GB2498194A

GB2498194A - Ice impact panel for a gas turbine engine

Info

Publication number: GB2498194A
Application number: GB1200095.6A
Authority: GB
Inventors: Clare Louise Pool; Julian Mark Reed
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2012-01-05
Filing date: 2012-01-05
Publication date: 2013-07-10
Also published as: GB201200095D0

Abstract

An ice impact panel 61 for a gas turbine engine comprises an abradable layer 69 and an ice impact layer 63, with a septum layer 67 disposed between the abradable layer and the ice impact layer. The thickness of the septum layer is selected so as to distribute the load of an ice impact into the ice impact layer so that substantially no damage is caused to the ice impact layer by the ice impact. The thickness of the septum layer may be selected such that, under blade impact, the septum layer provides effectively no resistance to the passage of the blade. The abradable layer may be adhesively bonded to one side of the septum layer and the ice impact layer may be adhesively bonded to the other side of the septum layer. The thickness of the septum layer may vary in an axial direction. The septum layer may be thicker at its axially rearward end than at its axially forward end. The thickness may be increased in discrete steps.

Description

AN ICE IMPACT PANEL

This invention relates to gas turbine engines. More specifically, it relates to the ice impact panels disposed within the fan casing of a gas turbine engine.

Ice can form on the rotor blades of a gas turbine engine during engine operation, and in particular on the fan blades. Due to the high centrifugal forces that are encountered during engine operation, the ice can be shed from the blades. Gas turbine engines conventionally include a fan case with acoustic panels and fan track liners that are vulnerable to damage when ice is shed during engine operation.

Rotor blades can become detached during engine operation, and the fan case also needs to cater for blade impact in the event of blade detachment. The fan case is a rotor blade containment assembly.

The characteristics of the containment assembly that are needed to cope with ice impact and rotor blade impact are somewhat different. In the case of ice impact, it is preferred that the containment assembly has high toughness to substantially prevent any penetration of the ice into the containment assembly, so that the panels and liners retain their functionality for noise and efficiency. In contrast, in the case of rotor blade impact it is preferred that the containment assembly has lower toughness to allow partial penetration of a detached rotor blade into the containment assembly. This partial penetration is necessary in particular to ensure effective containment of a fan blade in the event of fan blade detachment.

With conventional fan blade configurations, the ice impact and fan blade impact regions have typically been spaced apart axially through the gas turbine engine, with the ice impact region being downstream or aft of the fan blade impact region. It has thus been possible to vary the structure of the fan blade containment assembly in these different regions to provide the different characteristics that are needed to cope with ice impact and fan blade impact. However, some newly developed fan blade configurations have resulted in ice impact and fan blade impact occurring in the same region. This has led to difficulties with fan blade containment in the event of fan blade detachment.

Swept fan blades have a greater chord length at their central portion than conventional fan blades. Because of this, ice that forms on such a blade, although it follows the same rearward and outward path as on a conventional blade, may reach the radially outer tip of the blade before it reaches the trailing edge. It will therefore be shed from the blade tip and strike the fan track.

However, a conventional fan track is not strong enough to tolerate ice impact, and so conventional arrangements are not suitable for use with swept fan blades. It is not possible simply to strengthen the fan track to accommodate ice impact, because this would disrupt the blade trajectory during a blade-off event, and compromise the operation of the fan casing containment system.

It is an object of this invention to provide an ice impact panel able to be located directly outward of the fan blades, providing suitable mechanical properties both for the ice impact and for the blade containment functions.

Accordingly, the present invention provides an ice impact panel as set out in the claims.

Embodiments of the invention will now be described in more detail, with reference to the attached drawings, in which Figure 1 shows a section through a gas turbine engine of known type; Figure 2a shows a conventional fan blade of a gas turbine engine, and Figure 2b shows a swept fan blade of a gas turbine engine; Figure 3 shows an ice impact panel of known type; and Figure 4 shows an embodiment of an ice impact panel according to the invention.

Figure 1 shows a gas turbine engine 10 of conventional configuration. It comprises in axial flow series a fan 11, intermediate pressure compressor 12, high pressure compressor 13, combustor 14, high, intermediate and low pressure turbines 15, 16 and 17 respectively and an exhaust nozzle 18.

Air is accelerated by the fan 11 to produce two flows of air, the outer of which is exhausted from the engine 10 to provide propulsive thrust. The inner flow of air is directed into the intermediate pressure compressor 12 where it is compressed and then directed into the high pressure compressor 13 where further compression takes place.

The compressed air is then mixed with fuel in the combustor 14 and the mixture combusted. The resultant combustion products then expand through the high, intermediate and low pressure turbines 15, 16, 17 respectively before being exhausted to atmosphere through the exhaust nozzle 18 to provide additional propulsive thrust.

The high, intermediate and low pressure turbines 15, 16, 17 drive, respectively, the high and intermediate pressure compressors 13, 12 and the fan 11 via concentric driveshafts 19, 21, 22.

For efficient operation of the gas turbine engine, it is important that the gap 23 between the tips of the fan 11 blades and the fan casing 31 is as small as possible, to minimise leakage. To achieve this, the fan casing typically has an abradable liner 25, comprising a layer of material which can be abraded by the tips of the fan 11 blades as they rotate.

In this way, the fan blades cut a path, with a minimum clearance, in which to run.

A second, but equally important, function of the fan casing 31 in the region radially outward of the abradable liner 25 is to provide a containment system. This must absorb the energy of a released fan blade and any other debris, in a blade-off event. The energy absorption must be very carefully tailored to ensure that no material is released outward or forward of the engine, and that the material retained within the engine will cause minimal damage further downstream.

In use, ice can build up on the blades of the fan 11. The combination of centrifugal and gas loading causes pieces of ice to move radially outward and rearward (towards the right, in Fig. 1) until they are released from the blade at its trailing edge. An ice impact panel is provided at the location marked 33 on the inside of the fan casing 31. This ice impact panel, commonly made of (or wrapped in) glass-reinforced plastic (GRP), absorbs the energy of the released ice so that it will not damage components further downstream in the engine.

Figure 2a shows a conventional fan blade 41. A circumferential array of such blades rotates in use about an axis X-X. The arrow A shows a notional path followed by a piece of ice across the surface of the blade 41. The blade moves radially outwards because of centrifugal forces, and axially rearwards because of gas forces. The ice is released from the trailing edge 43 of the blade 41.

The arrow B shows a notional path followed by the fan blade 41, in the event that it is released from the engine. Because the fan blade is much heavier than a piece of ice, its trajectory is much less affected by gas forces, and therefore its path B is essentially radially outwards. Therefore, a released fan blade would impact the fan case 31 essentially radially outwards of the blade position (corresponding to the position of the abradable liner 25 in Figure 1), whereas a released piece of ice would impact the fan case 31 at a position axially rearwards, or downstream, of this, corresponding to the position of the ice impact panel 33 in Figure 1).

Figure 2b shows a swept fan blade 141. The swept design of blade is increasingly favoured in the gas turbine industry as it offers significant advantages in efficiency over the conventional blade 41 shown in Figure 2a. As for the conventional fan blade 41, a circumferential array of such blades rotates in use about an axis X-X. The arrow A' shows a notional path followed by a piece of ice across the surface of the blade 141.

This path is essentially the same as the path A followed by the ice across the surface of the conventional fan blade 41. However, it can be seen in Figure 2b that the greater chordal dimension of the swept blade 141 can cause ice to be released at the tip 145 of the blade, rather than at the trailing edge 143. With a conventional fan casing arrangement, as shown in Figure 1, this ice would then strike the fan track rather than the ice impact panel, causing damage.

Figure 3 shows in more detail an ice impact panel of conventional design, as introduced in Figure 1. The ice impact panel, shown generally at 51, is located within the fan casing 31. It comprises an aluminium honeycomb 53, part of which is tilled by an epoxy filler 55. The aluminium honeycomb 53 is covered by a glass-fibre wrap 57.

Figure 4 shows an ice impact panel 61 according to the invention. The panel 61 comprises an ice impact layer formed of aluminium or titanium honeycomb 63, the radially outer tace of which is bonded to the fan case 66. Bonded to the radially inner tace ot the ice impact layer 63 is a septum layer 67. An abradable layer 69, comprising a tilled Nomex® honeycomb, is bonded to the radially inner face ot the septum layer.

The abradable layer 69 is tilled with one ot the materials conventionally used for abradable liners, for example Araldite® 252A1B or a syntactic foam such as those sold under the brand name Synspand®.

The septum layer 67 is typically a sheet of fibre-reinforced composite material. It may be made trom a number of layers of composite material bonded together. It may be made trom tilament wound, woven tabric or tape lay-up material. The tibres may be glass, aramid, carbon or graphite. The septum layer 67 may be perforated. The perforation of the septum layer 67 helps with the bonding of the septum and the ice impact layer. The holes allow air to escape and the glue to run through the holes. If the septum is not perforated, air pockets can form which stop the septum bonding to the ice impact layer.

The function ot the septum layer in known ice impact panels is to provide a surface to which both the abradable layer and the ice impact layer can be bonded. This stabilises both, and (by providing shear stiffness across the axis of the honeycomb cells of the ice impact later) produces the synergistic effect that both layers are stronger than they would be without the septum layer. Accordingly the septum in known ice impact panels has been made as thin and light as possible so as not to interfere with the function of either the abradable layer or the ice impact layer.

In the ice impact panel according to the invention, by contrast, the thickness of the septum layer 67 is increased, so as to spread the load from an ice impact over a greater area of the honeycomb, so that the effective energy density seen by the honeycomb is lower than it would otherwise be. The aim is to enable the ice impact panel 51 to dissipate the energy of an ice impact while causing substantially no damage to the ice impact layer 63. The septum layer 67 therefore enhances the performance of the ice impact layer 63. In practice, the honeycomb of the ice impact layer should be able to sustain unlimited ice impacts without any detrimental effect on its properties or integrity.

However, set against this requirement, it is necessary that the septum layer 67 not be too thick, otherwise its strength would be so great that it would prevent the passage of a released blade. As explained above, it is desirable that a released blade partially penetrate the containment assembly so that it can be effectively contained. An excessively thick or strong septum layer would interfere with this containment function.

Accordingly, in the ice impact panel according to the invention, the septum layer must not be so thick that it would impede the passage of a released blade. The ice impact panel must present substantially no resistance to the passage of a released blade.

To achieve the necessary balance of properties, the thickness (and therefore the strength) of the septum layer must be balanced against the strength of the honeycomb in the ice impact layer. For example, a thinner septum may be combined with a stronger honeycomb (perhaps with smaller cells or thicker cell walls), or a thicker septum with a weaker honeycomb (with larger cells or thinner cell walls) to achieve the same overall strength. The design of the ice impact panel will also take into account the necessary properties to achieve the desired response to ice and blade impacts, as explained above. The exact properties of any given arrangement will depend on its operational requirements.

In a preferred embodiment of the invention, the thickness of the septum layer 67 can be varied along its axial length (i.e. in the direction generally from left to right in Figure 4) so that the strength of the septum layer is also varied. Referring back to Figures 2a and 2b, it will be seen that ice will generally impact (A) the containment assembly axially rearwards (downstream) of where a released blade would impact (B). Therefore, the upstream portion of the septum layer 67 may be made less strong, so as not to impede the passage of a released blade, whereas the downstream portion of the septum layer may be made stronger, so as to spread the energy of an ice impact more effectively over the honeycomb. The additional strength of the septum layer in this region is acceptable, because a released blade would not impact there. In this way, the performance of the ice impact panel may be further improved, to optimise the energy absorption from ice impacts without compromising the performance of the system under blade impact. In a particular embodiment, for example, the upstream (axially forward) part of the septum layer may be two layers thick, and the downstream (axially rearward) part may be four layers thick. For ease of manufacture it may be more convenient for the thickness of the septum layer to increase in discrete steps, rather than to have a steadily tapering septum layer.

Claims

<claim-text>CLAIMS1 An ice impact panel for a gas turbine engine, the engine comprising a plurality of blades rotatable in use about an axis, the panel potentially subject in use to ice impact and blade impact, the panel comprising an abradable layer and an ice impact layer, a septum layer being disposed between the abradable layer and the ice impact layer, the liner panel characterised in that the thickness of the septum layer is selected so as to distribute the load of an ice impact into the ice impact layer so that substantially no damage is caused to the ice impact layer by the ice impact.</claim-text> <claim-text>2 An ice impact panel as claimed in claim 1, in which the thickness of the septum layer is selected so that under a blade impact the septum layer provides effectively no resistance to the passage of the blade.</claim-text> <claim-text>3 An ice impact panel as claimed in claim 1 or claim 2, in which the abradable layer is adhesively bonded to one side of the septum layer and the ice impact layer is adhesively bonded to the other side of the septum layer.</claim-text> <claim-text>4 An ice impact panel as claimed in any one of the preceding claims, in which the thickness of the septum layer varies in the axial direction.</claim-text> <claim-text>5 An ice impact panel as claimed in claim 4, in which the septum layer is thicker at its axially rearward end than at its axially forward end.</claim-text> <claim-text>6 An ice impact panel as claimed in claim 4 or claim 5, in which the thickness of the septum layer increases in discrete steps in the axial direction.</claim-text> <claim-text>7 An ice impact panel substantially as described in this specification, with reference to and as shown in Figure 4 of the accompanying drawings.</claim-text>