Tip sealing for turbine rotor blade

Abstract: An unshrouded turbine rotor blade 24 for use particularly in gas turbine engines comprising an aerofoil 30 having a leading edge 32 and a trailing edge 34. The radially outer extremity of the aerofoil 30 having a passage 42 defined by a peripheral wall 44. An aperture is formed within the wall 44 in the proximity of the trailing edge 32 of the aerofoil portion 30. The walled passage 42 is provided to capture and retain air or gas flowing over the tip of the aerofoil 30 and redirect the flow through the aperture 46 at the trailing edge of the aerofoil 30.

Description: This invention relates to turbine rotor blades and in particular to rotor blades for use in gas turbine engines. The turbine of a gas turbine engine depends for its operation on the transfer of energy between the combustion gases and the turbine. The losses which prevent the turbine from being totally efficient are due at least in part to gas leakage over the turbine blade tips. Hence the efficiency of each rotor stage in a gas turbine engine is dependent on the amount of energy transmitted into the rotor stage and this is limited particularly in unshrouded blades by any leakage flow of working fluid ie. air or gas across the tips of the blades of the rotors. In turbines with unshrouded turbine rotor blades a portion of the working fluid flowing through the turbine tends to migrate from the concave pressure surface to the convex suction surface of the aerofoil portion of the blade through the gap between the tip of the aerofoil and the stationary shroud or casing. This leakage occurs because of a pressure difference which exists between the pressure and suction sides of the aerofoil. The leakage flow also causes flow disturbances to be set up over a large proportion of the height of the aerofoil which leads to losses in efficiency of the turbine. By controlling the leakage flow of air or gas across the tips of the blades it is possible to increase the efficiency of each rotor stage. There is disclosed in GB 2155558A an unshrouded rotor blade which has a recess at its radially outer extremity. The recess is defined by a peripheral wall and a number of transverse walls extending across the recess, thereby dividing the aerofoil into a number of chambers. These walls form a labyrinth seal and trapped vortices are set up in each of these chambers. The trapped vortices aim to reduce the leakage flow between the tip of the blade and the shroud or casing. The above arrangement traps the leakage flow within the recesses thereby reducing leakage flow across the tip of the blade. However the kinetic energy of this flow is still lost since it remains trapped within the chambers. This flow still forms a vortex in the main passage, albeit of reduced strength, which generates extra loss. In addition the prior art arrangement suffers from the disadvantage that most of the over tip leakage flow is over the rear part of the aerofoil where typically it is too thin to form within a cavity. It is also known, for instance from US-A-5 503 527, EP-A-0 684 364, DE-B-22 02 857, FR-A-2 074 130, GB-A-2 111 131 and EP-A-0 317 432, to provide an unshrouded turbine blade having a radially outer extremity which defines a passage. The so-defined passage is provided with an aperture in the proximity of the trailing edge of the aerofoil portion of the blade. Over tip leakage flows are directed into the defined passage, thereby alleviating the flow disturbances set up by the leakage flows. Additionally, the flows are redirected by the passage to flow from the leading edge of the aerofoil to the trailing edge, so recovering work that would otherwise have been lost by the flow. It is an object of the present invention to provide a rotor blade in which such an arrangement functions in the most effective manner. According to the present invention, an unshrouded rotor blade includes an aerofoil portion having a leading edge and a trailing edge, the radially outer extremity of said aerofoil portion having a passage defined by the peripheral wall of a gutter, said passages extending from said leading edge to said trailing edge, an aperture being formed within said wall in the proximity of the trailing edge of said aerofoil portion, wherein at each of the majority of positions between said leading and trailing edges of said rotor blade in a plane normal to the radial extent of said rotor blade, the width of said passage is greater than the width of the radially outer aerofoil portion adjacent thereto. Such an arrangement ensures that at least most of the flow contained in the gutter, that is the flow that forms between the casing and the suction side of the gutter and/or the existing secondary flow vortex (which passes between the casing and the pressure side of the gutter) passes through the gutter and the exit aperture of the gutter. The invention will be described more fully with reference to the accompanying drawings in which:- Figure 1 is diagrammatic view of a gas turbine engine which is partially cut away to show the turbine section. Figure 2 is an illustration of overtip leakage flow over a prior art turbine rotor blade. Figure 3 is another illustration of overtip leakage flow over a prior art turbine rotor blade. Figure 4 is an top view of the aerofoil portion of a rotor blade showing the walled portion. Figure 5 is a section through the tip of an aerofoil portion indicated by section line I-I of figure 3 incorporating the gutter. Figure 6 is another section through the tip of the aerofoil section of figure 3 indicated by II. A gas turbine engine 10 as shown in figure 1 comprises in flow series a fan 12, a compressor 14, a combustion system 16, a turbine section 18, and a nozzle 20. The turbine section 18 comprises a number of rotors 22 and stator vanes 26, each rotor 22 has a number of turbine blades 24 which extend radially therefrom. Figures 2 and 3 illustrate the leakage of hot air or gas over the tip of the aerofoil portions 30. The aerofoil 30 has a leading edge 32 and a trailing edge 34. In turbines with unshrouded turbine blades, as illustrated in fig 2 a portion of the flow of gas migrates from the concave pressure surface 36 to the convex suction surface 38 over the tip of the aerofoil portion of the blade 24. This leakage flow exists because of a pressure difference between the pressure and suction surfaces 36,38. The flow over the tip of the aerofoil forms a vortex indicated by arrow A. Figures 4 to 6 show the tip of an aerofoil section incorporating the gutter. In figure 4 the aerofoil section is indicated by line C. A gutter 40 is positioned over the tip of the aerofoil. It is envisaged that the gutter 40 may comprise two walls unconnected at the trailing edge and the leading edge (not shown). The gutter 40 provides a passage 42 defined by a peripheral wall 44 . An exit 46 is provided in the wall 44 at the trailing edge 34 of the aerofoil. The direction of leakage flow across the tip of the aerofoil is shown by the arrow D. The turbine casing 48 is in close proximity to the gutter 40 and overtip leakage flow is directed into the gutter in the direction of arrow E. The gutter 40 is in close proximity to the turbine casing 48 and the flow is directed between the casing and into the gutter 40 in the direction of arrow C and to the exit aperture 46. The exit aperture is at its widest at the 'trailing edge' of the gutter. In operation air enters the gas turbine engine 10 and flows through and is compressed by the fan 12 and the compressor 14. Fuel is burnt with the compressed air in the combustion system 16, and hot gases produced by combustion of the fuel and the air flow through the turbine section 18 and the nozzle 20 to atmosphere. The hot gases drive the turbines which in turn drive the fan 12 and compressors 14 via shafts. The turbine section 18 comprises stator vanes 26 and rotor blades 24 arranged alternately, each stator vane 26 directs the hot gases onto the aerofoil 30 of the rotor blade 24 at an optimum angle. Each rotor blade 24 takes kinetic energy from the hot gases as they flow through the turbine section 18 in order to drive the fan 12 and the compressor 14. The efficiency with which the rotor blades 24 take kinetic energy from hot gases determines the efficiency of the turbine and this is partially dependent upon the leakage flow of hot gases between tip of the aerofoil 30 and the turbine casing 48. The leakage flow across the tip of the aerofoil 30 is trapped within the passage formed by the gutter 40 positioned over the aerofoil tip. In the embodiment as indicated in Figure 5 this trapped flow forms a vortex A within the gutter. The flow is then redirected along the passage subsequently exhausting from the gutter trailing edge through the exit aperture 46. In this embodiment the exit aperture 46 comprises an area or width large enough to allow all the flow that occurs between the casing 48 and the pressure side portion of the gutter peripheral wall 44, to exit downstream. Since the area of the exit aperture 46 is of a size sufficient to allow all the tip leakage flow (D) pass through it (as a vortex A), this reduces the risk of some tip leakage flow continuing to exit over the suction side portion 50 of the gutter peripheral wall 44 into the main passage, as is the case for a rotor with a plain rotor tip. In another embodiment as illustrated in Figure 6 the overtip leakage flow D again forms a vortex A within the gutter 40, However in this embodiment, the gutter is large enough such that the passage vortex B also forms in the gutter itself. The passage vortex B is formed from the casing boundary layer flow which, in this embodiment, passes between the casing 48 and the suction side portion 50 of the gutter peripheral wall 44. The area of the exit aperture is of a width sufficient to allow both vortex flows A and B to pass through it. Thus, again, in this embodiment the exit aperture is of a size sufficient to allow both flows A and B to pass through it. The target velocity distribution of the flow in close proximity to the gutter 40, is for the flow to accelerate continuously to the trailing edge on both the pressure and suction surface sides and thus obtain the peak Mach number(minimum static pressure) at the trailing edge. The aim is for the static pressure in the gutter 40 to match that on the external suction surface 38 of the aerofoil, this will help prevent flow trapped within the gutter from flowing over the sides of the gutter. A vortex may form within the passage formed by the gutter 40. However, the vortex may be weaker than that formed if the overtip leakage flow had been allowed to penetrate the main flow. Interaction of the vortex formed within the gutter 40 will be prevented until the flow is exhausted from the gutter trailing edge. The flow F along the gutter 40 is established near the leading edge 32 and flows to the trailing edge 34. The flow already established in the gutter may act to reduce flow over the peripheral wall 44, nearer to the trailing edge 34 ie. act as an ever increasing cross-flow to later leakage flow. Thus the gutter 40 is as effective near the trailing edge as it is further upstream.

Claims: An unshrouded rotor blade (24) including an aerofoil portion (30) having a leading edge (32) and a trailing edge (34), the radially outer extremity of said aerofoil portion having a passage (42) defined by the peripheral wall (44,50) of a gutter (40), said passage (42) extending from said leading edge (32) to said trailing edge (34), wherein an aperture (46) is formed within said wall (44,50) in the proximity of the trailing edge (34) of said aerofoil portion (30), characterised in that at each of the majority of positions between said leading and trailing edges (32,34) of said rotor blade (24) in a plane normal to the radial extent of said rotor blade (24), the width of said passage (42) is greater than the width of the radially outer aerofoil portion (30) adjacent thereto. An unshrouded rotor blade (24) as claimed in claim 1, characterised in that the width of said passage (42) is progressively greater than the width of said radially outer aerofoil portion (30) adjacent thereto from said leading edge (32) of said rotor blade (24) to said trailing edge (34) thereof.

Inventors: HARVEY NEIL WILLIAM, HARVEY, NEIL WILLIAM
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1997
Main classification: F01D5
IPC codes: F01D5