Patent Publication Number: US-6220817-B1

Title: AFT flowing multi-tier airfoil cooling circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to cooling of turbine rotor blades and stator vanes in gas turbine engines and, more specifically, to serpentine cooling circuits therein. 
     2. Discussion of the Background Art 
     A gas turbine engine includes a compressor that compresses air which is channeled to a combustor wherein it is mixed with fuel and ignited for generating combustion gases. The combustion gases flow downstream through one or more stages of turbines which extract energy therefrom for powering the compressor and producing additional output power for driving a fan for powering an aircraft in flight for example. A turbine stage includes a row of turbine rotor blades secured to the outer perimeter of a rotor disk, with a stationary turbine nozzle having a plurality of stator vanes disposed upstream therefrom. The combustion gases flow between the stator vanes and between the turbine blades for extracting energy to rotate the rotor disk. Since the combustion gases are hot, the turbine vanes and blades are typically cooled with a portion of compressor air bled from the compressor for this purpose. Diverting any portion of the compressor air from use in the combustor necessarily decreases the overall efficiency of the engine. Accordingly, it is desired to cool the vanes and blades with as little compressor bleed air as possible. 
     Typical turbine vanes and blades include an airfoil over which the combustion gases flow. The airfoil typically includes one or more serpentine cooling passages therein through which the compressor bleed air is channeled for cooling the airfoil. The airfoil may include various turbulators therein for enhancing cooling effectiveness and, the cooling air is discharged from the passages through various film cooling holes disposed around the outer surface of the airfoil. 
     The temperature profile of the combustion gases channeled over the airfoil is typically center peaked at about 50% to about 80% of the radial height or span of the airfoil. Secondary flow fields between adjacent airfoils may sometimes cause the temperature profile of the combustion gases to shift radially outwardly on the pressure side of the airfoil. Accordingly, the airfoil typically experiences relatively high heat input loading on its pressure side above the airfoil mid-span. Since the serpentine cooling circuits introduce air into the airfoil from its root, the cooling air must be provided with a suitable flow rate to ensure that the outer portions of the airfoil experiencing the greatest heat input are adequately cooled for obtaining a useful life during operation. The inner portions of the airfoil may therefore be over-cooled which is an inefficient use of the valuable compressor bleed air. To overcome this drawback a “Multi-tier turbine airfoil”, disclosed in U.S. Pat. No. 5,591,007, was devised and is incorporated herein by reference. This patent discloses a turbine airfoil having a plurality of internal ribs defining at least two independent serpentine cooling circuits arranged in part in different longitudinal tiers, with an outer tier circuit being disposed in part longitudinally above an inner tier circuit for differentially longitudinally cooling the airfoil. More advanced turbine airfoil designs have been developed that could use a better cooling air distribution. 
     Typical mid-circuit cooling air, after picking up the heat in the serpentine passage, exits through film cooling holes. One or more rows of film cooling holes are placed on the pressure side and also on the suction side. New highly aerodynamically efficient airfoils in low through flow turbine designs are subject to an external gas path flow along the pressure side that has low velocity. This can result in a very high blowing ratio (mass flux ratio of film cooling air to gas flow) through the film cooling holes and very poor film cooling effectiveness (film blow-off) on the pressure side of the airfoil. Geometrical limitations of at least some of the cavities which supply the film cooling air prevent or make difficult the use of film holes on both pressure and suction sides that have relatively shallow angles from the surfaces of the sides. The use of larger angles would result in significant aerodynamic mixing losses and poor film cooling effectiveness because much of the film cooling air would flow out of the boundary layer. Therefore, it is desirable to have a circuit design which can avoid the use of film cooling in such areas of the airfoil and provide effective and efficient film and convective cooling of the entire airfoil. 
     SUMMARY OF THE INVENTION 
     A turbine airfoil includes a plurality of internal ribs defining at least two independent serpentine cooling circuits having outer and inner serpentine portions, respectively, in different longitudinal tiers with the outer serpentine position being disposed longitudinally above the inner tier serpentine position for differentially longitudinally cooling the airfoil. The outer and inner serpentine portions include outer and inner exits and entrances wherein the outer and inner exits are positioned aft of the outer and inner entrances, respectively, so as to have a chordal flow direction aftwards from the leading edge to the trailing edge within the serpentine portions. 
     The airfoil may include film cooling holes in an outer wall of the airfoil on the suction side of the airfoil and no film cooling holes on a pressure side of the outer wall along a mid-chord portion of the airfoil between the leading and trailing edges. 
     ADVANTAGES OF THE INVENTION 
     The present invention provides advantages that include a significant improvement in the cooling of not only, an upper span portion of the turbine airfoil outer wall, but also of a mid-chord portion of the suction and pressure sides of the outer wall. Furthermore, the use of separate leading edge and mid-circuits provides colder cooling air at the upper span portions of the airfoils. 
     The highly curved or arched contour of the airfoil has span ribs between span channels or cavities nearer to the leading edge that are wider than span ribs between span channels nearer the .trailing edge and, therefore, are also on the average further away from the external hot surfaces of the sides of the outer wall and generally have temperatures closer to the cooling air temperature in the channels. In a downstream wise serpentine circuit design as in the present invention, the cooling air temperature is colder than the cooling air temperature in the same cavities for upstream wise serpentine circuit design. Therefore, a downstreamwise serpentine circuit will have a colder average spanwise rib wall temperature than that of an upstreamwise serpentine circuit and, therefore, have an overall better cooling air temperature distribution in the chordwise direction and a better bulk temperature of the airfoil for better cooling of the entire airfoil. 
     The two tier circuit design offers additional flexibilities in distributing cooling air in a more efficient manner and also shortens the length of each pass and increases the number of turns which result in a higher heat transfer (cooling) coefficients inside the serpentine passages. The downstreamwise serpentine circuit design also provides an internal cooling air pressure which is more consistent with and tailored to the external gas pressure as the external gas expands in the chordwise or downstream direction through the turbine. This results in a better back flow margin for the blade and a more optimum use of internal cooling potential by trading more pressure consumption for better heat transfer. 
     Because the outer wall sides closer to the leading edge are cooled by colder fresher air than in those in the prior art, film cooling in this region may not be necessary. This will result in better turbine performance and lower cost in manufacturing. In addition, the film cooling holes closer to the trailing edge can have shallower flow angles from surface than those closer to the leading edge resulting in a better film cooling effectiveness. The external gas flow velocity closer to the trailing edge accelerates to a higher speed than at portions along the airfoil sides closer to the leading edge. Therefore, the airfoil cooling can be better tailored for conductive and convective cooling of portions of the sides of the outer wall closer to the leading edge and film cooling holes may be used for portions of the sides closer to the trailing edge where they will have smaller and, therefore, better blowing ratios and result in a better film cooling effectiveness and overall cooling efficiency. 
     Generally, design requirements for airfoils at the lower spans are driven by concerns for rupture at high stress levels at reduced metal temperature and at the upper regions by concerns over elevated surface temperature to avoid oxidation and fatigue crack initiation. The multi-tier downstream flowing serpentine design concept addresses these needs with the ability to better optimize cooling flow and blade life. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the present invention are set forth and differentiated in the claims. The invention, together with further objects and advantages thereof, is more particularly described in conjunction with the accompanying drawings in which: 
     FIG. 1 is a sectional view of an exemplary gas turbine engine airfoil laid out flat along its chord and having multi-tier serpentine cooling circuits with downstream flowing serpentine portions therein; and 
     FIG. 2 is a sectional view through the airfoil in FIG. 1 illustrated with an actual curved chord and taken along line  2 — 2 . 
    
    
     DETAILED DESCRIPTION 
     Illustrated in FIG. 1 is an exemplary turbine blade  10  for a gas turbine engine. The blade  10  includes an airfoil  12  and a conventional dovetail root  14  which is used to conventionally secure the blade  10  to a rotor disk of the engine. A cross-section of the airfoil  12  is illustrated in FIG.  2  and shows that the airfoil  12  includes an outer wall  15  with a pressure side  16  and a suction side  18  joined together along an upstream leading edge  20  and a downstream trailing edge  22  which is spaced axially or chordally therefrom. The airfoil  12  extends longitudinally along a longitudinal or radial axis  24  in a spanwise direction of the airfoil  12  from a radially inner base  26  to a radially outer airfoil tip  28  along a span S of the airfoil. The airfoil tip  28  is illustrated as a squealer tip having an outward extension from outer wall  15  or a squealer tip wall  29  extending longitudinally outward from and peripherally around an outer tip wall  31  forming a squealer tip cavity  33  therein. The inner base  26  is defined at a conventional platform  30  which forms the inner flow boundary of the airfoil  12  and below which extends the root  14 . 
     During operation of the blade  10 , combustion gases  32  are generated by a combustor (not shown) and flow downstream over both airfoil pressure and suction sides  16  and  18 , respectively, of the outer wall  15 . The radial or longitudinal temperature profile of the combustion gases  32  typically is center-peaked near a mid-span region above a mid-span chord CM of the airfoil from about 50% to about 80% thereof. Secondary flow fields between adjacent ones of the airfoils  12  may cause the temperature profile to shift radially outwardly over the airfoil pressure side  16  radially outwardly over a range of about 70% to about 85% of the radial height or span of the airfoil  12 . Accordingly, the pressure side  16  experiences its greatest heat input or load above the mid-span region in the 70% to 85% span height. 
     In accordance with the present invention, preferential radial or spanwise cooling of the airfoil  12  is effected to better match the distribution of the heat load thereto from the combustion gases  32 . Although an exemplary gas turbine rotor blade  10  is illustrated in the FIGS., the invention applies equally as well to turbine stator vanes having similar airfoils which may be similarly cooled in accordance with the present invention. 
     More specifically, the pressure and suction sides  16 ,  18  are spaced circumferentially or laterally apart from each other between the leading and trailing edges  20 ,  22  and are integrally joined together by a plurality of internal ribs indicated generally at  34  which define at least two independent cooling passages denoted as outer and inner tier cooling circuits  36  and  38 , respectively, having outer and inner tier serpentine portions  36   a  and  38   a , respectively, which are disposed generally above and below the mid-span chord CM, respectively. The outer and inner tier serpentine portions  36   a  and  38   a  are arranged in differential longitudinal decks or tiers, with the outer serpentine portion  36   a  being disposed in part longitudinally or radially above the inner serpentine portion  38   a  for differentially longitudinally cooling the airfoil  12  to better match the applied heat loads from the combustion gases  32 . Furthermore, the outer and inner tier serpentine portions  36   a  and  38   a  are arranged so as to cause serpentine cooling flow  35  within the respective outer and inner tier serpentine portions with a chordal flow direction  43  aftwards from the leading edge  20  to the trailing edge  22  within the serpentine portions. The outer and inner tier serpentine portions  36   a  and  38   a  include outer and inner entrances  36   b  and  38   b  and exits  36   c  and  38   c , respectively, which are arranged such that the outer and inner exits are positioned aft of the outer and inner entrances, respectively, to cause the serpentine cooling flow  35  to flow in the chordal flow direction  43  aftwards from the leading edge  20  to the trailing edge  22  to better match the applied heat loads from the combustion gases  32  and to even more effectively tailor the serpentine cooling flows  35  to the heat loading on and more effectively cool the airfoil  12 . 
     The internal ribs  34  illustrated in FIG. 1 are primarily in the form of chordally spaced apart and longitudinally extending short span ribs  34   a  which define the outer and inner tier serpentine portions  36   a  and  38   a  and long span ribs  34   b  which define a plurality of discrete span channels indicated generally at  40  and serpentine channels indicated generally at  41  which extend longitudinally inside the airfoil  12  for channeling cooling air  42  conventionally received from a compressor (not shown) inside the airfoil  12  for the cooling thereof. The internal ribs  34  also include preferably two or more (though one could suffice), mid-span chord ribs  34   x  extending chordally between adjacent ones of the short and long span ribs  34   a  and  34   b , respectively, to in part form outer and inner turning channels  37   a  and  37   b , respectively. The outer and inner turning channels  37   a  and  37   b , respectively, connect the span channels  40  and the serpentine channels  41  to form the outer and inner serpentine portions  36   a  and  38   a , respectively. Some of the outer and inner turning channels  37   a  and  37   b , respectively, near the mid-span chord CM are separated by the chordwise extending adjacent chord ribs  34   x  which may be longitudinally staggered or spaced apart. The staggered arrangement provides an additional advantage because it makes the airfoil stiffer with respect to the first mode of resonance which typically has a line of failure about which the airfoil tends to bend and fail through a mid-span region of the airfoil. Serpentine channels  41  of the outer serpentine portion  36   a  are preferably longitudinally aligned with corresponding ones of the inner serpentine portion  38   a  to further stiffen the airfoil  12  and simplify its construction. 
     In the exemplary embodiment illustrated in the FIGS., the outer and inner tier cooling circuits  36  and  38  are five-pass serpentine circuits. 
     Substantially, all of the outer tier cooling circuit  36  is disposed directly above a major portion of the inner tier cooling circuit  38  so that the airfoil  12  may be preferentially cooled in its longitudinal or radial direction for better matching the applied heat input from the combustion gases  32  flowing over the airfoil  12  during operation. The outer and inner tier cooling circuits  36 ,  38  may have any suitable number of serpentine passes as desired. 
     In the exemplary embodiment illustrated in FIG. 1, the outer and inner tier serpentine portions  36   a  and  38   a  are disposed near the mid-chord region M of the airfoil  12  between the leading edge  20  and the trailing edge  22  since experience has shown that the highest heat input in the airfoil  12  is in the mid-chord region near the airfoil tip  28  and on the pressure side  16  for example. In this specific embodiment, the chord ribs  34   x  are disposed at about mid-span of the airfoil  12  or at about 50% of the radial height. As indicated above in the Background section, the center-peaked temperature profile for the combustion gases  32  along with any radial migration will effect a maximum temperature on the pressure side  16  of the airfoil  12  in the range of about 70% to about 85% of the span height. Accordingly, by positioning the chord ribs  34   x  at mid-span M, the outer tier cooling circuit  36  extends upwardly to the airfoil tip  28  to provide cooling air directly to this highest heat input region for maximum cooling of the airfoil in this region. 
     The outer tier cooling circuit  36  further includes one of the span channels  40  which defines an outer inlet channel  40   b  extending radially upwardly to a first outer one  37   e  of the outer turning channels  37   a . The inner tier cooling circuit  38  similarly includes another one of the span channels  40  defining an inner inlet channel  40   a  extending radially upwardly to a first inner one  38   e  of the inner turning channels  37   b . The inner tier cooling circuit  38  also includes a third one of the span channels  40  defining an inner outlet channel  40   c  extending radially upwardly to and culminating at the outer tip wall  31 . 
     A leading edge cooling plenum  70  is formed between a forward most span rib  71  and the leading edge  20  of the outer wall  15 . A trailing edge cooling plenum  72  is formed between an aftward most span rib  75  and the trailing edge  22  of the outer wall  15 . Cooling air discharge apertures  74  in the forward most span rib  71  feeds cooling air from the outer inlet channel  40   b  to the leading edge cooling plenum  70  from where it is flowed through conventional leading edge shower head cooling holes  44 . Cooling air discharge apertures  74  in the aftward most span rib  75  feeds cooling air from an inner outlet channel  40   c  to the trailing edge cooling plenum  72  from where it is flowed through conventional trailing edge cooling holes  46 . This is used to cool the leading and trailing edges  20  and  22 , respectively. 
     The airfoil may include film cooling holes  48  along both sides of the outer wall  15  or, as in another more specific embodiment illustrated in FIG.  2 , may be constructed such that the suction side  18  has no film cooling holes and the pressure side  16  preferably has a plurality of downstream angled film cooling holes  48  in the outer wall  15  along a mid-chord portion of the airfoil between the leading and trailing edges. 
     Since the airfoil  12  in the exemplary embodiment illustrated in FIG. 1 receives the cooling air  42  from the root  14 , the inlet channels  40   b  and  40   a  of the outer and inner tier cooling circuits  36  and  38 , respectively, extend in lower part parallel to each other to separately feed the cooling air  42  to both the outer and inner tier cooling circuits  36  and  38 , respectively. 
     The airfoil squealer tip is cooled by tip cooling hole  59  in the outer tip wall  31  at locations which lead from the inner outlet channel  40   c  and the outer outlet channel  40   d  to the squealer tip cavity  33 . Thus, each of the outer and inner tier cooling circuits  36  and  38 , respectively, culminate with at least one of the tip cooling hole  59  and provide cooling air to the squealer tip cavity  33 . A refresher span channel  60  may be disposed through the root  14  to feed refresher cooling air  42   a  through at least one refresher hole  62  through a bottom most chord rib  64  between one of the inner turning channels  37   b  leading to the inner outlet channel  40   c  to feed additional fresh cooling air to the inner outlet channel and subsequently the trailing edge cooling plenum  72  for supplemental cooling of the trailing edge  22 . The outer tip wall  31  also caps the outer most portions of the cooling circuits, channels, and plenums. 
     One advantage of having each of the outer and inner tier cooling circuits  36  and  38 , respectively, culminate with the tip cooling holes  59  downstream of the forward most span rib  71  and the aftward most span rib  75  and their cooling air discharge apertures  74  is that there is always sufficient cooling air to feed the entire spanwise lengths of the leading and trailing edge cooling plenums  70  and  72 , respectively. Another advantage of this feature is that the forward most span rib  71  and the aftward most span rib  75  also referred to as the cold bridge and the warm bridge, respectively, remain cooler than in prior art designs so as to more effectively cool the outer wall  15  to which they are attached. This is a more effective cooling design than conventional cold and warm bridge designs which have cooled blades with an up pass which feeds the leading edge and trailing edge cooling air discharge apertures  74  or impingement holes and suction side film cooling holes  48  also referred to as gill holes. Very little coolant is left at the tip of these conventional feed cavities or passages. These bridges and passages also extend and cover a reasonably large surface length for which adequate cooling must be provided. 
     In the turbine blade embodiment illustrated in FIG. 1, the inlet channels  40   a  and  40   b  commence at a common plane below the inner base  26  inside the root  14  for conventionally receiving the cooling air  42 . The inner and outer outlet channels  40   c  and  40   d , respectively, terminate at a common plane at the airfoil tip  28 . In this way, the two tier cooling circuits  36 ,  38  are generally coextensive in the longitudinal direction with the outer tier  36  primarily cooling the outer portion of the airfoil  12 , with the inner tier cooling circuit  38  primarily cooling the inner portion of the airfoil  12 . Since less cooling is required below the blade mid-span, the cooling air  42  delivered to the inner tier cooling circuit  38  may be reduced in amount to only that which is needed for accommodating the lower heat input loads in that region. And, the cooling air  42  introduced into the outer tier cooling circuit  36  may be independently metered for providing a suitable amount of the cooling air  42  for accommodating the higher heat input loads in the upper region of the airfoil. In this way, the airfoil  12  may be preferentially cooled in the radial or longitudinal and axial or chordal directions of the airfoil using less total cooling air  42  where permitted without overcooling those regions as would occur in the prior art. The cooling air  42  is therefore used more efficiently and less cooling air is bled from the compressor for increasing the overall efficiency of operation of the gas turbine engine. 
     The airfoil  12  may have any other conventional features for enhancing the cooling thereof such as turbulators or pins (not shown) which are well known in the art. 
     Although the invention has been described with respect to the exemplary turbine blade  10  illustrated in the FIGS., it may also be used for turbine nozzle vanes which have similar airfoils which can benefit from preferential span-wise cooling thereof for better matching the radial applied temperature distribution from the combustion gases  32 . 
     The multi-tier serpentine cooling arrangement described above provides advantages of preferential span-wise and chordwise cooling for optimizing the usage of cooling air and to achieve a more desirable metal temperature distribution of the airfoil  12 . Although five-pass serpentine circuits are illustrated in FIG. 1, a three-pass serpentine circuit may also be used for the outer tier cooling circuit  36  depending on the design application and available cooling air pressure. The multi-tier serpentine outer and inner tier cooling circuits  36  and  38  may be readily manufactured using conventional casting techniques as are used for conventional multi-pass serpentine passages. 
     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. 
     Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims: