Patent Publication Number: US-2013251500-A1

Title: Gas turbine engine case with heating layer and method

Description:
TECHNICAL FIELD 
     The application relates generally to gas turbine engines and, more particularly, to a method and means to control the clearance between compressor or turbine blades and the corresponding compressor/engine case. 
     BACKGROUND OF THE ART 
     Compressor blade tip clearances are conventionally built larger than optimal to avoid blade rub in some flight conditions, such as takeoff and slam acceleration. The clearances are larger than optimal to compensate for a faster blade growth rate relative to the case growth rate because of a faster blade heat ramp rate and blade growth from centrifugal force. Compressor blade growth rate may also be faster than the case growth rate because the blades heat up faster due to their lower thermal mass, larger surface area to volume ratio, thinner wall thickness, and/or exposure to heat from all sides rather than from the gas side only as for the compressor case or engine case. 
     SUMMARY 
     In one aspect, there is provided an assembly of a rotor and case for a gas turbine engine comprising: a rotor having a plurality of circumferential blades, each of the blade having a blade tip; a case comprising at least one structural layer forming an annular body receiving therein the rotor, with a tip clearance being defined between the blade tips and an inner surface of the annular body, and a heating layer connected to the at least one structural layer, the heating layer having a resistivity to heat the structural layer when an electric current passes through the heating layer; and an electric power source connected to the heating layer and supplying current to the heating layer, the heating layer configured to in use change the diameter of the annular body to adjust the tip clearance between the rotor and the case. 
     In a second aspect, there is provided a method for adjusting a tip clearance between blade tips of a rotor and inner surface of a case in a gas turbine engine, comprising: applying a heating layer on the case; determining that gas turbine engine conditions require an increase in diameter of the case for tip clearance; and supplying electric current to the heating layer on the case to increase the diameter. 
     Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; and 
         FIG. 2  is a schematic sectional view of a clearance between a blade and a case, with an electrically-powered heating layer in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled within an engine case  13 , a multistage compressor  14  within a compressor case  15  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
     Referring to  FIG. 2 , a blade is shown enlarged at  20 . The blade  20  is one of multiple blades of a rotor, whether it be part of the fan  12  or the compressor  14 , The blade  20  is illustrated as having a blade tip  21 . The blade tip  21  is shown as having a straight profile. It is understood that the blade tip  21  may have any appropriate profile as alternatives to the straight profile of 
     A clearance A is defined between the blade tip  21  and an inner surface  30  of the case  13 / 15 . The case  13 / 15  is illustrated as consisting of one structural layer (e.g., sheet metal) and has an outer surface  31 . However, the case  13 / 15  may have additional structural layers, typically radially outward of the outer surface  31 . 
     A heating layer is generally shown at  32  and is against the outer surface  31 . The heating layer  32  is of the type heating up when an electrical current is passed through it. In accordance with an embodiment, the heating layer  32  is an electric heater coating that is applied (e.g., sprayed, painted, printed, etc.) to the outer surface  31 . One suitable type of coating used as heating layer  32  is a product provided by Datec Coating Corporation. In accordance with another embodiment, the heating layer  32  consists of heating foil that is applied against the outer surface  31 , In both embodiments, the installation of the heating layer  32  is readily effected when compared to existing prior art system using coils, etc. When heater coating is used, applying the coating may be similar to applying a coat of paint. Moreover, the heating layer  32  may be on either side of the case  13 / 15  where possible, of sandwiched between layers of the case  13 / 15 , etc. In both cases, the heating layer  32  must be made of a material having a minimum resistivity to generate sufficient heat from the electric current circulated therein. 
     The heating layer  32  is therefore wired to an electric power source  40 , for instance by way of wires  41  (i.e., leads, lead wires) on opposite sides of the layer  32 . Any appropriate type of arrangements may be used to allow a current supply through the heating layer  32  from the electric power source  40 . It is also considered to have multiple circuits for instance in parallel to heat up the heating layer  32  in segments. 
     According to the present disclosure, there is provided a smaller cold tip clearance between blade tip and the case then in conventional arrangements, such that the engine as the smallest tip clearance at cruise condition. The case is electrically heated for the case growth rate to better match the blade growth rate, in some flight conditions. For instance, at takeoff or at slam acceleration where blade rub could occur, the case is heated up by circulating current from the electric power source  40  in the heating layer  32 , thereby increasing the case growth rate. The electric heating from the electric power source  40  may be switched off at cruise condition. In other words, the case  13 / 15  is heated up when large tip clearance is needed to avoid rubbing, and the electric heating is switched off when the small clearance is needed for better performance. In an embodiment, the heating layer  32  is sandwiched between different layers of the case  13 / 15 . 
     The electric power source  40  may therefore comprise a controller or like processing unit to determine the flight conditions in which the gas turbine engine is operated. For instance, the electric power source  40  may have its controller connected to aircraft command center or to an engine control unit to receive this information. Alternatively, the electric power source  40  may have its controller connected to various probes to determine the flight conditions, such as temperature probes, manometers, etc. The electric power source  40  may therefore monitor the flight conditions to calculate an actual tip clearance. The electric power source  40  may then actively adjust the size of the case  13 / 15  by determining the amount of current required to reach the desired case size, as a function of the known resistivity of the heating layer  32 . 
     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 scope of the invention disclosed. For example, multiple heating layers (e.g., of different types) may be provided on the case to increase heating. Various wiring arrangements may be used to supply electric current to the heating layer  32 . The heating layer  32  may cover a portion or most of the case  13 / 15  opposite the rotor. 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.