Patent Publication Number: US-7721548-B2

Title: Combustor liner and heat shield assembly

Description:
TECHNICAL FIELD 
   The invention relates generally to gas turbine engine combustors and, more particularly, to combustor heat shield cooling. 
   BACKGROUND OF THE ART 
   Heat shields, which protect the dome panel of combustor shells, are exposed to hot gases in the primary combustion zone. The amount of coolant available for cooling the heat shields must be minimized to improve the combustion efficiency and to reduce the smoke, unburned hydrocarbon and CO/NOx emission. Example heat shields are shown in U.S. Pat. Nos. 4,934,145 and 5,419,115. 
   There is a continuing need for improved heat shields and cooling schemes. 
   SUMMARY 
   In one aspect, there is provided a heat shield for a combustor dome, the heat shield comprising a body defining a fuel nozzle hole, a back face adapted to be mounted adjacent and spaced-apart from a combustor dome, a ridge extending around the nozzle opening and extending from the back face substantially to a height adapted to substantially contact the combustor dome, the ridge thereby define a cooling compartment interior of the ridge between the body and the combustor dome, a plurality of slots provided through the ridge, the slots adapted to be closed by the combustor dome to provide cooling holes when heat shield is mounted on the combustor, the slots adapted to direct pressurized cooling air within the compartment therethrough to cool an adjacent region of the body exterior of the compartment. 
   In a second aspect, there is provided a combustor dome comprising at least one heat shield mounted to an annular dome panel, the at least one heat shield having a heat shield body extending between opposed lateral edges and radially inner and outer edges, at least one fuel nozzle opening defined in the heat shield body, the heat shield body having a back face facing the combustor dome, the back face and the dome panel defining an air space therebetween, a ridge extending from the back face of the heat shield body around the fuel nozzle opening and generally circumscribing a central annular area, a plurality of impingement holes defined in the dome panel and disposed to direct impingement jets in the central annular area upon the back face of the heat shield body, and at least one fluid passage extending through the ridge for discharging coolant from said central annular area to an adjacent region of the back face of the heat shield body. 
   Further details of these and other aspects will be apparent from the detailed description and figures included below. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the accompanying figures, in which: 
       FIG. 1  is a schematic cross-sectional view of a turbofan engine having a reverse flow annular combustor and dome panel heat shields; 
       FIG. 2  is an enlarged view of a combustor shell of the engine combustor shown in  FIG. 1 ; 
       FIG. 2   b  is an enlarged view of  FIG. 2 ; 
       FIG. 3  is an outside end view of the dome panel of the combustor shell, illustrating an impingement hole pattern; 
       FIG. 4  is a perspective view of a back face of a dome heat shield of the combustor; 
       FIG. 5  is a plan view of a back face of the heat shield shown in  FIG. 4 ; 
       FIG. 5   b  is a view similar to  FIG. 5   a , showing impingement areas and pin fin density regions; 
       FIG. 6  is a sectional view of the portion of  FIG. 5  indicated  6 - 6 ; and 
       FIG. 7  is a sectional view of the indicated portion of  FIG. 5  indicated  7 - 7 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a 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. 
   The combustor  16  is housed in a plenum  17  supplied with compressed air from compressor  14 . As shown in  FIG. 2 , the combustor  16  comprises a reverse flow annular combustor shell  20  composed of a radially inner liner  20   a  and a radially outer liner  20   b , defining a combustion chamber  22 . The combustor  16  has a bulkhead or inlet dome portion or panel  24  and an exit potion  26  for communicating combustion gases with the turbine section  18 . As shown in  FIG. 1  a plurality of fuel nozzles  28  are mounted to extend through the inlet dome end portion  24  of the combustor  20  to deliver a fuel-air mixture to the chamber  22 . 
   A plurality of effusion holes (not shown) are preferably defined in the inner and outer liners  20   a  and  20   b  for cooling purposes, and dilution holes (not shown) are also preferably provided for combustion purposes. Inner and outer liners  20   a  and  20   b  may have any suitable configuration, and thus are shown in dotted line only in  FIG. 2 . The inner and outer liners  20   a  and  20   b  are preferably made out of sheet metal, though any suitable material(s) and manufacturing method(s) may be used. A thermal barrier coating (not shown) is preferably applied to the inner or combustion facing surfaces  32 ,  34  of the liners  20   a  and  20   b  to protect them against the high temperatures prevailing in the combustion chamber  22 . 
   As shown in  FIG. 2 , the inner and outer liners  20   a  and  20   b  respectively include flanges  38  and  36  which overlap each other so as to form the dome panel  24  of the combustor shell  20  (Alternatively, any other suitable dome panel could be employed). The flanges  36  and  38  are directly fixedly secured together by a plurality of circumferentially distributed dome heat shields  40  mounted inside the combustion chamber  22  to protect the end wall of the dome  24  from the high temperatures in the combustion chamber  22 . The dome heat shields  40  are typically castings made out of high temperature materials. Each dome heat shield  40  has a plurality of threaded studs  42  (four according to the example shown in  FIG. 4 ) extending from a back face of the heat shield and through holes  44  ( FIG. 3 ) defined in flanges  36  and  38 . Self-locking nuts  46  are threadably engaged on the studs  42  from outside of the combustion chamber  22  for holding flanges  36  and  38  (and thus inner and outer liners  20   a  and  20   b ), and the dome heat shields  40  tightly together. 
   As shown in  FIGS. 2 and 3 , fuel nozzles openings  48  are defined through the dome panel  24  for allowing mounting of the fuel nozzles  28  to the combustor  16 . A central hole  52  is defined in each of the heat shields  40  and is aligned with a corresponding fuel nozzle opening  48  for accommodating an associated fuel nozzle therein. As illustrated in  FIG. 2 , a floating collar  54  is mounted in the nozzle opening  48  to provide sealing between the combustor shell  20  and the fuel nozzles  28  while allowing relative movement therebetween. The floating collar  54  has an anti-rotation tab (not shown) which fits within an anti-rotation slot  55  on heat shield  40  (see  FIG. 4 .) The fuel nozzles  28  are slidably received in the floating collars  54 . The floating collars  54  are axially trapped between the heat shields  40  and the end wall (i.e. flange  36 ) of the combustor dome  24 . The fuel nozzle openings  48  are slightly oversized relative to the floating collars  54 , thereby allowing limited radial movement of the collars  54  with the fuel nozzles  28  relative to the combustor shell  20 . 
   As shown in  FIG. 2   b , the heat shields  40  are spaced from the dome panel  24  by a distance of about 0.1 inch so as to define a heat shield back face cooling air space  60 . Relatively cool air from plenum  17  is admitted in the back cooling air space  60  via impingement holes  62  defined in the dome panel of the combustor shell  20  (see  FIG. 3 ). The impingement hole patterns are arranged in the dome panel of the combustor shell  20  to optimize the heat shield cooling, in co-operation with pin fins located on the heat shield, as will be described further below. As shown in  FIG. 3 , the impingement holes include a first set of holes  62   a  arranged on two circular paths concentrically arranged with fuel nozzle opening  48 . Preferably the inner circle of set  62   a  comprises holes equally spaced on a first pitch about the nozzle opening  48 , while the outer circle of set  62   a  comprises only 10 holes (6 outer and 4 inner) but on a pitch similar to the first pitch. Placement of the outer circle of set  62   a  will be discussed further below. Holes  62   b  are also provided in two rows extending laterally from each side of nozzle hole  48 , both rows concentric with the central axis of the annular combustor  20 . Preferably the holes  62  of inner row have an angular position which is staggered relative to the holes of the outer row of the set of  62   b . Preferably 8 holes are provided in the outer row, and 7 holes  62  on the inner row, in each set  62   b  on either side of nozzle hole  48 . Though this is the preferred embodiment, other hole placements and numbers maybe used. Placement of the holes  62  of set  62   b  will also be discussed further below. Holes  62  are preferably straight-through holes generally perpendicular to the dome panel face, thus having an axis generally parallel to the combustor (or engine) axis. By placing holes sets  62   a  and  62   b  in circular arrays allows the holes to be laser drilled using drilling-on-the-fly (DOF) techniques, which speeds manufacturing time. As will be discussed further below, impingement holes  62  are positioned and arranged directly (i.e. generally perpendicularly) above reduced-height pin fins  64   a  on the back face of the heat shield to improve cooling by minimising the resistance to the air flow, which facilitates combustor cooling where a low pressure drop or gradient is available to energize combustor cooling. This allows for an optimized cooling to be achieved on the heat shield while still providing enough momentum to the air exiting from behind the heat shield to form a uniform film around the circumference of the surfaces  32  and  34  of the inner and outer liners. 
     FIGS. 4 and 5  show an individual dome heat shield  40 . Each heat shield  40  is provided in the form of a circular sector having a radially inner lip  41 , having a plurality of ribs  72  discussed further below, a radially outer lip  43  and lateral edges  45 ,  47  extending between the inner and outer lips  41  and  43 . Heat exchange promoting protuberances, such as pin fins  64 , pedestals or other raised cooling structures, are provided preferably in rows, and preferably in staggered position from row to row, on the back face of the heat shields  40  for augmenting the heat transfer between the back face and the cooling air. The pin fin density and location are defined based on the heat shield hot spots and to minimize the pressure drop, as will be discussed further below. As will be discussed further below, the pin fins  64  have different heights, depending on their location on the back face of the heat shield  40 . The pin fin  64  height is preferably substantially the same as the distance between the heat shield back face and the inner surface of the dome panel (e.g. in this example, about 0.095″ to 0.1,″ from the back face). The pin fin-to-pin fin spacing is based on required cooling, and in the present embodiment ranges from 0.05 inch to 0.1 inch. Each pin fin  64  preferably has a frustoconical shape. 
   As shown in  FIGS. 4 and 5 , ribs or ridges  66  are provided extending from the back face of the heat shields  40  to strengthen the heat shield and direct the flow of cooling air as desired, as will be discussed further below. The ridges  66  preferably extend from the heat shield back face all the way into substantially sealing contact with the inner surface of the dome panel (e.g. in this example, about 0.095″ to 0.1″ from the back face), and thus more or less sealing engage the dome panel and thereby direct the cooling air from impingement hole sets  62   a  and  62   b  to the various regions of the heat shield, as will be described further below. The ridges  66  include a central circular ridge  66   a  concentrically disposed with the fuel nozzle opening  52 , a pair of generally diametrically opposed primary ridges  66   b  extending laterally from the central circular ridge  66   a , a pair of generally radially disposed ridges  66   c  extending radially outwardly from the central circular ridge  66   a , and a pair of generally radially disposed ridges  66   d  extending radially inwardly from the central circular ridge  66   a.    
   As shown in  FIG. 2 , the central circular ridge  66   a  preferably extends around fuel nozzle opening  52  in the heat shield in sealing contact with the inner surface of the dome panel. Referring to  FIG. 5   b , the areas of impingement by air passing through holes  62  of sets  62   a  and  62   b  are indicated by corresponding ellipses overlaid on the heat shield  40 . As can be seen from  FIG. 5   b , the rows of impingement holes  62   a  align with one on either side of the central circular ridge  66   a . Outer circle of holes  62  of set  62   a  generally align with the short pin fins  64   a  adjacent the central circular ridge  66   a . Referring again to  FIGS. 4 and 5 , the cooling air from holes  62  of the inner circle of the set  62   a  impinges upon the portion of the back surface of the heat shield  40  bounded by circular ridge  66   a  and is then mostly directed into cooling holes  67  extending through the heat shield  40  for exhausting through the face of the heat shield. A portion of the cooling air, however, is directed through (preferably) four grooves  63  defined at the radially outer side of the heat shield through the circular ridge  66   a . When mated against the combustor dome panel, the grooves  63  provide cooling holes or slots for allowing a portion of the cooling air to be discharged through the grooves  63  towards the radially outer lip  43  of the heat shield  40 , as shown in  FIG. 5 , and thereby cool an adjacent area  81  where no pin fins  64  are provided, due to space limitations on the heat shield back face for a given dome panel and fuel nozzle geometry. Grooves  63  also permit a proper radial airflow to exit the back of the heat shield and into the combustion chamber  22  (e.g. see upper arrows al in  FIGS. 2 ,  2   a ). The cooling air from holes  62  of the outer circle of the set  62   a  impinges upon the portion  80   a  ( FIG. 5 ) of the back surface of the heat shield  40 , and thus tends to be directed generally radially outwardly or radially inwardly, as the case may be, by the ridge  66   a  in co-operation with ridges  66   c  or  66   d , as the case may be, due to the substantially sealing contact provided by the ridges  66  with the combustor dome panel. Also as shown in  FIG. 4  (only), a circular array of short pin fins  64   a  may optionally be provided within circular ridge  66   a.    
   Referring back to  FIG. 4 , ridges  66   b  extend laterally from the central circular ridge  66   a  such as to divide the back surface area of the heat shield  40  into a radially outer half  68  and a radially inner half  70  (the term “half” is used approximately). Ridges  66   b  preferably extend parallely to impingement holes  62  of set  62   b , and disposed to be intermediate the inner and outer circle of holes  62  of set  62   b , as can be seen with reference to  FIG. 5   b . Inner and outer circle of holes  62  of set  62   b  generally align with a first row of the short pin fins  64   a  immediately adjacent ridges  66   b . Thus, the two rows (i.e. inner and outer circles) of impingement holes  62  of the set  62   b  in the dome panels are located one on either side of the ridges  66   b . The air from impingement holes  62   b  impinges upon the portions  80   b  of the back face of the shield adjacent the ridges  66   b , and thus tends to be directed generally radially outwardly or radially inwardly of the ridges  66   b , as the case may be, due to the substantially sealing contact provided by the ridges  66  with the combustor dome panel. 
   As mentioned, the ridges  66   c  extend generally radially outwardly from opposed sides of the central circular ridge  66   a  towards, but stopping preferably short of, the radially outer lip  43 . Likewise, the ridges  66   d  extend generally radially inwardly from opposed sides of the central circular ridge  66   a  towards, but stopping preferably short of, the radially inner lip  41 . The ridges  66   c  are thus preferably generally aligned with the ridges  66   d , and bound regions  80   a , for radially directing cooling air in that region. 
   As mentioned, and shown in  FIGS. 4 and 5 , the heat shield  40  provided with arrays of “full height” pin fins  64  (i.e. extending substantially, but preferably not quite, the entire distance between the heat shield back face and the combustor dome panel, or about 0.090″ to 0.1″ in this example, and more preferably to 0.090″ to 0.095″), however, in regions  80   a  and  80   b  (see  FIG. 5 ), adjacent to ridges  66 , partial height pin fins  64   a  are provided. Preferably, partial height pin fins  64   a  are about one half of the height of full-height pin fins  64 , but otherwise have the same shape and configuration (i.e. preferably partial height pin fins  64   a  appear as a “sawed off” version of pin fins  64 ). The pin fin height is reduced to improve the impingement cooling effectiveness while maximizing surface area for heat transfer. The ratio (R dh ) of the diameter of cooling hole to the height from the impingement surface should preferably greater than one and less than five (i.e. 1&lt;R dh &lt;5) for maximum impingement cooling effectiveness. Depending on pin fin density in the impingement zone, the height from the impingement surface may be considered to be the distance from the impingement holes to either the tops of the pin fins, the heat shield back face surface, or a suitable averaging of the two. Typically, the first (pin fin tops) will be used. Thus, this desired requirement would not be met with a full-height pin fin  64 , but in the current embodiment, the pin fins  64   a  and holes  62  can be respectively sized such that an optimum impingement height is achieved and an increased cooling surface area can still be provided in the impingement regions  80   a ,  80   b  of impingement holes  62 . In the present example, the tops of reduced-height pin fins  64   a  are in the range of 0.045-0.055 below the impingement holes, and the impingement holes have a diameter in the range of 0.025-0.035, thus providing an R dh  in the range of about 1.3 to 2.2 or, generally speaking, 1&lt;R dh &lt;3. 
   An area (unindicated) of pin fins  64  adjacent anti-rotation slot  55  may require height reduction to some extent, to avoid interference with the anti-rotation tab of floating collar  54 . 
   The skilled reader will appreciate that, in general, a higher pin fin density will increase surface area, and thus generally increase heat transfer. However, in situations were insufficient flow is available to overcome the additional flow resistance provided by increased pin fin density, improvements are needed to augment heat transfer. Referring to  FIG. 5   b , at hot spots regions of the heat shield, such as the peripheral regions  68   a  and  70   a , the pin fin  64  density is preferably reduced, relative to central regions  68   b  and  70   b , to increase the heat transfer coefficients by increasing the coolant flow in these peripheral areas, by reducing flow resistance by reducing pin fin density to increase the flow. Preferably, pin fin densities in regions  68   a  and  70   a  are between 0.4 and 0.7 of the densities in regions  65   b  and  70   b , respectively, however the exact densities will be determined based on cooling air flow and heat transfer requirements. For example, pin fin densities for regions  68   a ,  68   b ,  70   a , and  70   b  may be 144, 250, 170, 289 respectively. Due to lower overall pressure drop experienced in the hot spot regions  68   a  and  70   a  due to lower pin fin densities, the heat transfer is optimized by directing more coolant flow through these regions than would be possible if higher densities were used. It is noted that in this example, studs  42  correspond to regions  68   a  and  70   a , although this is not necessary. 
   Lateral ridges or ribs  69  and  71  are provided at lateral edges  45 ,  47  of each heat shield  40  provide a means for redirecting the flow of cooling air behind the heat shield away from the interface of mating sides of adjacent heat shields  40 , and thus impede leakage between adjacent heat shields. The cooling air directed through impingement holes  62  or set  62   b  is, thus, preferably eventually fully exhausted at the inner and outer lips  41  and  43  of each heat shield  40 . As shown in  FIG. 4 , straightener ribs  72  can be provided along the inner and outer lips  41  and  43  to straighten the cooling air flow before being discharged in the combustion chamber  22 . 
   In use, impingement holes  62   a  and  62   b  in the combustor dome allows air to pass into the cooling air space  60  between heat shield  40  back face and the combustor dome panel. The air from combustor impingement holes  62  of sets  62   a  impinges upon the partial height pin fins  64   a  in regions  80   a  on the back face area of the heat shield  40  adjacent circular ridge  66   a , and air from combustor impingement holes  62  of set  62   b  impinges upon the partial height pin fins  64   a  in regions  50   b  on the back face area of the heat shield  40  adjacent ridges  66   b . The partial height pins  64   a  provide sufficient clearance with the dome panel such that an optimal impingement height of 2-5 times the diameter of holes  62  is provided. After impinging the partial height pins  64   a , impingement air moves generally radially relative the heat shield, to move past full height pin fins  64 , in the case of air provided by holes  62  of set  62   b . The splashed air from impingement holes  62   b  is caused to flow over the pin fins towards the inner and the outer lips  41  and  43  by the ridges  66 ,  69  and  71 . This provides effective convection cooling. The air cools the back face of the heat shields by impingement and convection heat transfer. The cooling air is eventually discharged from the space  60  behind the heat shield at the inner and outer lips  41  and  43 , where the flow may be straightened by the straightener ribs  72  before being expelled into the combustion chamber  22  to travel downstream along the inner and outer liners of the combustor. Once travelling along the combustor liners, dilution holes, etc. (not shown) redirect the flow into a double torroidal flow, as indicated by arrows a 1  and a 2  in  FIG. 2 . Meanwhile, the majority of the air received within circular ridge  66   a  impingement cools the back face of the heat shield  40  before flowing through the holes  67 , preferably to provide cooling to the upstream face of the heat shield. The remaining portion of the air received within circular ridge  66   a  flows through grooves  63  to cool the back face of the heat shield radially outwardly therefrom, before being discharged radially at the outer lip  43 . 
   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 department from the scope of the invention disclosed. For example, the invention can be provided in any suitable heat shield configuration and in any suitable combustor configuration, and is not limited to application in turbofan engines. Still other 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.