Turbofan engine and method of operating same

The auxiliary duct can branch radially outwardly from the bypass duct, have a proximal end fluidly connecting the bypass duct and a distal end, a valve can be activatable to selectively open and close the auxiliary passage, and a structure can protrude 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.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, more particularly, to auxiliary ducts of turbofan engines.

BACKGROUND OF THE ART

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.

SUMMARY

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.

In a further aspect, there is provided an auxiliary duct system comprising: an auxiliary duct defining an auxiliary passage between a proximal end and a distal end, a valve activatable to selectively open and close the auxiliary passage, and a structure reducing the cross-sectional area of 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.

DETAILED DESCRIPTION

FIG.1illustrates a gas turbine engine10of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan12through which ambient air is propelled, a compressor section14for pressurizing the air, a combustor16in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases around the engine axis11, and a turbine section18for extracting energy from the combustion gases. The gas turbine engine10is of the turbofan type, and therefore has an annular bypass duct20. The bypass duct delimits a bypass passage22extending annularly around a the axis11. A forward and a rear directions can be defined along the axis11. The bypass passage22extends rearwardly from the fan12around and along an engine core24. A first portion of the flow from the fan12is directed to the engine core24as a working fluid for the turbine section18, and a second portion of the flow from the fan12is directed to the bypass passage22in the form of a bypass flow, for thrust.

In some embodiments, it can be required to provide the gas turbine engine10with an auxiliary duct26which branches off from the bypass duct20. Such an auxiliary duct26can serve to supply pressurized air to an aircraft system28such as an environmental cooling system (ECS) pre-cooler, or any other suitable aircraft system, for instance. The auxiliary duct26can have a proximal end30fluidly connecting the bypass duct20, and a distal end32which can directly or indirectly connect to the aircraft system28. The auxiliary duct26forms an auxiliary passage34from the proximal end30to the distal end32, the auxiliary passage34being fluidly connected to the bypass passage22at the proximal end30.

Depending on the embodiment, the amount of air to be supplied via the auxiliary passage34may 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 duct26can thus be associated to a trade-off of loss in engine efficiency. The aircraft system28may 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 system28, the loss may be more undesirable when the auxiliary air is not required by the aircraft system28.

To mitigate such undesired losses of engine efficiency, a valve36can be provided in the auxiliary duct26and used to selectively open or close the auxiliary passage34as a function of the auxiliary air requirements of the aircraft system28. For various reasons, it may not be feasible or otherwise suitable to position the valve directly at the proximal end30. In such cases, the valve36is positioned at the distal end32, or at an intermediate location along the length of the auxiliary passage34, between the proximal end30and the distal end32. In such cases, a portion of the auxiliary duct26extending between the proximal end30and the valve36can form a cavity38which is in fluid communication with the bypass passage22when 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 duct26, forcing it instead into the downstream portion of the bypass passage22where it can serve for thrust. However, the presence of the cavity38may not be ideal from an dynamic perspective. In particular, such an elongated cavity38can 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 duct22adjacent the proximal end30can 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 cavity38can be an undesired source of noise, or even of potential structural failure.

The interface between the auxiliary duct26and the bypass duct20can be configured in a manner that some dynamic pressure recovery from the bypass passage22occurs when the valve36is open. A typical curve of the pressure in the auxiliary duct26as a function of the mass flow circulating through it is depicted inFIG.2A. 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 inFIG.2B.

The more the interface between the auxiliary passage26and the bypass passage20at the proximal end30is 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 inFIG.3where the rear portion40of the auxiliary duct26penetrates radially to a certain extent into the bypass passage22forming a scoop42which redirects a portion of the axially incoming bypass flow into the radially outward orientation of a first section of the auxiliary passage34. Indeed, in this embodiment, to achieve sufficient flow, the scoop42is designed to reduce the pressure losses entering the auxiliary duct26and to recover some dynamic pressure from the bypass flow. Accordingly, circulating the auxiliary portion of the bypass flow through a curvilinear scoop42having a forward facing inlet (facing the fan12) can preserve flow momentum from the bypass flow in a first section of the auxiliary duct26.

In one embodiment, it can be desired to address the occurrences of resonances by perturbing the flow near the proximal end30. Such an approach can consist of introducing a fence or a louver directly at the proximal end30of the auxiliary duct26, 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 valve36is open. Accordingly, such an approach may impede flow rate momentum through the auxiliary duct26in situations where momentum through the auxiliary duct26is 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 structure44which generates asymmetrical losses to the circulation of air in the auxiliary duct26, and more specifically a greater loss in the reverse flow orientation (towards the proximal end30) than in the forward flow orientation (towards the valve36or distal end32) along the auxiliary passage34, 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 end30to the aircraft system28), which favors pressure recovery and momentum when the valve36is open, while increasing losses/impeding circulation in the reverse orientation (i.e. back to the proximal end30from the valve36), thus avoiding, or mitigating, the formation of a high energy resonance when the valve36is closed.

FIG.4presents an example of such a structure44. More specifically, the structure44protrudes partially from the auxiliary duct26into the auxiliary passage34, and in other words, restricts the cross-sectional area of the auxiliary passage34. The structure44can be located at an intermediate position between the proximal end30and the valve36. In other words, it can be spaced apart from both the proximal end30and from the valve36along the length45of the cavity38. In the embodiment presented inFIG.4, the structure44can be a Venturi-type structure extending axisymmetrically around a central axis or centerline of the auxiliary passage34, and presenting a greater pressure loss to reverse flow than to forward flow along the auxiliary passage34. 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 passage34can vary in alternate embodiments. For instance, the auxiliary passage34can 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 inFIG.4.

In some embodiments, the configuration of the auxiliary passage34relative to the bypass passage22may impart asymmetric features to the flow of air in the air passage34when the valve36is open. This can be the case, for instance, in the embodiment presented inFIGS.5A to5D.FIGS.5A and5B, in particular, show characteristics of the flow which can occur in typical operating conditions when the valve36is open. Given the fact that the auxiliary duct26branches off from the bypass duct20radially outwardly, a change in the traveling direction of the fluid, the pressurized air, occurs when the air traverses the inlet/proximal end30and circulates inside the first section of the duct. Such a change in flow orientation causes an asymmetry to the flow which can be seen inFIGS.5A and5B. Indeed, the speed and pressure of the fluid immediately downstream of the proximal end is significantly greater alongside the rear portion40of the auxiliary passage26than along the front portion47of the auxiliary passage26. In the reverse flow orientation (FIGS.5C,5D), however, such asymmetry may be absent, or even reversed. Indeed, in a scenario such as the one illustrated, where an oppositely oriented elbow48is defined between a first section50of the auxiliary duct26and a second section52. Indeed, in the illustrated embodiment, air progressing in the reverse direction will become pressed against the forward portion46of the auxiliary duct26rather than the rear portion40as it exits the elbow48. Indeed, in an embodiment such as illustratedFIG.5A, circulating the auxiliary flow through a curvilinear elbow48between the first section50and a second section52redirects 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 structure44which 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 inFIG.6. In the embodiment presented inFIG.6, a structure44is introduced in the form of a perforated plate which protrudes partially into the auxiliary passage34from the front portion47of the auxiliary duct26, such as presented in section A-A. Occupying the front portion47of the auxiliary duct26in a region where the forward flow is strongly compressed against the rear portion40(as best seen inFIGS.5A and5B), 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 inFIGS.5C and5D) which is compressed against the front portion47of the auxiliary duct as it exits the elbow48. 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 inFIG.7can use a structure44in the form of a louver51or a set of louvers to achieve a very similar aerodynamic effect. InFIG.7, the louver51extends obliquely into the auxiliary duct26leading to a progressively reducing surface area towards the valve36, along a given section of the auxiliary duct26. In other words, the louver51penetrates progressively deeper into the auxiliary passage34from the front portion47as it progresses the direction of the distal end or valve36.

Each of the embodiments presented inFIG.4,FIG.6andFIG.7use a structure44which generates asymmetric pressure losses to air flow depending on the orientation of the air flow. The embodiment presented inFIG.4intrinsically produces asymmetric pressure losses, and can work on a straight duct having uniform flow. The embodiments presented inFIGS.6and7use a structure44which is structurally asymmetrical relative to a central axis46of the auxiliary duct26, and which harnesses an asymmetry in the flow stemming from the irregular shape of the auxiliary duct26or of the flow within it such as which can stem from the configuration of the auxiliary passage26relative to the bypass passage22to 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 duct26bearing the structure44stems 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 scoop42and by the elbow48. More specifically, all these embodiments generate lesser pressure losses to air flowing towards the distal end32than pressure losses to air flowing towards the proximal end30.

In all the embodiments presented above, as evidenced inFIG.4for instance, the structure44generating the asymmetric pressure losses is not located at the proximal end30, but recessed therefrom into the auxiliary passage34. A first distance53between the structure44and the proximal end30of the auxiliary duct26can be of more than half of a cross-sectional dimension54of the air passage34at the proximal end30. The cross-sectional dimension54can be a diameter of the air passage at the proximal end30if the cross-sectional shape is circular. If the cross-sectional shape is non-circular, the cross-sectional dimension54can be a major dimension of the non-circular shape, a minor dimension of the non-circular shape, or an average between the two. The distance53between the structure44and the proximal end30of the auxiliary duct26can be of more than the entire cross-sectional dimension54of the air passage34at the proximal end30, or even more in other embodiments.

Referring toFIG.8, a method of operation can include a fan circulating100an annular bypass flow between a bypass duct20and an engine core24, branching off102an auxiliary portion of the bypass flow as an auxiliary flow, including circulating the auxiliary flow through a proximal end30of an auxiliary duct26, along the auxiliary duct26, and through a distal end32of the auxiliary duct26, closing104a valve36of the auxiliary duct26, thereby interrupting said auxiliary flow, and while the valve of the auxiliary duct26is closed, a structure44extending in the auxiliary duct26causing106first pressure losses to reverse flow from the valve36to the proximal end30, the structure causing second pressure losses to said auxiliary flow, the second pressure losses being lower than the first pressure losses, preferably 25% lower or more. The presence of the structure can impede acoustic resonances at a natural frequency cavity38formed by the auxiliary duct26between the proximal end30and the valve36when the valve36is closed.

In some embodiments, it can be preferred to avoid inducing not only resonances at a natural frequency of the cavity38, 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 structure44is placed along the length45of the cavity38. Indeed, positioning the structure at exactly half of the length45of the cavity38(the cavity38being is the “acoustically active” portion of the auxiliary duct26extending between the proximal end30and the valve36when the valve36is 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 structure44relatively close to the proximal end30, while maintaining the structure spaced from the proximal end30by at least half the cross-sectional dimension54of the auxiliary duct26, may be one way to achieve this in some embodiments. In particular, in an embodiment such as shown inFIGS.5A and5B, where a low velocity “bubble” is formed alongside the front portion46of the auxiliary duct26downstream of the proximal end30, it can be strategic to position the structure44within that area of forward flow low-velocity bubble. In most embodiments, it will be preferred for the distance53between the structure44and the proximal end30to be at least than 10%, preferably at least 20%, preferably at least 30% more or less than the length45of the cavity38or distance between the proximal end30and the valve36. To avoid higher energy harmonics, it can be preferred to position the structure at an irrational fraction of the length45of the cavity38. It can also be preferred to position the structure44closer to the proximal end30than to the valve36in some embodiments.

The embodiments described in this document provide non-limiting examples of possible implementations of the present technology. Upon review of the present disclosure, a person of ordinary skill in the art will recognize that changes may be made to the embodiments described herein without departing from the scope of the present technology. For example, the internal structures can be customized to non-circular ducts, the internal structures may be made of smart materials and may be intrinsically activated in response to specific flow parameters or topology, the internal structures may be machined, welded/brazed, moulded or 3D printed into the hosting duct. Yet further modifications could be implemented by a person of ordinary skill in the art in view of the present disclosure, which modifications would be within the scope of the present technology.