Abstract:
An air metering apparatus for a secondary air system of gas turbine engine includes a metering gap defined between surfaces comprised of one or more metals having one of similar coefficients of thermal expansion or similar thermal expansions at an operating temperature. Provided is an air metering gap which undergoes thermal growth in a way which provides a suitable air metering gap during engine running conditions.

Description:
TECHNICAL FIELD 
     The described subject matter relates generally to gas turbine engines and, more particularly, to an improved air metering for a gas turbine engine. 
     BACKGROUND OF THE ART 
     Gas turbine engines have secondary air systems which provide cooling to various engine parts. It is desirable to minimize cooling air requirements to improve engine efficiency, and hence secondary air is typically metered, such as through metering holes in an engine casing or similar structure. A simpler arrangement is sometime to meter air through gaps between adjacent parts in the secondary air system. However, since components tend to be made from differing materials, depending on their particular design requirements, this results in differing thermal growths, which may be significant depending on the materials, location in the engine, etc. The differing thermal expansion/contraction characteristics of various engine components can negatively affect the ability to meter air accurately, however, and hence the designer must optimize the gap size to, for example, provide a desired flow for a threshold or target operating segment of the aircraft cycle (e.g. cruise, take-off, etc.), and sub-optimal performance for other segments. Accordingly, there is a need to provide an improved air metering arrangement for gas turbine engines. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the described subject matter provides a gas turbine engine having an axis of rotation, the engine comprising a radially-extending annular wall comprised of a metal having a first thermal expansion at an engine running temperature; a cylindrical surface extending axially past the wall, the surface and wall cooperating to define adjacent secondary airflow cavities, one secondary airflow cavity on either side of the wall, the airflow cavities in serial communication with a source of pressurized secondary air, the cylindrical surface comprised of a metal having a second thermal expansion at an engine running temperature, the second thermal expansion unequal to the first coefficient; and an annular ring radially-slidably mounted to the radial wall, the ring cooperating with the cylindrical surface to define a non-zero gap between them, the gap providing a minimum opening between the cavities and hence in use metering serial secondary airflow communication between the cavities, the ring having a face which in use sealingly cooperates with the wall to impede secondary airflow between the ring and wall, the ring comprised of a metal having a thermal expansion at an engine running temperature substantially equal to the second thermal expansion. 
     According to another aspect, the described subject matter provides a gas turbine engine having an axis of rotation, the engine comprising a radially-extending annular wall having a terminal periphery, the radial wall comprised of a metal having a first coefficient of thermal expansion; a cylindrical surface extending axially past the terminal periphery of the wall, cylindrical surface comprised of a metal having a second coefficient of thermal expansion which is unequal to the first coefficient; and an annular ring extending around and radially-slidably mounted to the radial wall, the ring having a face sealingly cooperating with the wall to impede airflow therebetween when the engine is in use, the ring having a terminal periphery extending radially closer to the cylindrical surface than the terminal periphery of the wall, the terminal periphery of the ring spaced from the cylindrical surface to provide a non-zero gap between the ring and the cylindrical wall selected to meter a secondary airflow of the engine in communication with the gap, the ring comprised of a metal having a coefficient of thermal expansion substantially equal to the second coefficient of thermal expansion. 
     Further details of these and other aspects will be apparent from the detailed description and drawings included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying drawings depicting aspects of the described subject matter, in which: 
         FIG. 1  is a schematic cross sectional view of a gas turbine engine according to the present description; 
         FIG. 2  is a cross-sectional view of a portion of the gas turbine engine of  FIG. 1 ; 
         FIG. 3  is an enlarged view of a portion of  FIG. 2 ; 
         FIG. 4  is view similar to  FIG. 3 , of an alternate arrangement; and 
         FIG. 5  is an isometric view of a portion of the device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a gas turbine engine presented as an example of the application of the described subject matter includes a housing or nacelle  10 , a core casing  13 , a low pressure spool assembly which includes a fan assembly  14 , a low pressure compressor assembly  16  and a low pressure turbine assembly  18 , and a high pressure spool assembly which includes a high pressure compressor assembly  22  and a high pressure turbine assembly  24 . The core casing  13  surrounds the low and high pressure spool assemblies in order to define a main fluid path (not numbered) therethrough. In the main fluid path there is provided a combustor  28  to constitute a gas generator section  26 . Generally, those downstream of the gas generator section  26  are hot sections and any engine structures in the hot sections such as a mid-turbine frame  20  which is located between the high pressure turbine assembly  24  and the low pressure turbine assembly  18 . Referring to  FIG. 2 , a secondary air system (not indicated) delivers pressurized cooling air to the mid turbine frame  20  through a metering apparatus  30 . 
     Referring to  FIG. 3 , an example air metering apparatus  30  is depicted. The apparatus includes a first component such as a radial wall  32  as part of a stationary structure of the mid-turbine frame  20 , such as a flange extending from a vane platform of the mid-turbine frame  20 . The radial wall  32 , for example, includes opposed radial surfaces  34  and  36 , and extends to a radially-inward terminal periphery (not indicated). (Unless otherwise specified, the radial and axial directions are defined with respect to the engine axis shown in  FIG. 1 ). The apparatus  30  further includes a second component having at least one radial surface or face  44 , for example, a seal ring  40  having opposed radial surfaces  42 ,  44 . The ring  40  may extend around/along the entire circumference of wall  32 . A generally radially oriented slot  46  extends axially through the seal ring  40 , that is, between the opposed radial surfaces  42 ,  44 . The radially oriented slot  46  may define an opening (not indicated) in the outer periphery  48  of the seal ring  40 . As will be described further below, when the seal ring  40  abuts the surface  34  of the radial wall  32 , as shown in  FIG. 3 , sealing is provided such that air may pass only through a non-zero gap between the ring  40  and an engine case  41 , provided in this example by another portion of the mid turbine frame  20 , such as an inner bearing housing (not indicated). In a region of the ring  40 , the engine case  41  extends generally cylindrically and axially along the engine axis past the radial wall  32  and ring  40 . As mentioned, a radially-extending gap (not indicated) is defined between the ring  40  and the engine case  41 , the gap size being selected to meter a desired secondary cooling flow therethrough. 
     Radial wall  32  and engine case  41  are respectively made from metals suitable for their particular applications, such as Nickel alloys or superalloys, such as MAR-M-247, IN713, and so on. It will be understood that such materials may have a thermal growth mismatch. Ring  40  may be made either from the same material as engine case  41 , or from another material having the same or similar thermal coefficient of expansion, such that the engine case  41  and ring  40  will radially expand at a substantially the same rate when heated, as will be described further below. Material selection for main structural component, in this example radial wall  32  and engine case  41 , is made to meet structural and durability requirements for the part. Material selection for the ring  40  is then done to match, as close as possible, the thermal radial displacement of engine case  41 . It will be understood that the thermal displacement of ring  40  and engine case  41  will depend not only on the metal, but also on the metal temperature for each component, at specific engine running conditions (e.g.: cruise, take-off, etc). Thus, in a case where the same metal temperature exists for ring  40  and engine case  41  for a given engine running condition, the same material (or differing materials with substantially the same thermal expansion coefficients) may be selected, in order to provide a constant gap or spacing during engine running conditions. Alternately, where the same metals are used for wall  32  and engine case  41 , yet thermal mismatch occurs due to the differing temperature each experience, the temperature and desired expansion of the ring  40  may be considered when selecting a metal for ring  40 . Still alternately, respective materials having desired thermal expansion coefficients may be selected to one or increase or decrease the gap size during various engine running conditions, to thereby passively control secondary airflow requirements as desired. 
     A lug  50  with an enlarged head  56  extends from the radial wall  32 . The lug  50  may have any suitable shape, and is received within the slot  46  of ring  40  to radially-slidably mount the ring to the wall. The enlarged head  56  retrains axial movement of the ring  40  away from the radial wall  32 . The lugs and slots are preferably distributed substantially equally around the circumference of the engine, to facilitate maintaining a centering of the ring irrespective of any thermal growth experienced. The radial dimension of the slot  46  may be selected to allow for thermal radial expansion/contraction of the seal ring  40  independent from the connected radial wall  32 . 
     In use, engine operation results in pressurized air delivered to cavity  43  adjacent air metering apparatus  30 . The differential pressure between cavity  43  and a downstream cavity  45  of the secondary cooling system tends to press face  44  of ring  40  against radial wall  32 , thereby sealing therebetween. The secondary cooling air in cavity  43  is thus encouraged to flow through the gap between ring  40  and engine case  41 , to downstream cavity  45 , as indicated by the arrows in  FIG. 3 . Ambient heat from engine operation tends to cause thermal growth, in the form of radial growth, of the engine components, and in particular the ring  40 , the engine case  41  and the radial wall  32 . The lug  50 /slot  46  arrangement allows the ring to grow freely. The gap between the ring  40  and the engine case  41 , however, remains substantially constant due to the selection of materials for ring  40  and engine case  41  described above. The gap size, and hence air flow through the gap, is thus substantially unaffected by any thermal growth of the structure at any engine running condition, which thus allows air consumption to be optimized and any variation in cooling flow minimized for different engine running conditions. 
     Referring to  FIGS. 4 and 5 , in another example the lugs  50  are releasably mounted to the radial wall  32 . In this example, the lug  50  is inserted into a hole  38  in the wall  32 , and retained therein by a suitable fastener arrangements, such as a tack weld  62 , or threaded fastener (not shown), press fit with locking wire (not shown) etc., or other suitable fastener arrangement. Optionally, the lug  50  has a cylindrical stem  58  axially extending from the enlarged head  56  and a cylindrical end portion  60  extending axially from the stem  58  to form the end  54 . The end portion  60  may have a diameter less than the diameter of the stem  58 . The removable fastener permits the lug  50  to be removed from the engine for replacement or repairing during engine maintenance. The tack weld  62  can be removed by grinding, or other suitable method. 
     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 departure from the scope of the described subject matter. For example, the ring may have any suitable configuration. The arrangement of the engine case and radial wall may be reversed, such that the airflow passage is defined around an outer radius of the ring, and so on. Still, other modifications 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.