阅读说明：本技术 用于涡轮机器的叶片和护罩布置 (Blade and shroud arrangement for a turbomachine ) 是由 克里斯多夫·J·肖 于 2019-05-14 设计创作，主要内容包括：提出了一种用于涡轮机器的涡轮机,其中,在涡轮机叶轮(9)的进气口处,叶片(7)从喷嘴环(5)延伸穿过护罩(6)中的狭槽。叶片(7)形成有前部以及后部,该前部布置成接触相应的狭槽的前部,当叶片和狭槽的前部在一起时,该后部被成型为与狭槽的相应后部在室温下保持基大体恒定的间隔。该接触可以是点接触,例如,靠近叶片的前边缘。替代地,叶片可以包括前表面部分,该前表面部分与狭槽中的一个的相应的前表面部分的形状紧密一致。(A turbine for a turbo machine is proposed, in which, at the air inlet of the turbine wheel (9), vanes (7) extend from the nozzle ring (5) through slots in the shroud (6). The blade (7) is formed with a front portion arranged to contact the front portion of the respective slot and a rear portion shaped to maintain a substantially constant spacing from the respective rear portion of the slot at room temperature when the blade and the front portion of the slot are brought together. The contact may be a point contact, for example, near the leading edge of the blade. Alternatively, the blade may comprise a front surface portion that closely conforms to the shape of the respective front surface portion of one of the slots.)

1. A turbomachine, comprising:

(i) a turbine wheel, having an axis,

(ii) a turbine housing for defining a chamber for receiving the turbine wheel for rotation of the turbine wheel about the axis, the turbine housing further defining an inlet port and an annular inlet passage from the inlet port to the chamber,

(iii) an annular shroud defining a plurality of slots and surrounding the axis, each slot having a slot surface; and

(iv) a nozzle ring supporting a plurality of vanes extending from the nozzle ring parallel to the axis and projecting through respective ones of the slots;

each of the blades has:

an axially extending vane surface comprising (i) an outer vane surface facing a radially outer surface of a respective said slot, (ii) an opposing inner vane surface facing a radially inner surface of a respective said slot, and

a midline between the blade inner surface and the blade outer surface and extending from the leading end of the blade to the trailing end of the blade;

the vane is positionable such that a front surface portion of the vane inner surface contacts a respective front surface portion of a respective slot surface;

the blade inner surface further includes a rear surface portion extending along at least 33% of the midline, spaced apart from an opposing rear surface portion of the slot surface by a distance in the range of 10 microns to 250 microns at room temperature and when in contact with the front surface portion.

2. The turbomachine of claim 1 wherein the rear surface portions are spaced apart by a distance in a range of 25 microns to 100 microns at room temperature and when in contact with the front surface portion.

3. A turbine according to claim 1 or claim 2, wherein the rear portion of the blade surface extends along at least 50%, and more preferably at least 60%, at least 70% or at least 80% of the length of the mid-line.

4. A turbine according to claim 1 or claim 2 or claim 3, wherein the front surface portions of the vanes extend along at least 15% of the length of the mid-line, the respective profiles of the front surface portions of the vane surfaces and of the respective front surface portions of the respective slot surfaces differing from one another by no more than 1 to 50 microns, or more preferably 1 to 25 microns.

5. The turbomachine of claim 4 wherein said leading edge portion comprises a point at which said midline intersects said leading edge of said blade.

6. A turbine according to claim 1 or claim 2 or claim 3, wherein the front surface portion of the blade extends along less than 5% of the length of the mid-line.

7. A turbine according to any preceding claim, wherein, in use, a radially inner portion of the surface of the vanes and slots is at a lower pressure than a radially outer portion of the surface of the vanes and slots.

8. A turbocharger comprising a turbine according to any preceding claim.

Technical Field

The present invention relates to a vane and shroud arrangement for location at an air inlet of a turbo-machine, such as a turbocharger.

Background

Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost) pressure. Conventional turbochargers primarily include an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power. The turbocharger shaft is typically supported by journal and thrust bearings (including a suitable lubrication system) located within a central bearing housing connected between the turbine and compressor wheel housings.

In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passage defined between facing radial walls arranged around the turbine chamber; an inlet disposed about the inlet passage; and an outlet passage extending axially from the turbine chamber. The passageway and the chamber communicate such that pressurized exhaust gas entering the inlet chamber flows from the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel.

It is known to improve turbine performance by providing vanes, known as nozzle vanes, in the inlet passage to deflect gas flowing through the inlet passage towards the direction of rotation of the turbine wheel. Each vane is generally laminar and is positioned to have a radially outer surface which is arranged to oppose the movement of the exhaust gas within the inlet passage, i.e. the radially inward component of the movement of the exhaust gas in the inlet passage is such as to direct the exhaust gas onto the outer surface of the vane and then redirect it into circumferential movement.

The turbine may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the geometry of the inlet passageway can be varied to optimise the gas flow rate over a range of mass flows, so that the power output of the turbine can be varied to suit varying engine demands.

In one form of variable geometry turbocharger, the nozzle ring carries a plurality of axially extending vanes that extend into the inlet and through corresponding apertures ("slots") in a shroud that forms a radially extending wall of the inlet. The nozzle ring is axially movable by an actuator to control the width of the air passageway. The movement of the nozzle ring also controls the extent to which the vanes project through the respective slots.

An example of such a variable geometry turbocharger is shown in fig. 1(a) and 1(b) (from US 8,172,516). The variable geometry turbine shown comprises a turbine housing 1, the turbine housing 1 defining an inlet chamber 2, gas from an internal combustion engine (not shown) being delivered to the inlet chamber 2. The exhaust gas flows from the inlet chamber 2 to the outlet channel 3 via the annular inlet channel 4. One side of the inlet passageway 4 is defined by the surface of a movable annular wall member 5 constituting the nozzle ring, while the opposite side is defined by an annular shroud 6, which annular shroud 6 covers an opening facing an annular recess 8 in the wall.

Gas from the inlet chamber 2 to the outlet passage 3 flows through the turbine wheel 9 with the result that torque is applied to the turbocharger shaft 10 which is supported by a bearing assembly 14 which drives the compressor wheel 11. Rotation of the compressor wheel 11 about the axis of rotation 100 pressurizes ambient air present in the air inlet 12 and delivers the pressurized air to the air outlet 13, from which air outlet 13 it is fed to the internal combustion engine (not shown). The speed of the turbine wheel 9 is dependent on the speed of the gas passing through the annular inlet passage 4. For a fixed mass rate of gas flowing into the inlet passageway, the gas velocity is a function of the width of the inlet passageway 4, which can be adjusted by controlling the axial position of the nozzle ring 5. As the width of the inlet passage 4 decreases, the velocity of the gas passing through it increases. Fig. 1(a) shows the annular inlet channel 4 closed to a minimum width, while fig. 1(b) shows the inlet channel 4 fully open.

The nozzle ring 5 supports an array of circumferentially and equidistantly spaced vanes 7, each of the vanes 7 extending over the inlet passageway 4. The vanes 7 are oriented to deflect gas flowing through the inlet passage 4 towards the direction of rotation of the turbine wheel 9. As the nozzle ring 5 approaches the annular shroud 6 and the facing wall, the vanes 7 project through suitably configured slots in the shroud 6 and into the recesses 8. Each blade has an "inner" major surface closer to the axis of rotation 100 and an "outer" major surface further from the axis of rotation 100. The nozzle ring 5 and shroud 6 are both in a fixed angular position about the axis 100. The blade 7 is shown in fig. 1(a) and 1(b) as having a chamfered end (towards the right of the figure), but in most modern arrangements the blade is longitudinally symmetrical over its entire length, or is made up of two sections, each of which is longitudinally symmetrical but has a profile which differs from each other when viewed in the axial direction.

A pneumatically or hydraulically operated actuator 16 is operable to control the axial position of the nozzle ring 5 within an annular cavity 19 defined by a portion 26 of the turbine housing via an actuator output shaft (not shown) connected to a stirrup member (not shown). The stirrup member in turn engages axially extending guide rods (not shown) that support the nozzle ring 5. Thus, by appropriate control of the actuator 16, the axial position of the guide rods, and hence the nozzle ring 5, can be controlled. It should be understood that an electrically operated actuator may be used in place of the pneumatic or hydraulic actuator 16.

The nozzle ring 5 has axially extending inner and outer annular flanges 17, 18 respectively, the inner and outer annular flanges 17, 18 extending into an annular cavity 19, the annular cavity 19 being separated from the chamber 15 by a wall 27. Inner and outer sealing rings 20, 21, respectively, are provided to seal the nozzle ring 5 relative to the inner and outer annular surfaces of the annular cavity 19, while allowing the nozzle ring 5 to slide within the annular cavity 19. The inner sealing ring 20 is supported in an annular groove 22 formed in the inner surface of the cavity 19 and abuts the inner annular flange 17 of the nozzle ring 5, while the outer sealing ring 21 is supported in an annular flange 18 provided in the nozzle ring 5 and abuts the radially outermost inner surface of the cavity 19. It will be appreciated that the inner sealing ring 20 may be mounted in an annular groove in the flange 17 (rather than as shown) and/or the outer sealing ring 21 may be mounted in an annular groove provided in the outer surface of the cavity (rather than as shown). A first set of pressure balance apertures 25 are provided in the nozzle ring 5 within the vane passages defined between adjacent apertures, and a second set of pressure balance apertures 24 are provided outside the radius of the nozzle vane passages in the nozzle ring 5.

Note that in other known turbo machines the nozzle ring is axially fixed and an actuator is instead provided for translating the shroud in a direction parallel to the axis of rotation. This is referred to as a "moving shroud" arrangement.

In known variable geometry turbo machines that use vanes projecting through slots in the shroud, a gap is provided between the vanes and the edges of the slots to allow for thermal expansion of the vanes as the turbocharger becomes hotter. The blade and the slot have the same shape, viewed in the axial direction, but the blade is smaller than the slot. In a typical arrangement, the vanes are positioned with the axial centerline of each vane at the center of the respective slot such that the ratio of the distance from the centerline to the surface of the vane to the distance from the centerline to the edge of the respective slot is the same in all directions away from the centerline transverse to the axis of the turbine. The gap between the vane and the slot is typically set to at least about 0.5% of the distance of the vane center from the axis of rotation ("nozzle radius") around the entire periphery of the vane at room temperature (defined herein as 20 degrees celsius) (e.g., for a nozzle radius of 46.5mm, the gap may be 0.23mm or 0.5% of the nozzle radius). This means that if each of the blades is gradually thermally expanded perpendicular to the axial direction, all points around the periphery of the blade will simultaneously contact corresponding points on the slot. At all lower temperatures, there is a gap between the entire periphery of the vane and the edge of the respective slot.

Disclosure of Invention

The present invention is directed to new and useful vane and shroud assemblies for use in turbomachinery, and new and useful turbomachinery (particularly turbochargers) incorporating the vane assemblies.

In an earlier patent application (GB1619347.6, which was not published at the priority date of the present application), the present applicant proposed in a turbomachine of a turbo machine in which the vanes project from the nozzle through slots in the shroud at the inlet between the nozzle ring and the shroud, a "conformal" portion of the lateral surface (i.e. transverse to the axis of rotation) of each vane substantially conforming to the shape of a corresponding "conformal" portion of the lateral surface of the respective slot, so as to place the corresponding conformal portions of the surfaces relative to one another with only a small gap therebetween. This has the advantage that the gas flow between the corresponding conformal portion of the blade surface and the slot can be greatly reduced. This reduces leakage of gas from the nozzle ring into or out of the recess on the other side of the shroud. This leakage reduces the circumferential redirection of gas by the vanes and has been found to result in a significant loss in efficiency.

Although this proposal represents a significant technical advance in turbine technology, the inventors have found that in practice its advantages may not be fully realized. First, the formation of the vanes and slots is subject to tolerances, and therefore precise consistency between the vanes and slots may not be possible. Second, after the turbocharger has been in use for a period of time, the vanes are susceptible to Foreign Object Damage (FOD) due to debris in the exhaust gas, which reduces the quality of the consistency between the vane and slot shapes.

In general, the invention proposes that the vanes and the slots are formed and arranged such that there is contact between them at the front surface portion of the vanes. Away from the front surface portion, towards the rear edge of the vane, the vane and the slot comprise respective rear portions, which are spaced apart, for example by a substantially constant amount, and which conform to one another.

The present invention is motivated by the inventors' observation that FOD damage is not typically present in front of the radially inner surface of the vane and therefore it should be possible to achieve high quality contact in this region between the vane and the edge of the slot. However, if the rear of the blade is designed very close to the edge of the slot, a small amount of FOD damage thereto or a defect in that part of the blade or slot may cause the front of the blade to be disadvantageously spaced from the slot edge. This effect can be mitigated by forming the rear portion of the vane spaced from the slot edge.

Forming the slot and the rear of the blade surface can be considered similar to relief cutting used in mechanical cutting of objects, which reduces the risk of obstructing the cut by the object being remote from the part where the cut occurs.

Furthermore, arranging the blades and slots to be spaced apart in their respective rear portions may reduce the chance of the blades being caught on the slots due to thermal transients. This is because the uneven (differential) thermal expansion of the nozzle and the rear of the slot is less likely to cause them to collide with each other even though it causes the gap between them to decrease.

The present invention embodies a turbine comprising:

(i) a turbine wheel, having an axis,

(ii) a turbine housing for defining a chamber for receiving a turbine wheel for rotation about an axis, the turbine housing further defining an inlet port, and an annular inlet passage from the inlet port to the chamber,

(iii) an annular shroud defining a plurality of slots and surrounding an axis, each slot having a slot surface; and

(iv) a nozzle ring supporting a plurality of vanes extending from the nozzle ring parallel to the axis and projecting through respective ones of the slots;

each of the blades has:

an axially extending vane surface comprising (i) an outer vane surface facing the radially outer surface of the respective slot, (ii) an opposing inner vane surface facing the radially inner surface of the respective slot, and

a midline between the inner surface of the blade and the outer surface of the blade extending from the front end of the blade to the rear end of the blade;

the vane is positionable such that a front surface portion of the vane inner surface contacts a respective front surface portion of the respective slot surface;

the blade inner surface further comprises a rear surface portion extending along at least 33% of the mid-line and which is spaced apart from the opposite rear surface portion of the slot surface by a distance in the range of 10 to 250 microns, more preferably at least 25 microns and/or no more than 100 microns at room temperature and when in contact with the front surface portion.

This spacing may enable an effective compromise between a low spacing which may reduce gas leakage between the rear portions and a high spacing which will reduce the tendency for defects in the inner surface of the inner vane and the rear portion of the slot surface to cause these surfaces to meet.

Preferably, at room temperature, the respective profiles of the rear surface portion of the vane surface and the rear surface portion of the slot surface differ from each other by no more than 30 microns, 20 microns, or even 10 microns (for a nozzle radius of 48.1mm, they correspond to 0.05%, 0.04%, or even 0.02% of the nozzle radius).

The conformity of the vane surface and the rear surface portion of the slot surface may mean that each point on the rear surface portion of the vane is spaced from a respective corresponding point on the inner surface of the slot by a distance in the range of 0.1-0.3% of the radius of the nozzle. For a nozzle radius of 48.1mm, this would be a distance range of about 0.05mm to 0.15 mm.

In the first case, the front surface portion of the blade may be very short (e.g. no more than 5% of the length of the centre line), or even point contact. This may have the advantage of minimising the risk of the blades being caught on the slots due to thermal transients, since the dimensions of the regions where the blades are close to each other are small.

In the second case, the front surface portion of the blade may be longer (e.g., extending along at least 15% of the length of the midline). The length of the front surface portion may for example be less than 10% of the centre line along which the rear surface portion of the inner surface of the blade extends, subtracted from 100%. The front surface portions of the vanes may be arranged to closely conform to the shape of the front surface portions of the respective slots. They may be designed to have identical shapes. In practice, however, due to machining tolerances, the respective profiles of the front surface portions of the vane surfaces and the respective front surface portions of the respective slot surfaces may differ from each other by an amount in the range of 1 to 50 microns, or more preferably in the range of 1 to 25 microns. The difference is preferably less than the minimum spacing of the vane inner surface and the rear of the slot surface.

The front surface portion of the blade surface may extend along 15-20% of the length of the centre line or 15-25% of the length of the centre line.

The front surface portion of the blade may comprise a point at which the midline intersects the front edge of the blade. In fact, when the vane surface and the front surface portion of the slot surface are in contact, the vane surface and the slot surface may further be in contact with each other at least one point on the radially outer surface of the vane.

The rear portion of the blade surface may extend at least 50%, at least 60% or at least 70% of the length of the centre line.

In this document, the statement that the inner surface of the blade and the rear surface portion of the slot surface are spaced apart by a range of distances means that the respective distance from each point in the rear surface portion to the respective closest point of the rear surface portion of the rear slot surface is within this range. The statement refers to the portion of the inner surface of the vane that is axially aligned with the slot surface.

In this document, the statement that two lines differ from each other by no more than a certain distance x is to be understood as meaning that the lines can be placed such that they do not cross and that no point along any of the lines is further than the distance x from the other line. The statement that the front surface portion of the blade surface and the corresponding front surface portion of the slot surface differ from each other by no more than a certain distance x means that the front surface portions of the blade surface and the front surface portions of the slot surface are axially aligned with each other and show corresponding lines when viewed in the axial direction. In this view, the lines do not differ from each other by more than a distance x.

Preferably, the turbine is of the type in which the radially inner surfaces of the vanes and slots are at a lower pressure than the radially outer surfaces.

The turbine may include a rotation mechanism for generating a rotational torque for urging the nozzle ring to rotate relative to the shroud in the sense that the vanes and corresponding front surface portions of the slots are urged together. In some arrangements, the rotation mechanism is simply the force exerted by the exhaust gas on the vanes.

Drawings

Embodiments of the invention will now be described, for example purposes only, with reference to the following drawings, in which:

fig. 1 consists of fig. 1(a) and 1(b), fig. 1(a) being an axial cross-section of a known variable geometry turbine, fig. 1(b) being a cross-section of a portion of the turbine of fig. 1 (a);

FIG. 2 is an axial view of a nozzle ring that may be used in the known arrangement of FIG. 1;

FIG. 3 is an axial view of a shroud that may be used in the known arrangement of FIG. 1;

FIG. 4 illustrates the positional relationship between the nozzle ring of FIG. 2 and the shroud of FIG. 3;

FIG. 5 illustrates a possible positional relationship between the vanes and the corresponding slots;

FIG. 6 illustrates the formation of foreign object damage on a turbine blade;

FIG. 7 illustrates a region of the outer surface of the blade that is not damaged by foreign objects;

FIG. 8 illustrates how the positional arrangement of FIG. 5 may be modified due to foreign object damage;

fig. 9 shows the positional relationship of the vanes and the corresponding slots in the first embodiment of the present invention;

fig. 10, consisting of fig. 10(a) and 10(b), shows the profile of the slot in the first embodiment of the present invention; and

fig. 11 shows the positional relationship of the vanes and the corresponding slots in the second embodiment of the present invention.

Detailed Description

Referring to fig. 2, a nozzle ring is shown that may be used in the known turbocharger of fig. 1. The nozzle ring is viewed from a position between the nozzle ring 5 and the shroud 6 to the right in the axial direction shown in fig. 1(a) (this direction is also referred to herein as "from the turbine end of the turbocharger").

The axis about which the turbine wheel 9 (not shown in fig. 2, but visible in fig. 1 (a)) and the compressor wheel 11 (also not shown in fig. 2, but visible in fig. 1 (a)) rotate is indicated at 100.

Viewed in this axial direction, the generally planar annular nozzle ring 5 surrounds an axis 100. The vanes 7 project in the axial direction from the nozzle ring 5. By defining a circle 70 centrally located on the axis 100 and passing through the centre of the profile of the vane 7, we can define the nozzle radius 71 as the radius of the circle 70. The gas moves radially inwardly in the gap between the nozzle ring 5 and the shroud 6.

The nozzle ring 5 is moved axially by an actuator 16 (not shown in fig. 2, but visible in fig. 1 (a)) within an annular cavity (also not shown in fig. 2, but visible in fig. 1 (a)) defined by a portion 60 of the turbine housing. Each vane 7 is optionally longitudinally symmetrical (that is, its profile may be the same at all axial positions as viewed in the axial direction), although in some embodiments only a portion of the vane 7 is longitudinally symmetrical. The profile of the blade (or a longitudinally symmetrical part thereof), viewed along the longitudinal axis, is elongate, having two ends, with a median line extending between these ends. On either side of the midline is the major surface of the blade, with the profile having a relatively low curvature, while the curvature of the profile at either end of the midline is higher.

The actuator exerts a force on the nozzle ring 5 via two axially extending guide rods. In fig. 2, a portion 32 of the nozzle ring 5 is omitted so that the connection between the nozzle ring 5 and the first of the guide rods can be seen. The guide bar is not shown but is centred at the position marked 61. The guide bar is integrally formed with a bracket 33 (commonly referred to as a "foot"), which bracket 33 extends circumferentially from the guide bar to either side. The bracket 33 comprises two circular holes 62, 63. The face of the nozzle ring 5 facing away from the shroud 6 is formed with two bosses 34, 64 projecting from the nozzle ring 6. Each boss 34, 64 has a circular profile (viewed in the axial direction). Bosses 34, 64 are inserted into bores 62, 63, respectively, and bosses 34, 64 are sized such that boss 34 substantially fills bore 62, while boss 64 is narrower than bore 63. The connection between the bosses 34 and the apertures 62 fixes the circumferential position of the nozzle ring 5 relative to the bracket 33 (in a typical implementation, the relative circumferential movement of the nozzle ring 5 and shroud 6 about the axis 100 is no more than 0.05 degrees). However, if the guide rods are radially separated due to thermal expansion, the clearance between boss 64 and bore 63 allows the carrier 33 to rotate slightly about boss 34. Thus, the boss 34 is referred to as a "pivot".

The location at which the second of the guide rods is connected to the nozzle ring 5, seen in the axial direction, is shown at 31. The connection between the nozzle ring 5 and the second guide rods is due to the second brackets (not visible in fig. 2) being integrally attached to the second guide rods. A second bracket is attached to the rear surface of the nozzle ring 5 in the same manner as the bracket 33. The pivot of the second bracket is at position 35.

The apertures 24, 25 are balancing apertures provided in the nozzle ring 5 for balancing the pressure. They are provided to obtain the desired axial load (or force) on the nozzle ring 5.

Facing the nozzle ring 5 is a shroud 6 as shown in fig. 3. Fig. 3 is a view from the nozzle ring 5 toward the shroud 6 (i.e., toward the right in fig. 1). The shroud defines a slot 30 (that is, a through hole) for receiving a respective one of the vanes 7. The edge of each slot is an inwardly facing transverse (i.e., transverse to axis 100) slot surface. Note that in fig. 3, the slots 30 are not shown as having the same profile as the vanes 7 of fig. 2, but generally the respective profiles do have substantially the same shape, although the size of the slots is larger than the size of the vanes.

Fig. 4 is another view in the axial direction from the nozzle ring 5 toward the shroud 6 (i.e., toward the right in fig. 1 (a)), showing representative vanes 7 inserted into respective representative slots 30. The vanes 7 have a generally arcuate (crescent-shaped) profile, although in other forms the vanes are generally planar.

Specifically, the vanes 7 have vane inner surfaces 41 that are closer to the impeller. The vane inner surface 41 is generally concave in the axial direction, but may alternatively be planar. The vanes 7 also have a vane outer surface 42, the vane outer surface 42 being closer to the exhaust gas inlet of the turbine. Each of the blade inner and outer surfaces 41, 42 is a main surface of the blade. The vane outer surface 42 is generally convex in the axial direction, but may also be planar. The main surfaces 41, 42 of the blade 7 face in substantially opposite directions and are connected by two axially extending end surfaces 43, 44, which end surfaces 43, 44 have a radius of curvature, seen in the axial direction, which is smaller than the radius of curvature of either of the two surfaces 41, 42. The end surfaces 43, 44 are referred to as a leading edge surface 43 and a trailing edge surface 44, respectively.

In most arrangements, the vane outer surface 42 is arranged to oppose the movement of the exhaust gas in the inlet passage, i.e. the movement of the exhaust gas in the inlet passage causes the exhaust gas to be directed onto the vane outer surface. Accordingly, the blade outer surface 42 is typically at a higher pressure than the blade inner surface 41 and is referred to as a "high pressure" (or simply "pressure") surface, while the blade inner surface 41 is referred to as a "low pressure" (or "suction") surface. Corresponding portions of these opposing inwardly facing surfaces define the edges of slot 30 and are given the same corresponding names.

Each blade 7 has a centre line 51, seen in the axial direction, which extends from one end of the blade to the other (half the distance between the inner and outer surfaces 41, 42 of the blade, seen in the axial direction) and which has both a radial component and a circumferential component. We refer to the slot surface that the vane inner surface 41 faces as the slot inner surface 46 and the slot surface that the vane outer surface 42 faces as the slot outer surface 47. As shown in fig. 4, there is a gap having a substantially constant width between the periphery of the vane 7 and the surface of the slot 30. The gap comprises four parts: between the vane inner surface 41 and the slot inner surface 46; between blade outer surface 42 and slot outer surface 47; and between the leading edge surfaces 43 and trailing edge surfaces 44 of the vanes and the respective leading portions 49 and trailing portions 59 of the slot edges. Together, surfaces 46, 47, 49 and 59 constitute inwardly facing slot surfaces that define the slot.

Fig. 5 shows a possible positional arrangement between the vanes and the shroud slots proposed in GB 1619347.6. The turbine has the form shown in figures 1 and 2, with the difference being the shape and size of the vanes and/or slots in the shroud. In fig. 5, elements corresponding to those of fig. 1 to 4 are given reference numerals increased by 100. Thus, a representative vane 107 is depicted within the representative slot 130. The vane outer surface 142 faces the slot outer surface 147 and the vane inner surface 141 faces the slot inner surface 146. Alternatively, the vane 107 may be longitudinally symmetrical throughout its length (i.e., the profile is the same at all axial positions as viewed in the axial direction). In another possibility, only a portion of the vane 107 may be axially symmetric, e.g., including a portion insertable into the slot 130 when the vane 107 is in its most advanced position. In this case the blade part shown in fig. 5 is part of this axially symmetrical part of the blade. The vanes 107 are integrally formed with the nozzle ring 5 (e.g. by casting and/or machining) as a one-piece unit.

In contrast to the known vane of fig. 4, the vane 107 of fig. 5 has a relatively narrow gap between the vane inner surface 141 and the opposing slot inner surface 146. Instead, a wider gap exists between the vane outer surface 142 and the corresponding portion 147 of the slot outer surface 147. This means that exhaust gas entering the shroud recess 8 between the outer vane surface 142 and the slot outer surface 147 is largely prevented from exiting the shroud recess between the vane inner surface 141 and the slot inner surface 146.

To further achieve this effect, the blade surface and the slot surface are formed with a conformal portion 145 that extends along at least about 80% of the length of the midline 151. As shown in FIG. 5, the conformal portion 145 of the blade surface in FIG. 5 includes substantially all of the blade inner surface 141. The contour (i.e., shape as viewed in the axial direction) of the vane inner surface 141 and the corresponding portion of the slot inner surface 146 are very similar to each other so that they can be placed against each other along the entire length of the conformal portion 145 with little clearance therebetween. Specifically, at room temperature, the profile of vane inner surface 141 and the corresponding portion of slot inner surface 146 are such that they may be positioned against each other with a gap therebetween (e.g., transverse to the centerline) of no more than 0.35% of nozzle radius 71. The leading edge surface 143 of the vane contacts a corresponding portion 149 of the inner surface of the slot 130.

If there is uneven thermal expansion between the vanes 107 and the shroud (e.g., because they are formed of different materials and/or are subjected to different temperatures), conformal portions of the vanes 107 may be forced against the slot inner surfaces 146. The frictional forces therebetween may then prevent axial movement of the blades relative to the shroud. However, there is some freedom of movement (free play) in the system (e.g. because the nozzle ring 5 is connected to the rods (as shown in fig. 2), the nozzle ring may have some inherent freedom to rotate about the axis 100), which allows the vanes 107 to retract to some extent from the conformal surfaces of the slots.

Fig. 6 and 7 illustrate the formation of Foreign Object Damage (FOD) during use of the turbine of fig. 5. The large arrows indicate the general direction of rotation of the exhaust gas entering the turbine and the direction of rotation of the turbine wheel. Fig. 6 is a view of the shroud in an axial direction, and fig. 7 is an enlarged portion of fig. 6. The shroud defines slots 130, the slots 130 containing respective vanes 107. It has been found experimentally that for a given one of the vanes 107 (indicated 107a) there is a line 150 extending in an upstream direction (i.e. in a direction along the vane 107a opposite to the large arrow in fig. 6) from the trailing edge of an adjacent vane (indicated 107b) such that the vane 107b protects the vane 107a from FOD in a "front surface portion" 160 of the inner surface of the vane 107a radially outward of the line 150. In effect, the line 150 represents the trajectory of a debris particle that passes only through the inner end of the vane 107b and then impinges on the vane 107 a. All FOD damage to the leaf 107a is between the intercept of the line 150 and the inner surface of the leaf 107a and the rear edge 165 of the leaf 107 a.

In the case of a nozzle ring with a nozzle radius of 48.1mm and each vane having a length of 23mm (i.e. a mid-line length), it has been found that the undamaged portion of the vane inner surface 141 extends at least the first 4mm of the mid-line length (i.e. 17% of the vane length) from the end of the mid-line at the leading edge 167. Between 4mm and 5mm, there are some small impact pits and small indentations. At all points other than 5mm from the leading edge 167 of the vane 107a, the surface has the same condition. This effect is observed to be equal on all blades of the turbine. (Note that computer modeling showed that all FODs were at least 5.5262mm from the front 167, but this was found to be overestimated.)

Fig. 8 shows the results of FOD at the tail of the vane 107 a. Assuming that at point 161 near the trailing edge 165 of the vane 107a, the inner surface of the vane 107 is FOD (e.g., a convex notch), this results in the inner surface 141 of the vane 107a being spaced from the opposing slot inner surface 146 by a distance of 0.05mm near the damaged portion. It has been found that this may result in a greater separation (e.g., 0.15mm) between the vane 107a and the slot inner surface 146 at a point 162 near the leading edge 167 on the vane inner surface 141. Gas can pass through this gap (from the recess behind the shroud) to the low pressure side of the blades 107, reducing the efficiency of the turbine.

Turning to fig. 9, a portion of a first embodiment of the present invention is shown. This embodiment includes a representative blade 207 having at least a portion with longitudinal symmetry parallel to axis 100, and a representative slot 230 having longitudinal symmetry along direction 100. The view of fig. 9 is seen parallel to direction 100 and shows in cross section a longitudinally symmetric blade 207 (a longitudinally symmetric portion of blade 207). The blade 207 has opposite major surfaces (inner and outer surfaces) with a midline (not shown) midway between them and extending from the leading edge to the trailing edge of the blade 207. On either side of the midline is a major surface of the blade, with the contour having a relatively low curvature, and the curvature of the contour at either end of the midline is higher than the curvature on the major surface.

This embodiment is a turbine having the same construction as the turbine of the known system of fig. 1-3 (and therefore elements corresponding to corresponding elements of the vanes and slots of fig. 5-8 are given the reference numerals increased by 100), with the only difference being that the radially inner surface 241 of the vane 207 (vane inner surface) and/or the slot inner surface 246 of the slot 230 have a corresponding profile which differs from the known system of fig. 1-3.

First, in the front surface portion 260 of the blade 207, the blade inner surface 241 and the slot inner surface 246 closely fit each other. In particular, they may be designed to have identical shapes, but in practice differ from each other by 1 to 50 microns, or more preferably 1 to 25 microns.

Second, when the vane inner surface 241 and the slot inner surface 246 contact each other in the front portion 260, the vane inner surface 241 is spaced apart from the slot inner surface 246 at all locations on the vane inner surface 241 that are closer to the rear edge 265 than the front portion 260 (this set of locations is referred to as a "rear surface portion" 266 of the vane inner surface 241). The spacing in substantially all of the rear surface portion 266 may be at least 0.05mm, in the case of a nozzle ring of nozzle radius 48.1mm, which is approximately 0.1% of the nozzle radius. In practice, manufacturing tolerances of the vanes 207 or slots 230 may cause this spacing to decrease. Further, in use, the spacing is reduced at an isolated location within rear surface portion 266 due to crater damage on blade inner surface 241.

However, even if there is a FOD in the rear surface portion 266 that causes the surface of the blade inner surface 241 to rise by a height of 0.05, this does not cause the blade inner surface 241 to impact the slot inner surface 246 in the rear surface portion 266, and therefore does not cause the blade inner surface 241 to be spaced apart from the slot inner surface 246 in the front surface portion 260.

Similarly, if the inner surface 241 of the blade 207 in the rear surface portion 266 is deformed just in a direction towards the slot inner surface 246 by a distance of 0.05mm due to tolerances in the manufacture of the blade 207 and/or the slot 230, this will not cause the blade inner surface 241 to strike the slot inner surface 246 in the rear surface portion of the blade inner surface 241 and, therefore, it will not space the inner surface 241 from the slot inner surface 246 in the front surface portion 260. In practice, the manufacturing tolerances of the blade 207 and slot 230 may be as high as 0.1mm, so a spacing of 0.05mm only reduces the chance of the blade inner surface 241 being spaced apart from the slot inner surface 246 in the front surface portion 260. Accordingly, it may be preferred to provide a greater spacing, such as a 0.1mm spacing, between the blade inner surface 241 and the slot inner surface 246 in at least a majority of the rear surface portion 266.

The spacing in the aft surface portion 266 between the vane inner surface 241 and the slot inner surface 246 has the further advantage of reducing the risk of the vanes 207 sticking to the shroud due to thermal transients.

In fig. 9, it is shown that the extent of the vane slot and the contact portion of the slot surface includes all radially inner surfaces of the vane up to the front end 267 of the vane 207, but does not include any radially outer surfaces of the vane 207. However, in a variation of this embodiment, the radially outer portion of the blade 207 near the leading edge of the blade 207 may contact the slot surface (in the manner shown in FIG. 7).

Fig. 10(a) shows the profile of the slot 230 without the vanes. The view is parallel to the axis of rotation 100 and the slot is longitudinally symmetric in this direction. The slot inner surface 246 includes a front surface portion 2461, and when the vane 207 is present, the front surface portion 2461 lies along the front surface portion 260 of the vane inner surface 241. The slot inner surface 246 also includes a rear surface portion 2462, the rear surface portion 2462 being spaced apart from a corresponding rear surface portion of the blade inner surface 241 in use. As shown in fig. 10(b), which is an enlarged view of the portion of fig. 10(a), there is a transition region 2463 between the front surface portion 2461 and the rear surface portion 2462 of the slot inner surface 246 that includes a convex portion 2464 of the surface 246 and a concave portion 2465 of the surface 246. The length of the vane along portion 2461 may be about 5.33mm, and the radius of curvature of each of portions 2464, 2465 may be about 0.5mm (i.e., reduced by about a factor of 10).

Turning to fig. 11, a view of a second embodiment of the present invention is shown. Elements having the same meaning as elements of the first embodiment are given the reference numeral increased by 100. This embodiment includes a vane 307 having at least a portion that is longitudinally symmetric parallel to axis 100, and a slot 330, the slot 330 being longitudinally symmetric in direction 100. As in the first embodiment, the vane 307 has opposite major surfaces (inner and outer surfaces) with a midline located midway between them and extending from the leading edge to the trailing edge of the vane. The view of fig. 11 is seen parallel to direction 100 and shows in cross section a longitudinally symmetric blade 307 (a longitudinally symmetric portion of blade 307). A rear surface portion 366 of the blade inner surface 341 (again the low pressure side of the blade) conforms to the rear surface portion 3461 of the slot inner surface 346. The two rear surface portions are slightly spaced apart, e.g. with a substantially constant spacing between them. The spacing is typically in the range of 10 microns to 250 microns, and more preferably at least 25 microns and/or no greater than 100 microns. However, the vane inner surface 341 also includes a front surface portion 368 opposite the front surface portion 3462 of the slot inner surface 346 that gradually approaches the vane inner surface 341 until the two contact each other at a contact point 3463. The contact point 3463 may be on a line 3464, the line 3464 tangent to the profile of the blades 307 and passing through the axis of rotation 100 located at the center of the shroud. This position is selected to minimize (or substantially eliminate) the radial forces transmitted between the blades and the shroud.

From point 3463 toward the leading edge 367 of the lobe 307, the leading edge surface 343 of the lobe is spaced from the opposing respective portion 349 of the inner surface 346 of the slot 330. The contact point 3463 may be less than 10%, or even less than 5% of the length of the midline from the leading edge 367 of the blade. The contact between the vane 307 and the inner surface of the slot 330 extends in a direction along much less than 5% of the midline of the vane between its opposite major surfaces, for example along less than 1% of the length of the midline, or even along 0.1% of the length of the midline.

Since the rear surface portion 3461 of the slot inner surface 346 is spaced apart from the rear surface portion 366 of the blade inner surface 341, defects on the rear surface portion due to machining tolerances and/or due to FOD to the blade 307 do not cause the rear surface portions to contact each other. Accordingly, no force is generated between the rear surface portions separating the slot inner surface 346 and the vane inner surface 341 in their respective front surface portions 368, 3462 such that contact at the contact point 3463 is lost.

Since all the contact between the vane 307 and the slot 330 is on the narrow contact point 3463, there is little risk of the vane 307 becoming locked on the slot 330, so that the sliding movement of the vane 307 in the axial direction is impaired.