Impingement cooled airfoil tip

A turbine airfoil includes pressure and suction sidewalls joined together at leading and trailing edges, and extending from root to tip. The tip includes a tip rib extending from a tip cap enclosing an internal flow channel between the sidewalls. An aligned pair of first and second holes extend through the tip cap and rib, respectively, for discharging coolant from the flow channel.

BACKGROUND OF THE INVENTION
 The present invention relates generally to gas turbine engines, and, more
 specifically, to turbine blade cooling.
 In a gas turbine engine, air is pressurized in a compressor and mixed with
 fuel and ignited in a combustor for generating hot combustion gases. The
 gases flow through turbine stages which extract energy therefrom for
 powering the compressor and producing useful work, such as powering a fan
 for propelling an aircraft in flight.
 A turbine stage includes a row of turbine blades extending radially
 outwardly from a supporting rotor disk. Each blade includes an airfoil
 over which the combustion gases flow for extracting energy therefrom. The
 airfoil is hollow and is provided with air bled from the compressor for
 use as a coolant in cooling the blade during operation.
 Maximum efficiency of the turbine is obtained by closely positioning the
 radially outer tip of the airfoil adjacent a surrounding stationary
 turbine shroud for minimizing combustion gas leakage therebetween.
 However, differential thermal expansion and contraction between the blade
 tips and turbine shroud can cause rubbing therebetween. To accommodate
 this rubbing, the tip of the airfoil includes a squealer tip rib around
 its perimeter extending outwardly from a tip cap enclosing a coolant flow
 channel inside the airfoil. The squealer rib may be closely positioned
 adjacent the shroud and limits rubbing therebetween to the surface area of
 the rib itself.
 However, the squealer rib is thusly exposed on three sides to the hot
 combustion gases and is difficult to cool, and correspondingly affects
 useful blade life. The airfoil tip is typically cooled by providing tip
 holes through the tip cap which discharge a portion of the coolant into
 the tip cavity defined above the tip cap. And, inclined film cooling holes
 may extend through the concave or pressure sidewall of the airfoil just
 below the tip cap to provide film cooling air which bathes the pressure
 side portion of the tip with film cooling air. However, this form of tip
 cooling is limited in effectiveness.
 Accordingly, it is desired to provide a turbine blade having improved
 airfoil tip cooling.
 BRIEF SUMMARY OF THE INVENTION
 A turbine airfoil includes pressure and suction sidewalls joined together
 at leading and trailing edges, and extending from root to tip. The tip
 includes a tip rib extending from a tip cap enclosing an internal flow
 channel between the sidewalls. An aligned pair of first and second holes
 extend through the tip cap and rib, respectively, for discharging coolant
 from the flow channel.

DETAILED DESCRIPTION OF THE INVENTION
 Illustrated in FIG. 1 is an exemplary turbine rotor blade 10 for a gas
 turbine engine. The blade is one of many circumferentially spaced apart
 around the perimeter of a turbine rotor disk (not shown). Each blade
 includes a suitable dovetail 12 which retains the blade in a complementary
 dovetail slot formed in the perimeter of the disk. The blade has a radial
 or longitudinal axis, with an integral platform 14 and airfoil 16 disposed
 in turn radially above the dovetail.
 The blade is typically cast in a unitary or one-piece component, and
 includes an internal flow channel or circuit 18 for channeling a coolant
 20 therethrough. The flow channel 18 may have any conventional form, such
 as multi-pass serpentine channels, with the coolant 20 typically being a
 portion of air bled from the compressor of the engine.
 During operation, air pressurized in the compressor is mixed with fuel and
 ignited in a combustor (not shown) for generating hot combustion gases 22
 that flow over the airfoil 16 which extracts energy therefrom for rotating
 the rotor disk. The airfoil 16 includes a generally concave, first or
 pressure sidewall 24 spaced laterally or circumferentially in most part
 from a convex, second or suction sidewall 26. The sidewalls are joined
 together at axially opposite leading and trailing edges 28,30, and extend
 longitudinally or radially from a root 32 where the airfoil meets the
 platform to a radially outer tip 34.
 The blade or airfoil tip includes a squealer tip rib 36 which is integrally
 disposed along the pressure and suction sidewalls 24,26 to define an open
 tip cavity 38 extending radially outwardly from a tip floor or cap 40. The
 tip cap 40 encloses the top of the flow channel 18 between the sidewalls.
 In the preferred embodiment illustrated, the tip rib 36 extends from the
 perimeter of the tip cap 40 and is coextensive with the pressure and
 suction sidewalls 24,26 around the perimeter of the airfoil.
 As illustrated in FIGS. 1 and 2, the airfoil tip includes a plurality of
 coaxially aligned pairs of first and second tip holes 42,44 extending
 through the tip cap 40 and tip rib 36, respectively. The hole pairs 42,44
 are preferably disposed through the pressure-side portion of the tip rib
 36 on the airfoil pressure sidewall 24, and are spaced apart between the
 leading and trailing edges for providing improved cooling along the
 pressure side of the tip.
 The first and second tip holes in each pair are coaxially aligned with each
 other along a common hole axis in a serial or sequential arrangement. As
 shown in FIG. 2, the aligned pair of holes 42,44 are inclined together at
 a common acute inclination angle A, measured from the radial or
 longitudinal axis. In this way, the first hole 42 illustrated in FIG. 2 is
 disposed in flow communication with the internal flow channel 18 for
 discharging therefrom a portion of the coolant 20 toward the inner surface
 of the tip rib 36 for cooling thereof.
 The hole pair 42,44 are inclined through the tip rib and cap and are
 interrupted at an inside corner 46 defined between the tip cap 40 and the
 adjoining tip rib 36. The position and inclination of the hole pair is
 preferably selected so that the outlet of the first hole 42 is directed at
 the base of the tip rib 36 where it forms the inside corner 46 with the
 tip cap 40. In this way, the coolant 20 may be discharged from the flow
 channel 18 at the inside base of the tip rib 36 for providing enhanced
 cooling thereof.
 For example, a portion of the coolant discharged from the first hole 42 may
 impinge the inner surface of the tip rib 36 around the second hole 44 for
 providing impingement cooling thereof, while a portion of the discharged
 coolant also passes through the second hole 44 for internal convection
 cooling thereof, as well as film cooling upon leaving the second hole 44.
 FIG. 3 illustrates an alternate embodiment of the invention configured like
 the embodiment illustrated in FIG. 2, yet including a plug 48 disposed
 inside the second hole 44, while the first hole 42 remains empty. By
 plugging the second hole 44, the coolant discharged from the first hole 42
 may fully impinge the inner surface of the tip rib 36 at and near the plug
 44 for further enhancing cooling of the tip rib itself. The plug 48 is
 additionally shown in section in FIG. 4, and may be formed in any suitable
 manner such as by brazing or welding the initially empty second hole 44
 using a suitable filler material.
 In a preferred embodiment, the plug 48 has a different material composition
 than that of the tip rib 36 in which it is formed, with the plug 48
 preferably having a greater, and relatively high, thermal conductivity
 than that of the tip rib 36. In this way, impingement cooling of the plug
 48 may be used for extracting heat from the tip rib 36 during operation
 for further reducing its operating temperature to correspondingly increase
 useful blade life.
 In the exemplary embodiment illustrated in FIGS. 3 and 4, the first and
 second holes 42,44 have the same cross sectional shape, such as round or
 circular, with the plug 48 completely filling the second hole 44 to form a
 round or cylindrical pin therein.
 FIG. 5 illustrates an alternate embodiment of the invention wherein the
 first and second holes 42,44 have different cross sectional shapes for
 further improving cooling of the tip rib.
 More specifically, the second hole 44 preferably has a greater sectional
 perimeter or length than the first hole 42 with the latter preferably
 being round in cross section, and the former being preferably being lobed.
 As shown in FIG. 5, the second hole 44 preferably includes a plurality of
 circumferentially adjoining lobes of suitable number including two or
 more, or the four illustrated. Preferably two of the lobes of the second
 hole 44 are disposed longitudinally or radially symmetrically in the tip
 rib 36, and diverge away from each other from the underlying tip cap 40.
 Correspondingly, the plug 48 has complementary lobes which cooperate with
 the coolant which impinges against the inside surface of the plug during
 operation. As the coolant impinges on the plug, impingement cooling is
 provided, and a film of cooling air is formed as the air flows along the
 inner surface of the tip rib along the diverging lobes.
 Since the first and second tip holes 42,44 are relatively small in
 diameter, with an exemplary nominal diameter of about 15-20 mils (0.38-0.5
 mm), they are preferably formed after the initial casting of the blade
 itself.
 As shown in FIG. 2, the blade, including its airfoil, is initially formed
 by casting in any conventional manner, followed in turn by suitably
 forming the several hole pairs 42,44 through the cast tip rib 36 and tip
 cap 40. Each hold pair is preferably formed in a single operation by
 drilling using a laser, electrical discharge machine, or water jet, for
 example.
 Since the first hole 42 illustrated in FIG. 2 is inclined and hidden behind
 the pressure side tip rib 36, that hole may be formed only after the
 second hole 44 is formed through the tip rib. A common drilling operation
 may therefore be used to form in turn the second hole 44 followed by the
 first hole 42 for providing flow communication therethrough from the flow
 channel 18 to the outer surface of the tip rib 36.
 As shown in FIG. 3, the second hole 44 may then be plugged in any suitable
 manner such as by brazing or welding using a suitable filler material.
 Alternatively, the plug 48 may be preformed as a pin which itself is
 brazed in the second hole 44, or otherwise affixed therein.
 As indicated above, the lobed embodiment of the second hole 44 illustrated
 in FIG. 5 effects a cross section with a high perimeter to area ratio for
 enhancing cooling effectiveness of the impingement air. The differently
 configured first and second holes 42,44 may correspondingly be formed by
 first forming a common hole through both the tip rib 36 and cap, such as a
 round hole. The second hole 44 in the tip rib 36 may then be finally
 formed by enlarging the initially formed round hole greater in size than
 the first hole 42, with a suitable configuration or profile such as the
 lobed profile illustrated in FIG. 5. The lobed profile of the second hole
 44 may be obtained by using a correspondingly configured electrical
 discharge machining electrode, for example. The first tip holes 42
 described above are effective for impinging the coolant against the inner
 surface of the tip rib 36 for providing enhanced cooling thereof. The
 impingement air is protected by the tip rib 36 and is not directly
 subjected to the hot combustion gases surrounding the outer surface of the
 tip rib. Impingement cooling provides a greater amount of cooling than
 simple film cooling or convection cooling alone. Accordingly, the airfoil
 tip enjoys enhanced cooling for increased life during operation.
 While there have been described herein what are considered to be preferred
 and exemplary embodiments of the present invention, other modifications of
 the invention shall be apparent to those skilled in the art from the
 teachings herein, and it is, therefore, desired to be secured in the
 appended claims all such modifications as fall within the true spirit and
 scope of the invention.