Patent Application
20100077721
The technical field relates generally to a composite fan case for a turbofan gas turbine engine.
Turbofan engines typically have a fan with a hub and a plurality of fan blades disposed for rotation about a central axis. The casing surrounding the fan blades must be able to contain a broken fan blade propelled radially outwardly from the rotating hub at high speed.
Thus, the fan case includes a containment structure, which may have one of many various known designs, including designs employing composites, which can include a containment fabric layer, such as Kevlar®. The containment fabric is typically wrapped in multiple layers around a relatively thin, often penetrable supporting case, positioned between the blades and the fabric layer. Thus, a released blade will penetrate the support case and strike the fabric. The fabric deflects radially but largely remains intact to capture and contain the released blade.
However, improvements are desired.
There is provided a turbofan gas turbine engine comprising: a fan including a plurality of fan blades each having a blade tip oriented at an angle relative to a transverse reference axis; and a composite fan case radially spaced outwardly from said blade tips of the fan blades and extending longitudinally from a leading to a trailing edge thereof respectively disposed on opposite sides of at least the fan blades such as to surround the fan, the fan case having a blade containment zone surrounding and in longitudinal alignment with the fan blades for containing of a fan blade in the event of a blade release, the composite fan case including a structurally supporting outer composite shell and, in at least the containment zone thereof, an intermediate energy absorbing core disposed between the outer shell and an annular inner fabric layer, the inner fabric layer having fibres substantially uni-axially oriented at a fibre lay-up angle β relative to said transverse reference axis, the fibre lay-up angle β of the fibres within the inner fabric layer being substantially equal to a blade tip release angle α of the fan blade tips.
There is also provided a method of fabricating a composite fan case for a turbofan engine comprising the steps of: determining a predicted blade release angle α of a blade tip of a fan of the turbofan engine; providing a cylindrical fan case surrounding the fan and having a containment zone, the composite fan case including a composite outer shell and, in at least the containment zone, an energy absorbing core; and forming an inner fabric layer on an inner side of the cylindrical fan case within the containment zone and overlying at least the energy absorbing core, including uni-axially orienting fibres of the inner fabric layer at a fibre lay-up angle β, the fibre lay-up angle β being substantially equal to the blade release angle α.
There is further provided a turbofan engine comprising a composite fan case surrounding a fan having a plurality of fan blades, the composite fan case including a containment zone having an inner fabric layer composed of resin-impregnated fibres substantially uni-axially oriented along a common angle corresponding to a blade release angle of the fan blades, a composite outer shell, and an energy absorbing core disposed radially between the inner fabric layer and the composite outer shell, the energy absorbing core including non resin impregnated multidirectional fibres.
Further details will be apparent from the detailed description and figures included below.
Reference is now made to the accompanying figures, in which:
A composite (i.e. non metallic) fan case for a gas turbine engine is described below in detail. The case includes a containment zone having an inner fabric layer including uni-axially oriented fibres. An energy absorbing core may be superposed over (i.e. radially outward from) the inner fabric layer and including non resin impregnated fibres. More particularly, the fibres of the inner fabric layer are oriented substantially along a blade release angle direction of a blade of the gas turbine engine, while the fibres of the superposed energy core portion are multidirectional.
In the exemplary embodiment, at least the containment zone 41 of the fan case 40 is made of a composite (i.e. non-metallic) and includes an outer shell 42, an energy absorbing core 44 that is formed by non resin impregnated multidirectional fibres, an inner uni-axial fabric layer 46, and an abradable tip clearance control layer 48, all being superposed on one another and which together define the containment fan case 40.
As seen in
The fibres 47 of the inner fabric layer 46 can include strong synthetic fibres such as aramid fibres including Kevlar®. The fibres of the inner uni-axial fabric are impregnated with a resin, such as a thermosetting resin, in order to be bonded together.
Therefore, the fibre lay-up angle β is determined by analysis such as to correspond to the blade angle α of a tip 52 of the fan blade 24, upon release. As can be seen in
The lay-up angle β is the angle defined between the orientation of the fibres 47 of the inner fabric layer 46 and the reference axis 51, the reference axis 51 being substantially perpendicular to the main engine centerline axis 32. For instance and without being limitative, in one example the lay-up angle β can vary between 40 and 70 degrees relatively to the reference axis 51 of the fan assembly 12. However, it is to be understood that the lay-up angle β of the fibres 47 can vary depending on a number of factors, including engine size and configuration. Regardless, the angle β of the fibres 47 will always correspond to the blade release angle a of the rotating component, such as the fan blades, that the composite case 40 surrounds.
Referring back to
The composite containment case 40 operates somewhat similarly to a bullet-proof vest. The combination of uni-axially oriented fibres in the inner fabric layer 46, with an overlying dry aramid multidirectional fibre core 44, favours kinetic energy absorption. The energy absorbing core 44 absorbs the primary energy of a released fan blade or blade fragment. The orientation of fibres/plies versus blade angle mismatch in the energy absorbing core 44 is used to control energy absorption. As mentioned above, the energy absorbing core 44 includes fibrous materials such as Kevlar® which contain fibres with small “hooks” which can grab onto the released blade or blade fragment to slow its rotation. Blade rotation is where most of the kinetic energy is stored in a blade. Thus slowing rotation significantly de-energizes the released blade or blade fragment.
The aligned orientation (angle β) of the fibres 47 (ex.: Aramid fibres) of the inner fabric layer 46 and the blades allows a released blade or blade fragment to enter the containment zone, without damaging the outer shell 42 and while minimizing the damage/deformation to the structural integrity of the inner shell as the initial strain to the inner shell is not transmitted circumferentially, thus maintaining an adequate case stiffness.
The outer shell 42 of the case can then be a more cost effective fabric and flexible such as, for instance and without being limitative, lower grade multidirectional tow, since the direct impact energy transferred is dissipated in the energy absorbing core 44 instead of being transferred to the outer shell 42. The fan containment case 40 thereby substantially maintains its basic structural integrity after a blade or blade fragment release event. The outer shell 42 can thus include a lower modulus fibre weave, for instance a multi-directional [epoxy/vinyl ester] prepreg of carbon/graphite/E-glass, S-glass. It is to be understood that the term “prepreg” as used herein means a composite material that is “pre-impregnated” with a resin, for example a material including a combination of un-cured resin matrix and reinforcement fibers or fabrics.
The abradable tip clearance control layer 48, which may be provided on the innermost surface of the casing 40, is made of an abradable material which helps protect the fan blades 24 rotating within the casing 40. As per other abradable coatings which are used in gas turbine engines in order permit tip clearance gap control, the abradable layer 48 can be made from any suitable abradable material such as 3M's Scotch Weld™ or a similar and/or functionally equivalent epoxy based abradable compound.
Thus, in an embodiment, the fan containment case construction is a composite lay up of non resin impregnated multidirectional fibres 44, such as dry aramid/glass fabric, sandwiched between an inner uni-directional fabric layer 46 impregnated with a resin and an outer multi-directional layer 42.
Any suitable reinforcing fibre can be used to form the inner fabric layer 46 and the energy absorbing core 44 including, but not limited to, glass fibres, graphite fibres, carbon fibres, ceramic fibres, aromatic polyamid fibres, for example poly(p-phenyletherephtalamide) fibres (Kevlar® fibres), and mixtures thereof. Any suitable resin can be used in the inner fabric layer 46, for example, thermosetting polymeric resins such as vinyl ester resin, polyester resins, acrylic resins, polyurethane resins, and mixture thereof.
In an embodiment, the inner unidirectional fabric layer 46 includes an [epoxy/vinyl ester] prepreg.
In a non-limitative embodiment, the abradable tip clearance control layer 48 has a thickness ranging between about 1.5 and 4.5 millimetres (mm), the inner fabric layer 46 has a thickness ranging between about 1 and 3 mm, the core portion 44 has a thickness ranging between about 10 and 18 mm, and the outer shell 42 has a thickness ranging between about 2 and 7 mm. The fibre density in the outer shell 42, the core portion 44, and the inner fabric layer 46 can range between about 4 and 12 [oz/sq-yd]. However, it is to be understood that the thickness, density and other properties of each of the layers of the casing 40 can vary depending on a number of design factors, including engine size and configuration for example.
The fan containment case 40 is fabricated, in an exemplary embodiment, by laying-up each of the composite layers, consecutively, about a suitable cylindrical mandrel. Each layer is formed overtop of the radially inner one by continuously applying the composite fibres/prepreg and/or resin (when used), thereby bonding each layer with the next to create an integrally formed composite fan case. The containment zone 44 is sealed within an impervious skin during lay-up to ensure that it remains dry during the resin infusion process or to prevent bleed through during prepreg cure.
The composite fan case 40 described above is relatively light weight, provides a cost effective containment system, and provides a better vibration and sound damping structure over a hard walled composite. The primary containment is provided with an integral reinforcing fibre core 44 and the uni-axial inner tow 46 to direct the blades into the optimized containment zone. The uni-axial inner tow 46 potentially catches and restrains the blade fragments from falling back into the gas path and following blades.
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 inventions 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.
