Abstract:
An inner casing assembly for a turbine including: an annular inner casing including cooling passages, wherein each passage extends through a wall of the inner casing from a source of cooling fluid to an outer surface of the wall of the inner casing, and struts extending outward from the outer surface of the inner casing wherein the cooling passages are arranged on the inner casing such that a pair of the cooling passages is on opposite sides of each of the struts, and the cooling passages in each pair are equidistant to the corresponding strut.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to cooling of the exhaust section of a gas turbine, and, particularly, relates to cooling of the struts on a gas turbine inner casing in an exhaust section. 
         [0002]    A gas turbine engine combusts a mixture of fuel and compressed air to generate hot combustion gases, which drives turbine blades to rotate a shaft in the exhaust section supported by bearings and casings. The rotation of the shaft may generate significant amounts of heat in the turbine. Also, the hot turbine exhaust gases flowing through an exhaust section may transfer heat to the exhaust casings in the exhaust section. 
         [0003]    An inner casing in an exhaust section of a gas turbine is heated by the exhaust gas from the turbine engine. The inner casing may also experience thermal heating due to friction from the shaft in the casing. Inner casing in a turbine exhaust component may not be adequately and uniformly cooled due to differences in body mass throughout the inner casing, such as the flanges at the split line and at the roots of the struts connected to the inner casing. Uneven cooling of the struts may cause differences in thermal contraction and expansion in different areas of the inner casing, and induce damages associated with thermal stress. 
         [0004]    Methods of cooling turbine exhaust casing components have been described using a flow of cooling fluids (e.g. ambient air) through the exhaust section. Cooling systems are disclosed in U.S. Pat. Nos. 7,493,769; 6,578,363; 7,373,773; 2013/0064647; and 2013/0084172. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    An inner casing cooling system has been conceived and is disclosed herein to provide cooling flow in a turbine exhaust section for uniform cooling of the roots of the struts and the split line flanges on the inner casing. 
         [0006]    An inner casing assembly for a turbine is disclosed herein comprising: an annular inner casing including cooling passages, wherein each passage extends through a wall of the inner casing from a source of cooling fluid to an outer surface of the wall of the inner casing, and struts extending outward from the outer surface of the inner casing, wherein the cooling passages are arranged on the inner casing such that a pair of the cooling passages is on opposite sides of each of the struts, and the cooling passages in each pair are equidistant to the corresponding strut. 
         [0007]    The cooling passages may include a pair of cooling passages on opposite sides of a split line extending in an axial direction through the outer surface of the inner casing and the pair of cooling passages on opposite sides of the split line are each equidistant from the split line. The cooling passages may not be equidistantly arranged around a circumference of the inner casing. The cooling passages may include cooling passages arranged in annular arrays in front of and behind the struts along an axis of the inner casing. The cooling passages may be oriented to direct cooling flow through the passages towards the struts. 
         [0008]    A turbine exhaust section comprising: an outer annular duct configured to receive exhaust gas from a turbine and including an outer casing housing and an inner casing housing; struts extending between the inner casing housing and the outer annular casing housing, wherein the struts extend through the outer annular duct; an inner annular duct coaxial to the outer annular duct and configured to receive cooling air, wherein the inner annular duct provides cooling air to the inner casing housing, wherein the inner casing includes an outer wall with cooling passages for the cooling air, and each cooling passage extends through the outer wall to allow cooling air to flow to an outer surface of the outer wall, and the cooling passages are arranged on the inner casing such that a pair of the cooling passages is on opposite sides of each of the struts, and the cooling passages in each pair are equidistant to the corresponding strut. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a front view of a conventional front side of inner casing having cooling passages; 
           [0010]      FIG. 2  is a front view of a conventional back side of inner casing having cooling passages; 
           [0011]      FIG. 3  is a side view of an exhaust section of a gas turbine having an inner casing comprising cooling passages uniformly arranged near the struts; 
           [0012]      FIG. 4  is a front view of a front side of inner casing that shows an arrangement of cooling passages near the struts; 
           [0013]      FIG. 5  is a front view of a back side of inner casing that shows an arrangement of cooling passages near the struts; 
           [0014]      FIG. 6  is a side view that shows cooling passages and split line cooling passages; 
           [0015]      FIG. 7  is a magnified view of an inner casing with cooling passages uniformly arranged on either side of the split line of the inner casing 
           [0016]      FIG. 8  is a magnified view of an inner casing with cooling passages and split line cooling passages; and 
           [0017]      FIG. 9  is perspective view of an inner casing with cooling hole and split line cooling hole arrangements. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  shows a conventional inner casing  100  with cooling passages along the inner casing. The inner casing  100  includes semi-cylindrical casing housings, an upper inner casing housing  120  and a lower inner casing housing  130 . The casing housings are joined at the split line  106  (e.g. a seam between the casing housings) by connecting two upper flanges  122  and two lower flanges  132  at the split line  106 . 
         [0019]    Struts  102  are located on an outer circumference  108  of the upper inner casing housing  120  and the lower inner casing housing  130  of the inner casing  100 . The struts  102  located on the upper inner casing housing  120  and the lower inner casing housing  130  are symmetrical, and the struts  102  are typically equidistant from one another. 
         [0020]    As used in a conventional gas turbine exhaust section, an inner casing is situated such that heated exhaust flow from the gas turbine exits the exhaust section by flowing past the struts on the inner casing. An exhaust flow may be in the X-direction, flowing past the struts. Cooling passages supply cooling flow that may be used to cool the struts that are heated by the exhaust flow, and to cool the inner casing that is heated by the exhaust flow and by rotation of a shaft it is coupled to. 
         [0021]    On a conventional front side of inner casing  100 , the cooling passages  104  are typically equidistant from one another. The cooling passages  104  communicate and extend between the inner circumference  110  of the inner casing  100  and the outer circumference  108  of the inner casing  100 . Cooling flow may flow from an inner circumference  110  of the inner casing  100 , through the cooling passages  104 , and out of the outer circumference  108 . The upper inner casing housing  120  has x number of cooling passages  104  that are equidistant from one another along the circumference of the inner casing  100  from the split line  106 . The lower inner casing housing  130  has x number of cooling passages  104 . The inner casing  100  in  FIG. 1  has cooling passages  104  that do not coincide with the placements of the struts  102 . Specifically, the cooling passages  104  are not uniformly positioned between the struts. 
         [0022]    Similarly,  FIG. 2  shows a back side of inner casing  200  with cooling passages  204 . The back side of inner casing  200  also includes semi-cylindrical casing housings, an upper inner casing housing  220  and a lower inner casing housing  230 . The back side of inner casing  200  includes cooling passages  204  that extend and communicate between the inner circumference  210  and the outer circumference  208 . The cooling flow flows from the inner circumference  210  through the cooling passages  204  to supply cooling flow to the struts  202  located on the outer circumference  208  of the back side of inner casing  200 . 
         [0023]    The back side of inner casing  200  has a split line  206 . At the split line  206 , the upper inner casing housing  220  and the lower inner casing housing  230  are joined by connecting two upper flanges  222  and two lower flanges  232 . The back side of inner casing  200  has 8 cooling passages  204  in the upper inner casing housing  220 , and 8 cooling passages  204  in the lower inner casing housing  230 . The arrangement of cooling passages  204  also do not align with the placements of the struts  202 . That is, the cooling passages  104  are not uniformly positioned between the struts. 
         [0024]    The misalignment of the struts and the cooling supply passages have been found to cause uneven distribution of cooling flow to each strut and the flanges of the inner casing  100  and  200 . The uneven distribution of cooling passages may cause a high cooling flow variation and uneven cooling of the struts and the split line on an inner casing. Strut to strut flow variation may be as high as 60% on a conventional inner casing. Horizontal location struts, for example, would typically see a lower cooling flow rate due to a lesser number of supply passages per strut. 
         [0025]    In addition, cooling flow around the split line is typically distraught due to the structure of an inner casing split line. The split line is typically a larger structure than other parts of the casing housing, which includes the upper and lower flanges without placement of cooling passages around the split line. Thus, the split line structure would distraught cooling flow around the split line due to the lower number of cooling passages. 
         [0026]    A strut close to the split line would not receive adequate amount of cooling flow due to a lack of cooling passages in area. In comparison, other struts would have a higher cooling flow rate due to a higher number of cooling passages per strut in other areas of the inner casing. The uneven distribution of cooling passages with respect to the placements of the struts causes a reduction in reliability of the inner casing and struts in the turbine exhaust section due to inadequate cooling of the inner casing. 
         [0027]    The present invention provides an arrangement of cooling passages that increases the uniformity of cooling of the inner casing. Even distribution of the cooling flow may help reduce the exhaust frame out of roundness, reduce the bearing drop that impacts the rotor vibrating, and improve the reliability of the inner casing and struts. 
         [0028]    A gas turbine exhaust section  390  is shown with an inner casing  300  in  FIG. 3 . While in operation, a gas turbine engine compartment  380  would release a heated exhaust flow  382  that would flow from the turbine engine compartment  380  through an exhaust section  390 . As the exhaust flow  382  flows through the exhaust path  396 , the exhaust flow  382  may encounter struts  302  on an inner casing  300  and transfer heat from the exhaust flow  382  to the struts  302 . 
         [0029]    In the exhaust section  390 , the inner casing  300  may be coupled to a shaft  350  that is rotatable. The shaft  350  may provide support for a set of propeller  392  that is used to draw in ambient air as cooling flow  394  for the exhaust section  390 . Cooling flow  394  may convectively cool the exhaust section  390  and inner casing  300  to reduce thermal damage caused by the heat. 
         [0030]    After the cooling flow  394  is drawn into the inner casing  300 , the cooling flow  394  exits the inner casing  300  from cooling passages  304 . The cooling flow  394  convectively cools the inner casing  300 , including the struts  302 , and then joins the exhaust flow  382  in the exhaust path  396  to exit the exhaust section  390 . 
         [0031]    In  FIG. 4 , a forward of inner casing  400  has two casing housings, an upper inner casing housing  420  and a lower inner casing housing  430 . The upper inner casing housing  420  and the lower inner casing housing  430  is joined at the split line  406  by connecting upper flanges  422  and lower flanges  432 . 
         [0032]    Each of the upper inner casing housing  420  and lower inner casing housing  430  has a plurality of struts  402  protruding from an outer circumference  408  of the inner casing  400 . The inner casing  400  also includes cooling passages  404  that extend and communicate between the inner circumference  410  and the outer circumference  408  that allow cooling flow to pass through between the inner circumference  410  and the outer circumference  408 . 
         [0033]    On either side of each of the struts  402  on the outer circumference  408 , there is at least one pair of cooling passages  404 . The cooling passages  404  do not need to be equidistant from one another along the outer circumference  408  of the inner casing  400 . However, the cooling passages  404  are to be similarly distanced from each of the struts  402  that it is adjacent to. For example, with respect to exemplary strut  402 A, exemplary cooling passages  404 A and  404 B are placed on either side of the strut  402 A. Exemplary cooing flow passages  404 A and  404 B are placed equidistantly from exemplary strut  402 A. 
         [0034]    Similarly, in  FIG. 5 , a back side of inner casing  500  has an upper inner casing housing  520  and a lower inner casing housing  530 . The upper inner casing housing  520  and the lower inner casing housing  530  is joined at the split line  506  by connecting upper flanges  522  and lower flanges  532 . Each of the upper inner casing housing  520  and lower inner casing housing  530  has a plurality of struts  502  protruding from the outer circumference  508  of the inner casing  500 . 
         [0035]    The inner casing  500  has cooling passages  504  that extend and communicate between the inner circumference  510  and the outer circumference  508 . On either side of each of the struts  502  on the outer circumference  508 , there is at least one pair of cooling passages  504 . The cooling passages  504  do not need to be equidistant from one another along the outer circumference  508 , but the cooling passages  504  are to be similarly distanced from each of the struts  502  that it is adjacent to. For example, with respect to strut  502 A, cooling passages  504 A and  504 B are placed on either side of the strut  502 A. Cooing flow supply passages  504 A and  504 B are placed equidistantly from strut  502 A. 
         [0036]    In another embodiment, there may be more than four struts protruding from the outer circumference of an inner casing. The additional number of struts can be accommodated by placing the same number of cooling passages at similar distances on either side of each of the plurality of struts, such as described and shown  FIGS. 4 and 5  above. 
         [0037]    In an additional embodiment, there may be more than one pair of cooling passages on either side of each of the struts. There may be more than two cooling passages or more than three cooling passages on each side of the struts. The cooling passages are to be arranged symmetrically on either side of the struts to provide even and uniform cooling flow to each of the struts. 
         [0038]    Distances between struts and cooling passages are shown in  FIG. 6 , which provides a side view of an inner casing  600  that is coupled to a shaft  650  in a gas turbine. The inner casing  600  has an upper inner casing housing  620  and a lower inner casing housing  630 . The upper inner casing housing  620  and the lower inner casing housing  630  is joined at the split line  606  by connecting upper flanges  622  and lower flanges  632 . An outer circumference  608  of the inner casing  600  has a plurality of protruding struts  602 . 
         [0039]    The inner casing  600  comprises at least one pair of cooling passages  604  arranged on either side of each strut  602  along the outer circumference  608  of the inner casing  600 . The cooling passages  604  may be arranged such that each pair of the cooling passages  604  are the same distance M away from the center line S of the struts  602  on the outer circumference  608 . 
         [0040]    For example, exemplary strut  602 A has a center line S that extends from a center of mass of the strut towards the second rim  670 . Exemplary cooling passages  604 A and  604 B are arranged on either side of the exemplary strut  602 A along the second rim  670 , and each of the exemplary cooling passages  604 A and  604 B is the same distance M away from the center line S. This arrangement places the exemplary passages  604 A and  604 B equidistantly on either side of the exemplary strut  602 A. 
         [0041]    Alternatively, there may be more than one pair of cooling passages  604  on either side of the struts  602 . A number and pattern of cooling passages  604  on a first side of a strut  602  are symmetrical with respect to a number and pattern of cooling passages  604  placed on a second side of a strut  602  along the outer circumference  608 . 
         [0042]    Cooling passages  604  may be arranged along a first rim  660  of the inner casing  600  and along a second rim  670  of the inner casing  600 . A first set of cooling passages  604  are arranged substantially the same distance away from the first rim  660 , and a second set of cooling passages  604  are arranged a similar distance away from the second rim  670 . Alternatively, the first set of cooling passages  604  may be arranged in a pattern along the first rim  660 , and the second set of cooling passages  604  may be arranged in a similar pattern along the second rim  670  that is symmetrical to the first set of passages  604 . 
         [0043]    In another embodiment, in addition to the cooling passages  604  arranged adjacent to each strut  602 , there are split line cooling passages  614  placed along to the split line  606  on both the upper inner casing housing  620  and the lower inner casing housing  630 . The split line cooling passages  614  also extend and communicate between an inner circumference of the inner casing  600  and the outer circumference  608  of the inner casing  600 . Arrangement of the cooling passages  604  and split line cooling passages  614  is further shown in  FIG. 7 . 
         [0044]    An inner casing  700 , is magnified in  FIG. 7  to show the split line  706  with a strut  702  that is adjacent to the split line  706 , and a first rim  760  and a second rim  770  of the inner casing  700 . The inner casing  700  comprises an upper flange  722  on an upper inner casing housing  720  and a lower flange  732  on a lower inner casing housing  730 . The upper flange  722  and the lower flange  732  joins at the split line  706  to form the inner casing  700 . 
         [0045]    The upper flange  722  has a thickness of Q 2  along the outer circumference  708 . Similarly, the lower flange  732  has a thickness of R 2  along the outer circumference  708 . Split line cooling passages  714  are placed in close proximity to the split line  706 , adjacent to the upper flange  722  and the lower flange  732 . Split line cooling passages  714  are placed a distance Q 1  away from an edge of the upper flange  722  that is immediately adjacent to the cooling passages  714 . Similarly, split line cooling passages  714  are placed a distance R 1  away from an edge of the lower flange  732  that is immediately adjacent to the cooling passages  714 . The distances Q 1  and R 1  can be the same, or different if desired. 
         [0046]    Nonetheless, split line cooling passages  714  may not have the same distance away from the split line  706  on the upper inner casing housing  720  and the lower inner casing housing  730  if a thickness of the upper and lower flanges  722  and  732  are different. The split line cooling passages  714  are placed to aid in the cooling of the upper and lower flanges  722  and  732 . 
         [0047]    Unlike the split line cooling passages  714 , the cooling passages  704  are not placed with respect to the split line. The cooling passages  704  may not be the same distances away from the upper flange  720  as from the lower flange  730 , and may not be the same distances away from the split line  706 . Cooling passages  704  are placed according to the placements of the struts  702 . 
         [0048]    For example, cooling passages  704  may be a distance O away from the split line  706  on the upper inner casing housing  720 , and the cooling passages may be a distance P away from the split line  706  on the lower inner casing housing  730 . Distance O and distance P may be the same if the struts are placed equidistantly with respect to the outer circumference  708 , or distance O and distance P may not be the same if the struts are not placed equidistantly with respect to the outer circumference  708 . 
         [0049]    In addition, the split line cooling passages  714  may be placed symmetrically on the upper inner casing housing  720  and the lower inner casing housing  730 . For example, the exemplary cooling hole  704 A on the upper inner casing housing  720  are placed across the split line  706  from the exemplary cooling hole  704 B on the lower inner casing housing  730 . Exemplary cooling hole  704 A and exemplary cooling hole  704 B are placed symmetrically. Similarly, the exemplary split line cooling hole  714 A is placed across the split line  706  from the exemplary cooling hole  714 B. The exemplary split line cooling hole  714 A and exemplary split line cooling hole  714 B are placed symmetrically. 
         [0050]    A first set of cooling hole  704  and split line cooling hole  714  may be placed close to the first rim  770 , and a second set of cooling hole  704  and split line cooling hole  714  may be placed close to the second rim  760 . The first set and the second set are placed symmetrically. 
         [0051]    Alternatively, more than two split line cooling passages may be placed adjacent to the upper flange and the lower flange. A plurality of split line cooling passages may be located symmetrically on an upper inner casing housing and a lower inner casing housing such that the split line upper and lower flanges are cooled uniformly. The split line cooling passages may be placed equidistantly along the upper and lower flanges. 
         [0052]      FIG. 8  shows two exemplary cooling passages  804 A and  804 B adjacent to an exemplary strut  802  that have a size and orientation which may be advantageous in providing cooling flow to the struts and the upper and lower flanges at the split line. As shown in  FIGS. 4 and 5 , cooling passages extend between an inner circumference of the inner casing and an outer circumference of the inner casing. Thus the exemplary cooling passages  804 A and  804 B allows cooling flow  894  to pass through from the inner circumference to the outer circumference  808  of the inner casing  800 . 
         [0053]    The exemplary cooling passages  804 A and  804 B are placed on either side of the strut  802 , and the exemplary cooling passages  804 A and  804 B are oriented to direct cooling flow  894  towards the strut  802 . The exemplary cooling hole  804 A are oriented at an angle • 1  relative to an axis Z of the cooling hole, and the cooling hole  804 B are oriented at an angle • 2  relative to axis Z. 
         [0054]    For example, angles • 1  and • 2  may be symmetrically placed adjacent to the strut  802 . The exemplary cooling passages  804 A and  804 B are oriented such that the cooling flow  894  passes through and is directed towards the strut  802 . Cooling passages  804 A and  804 B may be angled at 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, or 165 degrees relative to an axis Z extending through the center of the hole. 
         [0055]    In addition, the cooling passages  804 A and  804 B may be in any kind of shape, such as conical, cylindrical, rectangular, spherical, hemispherical, and combinations thereof. 
         [0056]    Exemplary cooling passages  804 A and  804 B may be placed such that they are equidistant from the second rim  870  of the inner casing  800 , and also equidistant from the strut  802  on the outer circumference  808 . 
         [0057]    The inner casing  800  may also additionally comprise split line cooling passages  814 . The split line cooling passages  814  are oriented towards the split line  806 , and are placed adjacent to one of the flanges on the split line  806 , such as the upper flange  822 . The split line cooling passages  814  may be in any kind of shape, such as conical, cylindrical, rectangular, spherical, hemispherical, and combinations thereof. 
         [0058]    Split line cooling passages  814  may also be angled at 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, or 165 degrees relative to the axis Z. Preferably, the split line cooling passages  814  are oriented such that the cooling flow  894  passing through the split line cooling passages  814  is directed towards the split line  806 . 
         [0059]    Even though  FIG. 8  shows two cooling passages  804 A and  804 B that may be used to supply cooling flow to the exemplary strut  802 , the same limitations may be applied to other cooling passages to other struts on the inner casing  800  that are not shown in  FIG. 8 . Similarly, the inner casing  800  may comprise other split line cooling passages  814  to supply more cooling flow to the split line  806 . 
         [0060]    Advantages of the present invention include providing improved cooling of the inner casing, specifically at the roots of the struts and the flanges at the split line where the mass are different than at other locations on the inner casing. Cooling of struts  902  have been analyzed using an inner casing  900  shown in  FIG. 9 . 
         [0061]      FIG. 9  shows an inner casing  900  that comprises an upper inner casing housing  920  and a lower inner casing housing  930  that are joined at the split line  906 . The upper inner casing housing  920  includes 2 struts, S 3  and S 4 , and an upper flange  922 . Cooling passages  904  are placed on either side of the struts S 3  and S 4 , and split line cooling passages  914  are placed adjacent to the upper flange  922 . 
         [0062]    Similarly, the lower inner casing housing  930  includes 2 struts, S 1  and S 2 , and a lower flange  932 . Cooling passages  904  are placed on either side of the struts S 1  and S 2 , and split line cooling passages  914  are placed adjacent to the lower flange  931 . Cooling flow passes through the cooling passages  904  and the split line cooling passages  914  from an inner circumference  910  of the inner casing  900  to an outer circumference  908  of the inner casing  900 . The cooling flow is direct towards the struts S 1 , S 2 , S 3  and S 4  through the cooling passages  904 , and the cooling flow is directed towards the flanges  922  and  933  through the split line cooling passages  914 . 
         [0063]    An analysis was conducted to determine the variations in cooling flow for different types of inner casing: a conventional inner casing, an inner casing comprising the inventive cooling hole arrangement, and an inner casing comprising the inventive cooling hole and split line cooling hole arrangements. 
         [0064]    It was found that for a conventional inner casing, such as the inner casings  100  or  200  shown in  FIGS. 1 and 2 , the struts on the inner casing could see a strut to strut cooling flow variation that is as high as 60%. By positioning the cooling passages equidistantly on either side of each of the struts, cooling flow variation from strut to strut may be reduced to about 30%. Cooling flow variation from strut to strut may be reduced to about 15% for an inner casing that includes split line cooling passages placed adjacent to the split line in addition to cooling passages placed equidistantly on either side of each of the struts. 
         [0065]    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, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.