Patent Publication Number: US-11391213-B2

Title: Igniter for gas turbine engine

Description:
TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to igniters used for gas turbine engines. 
     BACKGROUND OF THE ART 
     Spark plugs are commonly used to ignite a mixture of air and fuel in a combustor of gas turbine engines. However, spark plugs for example have drawbacks. For instance, the spark plugs have been known to achieve less than full reliability in conditions such as when wet by exposure to condensation or washing fluid, or when the fuel and the engine are very cold. Cost is also a factor. There is always room for improvement. 
     SUMMARY 
     In one aspect, there is provided a method of servicing a gas turbine engine having a spark plug igniter connected to a spark plug igniter socket of the engine, the method comprising: removing the spark plug from the spark plug igniter socket of the gas turbine engine; inserting a glow plug igniter having a base configured for matingly engaging the spark plug igniter socket; and securing the glow plug igniter to the gas turbine engine. 
     In another aspect, there is provided an assembly configured to allow using a glow plug in a spark plug socket of a gas turbine engine, the spark plug socket defining socket threads, the assembly comprising: an adaptor including an adaptor body having a threaded aperture, an external portion of the adaptor body having external threads configured to threadingly engage the socket threads; and a glow plug having a base, the base having a threaded portion threadingly engaging the threaded aperture of the adaptor body, the glow plug having a heater rod extending along an axis away from the base. 
     In yet another aspect, there is provided a gas turbine engine comprising: a casing, a combustor liner provided inside the casing and enclosing a combustion chamber, a spark plug socket including a casing aperture defined through the casing and a liner aperture defined through the combustor liner, and an igniter received in the spark plug socket, the igniter having a base connected to the casing, a glow plug heater rod, the glow plug heater rod extending from the base along an axis and terminating in a rod end, the rod end exposed to the combustion chamber. 
     In still yet another aspect, there is provided a method of performing maintenance to a gas turbine engine having a casing and a combustion chamber liner, the combustion chamber liner enclosing a combustion chamber, and an igniter socket extending across the casing and the combustion chamber liner, the method comprising: securing a glow plug heater rod to the gas turbine engine with the glow plug heater rod extending into the igniter socket; and securing a flow impeding member to the gas turbine engine in a manner that the flow impeding member extends around the heater rod and penetrates into the combustion chamber liner. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures in which: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine; 
         FIG. 2  is an oblique view of an igniter in accordance with one embodiment; 
         FIG. 3  is a cross-sectional view of the igniter of  FIG. 2 , taken along a longitudinally-oriented plane, shown in an environment of use; 
         FIG. 4  is a cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 5  is a bottom view of the igniter of  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 7  is a bottom view of the igniter of  FIG. 6 ; 
         FIG. 8  is a view of an igniter in accordance with another embodiment; 
         FIG. 9  is a view of an igniter in accordance with another embodiment; 
         FIG. 10  is a schematic tridimensional view of an igniter in accordance with another embodiment; 
         FIG. 11  is a of an igniter in accordance with another embodiment; 
         FIG. 12  is an oblique view of an igniter in accordance with another embodiment; 
         FIG. 13  is an oblique view of an igniter in accordance with another embodiment; 
         FIG. 14  is a partial, schematic, cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 15  is a partial, schematic, cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 16  is a partial, schematic cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 17  is an oblique view of an igniter in accordance with another embodiment; 
         FIG. 18  is an oblique view of a swirler which can be used as part of the igniter of  FIG. 17 ; 
         FIG. 19  is a schematic cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 20  is a schematic cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 21  is a schematic cross-sectional view of an igniter in accordance with another embodiment; 
         FIG. 22  is a schematic bottom view of an igniter in accordance with another embodiment; 
         FIG. 22 a    is a schematic cross-sectional view of the igniter of  FIG. 22 ; 
         FIG. 23  is a schematic bottom view of an igniter in accordance with another embodiment; 
         FIG. 23 a    is a schematic cross-sectional view of the igniter of  FIG. 23 ; 
         FIG. 24  is a partial, schematic cross-sectional view of a heater rod of a glow plug in accordance with another embodiment; 
         FIG. 25  is a partial, schematic, cross-sectional view of an igniter in accordance with another embodiment; and 
         FIG. 26  is a partial, schematic cross-sectional view of a variation of the igniter of  FIG. 25 . 
     
    
    
     DETAILED DESCRIPTION 
       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 compressor section  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 , the compressor section  14 , and the turbine section  18  rotate about a central axis  11 . 
     In this embodiment, the gas turbine engine includes an engine casing  25  that is disposed radially outwardly of the combustor  16  relative to the central axis  11 . The combustor  16  has a combustor liner  16   a  that encloses a combustion chamber  16   b . The combustor liner  16   a  can form part of the engine casing  25  and not rotate with the rotors. The combustor liner  16   a  defines at least one igniter liner aperture  16   c  for receiving at least one igniter  20  ( FIG. 2 ), which is used for igniting a mixture of compressed air from the compressor section  14  and fuel injected by fuel injectors  22 . 
     The engine casing  25  can also have one or more igniter apertures  25   a , configured to receive the igniter  20  therein. As shown in greater detail in the example presented in  FIG. 3 , the igniter aperture  25   a  can be aligned with the igniter liner aperture  16   c  of the combustor liner  16   a . In one embodiment, an axis of the igniter aperture  25   a  is coincident with an axis of the igniter liner aperture  16   c . The igniter aperture  25   a  and the igniter liner aperture  16   c  can be said to form, collectively, an igniter socket. This alignment can allow an igniter to be received by both of the igniter liner aperture  16   c  and the igniter aperture  25   a  to reach the combustion chamber  16   b  of the combustor  16 . In gas turbine engine  10 , the igniter socket has a female thread designed to receive a mating male thread of a spark plug. Accordingly, a peripheral wall  25   b  of the igniter aperture  25   a  may include a threads  25   c  to be engaged by the igniter  20 . 
     As will be described in greater detail below, a glow plug can be used to ignite fuel in a gas turbine engine and in some cases, such glow-plug-based igniters can have advantages over spark plugs. The following paragraphs present various embodiments of glow-plug-based igniters for use in a gas turbine engine setting, and a discussion about various elements which may have to be taken into consideration when retrofitting a glow-plug-based igniter into a spark plug aperture of a gas turbine engine. 
     More specifically some igniters having a heater in the form of an exposed coil shaped resistors have been referred to as “glow plugs” in the past. In newer glow plug designs, the heater is typically encapsulated in a protective shell and the resulting assembly is referred to as a heater rod. Spark plugs ignite a mixture of air and fuel by generating a spark whereas glow plugs ignite such a mixture of air and fuel by having a tip section heated at a temperature above the fuel ignition temperature. At such elevated temperatures, the tip section “glows”, which led to the use of the familiar expression “glow plug”. A typical heater rod can have a surface made of non-oxidizing material that can withstand temperatures above 1000 degrees Celsius, which can impede carbon formation. 
     In the embodiment illustrated, the gas turbine engine  10  was initially designed for using a specific model of spark plugs as the igniters. The igniter presented in  FIG. 2  includes a heater rod, and was designed to be used instead of the spark plugs. 
     There can be dimensional issues to consider in using a heater rod-based igniter in a spark plug socket. Indeed, off-the-shelf glow plugs can be smaller in diameter than the spark plugs designed for the specific gas turbine engine  10 . It can be desired to control the depth of the heating tip of the heater rod. Moreover, it may be desired to provide the igniter with some additional feature or features, which may not be integrated with the off the shelf glow plugs, to allow them to be better suited for the gas turbine engine environment. 
     One avenue is to design glow plugs specifically for the intended use and context, which can include providing a body having suitable features which an off-the shelf glow-plug does not have. Another alternative is to design an adapter to a) fit a spark plug aperture, b) receive an off-the shelf glow plug and c) provide any additional feature useful in adapting the off-the-shelf glow plug to the specific gas turbine engine environment. In several of the embodiments described and illustrated, the avenue of using an igniter consisting of an off-the-shelf glow plug+adapter was preferred over the avenue of providing a new glow plug design, mostly because it was easier to design an adapter for an existing glow plug design than to design and producing a specific, new glow plug design specifically adapted to the application. Nonetheless, specific glow plug designs used without adapters can be preferred in some embodiments, and will be discussed further below. 
     In the embodiment shown in  FIG. 3 , the igniter liner aperture  16   c  and the igniter aperture  25   a  correspond to a spark plug socket PS configured for receiving therein a spark plug. A flow F′ of compressed air exits the compressor section  14  and is fed in an annular spacing S 1 , also referred to as a compressed gas passage, defined radially between the combustor liner  16   a  and the engine casing  25  relative to the central axis  11 . The combustor liner  16   a  can have a plurality of apertures (not shown) configured for allowing the flow F′ of compressed air to enter the combustion chamber  16   b  to be mixed with fuel and ignited. 
     Referring to  FIGS. 2-3 , the igniter  20  includes a glow plug that is generally shown at  24 . The disclosed glow plug  24  is of a “pencil-type”, and has a heater rod having a protective shell encapsulating a heater. The heater can be a coiled resistive element. As shown, the glow plug  24  extends along an axis A and has a body  24   a  and a heater rod  24   b  protruding from the body  24   a  along the axis A. As shown, the heater rod  24   b  extends across the spacing S 1 . In the depicted embodiment, the heater rod  24   b  has a ceramic portion which includes the “heater portion” of the glow plug  24  that becomes hot and “glows” to ignite the mixture of air and fuel. Various types of heater rods exist, in some types of heater rods, the heater rod  24   b  of the glow plug  24  may be made of a metallic material, in some other types, the glow plug heater rod  24   b  may be covered by a metallic material, such as Inconel™. The glow plug body  24   a  includes a main section  24   c  and an intermediate section  24   d . The intermediate section  24   d  is located axially between the main section  24   c  and the heater rod  24   b  relative to the axis A. A metal to ceramic junction J may be located at an intersection of the intermediate section  24   d  and the heater rod  24   b . As shown, a diameter of the intermediate section  24   d  is less than that of the main section  24   c  and greater than a diameter of the heater rod  24   b . The glow plug  24  includes connection means  24   e , which can include a positive and a negative connection terminals, configured to be electrically connected to a power source. The body  24   a  of the glow plug  24  includes a threaded portion  24   f  for securing the glow plug  24  to a structural element of the gas turbine engine  10  that may be, for instance, the threads  25   c  of the peripheral wall  25   b  of the igniter aperture  25   a  of the engine casing  25 . 
     The glow plug  24  has a heater located inside the heater rod  24   b  for heating the heater rod  24 . In the embodiment shown, the heater is a heating coil  24   i . Ceramic powder may be provided around the coil  24   i  to fill a gap between the coil  24   i  and an external shell of the heater rod  24   b . The heater rod  24   b  has a tip section  24   h  that extends from an end  24   g  of the heater rod toward the body  24   a  of the glow plug  24 . A length of the tip section  24   h  is less than that of the heater rod  24   b . The tip section  24   h  is also referred to as a heating section of the glow plug  24  as it is that section that “glows” for igniting the mixture of air and fuel. 
     The heating section  24   h  is typically the portion of the heater rod  24   b  that reaches temperature above 400 degrees Celsius in operation. The heating section  24   h  may reach a temperature of about 1100 degrees Celsius along a length of about 2 mm extending from the end  24   g  of the heater rod  24   b , and can be said to extend axially along a portion of the length of the heater rod, between the axial positions of the two opposite ends of the heater. 
     The combustor liner  16   a  has a collar  16   d  which surrounds a whole periphery/circumference of the igniter liner aperture  16   c . The collar  16   d  and the combustor liner  16   a  may be monolithic, e.g. via machining from a single component, or otherwise integral to one another, e.g. via soldering. The collar  16   d  extends from the combustor liner  16   a  toward the engine casing  25  and within an annular spacing S defined therebetween. The collar  16   d  is configured for receiving a portion of the igniter  20   
     Still referring to  FIGS. 2 and 3 , in this embodiment, the igniter  20  includes an off-the-shelf glow plug  24  and a gas turbine engine adaptor  30 , referred to hereinbelow simply as an adaptor  30 . As discussed above, a primary feature of the adaptor  30  can be to allow to fit an off-the-shelf glow plug to the female threads which were designed for a spark plug. However, the adaptor can also have one or more features providing additional functionality. 
     Indeed, in  FIGS. 2 and 3 , for instance, the adaptor  30  is also used to fill a gap that might otherwise exist between the peripheral wall  25   b  of the igniter aperture  25   a  of the engine casing  25  and the glow plug  24 . A sealing engagement may be created between the adaptor  30 , more specifically its outer surface  30   f  where it is threaded, and the igniter aperture  25   a  that may be correspondingly threaded. 
     In this embodiment, the adaptor  30  is hollow and defines a cavity  30   a  or socket for receiving therein the glow plug  24 . An inner surface  30   b  of the adaptor  30  may have a threaded portion  30   c  configured for being engaged by threads of the correspondingly threaded portion  24   f  of the body  24   a  of the glow plug  24 . Other means of securing the adaptor  30  to the body  24   a  of the glow plug  24  may be used without departing from the scope of the present disclosure. In a particular embodiment, a glow plug may have a body that is tailored to the igniter aperture  25   a  of the engine casing  25  or combustor liner  16   d  and may not require the adaptor  30 . Stated otherwise, the glow plug  24  and the adaptor  30  may be monolithic. 
     In the embodiment shown, the inner surface  30   b  of the adaptor  30  defines a constriction  30   h  that creates an abutment surface configured to be in contact with an end of the main section  24   c  of the body  24   a  of the glow plug  24 . The abutment of the glow plug against the inner surface  30   b  of the adaptor and at the constriction  30   h  may limit movements relative to the adaptor  30  of the glow plug  24  along its axis A and toward the combustion chamber  16   b.    
     In the embodiment shown, the adaptor  30  has a first section, also referred to as a casing portion,  30   d  and a second section  30   e  that are both annular and define portions of the adaptor conduit  30   a  for receiving the glow plug  24 . The second section  30   e  includes a securing mechanism, or assembly, SM configured to be matingly connected to the casing  25  of the gas turbine engine  10 . At an outer surface  30   f  of the adaptor  30 , the first section  30   d  has a diameter greater than that of the second section  30   e  to create an abutment surface  30   g  that contacts an outer side of the engine casing  25 . In other words, the abutment surface  30   g  is defined by a shoulder  30   g ′ of the adaptor  30 . The second section  30   e  extends axially relative to the axis A from the first section  30   d  toward an end of the adaptor  30  and is configured to be received within the igniter aperture  25   a  defined through the engine casing  25  and through the igniter liner aperture  16   c  of the combustor liner  16   a . More specifically, the second section  30   e  of the adaptor  30  has a threaded section  30   l  that is configured to engage the threaded section  25   c  of the peripheral wall  25   b  of the igniter aperture  25   a.    
     The portion of the igniter which is designed to be secured to the igniter aperture  25   a  can be referred to as the base, independently of whether the igniter is a specific, integral, glow plug design or of the “off-the-shelf glowplug”+adaptor type. 
     In the embodiment shown, rings also referred to as spacers  29  are located between the abutment surface  30   g  of the base and the engine casing  25  for adjusting a depth of penetration of the adaptor in the spacing S 1 . In other words, either a thickness of the rings  29  along the axis A and/or a number of the rings  29  may be varied to change the depth of penetration of the adaptor, and hence of the end  24   g  of the heater rod  24   b  within the combustion chamber. These spacers  29  were used for experimental purposes, allowing to easily test different depths of the igniter into the combustion chamber, and will likely be omitted from an industrial production of igniters (or adaptors), the industrial production being specifically designed to have an optimal distance between the shoulder engagement and the heater rod tip. 
     A challenge can arise in relation to the amount of fuel which will be exposed to the heating section of the heater rod, with the objective of reaching auto-ignition, and flame sustenance conditions. 
     Another challenge can stem from the body  24   a  of the glow plug  24 , more specifically its inter mediate section  24   d  (which can be a metal shell portion leading to a ceramic shell portion for instance), being less tolerant to high temperatures than the tip, and may need to be kept to a temperature that is substantially below the tip temperature and below a temperature inside the combustion chamber  16   b  of the combustor  16 . 
     Referring to  FIG. 3 , the igniter includes a sleeve  32 . The sleeve  32  may be connected to the base B independently of a structure of the gas turbine engine and protruding along the axis A from the base B toward the rod end  24   g . The sleeve  32  is disposed around the glow plug  24 . The sleeve member  32  has an outer surface  32   a  that faces away from the glow plug  24  and an inner surface  32   b  that faces the glow plug  24 . In the embodiment shown, the sleeve  32  is connected to an adaptor  30 . More specifically, the second section  30   e  of the adaptor  30  defines a mechanism  30   j  for holding the sleeve  32 . In the depicted embodiment, the mechanism  30   j  includes a threaded section  30   k  of the adaptor  30 . The sleeve outer surface  32   a  defines a threaded section  32   d  that is configured to cooperate with the threaded section  30   k  of the adaptor  30  to limit axial movements of the sleeve  32  with respect to the adaptor  30  relative to the axis A. This was provided for test purposes, to allow to easily test various sleeve designs, and in an industrial production, the sleeve can be integral to the base, for instance. In a particular embodiment, the sleeve  32  is axisymmetric around the axis A, and extends along a full circumference of the heater rod, but in alternate embodiments, the sleeve may extend only partially circumferentially around the heater rod or may intermittently extend circumferentially around the heater rod (e.g. be crenellated), for instance. 
     The sleeve can offer one or more of the following additional functionalities: 
     a) shielding the heater rod from the circulation of compressed gas between the combustion chamber liner and the gas turbine engine casing, 
     b) forming a constriction in a gap between the sleeve and the heater rod to impede combustion heat from accessing the metal-to-ceramic joint J, 
     c) fully or partially occupying a gap between which could otherwise be present between the combustion chamber liner and the igniter, and thereby impeding flow of compressed gas directly across the combustion liner aperture which could otherwise be detrimental to ignition or flame sustenance conditions, 
     to name a few examples. 
     As discussed above, the structure which is used to provide such additional functionality to the heater rod can be structurally connected to the base of the igniter, i.e. the portion of the igniter which is secured to the engine casing. To this end, the structure can be a) i) integrated to an adaptor designed to receive an off-the-shelf glow plug, or ii) be included in the design of a new glow plug design specifically adapted to these conditions, in which case it can be integral to the glow plug body. However, it will be understood that alternately such structure can b) form part of an adaptor member, distinct from the glow plug itself, which is designed to be secured to the combustion chamber liner, for instance, or even c) secured to the heater rod, such as by soldering or any suitable alternate form of securing. It will be understood that other structures providing other possibilities of added functionalities are described below, and that such other structures can be integrated to the igniter in accordance with either one of the options a)i), a)ii), b) or c) above. 
     In the embodiment shown, the sleeve  32  provides the added functionality of defining a fuel receiver R that can have a surface designed to be wetted by fuel, in a manner to favor ignition of the by the heater rod  24   b . As illustrated in  FIG. 3 , the sleeve also forms an annular spacing, or annular gap G that circumferentially extends a full circumference around the heater rod  24   b . The annular spacing can be designed in a manner to provide a pocket area of gaseous fuel and air mixture around the hot tip of the glow plug where the gas velocity is limited, to favor ignition and flame sustenance conditions. 
     In a particular embodiment, the sleeve  32  may be configured to protect the heater rod  24   b  from the hot compressed air that circulates in the spacing S 1  ( FIG. 1 ) and may limit fluid communication from the combustion chamber  16   b  enclosed by the combustor liner  16   c  toward the base B of the igniter  20 . In the embodiment shown, the sleeve  32  has a substantially cylindrical shape. It is understood that other shapes are contemplated. In a particular embodiment, the sleeve can form a constriction, axially upward from the annular spacing forming the pocket area, to protect the plug intermediate section  24   d  from excessive temperature due to elevated air temperatures in the spacing S 1  immediately after engine shutdown when there is no airflow through the engine. 
     In a particular embodiment, the fuel receiver R is an open cell structure, such as a porous media. In a particular embodiment, a distance along the axis between the rod end and the fuel receiver portion located closest to the rod end is less or equal to two times a length of the heating section. In a particular embodiment, the distance is less or equal to one and a half times the length of the heating section, preferably corresponds to the length of the heating section, preferably to about half the lend of the heating section, preferably to about a quarter of the length of the heating section. Such other embodiments are discussed further below. 
     In the embodiment shown, a threaded insert  132  is received in a cavity  30   i  defined within the base. As shown in  FIG. 3 , once this threaded insert is received within the cavity  30   i , a portion of the cavity  30   i  remains free of the threaded insert. 
     In a particular embodiment, the cavity  30   i  collects liquid fuel and fuel mist, such that when ignition occurs, said fuel is vaporized and is thereby pushed away from the sleeve  32  and adaptor  30  towards an area of combustion in the combustion chamber  16   b . This might lead to increased combustion near the ignitor  20  and potentially resulting in a jet of flame away from the ignitor  20  and towards the combustion chamber  16   b  and a spray generated by the fuel injectors  22  ( FIG. 1 ). 
     The sleeve  32  may be slidingly received within the collar  16   d . An external diameter of the sleeve  32  may be configured to correspond to an internal diameter of the collar  16   d  to limit the flow F′ of compressed air from entering the combustion chamber  16   b  via a gap between the sleeve  32  and the collar  16   d . Stated otherwise, the outer surface  32   a  of the sleeve  32  may be in abutment with an inner surface of the collar  16   d . The sleeve  32  is discussed in more detail below. 
     In the embodiment shown, the sleeve  32  is slidingly received within the collar  16   d  of the combustor liner  16   a . The outer surface  32   a  of the sleeve  32  may be in abutment against an inner surface of the collar  16   d . The engagement of the sleeve  32  and the collar  16   d  may be a sealing engagement that might impede fluid flow communication between the combustion chamber  16   b  and the spacing S 1  via the collar  16   d . The sealing engagement might avoid the combustion gases to leak from the combustion chamber  16   b  toward the spacing S 1  between the engine casing  25  and the combustor liner  16   a.    
     The sleeve  32  forms a radial, or annular gap G that circumferentially extends around a full circumference of the heater rod  24   b  in this embodiment, and more particularly around the tip section  24   h  of the glow plug  24 . The radial gap G extends radially from the heater rod  24   b  to at most the outer surface  32   a  of the sleeve  32  relative to the axis A. In a particular embodiment, a depth of the radial gap G taken along the axis A varies from zero to seven times a diameter of the heater rod  24   b  of the glow plug  24 . In a particular embodiment, the depth of the radial gap G taken along the axis A is equal to about a length of a portion of the heater rod  24   b  that is heated. In a particular embodiment, a value of the length of the portion of the heater rod  24   b  that is heated is approximately equal to a value of the diameter of the heater rod  24   b.    
     In the embodiment shown, a flow circulation area F is located near the heater rod  24   b . The end  24   g  of the heater rod  24   b  is positioned in the flow circulation area F. In the embodiment shown, the flow circulation area F is defined in part by the sleeve  32 . A constricted area C extends radially relative to the axis A between the sleeve inner surface  32   b  and the heater rod  24   b . The constricted area C is located axially between the flow circulation area F and the body  24   a  of the plug  24 . The constricted area C is designed to have a smaller transversal cross-sectional area than the cross-sectional area of the flow circulation area F. The transversal cross-sectional area is taken on a plane normal to the glow plug axis A. The annular gap G is fluidly connected to the fluid flow circulating area F. 
     In the embodiment shown, an axial position, relative to the axis A, of an end  24   g  of the glow plug heater rod  24   b  corresponds to that of a distal end  32   c  of the sleeve  32  that is located inside the chamber  16   b . In the embodiment shown, the cross-sectional area of the conduit decreases from a proximal end of the sleeve  432  to the constriction C. 
     In the depicted embodiment, the constricted area C is an annular gap that circumferentially and continuously extends all around the glow plug heater rod  24   b  in this embodiment. In the embodiment shown, the constricted area is axially offset from the tip section  24   h  of the heater rod  24   b  relative to the axis A of the glow plug  24 . As shown, the sleeve inner surface  32   b  at both of the flow circulation area F and the constricted area C is cylindrical and a diameter of the sleeve inner surface  32   b  at the flow circulation area F is greater than that at the constricted area C. In the embodiment shown, a diameter of the heater rod  24   b  of the glow plug  24  is slightly less than that of the inner surface  32   b  of the glow plug  24  at the constricted area C to allow for manufacturing tolerances of the diameter D 1  of the heater rod  24   b  of the glow plug  24  and other tolerances, which can be added together, such as concentricity of a surface of the heater rod  24   b  of the glow plug  24  relative to the axis A, concentricity of the constriction  30   h  relative to the inner threaded portion of the adaptor  30 . 
     In a particular embodiment, the sleeve  32  allows a sufficient quantity of the mixture of air and fuel to enter the flow circulation area F to be ignited and, at the same time, protect the glow plug body  24   d  from the very hot combustion gases within the combustion chamber. In a particular embodiment, the constricted area C impedes the hot combustion gases to flow toward the body  24   a  of the glow plug  24  thereby protecting the body  24   a  of the glow plug  24  against these gases. More specifically, and in accordance with a particular embodiment, the constricted area C reduces the heat transferred to the glow plug body when air temperature is very high due to flames or heat soak-back effects after start abort or shutdown. 
     In the embodiment shown, the end  24   g  of the heater rod  24   b  is axially aligned, relative to the axis A, with the igniter liner aperture  16   c . Stated otherwise, the end  24   g  is intersected by a projection of the combustor liner  16   a.    
     Still referring to  FIG. 3 , the igniter includes a metal portion M that is located closes to the heating section  24   h  of the heater rod  24   b . In the embodiment shown, the metal portion corresponds to the inner surface  32   b  of the sleeve  32  at the constricted area C. As shown, the heating section  24   h  protrudes beyond the metal portion. In other words, an entirety of the heating section is axially spaced apart from the metal portion. In a particular embodiment, a distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to two times a length along the axis A of the heating section  24   h . In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to one and a half time the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion corresponds to the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is about half the length of the heating section  24   h  taken along the axis A. 
     In the embodiment shown, the end  24   g  of the heater rod  24   b  is located inside the combustion chamber  16   b . That is, the end  24   g  traverses a projection  16   a ′ of the combustor liner  16   a ; the projection corresponding to where the combustion liner  16   a  would be if the igniter liner aperture  16   c  were not present. The projection  16   a ′ may be defined by an interpolation of the combustor liner  16   a  to fill the igniter liner aperture  16   c . When seen in a cross-section taken along a plane containing the central axis  11  of the gas turbine engine  10  (as shown on  FIG. 3 ), the projection  16   a ′ follows the peripheral wall of the igniter liner aperture  16   c . In a particular embodiment, the end  24   g  of the heater rod  24   b  is axially aligned relative to the axis A with the projection  16   a′.    
     Many possible embodiments for the sleeve are described herein. Nonetheless, it should be understood that still other variations are possible without departing from the scope of the present disclosure. 
     Referring now to  FIG. 4 , another embodiment of a sleeve is generally shown at  132  with the adaptor  30  and the glow plug  24  that may be identical to those shown in  FIG. 3 . 
     Referring now to  FIGS. 4 and 5 , the sleeve  132  defines fins  132   e  that extend axially from bases  132   f  to free tips  132   g  thereof and radially from the sleeve inner surface  132   b  to the sleeve outer surface  132   a . In the embodiment shown, the fins  132   e  are circumferentially interspaced around the axis A and are at equal distance from one another. The fins  132   e  may be non-uniformly distributed around the axis A such that a distance along a circumferential direction relative to the axis A and between two adjacent ones of the fins  132   e  may vary along the circumference of the sleeve  132 . The constricted area C is located at the bases  132   f  of the fins  132   e . In other words, the flow circulation area F axially starts where the fins  132   e  starts and extends radially from the glow plug heater rod  24   b  to the sleeve outer surface  132   a  via spacing S 2  between each of two consecutive ones of the fins  132   e . The constricted area C as shown is substantially from the heating section  24   h , but it can be located close to or in the heating section  24   h . The fins may extend intermittently around a full circumference of the heater rod  24   b . The sleeve including the fins need not be axisymmetric. In a particular embodiment, the fins form part of the liner portion, also referred to as the collar  16   d.    
     An axisymmetric design can be preferred in the context where the igniter is to be secured to the gas turbine engine by a threaded engagement concentric to the heater rod axis, but in certain cases, such as if the circumferential orientation of the igniter relative to the socket in the gas turbine engine is known, a non-axisymettric design can be preferred to adapt to the specific features of the environment, such as known position of incoming fuel mist, known position of heating air, known local orientation of gravity, etc. Accordingly, in one embodiment, the fins can extend intermittently around a full circumference of the heater rod, whereas in another embodiment, the fins can extend only in one or more portions, e.g. arcs, of the full circumference of the heater rod. 
     The sleeve  132  includes a surface from which the fins  132   e  protrudes; the surface containing the bases  132   f  of the fins  132   e  and extends from the inner surface  132   b  to the outer surface  132   a . In the embodiment shown, the surface from which the fins  132   e  protrude is not flat and is sloped such that a distance from the bases  132   f  of the fins  132   e  to the end  24   g  of the heater rod  24   b  taken along the axis A decreases from the inner surface  132   b  to the outer surface  132   a . The surface from which the fins  132   e  protrude may be alternatively flat. Other configurations are contemplated without departing from the scope of the present disclosure. 
     The sleeve  132  extends partially around the heater rod  24   b  as the fins  132   e  are circumferentially distributed all around the heater rod  24   b . In other words, in this embodiment the sleeve  132  does not continuously extends along a full circumference as gaps are present between two adjacent ones of the fins  132   e.    
     In the embodiment shown in  FIGS. 4 and 5 , the sleeve  132  includes eight fins  132   e . In a particular embodiment, more or less than eight fins may be used. More specifically, twelve fins may be used. In a particular embodiment, a thickness T 1  and a number of fins  132   e , and the space between the fins  132   e  which allows the entry of fuel/air mixture, affect ignition performance. In a particular embodiment, a minimum value for the thickness T 1  of the fins  132   e  is about 0.04 to 0.06. In a particular embodiment, the thickness T 1  of the fins  132   e  is about 0.01 inch, preferably 0.02 inch. 
     The fins  132   e  may be straight as illustrated on  FIG. 4  or may be define an “S-shape” or curved shape in a radial direction and/or an axial direction relative to the axis A. The fins  132   e  may extend in a circumferential direction relative to the axis such that they wrap around the heater rod  24   b . Other configurations are contemplated without departing from the scope of the present disclosure. 
     In a particular embodiment, the fins  132   e , and more particularly the spacing S 2  between the fins  132   e  promote an exposure of the heater rod  24   b  to the mixture of air and fuel. This might help in igniting said mixture. In a particular embodiment, sections of the circumference of the inner surface  132   b  of the sleeve  32  where the fins  132   e  are present amount to at least half of a full circumference of said inner surface  132   b.    
     As shown, the end  24   g  of the glow plug heater rod  24   b  extends axially beyond the sleeve  132  and out of the flow circulation area F. Stated otherwise, the end  24   g  of the heater rod  24   b  extends axially, relative to the axis A, beyond the igniter liner aperture  16   c  and into the combustion chamber  16   b . Stated otherwise, the end  24   g  traverses a projection of the combustor liner  16   a  to reach the combustion chamber. 
     In the embodiment shown, the end  24   g  of the heater rod  24   b  is located inside the combustion chamber  16   b . That is, the end  24   g  traverses a projection  16   a ′ of the combustor liner  16   a ; the projection corresponding to where the combustion liner  16   a  would be if the igniter liner aperture  16   c  were not present. In the embodiment shown, a major portion of the length of the heating section  24   h  of the heater rod  24   b  is located inside the combustion chamber  16   b.    
     In the embodiment shown, the metal portion M that is closest to the heating section  24   h  is the tips  132   g  of the fins  132   e . In a particular embodiment, a distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to two times a length along the axis A of the heating section  24   h . In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to one and a half time the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion corresponds to the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is about half the length of the heating section  24   h  taken along the axis A. 
     Referring to  FIGS. 6 and 7 , another embodiment of a sleeve is generally shown at  232 . More specifically, the sleeve  232  includes fins  232   e . The fins  232   e  include each a first section  232   e   1  that extends from the base  232   f  toward the tip  232   g  and a second section  232   e   2  that extends from the tip  232   g  toward the base  232   f . In the embodiment shown, a length of the second sections  232   e   2  of the fins  232   e  along the axis A is about one and a half times the diameter D 1  of the heater rod  24   b . The second sections  232   e   2  of the fins  232   e  of the sleeve  232  are grouped in groups  232   h  of fins  232   e  that are circumferentially spaced around the axis A. In the embodiment shown, the sleeve  232  includes four groups  232   h  of fins  232   e ; each group  232   h  including three fins  232   e  for a total of twelve fins. Other configurations are contemplated without departing from the scope of the present disclosure. For each of the groups  232   h , the fins  232   e  extend radially from a wall  232   i  to the outer surface  232   a  of the sleeve  232 . The wall  232   i  extends annularly around a portion of a circumference of the sleeve  232 . The flow circulation area extends from the heater rod  24   b  to the outer surface  232   a  via spacing S 2 ′ between each two consecutive ones of the groups  232   h  of fins  232   e.    
     Referring now to  FIG. 8 , another embodiment of a sleeve is generally shown at  332 . The sleeve  332  is similar to the sleeve depicted in  FIG. 6 , but includes twelve fins  332   e  instead of eight. Moreover, the end  24   g  of the glow plug  24  heater rod axially registers with a plane P containing the tips  332   g  of the fins  332   e . In the embodiment shown, the end  24   g  of the heater rod  24   b  is axially aligned, relative to the axis A, with the igniter liner aperture  16   c . Stated otherwise, the end  24   g  is intersected by a projection of the combustor liner  16   a.    
     In the embodiment shown, the metal portion M that is closest to the heating section  24   h  is the tips  332   g  of the fins  332   e . In the embodiment shown, a distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is zero. In a particular embodiment, the distance is less or equal to two times a length along the axis A of the heating section  24   h . In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to one and a half time the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion corresponds to the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is about half the length of the heating section  24   h  taken along the axis A. 
     In the embodiment shown, the end  24   g  of the heater rod  24   b  is axially aligned relative to the axis A with the projection  16   a ′ of the igniter liner aperture  16   a.    
     Referring now to  FIG. 9 , another embodiment of a sleeve is generally shown at  432 . The sleeve inner surface  432   b  at the flow circulation area has a frustoconical shape with a diameter that decreases from its distal end  432   c  to the constricted area C. As shown, the diameter decreases linearly from the distal end  432   c  of the sleeve  432  to the constricted area C. In the embodiment shown, the end  24   g  of the glow plug heater rods  24   b  protrudes axially beyond the sleeve  432 . 
     In the embodiment shown, the inner surface  432   b  of the sleeve  432  defines a Venturi. More specifically, the inner surface  432   b  of the sleeve  432  defines a conduit having a cross-sectional areal taken along a plane normally intersected by the axis A that increases from the constriction C to a distal end  432   c  of the sleeve  432 . 
     It is understood that different combinations of features described herein are possible in alternate embodiments. 
     In a particular embodiment, the constricted area C brings liquid fuel, which is on the inner surface  32   b  of the sleeve  32 , closer to the hot part of the heater rod  24   b , thereby encouraging vaporization, ignition, and further vaporization and combustion. In a particular embodiment, the presence of a small gap and cavity behind the constricted area C collects liquid fuel prior to ignition, which is then vaporized after ignition. This vaporization might push the fuel and vapor towards the hot part of the glow plug heater rod  24   b  where it ignites. This might result in a jet of flame that might help to ignite a fuel spray from the fuel injectors  22 . In a particular embodiment, this results in successful flame propagation and engine light-up. 
     The embodiments described herein include various means of collecting liquid fuel that can then be vaporized, either by the heat of the glow plug itself, or by the heat released following initial ignition. 
     In the embodiment shown, the metal portion M that is closest to the heating section  24   h  is the constriction C defined by the inner surface  432   b  of the sleeve  432 . In a particular embodiment, a distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to two times a length along the axis A of the heating section  24   h . In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to one and a half time the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion corresponds to the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is about half the length of the heating section  24   h  taken along the axis A. 
     Referring now to  FIG. 10 , another embodiment of a sleeve is generally shown at  532 . The sleeve  532  defines an annular plate  534  that faces axially toward the end  24   g  of the heater rod  24   b  that extends circumferentially around the heater rod  24   b  in this embodiment. In the embodiment shown, the sleeve  534  is recessed relative to the combustor liner  16   a  such as to define a fuel collector  536  that is delimited by the annular plate  534  and a peripheral wall  16   c ′ of the igniter liner aperture  16   c . The fuel collector  536  is configured for collecting fuel to be ignited by the heater rod  24   b . The fuel collector  536  may have a frustoconical shape. The fuel collector  536  forming a pool  536 ′ for receiving fuel. The peripheral wall  16   c ′ may extend circumferentially about a full circumference of the heater rod  24   b . The peripheral wall  16   c ′ may have a frustoconical shape. The peripheral wall  16   c ′ may extend axially beyond the rod end  24   g . In a particular embodiment, the peripheral wall may be defined by the adaptor. 
     Referring now to  FIG. 11 , another embodiment of a sleeve is generally shown at  632 . The sleeve  632  corresponds to the sleeve  532  of  FIG. 10  but further includes an annular wall  636  that is connected to a periphery of the annular plate  534 . The annular wall  636  extends circumferentially around the heater rod  24   b  in this embodiment. In the embodiment shown, the fuel collector  638  is delimited by the annular plate  534  and the annular wall  636 . The fuel collector  638  is configured for collecting fuel to be ignited by the heater rod  24   b . In one embodiment, the fuel collector can extend around a full circumference of the heater rod, continuously or intermittently, whereas in other embodiments, the fuel collector can extend only in one or more portions, or angular segment, of the full circumference of the heater rod. 
     Referring now to  FIG. 12 , another embodiment of a sleeve is generally shown at  732 . The sleeve  632  corresponds to the sleeve  532  of  FIG. 10  but slots  736  are defined in the annular plate  534 . The slots  736  are configured for receiving therein fuel to be ignited by the heater rod  24   b . Stated otherwise, the slots  736  act as fuel collectors. 
     Referring now to  FIG. 13 , another embodiment of a sleeve is generally shown at  832 . In the embodiment shown, the sleeve  832  has ridges  834  at the inner surface  832   b . A channel  836  is located between two adjacent ones of the ridges  834 . The channels  836  act as fuel collectors for containing fuel to be ignited by the heater rod  24   b . As shown, the channels  836  surround the heating section  24   h . The pocket may include the channels; the channels being axially distributed. 
     The sleeve includes pockets  836   a  defined by the channel  836 . The pockets  836   a  have a radial depth that extends radially relative to the axis A from a bottom of the channels  836  to an apex of the ridges  834 . In one embodiment, the pocket can extend around a full circumference of the heater rod, continuously or intermittently, whereas in other embodiments, the pocket can extend only in one or more portions, or angular segment, of the full circumference of the heater rod. The pocket may be defined between threads defined by the inner surface of the sleeve. The pocket may extend around a full circumference of the inner surface of the sleeve. 
     Referring now to  FIG. 14 , another embodiment of a sleeve is generally shown at  932 . In the embodiment shown, the sleeve  932  circumferentially extends at least partially around a portion of the heating section  24   h  of the heater rod  24   b . Herein, the expression “at least partially around” means that the sleeve is present a plurality of circumferential locations distributed all around the heating section, but the sleeve  932  need not continuously extend all around a full circumference about the axis A. Herein, the expression “a portion of the heating section  24   h ” means a portion taken along the axis A. In a particular embodiment, the portion of the heating section  24   h  includes at least half of a total length of the heating section  24   h  taken along the axis A. A porous medium, also referred to as a structure having open porosity,  934  is received within the sleeve  932  and extends radially between the heater rod  24   b  and the inner surface  932   b  of the sleeve  932  and at least partially circumferentially around the heater rod  24   b . The porous medium  934  may be metal foam, a porous ceramic such as a ceramic sponge, a  3 D printed lattice or any structure known in the art that defines porosities in which fuel may be received. The porous medium  934  is a structure having open porosities. In one embodiment, the structure having open porosity can extend around a full circumference of the heater rod, continuously or intermittently, whereas in other embodiments, the structure having open porosity can extend only in one or more portions, or angular segment, of the full circumference of the heater rod. In a particular embodiment, the foam is radially spaced apart from the heater rod  24   b  by a gap. The gap may extend around a full circumference of the heater rod  24   b  or around a portion of said circumference. This might allow the foam to expand when exposed to hot gas. In a particular embodiment, the structure having open porosity has an axial end being distal from the base; the axial end of the structure sloping inwardly and axially towards the base. The structure having open porosity may have a plurality of radially extending slots forming a fluid circulation area around the heating section of the heater rod. The foam need not be axisymmetric. In a particular embodiment, the structure having open porosity may form part of the liner. In other words, the structure having open porosity may be secured to the collar  16   d.    
     In the embodiment shown, the porous medium defines an annular surface  934   a  that circumferentially extends around the heater rod  24   b . In the embodiment the annular surface is angled such that as to face the heating section  24   h  of the heater rod  24   b . Stated otherwise, the annular surface  934   a  is an axial end that is distal from the glow plug body  24   a ; the axial end of the porous medium  934  sloping radially inwardly and axially towards the heater rod  24   b  relative to the axis A. This can offer a greater surface area to be heated by the heating section  24   h  compared to a configuration in which the annular surface  934   a  is perpendicular to the axis A. 
     In the embodiment shown, the porous medium  934  is connected to the inner surface  932   b  of the sleeve  932 . Alternatively, the porous medium  934  may be connected to an end wall  932   c  of the sleeve  932 ; the end  24   g  of the heater rod  24  protruding axially beyond the end wall  932   c . The porous medium  934  may be in abutment against the heating section  24   h  of the heater rod  24   b.    
     It is understood that the porous medium needs not to circumferentially extend around a full circumference of the sleeve  932 . For instance, the porous medium  932  may fill the spacing S 2  ( FIG. 5 ) located between two adjacent fins  132   e  ( FIG. 5 ) of the sleeve  132  of  FIG. 5 . In other words, there is presence of the porous medium at a plurality of circumferential locations around the axis A but the porous medium need not circumferentially extends along a full circumference. In a particular embodiment, sections of the circumference of the inner surface of the sleeve where the porous medium is present amount to at least half of a full circumference of said inner surface. 
     In the embodiment shown, the metal portion M that is closest to the heating section  24   h  is on the porous medium  934 . More specifically, the closes metal portion is located on the annular surface  934   a  of the porous medium  934  and at a radially inward-most point on said surface  934   a . In a particular embodiment, a distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to two times a length along the axis A of the heating section  24   h . In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to one and a half time the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion corresponds to the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is about half the length of the heating section  24   h  taken along the axis A. 
     In one embodiment, the sleeve can be designed to extend circumferentially around the heater rod, and radially between the heater rod and the aperture in the combustion chamber liner, essentially acting as a plug to limit or prevent the passage of compressed air between the heater rod and the combustion chamber liner. In some embodiments, especially if the sleeve is structurally connected to a base of the igniter, a small radial gap will nonetheless be left between the combustion chamber liner and the sleeve to facilitate insertion of the sleeve in the combustion chamber liner, but it may be preferred to otherwise minimize this radial gap. In one embodiment, the combustor liner plug extends continuously around the entire circumference of the heater rod. 
     The combustor liner plug extends circumferentially around the heater rod  24   b . The combustor liner plug is sized and configured to extend inside the combustor liner aperture of the gas turbine engine when the base B is connected to the casing  25  between the heater rod  24   b  and a periphery of the aperture  25   a . The combustor liner plug forms part of the sleeve which is structurally connected to the base B and protrudes along the axis A from the base B towards the rod end  24   g ; the combustor liner plug being at an end of the sleeve remote from the base. The combustor liner plug may have a portion connected to the heater rod. 
     Referring now to  FIG. 15 , the porous medium  1034  has a radial thickness relative to the axis A that is less than that of the porous medium  934  of  FIG. 14  such that an annular spacing  1036  is created radially between the heating section  24   h  of the heater rod  24   b  and the porous medium  1034 . This annular spacing  1036  might allow fuel to circulate between the porous medium  1034  and the heater rod  24   b  and offers a greater surface through which the fuel may be able to penetrate the porous medium  1034 . 
     Referring now  FIG. 16 , another embodiment of a sleeve is generally shown at  1132 . The sleeve  1132  is similar to the sleeve  932  of  FIGS. 14 and 15 , but the end wall  932   c  is axially offset from the porous medium  934 . This might allow the fuel to circulate through the porous medium  934  in an axial direction relative to the axis A to reach a portion of the heater rod  24   b  that is behind the porous medium  934 . 
     Referring now to  FIGS. 17 and 18 , in the embodiment shown, the adaptor  30  defines an inlet  30   m ′ provided in the form of slots  30   m  extending externally to the adaptor  30  and thereacross into the ventilation path  42 . The inlet  30   m ′ is fluidly connected to the spacing S 1 . The slots  30   m  are located axially between the proximal end of the sleeve  1132  and the constriction C. A swirler  40  is received in the portion of the cavity  30   i  that is free of the sleeve. The annular gap G around the heater rod  24   b  forms a ventilation path  42  that extends circumferentially around the heater rod  24   b  and is located radially between the swirler and the adaptor  30 . The ventilation path  42  extends axially along at least a portion of the heater rod  24   b . In one embodiment, the ventilation path extends around the entire circumference of the heater rod, in a manner to cool the heater rod in a relatively uniform manner. The swirler has an annular wall  40   a  and a plurality of helical vanes  40   b  that extend from the annular wall  40   a  toward a center of the annular wall  40   a . The helical vanes  40   b  extend both in the axial and circumferential directions relative to the axis A such that they wrap around the axis A. Spacing  40   c  are defined between the helical vanes  40   b , which are configured to impart a circumferential component to a flow circulating there between. 
     The ventilation path  42  extends from an inlet  42   a  to an outlet  42   b . The inlet  42   a  is fluidly connectable to the spacing S 1  between the engine casing  25  and the combustor liner  16   a . The outlet  42   b  extends circumferentially around the glow plug heater rod  24   b  and oriented axially relative to the axis A. The outlet is fluidly connectable to the combustion chamber. The ventilation path  42  may extend through the gap G. The ventilation path  42  may extend axially along a portion of the heater rod located between the heating section and the base. The inlet  42   a  of the ventilation path  42  may be connected to the outlet  42   b  via the constriction C. The ventilation path  42  may extend through a second broadening section  432   b   2  located upstream of the constriction C relative to the flow circulating in the ventilation path  42 . The ventilation path  42  may extend through the second broadening section, the constriction, and the broadening section. 
     In a particular embodiment, a tip portion of the sleeve is made of a different material having a higher resistance to heat than a remainder of the sleeve. 
     The inlet  42   a  of the ventilation path  42  may be extending radially through the sleeve. Gas may be drawn across this ventilation path  42  via a difference of pressure between the compressed gas path (e.g., spacing S 1 ) and the combustion chamber during normal operation. 
     In a particular embodiment, the ventilation path has an inlet segment which connects the compressed gas path with a gap between the sleeve and the heater rod, that gap may act as a second segment, and may broaden before reaching the heating section. This might reduce the likelihood of blowing out the flame. Alternately, the ventilation path  42  may exit across the sleeve. The ventilation path  42  may extend along the metal to ceramic junction J. 
     In a particular embodiment, the ventilation path may be formed within the sleeve and extend axially along a distal end portion of the sleeve. The sleeve may define a plurality of circumferentially interspaced conduits. The circumferential conduits may each extend along a portion of a circumference to induce a swirl in the air, to create a vortex. This might create a broadening flow. In a particular embodiment, a ventilation path may be created to blow fuel upward. 
     More specifically, the slots  30   m  are fluidly connected to the ventilation path  42 , which is fluidly connected to the spacing  40   c  between the helical vanes  40   b , which are fluidly connected to the flow circulation area F. The slots  30   m  receives compressed air from the spacing S 1  between the engine casing  25  and the combustor liner  16   a . The compressed air is directed along the axis A away from the end  24   g  of the heater rod  24   b . Then, the compressed air flows radially to reach the spacing  40   c  and flows along and around the axis A toward the end  24   g.    
     In the embodiment shown, the inner surface  432   b  of the sleeve  432  defines a convergent-divergent nozzle. More specifically, the inner surface  432   b  of the sleeve  432  defines a broadening section  432   b   1 . The cross-sectional area of the ventilation path  42  at the broadening section  432   b   1  increases past the constricted area C to decrease a velocity of the compressed air. This might reduce the cooling of the heater rod  24   b  in comparison to a configuration that does not present the increase in the cross-sectional area. In a particular embodiment, the swirler  40  increases a cooling capability of the compressed air around the intermediate section  24   d  of the glow plug body  24   a . In a particular embodiment, the increase in the cross-sectional area of the inner surface  432   b  of the sleeve  432  decreases a cooling capability of the cooling air around the heating section  24   h  of the heater rod  24   b.    
     In the embodiment shown, the swirler  40  has alignment features  40   d ′ provided in the form of pins  40   d  extending axially relative to the axis A and being secured to the wall  40   a . The pins  40   d  are slidingly received within corresponding aperture  30   n  of the adaptor  30  for avoiding the swirler  40  to rotate relative to the axis A. Stated otherwise, the pins maintain a circumferential alignment of the swirler with respect to the adaptor  30 . 
     In a particular embodiment, the swirler may be located radially between the heating section  24   h  of the heater rod  24   b  and the sleeve  432  relative to the axis A; the swirler being axially aligned with the heating section  24   h  relative to the axis A. In such a case, the sleeve  432  may define apertures locate axially above the heating section  24   h  to allow the compressed air to circulate within the sleeve  432  and downwardly toward the end  24   g  of the heater rod  24   b  via the spacing between the vanes of the swirler  40 . 
     In a particular embodiment, the compressed air that flows along the ventilation path  42  may be used to cool down the sleeve at locations proximate sensitive parts of the glow plug  24 . 
     Referring now to  FIG. 19 , a glow plug in accordance with another embodiment is generally shown at  124 . The glow plug  124  includes a body  124   a  and a heater rod  124   b . The glow plug body  124   a  is configured to be directly connected to the igniter aperture  25   a  of the engine casing  25  using threads. 
     In the embodiment shown, a sleeve  1232  is connected to the heater rod  24   b . The sleeve  1232  is provided in the form of a ring that circumferentially extends around a full circumference of the heater rod  24   b . The sleeve  1232  has an outer surface  1232   a  that is configured to be in abutment with the peripheral wall of the combustor liner aperture  16   c  of the combustor liner  16   a . An inner surface  1232   b  of the sleeve  1232  is in abutment with the heater rod  24 . In the embodiment shown, the cooperation of the combustor liner  16   a , the sleeve  1232 , and the heater rod  24   b  creates a sealing connection that prevents the combustion gases from leaking from the igniter liner aperture  16   c  toward the spacing S 1 . In other words, the sleeve  1232  fills a gap that would otherwise be present between the heater rod  24   b  and the combustor liner  16   a . It is understood that the sleeve  1232  may abut either one of an inner or outer surface of the combustor liner  16   a.    
     In a particular embodiment, the sleeve  1232  may define fins, may contain a porous medium, may define a cavity to act as a fuel collector. In a particular embodiment, the sleeve  1232  may be defined by the heater rod  24   b . The sleeve  1232  may be connected to the heater rod  24   b  by being heat shrunk there on. Any other methods of fastening the sleeve  1232  to the heater rod  24   b  known in the art may be used without departing from the scope of the present disclosure. 
     In the embodiment shown, the metal portion M that is closest to the heating section  24   h  is a face of the sleeve  1232  that faces away from the body  124   a . In a particular embodiment, a distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to two times a length along the axis A of the heating section  24   h . In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is less or equal to one and a half time the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion corresponds to the length of the heating section  24   h  taken along the axis A. In a particular embodiment, the distance taken along the axis A from the end  24   g  of the heater rod  24   g  to the metal portion is about half the length of the heating section  24   h  taken along the axis A. 
     Referring now to  FIG. 20 , the combustor liner  16   a  may define a gap  16   d  for allowing compressed air from flowing along an inner surface of the combustor liner  16   a . The compressed air is used to cool the combustor liner  16   a  via film cooling. However, the film of air created thereby might impede the ability of the heater section  24   h  of the heater rod  24   b  to ignite the mixture of air and oil. 
     In the embodiment shown, the sleeve  1332  is a flow impeding member  1332 ′ and includes an annular wall  1332   j  that circumferentially extends around the heater rod  24   b  to shield the heater rod form the film of cooling air. More specifically, the annular wall  1332   j  encloses a plenum  1332   k  that is fluidly connected to the combustion chamber  16   a  and in which a velocity of the fluid circulating therein is less than that in a remainder of the combustion chamber  16   b . The flow impeding member  1332 ′ may extend along the axis A and have a distal end spaced from the base B and extending axially relative to the axis A beyond the rod end  24   g . The flow impeding member  1332 ′ may be sized and configured such that the distal end extends into a combustion chamber of the gas turbine engine upon the base connected to the casing  25 . In a particular embodiment, the flow impeding member extends around at least haft of a circumference around the glow plug heater rod  24   b . The flow impeding member  1332 ′ may be structurally connected to the base B and may protrude along the axis A from the base toward the rod end  24   g . The flow impeding member  1332 ′ may have a portion connected to the heater rod  24   b  between the rod end  24   g  and the base B. The flow impeding member  1332 ′ may have a portion connected to the glow plug heater rod  24   b  between the rod end  24   g  and the base B. 
     Indeed, the combustor liner can be provided with cooling apertures designed to provide a curtain of cooling air along the inner surface of the combustor liner. The sleeve can have an annular wall which protrudes inwardly from the combustor liner and shields the heater rod from such a curtain of cooling air. In one embodiment, the protruding sleeve portion can extend around a full circumference of the heater rod, continuously, whereas in other embodiments, the protruding sleeve can extend only in one or more portions, or angular segments, of the full circumference of the heater rod. This can be the case, for instance, in a situation where the orientation of the igniter in its socket will be known beforehand, in which case it can be preferred to use a protruding sleeve portion only between the heater rod and the source of the curtain of cooling air, for instance. 
     In the embodiment shown, the sleeve  1332  has a flat end  1332   p  that defines a annular wall circumferentially extending around the heater rod  24   b ; the annular wall being normal to the axis A. Having such a flat end  1332   p  might allow the sleeve  1332  to contain more fuel than a configuration where the end  1332   p  is sloped. 
     Referring now to  FIG. 21 , a glow plug in accordance with another embodiment is generally shown at  224 . The glow plug  224  has a body  224   a  and a heater rod  224   b  protruding from the body  224   a . The heater rod  224   b  has a section  224   j  located between the heating section  224   h  and the body  224   a . The section  224   j  is configured to block the igniter liner aperture  16   c  to impede the combustion gases from flowing out of the combustion chamber  16   b  in the spacing S 1 . The body  224   a  may be fastened to the igniter aperture peripheral wall  25   a ′ via corresponding threads defined by the body  224   a.    
     In the embodiment shown, a blocking member BM is provided. The blocking member BM extends across the igniter aperture  25   a  and is configured to block fluid communication across the igniter aperture  25   a . The blocking member BM may be used to fluidly disconnect the igniter from a fuel source (e.g., fuel tank) such that no fuel is injected around the heating rod  224   b.    
     Referring now to  FIG. 22 , a coil element  50  may be received within the sleeve  32 . The coil element  50  may be in contact with the heater rod  24   b . The coil element  50  wraps around the heater rod  24   b  and extends radially from the heater rod  24   b  to the inner surface  32   b  of the sleeve  32 . In other words, by wrapping around, the coil element  50  extends both in a circumferential direction and a radial direction relative to the axis A. A plurality of gaps  50   a  are defined between portions of the coil element  50  and are configured for receiving therein fuel. The heater rod  24   b  transfers its heat to the coil element  50  thereby increasing a surface area being heated. This might help in igniting the mixture of fuel and air. As shown more clearly on  FIG. 22 a   , a radial depth relative to the axis A of the coil element  50  corresponds to a depth of the sleeve  32  relative to the axis A. 
     Referring now to  FIG. 23 , a coil structure  60  may be received within the sleeve  32 . The coil structure  60  includes ribs  60   a  that extends from the heater rod  24   b  to the inner surface  32   b  of the sleeve  32 . Coil elements  60   b  are each provided in an annular form and circumferentially extends around a full circumference of the heater rod  24   b . The coil elements  60   b  are connected to the ribs  60   a . The coil elements  60   b  are radially interspaced between the heater rod  24   b  and the sleeve  32  and define gaps  60   c  radially therebetween relative to the axis A. The gaps  60   c  are configured for receiving therein fuel. The heater rod  24   b  transfers its heat to the coil elements  60   b  thereby increasing a surface area being heated. This might help in igniting the mixture of fuel and air. As shown more clearly on  FIG. 23 a   , a radial depth relative to the axis A of the coil structure  60  corresponds to a depth of the sleeve  32  relative to the axis A. 
     Referring now to  FIG. 24 , an igniter  300  in accordance with a particular embodiment is shown. The igniter  300  includes a base (not shown) and a conductor  300   a  extending along an axis A from the base to an end  300   b . A heating element  300   c  is electrically connected to the end  300   c  of the conductor  300   a . An outer casing  300   d  circumferentially extends around the conductor  300   a . The outer casing  300   c  is in heat exchange relationship with the heating element  300   c  and radially spaced apart from the conductor  300   a  by a gap  300   e . The gap  300   e  is filled with an insulator. In the embodiment shown, electricity is routed from a power source (e.g., battery) to the heating element  300   c  via the conductor  300   a  and is directed from the heating element  300   c  back to the power source via the outer casing  300   d . The power source may be a source of direct current. The heating element  300   c  may be a ceramic. The heating element  300   c  may provide for a greater surface area that becomes incandescent than that of a tip of a conventional glow plug. In a particular embodiment, the electricity may be carried back and forth from the power source via the conductor  300   a.    
     In a particular embodiment, the conductor is made of copper. The heating element may be a conductive ceramic heating element. The housing may be in contact with the heating element. The heating element has a surface  300   f  facing away from the conductor. The surface may define a pattern. The pattern may be, for instance, grooves, and/or ridges that might increase the surface area in contact with the fuel. The heating element may be porous. In such a design, the heating element can be less vulnerable to damage than if the heating element were exposed. 
     In a particular embodiment, the disclosed igniter  300  allows for a greater surface area for a same power compared to the igniter  24  disclosed above. The increased surface area might improve the probability of igniting fuel/air mixtures by hot surface ignition. The heated surface may be wider than the igniter  24  disclosed above. This might allow the fuel/air mixture in the middle of the heated surface to reach the temperature needed to ignite. The ceramic might improve the life of the adapter by protecting the metal adapter. It might be possible to use a less expensive metal for the housing since the ceramic might protect the tip. 
     In a particular embodiment, the fins  132   e , the porous medium  934 , and the ridges/channels  834 ,  836  may increase a surface area that is heated. In other words, without the fins, the porous medium, or the ridges, only the heating section  24   h  of the heater rod  24   b  would be heated. By surrounding the heating section  24   h  by the fins, the porous medium, or the channels, heat is transferred by conduction and/or radiation from the heating section  24   h  to the fins, the porous medium, or the channels. Having more heated surface area might help in igniting the mixture of air and fuel. 
     In a particular embodiment, the sleeve  32  acts as a collector to collect fuel prior to be ignited by the glow plug  24 . In a particular embodiment, the sleeve  32  provides a plenum around the heater rod  24   b ; a velocity within the plenum being less than that outside the plenum. Such a plenum might help in igniting the mixture of air and fuel that enters the plenum. Moreover, by having a velocity of the mixture inside the plenum that is lower than that outside the plenum might avoid cooling the heating section  24   h , which would impair the ability of the heating section  24   h  to ignite the mixture. The sleeve  32  may therefore act as a flame stabilizer. 
     Referring now to  FIGS. 25 and 26 , there is disclosed an igniter including a heat spreader  1400  extending circumferentially around the glow plug heater rod  24   b . The heat spreader  1400  may be in heat exchange relationship with the glow plug heater rod  24   b  for dissipating heat generated by the glow plug heater rod  24   b . The heat spreader may be axially aligned with the heating section  24   h  of the glow plug heater rod  24   b.    
     The igniter further has a housing  1402  circumferentially surrounding the glow plug heater rod  24   b . The rod end  24   g  may protrude beyond the housing  1402 . The heat spreader  1400  may be secured to an end of the housing  1402 . 
     In the embodiment shown, the housing  1402  and the glow plug heater rod  24   b  are radially spaced from each other by a gap  1404  axially extending at least along a portion of the glow plug heater rod  24   b . The gap  1404  may be filled with an insulation material. In the depicted embodiment, the heat spreader circumferentially extends around a full circumference of the glow plug heater rod. 
     The heat spreader has an annular face  1400   a  circumferentially extending around the glow plug heater rod  24   b . The annular face  1400   a  may be beveled toward the glow plug heater rod  24   b.    
     The heat spreader  1400  may be made of a metallic material. The heat spreader  1400  may be made of a conductive ceramic material. The heat spreader  1400  may be secured to be in contact with the glow plug heater rod  24   b . The heat spreader  1400  may be made of Inconel™. The heat spreader  1400  may define porosities. 
     Referring now to  FIG. 26 , the heat spreader  1400  may be circumferentially surrounded by the housing  1402 . More specifically, the heat spreader  1400  has an cylindrical face  1400   b  that may be in contact with the housing  1402 . The housing  1402  may define a shoulder  1402   a  for abutment against the heat spreader  1400 . 
     In a particular embodiment, a catalyst may be deposited wherever fuel is expected to accumulate and where a temperature is expected to be high. The catalyst may stay hot due to the combustion process. The catalyst may, for instance, be located on the foam, the porous media, the spiral, the ridges, the inner surface of the sleeve, etc. 
     In some embodiments, the igniter can be secured to the casing by fasteners, for instance, rather than torque. In such other embodiments in particular, it can be easier to predetermine the circumferential orientation or the igniter around its axis, when assembled. In such cases, it can be preferred to use specifically provide the igniter with an asymmetrical design suited for the particular angular orientation. Accordingly, sleeve, flow impeding member, peripheral wall, ridges, grooves, heating element, heat spreader may be axisymmetric. 
     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 gas turbine engine  10  has been depicted as a turbofan, but the disclosed igniters may be used in any types of gas turbine engines, such as turboprop, turboshaft, auxiliary power unit, jet turbine combined heat and power generators, jet turbine powered surface vehicles. 
     Some of the igniter embodiments presented above have an axisymmetric design. The axisymmetricity is optional, and may be useful only in some embodiments. Some embodiments have non-axisymmetric designs. Indeed, axisymmetric designs can be particularly appropriate in a context where the igniter is to be secured to the gas turbine engine by a threaded engagement concentric to the heater rod axis, which is typically the case when retrofitting the igniter to an existing spark plug aperture, because in such context, the axissymmetricity ensures that there is no need to achieve a specific angular orientation relative to the thread. However, there are other cases where the circumferential orientation of the igniter relative to the socket in the gas turbine engine can be known, such as via a specific engine design for instance, and in which a non-axisymmetric design can be preferred and better adapted to the specific features of the environment of use. For instance, an igniter can be designed for use in a specific orientation relative to the axis of the socket which receives it, and/or for a specific circumferential position (e.g. 3 O&#39;clock, 6 O&#39;clock) in the engine, in which specific elements of the environment, such as known position of incoming fuel mist, known position of heating air, known local orientation of gravity, etc. can be predetermined. 
     Embodiments disclosed herein include: 
     A. A method of servicing a gas turbine engine having a spark plug igniter connected to a spark plug igniter socket of the engine, the method comprising: removing the spark plug from the spark plug igniter socket of the gas turbine engine; inserting a glow plug igniter having a base configured for matingly engaging the spark plug igniter socket; and securing the glow plug igniter to the gas turbine engine. 
     B. An assembly configured to allow using a glow plug in a spark plug socket of a gas turbine engine, the spark plug socket defining socket threads, the assembly comprising: an adaptor including an adaptor body having a threaded aperture, an external portion of the adaptor body having external threads configured to threadingly engage the socket threads; and a glow plug having a base, the base having a threaded portion threadingly engaging the threaded aperture of the adaptor body, the glow plug having a heater rod extending along an axis away from the base. 
     C. A gas turbine engine comprising: a casing, a combustor liner provided inside the casing and enclosing a combustion chamber, a spark plug socket including a casing aperture defined through the casing and a liner aperture defined through the combustor liner, and an igniter received in the spark plug socket, the igniter having a base connected to the casing, a glow plug heater rod, the glow plug heater rod extending from the base along an axis and terminating in a rod end, the rod end exposed to the combustion chamber. 
     D. A method of performing maintenance to a gas turbine engine having a casing and a combustion chamber liner, the combustion chamber liner enclosing a combustion chamber, and an igniter socket extending across the casing and the combustion chamber liner, the method comprising: securing a glow plug heater rod to the gas turbine engine with the glow plug heater rod extending into the igniter socket; and securing a flow impeding member to the gas turbine engine in a manner that the flow impeding member extends around the heater rod and penetrates into the combustion chamber liner. 
     Embodiments A, B, C, and D may include any of the following elements in any combinations: 
     Element 1: the spark plug igniter socket includes a threaded aperture, removing the spark plug from the spark plug igniter socket includes unfastening the park plug. Element 2: securing the glow plug includes fastening the glow plug in the threaded aperture. Element 3: fastening the glow plug includes fastening a male thread of the glow plug igniter within the igniter aperture until a shoulder face of the base abut a receiving face of a casing of the gas turbine engine. Element 4: the shoulder face and the receiving face of the casing are correspondingly beveled. Element 5: removing the spark plug includes removing a sleeve of the spark plug from a casing liner aperture, and said inserting includes introducing a sleeve of the igniter into the casing liner aperture, the sleeve extending around at least an axial portion of the heater rod. Element 6: securing a male thread of the glow plug igniter to a female thread of an adaptor, the base being part of the adaptor. Element 7: a sleeve extending at least partially axially along a heating section of the heater rod, the heating section extending between opposite ends of a heater contained within the heater rod, the sleeve extending at least partially circumferentially around the heating section. Element 8: the sleeve has a combustor liner plug configured to bridge a gap between a periphery of a combustion chamber liner aperture of the spark plug socket and the heater rod. Element 9: the sleeve is structurally connected to the adaptor body and extends axially therefrom and around the heater rod. Element 10: the sleeve has a portion connected to the heater rod between a rod end of the heater rod and the adaptor body. Element 11: the sleeve is matingly connectable to a liner of the gas turbine engine, the sleeve defining a part of the liner. Element 12: the heater rod has a heating section extending along an axial distance between axially opposite ends of a heater, wherein the sleeve extends at least partially axially along the heater section of the heater rod and is spaced therefrom. Element 13: the adaptor body defines a shoulder defining an abutment surface axially facing a rod end of the heater rod, the external threads located axially between the shoulder and the rod end. Element 14: the sleeve extends circumferentially around more than half the circumference of the heater rod. Element 15: the sleeve extends annularly around an entire circumference of the heater rod. Element 16: the base is connected to the casing via a threaded engagement. Element 17: the igniter includes a sleeve connected to the base independently of a structure of the gas turbine engine and protrudes along an axis from the base towards the rod end. Element 18: the steps of securing the glow plug heater rod and securing the sleeve are performed simultaneously by securing a base of an igniter having the glow plug heater rod to the casing of the gas turbine engine, the sleeve being structurally connected to the base independently of a structure of the gas turbine engine. Element 19: securing the base to the casing includes fastening a male thread of the base with a female thread in the casing until a shoulder face of a base of the igniter comes into abutment with a receiving face of the casing. Element 20: securing the base to the casing with a spacer trapped between the base and the casing. Element 21: the step of securing the sleeve is performed before the step of securing the heater rod and includes securing the sleeve to the combustion chamber liner. 
     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.