Cooled airfoil with reduced internal turn losses

Abstract

A cooled airfoil 10 has an internal fluid passage 34 that includes upstream and downstream legs, such as a co-centrifugal leg 40 and a counter-centrifugal leg 42. The legs are separated from each other by a rib 44 but are connected in series with each other. The airfoil also includes a vent passage 56 for venting fluid from the internal passage. The intake 60 to the vent passage resides in the counter-centrifugal leg. The vent at least partially counteracts the separation potential of a separation susceptible region 54 by allowing some of the coolant to vent from the passage leg 42.

Description

TECHNICAL FIELD

This invention relates to cooled airfoils of the type used in turbine engines and particularly to a cooled airfoil with reduced turning losses in an internal cooling passage of the airfoil.

BACKGROUND OF THE INVENTION

Turbine engines include one or more turbines for extracting energy from a stream of hot working medium gases. A typical turbine includes a rotatable hub with a set of circumferentially distributed blades projecting radially from the hub. Each blade includes an attachment for attaching the blade to the hub. Each blade also includes an airfoil that spans radially across a working medium flowpath from an airfoil root to an airfoil tip. A typical turbine also includes one or more arrays of stationary vanes axially spaced from the blades. Each vane includes an airfoil that spans radially across the flowpath and a hook or other feature for securing the vane to a case. Because the blades and vanes operate in a hot environment, it is common practice to provide internal coolant passages in at least the airfoils of the blades and vanes. During engine operation, coolant flows through the internal passages to protect the airfoils from the intense heat of the combustion gases. The coolant is usually relatively cool air that has been pressurized by a compressor powered by the turbine.

Some coolant passages are multi-pass passages. A multi-pass passage includes at least two spanwisely extending legs that are chordwisely adjacent to each other. A spanwisely extending rib separates the legs from each other. An elbow at the radially inner or outer ends of the legs wraps around one extremity of the rib to connect the legs in series.

During engine operation, a stream of coolant flows through one of the legs (the upstream leg), through the elbow and then through the other leg (the downstream leg). The elbow reverses the direction of coolant flow, for example from radially outwardly in the upstream leg to radially inwardly in the downstream leg. The coolant stream entering the downstream leg is susceptible to separation from the rib. The region of the leg susceptible to fluid separation extends chordwisely a considerable distance across the downstream leg and is characterized by high aerodynamic losses. These losses can imperil the durability of the airfoil by restricting coolant flow and/or by reducing the pressure of the coolant downstream of the region of separation. An engine designer can attempt to compensate for these effects by supplying higher pressure coolant to the passages. However such an approach may not be completely successful. Moreover, because the turbine itself is the source of energy for pressurizing the coolant, the use of higher pressure coolant degrades engine efficiency.

The above described susceptibility to separation arises in part from the severity of the turn from the upstream leg to the downstream leg. However other factors are also noteworthy, particularly in blades in which the elbow connects the radially outer ends of two legs of a multi-pass passage. One of these factors is that the geometry of many blades restricts the space available to accommodate an elbow near the airfoil tip. As a result, an elbow connecting the radially outer ends of two legs is typically squared off, rather than gently rounded, at the outside of the turn. This leads to higher pressure losses than would be experienced if the outside of the turn were rounded. Another factor arises from the fact that the blade rotates about the engine centerline. As a result, centrifugal effects resist the flow of coolant radially inwardly in the downstream passage.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, an airfoil has an internal fluid passage that includes upstream and downstream legs, such as a co-centrifugal leg and a counter-centrifugal leg. The legs are chordwisely separated from each other by a rib but are connected in series with each other. The airfoil also includes a vent passage for venting fluid from the internal passage. The intake to the vent passage resides in the counter-centrifugal leg.

The foregoing and other features will become more apparent from the following description of the best mode for carrying out the invention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side elevation view of a turbine blade with internal coolant passages in the airfoil portion thereof.

FIG. 2 is a view taken in the direction 2-2 of FIG. 1.

FIG. 3 is an enlarged view of the region 3-3 of FIG. 1 showing vent passages for moderating a region of separation and high loss in a downstream leg of a multi-pass coolant passage.

FIG. 4 is a view similar to FIG. 3 but without the vent passage.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2 show a cooled turbine blade 10 for the turbine of a gas turbine engine. The blade includes an attachment 12 for securing the blade to a hub, not shown. The hub is rotatable about an engine centerline or axis 14. The blade also includes a platform 16 and an airfoil 18. When the blade is installed in the hub, the airfoil spans radially across a working medium flowpath 20 from an airfoil root 24 to an airfoil tip 26. A notional chord line 28 (FIG. 2) extends from a leading edge 30 to a trailing edge 32 of the airfoil. Internal coolant passages, such as multi-pass serpentine passage 34, convey coolant 38 through the airfoil. The coolant protects the airfoil from the intense heat of combustion gases G flowing axially through the flowpath 20.

The passage 34 includes a spanwisely extending upstream leg 40 and a spanwisely extending downstream leg 42 chordwisely separated from the upstream leg by a spanwisely extending rib 44. The rib is truncated to accommodate an elbow 46 that connects the upstream and downstream legs in series flow relationship. The upstream leg 40 is a co-centrifugal leg because the rotation of the blade about axis 14 assists the flow of coolant from root end of the leg toward the tip end of the leg. The downstream leg 42 is a counter-centrifugal leg because the rotation of the blade about axis 14 resists the flow of coolant from tip end of the leg toward the root.

Referring principally to FIG. 4, the downstream leg 42 has a chordwise width W, as measured from the rib 44 to a chordwisely neighboring rib 48 that defines the opposing sidewall of the leg. The leg 42 also has a hydraulic diameter D_H, which is defined as four times the cross sectional area A of the leg divided by its wetted perimeter P (D_H=4A/P) where A and P are determined at the inlet 52 to the leg. A region 54 susceptible to fluid separation is present on the inside of the turn next to the rib 44. The chordwise dimension of the separation susceptible region increases with increasing lengthwise distance along the passage, and exhibits its maximum chordwise dimension at spanwise location M. The maximum chordwise dimension of the separation region is about: 50% of the chordwise width W. The region 54 then diminishes in chordwise dimension with additional lengthwise distance along the passage. The overall spanwise dimension of the illustrated separation region 54 is about two and one half to three hydraulic diameters. This is a typical spanwise dimension for the separation region, however the spanwise dimension can vary depending on the geometry of the passage leg and the fluid properties of the coolant.

Referring now to FIGS. 2 and 3, a vent passage 56 penetrates the suction sidewall 58 of the airfoil. The vent passage has an intake 60 residing in the region susceptible to separation and an exit 62 on the suction wall. The vent passage at least partially counteracts the separation potential of region 54 by allowing some of the coolant to vent from the passage leg 42. We believe that the vent passage will be most effective if its inlet 60 resides immediately adjacent to rib 44 as seen best in FIG. 3. However, we have concluded that a vent intake chordwisely spaced from the rib by as much as about half the maximum chordwise width of the unmoderated region 54 (FIG. 4) would also be quite effective. Because the widest portion of the unmoderated separation zone (seen in FIG. 4) occupies about 50% of the local passage width W, such a vent intake would be spaced from rib 44 by about 25% of the local passage width W. In addition, because the separation region typically extends lengthwise about two and one half to three hydraulic diameters lengthwise from the inlet 52, the vent intake should also be within about two and one half to three hydraulic diameters from the inlet. Ideally, the intake is at the lengthwise location M, where the chordwise dimension of the unmoderated separation susceptible region is widest.

The vent passage may be a single passage or it may be an array of passages, one example of which is the single linear array, seen in FIG. 3. The above described principles for positioning the passage intakes apply equally to a single passage or to an array of passages. In the case of a spanwisely extending row of passages, the row is centered at spanwise location M where the unmoderated separation susceptible region has its maximum chordwise dimension.

The vent passages may be installed by any suitable technique, for example laser drilling, electron beam drilling or electro-discharge machining. Since turbine blades are usually cast, the passages may also be provided for in the casting itself. One possible advantage to cast passages is the relative ease with which they may be precisely and repeatably positioned in a group of serially produced airfoils.

The particular vent passage shown in the illustrations is a film cooling hole that exhausts some of the coolant 38 to the surface of the suction wall 58 where it spreads out to form a thermally protective cooling film on the wall surface. Alternatively a film cooling hole that vents coolant to the surface of the pressure wall 64 would also be effective, provided the pressure difference across the passage is large enough to drive the coolant through the passage. Or, film cooling holes venting to both the suction and pressure sides would also be effective. Vent passages that do not also serve as film cooling holes will also suffice to moderate the region of separation.

Although this application has shown and described a specific embodiment of our airfoil, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the invention as set forth in the accompanying claims.

Claims

1. An airfoil having an internal fluid passage, the passage including a co-centrifugal leg and a counter-centrifugal leg in series with the co-centrifugal leg and separated therefrom by a rib, the airfoil also including a vent passage with an intake for venting fluid from the internal passage, the intake residing in the counter-centrifugal leg.

2. The airfoil of claim 1 wherein the intake resides in a region susceptible to fluid separation.

3. The airfoil of claim 1 wherein the counter-centrifugal leg has a width and the intake resides in a region susceptible to fluid separation extending chordwisely up to about 50% of the width of the counter-centrifugal leg as measured from the rib to an opposing sidewall of the counter-centrifugal leg.

4. The airfoil of claim 1 wherein the counter-centrifugal leg has a width and the intake is spaced from the rib by about 25% of the width as measured from the rib to an opposing sidewall of the counter-centrifugal leg.

5. The airfoil of claim 1 wherein the intake resides immediately adjacent to the rib.

6. The airfoil of claim 1 wherein the intake resides in a region susceptible to fluid separation, the region extending about three and one half hydraulic diameters lengthwise along the counter-centrifugal leg.

7. The airfoil of claim 1 wherein the counter-centrifugal leg has an inlet and the intake resides within a region extending about three and one half hydraulic diameters lengthwise from the inlet.

8. The airfoil of claim 1 wherein the intake resides lengthwisely at a location where a separation susceptible region has a maximum chordwise dimension.

9. The airfoil of claim 1 wherein the intake is an array of intakes centered at a location where a separation susceptible region has a maximum chordwise dimension.

10. The airfoil of claim 1 wherein the vent passage is a film cooling hole.

11. The airfoil of claim 1 wherein the vent exhausts to a suction side of the blade.