Patent Application
20140271226
The present disclosure relates generally to gas turbine engines, and, more specifically, to a gas turbine engine rotor blade having improved tip cooling.
A gas turbine engine includes one or more turbine blade rows disposed downstream of a combustor which extracts energy from combustion gases generated by the combustor. Disposed radially outwardly of the rotor blade tips may be a stator shroud which is spaced from the blade tips to provide a relatively small clearance between the blade tips and shroud for reducing leakage of the combustion gases over the blade tips during operation. Each of the rotor blades includes conventionally known pressure and suction sides which are preferentially aerodynamically contoured for extracting as much energy as possible from the combustion gases flowing over the rotor blades. The pressure and suction sides extend to the blade tip and are disposed as close as possible to the stator shroud for maximizing the amount of energy extracted from the combustion gases. The clearance, however, between the blade tips and the stator shroud must nevertheless be adequate to minimize the occurrence of blade tip rubs during operation, which may damage the blade tips.
Un-shrouded blades use a squealer tip to reduce hot gas leakage over the blade tip and reduce performance penalties. Such a tip design typically requires ribs, generally a pressure side rib and a suction side rib, to protrude from the blade tip floor. These ribs are relatively thin, which makes them difficult to cool effectively through conduction. Turbine blade tips and associated ribs, moreover, are exposed to the very high temperatures of combustion gasses flowing over their outside surfaces. These high temperatures and low cooling effectiveness lead to durability issues on the tip ribs and the potential for blade fallout at the end of the blade's life interval. Any tip ribs that suffer oxidation or cracks beyond the squealer floor will render a blade irreparable regardless of the overall airfoil condition.
Whether shrouded or un-shrouded, turbine rotor blades are typically hollow for channeling cooling air through the interior of the blade. This cooling air is provided from a conventional compressor of the gas turbine engine to cool the blades from the heat flux generated by the combustion gases flowing over the blades. The tip, or tip cap, portion of the blades is particularly susceptible to the damaging effects of the hot combustion gases and must be suitably cooled for reducing blade tip distress in the form of oxidation and thermal fatigue during operation. As the blade tip erodes during operation due to the blade tip distress, the pressure and/or suction sides of the blade are adversely affected, which decreases the aerodynamic efficiency of the blade used for extracting energy from the combustion gases. In addition, such erosion of the blade tip also increases the clearance between the blade tip and the stator shroud, which allows more of the combustion gases to leak over the blade tip, and, therefore, extraction of the energy therefrom is lost which also decreases aerodynamic efficiency.
Numerous conventional blade tip cap designs exist for maintaining the proper pressure and suction side flow surfaces of the blade at the tip cap as well as providing minimum clearances with the stator shroud. Numerous cooling configurations also exist for cooling the blade tips or blade tip caps for meeting life requirements of the blades without undesirable erosion thereof. Conventional design practice makes use of a tip shelf recess or an L-shaped trough defined by the tip shelf and a first tip wall disposed on the pressure side of the blade. The tip shelf may offer the advantage of providing a discontinuity on the airfoil pressure side of the blade tip, causing combustion gasses to separate from the surface of the blade tip, which may decreases the heat transfer capability of the hot gasses to the blade tip, and therefore may decrease the heat flux into the blade tip. Conventional design practice also makes use of straight round holes through the tip shelf for passing cooling gas from the hollow blade interior to the tip shelf and pressure side rib, with resultant tip cooling due to convective and film effects. The tip shelf recess provides a region for the cooling air exiting the interior of the blade to accumulate, thereby providing a film blanket of cooling air between the hot combustion gasses and the blade tip, thereby further cooling the blade tip.
Another approach to cooling the blade tip is to increase the total number of straight round cooling holes in the tip shelf to increase the total cooling flow and decrease the space available for hot gas to interact with the surface. Since cooling of the blade, including the blade tip, uses a portion of the compressed air from the gas turbine compressor, however, that air is unavailable for combustion in the combustor of the engine which decreases the overall efficiency of the gas turbine engine. Accordingly, cooling of the blade, including the blade tip, should be accomplished with as little compressed air as possible to minimize the loss in gas turbine engine efficiency.
Still another approach involves creating channels or indentations in the pressure side rib to direct cooling flow from the pressure side tip holes over the rim at desired locations to better cover the surface.
Yet another approach is to thicken the pressure side rim and drill cooling holes through the center and exit at the rim top face. It would be desirable to provide tip shelf cooling holes that are economical to install, provide an acceptable flow of cooling air over the blade tip shelf, and provide an improved film blanket of cooling air spread across the tip shelf, thereby better protecting the blade tip from hot combustion gasses.
According to the present disclosure, one or more diffuser cooling holes may be provided in the tip shelf of a turbine blade assembly. Diffuser cooling holes may allow the cooling gas to begin diffusing before exiting the cooling hole and covering a larger area than a straight hole would provide. The diffused cooling gas may then flow over the pressure side rail covering a larger surface area than is typical using straight round cooling holes. This increased coverage may provide more even cooling to the pressure side rail and less near-surface leakage paths for hot gas to occupy. The cooling gas diffusion also may serve to reduce the coolant exit velocity into the tip shelf cavity. The reduced velocity may increase the amount of cooling gas that is entrained in the shelf, thereby enhancing the overall cooling into the pressure side rail from the tip shelf region.
The following description is better understood when read in conjunction with the appended drawings.
Gas turbine blades having a cooling channel therein for channeling cooling air to the tip of the blade are generally known. As is known, the turbine blades typically include an airfoil including a first side joined to a second side at spaced apart leading and trailing edges to define therein a flow channel for channeling cooling air through the airfoil to cool the airfoil from combustion gases flowing over the first and second sides. The airfoil typically has a tip at its distal end and a root having a dovetail extending from the root for mounting the blade to a rotor disk. The airfoil tip typically includes a tip floor extending between the airfoil first and second sides and between the leading and trailing edges for enclosing the airfoil for containing cooling air in the air flow channel. A first tip wall typically extends from the tip floor at the airfoil first side to form an extension thereof. A second tip wall typically extends from the tip floor at the airfoil second side to form an extension thereof, and is spaced in part from the first tip wall to define therebetween an outwardly facing tip plenum. The first tip wall is typically recessed at least in part from the airfoil first side to define an outwardly facing tip shelf extending between the leading and trailing edges to provide a discontinuity in the airfoil first side, the first tip wall and the tip shelf defining therebetween a tip shelf recess or trough. In an alternative form, the tip shelf may extend from the leading edge to a point short of the trailing edge, a configuration sometimes referred to as a “partial tip shelf”
Referring to
As illustrated, the tip shelf 15 may be formed in a squealer tip rim 16 that is positioned at the blade tip. The tip shelf 15 may have positioned therealong one or more diffuser cooling holes 17. As further illustrated, the tip floor or plenum, generally 18, may include one or more tip floor cooling holes 19 distributed thereon. These diffuser cooling holes 17 and tip floor cooling holes 19 may be in flow communication with a substantially hollow interior 20 of the blade assembly, which may include a serpentine flow channel configuration formed by one or more internal ribs 21 for channeling cooling air, represented by the arrows “A” in
As more particularly shown in
As used herein, the term “diffuser cooling hole” is intended to mean a cooling hole that tends to diffuse and/or reduce the flow rate of cooling gas at the point where the cooling gas exits the cooling hole, as distinguished from fully straight-walled or cylindrical cooling holes, which do not perform in this manner. In one embodiment of the disclosure, the diffuser portion 24 may flare generally outwardly relative to the longitudinal axis AA of the diffuser cooling hole 17, and may be generally conical in shape in the axial direction and round in cross section, although other configurations for the diffuser portion 24, including, without limitation, parabolic, hyperbolic, semi-circular, semi-elliptical, and/or semi-oval, for example, in the axial direction, and elliptical, oval, square, rectangular, and/ or round, for example, in cross section, are also possible, provided the configuration tends to have an exit 26 with a greater area than a cross sectional area of the diffuser portion upstream of the exit, and tends to diffuse and/or reduce the flow rate of the cooling gas at the point 26 it exits the tip shelf, and tends to create a curtain of cooling gas along the tip shelf recess 22. It is also possible for the diffuser portion 24 of the diffuser cooling holes 17 to extend only a portion of the way around the cooling hole perimeter, e.g., in the case of a round diffuser in cross section, the diffuser portion 24 may extend 180° around the circumference, being half conical, for example, and half cylindrical, thereby creating a one-sided diffuser. As illustrated in
By way of further example, as illustrated in
As will now be appreciated, by varying the size and/or shape of the diffuser cooling holes 17 arrayed along the tip shelf 15, it may be possible to vary the flow rate and coverage of cooling gas in different regions of the tip shelf 15 with the objective of equalizing the temperature profile across the turbine tip. The flow rate is controlled by the size of the straight round portion 25 of the diffuser cooling holes 17. By increasing the size of the straight round portion 25, higher flow rates can be delivered to regions known to experience higher temperatures and vice versa. The diffuser portion 24 controls the spread and exit velocity of the flow. For a given flowrate, (i.e. fixed straight round portion 25), the diffuser portion 24 can be adjusted to tune the local temperatures. By making the diffuser portion 24 larger, the flow is spread out over a larger area providing better film coverage in regions known to experience higher temperatures. If the diffuser portion 24 is made smaller to approach the size and shape of the straight round portion 25, then the cooling benefits of the diffuser design are lessened.
Illustrated in
The disclosure may help to enhance film coverage over the pressure side tip rim, thereby reducing temperature gradients which are detrimental to LCF life. The disclosure may also help to distribute cooling air more evenly to the pressure side tip rim, thereby reducing overall surface temperatures. The use of diffuser shaped holes according to the present disclosure can lead to lower cooling flow usage relative to round straight holes for the same temperature limits, or equal cooling flow usage relative to round straight holes with decreased temperatures.
This written description uses examples to disclose the various embodiments, including the best mode, and also to enable any person of ordinary skill in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods or apparatus. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
