Patent Description:
Some turbofan engines have an auxiliary duct branching out from the bypass duct and configured to supply an aircraft system with a flow of pressurized air. The aircraft system can be a pre-cooler of an environment cooling system (ECS) for instance. In some cases, the auxiliary duct can be equipped with a valve to allow selectively opening or closing the auxiliary passage. When the valve is closed, a portion of the auxiliary duct open to the bypass flow can extend between the bypass duct and the valve. While existing auxiliary ducts were satisfactory to a certain degree, there always remains room for improvement. <CIT> discloses a flush inlet scoop design for an aircraft bleed air system.

In one aspect, there is provided a turbofan engine comprising : a bypass duct forming a bypass passage extending annularly around an axis, the bypass passage extending rearwardly from a fan and around an engine core; an auxiliary duct branching radially outwardly from the bypass duct relative the axis, the auxiliary duct having a proximal end fluidly connecting the bypass duct and a distal end, the auxiliary duct defining an auxiliary passage between the proximal end and the distal end, the auxiliary passage fluidly connecting the bypass passage, a valve activatable to selectively open and close the auxiliary passage, and a structure protruding partially from the auxiliary duct into the auxiliary passage, the structure spaced apart from the proximal end, between the proximal end and the valve, the structure generating lesser pressure losses when flow in the auxiliary passage is directed towards the distal end than when the flow is directed towards the proximal end.

In another aspect, there is provided a method of operating a turbofan engine comprising: a fan circulating an annular bypass flow between a bypass duct and an engine core; branching off an auxiliary portion of a bypass flow as an auxiliary flow, including circulating the auxiliary flow through a proximal end of an auxiliary duct, along the auxiliary duct, and through a distal end of the auxiliary duct, closing a valve of the auxiliary duct, thereby interrupting said auxiliary flow, and while the valve of the auxiliary duct is closed, a structure extending in the auxiliary duct causing first pressure losses to reverse flow from the valve to the proximal end, the structure causing second pressure losses to said auxiliary flow, the second pressure losses being lower than the first pressure losses.

<FIG> illustrates a gas turbine engine <NUM> 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 around the engine axis <NUM>, and a turbine section <NUM> for extracting energy from the combustion gases. The gas turbine engine <NUM> is of the turbofan type, and therefore has an annular bypass duct <NUM>. The bypass duct delimits a bypass passage <NUM> extending annularly around a the axis <NUM>. A forward and a rear directions can be defined along the axis <NUM>. The bypass passage <NUM> extends rearwardly from the fan <NUM> around and along an engine core <NUM>. A first portion of the flow from the fan <NUM> is directed to the engine core <NUM> as a working fluid for the turbine section <NUM>, and a second portion of the flow from the fan <NUM> is directed to the bypass passage <NUM> in the form of a bypass flow, for thrust.

In some embodiments, it can be required to provide the gas turbine engine <NUM> with an auxiliary duct <NUM> which branches off from the bypass duct <NUM>. Such an auxiliary duct <NUM> can serve to supply pressurized air to an aircraft system <NUM> such as an environmental cooling system (ECS) pre-cooler, or any other suitable aircraft system, for instance. The auxiliary duct <NUM> can have a proximal end <NUM> fluidly connecting the bypass duct <NUM>, and a distal end <NUM> which can directly or indirectly connect to the aircraft system <NUM>. The auxiliary duct <NUM> forms an auxiliary passage <NUM> from the proximal end <NUM> to the distal end <NUM>, the auxiliary passage <NUM> being fluidly connected to the bypass passage <NUM> at the proximal end <NUM>.

Depending on the embodiment, the amount of air to be supplied via the auxiliary passage <NUM> may need to be substantial. This auxiliary flow may be subject to various pressure losses or aerodynamic transformations which make it lose some or all of its efficiency from the point of view of propulsion. The circulation of air through the auxiliary duct <NUM> can thus be associated to a trade-off of loss in engine efficiency. The aircraft system <NUM> may not be continuously in operation, or may not continuously require the supply of auxiliary air. While the loss in engine efficiency may be considered a suitable trade-off during operation of the aircraft system <NUM>, the loss may be more undesirable when the auxiliary air is not required by the aircraft system <NUM>.

To mitigate such undesired losses of engine efficiency, a valve <NUM> can be provided in the auxiliary duct <NUM> and used to selectively open or close the auxiliary passage <NUM> as a function of the auxiliary air requirements of the aircraft system <NUM>. For various reasons, it may not be feasible or otherwise suitable to position the valve directly at the proximal end <NUM>. In such cases, the valve <NUM> is positioned at the distal end <NUM>, or at an intermediate location along the length of the auxiliary passage <NUM>, between the proximal end <NUM> and the distal end <NUM>. In such cases, a portion of the auxiliary duct <NUM> extending between the proximal end <NUM> and the valve <NUM> can form a cavity <NUM> which is in fluid communication with the bypass passage <NUM> when the valve is closed.

Such a configuration can help in successfully mitigating undesired losses by preventing or significantly limiting the flow of auxiliary air through the auxiliary duct <NUM>, forcing it instead into the downstream portion of the bypass passage <NUM> where it can serve for thrust. However, the presence of the cavity <NUM> may not be ideal from an dynamic perspective. In particular, such an elongated cavity <NUM> can have one or more resonance frequencies, such as a natural frequency and harmonics thereof, with the pressurized air medium, and the highly dynamic flow of air in the bypass duct <NUM> adjacent the proximal end <NUM> can represent a driving excitation for the resonance frequency. High energy air vibrations at the resonance frequency(ies) associated to variations of flow and of air pressure in the cavity <NUM> can be an undesired source of noise, or even of potential structural failure.

The interface between the auxiliary duct <NUM> and the bypass duct <NUM> can be configured in a manner that some dynamic pressure recovery from the bypass passage <NUM> occurs when the valve <NUM> is open. A typical curve of the pressure in the auxiliary duct <NUM> as a function of the mass flow circulating through it is depicted in <FIG>. Large forward flow is achieved when there is low back pressure, while large back pressure is required in order to reverse the flow. In an embodiment, a source of the resonance can be attributed to the sideways "s"-shape of the curve near zero flow, where for small forward flow larger pressure is seen in the auxiliary duct as there is some dynamic pressure recovery, while to drive flow in the reverse direction only sufficient pressure to overcome the static pressure in the bypass passage is required. When the valve is closed, a hysteresis can develop corresponding red-highlighted area in the curve which can lead to large intensity acoustic tones.

To eliminate the acoustic resonance it can be desired to modify the shape of the pressure curve such as to reduce or eliminate possibility of hysteresis as depicted in the curve in <FIG>.

The more the interface between the auxiliary passage <NUM> and the bypass passage <NUM> at the proximal end <NUM> is optimized from the point of view of pressure recovery, the greater the tendency to generate large acoustic tones may be. For instance, an example auxiliary passage shape is shown in <FIG> where the rear portion <NUM> of the auxiliary duct <NUM> penetrates radially to a certain extent into the bypass passage <NUM> forming a scoop <NUM> which redirects a portion of the axially incoming bypass flow into the radially outward orientation of a first section of the auxiliary passage <NUM>. Indeed, in this embodiment, to achieve sufficient flow, the scoop <NUM> is designed to reduce the pressure losses entering the auxiliary duct <NUM> and to recover some dynamic pressure from the bypass flow. Accordingly, circulating the auxiliary portion of the bypass flow through a curvilinear scoop <NUM> having a forward facing inlet (facing the fan <NUM>) can preserve flow momentum from the bypass flow in a first section of the auxiliary duct <NUM>.

In one embodiment, it can be desired to address the occurrences of resonances by perturbing the flow near the proximal end <NUM>. Such an approach can consist of introducing a fence or a louver directly at the proximal end <NUM> of the auxiliary duct <NUM>, at the inlet. Such an approach may have a suitable effect from the point of view of avoiding large intensity acoustic tones, but may, however, be detrimental from the point of view of pressure recovery when the valve <NUM> is open. Accordingly, such an approach may impede flow rate momentum through the auxiliary duct <NUM> in situations where momentum through the auxiliary duct <NUM> is preferably optimized. Such an approach may thus leave a want for improvement from the point of view of dynamic pressure recovery.

It was conceived that introducing a structure <NUM> which generates asymmetrical losses to the circulation of air in the auxiliary duct <NUM>, and more specifically a greater loss in the reverse flow orientation (towards the proximal end <NUM>) than in the forward flow orientation (towards the valve <NUM> or distal end <NUM>) along the auxiliary passage <NUM>, could be better than the previous approach. Indeed, such an approach can be optimized to limit losses in the forward flow orientation (i.e. from the proximal end <NUM> to the aircraft system <NUM>), which favors pressure recovery and momentum when the valve <NUM> is open, while increasing losses/impeding circulation in the reverse orientation (i.e. back to the proximal end <NUM> from the valve <NUM>), thus avoiding, or mitigating, the formation of a high energy resonance when the valve <NUM> is closed.

<FIG> presents an example of such a structure <NUM>. More specifically, the structure <NUM> protrudes partially from the auxiliary duct <NUM> into the auxiliary passage <NUM>, and in other words, restricts the cross-sectional area of the auxiliary passage <NUM>. The structure <NUM> can be located at an intermediate position between the proximal end <NUM> and the valve <NUM>. In other words, it can be spaced apart from both the proximal end <NUM> and from the valve <NUM> along the length <NUM> of the cavity <NUM>. In the embodiment presented in <FIG>, the structure <NUM> can be a Venturi-type structure extending axisymmetrically around a central axis or centerline of the auxiliary passage <NUM>, and presenting a greater pressure loss to reverse flow than to forward flow along the auxiliary passage <NUM>. It will be understood that other aerodynamic structures may lead to suitable effects and even be preferred in other embodiments. Moreover, the shape of the auxiliary passage <NUM> can vary in alternate embodiments. For instance, the auxiliary passage <NUM> can be straight instead of curvilinear in an alternate embodiment, and a Venturi-type structure provided within a straight passage can operate similarly to the one illustrated in <FIG>.

In some embodiments, the configuration of the auxiliary passage <NUM> relative to the bypass passage <NUM> may impart asymmetric features to the flow of air in the air passage <NUM> when the valve <NUM> is open. This can be the case, for instance, in the embodiment presented in <FIG>. <FIG>, in particular, show characteristics of the flow which can occur in typical operating conditions when the valve <NUM> is open. Given the fact that the auxiliary duct <NUM> branches off from the bypass duct <NUM> radially outwardly, a change in the traveling direction of the fluid, the pressurized air, occurs when the air traverses the inlet/proximal end <NUM> and circulates inside the first section of the duct. Such a change in flow orientation causes an asymmetry to the flow which can be seen in <FIG>. Indeed, the speed and pressure of the fluid immediately downstream of the proximal end is significantly greater alongside the rear portion <NUM> of the auxiliary passage <NUM> than along the front portion <NUM> of the auxiliary passage <NUM>. In the reverse flow orientation (<FIG>), however, such asymmetry may be absent, or even reversed. Indeed, in a scenario such as the one illustrated, where an oppositely oriented elbow <NUM> is defined between a first section <NUM> of the auxiliary duct <NUM> and a second section <NUM>. Indeed, in the illustrated embodiment, air progressing in the reverse direction will become pressed against the forward portion <NUM> of the auxiliary duct <NUM> rather than the rear portion <NUM> as it exits the elbow <NUM>. Indeed, in an embodiment such as illustrated <FIG>, circulating the auxiliary flow through a curvilinear elbow <NUM> between the first section <NUM> and a second section <NUM> redirects the flow towards an axial orientation, and circulating the flow in the reverse orientation through the same area redirects the flow towards a radially-inward orientation.

In a scenario such as the one illustrated, when the flow characteristics are not the same in the forward and the reverse directions, introducing a structure <NUM> which would have a symmetrical effect on a uniform flow may have a suitable asymmetrical effect given the differing flow configurations in the opposite flow orientations. Accordingly, such a concept can be harnessed to provide greater pressure losses in the reverse orientation than in the forward orientation.

An example of an embodiment harnessing this concept is illustrated in <FIG>. In the embodiment presented in <FIG>, a structure <NUM> is introduced in the form of a perforated plate which protrudes partially into the auxiliary passage <NUM> from the front portion <NUM> of the auxiliary duct <NUM>, such as presented in section A-A. Occupying the front portion <NUM> of the auxiliary duct <NUM> in a region where the forward flow is strongly compressed against the rear portion <NUM> (as best seen in <FIG>), the structure has a limited effect on the forward flow. The forward flow essentially "slips" under the structure. However, it has a significant effect on the rearward flow (as best seen in <FIG>) which is compressed against the front portion <NUM> of the auxiliary duct as it exits the elbow <NUM>. Sufficient energy can be dissipated by the perforated baffle as the flow exits in the reverse direction to reduce or eliminate the acoustic resonance.

The example of a perforated plate is but one of many possible alternative configurations. For instance, an embodiment presented in <FIG> can use a structure <NUM> in the form of a louver <NUM> or a set of louvers to achieve a very similar aerodynamic effect. In <FIG>, the louver <NUM> extends obliquely into the auxiliary duct <NUM> leading to a progressively reducing surface area towards the valve <NUM>, along a given section of the auxiliary duct <NUM>. In other words, the louver <NUM> penetrates progressively deeper into the auxiliary passage <NUM> from the front portion <NUM> as it progresses the direction of the distal end or valve <NUM>.

Each of the embodiments presented in <FIG>, <FIG> and <FIG> use a structure <NUM> which generates asymmetric pressure losses to air flow depending on the orientation of the air flow. The embodiment presented in <FIG> intrinsically produces asymmetric pressure losses, and can work on a straight duct having uniform flow. The embodiments presented in <FIG> and <FIG> use a structure <NUM> which is structurally asymmetrical relative to a central axis <NUM> of the auxiliary duct <NUM>, and which harnesses an asymmetry in the flow stemming from the irregular shape of the auxiliary duct <NUM> or of the flow within it such as which can stem from the configuration of the auxiliary passage <NUM> relative to the bypass passage <NUM> to impart asymmetric pressure losses in the different orientations. More specifically, in these embodiments, the asymmetry in both orientations of the flow in the portion of the auxiliary duct <NUM> bearing the structure <NUM> stems from the pressing of the forward flow and from the pressing of the rearward flow against opposite portions (forward or rearward) given the turning of the flow generated by the scoop <NUM> and by the elbow <NUM>. More specifically, all these embodiments generate lesser pressure losses to air flowing towards the distal end <NUM> than pressure losses to air flowing towards the proximal end <NUM>.

In all the embodiments presented above, as evidenced in <FIG> for instance, the structure <NUM> generating the asymmetric pressure losses is not located at the proximal end <NUM>, but recessed therefrom into the auxiliary passage <NUM>. A first distance <NUM> between the structure <NUM> and the proximal end <NUM> of the auxiliary duct <NUM> can be of more than half of a cross-sectional dimension <NUM> of the air passage <NUM> at the proximal end <NUM>. The cross-sectional dimension <NUM> can be a diameter of the air passage at the proximal end <NUM> if the cross-sectional shape is circular. If the cross-sectional shape is non-circular, the cross-sectional dimension <NUM> can be a major dimension of the non-circular shape, a minor dimension of the non-circular shape, or an average between the two. The distance <NUM> between the structure <NUM> and the proximal end <NUM> of the auxiliary duct <NUM> can be of more than the entire cross-sectional dimension <NUM> of the air passage <NUM> at the proximal end <NUM>, or even more in other embodiments.

Referring to <FIG>, a method of operation can include a fan circulating <NUM> an annular bypass flow between a bypass duct <NUM> and an engine core <NUM>, branching off <NUM> an auxiliary portion of the bypass flow as an auxiliary flow, including circulating the auxiliary flow through a proximal end <NUM> of an auxiliary duct <NUM>, along the auxiliary duct <NUM>, and through a distal end <NUM> of the auxiliary duct <NUM>, closing <NUM> a valve <NUM> of the auxiliary duct <NUM>, thereby interrupting said auxiliary flow, and while the valve of the auxiliary duct <NUM> is closed, a structure <NUM> extending in the auxiliary duct <NUM> causing <NUM> first pressure losses to reverse flow from the valve <NUM> to the proximal end <NUM>, the structure causing second pressure losses to said auxiliary flow, the second pressure losses being lower than the first pressure losses, preferably <NUM>% lower or more. The presence of the structure can impede acoustic resonances at a natural frequency cavity <NUM> formed by the auxiliary duct <NUM> between the proximal end <NUM> and the valve <NUM> when the valve <NUM> is closed.

In some embodiments, it can be preferred to avoid inducing not only resonances at a natural frequency of the cavity <NUM>, but also secondary resonances, which may be considered as "harmonics". This can be achieved in some embodiments by the choice of location at which the structure <NUM> is placed along the length <NUM> of the cavity <NUM>. Indeed, positioning the structure at exactly half of the length <NUM> of the cavity <NUM> (the cavity <NUM> being is the "acoustically active" portion of the auxiliary duct <NUM> extending between the proximal end <NUM> and the valve <NUM> when the valve <NUM> is closed) may successfully impede resonance at the natural frequency, while allowing relatively strong harmonics at twice the natural frequency or at other higher frequencies. It can be desired to strategically position the structure at a location where it will impede all significant resonances, which can be referred to as resonances occurring at frequencies of interest. Positioning the structure <NUM> relatively close to the proximal end <NUM>, while maintaining the structure spaced from the proximal end <NUM> by at least half the cross-sectional dimension <NUM> of the auxiliary duct <NUM>, may be one way to achieve this in some embodiments. In particular, in an embodiment such as shown in <FIG>, where a low velocity "bubble" is formed alongside the front portion <NUM> of the auxiliary duct <NUM> downstream of the proximal end <NUM>, it can be strategic to position the structure <NUM> within that area of forward flow low-velocity bubble. In most embodiments, it will be preferred for the distance <NUM> between the structure <NUM> and the proximal end <NUM> to be at least than <NUM>%, preferably at least <NUM>%, preferably at least <NUM>% more or less than the length <NUM> of the cavity <NUM> or distance between the proximal end <NUM> and the valve <NUM>. To avoid higher energy harmonics, it can be preferred to position the structure at an irrational fraction of the length <NUM> of the cavity <NUM>. It can also be preferred to position the structure <NUM> closer to the proximal end <NUM> than to the valve <NUM> in some embodiments.

Claim 1:
A turbofan engine (<NUM>) comprising:
a bypass duct (<NUM>) forming a bypass passage (<NUM>) extending annularly around an axis (<NUM>), the bypass passage (<NUM>) extending rearwardly from a fan (<NUM>) and around an engine core (<NUM>);
an auxiliary duct (<NUM>) branching radially outwardly from the bypass duct (<NUM>) relative the axis (<NUM>), the auxiliary duct (<NUM>) having a proximal end (<NUM>) fluidly connecting the bypass duct (<NUM>) and a distal end (<NUM>), the auxiliary duct (<NUM>) defining an auxiliary passage (<NUM>) between the proximal end (<NUM>) and the distal end (<NUM>), the auxiliary passage (<NUM>) fluidly connecting the bypass passage (<NUM>); and
a valve activatable to selectively open and close the auxiliary passage (<NUM>), characterised in that the engine (<NUM>) further comprises:
a structure (<NUM>) protruding partially from the auxiliary duct (<NUM>) into the auxiliary passage (<NUM>), the structure (<NUM>) spaced apart from the proximal end (<NUM>), between the proximal end (<NUM>) and the valve (<NUM>), the structure (<NUM>) generating lesser pressure losses when flow in the auxiliary passage (<NUM>) is directed towards the distal end (<NUM>) than when the flow is directed towards the proximal end (<NUM>).