Abstract:
The cylindrical compressor casing of a gas turbine engine has a plurality of radially inwardly and axially rearwardly opening anti-surge grooves disposed on a radially inner surface thereof whereby foreign objects ingested into the engine and entering the grooves are free to move axially rearwardly of the engine.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to gas turbine engines and more particularly to an improved compressor casing for a gas turbine engine that minimizes the deleterious effect of foreign object ingestion into the engine without compromising surge margin of the engine, thereby to enhance its utility as the power plant of an aircraft. 
     A typical gas turbine engine comprises a compressor, a combustor and a turbine in fluid flow relation. A variant of the typical engine includes a fan disposed forwardly of the compressor and an annular by-pass duct that surrounds the compressor. 
     One requirement of a jet engine in the aircraft environment is that it be capable of ingesting foreign objects without catastrophic damage. The problem of foreign object ingestion has been solved in the past by merely increasing the strength of the engine components exposed to impact damage. However, strength is generally equated with weight, which, in turn, compromises performance of the aircraft. Reconciliation of such seemingly divergent performance and safety requirements requires careful design of the aircraft&#39;s propulsion system coupled with airframe aerodynamics. 
     Another factor that must be considered when addressing the problem of foreign object ingestion, is preservation of the surge margin of the fan and/or compressor stages. Radially grooved compressor casings have been used heretofore on gas turbine fan and compressor stages to enhance their surge margin. Unfortunately, such heretofore-known radially grooved casings have increased fan and compressor stage susceptibility to foreign object damage. Specifically, since the radial component of velocity imparted to foreign objects by the fan or compressor blades is greater than the axial velocity thereof, radially extending casing grooves capture and entrap the debris, potentially causing catastrophic damage to the engine. Thus, there is a need for an improved casing for the fan or compressor of a gas turbine engine that minimizes entrapment of ingested debris while still offering fan and/or compressor surge margin during normal operation. 
     SUMMARY OF THE INVENTION 
     The present invention solves the aforesaid problem by utilizing a plurality of radially inwardly and axially rearwardly opening circumferential grooves in the compressor casing. The grooves are disposed slightly downstream of a line swept by the leading edge of the fan or compressor blade tip. The inclined grooves offer reduced target and entrapment area for debris. Axially spaced, circumferential fins defining the grooves are sufficiently deformable so as to close upon initial impact by debris, thus minimizing the opportunity for debris entrapment. The casing grooves are preferably used in conjunction with backswept fan or compressor blades and provide fan or compressor surge margin in the conventional manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a turbofan engine provided with a fan or compressor casing in accordance with the present invention; 
     FIG. 2 is a view of the engine of FIG. 1 partially in cross section; 
     FIG. 3 is an enlarged view taken within the circle “ 3 ” of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As seen in FIG. 1, a typical environment in which the present invention has utility comprises a by-pass turbofan engine  6  having a cylindrical casing  8  defining an air intake  9  at the front thereof and an annular by-pass duct  10  extending to the rear thereof. 
     As seen in FIG. 2, a low pressure spool assembly  12 , is rotatable about a central longitudinal axis  14  of the engine  6  and comprises a shaft  16  having a fan  18  and an intermediate pressure compressor stage  20  at the forward end thereof. An intermediate pressure turbine  22  and a low-pressure turbine  24  are disposed on the aft end of the shaft  16 . 
     A high pressure spool assembly  26  is telescoped over the low pressure spool  12  in coaxial relation thereto and comprises a shaft  32  having a high pressure compressor  34  at a forward end thereof and a high pressure turbine  36  at the aft end thereof. 
     An annular combustor  40  is disposed about the low and high-pressure spools  12  and  26 , respectively, between the high-pressure compressor  34  and high-pressure turbine  36 . 
     The flow of air induced by the fan  18  of the engine  6  is split, combustion air flowing to the low-pressure compressor  20  and by-pass air flowing to the by-pass duct  10 . Combustion air flows from the low-pressure compressor  20  to the high-pressure compressor  34 , thence to the combustor  40  wherein fuel is introduced and burned. Combustion gases pass through the high-pressure turbine  36 , thence through the intermediate and low pressure turbines  22  and  24 , respectively. 
     By pass air flows from the fan  18  through the by-pass duct  10  without additional heat energy being imparted thereto. However, because of the relatively high mass flow of air induced by the fan  18 , significant thrust is produced thereby. 
     In accordance with the present invention, and as best seen in FIG. 3, a forward end  70  of the engine casing  8  is provided with a plurality of radially inwardly and axially rearwardly opening annular grooves  72  on a radially inner surface  74  thereof. The grooves  72  are defined by fins  76  which extend radially inwardly and axially rearwardly from the casing  8 . Because the grooves  72  open rearwardly of the casing  8 , the axially rearward inertia component of a foreign object ingested into the engine  6  is utilized to clear the grooves  72 . Moreover, impact of a relatively heavy object against the radially inner edges of the fins  76  tends to bend the fins  76  radially outwardly and rearwardly so as to close the grooves  72  therebetween. 
     From the foregoing it should be apparent that entrapment of debris and resultant collateral damage caused by ingestion of a foreign object into a gas turbine engine  6  having a casing  8  in accordance with the present invention, is minimized. Moreover, the disclosed radially grooved casing  8  decreases the engine&#39;s susceptibility to foreign object damage while maintaining necessary surge margin. 
     While the preferred embodiment of the invention has been disclosed, it should be appreciated that the invention is susceptible of modification without departing from the scope of the following claims.