Patent Application
20090301680
The invention relates to gas turbine engines. More particularly, the invention relates to casting of cooled shrouds or blade outer air seals (BOAS).
BOAS segments may be internally cooled by bleed air. For example, there may be an upstream-to-downstream array of circumferentially-extending cooling passageway legs within the BOAS. Cooling air may be fed into the passageway legs from the outboard (OD) side of the BOAS (e.g., via one or more inlet ports at ends of the passageway legs). The cooling air may exit the legs through outlet ports in the circumferential ends (matefaces) of the BOAS so as to be vented into the adjacent inter-segment region. The vented air may, for example, help cool adjacent BOAS segments and purge the gap to prevent gas ingestion.
The BOAS segments may be cast via an investment casting process. In an exemplary casting process, a ceramic casting core is used to form the passageway legs. The core has legs corresponding to the passageway legs. The core legs extend between first and second end portions of the core. The core may be placed in a die. Wax may be molded in the die over the core legs to form a pattern. The pattern may be shelled (e.g., a stuccoing process to form a ceramic shell). The wax may be removed from the shell. Metal may be cast in the shell over the core. The shell and core may be destructively removed. After core removal, the core legs leave the passageway legs in the casting. The as-cast passageway legs are open at both circumferential ends of the raw BOAS casting. At least some of the end openings are closed via plug welding, braze pins, or other means. Air inlets to the passageway legs may be drilled from the OD side of the casting.
One aspect of the invention involves a blade outer air seal (BOAS) casting core. The core has first and second end portions and a plurality of legs. Of these legs, first legs each have: a proximal end joining the first end portion; a main body portion; and a free distal portion. Second legs each have: a proximal end joining the second end portion; a main body portion; and a free distal portion.
In various implementations, the distal portions of the first and second legs may project transverse to the main body portion. The core may be formed of refractory metal sheetstock. The core may have a ceramic coating. The proximal portions may each comprise a reduced cross-section neck. At least one third leg may connect to the first end portion to the second end portion. The at least one third leg may include first and second perimeter or edge legs. A plurality of connector branches may connect adjacent pairs of the legs. The connector branches may have minimum cross-sections smaller than adjacent cross-sections of the connected legs.
The core may be embedded in a shell and a casting cast partially over the core. The first and second end portions of the core may project from the casting into the shell. The first and second leg distal portions may project into the shell or may terminate in the casting.
The core may be manufactured by cutting from a refractory metal sheet. After the cutting, the first and second leg distal portions may be bent transverse to associated main body portions of those legs.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
A circumferential ring array of a plurality of the BOAS 22 may encircle an associated blade stage of a gas turbine engine. The assembled ID faces 32 thus locally bound an outboard extreme of the core flowpath 48 (
The BOAS may be air-cooled. For example, bleed air may be directed to a chamber 56 (
In operation, the inlet 66 feeds the leg 82 near a closed end 130 of the leg 82. The air flows down the leg 82 to an outlet 100 which is in a neck region at the other end 132 of the leg 82. Similarly, the inlet 60 feeds the leg 84 near a closed end 134. The outlet 110 is at a neck region at the other end 136. The inlets 68 and 70 feed the leg 86 near a closed end 138. The outlet 102 is formed at the other end 140. The inlet 62 feeds the leg 88 near a closed end 142. The outlet 112 is at the other end 144. The inlet 72 feeds the leg 90 near a closed end 146. The outlet 104 is in a neck region at the other end 148. The inlet 64 feeds the leg 92 near a closed end 150. The outlet 114 is formed in a neck region at the other end 152.
By using free distal ends of the RMC legs to cast closed passageway leg ends, the prior art plug welding step can be eliminated or reduced. However, the lack of local connection of the core leg free distal ends to the adjacent core end portion 202 or 204 may compromise structural integrity. To at least partially compensate, the RMC 200 has connecting portions 260, 262, 264, 266, and 268 connecting the main body portions of the adjacent legs. These connecting portions end up casting the passageways 120, 122, 124, 126, and 128, respectively.
From an airflow perspective, the connecting portions may advantageously be positioned at locations along the adjacent legs wherein air pressure in the cast passageway legs will be equal. This may minimize cross-flow and reduce losses. However, such location may provide less-than-desirable RMC strengthening. Thus, the connecting portions may be shifted (e.g., pushed circumferentially outward) relative to the optimal pressure balancing locations.
There may be one or more of several advantages to using the exemplary RMC 200 or modifications thereof. Use of the RMC with free distal leg portions may avoid or reduce the need for plug welding. Use of an RMC relative to a ceramic core may permit the casting of finer passageways. For example, core thickness and passageway height may be reduced relative to those of a baseline ceramic core and its cast passageways. Exemplary RMC thicknesses are less than 1.25 mm, more narrowly, 0.5-11.0 mm. The RMC may also readily be provided with features (e.g., stamped/embossed or laser etched recesses) for casting internal trip strips or other surface enhancements.
Further variations may involve radially constricting one to all of the interconnecting passageways (e.g., 120, 122, 124, 126, and 128) to have a smaller thickness (radial height) than characteristic thickness (e.g., mean, median, or modal) of the adjacent passageway legs. This may be provided by a corresponding thinning of the RMC connecting portions (e.g., 260, 262, 264, 266, and 268). Exemplary thinning may be from one or both RMC faces and may be performed as part of the main cutting of the RMC or later.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when implemented in the reengineering of a baseline BOAS, or using existing manufacturing techniques and equipment, details of the baseline BOAS or existing techniques or equipment may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.
