Patent Description:
Various factors exert pressures on turbine engine manufacturers to continually improve their designs. Design improvements take many factors into consideration, such as weight, structural optimization, durability, production costs, etc. Accordingly, while known turbine exhaust cases were satisfactory to a certain extent, there remained room for improvement.

<CIT> discloses heat shield based air dams for a turbine exhaust case.

According to an aspect of the present invention, there is provided a turbine exhaust case (TEC), according to claim <NUM>, comprising: an outer case; an inner case having a radially outer surface and an radially inner surface opposite the radially outer surface; an annular exhaust gas path between the outer case and the inner case, the radially outer surface of the inner case forming a radially inner boundary of the annular exhaust gas path; and a plurality of struts extending across the annular gas path and structurally connecting the inner case to the outer case, at least one of the plurality of struts having an airfoil body with a hollow core, the airfoil body having opposed sides extending chordwise from a leading edge to a trailing edge and spanwise from a radially inner end to a radially outer end; wherein the at least one of the plurality of struts has a leading edge stiffener at the radially inner end thereof, the leading edge stiffener projecting into the hollow core and merging with a stiffener ring projecting from a radially inner surface of the inner case, the leading edge stiffener extending radially outwardly relative to the radially inner boundary of the annular exhaust gas path.

Optionally, and in accordance with the above, the leading edge stiffener comprises a localized thickening of a leading edge wall of the airfoil body.

Optionally, and in accordance with any of the above, the leading edge stiffener projects radially inwardly beyond the airfoil body.

Optionally, and in accordance with any of the above, the annular exhaust gas path has a radial height (A) between the inner case and the outer case, wherein the leading edge stiffener has a radial height (D), and wherein (D) ≥ <NUM>/<NUM> x (A).

Optionally, and in accordance with any of the above, the stiffener ring has a radial height (C) and an axial length (B), and wherein (C) ≥ <NUM>/<NUM> x (B).

Optionally, and in accordance with any of the above, the localized thickening of the leading edge wall of the airfoil body provides a wall thickness at the radially inner end portion of the leading edge, which is at least twice that of an intermediate portion of the leading edge wall.

Optionally, and in accordance with the above, the stiffener ring extends circumferentially along a full circle, and wherein the leading edge stiffeners of the plurality of struts connect with the stiffener ring at circumferentially spaced-apart locations around the stiffener ring.

Optionally, and in accordance with any of the above, the stiffener ring axially spans the leading edges of the struts.

Optionally, and in accordance with any of the above, the stiffener ring and the leading edge stiffeners of the struts are integrally cast as a unitary body.

Optionally, and in accordance with any of the above, the stiffener ring has an axial length (B), and wherein (B) ≥ ½ x (D).

<FIG> illustrates an aircraft engine of a type preferably provided for use in subsonic flight, and generally comprising in serial flow communication an air inlet <NUM>, a compressor <NUM> for pressurizing the air from the air inlet <NUM>, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, a turbine <NUM> for extracting energy from the combustion gases, and a turbine exhaust case (TEC) <NUM> through which the combustion gases exit the engine <NUM>. The turbine <NUM> includes a low pressure (LP) or power turbine 14a drivingly connected to an input end of a reduction gearbox (RGB) <NUM>. The RGB <NUM> has an output end drivingly connected to an output shaft <NUM> configured to drive a rotatable load (not shown). For instance, the rotatable load can take the form of a propeller or a rotor, such as a helicopter main rotor. The engine <NUM> has an engine centerline <NUM>. According to the illustrated embodiment, the compressor and the turbine rotors are mounted in-line for rotation about the engine centerline <NUM>.

According to the embodiment shown in <FIG>, the TEC <NUM> terminates the core gas path <NUM> of the engine. The TEC <NUM> is disposed immediately downstream of the last stage of the low pressure turbine 14a for receiving hot gases therefrom and exhausting the hot gases to the atmosphere. The TEC <NUM> comprises an outer case <NUM> having a radially inner surface forming a radially outer delimitation (i.e. outer gas path wall) of an annular exhaust path 20a of the core gas path <NUM>, an inner case <NUM> having a radially outer wall forming a radially inner delimitation (i.e. inner gas path wall) of the annular exhaust path 20a of the core gas path <NUM>, and a plurality of turbine exhaust struts <NUM> (e.g. <NUM> struts in the embodiment shown in <FIG>) extending generally radially across the annular exhaust path 20a. As shown in <FIG>, the struts <NUM> are circumferentially interspaced from one another. The outer and inner cases <NUM>, <NUM> are provided in the form of outer and inner structural rings concentrically mounted about the engine centerline <NUM>. According to some embodiments, the outer case <NUM> may be bolted or otherwise suitably mounted to the downstream end of the turbine case via a flange connection. For instance, as exemplified in <FIG>, the outer case <NUM> can have an outer flange 22a bolted to a corresponding flange at the downstream end of the turbine case. The struts <NUM> structurally connect the inner case <NUM> to the outer case <NUM>. According to the illustrated embodiment, the inner case <NUM> is configured to support a bearing <NUM> of the low pressure turbine spool via a hairpin connection <NUM> or the like. The struts <NUM> provide a load path for transferring loads from the inner case <NUM> (and thus the bearing <NUM>) to the outer case <NUM>. According to some embodiments, the outer case <NUM>, the inner case <NUM> and the struts <NUM> are of unitary construction. For instance, the outer case <NUM>, the inner case <NUM> and the struts <NUM> can be integrally formed as a monolithic cast component.

Referring jointly to <FIG>, it can be appreciated that the exemplified struts <NUM> have an airfoil profile to serve as vanes for guiding the incoming flow of hot gases through the annular exhaust path 20a. According to the illustrated example, each of the struts <NUM> has an airfoil body with a hollow core <NUM>, the airfoil body having opposed pressure and suction sides <NUM>, <NUM> extending chordwise from a leading edge <NUM> to a trailing edge <NUM> and spanwise from a radially inner end <NUM> to a radially outer end <NUM> (<FIG> and <FIG>). As shown in <FIG>, the hollow core <NUM> of the struts <NUM> may provide an internal passageway for service lines L and the like.

It has been found that in certain engine running conditions, the thermal differential growth between the struts <NUM> and the cases <NUM>, <NUM> of the TEC may result in high stress concentration in the junction region J (<FIG>) of the leading edge <NUM> of the struts <NUM> and the inner case <NUM>. According to one aspect, the tensile stress in region J of the strut leading edge <NUM> can be reduced to an acceptable level by locally providing a leading edge stiffener <NUM> at the junction of the leading edge <NUM> with the inner case <NUM>.

According to some embodiments, the leading edge stiffener <NUM> is provided in the form of an internal core structure at the radially inner end <NUM> of the leading edge <NUM> of the struts <NUM>. The internal core structure is configured to locally reinforce the struts <NUM> where high stress concentrations have been observed. According to one aspect, the leading edge stiffener <NUM> is integrally cast with the associated strut <NUM> has an internal mass projecting into the hollow core <NUM> at the radially inner end <NUM> of the strut <NUM>. Such an embedded cast structure allows to locally increasing the wall thickness of the leading edge <NUM> at the inner end <NUM> of the strut to reduce the stress concentration thereat.

As can be appreciated from <FIG>, the leading edge stiffener <NUM> projects radially inwardly beyond the airfoil body of the struts <NUM> to merge with a stiffener ring <NUM> projecting from a radially inner surface <NUM> of the inner case <NUM>. As shown in <FIG>, the stiffener ring <NUM> extends along a full circumference of the inner case <NUM> and the leading stiffeners <NUM> radiate from different circumferential locations around the stiffener ring <NUM> into respective hollow cores <NUM> of the struts <NUM>. The leading edge stiffeners <NUM> of the struts <NUM> around the inner case <NUM> are, thus, structurally interconnected via the stiffener ring <NUM>. As best shown in <FIG>, the stiffener ring <NUM> is disposed to axially span the leading edge <NUM> of the airfoil body of the struts <NUM>. The combination of the leading edge stiffeners <NUM> of the struts <NUM> with the stiffener ring <NUM> on the inner case <NUM> allows distributing the loads outside the struts <NUM>, thereby relieving stress from the struts <NUM>. For instance, the leading edge stiffeners <NUM> and the stiffener ring <NUM> can cooperate to remove tensile stress in the strut leading edge <NUM> when there is a high delta temperature between the struts <NUM> and cases <NUM>, <NUM> of the TEC <NUM>. According to another aspect, the leading edge stiffeners <NUM> and the stiffener ring <NUM> eliminate the need for a heavy structural inner ring, thereby providing weight savings.

Referring to <FIG>, there is shown one possible configuration of the leading edge stiffener <NUM>. According to this example, the leading edge stiffener <NUM> has a radial height (D) which is greater than or equal to one-third of the radial height (A) of the annular exhaust gas path 20a. According to another aspect, the stiffener ring <NUM> has a radial height (C) which is greater than or equal to two-thirds of its axial length (B). According to another aspect, the leading edge stiffener <NUM> projects into the hollow core <NUM> by a distance (F) which is greater than or equal to the thickness (E) of the leading edge wall of the strut <NUM> at an intermediate location generally midway between the outer and inner cases <NUM>, <NUM>. In other words, the leading edge stiffener <NUM> at least locally doubles the leading edge wall thickness of the airfoil body of the strut <NUM>. According to another aspect, the axial length (B) of the stiffener ring <NUM> is greater than or equal to half the leading edge stiffener height (D). Various combinations of the above aspects are contemplated to reduce stress concentration at the leading edge of the struts <NUM>.

From <FIG>, it can be seen that the leading edge stiffener <NUM> has a width (W) in a circumferential direction. The width (W) generally corresponds to that of the leading edge <NUM>. That is the leading edge stiffener <NUM> is comprised between the opposed sides <NUM>, <NUM> of the airfoil body of the strut <NUM>.

Referring to <FIG>, <FIG>, it can be seen that the leading edge stiffener <NUM> may have a generally rectangular face facing the interior of the hollow airfoil body of the strut. Also, as shown in <FIG>, the leading edge stiffener <NUM> may taper in a radially outward direction (that is in a direction away from the stiffener ring <NUM>).

According to one aspect of some embodiments, the shape and position of the leading edge stiffener <NUM> inside the hollow core of the struts <NUM> is configured to act as a structural reinforcement which may on itself or in combination with the stiffener ring <NUM> be sufficient to allow the exhaust struts <NUM> to withstand the compressive stresses induced at the radially inner end portion of the strut leading edge when the strut are subject to thermal growth especially during engine transient conditions.

Claim 1:
A turbine exhaust case (<NUM>) comprising:
an outer case (<NUM>);
an inner case (<NUM>) having a radially outer surface and an radially inner surface (<NUM>) opposite the radially outer surface;
an annular exhaust gas path (20a) between the outer case (<NUM>) and the inner case (<NUM>), the radially outer surface of the inner case (<NUM>) forming a radially inner boundary of the annular exhaust gas path (20a); and
a plurality of struts (<NUM>) extending across the annular gas path (20a) and structurally connecting the inner case (<NUM>) to the outer case (<NUM>), at least one of the plurality of struts (<NUM>) having an airfoil body with a hollow core (<NUM>), the airfoil body having opposed sides (<NUM>, <NUM>) extending chordwise from a leading edge (<NUM>) to a trailing edge (<NUM>) and spanwise from a radially inner end (<NUM>) to a radially outer end (<NUM>),
characterised in that:
the at least one of the plurality of struts (<NUM>) has a leading edge stiffener (<NUM>) at the radially inner end (<NUM>) thereof, the leading edge stiffener (<NUM>) projecting into the hollow core (<NUM>) and merging with a stiffener ring (<NUM>) projecting from the radially inner surface (<NUM>) of the inner case (<NUM>), the leading edge stiffener (<NUM>) extending radially outwardly relative to the radially inner boundary of the annular exhaust gas path (20a).