Patent Application
20150003973
The present invention relates to an aft outer rim seal arrangement for a turbine blade in a gas turbine engine. In particular, the invention relates to flow guiding elements incorporated as part of the aft outer rim seal arrangement.
Gas turbine engine blades used in the engine's turbine section are typically cooled via internal cooling channels through which compressed air is forced. This compressed air is typically drawn from a supply of compressed air created by the engine's compressor. However, drawing of the compressed air for cooling reduces the amount of compressed air available for combustion. This, in turn, lowers engine efficiency. Consequently, minimizing the amount of cooling air withdrawn from the compressor for cooling is an important technology in modern gas turbine design.
In some gas turbine engine models downstream blades extend relatively far in the radial direction. Downstream blades may include, for example, a last row of blades. Cooling channels typically direct cooling air from a base of the blade toward a tip, where it is exhausted into a flow of combustion gases. By virtue of the cooling channel extending within the blade so far radially outward, rotation of the blade, and the cooling channel disposed therein, imparts a centrifugal force on the cooling air that urges the cooling air in the cooling channel radially outward. The cooling air exits the blade and this creates a flow of cooling air within the cooling channel. This flow within the cooling channel creates a suction that draws more cooling air from a rotor cavity around the base of the blade into the cooling channel. Consequently, unlike convention cooling where compressed air is forced through the cooling channels, air that is not compressed, such as ambient air present outside of the gas turbine engine, can be used to cool the downstream blades.
A static pressure of ambient air is sufficiently greater than a static pressure in the rotor cavity to produce a flow of cooling fluid from a source of ambient air toward the rotor cavity. Thus, a static pressure of ambient air may push a supply of ambient air toward the rotor cavity, where a suction generated by the rotation of the blades then draws the ambient air from the rotor cavity through the cooling channels in the turbine blades, thereby completing an ambient air cooling circuit. The suction force aids in drawing ambient air into the rotor cavity. In this manner a flow of ambient air throughout the cooling circuit can be maintained.
However, a static pressure of ambient air within the rotor cavity is not substantially greater than a static pressure of combustion gases in a radially inward region of the hot gas path. The static pressure of the combustion gases in a radially inward region of the hot gas path may vary circumferentially and there may be transient operating conditions that produce static pressure differences in the combustion gases. These conditions may lead to ingestion of hot gases through a rim seal separating the rotor cavity from the hot gases in the radially inward region of the hot gas path. Ingestion of hot gases may be detrimental to a life of the engine components. Thus, there is room for improvement in the art.
The invention is explained in the following description in view of the drawings that show:
The present inventors have devised an aft outer rim seal arrangement (rim seal) that includes various flow guiding elements that prevent ingestion of hot gases into an outer cavity adjacent to the rim seal, and the rotor cavity inward of the outer cavity, and minimize a purge flow from the outer cavity and into the hot gas path. Minimizing the purge flow leaves more cooling fluid available for cooling the turbine blade. The various flow guiding elements can be used individually or together within the rim seal. The aft outer rim seal arrangement can be used for a turbine blade cooled with compressed air or a turbine blade cooled using an ambient air cooling arrangement. The description herein describes the aft outer rim seal arrangement as used in an ambient air cooled arrangement, but the technology can also be applied directly to a compressed air cooled arrangement.
An aft outer rim seal arrangement 40 (rim seal) is disposed between an outer cavity 42 and a radially inward region 44 the hot gas path 34. During operation a static pressure Protorcavity in the rotor cavity 20 and a static pressure Poutercavity in the outer cavity 42 are slightly below a static pressure Pambient in the source 12 of the ambient air, and slightly above a static pressure Pinwardhotgases of the hot gases 36 in the radially inward region 44 the hot gas path 34. A static pressure difference between Poutercavity and Pinwardhotgases is enough to drive a purge flow 46 out of the outer cavity 42 through the rim seal 40. However, this static pressure difference may not be large enough to overcome transient static pressure conditions during operation, and as a result it is possible for hot gases 36 to flow from the radially inward region 44 the hot gas path 34, back through the rim seal 40, and into the outer cavity 42 and possibly into the rotor cavity 20.
The turbine blade 22 may have an aft side 60, a lower angel wing 62 having a lower angel wing aft end 64, and an upper angel wing 66 having an upper angel wing aft end 68. The lower angel wing 62 and the upper angel wing 66 may surround a stationary rim 70 that is annular shaped and centered about the longitudinal axis 30 of the rotor disc 31. The stationary rim 70 may have a rim fore-end 72, a rim outward-facing surface 74, and a rim inward-facing surface 76. The rim seal 40 may then have two seal gaps: a lower angel wing seal gap 80 between and defined by the lower angel wing aft end 64 and the rim inward facing surface 76; and an upper angel wing seal gap 82 between and defined by the upper angel wing aft end 68 and the rim outward facing surface 74. In an exemplary embodiment the lower angel wing seal gap 80 may be approximately 9.0 mm, and the upper angel wing seal gap 82 may be approximately 4 mm.
In operation the static pressure Pinwardhotgases of the hot gases 36 in the radially inward region 44 the hot gas path 34 is slightly lower than the static pressure Pambient in the source 12 of the ambient air, and this moves cooling fluid 28 from the source 12 of ambient air, through the air supply passage 14, and through the pre-swirler 18 where it is swirled about the longitudinal axis 30 of the rotor disc 31 as it enters the rotor cavity 20. Once in the rotor cavity 20 the lower static pressure Pinwardhotgases of the hot gases 36 in the radially inward region 44 the hot gas path 34 may draw some of cooling fluid 28 along a first cooling fluid path 90 that is external to the turbine blade 22, from the rotor cavity 20, through the lower angel wing seal gap 80, into the outer cavity 42, and through the upper angel wing seal gap 82, where it exhausts into the hot gas path 34. Some of the cooling fluid 28 may be drawn along a second cooling fluid path 92 from the rotor cavity 20, through the dovetail gap 54, into the dead rim cooling channels (not shown) adjacent the entry passages 56, to the dead rim cooling channel outlet 58, to the outer cavity 42, and through the upper angel wing seal gap 82, where it exhausts into the hot gas path 34. Yet another portion of the cooling fluid 28 may be drawn along a third cooling fluid path 94 from the rotor cavity 20, through the dovetail gap 54, and into one of the entry passages 56 leading to the cooling channel 26, where it then exhausts into the hot gas path 34.
Hot gas ingestion into the third cooling fluid path 94 through the turbine blade 22 is less of a concern due to the rotation of the turbine blades 22 that mechanically introduces the necessary static pressures and centrifugal force to the cooling fluid 28 in the third cooling fluid path 94 to keep the hot gases 36 from entering. However, the transient static pressure variations in the hot gas path 34, and even the suction created in the third cooling fluid path 94 that leads to the rotor cavity 20, which, in turn, is in fluid communication with the outer cavity 42, could result in a situation where the static pressure Protorcavity in the rotor cavity 20 and/or the static pressure Poutercavity in the outer cavity 42 could drop below the static pressure Pinwardhotgases of the hot gases 36 in the radially inward region 44 the hot gas path 34. This would invite ingestion of the hot gases 36 from the hot gas path 34. This reversal of flow in across the lower angel wing seal gap 80 and possibly the upper angel wing seal gap 82 may be a greater concern due to the reliance on the static pressure Pambient in the source 12 of the ambient air, and its relatively small driving force due to the relatively small static pressure difference between Poutercavity and Pinwardhotgases.
The inventors have developed various flow guiding elements that are configured to prevent the ingestion of the hot gases 36 across the lower angel wing seal gap 80 and possibly the upper angel wing seal gap 82. The flow guiding elements include guide vanes 100, pumping fins 102, and a discourager tooth 104. In an exemplary embodiment the guide vanes 100 may be disposed on the rim inward facing surface 76, which is stationary, within the lower angel wing seal gap 80. The guide vanes 100 act similar to the pre-swirler 18 in that the guide vanes 100 impart swirl to cooling fluid 28 traversing the lower angel wing seal gap 80, which provides for a better match between the cooling fluid 28 traversing the lower angel wing seal gap 80 and the rotating turbine blades 22.
In an exemplary embodiment the pumping fins 102 may be disposed on a radially inward side 106 of the upper angel wing aft end 68 in the upper angel wing seal gap 82 and take advantage of the existing rotation of the turbine blades 22 to generate a pumping action on the cooling fluid 28 present in the outer cavity 42. This pumping action pumps the cooling fluid 28 through the upper angel wing seal gap 82, and this reduces the chances of ingestion of the hot gases 36. A discourager tooth 104 may be disposed anywhere a large enough gap remains. In an exemplary embodiment, the discourager tooth 104 may be disposed on the rim outward facing surface 74 and toward the rim fore-end 72, also in the upper angel wing seal gap 82 adjacent the pumping fins 102. This discourager tooth 104 presents a physical barrier to hot gases 36 present in the radially inward region 44 of the hot gas path 34, which would mitigate ingestion. The discourager tooth 104 also presents the same physical barrier to cooling fluid 28 present in the outer cavity 42. As a result less cooling fluid 28 may be lost as purge flow 46 while chances of ingestion of the hot gases 36 are also reduced.
The pumping action of the pumping fins 102 would create a second suction on the cooling fluid 28, in addition to that created by the rotation of the turbine blades 22. This would help draw some cooling fluid 28 through the outer cavity 42. This, in turn, would help draw cooling fluid 28 through the dead rim cooling channels, which might otherwise tend to stagnate. This would result in a greater portion of the purge flow 46 coming directly from the rotor cavity 20, as opposed to coming both directly from the rotor cavity 20 and via the dead rim cooling channels. Thus, the pumping fins 102 not only resist ingestion, they encourage flow through the dead rim cooling channels. In an exemplary embodiment the pumping fins 102 may extend approximately 2.0 mm into the upper angel wing seal gap 82.
When the pumping fins are used in conjunction with the discourager tooth 104, the upper angel wing seal gap is reduced in size to a toothed upper angel wing seal gap 140. This reduction in size provides a smaller opening which is more difficult for ingested gases to traverse. It further reduces a total volume of the purge flow 46, thereby leaving more cooling fluid 28 for the turbine blade 22. In an exemplary embodiment the discourager tooth 104 may extend approximately 4.5 mm into the upper angel wing seal gap 82.
From the foregoing, it has been shown that the present inventors have developed various flow guiding elements that prevent ingestion of hot gases through the rim seal. These flow guiding elements can be used by themselves, or together as part of an outer rim seal arrangement. The flow guiding elements are simple to manufacture, yet effective in helping to prevent ingestion of hot gases that shorten a service life of the engine components. As a result, the outer rim seal arrangement disclosed herein 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.
Development for this invention was supported in part by Contract No. DE-FC26-05NT42644, awarded by the United States Department of Energy. Accordingly, the United States Government may have certain rights in this invention.
