Patent Publication Number: US-9903209-B2

Title: Rotor blade and guide vane airfoil for a gas turbine engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 13198810.7 filed Dec. 20, 2013, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to the field of heat transfer characteristics of a flowing passage with pins and ribs and improving heat transfer coefficient. 
     A rotor blade or guide vane airfoil for a gas turbine engine having a longitudinal axis and a source of cooling fluid, the airfoil having a pressure wall, a suction wall, a leading edge, a trailing edge and at least one cooling fluid flow passage, whereas the cooling fluid flow passage in fluid communication with the source of cooling fluid and providing means for directing cooling fluid at least to the trailing edge, whereas the cooling fluid flow passage including: a plurality of axially extending walls, each of the walls extending laterally between the pressure wall and suction wall, whereas the plurality of walls being radially spaced within the cooling fluid flow passage such that adjacent pairs of walls define a channel, whereas the axial spacing between the adjacent walls comprising in radial direction of the airfoil a pins and a ribs structure. 
     BACKGROUND 
     The gas turbine community continually seeks to increase the thermal efficiency and power output by increasing the turbine inlet temperature to beyond the melting temperature of turbine airfoil vanes and blades. Effective cooling schemes are required to protect the gas turbine components from failure. Many cooling techniques for example film cooling, pin fins cooling and rib-turbulated cooling are employed to protect the airfoils, preventing the airfoils from failure while extending durability. 
     According to EP 1 508 746 A1 a heat exchange wall includes a base plate, a plurality of first protrusions distributed on a surface of the base plate, and a plurality of second protrusions distributed on the base plate surface. The height of the second protrusion in a normal direction of the base plate is desirably less than ½ of a height of the first protrusion in the normal direction. The height of the second protrusion in the normal direction is desirably between 1/20 and ¼ of the height of the first protrusion in the normal direction. More desirably, the height of the second protrusion in the normal direction is 1/10 of the height of the first protrusion in the normal direction. 
     According to the document ASME 2001-GT-0178 pin fins are normally used for cooling the trailing edge region of a turbine, where their aspect ratio (height H/diameter D) is characteristically low. In small turbine vanes and blades, however, pin fins may also be located in the middle region of the airfoil. In this case, the aspect ratio can be quite large, usually obtaining values greater than 4. Heat transfer tests, which are conducted under atmospheric conditions for the cooling design of turbine vanes and blades, may overestimate the heat transfer coefficient of the pin-finned flow channel for such long pin fins. The fin efficiency of a long pin fin is almost unity in a low heat transfer situation as it would be encountered under atmospheric conditions, but can be considerably lower under high heat transfer conditions and for pin fins made of low conductively material. 
     Referring to ASME GT 2011-46078 a pin-fin array is usually rows of short circular cylindrical elements generally arranged in staggered configurations in a narrow channel with cooling fluid passing over the array. This appears to be an effective heat transfer enhancement method, but is accompanied with a pressure loss. Pin fins are usually attached perpendicularly to both end-walls inside the narrow cooling channel, for example of a gas turbine airfoil. According to this document,  FIG. 2  shows schematically a pin-rib geometry viewed from the top and the side of the channel. Various Figures show the top view of the top end-wall mounted with pin-fins. A further Figure illustrates the side view of the staggered pin-fins configuration in the test section. The top and bottom end-wall are identical and the bottom end-wall is arranged by shifting on pitch downstream of the top end-wall. 
     Generally, referring to the pressure loss coefficient it is noted that the heat transfer enhancement is usually accompanied by penalty of additional pressure loss. Any element protruding from the end-wall, i.e. pin fins and ribs, will obstruct the flow causing drag and head loss in the system. 
     SUMMARY 
     Accordingly, an object of the invention as defined in the claims is to provide improvements over state of the art in connection with an implementation of pins with ribs in a channel to cool turbine vanes and blades aft part. 
     An advantageous embodiment provides a converging channel as needed in aft part of turbine vanes or rotor blades. Furthermore, depending of the operational use, the sectional bodies of the cooling channel can have be shaped with a continually increasing or decreasing cone angle in the direction of flow along the channel. It can be envisaged that the bodies shaping the structure of the flow channel each have a cylindrical initial part. 
     Pins are connected with ribs for a better castability, and the pin diameter is adapted to channel height. Ribs enhance heat transfer coefficient in the required area, where the pin height is larger and the coolant velocity smaller. 
     Pin span-wise pitch is decreased, where the channel height and the pin diameter gets smaller get required heat transfer coefficient, but the staggered arrangement is kept. In order to keep a regular pattern the span-wise pitch of the larger pins should be equal or multiple of the pitch of the smaller pins downstream. 
     Rib height (h) is adapted to pin height, wherein rib height (h) is adapted to certain fraction of pin height. Rib width (w) at the bottom is adapted to castability requirement, wherein the width should be larger than 60% of the height. 
     When the height of the rib is low, turbulence generated by the top portion of the rib reaches the base plate surface to promote heat exchange. This embodiment is effective in case the pin has a low thermal conductivity. The reason of this result is because the base plate of the channel can be cooled more efficiently by cooling the surface of the base plate directly rather than cooling the side face of the pin of the low thermal conductivity. When the diameter of round pin is small, the projection area in the direction of the cooling air flow decreases so that the pressure loss can be suppressed. 
     The height of the ribs is limited relative to the height of the pins, wherein the pins extend over the whole opening of the channel. The top and bottom end-wall comprise individually a ribs structure in connection with each adjacent pins. 
     The ribs have a square or rectangular or trapezoidal cross-section, adapting to castability requirement; moreover, the leading face is provided along the entire length of the rib between two adjacent spin with an inclined or tapered surface in the flow direction of the cooling medium. Accordingly, in this case flowing of the inclined or tapered surface corresponds to one side aligned vortex generator. 
     Additionally, the flowed surface of the ribs in the direction of flow corresponds to a vortex generator comprises a tapered surface along the entire length of the rib between two adjacent spins. 
     Moreover, the flowed surface of the ribs in the direction of flow corresponds to a vortex generator essentially comprises three triangular surfaces around which flow occurs. Accordingly, the length of the ribs between two adjacent ribs may be formed by a number of such generators. These are a top surface and two side surfaces. In their longitudinal extent, these surfaces run at certain angles in the direction of flow. The side walls of the vortex generators, which preferably consist of right-angled triangles, are fixed, preferably gastight, with their longitudinal sides to the channel wall already above discussed. They are orientated in such a way that they form a face at their narrow sides while enclosing an acute or arrow angle. The face is embodied as a sharp connecting edge and is perpendicular to every channel wall with which the side surfaces are flush. The two side surfaces enclosing the arrow angle are symmetrical in form, size and orientation and they are arranged on both sides of a symmetry axis which is equi-directional to the duct axis. 
     The mode of operation of the vortex generator is as follows: when flow occurs around the edges, the main flow is converted into a pair of oppositely directed vortices. The vortex axes lie in the axis of the main flow. The swirl number and the location of the vortex breakdown, provided the latter is intended, are determined by corresponding selection of the setting angle and the arrow angle. The vortex intensity and the swirl number increase as the angles increase, and the location of the swirl breakdown is displaced upstream right into the region of the vortex generator itself. Depending on the use, these cited two angles being predetermined by design conditions and by the process itself. These vortex generators need only be adapted in respect of length and height. 
     The vortex to be produced along the alveolar structure of the ribs in flow-direction of the cooling medium is ultimately decisive for the selection of the number and the arrangement of the adapted ribs having the form of a vortex generator. 
     Where the cooling channel is sufficiently narrow, ribs are not required anymore. The higher flow velocity provides enough heat transfer coefficient. 
     The pins are radially spaced with respect to the flow direction of cooling fluid and extend laterally between the flowed walls. Each of the pins is disposed downstream of a radially aligned with one of the channels of an airfoil. In this way, each of the pins provides an obstruction in the flow exiting each of the sub-channels. Each of the pins is circular in cross section and equal in radial dimension. It should be apparent that a mixture of pins of various shapes and sizes may be used. 
     Cooling fluid exiting the channels impinges upon one of the pins disposed along the cooled channel. The cooling process results in the one hand in heat being transferred between the pin and the cooling fluid and also results in vortices being generated in the flow flowing past the pins. The vortices generated result in additional heat transfer from the channel surfaces to the cooling fluid. The cooling fluid flowing around the pins then impinges upon the flowed surface of the ribs in the direction of flow. In the other hand, this impingement again results in heat transfer and in the generation of flow vortices with respect to the channel surfaces between the alveolar structures of the ribs. 
     The spacing between the alveolar structures of the ribs defines an interruption in each of the cooled channels. The interruptions permit cross flow between channels. The cross flow ensures that, in the event that one of the first plurality of cooled channels becomes blocked, cooling fluid will continue to be distributed over the adjacent extent of the channel space. The cross flow through the interruption provides a means to backfill each of the second plurality of sub-channels which is downstream of a blocked first sub-channel of the airfoil. In addition, each of the pins provides an obstruction within the channel which encourages cross flow between channels and facilitates distribution of cooling flow to the whole extension of the channel. 
     By diffusing the cooling fluid in connection with a channel of a trailing edge, the velocity of the exiting cooling fluid is lowered to reduce the likelihood of separation of the cooling fluid from the trailing edge. 
     The main advantage of the invention consists in the fact that the cooling structure improved in an essentially measure the heat transfer and reduced consistently cooling air consumption, which leads to a better performance of the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention shall subsequently be explained in more detail based on exemplary embodiments in conjunction with the drawings. In the drawings: 
         FIG. 1  presents a cross sectional view showing a known to all rotor blade of a gas turbine with a heat exchange wall of the present invention; 
         FIG. 2  shows a cross section of the cooling channel comprising pins and ribs; 
         FIG. 3  shows a plan view of the pins and ribs structure along the cooling channel; 
         FIG. 4  shows a section of a trapezoidal rib; 
         FIG. 5  shows a section of a rib with an inclined surface 
         FIG. 6  shows a three-dimensional view of a further rib as vortex generator. 
     
    
    
     DETAILED DESCRIPTION 
     In connection with  FIG. 1 , a cooling channel  100   a  is provided in a rotor blade or guide vane (in the following, for simplicity, is spoken by a rotor blade) of the gas turbine to send a cooling medium  130  therein. The inner wall of the flow cooling path  100  is covered with the heat exchange walls  110   a  and  111   a  in which the pins (see  FIG. 2 ) are provided towards the inner side of the cooling channel  100   a . The structure of the heat exchange walls  110   a  and  111   a  can be the same as the structure of any other cooling path  101   a ,  102   a.    
     When the gas turbine is operated, a high temperature gas  120  is blown towards the rotor blade, and the rotor blade is rotated around a rotation shaft (not shown). The cooling medium  130  is supplied from the base portion of the rotor blade into the cooling channel  100   a . The cooling medium  130  takes away the heat from the rotor blade and is discharged to a path  131  through which the high temperature gas  120  flows. The heat exchange walls  110   a ,  111   a  are provided on the inner wall of the cooling channel  100   a  to efficiently transfer the heat of the rotor blade to the cooling medium  130 . 
     Since the rotor blade is efficiently cooled by the heat exchange along the channels  100   a ,  101   a ,  102   a , it is preferably used in the gas turbine in which the higher temperature gas  120  is used. Or, the flow rate of the cooling medium  130  is little as compared with the gas turbine to which the temperature of the combustion gas  120  is equal. 
       FIG. 2  shows a cross section of the cooling channel  100  in the region of the trailing edge of the rotor vane or guide vane comprising pins  200  and ribs  300 . Rib height h is adapted to pin height, wherein rib height is adapted to certain fraction of pin height. Rib width w at the bottom  201  (see  FIG. 4 ) is adapted to castability requirement, wherein the width should be larger than 60% of the height. 
     When the height of the rib is low, turbulence generated by the top portion of the rib reaches the base wall  110  and bottom wall  111  plate surfaces to promote heat exchange. The wall  110  and  111  correspond to the pressure side and suction side of the rotor blade or guide vane. This embodiment is effective in case the pin has a low thermal conductivity. The reason of this result is because the base plate of the channel can be cooled more efficiently by cooling the surface of the base plate directly rather than cooling the side face of the pin of the low thermal conductivity. When the diameter of round rib is small, the projection area in the direction of the cooling air flow decreases so that the pressure loss can be suppressed. 
     The pins  200  are radially or quasi-radially spaced along the channel  100  with respect to the flow direction of cooling medium  130  and extend laterally between the flowed surfaces  110 ,  111 . Each of the pins  200  is transversely disposed to the flow direction of the cooling fluid along the trailing edge of the rotor or guide vane. In this way, each of the pins  200  provides an obstruction in the flow exiting of the flowed channel  100 . Each of the pins  200  is circular in cross section and equal in radial dimension. It should be apparent that a mixture of pins of various shapes and sizes may be used. 
       FIG. 3  shows a plan view of the pins  200  and ribs  300  structure along the cooling channel  100  resp.  100   a  (see  FIG. 1 ). The rib  300  is disposed along the cooling channel  100  between the pins configuration forming an alveolar or quasi-alveolar structure. This structure of the ribs defines an interruption in each of the cooled channels  100 . The interruptions permit cross-flow within cooling channel  100 . The cross-flow ensures that, in the event that one of the first portions of cooled channels becomes blocked, cooling fluid will continue to be distributed over the adjacent extent of the channel space. The cross-flow through the interruption provides a means to backfill each of the second plurality of sub-channels (see  FIG. 1 ) which is downstream of a blocked first sub-channel of the airfoil. In addition, each of the pins  200  provides an obstruction within the channel which encourages cross flow between channels and facilitates distribution of cooling flow to the whole extension of the channel. Where the cooling channel  100  is sufficiently narrow, ribs  300  are not required anymore. The higher flow velocity provides enough heat transfer coefficient. 
       FIG. 4  shows a section of a trapezoidal rib  300   a  enclosing the width w and height h configuration. 
       FIG. 5  shows a section of a rib between two pins with an inclined surface  300   b.    
     According to  FIG. 6 , the flow of the cooling medium  130  is shown by an arrow (see  FIG. 2 ), whereby the direction of flow is also predetermined. According to  FIG. 6 , a vortex generator  300   c  essentially comprises three triangular surfaces around which flow occurs. These are a top surface  310  and two side surfaces  311  and  313 . In their longitudinal extent, these surfaces run at certain angles in the direction of flow. The side walls of the vortex generators  300   c , which preferably consist of right-angled triangles, are fixed, preferably gastight, with their longitudinal sides to the channel or duct wall  110 . They are orientated in such a way that they form a face at their narrow sides while enclosing an acute or arrow angle α. The face is embodied as a sharp connecting edge  316  and is perpendicular to every duct wall  110  with which the side surfaces are flush. The two side surfaces  311 ,  313  enclosing the arrow angle αare symmetrical in form, size and orientation and they are arranged on both sides of a symmetry axis  317  which is equi-directional to the duct axis. 
     With a very narrow edge  315  running transversely to the duct through which flow occurs, the top surface  310  bears against the same duct wall  110  as the side surfaces  311 ,  313 . Its longitudinally directed edges  312 ,  314  are flush with the longitudinally directed edges of the side surfaces  311 ,  313  projecting into the flow duct. The top surface  310  runs at a setting angle γ to the duct wall  110 , the longitudinal edges  312 ,  314  of which form a point  318  together with the connecting edge  316 . The vortex generator  300   c  can of course also be provided with a base surface with which it is fastened to the duct wall  110  in a suitable manner. However, such a base surface is in no way connected with the mode of operation of the element. 
     The mode of operation of the vortex generator  300   c  is as follows: when flow occurs around the edges  312  and  314 , the main flow is converted into a pair of oppositely directed vortices, as shown schematically in the figures. The vortex axes lie in the axis of the main flow. The swirl number and the location of the vortex breakdown, provided the latter is intended, are determined by corresponding selection of the setting angle γ and the arrow angle α. The vortex intensity and the swirl number increase as the angles increase, and the location of the swirl breakdown is displaced upstream right into the region of the vortex generator  300   c  itself. Depending on the operational use, these two angles α and γ are predetermined by design conditions and by the process itself. This vortex generator need only be adapted in respect of length, width and height. 
     In  FIG. 6 , the connecting edge  316  of the two side surfaces  311 ,  313  forms the downstream edge of the vortex generator  300   c . The edge  315  of the top surface  310  running transversely to the duct through which flow occurs is therefore the edge acted upon first by the duct flow. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment(s), but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted under the law. Furthermore it should be understood that while the use of the word preferable, preferably, preferred or advantageously in the description above indicates that feature so described may be more desirable, it nonetheless may not be necessary and any embodiment lacking the same may be contemplated as within the scope of the invention, that scope being defined by the claims that follow. In reading the claims it is intended that when words such as “a,” “an,” “at least one” and “at least a portion” are used, there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. Further, when the language “at least a portion” and/or “a portion” is used the item may include a portion and/or the entire item unless specifically stated to the contrary.