspective view typical of an axial flow compressor or fan for a turbine engine and showing a grooved casing according to one aspect of the present invention. Figures 2A and 2B are views similar to figure 1 schematically illustrating the distribution of fluid flow into a casing treatment groove at an extraction site and out of the casing treatment groove at an injection site circumferentially offset from the extraction site. Figures 3-5 are views similar to Fig. 1 illustrating alternative embodiments of the grooved casing. Figures 6 and 6A are schematic side views of a turbine engine with the engine casing partially broken away to expose a centrifugal compressor employing a grooved casing of the present invention. Figures 7A and 7B are graphs showing the influence of the grooved casing on compressor stability and efficiency respectively. Figure 1 schematically illustrates a portion of an axial flow compressor representative of those used in turbine engines. In the context of a turbine engine the term "compressor", as used throughout this specification, refers to both the core engine compressors and to the relatively large diameter, low compression ratio fans employed on many engine models. The compressor includes a hub 12 rotatable about a compressor rotational axis 14 and an array of blades 16 extending radially outwardly from the hub. The blades 16 span a compressor flowpath 18 that extends substantially parallel to the rotational axis 14 and channels a stream of air or other working medium fluid 20 through the compressor. Each blade has a root 22 , a tip 24 , a leading edge 26 and a trailing edge 28 . As seen best in Fig. 1A , each blade has suction and pressure surfaces 32, 34 extending from the leading edge to the trailing edge and spaced apart by an axially nonuniform blade thickness T . Each blade also has a mean camber line MCL , which is a locus midway between the pressure and suction surfaces as measured perpendicular to the mean camber line. A chord line C , which is a locus that extends linearly from the leading edge to the trailing edge, joins the ends of the mean camber line. A projected chord C P , is the chord line C projected onto a plane that contains the rotational axis 14 . The compressor also includes a casing 36 having a radially inner flowpath surface 38 . The flowpath surface circumscribes the blade array and is spanwisely or radially spaced from the blade tips by a small clearance gap G . The casing includes a circumferentially continuous groove 40 defined by axially spaced apart upstream and downstream walls 42, 44, each of which extends from a groove floor 46 and adjoins the flowpath surface at respective upstream and downstream lips 48, 50 . The lips define a groove mouth 54 that places the groove in fluid communication exclusively with the flowpath 18 . The upstream wall 42 is oriented at an acute angle  A relative to the flowpath surface 38 and the downstream wall 44 is oriented at an obtuse angle  O relative to the flowpath surface. Figure 2 depicts the fluid flow patterns attributable to the grooved casing treatment. The blade array represented by the single blade 16 rotates in direction R to pressurize the fluid stream 20 , compelling the fluid to flow through the flowpath against an adverse pressure gradient. If the pressure loading of the blade tip region is excessive, the groove 40 provides a path for indigenous fluid to migrate circumferentially from the region of high loading (and correspondingly high pressure and low streamwise momentum) to another region where the local loading is more moderate, the flowpath pressure is less severe and the streamwise momentum of the fluid is greater. As used herein, the term "indigenous fluid" refers to fluid in the groove and in the flowpath in the vicinity of the groove as opposed to fluid supplied from a remote portion of the flowpath or from an external source. More specifically, fluid exits the flowpath and flows into the groove at an extraction site 56 , proceeds circumferentially as shown by the fluid flow arrows 20a , and discharges into the flowpath at an injection site 58 substantially axially aligned with and circumferentially offset from the extraction site 56 . The fluid flows as indicated by arrows 20a because the pressure of the fluid in the flowpath is higher at the extraction site than it is at the injection site. In particular, the flowpath fluid pressure at the injection site is lower than the flowpath fluid pressure adjacent the pressure surface of the blade at the extraction site. The migrated fluid is thus better positioned to advance against the flowpath adverse pressure gradient. The circumferential fluid migration also relieves the excessive blade tip loading at the extraction site and reduces the likelihood of tip vortex induced compressor stall or surge. The groove walls are inclined at angles  A and  O , so that fluid entering the flowpath at the injection site does so with an appreciable streamwise directional component. As a result, the high mixing losses that can arise from transverse fluid injection are at least partially avoided. In addition, the groove inclination and the accompanying streamwise directional component of fluid discharge help overcome any tendency of the fluid to recirculate unbeneficially into and out of the groove. Thus, the inventive casing treatment offers a stability improvement without exacting a significant penalty in compressor efficiency. Figures 2A and 2B illustrate that the axial distribution of fluid flow into the groove at the extraction site 56 (Fig. 2A ) may differ from the distribution of fluid flow out of the groove at the injection site 58 (Fig. 2B ). At the extraction site 56, flowpath fluid pressure increases from P 1E near the groove upstream wall 42 to P 2E near the groove downstream wall 44 . Since fluid flow into the groove is dominated by higher flowpath pressure, the mass flow rate of fluid entering the groove is distributed preferentially toward the downstream wall 44 as suggested by the schematic flow distribution diagram superimposed at the mouth 54 of the groove on Figure 2A . At the injection site 58 , flowpath fluid pressure increases from P 1I near the upstream wall to P 2I near the downstream wall. The lower pressure P 2I dominates fluid discharge at the injection site by offering less resistance than the higher pressure P 2I . Accordingly, fluid discharge into the flowpath is distributed preferentially toward the upstream wall 42 as indicated by the flow distribution diagram of Fig. 2B . It should be appreciated that the distribution diagrams of Figures 2A and 2B are schematic. The actual fluid flow distributions are influenced by the local streamwise pressure gradients at the extraction and injection sites and by the magnitude of the circumferential pressure gradient in the flowpath. Moreover, it should be appreciated that the actual fluid dynamics are extremely complex, and that the distribution diagrams indicate the predominant fluid flow patterns. In practice some amount of fluid may discharge from the groove at the extraction site and may enter the groove at the injection site. The positioning and length of the groove mouth, the groove orientation and the groove depth will vary depending on the operating characteristics and physical constraints of the compressor. Nevertheless certain general observations can be made. Referring primarily to Figure 1 , the groove mouth 54 should be situated so that its downstream lip 50 is no further upstream than the leading edge 26 of the blade array at the blade tips. Such placement positions the groove to receive flowpath fluid that leaks over the blade tips and threatens to develop into a potentially destabilizing tip vortex. Since tip leakage vortices extend downstream of the blade tailing edges, the mouth may be situated so that its upstream lip 48 is downstream of the trailing edge 28 of the blade array at the blade tips. However it is anticipated that the groove will be most effective if its upstream lip 48 is no further downstream than the trailing edge 28 of the blade array at the blade tips. Thus, it is expected that the best benefits will be achieved if the groove mouth is positioned so that at least a portion of the mouth is streamwisely coextensive with the projected tip chord C P , i.e. with the groove downstream lip 50 no further upstream than the leading edge 26 of the blade array at the blade tips and the upstream lip 48 no further downstream than the trailing edge 28 of the blade array at the blade tips. The axial length L of the groove mouth 54 should be long enough to ensure that the mouth can capture a quantity of flowpath fluid sufficient to alleviate excessive blade loading. However since the mouth represents a discontinuity in the flowpath surface 38 , the mouth length should be small enough to preclude fluid separation from the flowpath surface and concomitant fluid dynamic losses. The groove orientation depends on both fluid dynamic and manufacturing considerations. As noted above, fluid discharge into the flowpath is distributed preferentially toward the upstream wall 42 . Accordingly, the upstream wall strongly influences the direction of fluid discharge. Since it is desirable to accentuate the streamwise directional component of fluid discharge, the acute angle  A should be made as small as practicable. Manufacture of a case with a small acute angle  A , nonparallel walls 42, 44, or other complex geometry may be facilitated by constructing the case of forward and aft portions that are mated together at an interface 59 . If desired, the groove may instead be machined into a single piece case, however it has proved difficult to machine a groove having an acute angle  A of less than about 30°. If the groove is machined into a single piece case, it is desirable to facilitate manufacture by making the upstream and downstream walls 42, 44 parallel to each other so that the groove has a uniform axial width W. The groove depth D is a compromise between fluid dynamic considerations, case structural integrity, space constraints and producibility. The groove must be shallow enough that the structural integrity of the casing is not compromised. However, if the groove is too shallow, the performance of the casing approaches that of a smoothwall case -- one that preserves compressor efficiency but fails to improve the compressor's tolerance to tip vortices. By contrast, a deep groove has a greater capacity to carry fluid from the extraction site to the injection site, and therefore has a more beneficial effect on compressor stability. However it is believed that the stability benefit does not accrue without limit. Moreover, the groove depth is obviously limited by the thickness of the casing and any other radial space constraints. Experience with currently available machining techniques has demonstrated that it is possible to produce a groove whose depth D is at least about three times the mouth length L . In one specific arrangement contemplated for a turbine engine being developed by the present application, the grooved casing treatment is applied to four of five compression stages in one of the engine's two core compressors. Each of the four blade arrays is circumscribed by a circumferentially extending groove whose upstream lip is situated at about 25% of the projected tip chord and whose downstream lip is situated at about 55% of the projected tip chord. The groove has parallel upstream and downstream walls and the upstream wall is oriented at an acute angle  A of about 30°. The groove depth is about two times the mouth length. In view of the foregoing discussion, certain additional details of the grooved casing treatment can now be appreciated. As already noted, the orientation of the upstream wall 42 is thought to be more critical than the orientation of the downstream wall 44 in imparting a streamwise directional component to the discharged fluid. Therefore, it may be desirable to construct the casing, or at least the portion of the casing near the upstream lip 48 , of a material capable of resisting erosion and abrasion. Otherwise the upstream lip may be chipped or worn away by foreign objects entrained in the fluid stream 20 or, more likely, by occasional contact with the blade tips during compressor operation. Either way, erosion of the lip 48 can allow fluid to enter the flowpath with a substantially diminished streamwise directional component, sacrificing much of the benefit of the invention. The downstream lip 50 also influences fluid discharge into the flowpath. Ideally, the lip 50 is a smooth curve rather than a sharp corner defined by the prolongations of the flowpath surface 38 and the downstream wall 44 . The curvature exploits the Coanda effect in which fluid immediately adjacent to a curved surface depressurizes and accelerates as it flows over the surface. Nearby higher pressure fluid not subject to the Coanda effect urges the affected fluid to follow the surface contour. As seen best in Fig. 1 , the lip 50 is gently curved to take advantage of the Coanda effect and urge fluid discharging from the groove to hug the lip and turn in the streamwise direction. It has also been determined that the stability enhancing effect of the casing treatment might be augmented by groove walls that exhibit a surface roughness that exceeds about 75 AA microinches. The AA surface roughness measure, also known as the roughness average (R a ) or centerline average (CLA), is defined in ANSI specification B46.1-1995 available from the American Society of Mechanical Engineers. The observation that surface roughness may be influential was made in the course of testing a turbine engine with a groove 40 machined into the fan casing 36 radially outboard of a single array of fan blades. In one test configuration the portion of the casing outboard of the fan blades was made of an abradable material (adhesive EC-3524B/A available from the 3M Company, St. Paul Minnesota, USA). Because of roughness inherent in the abradable material, the machined groove had a perceptible but indeterminate surface roughness. In a second configuration, the groove was machined into an aluminum case, resulting in relatively smooth walls having a surface roughness of only about 75 AA micrcinches in the axial direction and no more than about 16 AA microinches in the circumferential direction. During testing, the first configuration demonstrated better fan stability than the second configuration, suggesting that the surface roughness may be beneficial. A third configuration was tested to verify the benefit. The third configuration was a modified version of the second configuration in which ordinary paint was sprayed onto the groove walls. The spray gun used to apply the paint was positioned far enough away from the walls that the spray droplets partially congealed prior to contacting the walls. Upon striking the walls, the partially congealed droplets adhered to the wall surfaces to give the walls a granular texture whose roughness was determined to be about 300-400 AA microinches. Testing of the third configuration revealed fan stability similar to that of the first configuration, tending to confirm the desirability of surface texture. In practice, it will be necessary to use a more suitable, controllable and repeatable means of introducing a durable surface texture. Figures 3, 4 and 5 depict alternative embodiments of the grooved casing treatment. In figure 3 , the wall orientation angles  A ,  O , are selected so that the upstream and downstream walls 42, 44 of the groove 40 define a tapered groove whose width W diminishes with increasing groove depth D . The diminishing width of the tapered groove slightly compresses fluid that flows into the groove at the extraction site so that the fluid will be more forcibly expelled into the flowpath at the injection site, thereby enhancing the benefit of the streamwise directional component. Figure 4 shows a grooved casing treatment in which the upstream and downstream walls 42, 44 define a contoured groove 40 for imparting a streamwise directional component to fluid entering the flowpath at the injection site. The contour is such that the slope of groove mean line M (a line midway between the upstream and downstream walls as measured perpendicular to the mean line) approaches an orientation more perpendicular than parallel to the streamwise direction near the groove floor 46 and more parallel than perpendicular to the streamwise direction near the groove mouth 54 . Figure 5 shows a casing treatment comprising multiple grooves 40 . Each groove is similar to the groove depicted in Figures 1, 2, 2A and 2B , however in practice each groove may have its own unique geometry (depth, width and orientation). Multiple grooves, whether of similar or dissimilar geometry, may be useful for selectively relieving excessive blade loading at multiple, axially distinct locations. Figures 6 and 6A illustrates the grooved casing treatment as it might be applied to a centrifugal compressor in a turbine engine. Primed reference characters are used to designate features of the centrifugal compressor analogous to those already described for an axial flow compressor. In the centrifugal compressor at least a portion of the compressor flowpath 18 ' extends radially, i.e. approximately perpendicular, relative to the compressor rotational axis 14 '. However the grooved casing treatment is similar in all respects to the grooved casing treatment for an axial flow compressor. An aircraft turbine engine with a casing treatment similar to that illustrated in Fig 1 has been tested by the assignee of the present application. The casing treatment groove 40 in the tested engine was situated outboard of an array of fan blades 16 with the groove upstream lip 48 at about 50% of the projected tip chord, and the groove downstream lip 50 at about 90% of the projected tip chord. The upstream and downstream walls 42, 44 were parallel to each other, the acute orientation angle  A was about 30° and the obtuse, angle  O was about 150°. The groove depth was about three times the groove width. For comparison, tests were also conducted with a smoothwall case (one not having a casing treatment) and with a conventional casing treatment comprising an array of six transverse grooves (i.e.  A and  O both equal to 90°) that allow fluid to enter the flowpath without any appreciable streamwise directional component. The tests were repeated with different clearance gaps G separating the blade tips 16 from the flowpath surface 38 , the smallest or tightest of those clearances being representative of the clearance in a revenue service engine operating at its steady state design point. Testing at the larger clearances is significant because the blade tip clearance gap is usually at least slightly enlarged for brief time intervals during normal engine operation. Unfortunately, these enlarged clearances, which are detrimental to fluid dynamic stability, often occur in an aircraft engine at engine power levels and operating conditions where the fan is simultaneously exposed to other stability threats. Results of the engine testing are displayed in Figures 7A and 7B . Figure 7A shows the results of tests with a moderately enlarged tip clearance of about 1.4% of blade chord C . During the testing engine power was gradually increased until the fan surged. Fan stability is represented on the Figure as the percent of compressor rotational speed at which stall occurred (100% speed is the mechanical redline speed). As seen in Fig. 7A , fan stability was significantly better with the inventive grooved casing than with a smoothwall case despite the somewhat enlarged tip clearance. Figure 7B shows how steady state fan efficiency is affected by the casing treatments. Tip clearance is expressed in the Figure as a percentage of blade span S as seen in Figure 1). The graph reveals that the efficiency penalty attributable to the inventive grooved casing treatment is appreciably less than that attributable to the conventional grooved treatment, especially at the tightest tip clearance. The less dramatic benefit at the enlarged clearances is not troublesome since a turbine engine fan or compressor normally operates with loose clearances for only brief periods of time. When the engine is operated at its design condition, the clearances are tight. In combination, Figures 7A and 7B demonstrate that the inventive grooved casing treatment offers a significant improvement in stability with only a modest penalty to compressor efficiency. Although the invention has been described with reference to exemplary embodiments thereof, those skilled in the art will appreciate that various changes and adaptations may be made without departing from the invention as set forth in the accompanying claims.

Claims: A fluid compressor, comprising: a blade array (16;16') rotatable about a rotational axis (14;14'), each blade of the array having a root (22), a tip (24), a leading edge (26) and a trailing edge (28), and each blade spanning a fluid flowpath (18;18') that channels a stream of fluid through the compressor; and a casing (36) having a flowpath surface (38;38') circumscribing and spanwisely spaced from the blade tips (24), the casing having a circumferentially extending groove (40;40') in fluid flow communication with the flowpath (18;18') for receiving fluid from the flowpath at a fluid extraction site (56) and for discharging fluid into the flowpath at a fluid injection site (58) characterised in that the groove is constructed and arranged such that the discharged fluid enters the flowpath with a streamwise directional component, so that the fluid injection site (58) is circumferentially offset from and substantially axially aligned with the fluid extraction site (56). The fluid compressor of claim 1, wherein: the groove (40;40') is defined at least in part by an upstream wall (42;42') and a downstream wall (44;44'), both walls extending to and adjoining the flowpath surface (38;38') at respective upstream and downstream lips (48,50;48',50'), the lips forming a mouth (54;54') of the groove (40;40'), the upstream wall being oriented at an acute angle ( A ;  A' ) relative to the adjoining flowpath surface, and the downstream wall being oriented at an obtuse angle ( O ;  O' ) relative to the adjoining flowpath surface (38;38') so that the discharged fluid enters the flowpath (18;18') with a streamwise directional component. The fluid compressor of claim 2 wherein the acute and obtuse angles ( O ;  O' ;  A ;  A' ) are selected so that the walls (42,44;42',44') are parallel to each other and define a groove (40;40') of uniform width. The fluid compressor of claim 2 wherein the acute and obtuse angles ( O ;  O' ;  A ;  A' ) are selected so that the (42,44;42,44') walls define a tapered groove (40;40') whose width diminishes with increasing groove depth. The fluid compressor of claim 2 wherein the upstream and downstream walls (42,44;42'44') define a contoured groove (40;40') having a floor, a mouth and a mean line whose slope approaches an orientation more perpendicular than parallel to the streamwise direction near the groove floor and more parallel than perpendicular to the streamwise direction near the groove mouth for imparting a streamwise directional component to fluid entering the flowpath at the injection site. The fluid compressor of any of claims 2 to 5 wherein the groove downstream lip (50;50') is no further upstream than the leading edge (26) of the blade array at the blade tips (26) . The fluid compressor of any of claims 2 to 6 wherein the groove upstream lip (48;48') is no further downstream than the trailing edge (28) of the blade array at the blade tips (24) . The fluid compressor of any of claim 2 to 7 wherein the mouth (54;54') has a streamwise length (L) and the groove (40;40') has a depth (D) of up to about three times the mouth length (L). The fluid compressor of any of claims 2 to 8 wherein the downstream lip (50;50') is curved to encourage fluid discharging from the groove (40;40') to turn in the streamwise direction. The fluid compressor of any of claims 2 to 9 wherein the flowpath (18) extends substantially parallel to the rotational axis (14) , the groove upstream lip (48) is situated at about 25% of the projected tip chord (Cp) of the blade, the groove downstream lip (50) is situated at about 55% of the projected tip chord (Cp), the acute angle ( O ) is approximately 30 degrees, the obtuse angle ( A ) is approximately 150 degrees, the mouth (54) has a streamwise length (L) and the groove has a depth of approximately two times the mouth length (L). The fluid compressor of any of claims 2 to 9 wherein at least a portion of the flowpath (18') extends approximately normal to the rotational axis. The fluid compressor of any of claims 2 to 11 wherein the groove walls (42,44;42',44') have a surface roughness of at least about 75 AA microinches. The fluid compressor of claim 12 wherein the surface roughness is between about 300 AA microinches and about 400 AA microinches. The fluid compressor of claim 1 for a turbine engine, further comprising: a hub (12;12') rotatable about a rotational axis (14;14'); a blade array (16;16') extending outwardly from the hub (12;12'), each blade of the array further having a projected tip chord (Cp); wherein the circumferentially extending groove (40;40') is in fluid flow communication exclusively with the flowpath for receiving indigenous fluid from the flowpath at the fluid extraction site (56) and discharges indigenous fluid into the flowpath at the fluid injection site (58) substantially streamwisely aligned with the extraction site; the groove (40;40') comprising streamwisely spaced apart upstream and downstream walls (42,44;42,44') each extending to and adjoining the flowpath surface (38;38') to define respective upstream and downstream lips (48,50;48,50'), the lips defining a mouth (54;54') of the groove, the upstream wall being oriented at an acute angle ( A ;  A' ) relative to the adjoining flowpath surface, the downstream wall being oriented at an obtuse angle ( O ;  O' ) relative to the adjoining flowpath surface, the groove mouth (54;54') being positioned so that at least a portion of the mouth (54;54') is streamwisely coextensive with the projected tip chord (Cp). A method of augmenting fluid flow stability of a compressor, the compressor having a blade array (16;16') rotatable about an axis (14;14') , each blade of the array extending across a flowpath (18;18') that channels a stream of fluid through the compressor, each blade also having a blade tip (24), the compressor also having a casing (36;36') with a flowpath surface (38;38') spaced apart from and circumscribing the blade tips, the fluid stream having a circumferentially nonuniform, streamwisely adverse pressure gradient, the method comprising: diverting indigenous fluid from the flowpath at an extraction site (56) circumferentially aligned with a relatively high flowpath fluid pressure ; directing the indigeneous fluid circumferentially to an injection site (58) circumferentially aligned with a relatively low flowpath fluid pressure ; and discharging the indigenous fluid into the flowpath at the injection site so that the discharged fluid enters the flowpath with a streamwise directional component, said injection site being circumferentially offset from, and substantially axially aligned with said extraction site. The method of augmenting fluid flow stability of a compressor of claim 15 wherein, each blade of the array also having a blade tip (24) a pressure surface (34) and a suction surface (32) , wherein the extraction site ( 56 ) is circumferentially aligned with a relatively high circumferential pressure difference across a blade tip; and the injection site (58) circumferentially aligned with a flowpath fluid pressure lower than the flowpath fluid pressure adjacent the pressure surface of the blade at the extraction site.

Inventors: BARNETT MARK, GRAF MARTIN, RAW JOHN A, SHARMA OM, SPROUT WILLIAM D, BARNETT, MARK, GRAF, MARTIN, RAW, JOHN A., SHARMA, OM, SPROUT, WILLIAM D.
Applicants: PRATT & WHITNEY CANADA, UNITED TECHNOLOGIES CORP, PRATT & WHITNEY CANADA INC., UNITED TECHNOLOGIES CORPORATION
Application year: 1999
Main classification: F04D27
IPC codes: F04D27, F04D17, F04D29