Abstract:
A system and method for achieving clearance control for a high-pressure turbine by means of casing mechanical deflection. An active clearance control system is provided to act on a blade that rotates near a shroud. The shroud is attached to a case at a shroud supporting location, or shroud hanger. A clearance is required between a tip of the blade and the shroud. The blade tip and shroud are surrounded with an elastic case. This case can deflect radially in response not only to thermal expansion, but also to a difference in pressures acting on the inner and outer diameters of the case.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the active clearance control system of a high-pressure turbine and, more particularly, to casing mechanical deflection for the high-pressure turbine.  
           [0002]    The active clearance control system (ACC) of a high-pressure turbine (HPT) has two basic functions. The first is to maintain tight blade-shroud clearances during transient operation, to minimize exhaust gas temperature (EGT). The second is to close the tip clearances during steady-state operation to increase turbine efficiency and reduce fuel burn.  
           [0003]    For both types of designs, i.e., single and dual stage, the case will shrink or grow, depending on the air-cooling temperature and the effect on the temperature of the case. Changing the case temperature will result in a clearance change. The thermal part of the clearance system is a slow response deflection approximately 30-60 seconds.  
           [0004]    State of the art active clearance control systems account for disk elastic deflection and blade thermal growth from idle conditions to take off by having a large clearance at idle. Such a system requires a large change in temperature at steady state conditions to reduce clearance to a minimum level. However, the desired case temperature change can be beyond system capabilities. In addition, it is difficult for the state of the art system to respond in time to overcome any rotor elastic stretch due to an instantaneous acceleration or re-acceleration (reburst) resulting in airfoil to shroud contact or rub.  
           [0005]    It would be desirable to provide an improved active clearance control system and method for a high-pressure turbine that overcomes problems in the existing art.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0006]    A system and method are proposed wherein casing elastic deflection is used to improve active clearance control of a high-pressure turbine.  
           [0007]    Accordingly, the present invention provides a system and method for achieving clearance control for a high-pressure turbine by means of casing mechanical deflection. An active clearance control system is provided to act on a blade that rotates near a shroud. The shroud is attached to a case at a shroud supporting location, or shroud hanger. A clearance is required between a tip of the blade and the shroud. The blade tip and shroud are surrounded with an elastic case. This case can deflect radially in response not only to thermal expansion, but also to a difference in pressures acting on the inner and outer diameters of the case. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic illustration of a single stage active clearance control system of the type that may employ the casing mechanical deflection technique of the present invention;  
         [0009]    [0009]FIG. 2 is a schematic illustration of a dual stage active clearance control system of the type that may employ the casing mechanical deflection technique of the present invention;  
         [0010]    [0010]FIG. 3 illustrates the thin case active clearance control according to the present invention;  
         [0011]    [0011]FIG. 4 is a diagram that shows the relation between pressure and rotor speed for idle to cruise conditions;  
         [0012]    [0012]FIG. 5 is a diagram comparing the radial deflection of the rotor and stator for a state of the art system and for the system applying the present invention; and  
         [0013]    [0013]FIG. 6 illustrates an alternative embodiment of the thin case active clearance control according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Modern gas turbine engine control systems typically require an active clearance control system for maintaining blade-shroud clearances and tip clearances during operation. For a single and dual stage HPT, illustrated in FIGS. 1 and 2, respectively, the appropriate clearance  18  between the blade  14  and the shroud  16  is achieved by controlling the case  10  temperature. For a single stage high-pressure turbine type, the case is heated and cooled by air coming from compressor mid stage  12  and discharge pressure source. Moreover, for a dual stage high-pressure turbine type, the first stage turbine case is controlled by compressor discharge pressure air. The second stage is controlled by compressor inter-stage bleed air. At appropriate times, the case is cooled by fan air in order to reduce case ring  25  temperatures.  
         [0015]    In FIGS. 1 and 2, the blade  14  and blade tip rotate as a result of the hot air flowing through the turbine. The shroud  16  is a piece of metal that defines the distances, or clearances, between the blade  14  tip and the shroud  16  itself. The purpose of the active clearance control system is to minimize clearance  18 . The larger the clearance, the less efficient the turbine will be. The shroud  16  is attached to the case of the ACC by a hanger  22 . Case growth causes the shroud  16  to move radially. In the existing art, the case  10  grows only by thermal expansion. With the present invention, the case will deflect due to thermal expansion and pressure acting on the outer and inner diameter of the case.  
         [0016]    The present invention proposes a system and method for improving an existing high-pressure turbine active clearance control system by modifying the turbine case, as illustrated in FIG. 3. In accordance with the present invention, the elastic case  24  will be a continuous 360-degree shell flexible enough to deflect radially due to the difference between the pressures P low  acting on the outer diameter of the case  24  and P high  acting on the inner diameter of the case  24 . The case  24  flexibility will be achieved by making the case average thickness in the locations supporting the hangers thin, so that the casing elastic deflection is increased from the prior art design. Although the specific thickness can vary, in the prior art the thickness of the casing at the location where the shroud supports are attached to the casing will be substantially thicker than that proposed by the present invention, with the prior art configuration therefore having negligible casing deflection. In a preferred embodiment of the present invention, the thickness will be thinner than the current design by eliminating the case rings  25 , typically, by way of example only, on the order of approximately 0.1 inches to 0.2 inches, or otherwise significantly thinner than the 1 to 2 inch thickness of existing casings. It will be obvious to those skilled in the art, however, that the thickness can vary beyond the thickness of a preferred embodiment, still being thinner than the existing art provides for, without departing from the scope of the invention. As with existing systems, the shrouds  16  will be attached to the case  24  by shroud hangers  22 . The shroud and the case will be made of a high temperature alloy.  
         [0017]    With the application of the present invention, the blade  14  to shroud  16  clearance  18  will change when the case  24  deflects radially due to pressure. The blade tip to shroud clearance will depend on the magnitude of the pressure acting on the case. The pressure acting on the case depends on the engine operating condition. Referring now to FIG. 4 the relationship between pressure and speed is shown. The present invention takes advantage of this pressure and speed, resulting in the illustration of FIG. 5. In FIG. 4, the pressure is at a minimum when the engine is at idle conditions, in region  26 . It will reach a maximum during high power at low altitude, in region  28 . At cruise conditions, in region  30 , when the engine is at high altitudes, the pressure will decrease (˜30% change) while the speed remains almost constant (˜10% change). According to this relationship between pressure and speed, illustrated in FIG. 4, the clearance will increase when the engine goes from idle to take off conditions. This pressure to speed relationship will allow the system to compensate for some disk elastic stretch and blade thermal expansion without the necessity of having large clearance at idle. Moreover, at high altitudes the pressure acting on the case will decrease, causing the case to shrink while the rotor speed change is small, thus maintaining high elastic stretch. This will result in smaller clearance at cruise relative to clearances required by current state of the art systems.  
         [0018]    The present invention takes advantage of the relationship between pressure and speed. FIG. 5 illustrates the stator and rotor deflection when the elastic case of the present invention is applied to a single or dual stage high pressure turbine. The elasticity of the case is indicated by dotted line  32  in FIG. 5. The prior art stator response is shown by line  34 , indicating the thermal expansion. The rotor response for both the invention and the prior art is shown by line  36 , indicating disk elastic stretch and blade thermal expansion during periods of idle, acceleration and cruise. The present invention will provide a protection against airfoil to shroud contact due to instantaneous acceleration (reburst). The pressure will increase at nearly the same rate as the rotor speed during an instantaneous acceleration allowing the case to deflect to avoid airfoil to shroud contact (rubs).  
         [0019]    Referring now to FIG. 6, an alternative embodiment for the thin case active clearance control can be applied by modifying the case elastic deflection to account for airfoil tip loss over operation time. The alternative embodiment comprises a band  38  attached to the case outer diameter. The band is preferably comprised of any suitable high temperature alloy or coating. The band thickness will be sized depending on the amount of airfoil material loss. The band will cause the case elastic deflection to be less by the same amount of airfoil material loss.  
         [0020]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.