Shroud configuration for turbine blades

Abstract: The shroud (26) of a shrouded turbine blade (22) has a convex leading edge (30) the apex of which extends towards the trailing edge of the shroud (24) of an adjacent, preceding guide vane. The extension enables the containment of a reverse flow of high pressure gas from the juncture of the blade aerofoil and shroud, until the pressure has dropped sufficiently to avoid nullifying gas flow from the corresponding portion of the guide vane (20) and thereby reduces excessive hot gas leakage from between the opposing shrouds edges.

Description: The present invention relates to shrouded turbine blades of the kind used in gas turbine engines. The invention has particular efficacy if used in gas turbine engines of the kind used to power aircraft. During the operation of a gas turbine engine, combustion gases pass through a stage of guide vanes, and are directed thereby on to a rotary stage of turbine blades in know manner. The stage of blades is thus rotated and, via connecting shafting, in turn, rotate a compressor and/or fan. The side edges of the shrouds of the turbine blades are sealed one to the other, in known manner, by means which allows the blades to move away laterally from each other, under the influence of, inter alia, centrifugal forces, without producing gaps through which gases would escape. However, axial spaces must be provided between the downstream edges of shrouds surrounding the stage of guide vanes, and the upstream edges of the turbine blades shrouds, so as to avoid friction between the static and rotating stages. During operational rotation of the stage of turbine blades, the rotational loads exerted on the gas flow, added to by the friction between the gas and the juncture of the blades aerofoil portions and their respective shroud under surfaces, causes the generation of a volume of relatively high pressure gas, which radiates outwardly of the blades surfaces, in directions other than in a downstream direction, but mainly in a direction forwardly of the blades leading edges, ie upstream, in the form of a jet. The force of the jet, under some operating regimes of an associated engine is sufficient to overcome the force of the gas leaving the corresponding area on the guide vanes, ie the juncture of their aerofoils and shrouds, in a downstream direction. There results an outwardly turned flow of gases through the gap between the guide vanes and turbine, which flow then reverses, to flow over the exterior of the turbine blade shrouds. US-A-5 466 123 discloses an annular array of nozzle guide vanes having radially inner platforms whose downstream edges are so configured as to define a plurality of radially directed convex and concave portions. The configurations of these portions are so arranged that they vary the static pressure in the field immediately downstream of the vanes. The effect is to provide substantially circumferentially uniform static pressure in that field. The present invention seeks to provide a shrouded turbine blade having improved gas flow control. According to the present invention, a shrouded turbine blade for cooperation with an opposing shrouded guide vane comprises an aerofoil having a shroud on its radially outer end with respect to an axis of operational rotation thereof, whereby the local shape of the leading edge of said shroud is convex over a major portion of the width extremities of its leading edge, the apex of said convex local shape being operationally closest to the shroud of said opposing guide vane and directly in front of the leading edge of its associated blade aerofoil, said convex local shape being effectively identical with the pressure wave pattern defined by gases in the vicinity of said aerofoil leading edge, whereby combustion gas flows which extend from the radially outer leading edge portion of said turbine blade aerofoil in directions other than downstream during in situ operational rotation of said turbine blade in a gas turbine engine are at least substantially contained. The invention will now be described, by way of example and with reference to figures 1, 4 and 5 of the accompany drawings in which: Fig 1 is a diagrammatic view of a gas turbine engine incorporating shrouded guide vanes and shrouded turbine blades positioned in known manner, with or without the features of the present invention. Fig 2 is an enlarged part view of a co-operating guide vane and prior art shrouded turbine blade. Fig 3 is a view in the direction of arrow 3 in fig 2. Fig 4 is as fig 3 but of a shrouded turbine blade including an example of the present invention, and Fig 5 is a view in the direction of arrow 5 in fig 4. Referring to fig 1. A gas turbine engine 10 has a compressor 12, combustion equipment 14, a turbine section 16, and an exhaust nozzle 18, all of which are arranged in flow series. The turbine stage 16 comprises a stage of non-rotatable guide vanes 20, followed in the said flow series by a rotatable stage of turbine blades 22, which are spaced from the stage of guide vanes 20, in a direction axially of the engine 10. Each vane 20 and blade 22 is provided with a respective shroud 24 and 26, which together, form axial portions of the outer wall of the gas annulus 28. Referring now to fig 2, which is an illustration of a drawback experienced during use of prior art turbine blades 22, in conjunction with guide vanes 20. During rotational operation of the turbine blades 22, friction between the gas and blades aerofoils and shrouds underside surfaces causes local slowing of the gas flow in those regions. There results a build up of pressure in the slowed gas flow at the juncture of the respective blades aerofoils and shrouds, which develops into a flow which radiates outwardly, around the leading edge of each blade, in directions laterally and forwardly thereof. The maximum pressure in the radiated flow is in that portion of the gas which flows off the leading edge of each blade, in a direction upstream, ie a reversed flow, relative to the main flow through the turbine. Brief references to fig 3 shows the pressure pattern 29 in the slowed gas flow as having a pseudo sinewave form, which rotates with the turbine stage. Referring again to fig 2. The drawback mentioned hereinbefore, is that the pressure of the reversed gas flow from the leading edges of the blades is so high, that it overcomes the gas flow leaving the corresponding areas of the guide vanes 20, in the opposite direction with the result that both gas flows turn outwardly through the clearance space between their opposing shroud edges, and there, mix with cooling air (arrows 25) which is flowing over the outer surfaces of the blades shrouds 26. The effectiveness of the cooling air is thus degraded with the result that, in prior art turbines, extra cooling air has to be provided, in order to maintain appropriate cooling, and thereby, blade shroud life. However, the use of more air to achieve the required cooling, leaves less air for combustion, and a consequence of this is that the engine efficiency is reduced. Referring now to fig 4. The present invention is exemplified by modifying the leading edge 30 of each turbine blade shroud 26, such that part thereof which is immediately in front of the leading edge of its respective blade, is sufficiently extended as to cover the highest pressure portion of the reversed gas flow, until such time that its pressure has dropped to a value too low to adversely affect the opposing gas flow leaving the corresponding areas of the guide vanes 20. By this means, leakage to the exterior of the turbine is reduced to an acceptable level, and the need for excessive cooling air obviated. In the example of the present invention as illustrated, the leading edge 30 of each blade shroud 26 is convexly curved, the apex of each curve being closest to the opposing guide vane shroud, and directly in front of the leading edge of its associated blade aerofoil. The remainder of the curve approximately follows the pressure patterns depicted in the fig 3 prior art. Referring to fig 5. The reduced pressure, reversed gas flow is represented by the differing sized arrows.

Claims: A shrouded turbine blade (22) for cooperation with an opposing shrouded guide vane (20) comprising an aerofoil having a shroud (26) on its radially outer end with respect to an axis of operational rotation thereof, characterised in that the local shape of the leading edge (30) of said shroud (26) is convex over a major portion of the width extremities of its leading edge (30), the apex of said convex local shape being operationally closest to the shroud (24) of said opposing guide vane (20) and directly in front of the leading edge of its associated blade (22) aerofoil, said convex local shape being effectively identical with the pressure wave pattern defined by gases in the vicinity of said aerofoil (22) leading edge, whereby combustion gas flows which extend from the radially outer leading edge portion of said turbine blade (22) aerofoil in directions other than downstream during in situ operational rotation of said turbine blade (22) in a gas turbine engine are at least substantially contained. A turbine disc characterised in that said disc is fitted with shrouded turbine blades as claimed in claim 1. A gas turbine engine provided with a turbine disc, characterised in that said disc is as claimed in claim 2.

Inventors: DANCER STEPHEN NICHOLAS, DODD ALEC GEORGE, DANCER, STEPHEN NICHOLAS, DODD, ALEC GEORGE
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 2000
Main classification: F01D5
IPC codes: F01D5