Patent Description:
Aircraft engines, such as gas turbine engines, use secondary air systems to provide air to the engine for non-combustion purposes, for example for internal engine cooling, sealing bearing cavities, etc. This non-combustion air therefore needs to be fed across the main gas path to the engine shafts and bearing cavities within the inner core of the engine. In certain engine configurations, a hollow strut is used as a conduit for the secondary airflow. This hollow strut is typically relatively large and airfoil shaped, and bridges from the outer casing of the main gas path to an inner surface of the main gas path, thereby providing support and also allowing the non-combustion secondary air to be routed through the interior of the strut to the inner core of the engine (and thus to bearing cavities, etc.).

While such secondary air systems and their associated structures and/or architecture are suitable for their intended purposes, continuous improvement is always sought. Amongst other things, there is a desire to be able to increase the supply capacity of the secondary airflow, should the need arise, while limiting the secondary airflow at high power engine conditions.

<CIT> discloses a prior art secondary air system of an aircraft gas turbine engine as set forth in the preamble of claim <NUM>.

<CIT> discloses prior art compressor rim thermal management.

In one aspect, there is provided a secondary air system (SAS) of an aircraft gas turbine engine as recited in claim <NUM>.

In another aspect, there is provided a method of operating an aircraft gas turbine engine having a secondary air system (SAS) as recited in claim <NUM>.

<FIG> illustrates an aircraft engine <NUM> (or simply "engine" <NUM>), which in this case is a gas turbine engine of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan <NUM> through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section <NUM> for extracting energy from the combustion gases. engine <NUM> has a longitudinal center axis <NUM>.

The engine <NUM> depicted in <FIG> is a turbofan engine, and therefore includes a bypass duct <NUM> that surrounds a core <NUM> of the engine, the engine core <NUM> including for example the compressor section <NUM>, the combustor <NUM> and the turbine section <NUM>. The fan <NUM> propels air through both the central engine core <NUM> and through the radially outer bypass duct <NUM>. However, it is to be understood that the present disclosure may also be applicable to other types of gas turbine engines, including turboshafts and turboprops for example. Indeed, the present disclosure may also be applicable to other types of airborne aircraft engines which have a secondary air system, whether or not they are traditional gas turbine engines. For example, the present SAS system may also be used in hybrid, alternate fuel and/or electric aircraft engines, provided that they include a secondary air system.

The engine <NUM> defines a main gas path <NUM> of combustion gasses flowing through the engine core <NUM>. The exemplified engine <NUM> shown is a "through-flow" type gas turbine engine, in which gases flow through the central core <NUM> of the engine from the air inlet <NUM> located at the forward end of the engine to the exhaust <NUM> located at the rearward (aft) end of the engine <NUM>. In the depicted embodiment, this direction of airflow along the main gas path <NUM> and through the core <NUM> of the engine <NUM> is generally in a direction opposite to the direction of travel D of the aircraft, in that the thrust T produced by the engine <NUM> in the aft direction propels the aircraft forward in the direction D.

However, the features of the secondary air system (SAS) <NUM> as described herein are similarly applicable to a "reverse-flow" turboprop or turboshaft engine, for example, wherein gases flow through the core of the engine from an inlet located at or near the rear (aft) end of the engine to the exhaust outlet located at a forward end (i.e. relative to the direction of travel of the aircraft) of the engine. In such a reverse flow engine configuration, the direction of airflow through the core of the engine is therefore generally in the same direction as the direction of travel of the aircraft.

It will be appreciated that the expressions "forward" and "aft" used herein refer to the relative disposition of components of the engine <NUM>, in correspondence to the "forward" and "aft" directions of the engine <NUM> and an aircraft including the engine <NUM> as defined with respect to the direction of travel D of the aircraft. In the embodiment shown in <FIG>, a component of the engine <NUM> that is "forward" of another component is arranged within the engine <NUM> such that it is located closer to the fan <NUM>. Similarly, a component of the engine <NUM> that is "aft" of another component is arranged within the engine <NUM> such that it is further away from the fan <NUM>. Similarly, unless indicated otherwise, the expressions "upstream" and "downstream" as used herein refer to similar relative axial dispositions of components of the engine relative to the direction of the main gas path <NUM> through the engine, from the air inlet <NUM> to the exhaust <NUM>.

The engine <NUM> includes a secondary air system (SAS) <NUM> to provide compressed air to the engine for non-combustion purposes, for example for example for internal engine cooling, sealing bearing cavities, feeding pneumatic systems, de-icing, meeting environmental control system requirements of the aircraft, etc. This compressed air used for non-combustion purposes will be referred to herein as "secondary airflow" or "secondary air". The SAS <NUM> is accordingly operable to bleed, distribute, handle and/or regulate the secondary air flow to and/or for one or more of such non-combustive air flow systems.

Depending on the engine configuration, the source of this secondary air distributed by the SAS <NUM> may include air bled off from the compressor section <NUM> of the engine <NUM> or air bled off from the bypass airflow flowing through the bypass duct <NUM> of the engine <NUM> (in the case of a turbofan engine, as depicted). For air bled from the compressor section, secondary air used for internal engine cooling and bearing cavity sealing may be bled from, for example, one or more locations near a high pressure compressor of the compressor section <NUM>. For example, compressor bleed air may be extracted from a location downstream from the outlet of the high pressure compressor (i.e. air from station <NUM> station of the engine, or "P3" air) and/or from a location upstream of the inlet of the high pressure compressor (i.e. air from station <NUM> of the engine, or "P2. <NUM>" air).

In the case of the turbofan engine <NUM> of <FIG>, the source of the secondary air for the SAS <NUM> may include bypass air which is withdrawn or bled from the outer bypass duct <NUM> of the engine <NUM>. Any suitable port or take-off may be used to direct the bypass air into the SAS <NUM>. In one particular embodiment, the bypass air may be initially cooled by a bypass air cooler (BAC) <NUM>, located for example in the bypass duct <NUM>, before being directed into the SAS <NUM> and thus providing the secondary airflow.

Referring to <FIG>, the compressor section <NUM> of the engine <NUM> may include an inter-compressor case (ICC) <NUM> within which certain elements of the SAS <NUM> as described herein may be integrated. Although the SAS <NUM> is described below in the context of the compressor <NUM> of the engine <NUM>, it is to be understood that the present SAS <NUM> can also be used elsewhere within the engine <NUM>, for example within the turbine section <NUM>. The ICC <NUM> includes generally a radially outer casing <NUM> and a radially inner casing <NUM> that are radially spaced apart, relative to the longitudinal central axis <NUM> of the engine <NUM>. In the depicted embodiment, at least two struts <NUM> extend radially between the radially inner and outer casings <NUM>, <NUM> of the ICC <NUM>. The struts <NUM> are hollow and therefore form conduits which define secondary air flow paths <NUM> radially through the struts. In at least the depicted embodiment, the secondary air flowing along the secondary air flow paths <NUM> flows radially inwardly through the struts <NUM>, that is from a radially outer end to a radially inner end of each of the struts <NUM>. From the radially inner end of each strut <NUM> (i.e. the outlet of the secondary air), the secondary airflow is direction along paths <NUM> to feed the secondary air to seals <NUM> and/or bearing cavities <NUM> within the inner core of the engine, proximate the main engine shafts. In alternate embodiments and engine configurations, however, it is understood that the secondary air may also and/or instead flow radially outwardly through the struts <NUM> of the ICC <NUM>. The external surfaces of the struts <NUM> may be airfoil shaped, given that these struts <NUM> extend through the main gas path of the compressor section <NUM>. The struts <NUM> may also provide structural support for bearing housings or bearing cavities <NUM> and/or other components located radially inward of the main gaspath <NUM> and/or for the outer casing <NUM> of the ICC <NUM> located radially outward of the main gaspath. The ICC <NUM> may also define a portion of the inner wall of the outer bypass duct <NUM>, which is disposed radially outward from the ICC <NUM> and the engine core <NUM>.

As mentioned above, in certain prior art engine configurations, a single, large, hollow strut is typically used as a conduit for the secondary airflow, wherein compressed air is directed through a single hollow strut that passes through the main gas path of the engine in order to feed compressed secondary air to the inner core of the engine. However, using a single strut to supply the secondary airflow may leave little margin for increasing the supply capacity of the secondary air, should the need arise. This can be alleviated, as per the present disclosure, by providing one or more flow paths, which may include using multiple struts instead of a single one, and limiting the flow within each of these flow paths using sonic orifices to control the flow therethrough. The need to limit the flow is driven by the air system supply requirements, particularly at high power operating regime when excess flow consumption may otherwise have detrimental effect on the performance of the engine. However, if other types of control orifices (i.e. not sonic orifices, as described herein) are used as means to limit the secondary flow, they could further decrease the amount of air supply available at low power regime, when typically the largest amount of flow possible is desired.

Referring now to <FIG>, the SAS <NUM> of the present disclosure accordingly includes a SAS feed pipe configuration <NUM>, which includes, in at least one particular embodiment, two or more separate SAS feed pipes, such as the two SAS feed pipes <NUM>, <NUM>' as will be explained below, for feeding the secondary air from the secondary air source into the inner core <NUM> of the engine <NUM>, via a corresponding number of hollow struts <NUM>. Additionally, as will also be explained in more detail below with reference to <FIG>, each of the SAS feed pipes <NUM> includes a sonic orifice <NUM> therein.

Although the SAS <NUM> will be described below with general reference to an embodiment wherein two SAS feed pipes <NUM>, <NUM>' are provided, it is to be understood that in another possible embodiment, a single SAS feed pipe <NUM> may be used, or alternately still more than two SAS feed pipes may be provided. Therefore, although in the depicted embodiment the SAS feed-pipe configuration <NUM> of the SAS <NUM> is a twin-pipe configuration, in that there are two SAS feed pipes <NUM>, <NUM>', it is to be understood that a single feed pipe <NUM> or more than two of the SAS feed pipes <NUM>, <NUM>' may also be provided. Regardless of the number of SAS feed pipes, however, each of the SAS feed pipes will include a sonic orifice <NUM> therein, as described below. Accordingly, any number of parallel flow paths <NUM>, <NUM>' (formed by the SAS feed pipes <NUM>, <NUM>' and their respective hollow struts <NUM>) may be provided, and can be selected depending on the flow demands of the particular engine <NUM> and/or the geometry and space envelope available. In the case of a single SAS feed pipe <NUM>, it may feed either a single corresponding hollow strut <NUM> or alternately a single SAS feed pipe <NUM> may feed secondary air to multiple (two or more) hollow struts <NUM>.

As seen in <FIG>, in the depicted embodiment, incoming secondary air flow <NUM> received from the secondary air source, which in this case is the BAC <NUM> in the outer bypass duct <NUM>, is split into two separate streams by a Y-junction or flow splitter <NUM>. According the incoming secondary air flow <NUM> from the source is split into a first secondary air stream <NUM> and a second secondary air stream <NUM>' by the flow splitter <NUM>, such that downstream of the flow splitter <NUM> the first and second secondary air streams <NUM>, <NUM>' flow through separate conduits. More particularly, as noted above, the feed-pipe configuration <NUM> of the SAS <NUM> includes a first SAS feed pipe <NUM> and, in the present embodiment, at least a second SAS feed pipe <NUM>'. The first SAS feed pipe <NUM> has an inlet <NUM> that is connected in fluid flow communication to a first outlet <NUM> of the flow splitter <NUM>, and the second SAS feed pipe <NUM>' has an inlet <NUM>' that is connected in fluid flow communication to a second outlet <NUM>' of the flow splitter <NUM>. As such, the first SAS feed pipe <NUM> contains and defines the first secondary air stream <NUM> and the second SAS feed pipe <NUM>' contains and defines the second secondary air stream <NUM>', both flowing through discrete conduits downstream of the flow splitter <NUM>.

In an alternate embodiment, however, the SAS may not include the flow splitter <NUM>, and instead each of the SAS feed pipes <NUM>, <NUM>' may be fed directly from either the same pressurized air source or from separate pressurized air sources. For example, each of the SAS feed pipes <NUM>, <NUM>' can be directly connected to the BAC <NUM> and/or the bypass duct <NUM>, with each having its own bleed or take-off port feeding bypass duct air into the two SAS feed pipes <NUM>, <NUM>'.

As best seen in <FIG>, the first and second SAS feed pipes <NUM>, <NUM>' have outlets <NUM>, <NUM>' which are connected in fluid flow communication with different hollow struts <NUM> of the ICC <NUM>. More particularly, the first SAS feed pipe <NUM> has an outlet <NUM> which is connected in fluid flow communication with an inlet <NUM> of a first strut conduit extending <NUM> through a first strut <NUM> and the second SAS feed pipe <NUM>' has an outlet <NUM>' that is connected in fluid flow communication with an inlet <NUM>' of a second strut conduit <NUM>' extending through a second strut <NUM>', whereby the first secondary air stream <NUM> is fed into the first strut conduit <NUM> of the first strut <NUM> by the first SAS feed pipe <NUM> and the second secondary air stream <NUM>' is fed into the second strut conduit <NUM>' of the second strut <NUM>' by the second SAS feed pipe <NUM>. In the depicted embodiment, the inlets <NUM>, <NUM>' of the first and second struts <NUM>, <NUM>' are located at their radially outer ends, such that the first and second secondary air streams <NUM>, <NUM>' flow through the first and second strut conduits <NUM>, <NUM>' of the hollow struts <NUM>, <NUM>' in a radially inward direction toward the center core <NUM> of the engine <NUM>.

Referring still to <FIG>, the hollow struts <NUM>, <NUM>' have respective outlets <NUM>, <NUM>' at their downstream ends (with respect to the direction of the secondary air flow through the struts). The downstream ends of the hollow struts <NUM>, <NUM>', which in the depicted embodiment are the radially inner ends of the struts, are connected in fluid flow communication with a single, common plenum in the form of a buffer cavity <NUM>. In the depicted embodiment, the buffer cavity <NUM> is arcuate and extends partially circumferentially within the radially inner casing <NUM> of the ICC <NUM> such as to fluidly interconnect the two outlets <NUM>, <NUM>' of the inner conduits of the two hollow struts <NUM>, <NUM>'. This buffer cavity <NUM> is accordingly located under, that is radially inward of, the main gas path <NUM> through the engine core <NUM>. The first and second secondary air streams <NUM>, <NUM>' , which respectively flow through the first and second SAS feed pipes <NUM>, <NUM>' and the first and second hollow struts <NUM>, <NUM>', accordingly reunite at the buffer cavity <NUM>.

From the buffer cavity <NUM>, the re-united secondary air is fed downstream (relative to the flow of secondary air) to the bearing cavities <NUM> and/or seals <NUM> of the engine core <NUM> along secondary flow paths <NUM> as shown in <FIG>.

Referring now back to <FIG>, each of the two SAS feed pipes <NUM>, <NUM>' includes a sonic orifice <NUM> therein, located between the inlets <NUM>, <NUM>' and the outlets <NUM>, <NUM>' of the pipes <NUM>, <NUM>'. The sonic orifices <NUM> are static and have no moving parts, but effectively provide a different flow restriction at various engine operating conditions, such that the secondary air flow through the SAS feed pipes <NUM>, <NUM>' can be more restricted at higher secondary air flow rates but less restricted at lower secondary air flow rates.

The sonic orifices <NUM> are accordingly operable to create a flow restriction when the engine <NUM> is operating at high power and/or high engine speed, such as during take-off, flight cruise, etc. More particularly, at such high power engine regimes when flow rates of the secondary air flow through the SAS feed pipes <NUM>, <NUM>' is relatively high, the sonic orifices <NUM> lead to a compressibility-driven restriction at the throats <NUM> within the orifices <NUM>. Thus, flow is limited due to choking of the flow.

However, when the engine <NUM> is operating at lower power and/or speed, such as at engine idle for example, when the flow rates of the secondary air flow are relatively lower, the secondary air flow through the SAS feed pipes <NUM>, <NUM>' is less restricted (in comparison with the degree or amount of restriction created at higher flow rates, as noted above). At low secondary air flow rates, which may occur during engine idle for example, significantly reduced flow restriction is therefore provided by the sonic orifices <NUM> and, thus, the secondary airflow can be maximized at low engine power. The flow restrictions provided by the sonic orifices <NUM> at high engine power however help to constrain the secondary air flow at such high engine power regimes to prevent excessive quantities of secondary airflow or secondary airflow having too high a pressure to be fed to the engine core <NUM> for sealing and/or cooling purposes during high engine power regimes. Thus, for a given air flow supply or specific fuel consumption (SFC) of the engine <NUM> running at high power, the shape of the sonic orifices <NUM> maximizes secondary air flow at engine idle (e.g. low power) when bearing cavity pressurization needs it the most. Stated differently, the sonic orifices <NUM> offer reduced flow restriction penalty at low engine speeds (e.g. at engine idle), while still providing flow restriction at the throats of the sonic orifices <NUM> at high power (because the air flow is choked by the sonic orifice thereby causing a compressibility-driven restriction).

The sonic orifices <NUM> can be tailored to have negligible flow restriction at low power (to promote greater air-supply) and to restrict the secondary air flow at higher engine power regimes, and this can be selected and/or tailored depending upon the secondary air requirements of the particular engine.

Accordingly, the sonic orifices <NUM> are operable to limit the secondary airflow at high engine power, relative to what would otherwise be possible (i.e. if other components, such as struts, etc. were to act as flow restrictors). Stated differently, the sonic orifices <NUM> are selected such as to act as the dominant flow restriction/limitation within the secondary airflow path between the source and the buffer cavity. The quantity of parallel flow paths defined by the SAS feed pipes and their respective hollow struts downstream thereof is selected as required, and may be dependent upon a total SAS secondary airflow demand and/or restriction of existing passages either upstream of downstream of the SAS feed pipes. The quantity of parallel paths may thus increase with engine flow demand and may be reduced with available passage sizes. For example, in a particular embodiment, it may be possible to use a single flow path - i.e. a single SAS feed pipe <NUM> - if it is sufficient to meet the secondary airflow demands of the engine when operating a low engine power. In all cases, however, and in each parallel path, the sonic orifice <NUM> is the dominant restrictor at high power, while not unduly limiting secondary airflow at lower powers - when the mass flow and/or pressure of the secondary airflow is lower.

As seen in <FIG>, the sonic orifices <NUM> are formed by converging-diverging nozzles. More particularly, in the embodiment depicted in <FIG>, the converging portion <NUM> the converging-diverging nozzle is shorter in axial length (i.e. in the direction of flow <NUM>, <NUM>') than the longer diverging portion <NUM> of the converging-diverging nozzle.

In an alternate embodiment, the sonic orifices <NUM> may include and/or be replaced with flat plate orifices or other suitable and similar flow restrictors. However, such flat plate orifices may offer less advantages, as there would be less flow supply at idle and therefore the flow restriction provided by such flat-plate orifices may be substantially uniform regardless of the flow rates. From a flow supply perspective, therefore, using sonic orifices <NUM> (comprising converging-diverging nozzles for example) offers a more optimal solution because the secondary airflow is restricted less at low power regimes (flow rates) and more at higher power regimes (flow rates).

As can be see in <FIG>, each of the two SAS feed pipes <NUM>, <NUM>' may, in one particular embodiment, include a first flexible hose portion <NUM> at an upstream end (relative to the flow of secondary air therethrough) of the SAS feed pipe <NUM>, <NUM>' and a second rigid pipe portion <NUM> at a downstream end ((relative to the flow of secondary air therethrough) of the SAS feed pipe <NUM>, <NUM>'. The sonic orifice <NUM> may, as shown in the depicted embodiment, be located within the second rigid pipe portion <NUM> of the SAS feed pipe <NUM>, <NUM>'. More particularly still, the sonic orifice <NUM> may be located at the most upstream end of the second rigid pipe portion <NUM>, proximate the junction between the first flexible hose portion <NUM> and the second rigid pipe portion <NUM>. The use of the first flexible hose portions <NUM> may enable the SAS feed pipes <NUM>, <NUM>' to be supported by, or routed-through, existing engine hardware.

The secondary air system (SAS) <NUM> as described herein therefore uses a feed-pipe configuration <NUM> (which in a particular embodiment is a multi-pipe configuration composed of two or more SAS feed pipes) for feeding the SAS air into the engine core <NUM>, with sonic orifices <NUM> provided between the inlets <NUM>, <NUM>' and the outlets <NUM>, <NUM>' of each of the SAS feed pipes <NUM>, <NUM>'.

Although the concepts described herein with respect to the SAS feed pipes <NUM>, <NUM>' are done in the context of the SAS <NUM> in general, and the secondary air inlet feed to the engine core <NUM> in particular, it is to be understood that twin pipe configuration <NUM> having the sonic orifices <NUM> therein may be extended and/or applied to other feeder pipes within the engine <NUM>, including but not limited to other secondary air flow passages - for example those in the turbine section of the engine, or elsewhere.

With reference to <FIG> and further to the embodiments described above, a method <NUM> of operating the gas turbine engine <NUM> having the SAS <NUM> as described herein may also include, generally, the steps of: receiving, at <NUM>, secondary airflow from a source provided by the aircraft engine, the secondary airflow including one or more secondary air streams flowing downstream of the source; flowing, at <NUM>, each of the secondary air streams through a respective SAS feed pipe, wherein an outlet of the SAS feed pipe is in fluid communication with a buffer cavity that receives therein the secondary air streams therein; and generating, at <NUM>, a first flow restriction in each of the secondary air streams during a first (high) power regime of the engine, and generating a second flow restriction in the secondary air streams during a second (lower) power regime of the engine, the second power regime being lower than the first power regime, and the second flow restriction being less than the first flow restriction.

The step <NUM> of generating may further comprise, in certain embodiments, using sonic orifices located in each of the SAS pipes to generate the first and second flow restrictions. The method <NUM> may also further comprises, downstream of the SAS feed pipes, flowing the secondary air streams through a respective hollow strut, wherein outlets of the hollow struts are in fluid communication with the buffer cavity.

Claim 1:
A secondary air system (SAS) (<NUM>) of an aircraft gas turbine engine (<NUM>) having a main gas path (<NUM>) extending through an engine core (<NUM>), the aircraft engine (<NUM>) producing secondary airflow (<NUM>; <NUM>) from a source of secondary air (<NUM>), the SAS (<NUM>) comprising:
a hollow strut (<NUM>; <NUM>') configured to extend radially through the main gas path (<NUM>), the hollow strut (<NUM>; <NUM>') defining therein a strut conduit (<NUM>; <NUM>') extending between a strut inlet (<NUM>; <NUM>') and a strut outlet (<NUM>; <NUM>') at opposite ends of the hollow strut (<NUM>; <NUM>'); and
a buffer cavity (<NUM>), wherein the strut outlet (<NUM>; <NUM>') is in fluid flow communication with the buffer cavity (<NUM>) for feeding the secondary airflow (<NUM>; <NUM>) to the engine core (<NUM>);
characterised in that the SAS comprises:
two or more SAS feed pipes (<NUM>; <NUM>') each having an inlet (<NUM>; <NUM>') receiving the secondary airflow (<NUM>; <NUM>) from the source of secondary air (<NUM>) and respectively defining therein a secondary air stream (<NUM>, <NUM>') flowing in parallel, and an outlet (<NUM>; <NUM>') in fluid flow communication with the strut inlet (<NUM>; <NUM>') to feed the secondary airflow (<NUM>; <NUM>) into the strut conduit (<NUM>; <NUM>'), wherein each of the SAS feed pipes (<NUM>; <NUM>') has a sonic orifice (<NUM>) therein between the inlet (<NUM>; <NUM>') and the outlet (<NUM>; <NUM>') thereof, wherein the sonic orifice (<NUM>) is formed by a converging-diverging nozzle and is configured such that: at high power and/or high engine speed, the sonic orifice (<NUM>) forms a compressibility-driven restriction for to limit the secondary airflow (<NUM>; <NUM>) due to choking; and at low power and/or low engine speed the sonic orifice (<NUM>) forms a significantly reduced flow restriction for maximizing the secondary airflow (<NUM>; <NUM>).