Patent Application
20140147250
The invention relates to improving an interaction of rotor cavity purge cooling air as it enters a flow of combustion gases. In particular, the invention relates to pumping features disposed in a turbine blade angel wing that impart a swirl to the flow of cooling air.
Gas turbine engines conventionally include a rotor shaft and several rows of rotor blades, each row including multiple blades distributed circumferentially about the rotor shaft. In between the rows of blades are rows of stationary vanes. Combustion gases flow along the gas turbine engine longitudinal axis in an annular flow path defined by the blades and vanes. The rotor shaft lies radially inward of the annular flow path and a rotor cavity is formed between the rotor disk and a stator structure holding the stationary vanes. Cooling air, or rotor purge air is often directed into the rotor cavity. The purge air cools components within the rotor cavity that support the blades and vanes, after which the purge air typically exits the rotor cavity through a gap between the vanes and the blades on a radially inward end of the vanes and blades.
Combustion gases traveling in the annular flow path tend to form a “bow wave” immediately upstream of any components the gases encounter, such as a blade or vane. As a result, pressure builds up within the combustion gases immediately upstream of each blade. The bow waves are distributed circumferentially about the gas turbine engine, just radially outward of the gap. In order to prevent ingestion of the combustion gases into the gap and the rotor cavity, flow discouraging seals are often formed just inside the gap, slightly upstream of an outlet of the gap.
Flow discouraging seals may be formed via an angel wing, which uses a platform that extends axially from a base of the blade, together with a radially raised lip extending radially outward from a tip of the axial platform, to form a restriction in the gap intended to limit the flow of purge air outward, and combustion gases inward. The radially raised tip is conventionally axially aligned with an opposing surface, such as a surface on the stationary vane, which forms the restriction that acts as the flow discouraging seal.
It is known that the purge air has an aerodynamic impact on the flow of combustion gases where they interface, and various approaches have been taken to mitigate the impact. For example, U.S. Pat. No. 8,083,475 to Bulgrin et al. discloses an angel wing compression seal that guides the rotor air traversing the angel wing to a region in front of the respective blade. However, this patent appears to be limited to addressing the bow wave. Addressing other aerodynamic impacts, as well as addressing aerodynamic impacts for different blade geometries, leaves room in the art for improvement.
The invention is explained in the following description in view of the drawings that show:
The present inventors have recognized that the aerodynamic impact of the merging of rotor purge air with the combustion gases creates vortices. These vortices tend to traverse along the suction side of the blades, from front to back and from base to tip. This causes aerodynamic losses and an associated reduction in the energy that can be extracted from the combustion gases. During operation of the gas turbine engine the rotor blades are rotating about the gas turbine engine longitudinal axis. Prior to entering the combustion gas flow, the axially flowing rotor purge air is flowing at a negative angle of incidence with respect to a leading edge of a blade. The inventors have discovered that these vortices are formed, at least in part, due to axially flowing cooling air encountering combustion gases that are flowing helically about a gas turbine engine longitudinal axis, creating a large angle of encounter. In response, the inventors have developed pumping features integral to the angel wing that impart a swirl into the rotor purge air as the purge air traverses the angel wing. When the swirl is imparted to the axially traveling rotor purge air the rotor purge air ends up traveling in a helical manner about the gas turbine engine longitudinal axis. When the helically moving rotor purge air merges with the helically moving combustion gas at a smaller angle of encounter, the vortices are reduced. This, in turn, increases the efficiency with which the blade can extract energy from the combustion gases.
In the shown configuration there is a forward lower angel wing 30 and a forward upper angel wing 32 on an upstream side 34 of a base 36 of the blade 10. Each forward angel wing 30, 32 includes a radially raised lip 38. Radially outward of (i.e. axially opposite) the radially raised lip 38 of the forward upper angel wing 32 is an opposing surface 40, and the radially raised lip 38 and the opposing surface 40 together form a narrowed gap of the cooling fluid path 24 known as a flow discourager seal clearance 42. A vertical wall 44 and an overhang 46 are disposed proximate an outlet 48 of the cooling fluid path 24. Due to the vertical wall 44 and the overhang 46, even if the angle of encounter were previously recognized as causing a reduction in efficiency, it would have been impossible to impart any helical movement about the gas turbine engine longitudinal axis 18 to the rotor purge air as it merges with the combustion gas 16 because the vertical wall 44 and the overhang 46 would block any axial movement of the rotor purge air.
As can be seen in
When assembled and rotating in the gas turbine engine, the angel wing 62 defines a sweep defined by space that axial platform 72 and the radially raised lip 64 occupy as they rotate. Given the rotation about the gas turbine engine longitudinal axis 18, the outer surfaces of the angel wing 62 define the sweep, and a cross section of the sweep, which has an annular shape, would resemble that a cross section of the angel wing 62 at the same location. For example, the sealing surfaces 86 define a sealing surface sweep 136 of a constant diameter. (The amount of curvature in the figure has been exaggerated for sake of explanation.) Thus the outer most surfaces define the shape of the sweep. As can be seen, the pumping features 130 are disposed entirely within the sweep defined by the angel wing 62, as evidenced by the example sealing surface sweep 136. Stated another way, no material is added to the angel wing 62 of
Due to its position within the flow discourager seal clearance 42, which is the narrowest gap in the cooling fluid path 24, the opposing surface 40 that also defines the flow discourager seal clearance 42 prevents the purge air from moving radially outward as it passes over the first pumping surfaces. Consequently, due to the unique configuration, instead of simply passing over the pumping features 130, the rotor purge is forced to rotate with the first pumping surface 132. This imparts the swirl into the rotor purge air which, together with the existing axial movement of the rotor purge air, produces the desired helical movement within the rotor purge air as it merges with the combustion gas 16. The annular flow of rotor fluid that is moving in a helical direction is also characterized by an essentially uniform circumferential distribution of pressure as it exits the cooling fluid path 24. As a result of the foregoing, the flow of rotor purge air tends to remain more attached to the blade platform 70, which reduces the amount of radial rise of the vortices. This, in turn, prevents the vortices from migrating toward the upper span of the suction side 106, which increases aerodynamic efficiency of the blade 60. Still further, more purge flow adheres to the blade platform 70, and the adhering purge flow also penetrates axially farther down the blade platform 70, allowing the blade platform 70 to remain cooler, thereby extending a service life of the blade 60. The performance has been demonstrated to be effective through computational fluid dynamic analysis.
In operation, a portion of the rotor purge air enters (i.e. is scooped into) the scoop flow path 150 where it is accelerated and where circumferential motion is imparted. The scooped rotor purge air is ejected radially outward and tangentially forward with respect to the direction of rotation 134, where it meets with rotor purge air that bypassed the scoop 148. The merging of the scooped rotor purge air with the rotor purge air that bypassed the scoop 148 causes the merged rotor purge flow to flow in a helical movement about the gas turbine engine longitudinal axis 18. As a result, when the merged rotor purge air merges with the combustion gas 16, the sought after smaller second angle of encounter 122 is effected.
Although the invention has been shown in two exemplary embodiments, any geometry capable of imparting the swirl as disclosed and within the sweep of the angel wing is considered within the scope of the disclosure. This includes orienting the first pumping surface 132 more tangentially forward facing, less tangentially forward facing, or completely tangentially forward facing. This further includes moving the scoop inlet end 152 to any position on the angel wing 62 suited for receiving rotor purge air, reconfiguring the scoop flow path 150 as necessary, and locating the scoop outlet end 156 to any position and orientation suitable for ejecting the scooped rotor purge air with a tangential component.
It has been disclosed that the inventors have found a simple and cost effective technique for inducing helical motion into rotor purge air prior to its merging with combustion gas. As a result, aerodynamic efficiency of the blade is improved, thereby increasing efficiency of the engine, and the blade platform is kept cooler, thereby increasing service life of the blade. Further, the pumping features disclosed herein can be incorporated into existing blades via a simple machining operation. In light of the foregoing, this represents an improvement in the art.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
