Turbine shroud cooling

Abstract: Shrouds (40) surrounding a stage of turbine blades (44) are cooled by a compressor airflow which is led to the downstream end of the shrouds before contacting them. The airflow passes through apertures (56) in plates (50) then over the shrouds (40) in an upstream direction, to exit from apertures (62) in the shrouds (40) in parallel with and in the same direction as the gasflow. Airflow needed is reduced relative to prior art needs, resulting in improved engine efficiency, and ejection of the air does not disturb the gasflow.

Description: The present invention relates to turbine machinery, particularly of the kind utilised in gas turbine propulsion engines. More specifically, the invention relates to the improved cooling of such devices, especially in those engines used for the propulsion of aircraft, though not restrictively so. It is the common practice to provide compressor air for the purpose of cooling a multiplicity of turbine machinery parts. Thus, it is known to provide compressor air to shrouds, which in situ, surround a stage of turbine blades, and thereby form an outer wall of an associated turbine annulus in which in operation, the stage of blades rotates. A first drawback to the known system is that it necessitates the provision of duel airflows, one for cooling the downstream ends of the shrouds, and another for cooling the upstream ends thereof. This results in the use of a quantity of air which consequently cannot be used for combustion, and further results in a noticeable drop in engine efficiency. A second drawback in known structures is that the air used for cooling the upstream ends of the shrouds, by virtue of structure, could not be finally ejected into the gas stream in the annulus, without disturbing the flow. This added further to efficiency losses. It is known from US-A-3990807 to provide a turbine blade shroud capped by a cover in which cooling air is directed through the space defined between the shroud and cover before being ejected into the gas annulus in the same direction as the gases flowing through that annulus. However, there can be some loss of efficiency in the ejection of cooling air into the gas annulus. EP-A-0709550 discloses a similar shroud cooling arrangement, differing in that an elongated rib is provided on the radially outer surface of the shroud in order to inhibit the mixing of cooling air flows over the shroud radially outer surface. However, there are still efficiency losses incurred in the ejection of the cooling air into the gas annulus. It is an object of the present invention to provide such an arrangement having enhanced efficiency. According to the present invention, turbomachinery for a gas turbine engine comprises a turbine blade shroud capped by a cover which is in spaced apart relationship therewith intermediate the shroud ends to define a space, said space being connectable via apertures provided in the downstream end of said cover to a cooling air supply from a compressor of said engine, and connectable via exit apertures provided in said shroud near the shroud upstream end to the gas annulus of said engine so as to, on operation of said engine, when associated therewith, a plurality of fences being provided which extend for the full length of the space axially to a said engine when associated therewith, which fences channel said airflow, characterised in that said fences are forked at their upstream ends to define two legs, one of said legs having a cut-out at its end extremity so as to enable said cooling airflow to enter between the legs and flow in a reverse direction to said exit apertures provided in said shroud. The invention will now be described, by way of example, and with reference to the accompanying drawings in which: Fig 1 is a diagrammatic view of a gas turbine engine including turbine machinery of the present invention. Fig 2 is an axial cross section view through the turbine section of the engine of Fig 1. Fig 3 is a view in the direction of arrow 3 in Fig 2. Fig 4 is an enlarged pictorial part view of Fig 2. Fig 5 is a pictorial part view of an alternative configuration to the device of Figs 2 and 3. Fig 6 is a pictorial part view of a further alternative configuration to the devices of Figs 2, 3, 4 and 5. Fig 7 is a cross sectional part view of a mode of retention of the cover of Fig 2. Referring to Fig 1. A gas turbine engine which includes turbine machinery in accordance with the present invention, has a compressor section 10, a combustion and fuel entry section 12 and a turbine section 14. The engine terminates in an exhaust nozzle 16. Referring now to Fig 2. The turbine section 12, has an outer casing 18 which includes internal annular flanges 20, 22. Each flange 20, 22 have respective annular grooves 24, 26. Groove 24 supports the outer rim 28 of an annular groove 30 in the upstream end of the integral shroud 32 of a non rotatable guide vane 34. By 'upstream' and 'downstream' is meant with respect to the direction of flow of gases through the engine of Fig 1. The inner rim 36 of the guide vane shroud 32 supports the downstream end 38 of a turbine blade shroud 40 in sliding relationship. Shaped sealing strips 39, 41 are fitted therebetween. The shroud 40 is spaced from the tip 42 of a turbine blade 41, and extends upstream and downstream thereof. The inner rim 26 of the flange 22 supports the upstream end of the shroud 40, which in turn, carries an annular airflow restrictor 46, the operation of which is explained later in this specification. A shaped sealing strip 47 is fitted therebetween. The shroud 40 has a number of fences 48 extending over the major portion of its length, intermediate its thickened ends, which fences are covered by a plate 50 which, with the fences 48, forms a number of closed, elongated passages 52 lying axially of the engine. The passages 52 are best seen in Fig 3. Only two passages 52 are shown therein, but in practice, there would be sufficient fences 48 to provide passages which would span the major portion of the width of the shroud 40 in a direction circumferentially of the turbine stage of which blade 44 forms a part. The upstream ends of the fences 48 are forked, for reasons which are explained hereinafter. Referring back to Fig 2. In operation of the gas turbine engine, air is bled from the compressor 10 (Fig 1) and led via a circumferential row of holes 54 in the restrictor 46, the space defined by the turbine casing 18 and plate 50, to a further row of holes 56, spanning the plate 50 at its downstream end. The air passes inwardly through the holes 56, into the passages 52, reversing its direction of flow, to flow along the passages 52, to their upstream end portions. On reaching the upstream end portions of the passages 52, the airflow is constricted by narrowed passages defined by the forked portions of the fences 48. As a result, the airflow is re-energised at least in some small degree, prior to reaching a cutout 58 in the end extremity of one leg 60 in each fork. The cut out 58 is more clearly, seen in Fig 4. The air passes through the cut outs 58 and again reverses its direction of flow, to exit from a row of holes 62 in the shroud 40, in the same direction as the gas flow through the turbine section 14, as is indicated by the arrow 64. It will be seen from the foregoing description how a single compressor air supply can be utilised to cool both outer and inner surfaces of a turbine blade shroud, and further, be ejected therefrom into a region of the gas flow annulus, without disturbing the gas flow itself. The present invention has been described with reference to only one shroud and an associated blade. However, the turbine stage will of course comprise a ring of turbine blades 44, and a ring of shrouds 40. Each shroud 40 may span one or more blades 44 in known manner, and, in accordance with the example of the particular present invention, will be provided with its own plate 50. The fences 48 may be cast on the shroud 40 at the manufacturing stage thereof. However, the shroud 40 is not a rotating part, and consequently, to achieve minimum weight, appropriately shaped thin metal strips may be brazed or otherwise fixed thereon, to form the fences. An alternative structure comprising a honeycomb core 66 (Fig 5) could be substituted for the fences 48. The walls of the core 66 would have holes 70 in them, for the passage of cooling air through the core, towards the upstream end of the shroud 40. A further alternative to fences would provide pillars 72 depicted in Fig 6 and which would separate the shroud 40 and plate 50. Air passing through the holes 56 would flow around the pillars 72 in a generally upstream direction, until it reached the exit holes 62 in the shroud 40. The pillars 72 are illustrated in straight form, but they could have any form, regular or irregular. It is the common practice, to seal the gap between adjacent shrouds against gas leakage by providing opposing slots in opposing shroud edges, and fitting a metal strip 74 therein, to bridge the gap. This is depicted in Fig 7 and per se forms no part of the present invention. However, Fig 7 also depicts a mode of retaining each plate 50 on its respective shroud 40, and consists of grooved edge portions 76 being provided along the side edges of each shroud 40, into which the side edges of the plate 50 are sprung. The grooved edge portions need not extend the full length of the respective shrouds, but preferably would extend over a substantial portion of the edges of the aforementioned intermediate part thereof. Despite the serpentine path which the compressor driven air has to follow between holes 54 and 62, a positive flow is maintained, helped by a pressure drop created at the exit ends of holes 62, by the passage of the high velocity gases thereby, in the direction indicated by arrow 64. The cooling air is bled from one stage of compressor blades (not shown) in the associated engine, into a plenum (not shown) from where the cooling air passes to the apertures 54, then to the apertures 56. However, the cooling air could be piped from the plenum (not shown) by pipes (not shown) equal in number to apertures 56, and connected thereto, one pipe to one aperture 56. Hereinbefore, the cover is represented by plate 50. It could however, be a frusto conical member (not shown) having an axial cross-sectional shape identical with that of plate 50, as illustrated in Figure 2.

Claims: Turbo machinery for a gas turbine engine comprising a turbine blade shroud (40) capped by a cover (50) which is in spaced relationship therewith intermediate the shroud (40) ends to define a space, said space being connected via apertures (56) provided in the downstream end of said cover (50) to a cooling air supply from a compressor (10) of said engine, and connectable via exit apertures (62) provided in said shroud (40) near the shroud (40) upstream end to the gas annulus of said engine so as to, on operation of the said engine when associated therewith, eject said cooling airflow when effected into the gas annulus, in parallel with, and in the same direction as, the gasflow (64) therethrough, a plurality of fences (48) being provided which extend for the full length of the space axially to said engine when associated therewith, which fences (48) channel said cooling airflow, characterised in that said fences (48) are forked at their upstream ends to define two legs, one of said legs (60) having a cut-out (58) at its end extremity so as to enable said cooling airflow to enter between the legs and flow in a reverse direction to said exit apertures (62) provided in said shroud (40). Turbo machinery as claimed in claim 1 characterised in that said cooling air flow is directed to said cover inlet apertures (56) via a flow path defined by the exterior surface of the cover (50) and an interim surface portion of the turbine casing (14) of a said gas turbine engine when associated therewith. Turbo machinery as claimed in claim 1 or claim 2 characterised in that the cover (50) is a plate shaped to span at least one shroud (40) circumferentially thereof.

Inventors: BHANGU JAGNANDAN KUMAR, DODD ALEC GEORGE, BHANGU, JAGNANDAN KUMAR, DODD, ALEC GEORGE
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1999
Main classification: F01D25
IPC codes: F01D25, F01D9