BACKGROUND OF THE INVENTION
Components in the hot gas flow path of gas turbines often have cooling channels. Cooling effectiveness is important to minimize thermal stress on these components, and cooling efficiency is important to minimize the volume of air diverted from the compressor for cooling. Film cooling provides a film of cooling air on outer surfaces of a component via holes from internal cooling channels. Film cooling can be inefficient, because a high volume of cooling air is required. Thus, film cooling has been used selectively in combination with other techniques. Impingement cooling is a technique in which perforated baffles are spaced from a surface to create impingement jets of cooling air against the surface. Serpentine cooling channels have been provided in turbine components, including airfoils such as blades and vanes. The present invention increases effectiveness and efficiency in cooling channels.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in the following description in view of the drawings that show:
FIG. 1 is a sectional side view of a turbine blade with cooling channels.
FIG. 2 is a sectional view of an airfoil trailing edge taken on line 2-2 of FIG. 1, with cooling channels showing aspects of the invention.
FIG. 3 is a transverse profile of a cooling channel per aspects of the invention.
FIG. 4 is a sectional view of one-sided near-wall cooling channels.
FIG. 5 is a sectional view of cooling channels in a tapered component.
FIG. 6 is a transverse sectional view of a turbine airfoil with hourglass shaped cooling channels.
FIG. 7 shows a process of molding ceramic cores for a mold for hourglass shaped cooling channels.
FIG. 8 shows a transverse sectional view of an hourglass shaped cooling channel with converging side surfaces defined by peaked turbulators.
FIG. 9 shows an embodiment as in FIG. 8 combined with fins on the near-wall inner surfaces.
FIG. 10 is a view taken along line 10-10 of FIG. 8 showing peaked turbulators with convex upstream sides.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a sectional view of a turbine blade 20 having a leading edge 21 and a trailing edge 23. Cooling air 22 from the turbine compressor enters an inlet 24 in the blade root 26, and flows through channels 28, 29, 30, 31 in the blade. Some of the coolant may exit film cooling holes 32. A trailing edge portion TE of the blade may have turbulator pins 34 and exit channels 36. Each arrow 22 indicates an overall coolant flow direction at the arrow, meaning a predominant or average flow direction at that point.
FIG. 2 is a sectional view of a turbine airfoil trailing edge portion TE taken along line 2-2 of FIG. 1. The trailing edge portion has first and second exterior surfaces 40, 42 on suction and pressure side walls 41, 43 of the airfoil. Cooling channels 36 may have fins 44 on inner surfaces 48, 50 of the exterior walls 41, 43 according to aspects of the invention. These inner surfaces 48 and 50 are called “near-wall inner surfaces” in the art, meaning an interior surface of a cooling channel that is closest to a cooled exterior surface. Gaps G between the channels produce gaps in cooling efficiency and uniformity. The inventors recognized that cooling effectiveness, efficiency, and uniformity could be improved by increasing the cooling rate in the corners C of the cooling channels, since these corners are nearest to the gaps G. One way to accomplish this preferential cooling is to provide an hourglass-shaped channel profile in which the side surfaces 52, 54 of the channel form a waist that is narrower than a width of each of the first and second inner surfaces 48 and 50. The waist functions to increase the flow resistance in the center of the channel, thereby urging the coolant toward the corners of the channel. Since coolant flow in the center of the channel does not contact a heat transfer surface whereas flow in the corners does function to remove heat, the present invention is effective to increase the efficiency of the cooling.
FIG. 3 is a transverse sectional profile 46 of a cooling channel that is shaped to efficiently cool two opposed exterior surfaces. The channel may be a trailing edge channel 36 or any other cooling channel, such as channels 29 and 30 in FIG. 1. It has two opposed near-wall inner surfaces 48, 50, which may be parallel to the respective exterior surfaces 40, 42 of FIG. 2. Here “parallel” means with respect to the portions of the near-wall inner surface closest to the exterior surface, not considering the fins 44. The channel has widths W1, W3 at the near-wall inner surfaces 48, 50. Two interior side surfaces 52, 54 taper toward each other from the sides of the inner surfaces 48, 50, defining a minimum channel width W2 or waist in the side surfaces. The inner surface widths W1 and W3 are greater than the waist width W2, so the channel profile 46 has an hourglass shape formed by convexity of the side surfaces 52, 54. This shape increases the coolant flow 25 toward the corners C of the channel. The overall coolant flow direction is normal to the page in this view. The arrows 25 illustrate a flow-increasing aspect of the profile 46 relative to a channel without an hourglass shape and/or without fins next described.
Fins 44 may be provided on the inner surfaces 48, 50. The fins may be aligned with the overall flow direction 22 (FIG. 1) which is normal to the plane of FIG. 3. If fins are provided, they may have heights that follow a convex profile such as 56A or 56B, providing a maximum fin height H at mid-width of the near-wall inner surface 48 and/or 50. These fins 44 increase the surface area of the near-wall surfaces 48, 50, and also increase the flow 25 in the corners C. The taller middle fins reduce the flow centrally, while the shorter distal fins encourage flow 25 in the corners C. The combination of convex sides 52, 54 and a convex fin height profile 56A, 56B provides synergy that focuses cooling toward the channel corners C.
Dimensions of the channel profile 46 may be selected using known engineering methods. The illustrated proportions are provided as an example only. The following length units are dimensionless and may be sized proportionately in any unit of measurement, since proportion is the relevant aspect exemplified in this drawing. In one embodiment the relative dimensions are B=1.00, D=0.05, H=0.20, W1=1.00, W2=0.60. The side taper angle A=−30° in this example. Herein, a negative taper angle A of sides 52, 54 in the profile 46 means the sides converge toward each other toward an intermediate position between the inner surfaces 48, 50, forming a waist W2 as shown. In some embodiments the taper angle A may range from −1° to −30°. The waist width W2 may be determined by the taper angle. Alternately it may be 80% or less of one or both of the near wall widths W1, W3, or 65% or less in certain embodiments. One or more proportions and/or dimensions may vary along the length of the cooling channel. For example, dimension B may vary with the thickness of the airfoil. The widths W1, W3 of the two inner surfaces 48 and 50 may differ from each other in some embodiments. In this case, the waist W2 may be narrower than each of the widths W1, W3.
FIG. 4 shows a cooling channel 36B shaped to cool a single exterior surface 40 or 42. It uses the fin and taper angle concepts of the cooling channel 36 previously described. The near-wall inner surface width W1 is greater than the minimum channel width W2 due to tapered interior side surfaces 52, 54. Fins 44 may be provided on the near-wall inner surface 48, and they may have a convex height profile centered on the width W1 of the near-wall inner surface. Such cooling channels 36B may be used for example in a relatively thicker part of a trailing edge portion TE of an airfoil rather than the relatively thinner part of the trailing edge portion TE where a cooling profile 46 as in FIG. 3 might be used. The transverse sectional profile of this embodiment may be trapezoidal, in which the near-wall inner surface 48 defines a longest side thereof.
FIG. 5 shows that the exterior surfaces 40 and 42 may be non-parallel in a transverse section plane of the channel 36. The near-wall inner surfaces 48, 50 may be parallel to the exterior surfaces 40, 42.
FIG. 6 shows a transverse section of a turbine airfoil 60 with hourglass-shaped span-wise cooling channels 63, 64, 65, and 66. Herein “span-wise” means the channel is oriented in a direction between radially inner and outer ends of the airfoil. “Radial” is with respect to the turbine axis of rotation. For example, in FIG. 1 channels 28, 29, 30, and 31 are span-wise channels. These channels may optionally have fins 44 as previously described regarding FIG. 3.
FIG. 7 shows a process of forming ceramic cores 74, 75 for an airfoil mold. The cores may be chemically removed after casting of the airfoil 60. Flexible dies 84A, 84B, 85A, 85B or dies with flexible liners may be used to form the cores 74, 75 of a green-body ceramic that is stiff enough for pulling 89 of the dies elastically past interference points 91. Such technology is taught for example in U.S. Pat. Nos. 7,141,812 and 7,410,606 and 7,411,204 assigned to Mikro Systems Inc. of Charlottesville, Va. Even small negative taper angles such as −1 to −3 degrees are significant and useful for cooling efficiency compared to the positive taper angles required for removal of conventional rigid dies.
FIG. 8 shows a transverse sectional view of an hourglass shaped cooling channel 65 with converging side surfaces 52, 54 defined by turbulators 92. Each turbulator has a peak 97 in a middle portion thereof that defines the waist of the cooling channel. The side surfaces 52, 54 on the turbulators may have the taper range previously described, or especially in the range of −2 to −5 degrees (−5 degrees shown). The turbulators 92 may alternate with surfaces 95, 96 that are flat (shown) or have positive taper (not shown).
FIG. 9 shows an embodiment as in FIG. 8 combined with profiled fins 44 on the near-wall inner surfaces 48, 50 as previously described.
FIG. 10 is a view taken along line 10-10 of FIG. 8 showing peaked turbulators 92 with convex upstream sides 93 and straight downstream sides 94. The convex upstream sides 93 urge the flow 22 toward the corners C. The straight downstream sides 94 facilitate pulling the dies 84A, 84B, 85A, 85B of FIG. 7 straight out, normal to the cores 74, 75. Alternately, the downstream sides 94 of the turbulators may be convex (not shown) such as parallel to the upstream sides 93.
The embodiments of FIGS. 8-10 can be fabricated using the cost-effective process of FIG. 7. The turbulators 92 concentrate the coolant flow toward the near-wall inner surfaces 48 and 50 and into the corners C. The combination features shown in FIG. 9 is especially effective and efficient, since the turbulators 92 slow the flow 22 centrally while concentrating it toward the inner surfaces 48 and 50, where the ribs 44 transfer heat from the exterior surfaces 40, 42, and increase the flow 22 toward the corners C.
The present hourglass-shaped channels are useful in any near-wall cooling application, such as in vanes, blades, shrouds, and possibly in combustors and transition ducts of gas turbines. They increase uniformity of cooling, especially in a parallel series of channels with either parallel flows or alternating serpentine flows. The present channels may be formed by known fabrication techniques—for example by casting an airfoil over a positive ceramic core that is chemically removed after casting.
A benefit of the invention is that the near-wall distal corners C of the channels remove more heat than prior cooling channels for a given coolant flow volume. This improves efficiency, effectiveness, and uniformity of cooling by overcoming the tendency of coolant to flow more slowly in the corners. Increasing the corner cooling helps compensate for the cooling gaps G between channels. The invention also provides increased heat transfer from the primary surfaces 40, 42 to be cooled through the use of the fins 44.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
Patent Application
20130149169
