The application relates generally to gas turbine engine and, more particularly, to systems and method used to cool hot engine fluids.
Gas turbine engines, more specifically turbofan engines, comprise a fan case having a by-pass duct for receiving an annular by-pass flow surrounding the engine core. During operation, the temperature of the annular by-pass flow can be sufficiently lower than the temperatures of the engine core that a surface cooler can be used to provide heat transfer between a hot engine fluid and colder air of the annular by-pass flow. Such hot engine fluid may be, for instance, lubricating fluids or oil from engine systems.
Surface coolers typically have a plurality of fins which are usually rectangular and protrude radially into the by-pass duct. Although the fins are disposed parallel to the annular by-pass flow, they generate drag and associated losses.
In one aspect, there is provided a gas turbine engine, comprising: an engine core having a compressor, a turbine, a combustor, and a rotation axis; a fan case wall extending circumferentially and having a radially-inner surface; a by-pass duct between the fan case wall and the engine core, a plurality of struts circumferentially distributed and extending across the by-pass duct; a surface cooler adjacent to the radially-inner surface of the fan case wall and configured to be exposed to a by-pass air flow through the by-pass duct during operation of the gas turbine engine, the surface cooler fluidly communicating with a fluid circuit of the engine requiring cooling of a fluid; and a recirculation conduit extending between an inlet in the radially-inner surface of the fan case wall disposed downstream of the surface cooler and an outlet in the radially-inner surface of the fan case wall disposed upstream of the surface cooler.
In another aspect, there is provided a fan case assembly of a gas turbine engine, comprising: a fan case wall circumferentially extending around a longitudinal axis of the gas turbine engine and having a radially-inner surface; a recirculation conduit circumferentially extending between an inlet and an outlet defined in the fan case wall, the inlet disposed downstream of the outlet, the recirculation conduit configured to deliver by-pass air from the inlet to the outlet; and a surface cooler mounted adjacent the radially-inner surface between the inlet and the outlet, the surface cooler communicating with a fluid circuit of the engine requiring cooling of a fluid.
In yet another aspect, there is provided a method for cooling an engine fluid circulating in an engine core of a gas turbine engine, comprising: receiving a by-pass flow in a by-pass duct defined by a fan case wall surrounding the engine core; transferring heat from the engine fluid to the by-pass flow by convection with a surface cooler adjacent to a radially-inner surface of the fan case wall; bleeding the by-pass flow downstream of the surface cooler through an outlet in the radially-inner surface; and recirculating an extracted portion of the by-pass flow and injecting it back into the by-pass flow at a position upstream of the surface cooler through an inlet in the radially-inner surface, the inlet being fluidly connected to the outlet by a recirculation conduit.
Reference is now made to the accompanying figures in which:
The gas turbine engine 10 further comprises a fan case defining a by-pass duct 22 surrounding the engine core that comprises the compressor 14, combustor 16, and turbine 18. A plurality of struts or vanes 24 are circumferentially disposed around the engine core and extend from a case of the engine core 26 toward the fan case. The struts 24 are disposed downstream of the fan 12 relative to a direction of the flow D. The struts 24 are configured for structurally positioning the fan case wall 30 relative to the engine core case 26.
In a particular embodiment, the engine 10 comprises a radially-outer nacelle wall 28 and a radially-inner fan case wall 30 defined by the fan section 20. The nacelle wall 28 is radially spaced-apart from the fan case wall 30. In a particular embodiment, the gas turbine engine 10 further comprises a surface cooler 32 circumferentially extending around the fan case wall 30. In a particular embodiment, the surface cooler 32 is mounted to the fan case wall 30.
Surface coolers can be used to remove heat from engine air and oil systems and are typically mounted in engine bypass duct 22 where high moving air mass is available. Heat transfer occurs by a process of convection with a flow of air circulating in the by-pass duct 22. Surface cooler thermal performance is thus highly dependent on Reynold no/local Mach number and its total wetted surface area. Surface coolers are more specifically used as air-cool-oil-cooler (ACOC) system and air cooler for the integrated drive gear (IDG) system. The IDG system is used to provide electrical power to the aircraft and has a gear box. Oil of the gear box is cooled with the surface cooler.
For a given bypass Mach number a balance is required for cooler surface area needed for heat rejection relative to the losses generated by skin friction. Others factors such as space restriction, ease of accessibility/installation also dictate the surface cooler geometry and dimensions.
For turbofan engines bypass duct losses can play an important role in engine specific fuel consumption (SFC). The losses in the duct 22 can have a very large exchange change rate to SFC and even be higher than that of fan or the low pressure compressor. It can be a suitable place in the engine where SFC can be readily recovered or reduced. Unfortunately, in reality when mechanical/manufacturing limits are imposed, most by-pass components, such as surface coolers, affect engine SFC.
In a particular embodiment, the gas turbine engine 10 further comprises a fluid recirculation sub-system comprising pipes 25 carrying an engine fluid from the engine core to the surface cooler 32 to be cooled. In one embodiment, the pipes 25 are disposed within a hollow portion of the struts 24. The pipes 25 comprises a first pipe 25a to carry the hot fluid from the engine core to the surface cooler 32. Another pipe 25b is used to carry the hot fluid that has been cooled from the surface cooler 32 back to the engine core. Others mean known in the art may be used without departing from the scope of the present disclosure. Such hot fluid may be lubricating oil or other fluids from systems of the engine 10. Oil is re-circulated through tubes 25 running circumferentially outside of surface cooler fin. The cooler fin on the air side carries heat away by process of convection. Cooled oil is returned to oil tank and is then pumped through pipe 25 to supply oil to bearings.
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In a particular embodiment, the fins 40 have a rectangular cross-section taken along an axial plane. Any other shape may be used without departing from the scope of the present disclosure. In one embodiment, the surface cooler 32 is circumferentially mounted and extends around the entire circumference of the by-pass duct 22 (over 360 degrees). The surface cooler 32 axially extends to cover a portion, about 30% in this embodiment, of a total length of the fan case wall 30 taken along the longitudinal direction L. In an alternate embodiment, the whole radially-inner fan case wall 30 is covered by the surface cooler 32. In another alternate embodiment, the surface cooler 32 only extends about a portion of the circumference of the fan case wall 30.
Although an attempt is made to make the fins aerodynamics, it remains that the fins of the surface cooler 32 tend to cause the flow in the by-pass duct 22 to lift off pass the leading edge 42 of the surface cooler 32. Such phenomenon in one part leads to mixing loss with the flow of the engine core, but also starves the surface cooler 32 of air when the flow reaches the trailing edge 43 of the surface cooler 32. Indeed, the mass flow rate circulating between two consecutive fins decreases with the direction of the by-pass flow because the flow deviates radially away from the fan case wall 30. The local heat transfer coefficient along the direction of the flow thus decreases since less air is available to receive the heat of the fins 40. A discussion regarding this phenomenon is presented herein below.
A typical remedy is to increase the surface cooler wetted area by increasing fin density, and the dimensions of the fins 40. However, such practice tends to exacerbate the spillage problem thereby leading to even more by-pass performance loss. Furthermore, large spillage flow also creates large back pressure on upstream components of the engine moving them away from their optimum operational position. Moreover, the size of the surface cooler 32 has a direct weight impact since their mounting hardware are made of thick heavy material.
Currently surface coolers are sized based on given Mach number in the by-pass duct 22, the temperature and the pressure in said duct 22, and based on the heat rejection requirements. However, shaping a surface cooler to be more aerodynamic would result in a more expensive machining process. It was found that in at least some embodiments, it was possible to improve the surface cooler performance by bleeding the flow downstream of the surface cooler.
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In a particular embodiment, the fan case wall 30 further defines an inlet 48 for re-injecting the flow that has been extracted from the by-pass duct 22 through the outlet 44. The inlet 48 is located upstream of the surface cooler 32 relative to the direction D. In one embodiment, the inlet 48 is located upstream of the surface cooler 32 and of the struts 24 of the gas turbine engine 10. The inlet 48 present a plurality of possible embodiments that are discussed herein below.
In a particular embodiment, an air pressure of the flow in the by-pass duct 22 varies along the direction of the flow D. Accordingly, by increasing a distance between the inlet 48 and the outlet 44, the pressure differential between the inlet and the outlet increases. Such pressure difference causes the flow downstream of the surface cooler 32 to be sucked in the recirculation conduit 46 through the outlet 44 to be re-injected upstream through the inlet 48. A greater pressure differential thus results in a greater mass flow rate in the recirculation conduit 46.
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In one embodiment, the bleeding slot 44 is fluidly connected to a recirculation conduit 46. In one embodiment, the recirculation conduit 46 is a cavity circumferentially extending 360 degrees around the by-pass duct and between the fan case wall 30 and the nacelle wall 28 of the fan section 20. In an alternate embodiment, the recirculation conduit can be a cavity extending only partially around the by-pass duct, for instance.
In a particular embodiment, the recirculation conduit comprises guide vanes 50 to guide a flow of the recirculation conduit 46. The guide vanes 50 are disposed across the conduit 46 through the flow circulating therein and have the objective to guide the flow such that a direction of the flow exiting the recirculation conduit 46 is locally parallel to the flow in the by-pass duct 22.
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The bleeding conduit 102 has two walls 102A and 102B axially spaced from one another. The downstream-most wall 102A is a continuity of the circumferential wall 100A of the main conduit 100. Similarly, the upstream-most wall 102B is a continuity of the circumferential wall 100B of the main conduit 100. The radial walls 102A and 102B are angled such that the flow enters at an angle θ1 relative to the fan case wall 30. The angle θ1 is smaller than 90 degrees to provide a bleeding direction DB to facilitate extraction of air from the by-pass duct 22. In one embodiment, the angle θ1 is between 25 degrees and 50 degrees. In another embodiment, the outlet 44 are bleeding apertures, or nozzles, circumferentially distributed downstream of the surface cooler 32 and extending through the inner surface of the fan case wall 30.
The injecting conduit 104 also defines two walls 104A and 104B axially spaced from one another. The upstream-most wall 104A and the downstream-most wall 104B are continuities of the circumferential wall 100A and 100B of the main conduit 100, respectively. In accordance with one embodiment, the upstream-most and downstream-most walls 104A and 104B of the injection conduit 104 are angled such that the exiting flow defines an angle θ2 relative to the fan case wall 30. The angle θ2 is also smaller than 90 degrees. In one embodiment, the angle θ2 is less or equal than 20 degrees. Such angle has the objective to inject the flow at a direction DI substantially parallel to a local direction of the by-pass flow in the by-pass duct 22. In a particular embodiment, a distance between the upstream-most 104A and downstream-most 104B walls of the injection conduit decreases with the direction of the flow DI to accelerate the flow toward its re-entry in the by-pass duct 22.
In a particular embodiment, a ratio between a distance LIO between the inlet and the outlet of the recirculation conduit and the height h of the by-pass duct between the engine core and the radially-inner wall is greater than 2.
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In accordance with a particular embodiment, the apertures 52 define an angle a to guide the flow such that a direction of the flow exiting the apertures 52 is parallel to the flow in the by-pass duct 22. The angle a is smaller than 90°. In a particular embodiment, the angle a is between 0° and 20°. In a particular embodiment, a guide vane as described herein above extends in the recirculation conduit 46 across the flow circulating therein.
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Accordingly, in such an embodiment, the outlet 48 is an annular surface 60 defined between the wall section 106 and the radially-inner fan case wall 30. In such an embodiment, the flow exits the recirculation conduit 46, and the annular conduit 58, parallel to the fan case wall 30. In a particular embodiment, the annular conduit 58 intersects with the vanes 24A.
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Referring to all figures, a method for cooling an engine fluid circulating in an engine core of a gas turbine engine 10 is also disclosed. The method comprises the step of receiving a by-pass flow in a by-pass duct 22 defined by a fan case wall 30 surrounding the engine core. Then, heat is transferred from the engine fluid to the by-pass flow by convection with a surface cooler 32 affixed to a radially-inner fan case wall 30. The surface cooler 32 has a plurality of circumferentially spaced-apart, and longitudinally extending fins 40.
The air is also bled downstream of the surface cooler 32 relative to the by-pass flow through an outlet 44 defined through the radially-inner fan case wall 30 and re-injected at a position upstream of the surface cooler 32 through an inlet 48 defined through the radially-inner fan case wall 30. In a particular embodiment, the inlet 48 is fluidly connected to the outlet 44 by a recirculation conduit 46. In one embodiment, the recirculation conduit 46 is a cavity extending between a radially-outer nacelle wall 28 of the fan section 20 and the radially-inner fan case wall 30.
According to a particular embodiment, the method further comprises the step of guiding the extracted air parallel to a local direction of the flow of the by-pass duct 22. In one embodiment, this step is carried by a shape of the recirculation conduit 46 that has an injection conduit 104 oriented toward a direction of the flow in the by-pass duct 22.
In one embodiment, the flow of the recirculation conduit 46 is accelerated before its re-entry in the by-pass duct 22. The injection conduit 104 may have a cross-sectional area decreasing with the direction of the flow. In a particular embodiment, the injection conduit 104 is fluidly connected to a plurality of apertures 52 having a decreasing cross-sectional area along a direction of the flow circulating therein. The apertures are defined through a thickness of the radially-inner fan case wall 30.
In an alternate embodiment, the recirculation conduit radially extend over the radially-outer nacelle wall 28. Also, the recirculation conduit 46 may also be a plurality of pipes axially extending between a position downstream of the surface cooler and a second position upstream of the surface cooler.
In an alternate embodiment, the surface cooler is mounted around the engine core case instead of the fan case. In such an embodiment, the fins of the surface coolers extend outwardly from the engine core case toward the fan case. The recirculation conduit is a cavity extending within the engine core case.
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 departing from the scope of the invention disclosed. Still other modifications which fall within the scope of the present invention 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 appended claims.