Abstract:
A gas turbine engine comprising a nacelle having an intake, the intake defining a generally annular chamber, an engine accessory and a heat exchanger for cooling a fluid of the accessory. The chamber is closed and the heat exchanger is disposed within the chamber operable to provide heat to prevent ice forming on the intake during engine operation.

Description:
TITLE OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an intake of an aeroengine nacelle and in particular to providing a heated intake for prevention of ice build up thereon. 
   2. Background of the Invention 
   The leading edge of the intake of aeroengines is particularly prone to icing during flight through clouds containing super-cooled water droplets or ground operation in freezing fog. Protection against ice formation may be required since icing of the intake may significantly affect the airflow through the engine, causing a loss of performance and possible malfunction of the engine. In addition, damage may result from ice breaking away and being ingested into the engine or impacting acoustic liners around the intake duct. 
   It is well known to provide the intake with an electrical element heating system, ducting to supply the intake with heated core engine air or piping to supply hot engine oil around to the intake prevent ice build up. Embodiments of these systems are disclosed in “The Jet Engine”, Rolls-Royce Plc, 1986, ISBN 0902121235. 
   Electrical heating systems require a substantial input of electricity, which is generated by an auxiliary power unit, as known in the art. Such a system inefficient as it requires additional electrical generation. Current heated air anti-ice systems require substantial ducting and control equipment to bleed air from the core engine only when anti-icing measures are required. These systems are disadvantaged in that they use air bled from the compressor thereby reducing engine efficiency. As both these systems impose significant engine power requirements both use either pilot or control input to switch on the anti-icing devices. 
   GB2136880 relates to a turbo-prop engine where heat from a reduction gearbox is transferred by conduction through the engine casing to the intake lip. An annular air intake comprises a number of stator vanes having heat pipes therethrough connecting the interior of the reduction gearbox with a heat sink situated adjacent the upstream lip of the intake. Heat from the hot oil mist within the gearbox is thus conducted to the heat sink and serves to heat up the intake lip to prevent the formation of ice thereon, while at the same time the oil mist is cooled. As this system employs engine oil, failure of the system is likely to compromise engine operability. A further problem is that the heat sink and piping imposes a significant weight penalty to the engine and nacelle assembly. 
   GB2204361 discloses the transfer of hot oil from a reduction gearbox to a heat exchanger and with an air scoop that directs external air through the heat exchanger and then the warmed air is used to de-ice the intake. However, a serious disadvantage with this arrangement is that the air scoop causes additional aerodynamic drag, therefore reducing the overall efficiency of the engine. 
   Therefore it is an object of the present invention to provide an anti-icing system for at least the intake of a gas turbine engine that does not cause an aerodynamic drag, does not use engine oil or air bled from a compressor and is a closed system. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention a gas turbine engine is provided that comprises a nacelle having an intake, the intake defining a generally annular chamber, an engine accessory and a heat exchanger for cooling a fluid of the accessory characterised in that the chamber is closed and the heat exchanger is disposed within the chamber operable to provide heat to prevent ice forming on the intake during engine operation. 
   Preferably, a means for circulating air around the chamber and driving the air through the heat exchanger is provided within the chamber and the means for circulating air comprising at least one electrically driven fan. 
   Preferably, the accessory is a generator operable to generate electricity during engine operation. 
   Alternatively, the accessory is a starter/generator operable to drive the engine at start up and generate electricity during engine operation. 
   Alternatively, the accessory is a gearbox for transferring drive between an engine shaft and the accessory. 
   Preferably, a second heat exchanger is included in the chamber and associated to a second accessory. 
   Preferably, the intake comprises at least one rib extending substantially around the circumference of and into the chamber. 
   Preferably, the at least one rib increases in extent into the chamber generally from the heat exchanger thereby increasing its surface area for heat exchanging to provide a more constant heat exchange around the circumference of the intake. 
   Alternatively, further ribs are provided around the circumference to increase the surface area for heat exchanging around the intake to provide a more constant heat exchange around the circumference of the intake. 
   Preferably, the ribs also provide increased rigidity of the intake to protect against foreign body impacts. 
   Furthermore and in accordance with the present invention there is provided an intake for a gas turbine engine, the intake defining a generally annular chamber and capable of housing an engine accessory and a heat exchanger capable of cooling a fluid of the accessory characterised in that the chamber is closed and the heat exchanger operable to provide heat to the intake. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully described by way of example with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic section of part of a ducted fan gas turbine engine incorporating the present invention; 
       FIG. 2  is a view on section A-A of  FIG. 1 ; 
       FIG. 3  is a schematic section of part of a ducted fan gas turbine engine incorporating the present invention; 
       FIG. 4  is a view on arrow B of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIGS. 1 and 2 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , and intermediate pressure turbine  18 , a low-pressure turbine  19  and an exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and the exhaust nozzle  20 . 
   The gas turbine engine  10  works in the conventional manner so that air entering the intake  11  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
   The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines  17 ,  18 ,  19  respectively drive the high and intermediate pressure compressors  15 ,  14  and the fan  13  by suitable interconnecting shafts  23 ,  24 ,  25 . 
   The fan  13  is circumferentially surrounded by a structural member in the form of a fan casing  26 , which is supported by an annular array of outlet guide vanes  27 . 
   The engine  10  further comprises a gearbox/generator assembly  28  used for engine start up and for generating electricity once the engine has been started and working in convention fashion. The generated electricity is used for engine and associated aircraft electrical accessories as well known in the art. The gearbox/generator assembly  28  is drivingly connected to the high-pressure shaft  24 , however, in other embodiments may be driven by any one or more of the shafts  24 ,  25 . In this embodiment, the gearbox/generator assembly  28  comprises an internal gearbox  29  connecting a first drive shaft  30  to the high-pressure shaft  23 , an intermediate gearbox  31  connecting the first drive shaft  30  to a second drive shaft  32  and an external gearbox  33  drivingly connected to the second drive shaft  32 . The external gearbox  33  is drivingly connected to a generator  34  that is capable of the aforesaid engine operation. The generator  34  and external gearbox  33  are housed within the nacelle  21 . The first drive shaft  30 , intermediate gearbox  31  and the second drive shaft  32  are housed within a bypass duct splitter fairing  40 . 
   The nacelle  21  comprises the intake  12  at its forward end; the intake  12  defines a generally D-shaped and generally annular chamber  35 . An oil heat exchanger  36 , which serves to cool the oil used in the generator  34  and gearbox  33 , is located in the chamber  35 . A pipe  39  fluidly connects the heat exchanger  36  to the generator  34 . It should be appreciated by those skilled in the art, that any suitable heat exchanger  36  is usable. The present invention relates to the positioning of the heat exchanger  36  in the D-shaped chamber  35  and that the chamber  35  is closed. The air inside the chamber  35  is circulated, removing heat from the oil in the heat exchanger  36  and distributing the heat around the chamber  35  to heat the intake  12  and thereby prevent ice-build up on the intake  12 . A means for circulating air  37  and driving the air through the heat exchanger  36  is provided in the form of an electrically driven fan  37 , however, more than one fan  37  may be provided in the chamber  35 . Other means for driving the fan  37  may be employed, such as, using pressurised oil or air. 
   The present invention is particularly advantageous in that the heat exchanger  36  is located within the intake  12  meaning no additional pipe work or ducting is required for transferring heated air to the intake  12 . A second advantage is present in that the arrangement is a closed system i.e. there is no external air scoop to feed air into the heat exchanger and therefore no aero drag penalty. Furthermore the chamber  35  circulating air is not vented overboard that would otherwise cause an aerodynamic drag penalty. A further advantage is that the generator oil is used as the heat source as opposed to engine oil. Hence impact and subsequent damage from a foreign object, such as a bird, would not be critical to continued engine  10  operation. 
   Importantly, the present invention provides an anti-icing system that is always active so there is no requirement for pilot interaction or electronic control. 
   To assist in transferring heat to the intake  12  the inner surface of the intake  12  comprises a means for heat exchange  38  between the air circulating within the chamber  35  and the walls of the intake  12 . In this embodiment the means for heat exchange  38  comprises a plurality of generally annular extending ribs  38  that also extend from the inner walls of the intake  12  into the chamber  35 . It should be appreciated that the ribs  38  may take many different forms to provide heat exchange and as such all provide the essential function of transferring heat from the circulating air to the intake  12  to prevent ice build up and to provide a relatively cool air supply to the heat exchanger  36 . As the heated air is circulated around the chamber  35  it reduces in temperature, so to provide a more even distribution of heat around the intake, the ribs  38  generally increase in number from the outlet of the heat exchanger to its inlet. Alternatively and as shown in  FIG. 4 , each rib  38 A increases in height and thus surface area to accommodate and increase heat transfer of the reduction in air temperature around the intake  12  from the outlet of the heat exchanger to its inlet. The inclusion of ribs  38  further enhances the strength of the intake  12  and reduces the damage caused by a foreign object striking the intake  12 . 
   The present invention provides further advantages over the prior art in that the prior art engine comprises a thermal anti-ice pipe and outlet duct that are omitted in the arrangement of the present invention thereby giving an improved weight saving and improved arrangement of the engine&#39;s remaining piping and other accessories. Prior art systems supply hot engine core air to a D-shaped chamber at the front of the intakes via a large pressurised duct. In the event of duct rupture the nacelle fan cowl doors are protected from failure either by a pressure relief door or a double walled pressurised pipe. The present invention eliminates the requirement for these additional features. The space taken up by conventional cooler scoops in the rear fancase can provide additional acoustic lining. This also provides a performance advantage since bypass air not lost overboard and core air not lost for anti ice purposes. 
   In a further embodiment of the present invention, the accessory ( 33 ,  34 ) is a starter/generator ( 36 ) that is operable to drive the engine ( 10 ) at start up and generate electricity during engine ( 10 ) operation. 
     FIGS. 3 and 4  show the heat exchanger  36  and the fans  37  further rearward in the chamber  35 . This is a preferable configuration where the heat exchanger  36  is significantly heavy and is required to be moved rearward to reduce the bending moment about the outlet guide vanes  27 . As can be seen, the chamber  35  is curved axially and locally to meet and supply cooling air through the heat exchanger  36 . 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.