Patent Application
20100054915
The subject matter of this application relates generally to gas turbine engines, and specifically to cooling techniques for the airfoil sections of turbine blades and vanes. In particular, the invention concerns an insert for convective cooling the interior surfaces of turbine airfoils exposed to high-temperature working fluid flow.
Gas turbine engines are built around a power core comprising a compressor, a combustor and a turbine, which are arranged in flow series with a forward (upstream) inlet and an aft (downstream) exhaust. The compressor compresses air from the inlet, which is mixed with fuel in the combustor and ignited to produce hot combustion gases. The hot combustion gases drive the turbine section, and are exhausted with the downstream flow.
The turbine drives the compressor via a shaft or a series of coaxially nested shaft spools, each driven at different pressures and speeds. The spools employ a number of stages comprised of alternating rotor blades and stator vanes. The vanes and blades typically have airfoil cross sections, in order to facilitate compression of the incoming air and extraction of rotational energy in the turbine.
In addition to its use as an oxidant, compressor air is also utilized to cool downstream engine components, particular turbine and exhaust parts exposed to hot gas flow. Depending upon application, compressed air is also utilized for environmental control, pneumatics and other accessory functions.
Energy not used to drive the compressor and provide cooling or accessory functions is available for external use. In ground-based industrial gas turbines, for example, power is typically provided in the form of rotational energy, which is used to drive an electrical generator or other mechanical load. In aviation applications, on the other hand, reactive thrust is generated directly from the exhaust.
In turbofan engines, one of the spools also drives a forward fan or ducted propeller, producing additional thrust from bypass flow directed around the engine core. Turbofans are either directly driven by the low-pressure turbine spool, or employ a reduction gearbox to slow the fan, reducing noise and increasing efficiency.
The majority of commercial aircraft employ high-bypass turbofan engines, which generate most of their thrust from bypass flow. Supersonic jets and other high-performance aircraft typically employ low-bypass turbofans, which rely primarily on reactive thrust from the exhaust. Low-bypass turbofans are louder and less fuel efficient, but provide greater response and specific thrust. Low-bypass turbofans are also commonly configured for afterburning, in which additional fuel is injected into in an augmentor assembly downstream of the turbine, where it is ignited to provide additional thrust.
The main goals in gas turbine engine design are efficiency, performance and reliability. Efficiency and performance both favor high combustion temperatures, which increase the engine's thermodynamic efficiency, specific thrust, and maximum power output. Unfortunately, high combustion temperatures also increase thermal and mechanical loads, particularly on turbine airfoils downstream of the combustor. This reduces service life and reliability, and increases operational costs associated with maintenance and repairs.
There is thus a need for enhanced turbine airfoil cooling techniques. In particular, there is a need for techniques that not only improve cooling efficiency, reducing the required cooling fluid mass flow rate and resulting energy losses, but also improve cooling efficacy, reducing thermal stresses, extending service life and increasing reliability.
This disclosure concerns an airfoil insert for convective cooling and a method of cooling using the insert. The insert comprises an insert wall, a contact element and a flow director. The insert wall defines an interior region extending inside the insert from a first end to a generally opposed second end, and an exterior region extending along the outside of the insert from the first end to the second end.
The contact element is formed on the exterior of the insert wall between the first end and the second end, for attaching the insert to an airfoil and for spacing the insert wall from the internal airfoil surfaces. The flow director is formed on the insert wall at a boundary between the exterior and interior, and increases the heat transfer coefficient of convective flow along the insert by directing the convective flow to the exterior region, between the insert wall and the surface of the cooling passage.
The method comprises positioning the insert along an internal surface of an airfoil, spacing the insert from the internal surface, and restricting convective flow along the insert to a region between the insert and the inner surface. This increases the Reynolds number of the convective flow, which in turn increases the heat transfer coefficient.
Airfoil 20 comprises airfoil section 21, inner platform 22 and outer platform 23. Airfoil section 21 extends axially between leading edge 24 and trailing edge 25, and radially between platforms 22 and 23. Internal cooling passages 26A, 26B, 26C and 26D are defined along internal surfaces 27 of airfoil section 21. Augmentors such as trip strips 28 and pedestals 29 are formed along internal surfaces 27 of passages 26A, 26B and 26C, in order to increase turbulence and improve internal cooling. Film cooling holes 30A, 30B and 30C provide cooling fluid to external surfaces of airfoil section 21 that are exposed to hot working fluid flow.
In the particular embodiment of
Alternatively, airfoil 20 is formed as a rotating blade. In these embodiments, airfoil section 21 is typically formed into a tip at outer platform 23, and inner platform 22 accommodates a root structure or other means of attachment to a rotating shaft. In further embodiments, airfoil 20 is provided with additional structures for improved working fluid flow control, including, but not limited to, platform seals, knife edge seals, tip caps and squealer tips.
Airfoil 20 is exposed to a generally axial flow of combustion gas F, which flows across airfoil section 21 from leading edge 24 to trailing edge 25. Flow F has a radially inner flow margin at inner platform 22, and a radially outer flow margin at outer platform 23, or, in blade embodiments, at the blade tip.
To protect airfoil 20 from wear and tear due to the working fluid flow, its various components are manufactured from durable, heat-resistant materials such as high-temperature alloys and superalloys. Typically, surfaces that are directly exposed to hot gas are also coated with a protective coating such as a ceramic thermal barrier coating (TBC), an aluminide coating, a metal oxide coating, a metal alloy coating, a superalloy coating, or a combination thereof.
Airfoil 20 is manufactured as a hollow structure with internal cooling passages 26A, 26B, 26C and 26D. The cooling passages are defined along internal surfaces 27, forming channels or conduits for cooling fluid flow through airfoil section 21. In turbofan embodiments, the cooling fluid is usually provided from a compressed air source such as compressor bleed air. In ground-based industrial gas turbine embodiments, other fluids such as steam are also used.
The flow of cooling fluid depends upon the particular configuration of airfoil 20. In the embodiment of
As shown in
Airfoil 20 also employs external cooling, in which additional cooling fluid is directed from cold internal surface(s) 27 to the hot (outer) surfaces of airfoil section 21. The cooling fluid is directed through a combination of film cooling hole structures, including, but not limited to, “showerhead” openings 30A in cooling passage 26A along leading edge 24, film cooling holes 30B in passage 26D, and film cooling slots 30C along trailing edge 25.
Like airfoil 20, insert 10 is manufactured from a durable, heat-resistant material such as a metal alloy or superalloy. In typical embodiments, insert 10 is not provided with a TBC coating because insert wall 11 is not directly exposed to hot combustion gas flow. In other embodiments, an anti-oxidant coating or other protective coating is applied. In further embodiments, insert 10 is provided with a flame spray coating or other coating for forming a fluid seal, a contact surface, a coupling or a mechanical attachment between insert wall 11 and airfoil section 21.
As shown in
Insert wall 11 defines internal region 15, extending along the inside (interior) of insert 10 between first end 12 and second end 13, and external region 16, extending along the outside (exterior) of insert 10 between first end 12 and second end 13. External region 16 also extends along cooling passage 26A, between insert wall 11 and internal surface 27 of airfoil section 21.
In some embodiments, insert wall 11 has a closed-tube geometry in which interior 15 and exterior 16 are defined along a substantially complete cross-sectional perimeter (see, e.g.,
The particular form of contact element 14 varies. In the closed-geometry embodiment of
Insert 10 increases the heat transfer coefficient within airfoil 20 by directing flow around insert wall 11, excluding convective flow from internal region 15 and restricting it to external region 16. Essentially, the heat transfer coefficient varies with the Reynolds number, which describes the ratio of inertial to viscous forces (inertial and viscous effects) in the flow. When convective flow is restricted to external region 16, the flow area is reduced and the impedance rises. The convective flow becomes less laminar and more turbulent, increasing the Reynolds number. This enhances flow interactions along internal surfaces 27 of cooling passage 26A, allowing greater thermal transfer from airfoil section 21.
In some embodiments, convective flow is also described in terms of Mach number. In these embodiments, insert 10 produces Mach acceleration in the convective flow, increasing the heat transfer coefficient by generating greater turbulence and other flow interactions in the region between insert wall 11 and internal airfoil surfaces 27.
Increasing the heat transfer coefficient enhances convective cooling within airfoil section 21. In particular, insert 10 lowers operating temperatures and thermal gradients, reducing thermal stresses and increasing service life. Insert 10 also reduces the cooling flow required to achieve these benefits, improving cooling efficiency and reserving capacity for additional downstream cooling loads.
Airfoil section 21 extends axially between leading edge 24 and trailing edge 25, along pressure or concave side 33 (the upper surface in
As shown in
In this particular embodiment, Insert 10 is located in first midbody cooling passage 26B. Collar 31 provides means for positioning insert 10 within cooling passage 26B, such that collar 31 extends outside of cooling passage 26B and insert wall 11 is spaced within cooling passage 26B. Internal surface 27 defines the outside edge of cooling passage 26B, which is located between the outside edge of insert 11 (inside the cooling passage) and the outside edge of collar 31 (outside the cooling passage).
Collar 31 also provides means for attaching insert 10 to airfoil section 21, for example by welding or brazing, by thermal or friction fitting, or by forming a mechanical attachment via a flame spray coating or other material applied to insert wall 11 or to internal surface 27 of cooling passage 26B. In some embodiments, the attachment features are constructed such that insert 10 is removable for adjustment and repair. In other embodiments, insert 10 is permanently attached to airfoil 21. In these embodiments, insert 10 also functions as a cast-in wall within cooling passage 26B, without requiring the same complex manufacturing steps.
In the particular embodiment of
Inlet 32 is typically formed by stamping, drilling, cutting or machining an opening or orifice on the upstream end of insert wall 11. Inlet 32 defines a boundary between the interior and exterior of insert wall 11, and directs flow through this boundary into insert 10. The internal flow is provided with a positive overpressure, as compared to the exterior flow, and is utilized for downstream cooling functions. In some embodiments, for example, the internal flow is converted to convective flow via a number of indirect (downstream) apertures in insert wall 11, and in other embodiments the internal flow conducted through an outlet to a downstream cooling load (see
Spacing tabs 35 comprise spacing and contact or coupling members formed onto the exterior of insert wall 11 via techniques such as stamping, welding, brazing, milling, molding, cutting and machining, as described above for collar 31 of
In some embodiments, spacing tabs (or fingers) 35 extend within the cross-sectional area of cooling passage 26A, and are provided anywhere along the length of insert 10 between the upstream and downstream ends of insert wall 11. Alternatively, spacing tabs 35 are provided at the first (upstream) end of insert wall 11, and sometimes extend beyond the cross-sectional area of cooling passage 26A, as described above for collar 31 of
Direct-flow convection apertures 37 are sometimes co-formed with contact element 14, for example by defining the apertures between tabs 35. Apertures 37 direct cooling fluid flow around insert 10, into the convective flow region between insert wall 11 and internal surface(s) 27 of airfoil section 21.
In some embodiments, spacing tabs 35 are adjustable by bending, twisting, turning or similar mechanical manipulation, in order to change the spacing between insert wall 11 and cooling passage 26A, or to change the dimensions of apertures 37. This allows the convective flow rate and heat transfer coefficient to be adjusted to suit the particular cooling needs of airfoil section 21.
Spacing ribs 38 comprise spacing and attachment members formed by casting, welding, brazing, milling, molding, cutting or machining internal cooling passage 26A. Spacing ribs 38 provide means for spacing insert wall 11 from internal surface(s) 27 of cooling passage 26A, and for attaching airfoil section 21 to insert 10 along contact element 14.
As shown in
Flow stop 36 comprises a plug, wall, baffle, flow block or other structure formed across insert wall 11, using any of the mechanical fabrication techniques described above. As shown in
Flow stop 36 blocks flow to the interior of insert wall 11, stopping the internal flow through insert 10. Convective flow is directed through apertures 37 to the exterior of insert 10, to the convective region along cooling passage 26A, between insert wall 11 and internal surfaces 27 of airfoil section 21.
Indirect convective flow apertures 41 are formed into insert wall 11 downstream of inlet 32, using one or more of the mechanical techniques described above. In contrast to direct-flow apertures, however, which direct convective flow around insert wall 11, indirect-flow apertures 41 generate flow through insert wall 11, converting the internal (non-convective) flow inside insert 10 to an external (convective) flow on the outside of insert 10.
The number, sizes, shapes and locations of indirect-flow apertures 41 vary, depending upon the desired convective flow rate along insert wall 11. In the embodiment of
Convective flow apertures 41 are distinguished from impingement holes and related structures, which direct relatively high-pressure flow away from insert wall 11 in order to impinge on interior airfoil surfaces. In contrast, insert 10 maintains a relatively low overpressure, so that the flow through apertures 41 is generated with a generally parallel sense along the exterior of insert wall 11. This produces a substantially convective flow, rather than an impingement flow. Similarly, direct flow apertures 37 (above) generate convective flow that is directed along insert wall 11, rather than impingement jets that are directed away from insert wall 11 and toward interior surfaces of the airfoil.
In the embodiment of
Alternatively, insert 10 is provided with downstream flow stop 36, as shown in
The cross-sectional profile of insert 10 is configurable to control heat pick up and absorption, to augment heat transfer, and to limit pressure losses along the internal cooling passage, while maintaining a positive overpressure within insert wall 11. Thus insert 10 and insert wall 11 take on a variety of shapes and forms.
In
In each of the above embodiments, insert 10 exhibits a generally closed geometry in which insert wall 11 has a generally tubular or annular cross section, which forms a substantially complete perimeter boundary between the interior (non-convective) and exterior (convective) regions. In other embodiments, insert wall 11 exhibits an open configuration, in which the boundary extends to at least one interior surface of the airfoil.
The overpressure within insert wall 11 is maintained via seals 5l along internal surface 27 of cooling passage 26A, which complete the boundary between the interior (non-convective) and exterior (convective) flow regions of insert 10. The seals typically extend from first end 12 to second end 13 of insert wall 11 (see
Flow stop 36 works in cooperation with seals 51 to direct flow from feed 52 through apertures 37, but restricts convective flow to internal surfaces 27 of cooling passage 26A opposite leading edge 24, pressure side 33 and suction side 34. This increases cooling efficiency by reducing flow along the interior wall between cooling passages 26A and 26B, where there is no exposure to hot working fluid.
As shown in
The present invention has been described with reference to preferred embodiments. The terminology used is for the purposes of description, not limitation, and workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
