Abstract:
The combustor has a laser ignitor mounted to the casing, remotely from the liner of the combustion chamber. The laser ignitor has an igniter beam path for igniting the fuel and air mixture in the combustion chamber, the igniter beam path extending at least partially across the air plenum surrounding the liner and into the combustion chamber through a corresponding beam path aperture provided in the liner.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to gas turbine engines and, more particularly, to combustors therefore which have laser igniters. 
       BACKGROUND OF THE ART 
       [0002]    Spark plugs have been the traditional means to ignite the fuel/air mixture inside combustion chambers of gas turbine engines for many decades. The spark plugs were housed in struts extending across the compressed air plenum, between the combustor casing and the combustion chamber liner. The liner is subjected to extreme high temperatures and associated thermal growth whereas the casing is subjected to relatively cool compressed air and therefore to much less thermal growth. The end of the strut mounted to the liner needed to accommodate the relative thermal growth of the liner, which was achieved by a floating collar but nonetheless produced fretting and required regular maintenance. 
         [0003]    In more recent years, spark plugs have been replaced by laser igniters. The laser igniters were at least partially housed in the strut and typically had a focusing lens, or window, exposed to soot from the combustion chamber. Henceforth, although the laser igniters procured some advantages over spark plugs, they continued to require maintenance required from the fretting of the strut and exposure of optical components to soot. Accordingly, there remained room for improvement in addressing the level of maintenance required from the fretting and exposure to soot. 
       SUMMARY 
       [0004]    In one aspect, there is provided a combustor for a gas turbine engine, the combustor comprising: a casing forming a pressure vessel and having an inlet for receiving compressed air; a liner held inside the casing and delimiting a subchamber of the pressure vessel from a plenum extending between the liner and the casing, the liner having a plurality of apertures formed therethrough and allowing controlled fluid flow communication of the compressed air from the plenum into the subchamber, with at least one of the plurality of apertures being a beam path aperture, and an outlet leading to a turbine stage of the gas turbine engine; at least one fuel nozzle mounted to the liner, for injecting fuel into the subchamber; and at least one laser ignitor mounted to the casing, remotely from the liner, the at least one laser ignitor having an igniter beam path for igniting the fuel and air mixture in the subchamber, the igniter beam path extending at least partially across the plenum and into the subchamber through a corresponding one of the at least one beam path apertures in the liner. 
         [0005]    In a second aspect, there is provided a gas turbine engine comprising: a compressor section for compressing incoming air, a combustor having a casing forming a pressure vessel and having an inlet for receiving compressed air from the compressor section; a liner held inside the casing and delimiting a subchamber of the pressure vessel from a plenum extending between the liner and the casing, the liner having a plurality of apertures formed therethrough and allowing controlled fluid flow communication of the compressed air from the plenum into the subchamber, with at least one of the plurality of apertures being a beam path aperture, and an outlet; at least one fuel nozzle mounted to the liner, for injecting fuel into the subchamber; at least one laser ignitor mounted to the casing, remotely from the liner, the at least one laser ignitor having an igniter beam path for igniting the fuel and air mixture in the subchamber and generating a hot stream of combustion gasses through the outlet, the igniter beam path extending at least partially across the plenum and into the subchamber through a corresponding one of the at least one beam path apertures in the liner; and a turbine section for extracting energy from the hot stream of combustion gasses, the turbine section being in fluid flow communication with the outlet of the combustor. 
         [0006]    In a third aspect, there is provided a combustion chamber for a gas turbine engine, the combustion chamber comprising: a casing forming a pressure vessel and having an inlet for receiving compressed air; a liner held inside the casing and delimiting a subchamber of the pressure vessel from a plenum extending between the liner and the casing, the liner having a plurality of apertures formed therethrough and allowing controlled fluid flow communication of the compressed air from the plenum into the subchamber, with at least one of the plurality of apertures being a beam path aperture, and an outlet leading to a turbine stage of the gas turbine engine; at least one fuel nozzle mounted to the liner, for injecting fuel into the subchamber; at least one laser ignitor window made integral to the casing, at least one laser igniter emitter located externally from the laser ignitor window for generating an igniter laser beam along an igniter beam path, the igniter beam path extending across a corresponding one of the at least one laser ignitor window, at least partially across the plenum, and into the subchamber through a corresponding one of the at least one beam path apertures in the liner, the laser beam path having at least one focussed energy kernel inside the subchamber for igniting the fuel and air mixture. 
         [0007]    Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference is now made to the accompanying figures, in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
           [0010]      FIG. 2  is a cross-sectional view of a combustor of a gas turbine engine. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      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. The fan  12 , compressor  14 , combustor  16  and turbine section  18  are generally annular around a main axis  11  of the gas turbine engine  10 . 
         [0012]    The enlarged view provided in  FIG. 2  shows a combustor  16  in greater detail. More particularly, as can be seen in the figure, the combustor  16  generally includes a case or casing  20  forming an outer shell and serving as a pressure vessel to withstand the important pressure difference which can exist between the compressed air and the environment. To this end, the casing  20  is engineered to be highly leak proof. In the case of the illustrated gas turbine engine  10 , the casing  20  has an annular inlet  22  at the front thereof and through which high-pressure compressed air from the compressor  14  is received and diffused. 
         [0013]    The combustor  16  also includes a liner  24  which forms a subchamber  26  of the pressure vessel commonly referred to as the combustion chamber. The volume between the liner  24  and the casing  20 , where relatively cool compressed air circulates, can be referred to as the plenum  28 . In this embodiment, the liner  24  has an annular aperture at the rear forming an outlet  30  of the combustor  16  leading to the turbine section  18 . A plurality of circumferentially interspaced fuel nozzles  32  are mounted to the liner  24  to inject fuel into the combustion chamber. In this embodiment, the fuel nozzles  32  are mounted to an end of the liner  24  opposite the outlet  30 . The fuel nozzles  32  are connected by a fuel line which extends inside a strut  34  having one end connected to the fuel nozzle  32  and the other end mounted to the casing  20  in a sealed manner. The fuel line strut  34  is exposed to the compressed air which is significantly cooler than the combustion gasses inside the combustion chamber. 
         [0014]    The liner  24  can have a large quantity of apertures formed therethrough and interspaced across its surface, allowing the controlled mass transfer of compressed air between the plenum  28  and the subchamber  26 . The control of the compressed air transfer is an elaborate science which aims a plurality of objectives, and any change in the configuration of apertures can have important effects on the dynamics of combustion. One objective is to achieve a high efficiency of combustion. Another objective is to control the temperature of the liner  24 . Accordingly, on many types of gas turbine engines, the liners use a plurality of small apertures interspaced from one another across the surface of the liner, which serve both to allow a regular feed of compressed air to the combustion and to cool their immediate viscinity by way of the flow of cooler air. In the illustrated embodiment, these small apertures can have a diameter in the order of 0.030″ or 0.040″ for instance. 
         [0015]    Other types of apertures are used as well in the illustrated liner. For instance, conveniently positioned apertures having a diameter in the order of 0.100″ can be used as nozzle cooling apertures. Moreover, in this specific embodiment, dilution apertures  36  having a size of 0.5″ or more are used, one set of which being positioned on the radially outer wall  38  of the liner, about midway between the fuel nozzle  32  and the outlet  30 . 
         [0016]    In this example, a laser igniter  40  is mounted to the radially-outer wall  42  of the casing  20 , remotely from the combustion chamber liner  24 . The laser igniter  40  is not directly mounted to the combustion chamber liner  24 , but held at a relative position and orientation therefrom by the casing  20 . In the depicted configuration, the laser igniter  40  has a laser emitter (not shown) which directs a laser beam through a focusing lens which is located at an end  43  of a neck portion  44  of the laser igniter  40 . The focusing lens is thus exposed to the relatively cool compressed air circulating in the plenum  28 , and is virtually unexposed to the harsh combustion conditions occurring in the subchamber  26  during operation. In this embodiment, the end  43  of the neck is located in the plenum  28 . In alternate embodiments, the end  43  of the neck can be mounted flush with the casing  20  or even recessed therein, for instance. 
         [0017]    The laser igniter directs a laser beam along a beam path  46  which penetrates into the subchamber  26  via a beam path aperture  48  provided in the liner  24 . In this embodiment, one of the dilution holes  36  is used as the beam path aperture  48 . The beam path  46  is adjusted and oriented in a manner that laser energy kernels  50  of the focused laser beam path  46  occur at useful locations inside the subchamber  26  for efficient ignition. The fine tuning concerning the exact orientation of the beam path  46  and the position of the laser energy kernels  50  can be done by persons of ordinary skill in the art in the light of this disclosure. 
         [0018]    The aperture or hole in the liner  24  through which the laser beam path  46  extends is referred to herein as the beam path aperture  48 . In alternate embodiments, the position and size of the beam path aperture  48  can vary, and the location and specifics of the laser ignitor  40  can vary as well. For instance, another type of existing aperture than a dilution hole  36  could be used as the beam path aperture, or the liner could be designed with a specific aperture intended to be used as a beam path aperture and also designed with the effect upon combustion dynamics in mind. The size of the beam path aperture can be selected to be sufficiently big to maintain alignment with the laser beam path as the relative position, orientation and size of the beam path apertures varies due to thermal growth of the liner. In the embodiment illustrated and described herein, a aperture having a length of 0.5″ (e.g. a surface area of 0.2 square inches, preferably above 0.5 square inches) along the cross-section of the liner was considered to be a practical minimum to allow to maintain this alignment during normal thermal growth variations. 
         [0019]    Positioning the laser igniter remotely from the liner and orienting the laser beam path through an appropriate aperture in the liner eliminate the needs for a floating connection. Also, it avoids exposing the focusing lens, window, or other optics of the laser igniter to combustion soot which is produced in the subchamber  26  during combustion. The instant disclosure and the attached figures are intended to show one preferred example of how this can be achieved, though it will be understood that the exact configuration can vary in alternate embodiments. In the illustrated embodiment, for instance, the laser igniter has a neck  44  which protrudes into the plenum  24 . It will be understood that the distance  52  between the end  43  of the neck  44  and the liner can vary in alternate embodiments. One consideration which is satisfied in the illustrated embodiment is to maintain the leak-proof characteristics of the casing at the location where the laser igniter  40  is mounted to the casing. In the illustrated embodiment, the focusing lens provided inside the neck  44  of the laser igniter also serves as a leak-proof (pressuretight) and optically transparent seal, which can be referred to herein as a window, which prevents the compressed air from leaking into the laser igniter via the neck. In an alternate embodiment, for instance, the laser emitter of the laser igniter can be held at a predetermined location externally from the casing, and the casing radially outer wall can simply include a leak-proof window across which the laser beam path can extend to penetrate into the beam path aperture and into the combustion chamber. The window can have focusing properties, or the focusing lens can be provided separately from the window, for instance. The window is the last optical component which the light traverses prior to exiting the laser igniter and reaching the plenum. Still alternately, the leak-proof or pressure tight characteristics of the laser igniter can be provided by a leak-proof housing which houses the focusing optics, for instance. Other optical components can be added to orient and/or modulate the laser beam path as desired. 
         [0020]    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. For example, the remote configuration of the laser ignitor relative to the liner described herein and illustrated can be adapted to other types of gas turbine engines than a turbofan such as illustrated herein. 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 scope of the appended claims.