Turbine airfoil with internal cooling

The flow of cooling fluid through core tie holes formed between the internal cooling passageways of turbine airfoils is reduced by disposing flow deflectors on the wall adjacent to the holes. The deflectors alter the local static pressure near the holes, thereby minimizing the pressure differential across the holes so as to reduce the flow of cooling fluid.

BACKGROUND OF THE INVENTION
 The present invention relates generally to gas turbine engines, and more
 particularly to internally cooled airfoils used in such engines.
 Gas turbine engines, such as aircraft jet engines, include many components
 (e.g., turbines, compressors, fans and the like) that utilize airfoils.
 Turbine airfoils, such as turbine blades and nozzle vanes, which are
 exposed to the highest operating temperatures, typically employ internal
 cooling to keep the airfoil temperatures within certain design limits. A
 turbine rotor blade, for example, has a shank portion that is attached to
 a rotating turbine rotor disk and an airfoil blade portion which is
 employed to extract useful work from the hot gases exiting the engine's
 combustor. The airfoil includes a blade root that is attached to the shank
 and a blade tip that is the free end of the airfoil blade. Typically, the
 airfoil of the turbine rotor blade is cooled by air (normally bled from
 the engine's compressor) passing through an internal circuit, with the air
 entering near the airfoil blade root and exiting near the airfoil blade
 tip as well as through film cooling holes near the airfoil blade's leading
 edge and through trailing edge cooling holes. Known turbine blade cooling
 circuits include a plurality of radially-oriented passageways that are
 series-connected to produce a serpentine flow path, thereby increasing
 cooling effectiveness by extending the length of the coolant flow path. It
 is also known to provide additional, unconnected passageways adjacent to
 the serpentine cooling circuit.
 Turbine rotor blades with internal cooling circuits are typically
 manufactured using an investment casting process commonly referred to as
 the lost wax process. This process comprises enveloping a ceramic core
 defining the internal cooling circuit in wax shaped to the desired
 configuration of the turbine blade. The wax assembly is then repeatedly
 dipped into a liquid ceramic solution such that a hard ceramic shell is
 formed thereon. Next, the wax is melted out of the shell so that the
 remaining mold consists of the internal ceramic core, the external ceramic
 shell and the space therebetween, previously filled with wax. The empty
 space is then filled with molten metal. After the metal cools and
 solidifies, the external shell is broken and removed, exposing the metal
 that has taken the shape of the void created by the removal of the wax.
 The internal ceramic core is dissolved via a leaching process. The metal
 component now has the desired shape of the turbine blade with the internal
 cooling circuit.
 In casting turbine blades with serpentine cooling circuits, the internal
 ceramic core is formed as a serpentine element having a number of long,
 thin branches. This presents the challenge of making the core sturdy
 enough to survive the pouring of the metal while maintaining the stringent
 requirements for positioning the core. Furthermore, the thin branches of
 the serpentine core can experience relative movement if not stabilized in
 some manner. Thus, core ties (i.e., small ceramic pins connecting various
 branches) are used to accurately position the core and prevent relative
 movement of the core branches such that the thicknesses of the walls
 separating adjacent passageways of the serpentine cooling circuit are
 controlled better. After casting, when they have been removed along with
 the core, the core ties leave holes in the walls. These core tie holes
 provide unwanted flow communication between adjacent passageways due to a
 pressure differential that typically exists between the two passageways.
 That is, cooling fluid in the higher pressure passageway will flow into
 the lower pressure passageway through the core tie hole. This will result
 in an undesirable cooling flow distribution compared to the original
 design intent.
 Accordingly, there is a need for a turbine airfoil in which cooling fluid
 flow through core tie holes is minimized.
 SUMMARY OF THE INVENTION
 The above-mentioned needs are met by the present invention which provides
 an airfoil comprising at least two internal cooling passageways separated
 by a wall having a core tie hole formed therein. Flow deflectors disposed
 on the wall adjacent to the hole alter the local static pressure near the
 hole to minimize the pressure differential across the hole. This reduces
 the flow of cooling fluid through the core tie hole.
 Other objects and advantages of the present invention will become apparent
 upon reading the following detailed description and the appended claims
 with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 Referring to the drawings wherein identical reference numerals denote the
 same elements throughout the various views, FIG. 1 shows a prior art gas
 turbine engine rotor blade 10 having a hollow airfoil 12 and an integral
 shank 14 for mounting the airfoil 12 to a rotor disk (not shown) in a
 conventionally known manner. The airfoil 12 extends longitudinally or
 radially upwardly from a blade root 16 disposed at the top of the shank 14
 to a blade tip 18. The airfoil 12 includes an internal serpentine coolant
 circuit having five series-connected, generally radially extending coolant
 passageways 20-24.
 The first passageway 20 receives a cooling fluid (usually a portion of
 relatively cool compressed air bled from the compressor (not shown) of the
 gas turbine engine) through the shank 14. The cooling fluid travels
 radially outwardly through the first passageway 20, passes into the second
 passageway 21 and then flows radially inwardly through the second
 passageway 21. From there, the cooling fluid similarly passes in series
 through the other passageways 22-24, thereby cooling the airfoil 12 from
 the heating effect of the combustion gases flowing over the outer surfaces
 thereof. The cooling fluid exits the airfoil 12 through an opening 26 in
 the blade tip 18.
 The airfoil 12 includes a leading edge coolant passageway 28 in addition to
 the serpentine cooling circuit. The leading edge passageway 28 extends
 radially between the airfoil leading edge 30 and the first passageway 20
 and is not connected to the serpentine cooling circuit. A separate flow of
 cooling fluid is introduced through the shank 14. The cooling fluid flows
 radially through the leading edge passageway 28 and is discharged from the
 airfoil 12 through conventional film cooling holes and/or a tip hole (not
 shown) formed through the exterior wall of the airfoil 12. Similarly, a
 radially extending trailing edge coolant passageway 32 is disposed between
 the airfoil trailing edge 34 and the fifth passageway 24 of the serpentine
 cooling circuit. The trailing edge passageway 32 is also not connected to
 the serpentine cooling circuit and receives another separate flow of
 cooling fluid through the shank 14. This cooling fluid flows radially
 through the trailing edge passageway 32 and is discharged in part from the
 airfoil 12 through a conventional row of trailing edge holes and/or a tip
 hole (not shown). The arrows in FIG. 1 indicate the various paths of
 cooling fluid flow.
 As seen in FIG. 1, each one of the passageways 20-24, 28, 32 is separated
 from adjacent passageways by six radially extending walls 36-41. That is,
 the leading edge passageway 28 and the first passageway 20 of the
 serpentine cooling circuit are separated by a first wall 36, the first
 passageway 20 and the second passageway 21 are separated by a second wall
 37, and so on. At least some of the walls 36-41 have a core tie hole 42
 formed therein due to the use of core ties in the casting process.
 Specifically, the prior art blade 10 of FIG. 1 has core tie holes 42
 formed in the first wall 36, the third wall 38, the fifth wall 40 and the
 sixth wall 41, although other configurations are possible depending on how
 the core ties are deployed during the casting process. Core tie holes,
 which are often elliptical in cross-section, typically have an equivalent
 diameter of about 30-100 mils.
 Each core tie hole 42 will provide an unwanted flow of cooling fluid
 between the adjacent passageways it connects if there is a pressure
 differential between the two passageways. The passageways 20-24 of the
 serpentine cooling circuit will generally have pressure differentials
 because the pressure tends to decrease along the serpentine flow path due
 to friction and turning losses as the cooling fluid passes into successive
 passageways. Thus, the pressure in the first passageway 20 is greater than
 the pressure in the second passageway 21, which in turn is greater than
 the pressure in the third passageway 22 and so on to the fifth passageway
 24 which has the lowest pressure. The pressures in the leading edge
 passageway 28 and the trailing edge passageway 32 will be substantially
 equal to the pressure in the first passageway 20 because each of these
 passageways is directly connected to the inlet of cooling fluid through
 the shank 14. Accordingly, the pressure in the fifth passageway 24 will be
 less than the pressure in the trailing edge passageway 32.
 Because of these pressure differentials, cooling fluid will pass from the
 second passageway 21 to the third passageway 22 through the core tie hole
 42 in the third wall 38, from the fourth passageway 23 to the fifth
 passageway 24 through the core tie hole 42 in the fifth wall 40, and from
 the trailing edge passageway 32 to the fifth passageway 24 through the
 core tie hole 42 in the sixth wall 41. The flow of cooling fluid through
 the core tie holes 42 produces an undesirable cooling flow distribution.
 Cooling fluid will generally not flow through the core tie hole 42 in the
 first wall 36 because the pressures in the leading edge passageway 28 and
 the first passageway 20 are substantially equal.
 Referring now to FIG. 2, a turbine blade 110 is shown in which cooling
 fluid flow through core tie holes is minimized. For purposes of
 illustration only, the blade 110 has the same cooling circuit
 configuration as the blade 10 of FIG. 1. However, it should be noted that
 the present invention is applicable to turbine blades having any type of
 cooling circuit configuration containing core tie holes. Furthermore, as
 will become apparent from the following description, the present invention
 is not limited to turbine blades and could be used with other types of
 airfoil components such as turbine nozzles.
 Accordingly, the blade 110 has a hollow airfoil 112 and an integral shank
 (not shown in FIG. 2 but essentially identical to the shank 14 of FIG. 1).
 The airfoil 112 includes a serpentine cooling circuit having five
 series-connected, generally radially extending coolant passageways
 120-124, a leading edge coolant passageway 128 extending radially between
 airfoil leading edge 130 and the first passageway 120, and a radially
 extending trailing edge coolant passageway 132 disposed between airfoil
 trailing edge 134 and the fifth passageway 124. The passageways 120-124,
 128, 132 are supplied with cooling fluid through the shank of the blade
 110 in the same manner as described above with respect to the conventional
 blade 10. Each one of the passageways 120-124, 128, 132 is separated from
 adjacent passageways by six radially extending walls 136-141. A core tie
 hole 142 is formed in the first wall 136, the third wall 138, the fifth
 wall 140 and the sixth wall 141, although other configurations are
 possible depending on how the core ties are deployed during the casting
 process.
 The present invention uses a plurality of flow deflectors 144 to change
 local pressures around the core tie holes 142 so as to reduce cooling
 fluid flow through the core tie holes 142. Because core tie holes are
 perpendicular to the direction of flow in the passageways, the unwanted
 flow of cooling fluid through these holes in conventional airfoils is due
 to differences in static pressure only. Thus, the deflectors 144, which
 are preferably integral parts of the casting, are positioned on the
 separating walls so as to have a favorable impact on the static pressure
 in the vicinity of the core tie holes 142. Specifically, the deflectors
 144 minimize the pressure differential across the core tie holes 142.
 Generally, each core tie hole 142 has two of the deflectors 144 associated
 with it. Regarding the core tie hole 142 in the third wall 138, which
 separates the second and third passageways 121, 122, one deflector 144 is
 disposed in the second passageway 121, adjacent to and upstream of the
 core tie hole 142, and another deflector 144 is disposed in the third
 passageway 122, adjacent to and downstream of the core tie hole 142. By
 being located upstream of the core tie hole 142 in the second passageway
 121, which is the higher pressure passageway, the deflector 144 will
 accelerate the flow at this point and thereby decrease the local static
 pressure at the core tie hole 142 in the second passageway 121.
 Conversely, the deflector 144 in the lower pressure third passageway 122,
 which is located downstream of the core tie hole 142, will create a small
 stagnation point so as to "capture" some of the dynamic pressure and
 increase the local static pressure at the core tie hole 142 in the third
 passageway 122. By decreasing the local static pressure in the higher
 pressure second passageway 121 and increasing the local static pressure in
 the lower pressure third passageway 122, the static pressure differential
 across the core tie hole 142 will be minimized, which in turn will
 minimize the flow of cooling fluid through the core tie hole 142.
 The present invention is applicable to both counter flow passageways, such
 as the second and third passageways 121, 122, and parallel flow
 passageways. Thus, the deflectors 144 associated with the core tie hole
 142 in the fifth wall 140, which separates the counter flowing fourth and
 fifth passageways 123, 124, are similar in appearance to the deflectors
 144 of the core tie hole 142 in the third wall 138 in that the deflectors
 144 of each set are located at the same radial position. However, the
 deflectors 144 associated with the core tie hole 142 in the sixth wall
 141, which separates the parallel flowing fifth and trailing edge
 passageways 124, 132, are located at different radial positions. This
 difference is due to the fact that the fifth and trailing edge passageways
 124, 132 have parallel flows. That is, the deflector 144 in the higher
 pressure passageway, the trailing edge passageway 132, is located upstream
 of the core tie hole 142, and the deflector 144 in the lower pressure
 fifth passageway 124 is located downstream of the core tie hole 142.
 Because of the parallel flow, the two deflectors 144 are on radially
 opposite sides of the core tie hole 142. Despite this difference, the
 deflectors 144 disposed on the sixth wall 141 function in the same manner
 as the other deflectors. Specifically, the deflector 144 in the higher
 pressure trailing edge passageway 132 accelerates the flow so as to
 decrease the local static pressure at the core tie hole 142 in the
 trailing edge passageway 132. The deflector 144 in the lower pressure
 fifth passageway 124 creates a small stagnation point, thereby increasing
 the local static pressure at the core tie hole 142 in the fifth passageway
 124. Accordingly, the flow of cooling fluid through the core tie hole 142
 is decreased because the static pressure differential across the core tie
 hole 142 is minimized.
 The core tie hole 142 in the first wall 136, which separates the leading
 edge passageway 128 from the first passageway 120, does not have any
 deflectors associated therewith because the first and leading edge
 passageways 120, 128 have substantially equal pressures.
 As seen in FIG. 2, the deflectors 144 preferably have a shape in which one
 side presents a sloped surface and the other side is more blunt. Not only
 does this configuration facilitate casting of the deflectors 144, but it
 also enhances the intended function of the deflectors 144 as follows. The
 deflectors 144 disposed in higher pressure passageways are arranged with
 the sloped side facing the flow (i.e., facing upstream). This way, the
 deflectors 144 are able to accelerate flow without causing an excessive
 pressure drop beyond the desired change in the local static pressure. The
 deflectors 144 disposed in lower pressure passageways are arranged with
 the blunt side facing the flow (i.e., facing upstream) so as to provide a
 sufficient stagnation point.
 The deflectors 144 are sized so as to perform their intended function
 without adversely affecting the cooling flow. Preferably, the deflectors
 144 will have a "height" (i.e., the distance a deflector protrudes from
 the wall it is disposed on) which is substantially equal to the diameter
 of the associated core tie hole 142. However, this is provided that the
 deflectors 144 will preferably block less than 20% of the flow area, and
 more preferably less than 10% of the flow area.
 The foregoing has described a turbine airfoil in which cooling fluid flow
 through core tie holes is minimized. While specific embodiments of the
 present invention have been described, it will be apparent to those
 skilled in the art that various modifications thereto can be made without
 departing from the spirit and scope of the invention as defined in the
 appended claims.