Abstract:
A cooled airfoil has an internal cooling passage in which a plurality of trip strips are arranged to effect variable coolant flow and heat transfer coefficient distribution so as to advantageously minimize the amount of coolant flow required to adequately cool the airfoil structure. In one embodiment, this is accomplished by varying the dimensions of the trip strips along a transversal axis relative to the cooling passage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the cooling of components exposed to hot gas atmosphere and, more particularly, pertains to internally convectively cooled airfoil structures. 
     2. Description of the Prior Art 
     It is well known to cool airfoil structures, such as gas turbine blades or vanes, exposed to a hot gas atmosphere by circulating a cooling fluid through internal cooling passages defined within the airfoil structures in order to reduce the level of thermal stresses and reduce the peak airfoil temperatures in the airfoil structures and, thus, preserve the structural integrity and the service life thereof. 
     In gas turbine applications, the airfoil structures are typically air cooled by a portion of the pressurized air emanating from a compressor of the gas turbine engine. In order to preserve the overall gas turbine engine efficiency, it is desirable to use as little of pressurized air as possible to cool the airfoil structures. Accordingly, efforts have been made to efficiently use the cooling air. For instance, GB laid-open Patent Application No. 2,112,467 filed on Dec. 3, 1981 in the names of Schwarzmann et al. discloses a coolable airfoil having a leading edge cooling passage in which a plurality of identical and uniform sized trip strips are oriented at an angle to a longitudinal axis of the cooling passage in order to increase turbulence in the leading edge region of the blade, which is typically the most thermally solicited portion of the airfoil. 
     U.S. Pat. No. 4,416,585 issued on Nov. 22, 1983 to Abdel-Messeh and U.S. Pat. No. 4,514,144 issued on Apr. 30, 1985 to Lee both disclose a cooled blade having an internal cooling passage in which pairs of uniform sized ribs are angularly disposed to form a channel therebetween for channeling the cooling fluid along a selected flow path in order to increase heat transfer coefficient while at the same time minimizing the cooling fluid pressure drop in the internal cooling passage. 
     Although the heat transfer promotion structures described in the above-mentioned references are effective, it has been found that there is a need for a new and improved heat transfer promotion structure which allows for variable coolant flow and heat transfer coefficient distribution which can be set in accordance with a non-uniform external heat load. 
     SUMMARY OF THE INVENTION 
     It is therefore an aim of the present invention to provide a new and improved heat transfer promotion structure which is adapted to efficiently use cooling fluid to convectively cool a gas turbine airfoil structure. 
     It is also an aim of the present invention to provide such a heat transfer promotion structure which allows for variable cooling flow and heat transfer coefficient distributions. 
     Therefore, in accordance with the present invention there is provided a coolable gas turbine airfoil structure having a leading edge, a leading edge internal cooling passage through which a cooling fluid is circulated to convectively cool the airfoil structure, and a heat transfer promotion structure provided within the leading edge internal cooling passage. The heat transfer promotion structure comprises a plurality of trip strips arranged to cause the cooling fluid to flow towards the leading edge in a pair of counter-rotating vortices, thereby promoting heat transfer at the leading edge. 
     In accordance with a further general aspect of the present invention, there is provided a cooled airfoil structure for a gas turbine engine, comprising first and second opposed side walls joined together at longitudinally extending leading and trailing edges, at least one longitudinally extending internal cooling passage for passing a cooling fluid therethrough to convectively cool the airfoil structure, and a heat transfer promotion structure provided within the internal cooling passage. The heat transfer promotion structure includes a plurality of trip strips arranged inside the internal cooling passage to effect a variable heat transfer coefficient distribution. Each of the trip strips has a height (h) and a width (w) defining a w/h ratio. Within the plurality of trip strips, at least one of the height (h), the width (w) and the w/h ratio is varied along a transversal axis relative to the internal cooling passage. This advantageously provides variable flow and heat transfer coefficient distribution, thereby allowing to reduce cooling flow requirements. 
     In accordance with a further general aspect of the present invention, there is provided a method of cooling a leading edge of a gas turbine engine airfoil having a leading edge internal cooling passage extending between first and second side walls, comprising the steps of: providing a heat transfer promotion structure within the leading edge internal cooling passage, directing a cooling fluid into the leading edge internal cooling passage, and causing said cooling fluid to flow towards the leading edge in a pair of counter-rotating vortices, thereby promoting heat transfer at the leading edge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof, and in which: 
     FIG. 1 is a partly broken away longitudinal sectional view of an internally convectively cooled blade in accordance with a first embodiment of the present invention; 
     FIG. 2 a  is a cross-sectional view taken along line  2   a — 2   a  of FIG. 1; 
     FIG. 2 b  is a cross-sectional view taken along line  2   b — 2   b  of FIG. 1; 
     FIG. 3 is a partly broken away longitudinal sectional view of an internally convectively cooled blade in accordance with a second embodiment of the present invention; 
     FIG. 4 is an enlarged cross-sectional view taken along line  4 — 4  of FIG. 3; and 
     FIG. 5 is a partly broken away longitudinal sectional view of an internally convectively cooled blade in accordance with a third embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to FIGS. 1,  2   a  and  2   b , there is shown an internally convectively cooled blade  10  suited for used as a turbine blade of a conventional gas turbine engine (not shown). 
     The cooled blade  10  comprises a root section  12 , a platform section  14  and a hollow airfoil section  16  over which flows hot combustion gases emanating from a combustor (not shown) forming part of the gas turbine engine. The root section  12 , the platform section  14  and the airfoil section  16  are typically integrally cast as a unitary structure. 
     According to one application of the present invention, the cooled blade  10  extends radially from a rotor (not shown) and is connected thereto via the root section  12 . The root section  12  defines a fluid passage  18  which is in fluid communication with a source of pressurized cooling fluid, typically pressurized air emanating from a compressor (not shown) of the gas turbine engine. 
     The hollow airfoil section  16  includes a pressure side wall  20  and a suction side wall  22  joined together at longitudinally extending leading and trailing edges  24  and  26 . The airfoil section  16  further includes a tip wall  28  at a distal end thereof. As seen in FIGS. 1,  2   a  and  2   b , the airfoil section  16  defines an internal cooling passageway  29  arranged in a serpentine fashion and through which the cooling air is passed to convectively cool the blade  10 , as depicted by arrows  27  in FIG.  1 . 
     The cooling passageway  29  includes a leading edge cooling passage  30  extending in the spanwise or longitudinal direction of the blade  10  adjacent the leading edge wall  24  thereof. The leading edge cooling passage  30  is in flow communication with passage  18  and extends to the tip wall  28  of the blade  10  where the coolant air is deviated 180° degrees into a central cooling passage  32 , as seen in FIG.  1 . The cooling air then flows longitudinally into the central cooling passage  32  towards the root section  12  of the blade  10  before being deviated 180° degrees longitudinally into a trailing edge cooling passage  34  which extends to the tip wall  28  and in which a plurality of spaced-apart pedestals  36  are provided between the pressure and suction side walls  20  and  22  of the cooled blade  10 . The cooling air is typically discharged from the trailing edge cooling passage  34  via a plurality of exhaust ports  38  defined at selected locations through the trailing edge  26 , as seen in FIGS. 2 a  and  2   b.    
     The leading edge cooling passage  30  is delimited by the pressure and suction side walls  20  and  22 , the leading edge wall  24  and a partition wall  40  extending in the longitudinal direction of the blade  10  between the pressure and suction side walls  20  and  22 . As seen in FIG. 1, the partition wall  40  forms a gap with the tip wall  28  for allowing the cooling air to flow from the leading edge cooling passage  30  into the central or midchord cooling passage  32 . Similarly, a second partition wall  42  (see FIGS. 2 a  and  2   b ) extends longitudinally from the tip wall  28  of the cooled blade  10  towards the root section  12  between the pressure and suction side walls  20  and  22  for separating the central cooling passage  32  from the trailing edge cooling passage  34  and, thus, cause the cooling air to flow in a serpentine fashion towards the exhaust ports  38  defined through the trailing edge  26  of the cooled blade  10 . 
     The external heat load is usually more important at the leading edge  24  and, more particularly, at a stagnation point P located thereon. Furthermore, the external surface of the leading edge region of the airfoil section  16  which is exposed to the hot gas is large compared to that exposed to the cooling air. Therefore, it is desirable to promote heat transfer to the cooling air in the leading edge region of the blade  10  in order to keep the cooling flow requirements to a minimum. 
     It has been found that by causing the cooling air to flow towards the leading edge  24  in a pair of counter-rotating vortices V 1  and V 2  (see FIG.  4 ), an efficient cooling of this region of the blade  10  can be achieved. 
     According to one embodiment of the present invention, this is accomplished by providing a heat transfer promotion structure comprising a plurality of trip-strips or ribs having variable dimensions in a lengthwise direction thereof, the dimensions of the trip strips being set to produce the desired flow pattern and augmentation in local heat transfer coefficient in accordance with the non-uniform external heat load exerted on the blade  10 . 
     More specifically, as seen in FIGS. 1,  2   a  and  2   b , a first array of parallel trip strips or ribs  44 s of variable dimensions extend from an inner surface of the suction side wall  22  at angle θ with respect to a longitudinal axis of the leading edge cooling passage  30  or to the direction of the cooling flow. The value of θ may be comprised in a range of about 20° degrees to about 60° degrees. However, the preferred range of angle θ is between 40° degrees to 50° degrees. As seen in FIGS. 2 a  and  2   b , a second array of parallel trip strips or ribs  44   p  of variable dimensions extend from an inner surface of the pressure side wall  20 . The trip strips  44   p  are parallel and staggered with respect to the trip strips  44   s  such that the trip strips  44   p  and  44   s  extend alternately in succession across the leading edge cooling passage  30 . 
     The trip strips  44   p  and  44   s  may or may not extend to the partition wall  40  and are spaced from the leading edge wall  24 . 
     The leading edge cooling passage  30  has a generally triangular cross-section and has a height (H) at any point along a line which is perpendicular to a meanline of the leading edge cooling passage  30 , as seen in FIG. 2 a . The trip strips  44   p  and  44   s  have a height (h) (see FIG. 2 a ) and a width (w)(see FIG. 1) defining a w/h ratio. The preferred value of the ratio w/h is comprised in a range of 0.05 to 20 inclusively. The preferred value of the strip-to-passage height ratio h/H is comprised in a range of 0.05 to 1.0 inclusively. 
     The dimensions of each trip strips  44   s  and  44   p  generally gradually decrease from a first end  46  to a second end  48  thereof, the second end being disposed upstream of the first end  46  and closer to the leading edge  24 . The width (w), the height (h) and/or the w/h ratio may be varied along the length of each trip strips  44   s  and  44   p  to induce the desired flow pattern which will promote heat transfer in the leading edge region of, the blade  10 . 
     The trip strips  44   p  and  44   s  are typically integrally cast with the associated side wall  20  and  22 . 
     Conventional trip strips  48   p  and  48   s  of uniform sizes can be provided in the central cooling passage  32  to promote heat transfer therein. The orientation of trip strips  44   p ,  44   s ,  48   p  and  48   s  can generally be the same. It is understood that the swirling movement of the air may be carried over from one passage to the next. However, this is not necessarily the case, as it may be eradicated by a 180° turn and then re-started by the next set of trip strips. 
     According to a second embodiment of the present invention which is illustrated in FIGS. 3 and 4, the cooling air may be caused to flow in a pair counter-rotating vortices V 1  and V 2  within a triangular or trapezoidal passage by providing a plurality of trip strips  144   s  and  144   p  of uniform but different dimensions within the passage. For simplicity and brevity, components which are identical in function and identical or similar in structure to corresponding components of the first embodiment are given the same reference numerals in the hundreds, and a description of these components is not repeated. 
     More specifically, as seen in FIG. 3, a first array of parallel trip strips  144   s  extend from the suction side wall  122  and the partition wall  140  in a crosswise direction with respect to the flow direction and the longitudinal axis of the leading edge cooling passage  130 . However, it is understood that the trip strips  144   s  do not necessarily have to extend to the partition wall  140 . Each trip strips  144   s  is of uniform dimensions. The trip strips  144   s  are uniformly distributed along the longitudinal axis of the leading edge cooling passage  130 . A second array of parallel trip strips  145   s , which are spaced from the distal end of the first trip strips  144   s , extend from the suction side wall  122 . The trip strips  145   s  are disposed closer to the leading edge  124  than the first array of trip strips  144   s  . Each trip strips  145   s  is of uniform dimensions. The second trip strips  145   s  are generally smaller than the first trip strips  144   s . The height (h) and the width (w) of the trip strips  145   s  are less than the height (h) and the width (w) of the trip strips  144   s . The dimensions of the trip strips  144   s  and  145   s  are set to provide the desired variable heat transfer coefficient distribution across the leading edge cooling passage  130 . 
     As seen in FIG. 3, the second trip strips  145   s  are uniformly longitudinally distributed within the leading edge cooling passage  130 . The spacing between adjacent trip strips  145   s  is less than the spacing between adjacent trip strips  144   s.    
     As seen in FIG. 4, third and fourth corresponding arrays of trip strips  144   p  and  145   p  of uniform but different dimensions extend from the pressure side wall  120  inwardly into the leading edge cooling passage  130 . The third and fourth arrays of trip strips  144   p  and  145   p  are respectively longitudinally staggered with respect to corresponding first and second arrays of trip strips  144   s  and  145   s.    
     In the leading edge cooling passage  130 , the provision of the trip strips  144   s ,  144   p ,  145   s  and  145   p  causes the cooling air to flow in a pair of counter-rotating vortices V 1  and V 2 . The first vortex V 1  defines a vortex line extending from the leading edge area generally in parallel with an inner surface of the pressure side wall  120  and then back towards the leading edge area. Likewise, the second vortex V 2  defines a vortex line which extends from the leading edge area generally in parallel to an inner surface of the suction side wall  122  and then back towards the leading edge area. 
     In addition to the benefits of the first embodiment, the second embodiment has the advantages of being easier to manufacture and to allow for different spacing for different sized trip strips. 
     FIG. 5 illustrates a third embodiment of the present invention, wherein for simplicity and brevity, components which are identical in function and identical or similar in structure to corresponding components of the first embodiment are given the same reference numerals raised by the two hundred, and a description of these components is not repeated. According to the embodiment illustrated in FIG. 5, a first array of trip strips  244  of variable dimensions and a second array of uniformed sized trip strips  245  extend from the pressure side wall  220  as well as from the opposed suction side wall (not shown) of the cooled blade  200 . It is understood that any permutation of the first two embodiments of the present invention may be used in a same passage to produce the desired results. 
     It is understood that the present invention could apply to a variety of cooling schemes, including leading edge cooling passages that only extend half way up the leading edge. Also, the leading edge passage may end in a 90° turn, instead of a 180° turn, as described hereinbefore. It is also understood that the remainder of the cooling scheme, i.e. past the leading cooling passage, is immaterial to the functioning of the present invention. Finally, it is understood that the present invention is not restricted to large trip strips near the root of the airfoil and smaller ones near the tip thereof.