Patent Publication Number: US-8985938-B2

Title: Fan blade tip clearance control via Z-bands

Description:
BACKGROUND 
     The present disclosure relates to a blade tip clearance control system, more specifically a fan blade tip clearance control system, to be used in engines such as gas turbine engines. 
     In a gas turbine engine, fan blades may be formed from an aluminum material, while the casing surrounding the fan blades may be formed from a composite material. There can be large differences in the thermal growth of these two materials. As a result, blade tip clearances may go beyond a desired range and fan efficiency may decrease. 
     SUMMARY 
     A system which helps maintain control of the blade tip clearance is highly desirable from the standpoint of obtaining fan efficiency. 
     In accordance with the present disclosure, there is provided an engine which broadly comprises a blade, a casing surrounding the blade, a seal ring, and a passive system for connecting the seal ring to the casing and for accommodating thermal expansion of the seal ring relative to the casing so as to maintain blade tip clearance control. 
     Further in accordance with the present disclosure, there is provided a method for maintaining blade clearance tip control in a fan section of an engine, which method broadly comprises the steps of: providing a fan casing formed from a composite material and a plurality of fan blades formed from an aluminum containing material; providing an annular seal ring; and providing a passive system for connecting the seal ring to the casing and for accommodating thermal expansion of the seal ring relative to the casing so as to maintain the blade tip clearance control. 
     Other details of the fan blade tip clearance control via Z-bands are set forth in the following detailed description and the accompanying drawings, wherein like reference numerals depict like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a gas turbine engine having a fan section; and 
         FIG. 2  is a sectional view of a system for maintaining blade tip clearance control. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a gas turbine engine  10  is diagrammatically shown. The gas turbine engine  10  includes a fan section  12 , a compressor section  14 , a combustor section  16 , and a turbine section  18 . The gas turbine engine  10  has an axially extending centerline  22 . Ambient air enters the engine  10  through the fan section  12 . A fraction of that air subsequently travels through the compressor, combustor and turbine sections  14 ,  16 , and  18  as core gas flow before exiting through a nozzle. 
     The fan section  12  includes a fan casing  30  and a plurality of fan blades  32  which rotate about the centerline  22 . The fan blades  32  are each connected to a fan rotor disk  34  which may be driven by a spool or shaft  33  connected to a low pressure turbine array  35  in the turbine section  18 . If desired, each fan blade  32  may be formed from an aluminum containing material such as an aluminum or an aluminum alloy where aluminum is present in an amount greater than  50  percent by weight. 
     The fan casing  30  may be formed from any suitable material. If desired, the fan casing  30  may be formed from a composite material such as an organic matrix composite material. 
     Referring now to  FIG. 2 , the fan casing  30  is provided with a seal ring  40  such as an abradable seal ring. The seal ring  40  may comprise an annular rub strip  42  formed from an abradable material and an annular backing ring  44 . The backing ring  44  may be formed from a metallic material such as an aluminum containing material including, but not limited to, aluminum and aluminum alloys where aluminum is present in an amount greater than  50  percent by weight. The backing ring  44  thus passively matches the thermal growth of the fan blades  32 . 
     It is desirable to maintain a clearance distance between the seal ring  40  and the tip  46  of each fan blade  32 . To this end, a passive system  48  for connecting the seal ring  40  to the fan casing  30  is provided. The passive system  48  accommodates thermal expansion of the seal ring  40  relative to the fan casing  30  so as to maintain blade tip clearance control. The system  48  is passive because it does not require the use of sensors, heating elements, piezoelectric materials, shape memory metal elements, fluid control systems, and the like. 
     The passive system  48  may comprise a plurality of Z-bands  50  extending between an inner wall  52  of an annular duct portion  54  of the fan casing  30  and the abradable seal ring  40 . Each Z-band  50  may be formed from a non-corrugated, solid piece of metallic material such as nickel sheet material, a nickel alloy sheet material such as INCO 718, a steel sheet material, a titanium sheet material, an aluminum sheet material or a composite sheet material. The material which is used for each Z-band may have a thermal growth which falls between the thermal growth of the material forming the fan casing and the thermal growth of aluminum. Each Z-band  50  may have an annular configuration and extend about the entire inner periphery of the duct portion  54 . Alternatively, each Z-band  50  may comprise an arc segment which extends about a portion of the inner periphery of the duct portion  54 . If metallic, each Z-band may have a thickness in the range of 0.015 to 0.030 inches. If composite, each Z-band may have a thickness in a wider range of 0.015 to 0.060 inches due to fiber orientation and lay-up possibilities. 
     Each Z-band  50  may be attached to the inner wall  52  and to the exterior surface  56  of the backing ring  44 . Any suitable means may be used to attach each Z-band  50  to the inner wall  52  and to the exterior surface  56 . For example, nuts and bolts may be used to join each Z-band  50  to the inner wall  52  and the exterior surface  56 . 
     As can be seen from the foregoing discussion, the fan casing  30  and the backing ring  44  are formed by different materials having different thermal expansion coefficients. 
     The Z-bands  50  allow the backing ring  44  to thermally expand relative to the fan casing  30 . Each Z-band  50  may be provided with a thickness which is sufficiently thin so that the alpha thermal differences between the Z-bands  50  and the backing ring  44  have minimal influence on the backing ring  44  and hence blade tip clearance is maintained. 
     If desired, the passive system  48  may comprise multiple Z-bands, for example, three Z-bands  50  with a first of the Z-bands  50  being attached to a leading edge portion  60  of the seal ring  40 , a second of the Z-bands  50  being attached to a trailing edge portion  62  of the seal ring  40 , and a third of the Z-bands  50  is attached to the seal ring  40  intermediate of the first and second ones of the Z-bands  50 . 
     The passive system  48  further comprises a slip joint  64  between the fan casing  30  and the abradable seal ring  40 . The slip joint  64  may be located adjacent the leading edge and trailing edge portions  60  and  62  respectively of the seal ring  40  and a wall  70  of the fan casing  30 . 
     The passive system  48  allows the seal ring  40  to grow different from the fan casing  30  and move thermally independent of the fan casing  30 . As a result, increases in fan efficiencies may be obtained. 
     There has been provided in accordance with the instant disclosure a blade tip clearance control via z-bands. While the fan blade tip clearance control via z-bands has been described in the context of specific embodiments thereof, other unforeseen alternatives, modifications, and variations may become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.