Abstract:
A mid-turbine frame module comprises an outer structural ring, an inner structural ring and a plurality of circumferentially spaced-apart spokes structurally interconnecting the inner structural ring to the outer structural ring. At least one of the tubular spokes accommodates a service line. The remaining spokes with no service line have an internal architecture which mimics an air cooling scheme of the at least one spoke housing a service line in order to provide temperature uniformity across all spokes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority on US Provisional Patent application No. 62/196,380 filed on July 24, 2015, U.S. Provisional Patent Application No. 62/196,500 filed on Jul. 24, 2015 and US Provisional Patent Application No 62/196,368 filed on Jul. 24, 2015, the entire content of the above applications is herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The application relates generally to gas turbine engines and, more particularly, to a cooling arrangement for cooling the structural spokes of a mid-turbine frame module. 
       BACKGROUND OF THE ART 
       [0003]    It is known to use structural spokes to transfer loads from a bearing casing to an outer structural ring of a gas turbine engine. For instance, such spokes may be found in mid-turbine frame modules. Each spoke typically extends radially from the outer ring through a strut in the gaspath to an inner ring supporting the bearing casing. During engine operation, the spokes all around the module must be maintained at substantially the same temperature in order to prevent the bearing from becoming off-centered as a result of differential thermal growth between the spokes. 
       SUMMARY 
       [0004]    In one aspect, there is provided a mid-turbine frame module comprising an outer structural ring, an inner structural ring supporting a bearing, a plurality of circumferentially spaced-apart tubular spokes structurally interconnecting the inner structural ring to the outer structural ring, at least one of said tubular spokes accommodating a bearing service line, the remaining tubular spokes with no bearing service line having an internal architecture which mimics an air cooling scheme of the at least one spoke to provide temperature uniformity across all the spokes. 
         [0005]    In accordance with another aspect, there is provided a tubular insert inside the tubular spokes, which house no service line, an annular gap being defined between said spokes and the insert. 
         [0006]    In accordance with another aspect, flow calibration holes are provided to calibrate the cooling air through the annular gap. 
         [0007]    In accordance with a further aspect, there is provided a mid-turbine frame for a gas turbine engine, the mid-turbine frame comprising: an outer structural ring, an inner structural ring, an annular gas path between the inner and outer structural ring, a plurality of circumferentially spaced-apart hollow struts extending radially across the gas path, a plurality of circumferentially spaced-apart tubular spokes respectively extending internally through the hollow struts, the tubular spokes structurally connected to the inner structural ring and to the outer structural ring, at least one of the tubular spokes housing a service line, a remainder of the tubular spokes having a sleeve extending therethrough, an internal coolant flow passage defined centrally through the sleeve and an annular coolant flow passage defined between the sleeve and the tubular spoke, the internal coolant flow passage and the annular coolant flow passage connected in serial flow communication at respective adjacent ends thereof and with a source of coolant liquid to provide a coolant reverse flow path from a radially inward direction to a radially outward direction. 
         [0008]    In accordance with another further aspect, there is provided a spoke cooling arrangement for a gas turbine engine mid-turbine frame module comprising a plurality of circumferentially spaced-apart tubular spokes structurally interconnecting an inner structural ring to an outer structural ring, at least one of the tubular spokes housing a service line, the spoke cooling arrangement comprising: a main coolant flow passage extending through each of the spokes having no service line, and a reverse flow passage serially interconnected to the main coolant flow passage for recirculating at least a portion of the coolant back into the associated spoke in a direction opposite to that of the main coolant flow passage. 
         [0009]    In accordance with a still further general aspect, there is provided a method of cooling structural spokes of a gas turbine engine mid-turbine frame module, wherein at least one of the structural spokes houses a service line; for each of the structural spokes housing no service line, the method comprising: directing a coolant flow radially inwardly through a main flow passage defined axially through the structural spokes, and redirecting at least a portion of the coolant flow received from the main flow passage radially outwardly into a reverse flow passage extending axially through each of the structural spokes with no service line. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    Reference is now made to the accompanying figures in which: 
           [0011]      FIG. 1  is a schematic cross-section view of a gas turbine engine; 
           [0012]      FIG. 2  is an isometric view of a mid-turbine frame module mounted in an engine outer case; 
           [0013]      FIG. 3  is an isometric view of the mid-turbine frame shown without the engine outer case; 
           [0014]      FIG. 4  is an enlarged view of a portion of the mid-turbine frame illustrating an air intake arrangement for uniformly distributing cooling air all around the module and avoid the formation of a local cold spot in the module; 
           [0015]      FIG. 5  is a cross-section view of the air intake arrangement shown in  FIG. 4 ; 
           [0016]      FIG. 6 a    is a cross-section view of a portion of the mid-turbine frame module illustrating a cooling flow scheme through one of the spokes; 
           [0017]      FIG. 6 b    is an enlarged view of a radially inner end portion of the spoke cooling flow scheme shown in  FIG. 6   a;    
           [0018]      FIG. 6 c    is an enlarged view of a radially outer end portion of the spoke cooling flow scheme shown in  FIG. 6   a;    
           [0019]      FIG. 7 a    is an end view of the mid-turbine frame module illustrating a first cooling circuit for structurally dedicated spokes, which do not accommodate any service lines, and a second cooling circuit for the top and bottom spokes, which integrate bearing housing service lines, the two circuits being separated to avoid air contamination; 
           [0020]      FIG. 7 b    is an enlarged cross-section view of a radially inner outlet end portion of the first cooling circuit; and 
           [0021]      FIG. 7 c    is an enlarged cross-section view of a radially outer outlet end portion of the second cooling circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
         [0023]      FIGS. 2 and 3  show a portion of the turbine section  18 . More particularly,  FIG. 2  illustrates a mid-turbine frame module  20  housed within an engine outer case  21 . As shown in  FIG. 3 , the mid-turbine frame module  20  comprises an inner structural ring  22  adapted to receive and support a bearing casing  23 , which is, in turn, adapted to support the main shafts of the engine  10 . The bearing casing  23  may be detachably mounted to the inner ring  22  by means of bolts or the like. 
         [0024]    The inner bearing support ring  22  is structurally supported by an outer structural ring  24  by means of a plurality of circumferentially distributed tubular spokes  26  ( 6  in the illustrated embodiment). In addition of transferring the loads from the inner ring  22  to the outer ring  24 , the spokes  26  centralize the inner ring  22  and, thus, the bearing casing  23  relative to the outer ring  24 . The term “tubular spoke” is herein intended to generally refer to a hollow spoke structure and is not limited to any specific cross-sectional shape. 
         [0025]    Each spoke  26  may extend radially through a hollow strut  29   a,  b ( FIG. 6 a   ) of a non-structural integrated strut-vane (ISV) casing  28  “floatingly” mounted between the inner and outer structural rings  22  and  24  for guiding the combustion gases between two axially adjacent turbine stages. The ISV casing  28  has a radially outer and a radially inner gaspath walls  28   a ,  28   b  ( FIGS. 5 and 6   a ) defining therebetween a portion of the gaspath of the turbine section  18 . According to the illustrated embodiment, the ISV casing  28  does not play a structural role. That is loads from the bearing casing  23  are not transmitted to the outer casing  24  via the ISV casing  28 . The loads are rather transmitted through the spokes  26 , which are shielded from the hot combustion gases by the hollow struts  29  of the ISV casing  28 . In such an arrangement, the spokes can be referred to as cold spokes. 
         [0026]    During engine operation, all the spokes  26  need to be kept at substantially the same temperature in order to prevent the bearing casing  23  from becoming off-centered. Indeed, if the spokes  26  have different thermal growths, the concentricity of the inner ring  22  relative to the outer ring  24  may be lost and consequently the bearing centralization compromised. Accordingly, there is a need for a way to uniformly distribute coolant to the spokes  26  all around the module  20  so that the temperature of all the spokes  26  is substantially the same. Moreover, when introducing coolant (e.g. compressor bleed air) in module  20 , the coolant should be directed such as to avoid creating local cold spots on the outer ring  24 , which could also affect the bearing centralization. 
         [0027]    According to one embodiment, a single external pipe (not shown) may be used to direct coolant, such as bleed air from the compressor of the engine  10 , to the mid-turbine frame module  20 . As shown in  FIG. 2 , a port  30  is provided on the engine outer case  21  for receiving cooling air from the external pipe. Cooling air from the engine outer case intake port  30  is then directed into an intake duct  32  mounted to the outer structural ring  24 . According to the embodiment illustrated in  FIG. 4 , the intake duct  32  may be provided in the form of a generally T-shaped duct having an inlet branch  32   a  extending radially through a hole  34  defined in the outer ring  24  and a pair of outlet branches  32   b  extending laterally from opposed sides of the inlet branch  32   a  on a radially inner side of the outer ring  24 . The outlet branches  32   b  generally extend in circumferentially opposite directions and have respective outlet ends connected to outlet ports  36  provided on the outer ring  24  on opposed sides of the hole  34 . The intake duct  32  may be made in sheet metal, casting or any other suitable materials. 
         [0028]    As shown in  FIG. 5 , the outlet branches  32   b  of the air intake duct discharge the cooling air in circumferentially opposed directions into an annular cavity  40  defined between the engine outer case  21  and the outer ring  24 . The annular cavity  40  forms an air plenum all around the module. As shown in  FIG. 3 , the air plenum is in flow communication with the spokes and the hollow struts in which the spokes  26  are positioned. By building an air pressure in the annular air plenum, cooling air may be uniformly distributed to the spokes  26  all around the cavity  40 . It provides for an internal core passage architecture that distributes the cooling air in a circumferential manner to avoid unequal metal temperature in the mid-turbine frame module outer ring structure. Also, it can be appreciated that the air intake duct  32  prevents the incoming cooling air to be locally discharged directly against the outer ring  24 , thereby avoiding the creation of a local cold spot thereon adjacent one of the spokes  26 . The air intake duct  32  rather splits the incoming flow of cooling air and redirects it with a radially outward and a circumferential component into the annular cavity  40  between the outer ring  24  and the engine outer case  21 . The air impacts upon the engine outer case  21  and, thus, not on the outer ring  24 , which is used to centralize the inner bearing casing  23  with the spokes  26 . This contributes preserving the bearing centralization. 
         [0029]    Also the above embodiment eliminates the use of multiple air cooling feed pipes, which may have a non-negligible impact on the overall weight of the engine. It also allows the introduction of cooling air in a restricted area. The air duct internal intake can also be easily replaced. 
         [0030]    According to an embodiment, six spokes are used to support and centralize the bearing casing  23 . Two of the spokes  26  (one at the bottom and one at the top of the module) are also used to accommodate bearing housing service lines  50 , such as oil tubes.  FIG. 6 a    illustrates an example of a first hollow airfoil strut  29   a  containing a combined structural spoke  26   a  and bearing housing service line  50  and a second hollow airfoil strut  29   b  containing a structurally dedicated spoke  26   b  (spoke with no oil service lines). The two structural spokes  26   a  with their internal bearing service lines  50  and the four structurally dedicated spokes  26   b  must be kept at substantially the same temperature to ensure rotor centralization. This may be achieved by providing in each of the 4 structurally dedicated spokes with an internal architecture that mimics the air circulation through the 2 spokes accommodating the bearing service lines  50 . 
         [0031]    Referring concurrently to  FIGS. 6 a  to 6 c   , it can be appreciated that a sleeve or tubular insert  52  may be provided in each of the  4  structurally dedicated spokes  26   b  to form an internal annular gap or annular reverse flow passage  54 , which generally corresponds to the one between the combined spoke  26   a  and bearing housing service line  50  and associated surrounding strut  29   a.  Referring concurrently to  FIGS. 6 a  to 6 c  and 7 a   , it can be appreciated that a first cooling circuit is formed between the annular cavity  40  and the  4  structurally dedicated spokes  26   b.  The cooling air flows from the annular cavity  40  radially inwardly through the internal main coolant flow passage defined by the tubular insert  52  mounted inside each of the structurally dedicated spokes  26   b.  As shown in  FIG. 7 b   , the air discharged from the insert  52  of each spoke  26   b  is received in a chamber  80  defined between the inner ring  22  and the radially inner end of each spoke  26   b.  A first portion of this air is discharged through holes  82  in the inner ring  22  and then directed to purge the upstream disc cavity  93  of an adjacent turbine rotor  95 . As best shown in  FIG. 6 b   , the remaining portion of the cooling air discharged from each insert  52  is recirculated back through the spokes  26   b  in the annular reverse flow passage  54 . Flows calibrating holes or other suitable flow calibration devices  56  are provided at the radially outer end of each spoke  26   b  to calibrate the flow of cooling air passing through each of the annular gaps  54 . The holes  56  are calibrated so that the portion of the cooling air flowing radially outwardly through the annular gap  54  maintains the spokes  26   b  substantially at the same temperature as the top and bottom spokes  26   a  housing the internal bearing service lines  50 . As shown in  FIG. 6 c   , outlet holes  58  are defined in the radially outer end portion of the spokes  26   b  to discharge the cooling air between the ISV casing  28  and the outer ring  24 . This flow path mimics the cooling flow path around the top and bottom spokes  26   a  ( FIG. 7 a   ) used for the oil tubes/bearing service lines  50 . This configuration ensures that all the structural spokes  26  with and without bearing housing service lines are kept at the same temperature, thereby ensuring bearing housing centralization throughout the engine operating envelope. In the prior art, separate struts had to be used for the structural spokes and the bearing service lines. With the new proposed arrangement, a service line and a spoke can be positioned in a same hollow strut. This reduces the number of large, hollow struts in the gaspath. It allows the cold spoke design mid-turbine frame to be used in physically smaller engines. The uniformity of the cooling flow between the different types of spokes ensures bearing housing concentricity while allowing various hardware combinations to transverse the ISV gaspath combinations. 
         [0032]    Referring to  FIGS. 7 a  and 7 c   , it can be appreciated that the cooling system comprises a second cooling circuit which is separate from the first cooling circuit described above for the  4  structurally dedicated spokes  26   b.  The second cooling circuit provides cooling to the top and bottom spokes  26   a  housing the service lines  50 . As can be appreciated from  FIG. 7 a   , the annular gap between the bottom spoke  26   a  and the service line  50  extending therethrough is connected in fluid flow communication with the annular cavity or air plenum  40 . The air is discharged from the bottom spoke  26   a  into a sealed annular chamber or cavity  90  defined between the inner ring  22 , the bearing casing  23  and a rear cover  92  ( FIG. 7 b   ) bolted to the inner ring  22 . The cooling air travels circumferentially through the annular cavity  90  from the bottom spoke  26   a  to the top spoke  26   a.  As shown in  FIG. 7 a   , the cooling air exits the annular cavity  90  via the annular gap defined between the top spoke  26   a  and the service line  50  extending therethrough. As shown in  FIG. 7 c   , the air is discharged at a radially outer end of the service line  50  through outlet holes  94 . The person skilled in the art will appreciate that the top and bottom spokes  26   a  are used to feed/purge air and oil of a scupper line in the case of oil failure. The air in the first circuit through the  4  structurally dedicated spokes  26   b  will not be contaminated by the air flowing through the top and bottom spoke housing the service lines  50  in the event of oil leakage. 
         [0033]    The use of the 4 structurally dedicated spokes  26   b  to feed secondary cooling air from the cavity  40  to the cavity disc of the upstream rotor also contributes to reduce the number of pipes and tubes. Indeed, the spokes are used as air feed tubes to direct cooling air to adjacent turbine components, thereby reducing the number of parts to be installed on the engine. 
         [0034]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Any modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.