Patent Application
20150267539
The present disclosure concerns fluidfoils, gas turbine engines and methods of managing flow tip momentum transfer. More specifically the disclosure concerns, in part, particular fluidfoil fence arrangements. The disclosure may have particular utility when applied to some or all of the propeller blades of open rotor gas turbine engines having contra-rotating propeller stages, but this is not intended to be limiting.
It is known to provide the tips of fluidfoils with fences in the form of an aerodynamic fence at the end of the fluidfoil. The fence is leant at a particular lean angle, either towards the suction side of the fluidfoil (dihedral fence) or towards the pressure side of the fluidfoil (anhedral fence). Some fluidfoil tips may also have both an anhedral and a dihedral fence.
Fences increase efficiency by reducing fluid momentum in the radial direction as it passes the fluidfoil.
Fences also serve to reduce vortices at the fluidfoil tips. Vortices are inefficient and can be acoustically undesirable, especially where downstream structures intercept the vortices. A fence inhibits the flow of fluid from the pressure side of the fluidfoil (where the pressure is greater) to the suction side (where the pressure is lower). This tends to limit or prevent the creation of a spiralling trail of fluid (vortex) leaving the fluidfoil tip.
Where however momentum transfer from the pressure side to the suction side is insufficient in view of the presence of a fence, a vortex breakdown occurs and a bubble is formed. Vortex breakdown is an abrupt change of flow structure that occurs in swirling flows. If the fence causes sufficient reduction in momentum transfer, there may be a rapid deceleration of the flow in the axial direction in the vicinity of the fence. Further there may be a rapid expansion of the vortex core at approximately 50% of the tip chord, giving rise to a bubble like structure. Although the breakdown causes the vortex to be weaker, it does not imply the decay of vorticity itself. The swirling is in fact redistributed over a larger area, potentially worsening the downstream wake. Vortex breakdown tends therefore to be even less desirable than a standard vortex from an acoustic viewpoint, particularly tending to increase broadband noise. Moreover, the overall direction of axial flow is usually reversed inside the bubble, thereby compromising, to some extent, aerodynamic performance.
Unless otherwise specified, the word axial is used throughout this specification to refer to the direction of the main rotational axis of a gas turbine engine to which fluidfoils would, in use, be attached. Similarly the word radial is used with respect to a gas turbine engine to which fluidfoils would, in use, be attached. Specifically radial refers to directions perpendicular to the main rotational axis of the gas turbine engine.
According to a first aspect of the invention there is provided a fluidfoil comprising a main body and a fence, wherein the fence has different lean angles with respect to the main body at different chordwise positions.
The lean angle corresponds to the approximate angle that the fence or fence portion makes with a main body of the fluidfoil. More specifically the lean angle at a particular chordwise position may be defined as: 90° plus the angle subtended by:
A fence with lean angle variation from the front to the back of the fence (in the axial direction) may offer the potential for control of the flow tip momentum transfer between the pressure and suction sides of the fluidfoil. By deliberate design of the lean angle variation it may be possible to control the evolution of a leakage vortex, retaining it at the incipience of a breakdown event (i.e. low energy vortex). Such a fence may therefore give the efficiency benefits of a standard fence, without instigating a potentially detrimental vortex breakdown.
In some embodiments the fence is divided along its chordwise extent into at least two portions, where a first portion nearer a leading edge of the fence has a smaller lean angle than a second portion further from the leading edge. The first portion may reduce momentum transfer to reduce the tip vortex energy towards a critical condition of incipient vortex breakdown. Thereafter the second portion may prevent breakdown through a controlled increase in momentum transfer brought about by its increased lean angle.
In some embodiments the first portion is the nearest portion to the leading edge.
In some embodiments the first and second portions are immediately adjacent.
In some embodiments a third portion, further from the leading edge than the second portion, has a greater lean angle than the second portion. The third portion may therefore stabilise momentum transfer close to the breakdown point and prevent an increase in vortex energy from a trailing edge of the second portion.
In some embodiments the second and third portions are immediately adjacent.
In some embodiments a first portion, nearest a leading edge of the fence, has the same lean angle as a second portion immediately adjacent the first portion and the second portion is shorter than the first portion. Shortening the second portion may provide a convenient alternative to increasing its lean angle with respect to the first portion. Shortening a portion may have similar results to increasing its lean angle, as the impediment to flow from the pressure to the suction side of the fluidfoil will be correspondingly reduced.
In some embodiments a third portion, immediately adjacent the second portion, has a greater lean angle than the first and second portions. The third portion may therefore stabilise momentum transfer close to the breakdown point and prevent an increase in vortex energy from a trailing edge of the second portion.
In some embodiments at least two portions are located at different spanwise positions along the fluidfoil.
In some embodiments the lean angle within at least one portion is substantially consistent. This may facilitate easier manufacturing of the fluidfoil. It may also make the momentum transfer difference that will occur as a consequence of that portion more predictable.
In some embodiments the lean angle within at least one portion varies. This may allow closer control over vortex energy.
In some embodiments there is a step-wise discontinuity between at least two of the portions. This may be easier to manufacture than providing the portions with complimentary ramp regions to join them.
In some embodiments at least two portions have complimentary ramp regions which meet to smoothly join the portions. This may reduce stress occurring between portions.
In some embodiments there is continuous smooth variation in the lean angle across the fence in the chordwise direction. In this case it might be considered that there are an infinite number of portions each with a different lean angle.
In some embodiments the fence has different spanwise lengths at different chordwise positions. It may be for example that where the fence is divided into portions as previously described, each portion has a consistent spanwise length, but at least two portions have different spanwise lengths. Alternatively it may be that there is continuous smooth variation in the spanwise length across the fence in the chordwise direction. Fence spanwise length variation may be of further assistance in managing the degree of impediment to momentum transfer and therefore vortex energy.
In some embodiments the fluidfoil is an open-rotor propeller blade. In other embodiments however the fluidfoil may for example be a blade or rotor blade, or specifically a ducted fan blade, a compressor blade, a turbine blade or a propeller blade.
According to a second aspect of the invention there is provided a gas turbine engine comprising one or more fluidfoils in accordance with the first aspect.
In some embodiments the gas turbine engine comprises contra-rotating forward and aft propeller stages, where the forward propeller is provided with at least one of the fluidfoils. The fluidfoil may be particularly advantageous when used in the forward propeller stage of a contra-rotating propeller system. The potential control exerted over the vortex produced by the forward propeller fluidfoils may mean that the impulsiveness with which the aft propeller intersects the vortex is decreased, with consequent acoustic benefits.
In some embodiments the gas turbine engine is an open rotor engine.
According to a third aspect of the invention there is provided a method of managing the flow tip momentum transfer from the pressure side to the suction side along the tip of a fluidfoil comprising providing different degrees of impediment to fluid flow from the pressure side to the suction side at different points along the tip in a chordwise direction.
In some embodiments the method further comprises arranging the impediments so that in use they cause an approach to incipient vortex breakdown and maintain this condition.
In some embodiments the method further comprises providing a first upstream impediment to fluid flow from the pressure side to the suction side and providing a second reduced impediment downstream of this. Optionally the method further comprises providing a third increased impediment downstream of the second reduced impediment.
The skilled person will appreciate that a feature described in relation to any one of the above aspects of the invention may be applied mutatis mutandis to any other aspect of the invention.
Embodiments of the invention will now be described by way of example only, with reference to the Figures, in which:
Referring to
The gas turbine engine 10 works in a conventional manner so that air entering the intake 12 is accelerated and compressed by the intermediate pressure compressor 14 and directed into the high-pressure compressor 15 where further compression takes place. The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high-pressure, intermediate pressure and free power turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high-pressure, intermediate pressure and free power turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the propellers 23, 24 by suitable interconnecting shafts. The propellers 23, 24 normally provide the majority of the propulsive thrust. In the embodiments herein described the propellers 23, 24 rotate in opposite senses so that one rotates clockwise and the other anti-clockwise around the engine's rotational axis 9.
With reference to
Referring specifically now to
Blade 32 has a pressure surface 34 and a suction surface 36. The blade 32 comprises a main body 38, a fence 40 and a blend region 42 disposed between the main body 38 and fence 40. The fence 40 is anhedral with respect to the main body 38, that is the fence 40 is leant towards the pressure surface 34 of the main body 38. As will be appreciated, in other embodiments the fence may be dihedral or have both anhedral and dihedral portions. The blend region 42 provides a curved transition between a tip 44 of the main body 38 and the fence 40, which is leant with respect to the main body 38. Both the blend region 42 and fence 40 retain the fluidfoil cross-section of the main body 38.
The blade 32 is divided into two portions, a first portion 46 nearest a leading edge 48 of the fence 40 and a second portion 50 immediately adjacent and downstream of the first portion 46 and extending to a trailing edge 52 of the fence 40. The first portion 46 and second portion 50 are immediately adjacent. The two portions 46, 50 have different lean angles. The lean angle of each portion 46, 50 corresponds to the approximate angle that the portion makes with the main body 38 of the blade 32.
More specifically the lean angle for the first portion 46 at the particular chordwise position corresponding to the leading edge 48 is: 90° plus an angle 54 subtended by:
The lean angle for the second portion 50 at its upstream end (90° plus an angle 64) can be calculated in a similar way, taking account of the shift in the particular chordwise position.
Because the first portion 46 has a consistent lean angle throughout its extent, the lean angle calculated for the leading edge 48 chordwise position will also be applicable at all other chordwise positions throughout the chordwise extent of the first portion 46. This is also true for the second portion 50, albeit at a different consistent lean angle.
The lean angle of the first portion 46 is smaller than the lean angle of the second portion 50. The first portion 46 is therefore leant further towards pressure surface 34 than the second portion 50. Because the lean angle within each of the portions 46, 50 is consistent throughout that portion 46, 50, there is a step-wise discontinuity 66 at the transition between the two. In other embodiments however the portions 46, 50 could be provided with complimentary ramp regions which meet to smoothly join the portions 46, 50.
In the embodiment of
When the blade 32 is rotating in use, air will flow around the blade 32 in the axial direction 28, relatively high pressure existing adjacent the pressure side 34 of the blade 32 and a relatively low pressure existing adjacent the suction side 36 of the blade 32. The pressure differential would therefore tend to cause a flow of air from the pressure side 34 to the suction side 36 at the tip 44 of the main body 38. As the air flows axially rearward from the leading edge 48, the first portion 46, with its relatively small lean angle, presents an impediment to fluid flow and decreases the flow tip momentum transfer from the pressure side 34 to the suction side 36. The momentum transfer decrease, decreases the energy of a vortex formed by the blade 32 towards the incipience of vortex breakdown. The second portion 50, with its larger lean angle, tends to reduce the impediment to fluid flow, allowing a modest increase in flow tip momentum transfer and preventing vortex breakdown from actually occurring. In embodiments where a third and possibly more portions are provided, these may be used to further control the momentum transfer with a view to maintaining the vortex energy at the incipience of breakdown. This may be achieved by carefully selecting the lean angle and/or spanwise length of the additional portion(s).
The maintenance of the vortex at the incipience of breakdown may be particularly advantageous where any vortex is likely to be intercepted by the blades of a downstream rotor (e.g. in the case of contra-rotating rotor rows) or other structures. This is because a low energy vortex may give rise to lower noise levels than a relatively high energy vortex or vortex breakdown when intercepted.
Referring now to
The lean angle of the fence 72 increases steadily from the leading edge 80 to a trailing edge 82 of the blade 70. The lean angle variation is continuous and smooth throughout and across all of the portions 74, 76, 78. Consequently there are no step-wise discontinuities between the portions 74, 76, 78, despite there being no provision of complimentary ramp regions. At the leading edge 80 the lean angle is approximately 90°, while at the trailing edge 82 the lean angle is approximately 140°.
The second portion 76 is shorter than the first 74 and third 78 portions. In the embodiment of
In the embodiment of
It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the various concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein in any form of fluidfoil or gas turbine engine.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1404938.1 | Mar 2014 | GB | national |
