Abstract:
A method for attenuating resonance frequency responses is disclosed. The method includes encasing at least one conducting element within a protective sheath and disposing a damping element about the at least one conducting element. The damping element is fixedly secured to the at least one conducting element for inhibiting relative movement between the damping element and the conduit. The method also includes positioning the at least one conducting element concentrically within a conduit to define a gap between the at least one conducting element and the conduit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to vibratory environments, and more particularly, to methods and apparatus for resonance frequency response attenuation. 
         [0002]    Resonance frequency activity of cables is determined by the mass and stiffness of the cable and cable/clamp support system. Resonance frequency response activity, of cables in phase and amplitude with engine imbalance forces, is dependent on phase and amplitude of the forcing function and may lead to early failure if the forcing function coincides with the modal response of the cables. Electrical conductors and cables, as installed on engines, require effective damping and support constraint to survive the high level vibratory environment in these applications. 
         [0003]    Electrical cables are bundled and shrouded with flexible conduits allowing routing to accommodate pre-existing cable clamp/bracket locations, and have low bending rigidity. The damping characteristics must be effective over broadband frequency and thermal ranges to control mechanically induced vibratory excitation. Electrical cable routing configurations are generally tuned to be quiescent by application specific means and the vibration stability of each application is verified individually by testing, monitoring and trending. The free span clamp lengths are defined to control vibration frequency response acceptable levels. Solutions to these issues require extensive data characterization, are reactive in nature and require extensive resources to resolve. 
         [0004]    Electrical cables and cable-like sensors, such as TNACs, are made of an inner conducting wire and an outer protective jacket. The TNAC outer protective jacket is made of Nickel-200 and flexes repeatedly due to vibratory excitation of a gas turbine engine. As the outer protective jacket repeatedly flexes, it work-hardens, becomes brittle and breaks. When the Nickel-200 outer jacket breaks the inner sensor wire is directly exposed to the harsh operating environment of the gas turbine engine and is quickly damaged. 
         [0005]    Gas turbine engine components like the TNAC are required to satisfy on-wing life expectancy requirements by functioning for up to fifty thousand operating hours without failure. However, the average on-wing life for the TNAC is only three thousand operating hours. Consequently, the TNAC fails to meet on-wing life expectancy requirements. To avoid damaging the entire sensor and at the same time satisfy on-wing expectancy requirements, the inner sensor wire must be immobilized and protected from the outside environment. The environment includes the vibratory and temperature conditions of the engine and other miscellaneous loads such as tools hung on the outer protective jacket by maintenance workers. 
         [0006]    Consequently, there is needed an improved damping system effective over a wide range of frequencies and applications specific to a temperature range that attenuates all vibratory activity without the need to tune to a specific frequency. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    In one exemplary embodiment, a method for attenuating resonance frequency responses is disclosed. The method includes encasing at least one conducting element within a protective sheath and disposing a damping element about the at least one conducting element. The damping element is fixedly secured to the at least one conducting element for inhibiting relative movement between the conducting element and a conduit. The method also includes positioning the at least one conducting element concentrically within the conduit to define a gap between the at least one conducting element and the conduit. 
         [0008]    In another exemplary embodiment a resonance frequency response attenuation apparatus is disclosed. The apparatus includes a conduit, at least one conducting element encased in a protective sheath and a damping element disposed about the at least one conducting element. The damping element is fixedly secured to the at least one conducting element for inhibiting relative movement between the conducting element and the conduit, and the at least one conducting element is disposed concentrically within the conduit and defines a gap between the conduit and the at least one conducting element. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a gas turbine engine; 
           [0010]      FIG. 2  is a schematic cross sectional view of a gas turbine engine; 
           [0011]      FIG. 3  illustrates a gas turbine engine with a duct mounted thereon and a conducting element mounted on the duct; 
           [0012]      FIG. 4  illustrates a blown-up view of the mounting structure used for attaching the conducting element to the engine duct of  FIG. 3 . 
           [0013]      FIG. 5  illustrates an exemplary embodiment of a resonance frequency response attenuation apparatus; 
           [0014]      FIG. 6  illustrates another exemplary embodiment of a resonance frequency response attenuation apparatus; 
           [0015]      FIG. 7  illustrates yet another exemplary embodiment of a resonance frequency response attenuation apparatus; 
           [0016]      FIG. 8  illustrates yet another exemplary embodiment of a resonance frequency response attenuation apparatus. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1  shows a schematic illustration of a gas turbine engine  10  including a low pressure compressor  12 , a high pressure compressor  14  and a combustor  16 . The gas turbine engine  10  also includes a high pressure turbine  18 , a low pressure turbine  20  and a turbine nozzle assembly  30 . 
         [0018]    In operation, air flows through low pressure compressor  12  and then compressed air is supplied from low pressure compressor  12  to high pressure compressor  14 . Each of the low pressure compressor  12  and the high pressure compressor  14  includes variable stator vanes  22  that control the incidence angle of the air as it enters the compressors  12 ,  14 , allowing the gas turbine engine  10  to operate more efficiently. A conventional fuel system  28  provides fuel that is combined with high pressure air and burned in the combustor  16 . The resulting high temperature combustion gases are delivered from the combustor  16  to the turbine nozzle assembly  30 . Airflow (not shown in  FIG. 1 ) from the combustor  16  drives the high pressure turbine  18  and the low pressure turbine  20 . 
         [0019]      FIG. 2  illustrates a cross sectional view of the gas turbine engine  10  of  FIG. 1 . The low pressure compressor  12 , high pressure compressor  14 , high pressure turbine  18  and low pressure turbine  20 , each includes a rotor assembly. A rotor assembly, for example turbine  18 , includes a set of rotor blades  36 , wherein each rotor blade  36  is coupled to a rotor disk (not shown in  FIG. 2 ) that is rotatably coupled to a rotor shaft  42 , such that blades  36  are spaced about a circumference of the rotor disk. A plurality of circumferentially-spaced rotor blades  32 ,  34 ,  36 ,  38  is generally referred to as a bucket. During gas turbine engine  10  operation the rotor assemblies induce mechanical and aerodynamic vibratory excitation in the gas turbine engine  10  structure. 
         [0020]    Mechanical vibratory excitation is induced in the gas turbine engine  10  by the rotor assemblies of the compressors  12 ,  14  and of the turbines  18 ,  20 . During gas turbine engine  10  operation, the rotor assemblies experience imbalance during rotation, thus imparting mechanical vibratory excitation to the stationary members of the gas turbine engine  10 . The primary rotor assembly imbalance occurs at one per revolution of the machine. Foreign object damage also causes mechanical vibratory excitation. During operation, the gas turbine engine  10  may ingest some type of foreign object or debris, such as ice, birds and mechanics tools from the runway. Depending on the size of the ingested object or debris, a rotor blade  32 ,  34 ,  36 ,  38  may break off causing further rotor assembly and rotor blade imbalance. Additionally, the rotor blades  32 ,  34 ,  36 ,  38  generate further vibratory excitation through harmonics that develop at multiples of one per revolution. 
         [0021]    Aerodynamic vibratory excitation is developed by the passing frequency of the rotor blades  32 ,  34 ,  36 ,  38 . As the rotor blades  32 ,  34 ,  36 ,  38  pass stationary members of the gas turbine engine  10  aerodynamic forces are generated. These aerodynamic forces create vibratory signatures on stationary members of the engine  10 , such as the nozzle assembly  30 , and are then transmitted throughout the gas turbine engine casing  40 . 
         [0022]      FIG. 3  shows a conducting element  42  mounted on a gas turbine engine duct  11 . In one exemplary embodiment the conducting element  42  may be a resistance temperature detector, also known as an RTD. However, it should be appreciated that, in other various exemplary embodiments, the conducting element  42  may be any type of conducting element and is not limited to being an RTD. Conducting elements  42 , such as the RTD, in high temperature applications are protected by metallic sheaths that control internal cable temperature to acceptable limits. High temperature applications apply to maximum temperatures greater than five hundred degrees Fahrenheit (260 degrees Celsius). The conducting elements  42  are routed in three dimensional space to conform to the external gas turbine engine  10  configuration. The conducting element  42  routing includes straight segments interconnected by intersection bend radii, simple bends and terminations. 
         [0023]    In the exemplary embodiment, the conducting element  42  may be a specific type of RTD called an area averaging resistance temperature detector.  FIG. 4  shows the TNAC mounted on a gas turbine engine duct  11  using a series of small clamps  44 . 
         [0024]      FIG. 5  shows a perspective view of a cable  50 . In the exemplary embodiment the cable  50  has a vibration damping element designed to attenuate all engine vibratory activity without the need to tune to a specific frequency. In the exemplary embodiment the cable  50  includes two conducting elements, or sensor elements  52 , made from platinum encased within a Nickel-200 sheath  54  and is disposed within an external conduit  60 . The overall conductor is 0.118 inches (0.3 cm) wide, 0.053 inches (0.13 cm) high and 57.60 inches (146 cm) long. It should be appreciated that the sensing elements  52  may have any cross sectional shape. 
         [0025]    The external conduit  60  is made from inconel, a nickel-ferrous alloy, has a circular cross sectional area and extends for the full length of the sensing elements  52 . Additionally, the external conduit  60  has an inner surface defined by an inside diameter and an outer surface defined by an outside diameter. In the exemplary embodiment, the inside diameter of the conduit  60  is 0.18 inches (0.46 cm). It should be appreciated that the external conduit  60  may be made from other materials suitable for other applications and their respective operating environments. Consequently, in other various exemplary embodiments the external conduit may be made from material such as, but not limited to, stainless steel, aluminum, and PVC. It should also be appreciated that the inside and outside diameters of external conduit  60 , and corresponding cross sectional areas, may be any size so long as the inside diameter is less than the outside diameter. However, the size of the external conduit  60  outside diameter may be limited due to the space available in the gas turbine engine  10 . Further, it should be appreciated that although in the exemplary embodiment the external conduit  60  has a circular cross section, the external conduit  60  may have any cross sectional shape. 
         [0026]    The sensing elements  52  are sensitive to the harsh operating environment of gas turbine engines  10 , so a protective sheath  54  is disposed about each sensing element  52 , effectively encasing the sensing element  52  within the protective sheath  54 . To further protect the sensing elements  52 , wherein each sensing element  52  is encased within a respective protecting sheath, the sensing elements  52  are concentrically positioned within the external conduit  60  to define a gap  62  between the outside surface of the protective sheaths  54  and the inside surface of the external conduit  60 . In the exemplary embodiment, the gap  62  is between 0.0025 inches (0.0064 cm) and 0.0065 inches (0.1651 cm). The gap  62  preferably has a constant width and extends for the full length of the sensing element  52 . It should be appreciated that the number of sensing elements  52  is not limited to two elements  52  and that any number of sensing elements  52  may be used. However, the number of sensing elements  52  may be limited by the size of the inside diameter of the external conduit  60 . It should be further appreciated that the width of gap  62  may vary depending on the inside diameter of the external conduit  60  and the number of sensing elements  52  disposed within the conduit  60 . 
         [0027]      FIG. 5  also shows the spacer wires  56  disposed between the protective sheaths  54 . The spacer wires  56  are made from the same material as the protective sheath  54 , Nickel-200, and have an outside diameter of 0.0285 inches (0.072 cm). It should be appreciated that the spacer wires  56  may be made from other materials so long as the material does not damage the other components, including the damper wires  58  (discussed below) and the external conduit  60 . The spacer wires  56  facilitate filling the gap  62  between the outside surface of the protective sheaths  54  and the inner surface of the external conduit  60 , thus creating a more circular cross sectional shape to meet the inner cross sectional shape of the external conduit  60 . It should be appreciated that as the number of sensing elements increases, fewer spacer wires  56  are needed to meet the inner cross sectional shape of the external conduit  60 . 
         [0028]    The sensing elements  52  encased in protective sheaths  54  and being separated by the spacer wires  56  form a composite structure  64 . A vibration damping element, such as the damper wire  58 , is disposed about the composite structure  64  and is in firm contact with the inside surface of the external conduit, thus concentrically positioning and damping vibrations of the sensing elements  52 . The damper wire  58  has a circular cross section, has an outside diameter of 0.0285 inches (0.072 cm) and is disposed within the external conduit  60  and within the gap  62  between the outside surface of the protective sheath  54  and the inner surface of the external conduit  60 . Additionally, the damper wire  58  is wound helically about the composite structure  64  and circumscribes the composite structure  64 . Further, the damper wire  58  may be brazed to the protective sheaths  54  of the composite structure  64  to constrain any relative movement, due to relative thermal growth, vibration or other effects, between the damper wire  58  and the protective sheath  54 . Doing so prevents the damper wire  58  from chafing the protective sheath  54  and possibly shorting the sensing element  52  to ground. The damper wire  58  is made from Nickel-200 or a nickel material derivative characterized as a softer material and should be no harder than the protective sheath  54  material. It should be appreciated that the damper wire  58  may be made from other materials suitable to other applications and their respective operating environments. Consequently, in other various exemplary embodiments, the damper wires  58  may be made from materials such as, but not limited to, nickel, rubber, copper, steel, and a visco-elastic material. The damper wire  58  should be lightweight and economic. 
         [0029]    Each gas turbine engine  10  generates different vibratory excitations at different frequencies or sine functions, so the cable routing configurations are separately tuned. However, gas turbine engines  10  uniformly experience different operating vibratory excitation frequencies corresponding to different stages of operation. For example, gas turbine engines  10  have a warming-up stage with a corresponding frequency of about 60 Hz and a constant continuous flying stage with a corresponding frequency of about 180 Hz. In the exemplary embodiment, the resonance frequency response of the sensing element  52  is controlled, or tuned, by adjusting the helical pitch spacing of the damper wires  58 , adjusting the width of the gap  62  and adjusting the number of damper wires  58 . 
         [0030]    Disposing the damper wire  58  to circumscribe the composite structure  64  in a helical pattern facilitates tuning the sensing element  52 . Specifically, the helical pitch spacing of the damper wires  58  controls the resonance frequency of the sensing element  52 . Thus, decreasing helical pitch spacing increases the resonance frequency of the sensing element  52 . By the same token, increasing the helical pitch spacing decreases the resonance frequency of the sensing element  52 . The helical pitch spacing is designed to control the constant continuous flying stage resonance frequency response at about 180 Hz for gas turbine engines  10 , thus eliminating the need for tuning to a specific frequency. In the exemplary embodiment, the helical pitch spacing of the damper wires  58  is 6.25 inches (15.88 cm). 
         [0031]    Providing the gap  62  in the exemplary embodiment additionally controls the resonance frequency response of the sensing element  52 . The resonance frequency response is further controlled, or the sensing element  52  is further tuned, by adjusting the width of the gap  62  between the outside surface of the protective sheaths  54  and the inner surface of the external conduit  60 . Decreasing the gap  62  width increases the resonance frequency response of the sensing element  52 . Increasing the gap  62  width decreases the resonance frequency response of the sensing element  52 . 
         [0032]    In the exemplary embodiment, the resonance frequency response of the sensing element  52  is also controlled by the number of damper wires  58  disposed about the composite structure  64 . Increasing the number of damper wires  58  decreases the resonance frequency response of the sensing elements  52 . Decreasing the number of damper wires  58  increases the resonance frequency response of the sensing elements  52 . 
         [0033]    In the exemplary embodiment, the composite structure  64  is manufactured in a straight length configuration. The external conduit  60  has a circular cross section area and is also manufactured in straight length configurations. The composite structure  64 , with damper wire  58 , is inserted into the external conduit  60  before the assembled cable  50  is conformally routed throughout the engine  10 . It should be appreciated that situating the composite structure  64  and the damper  58  within the external conduit  60  does not constitute a swaged fit. Because the damper wires  58  are in firm contact with the inside surface of the external conduit  60 , the damper wires  58  also reinforce the external conduit  60 . 
         [0034]    It should be appreciated that although a single damper wire  58  is used in the exemplary embodiment, any number of damper wires  58  may circumscribe and be brazed to the composite structure  64  in a helical pattern. It should be further appreciated that vibration damping elements other than a damping wire  58  may be used and disposed in the gap  62 . 
         [0035]      FIG. 6  shows another exemplary embodiment wherein the vibration damping element includes two damper wires  58  disposed adjacent and parallel to each other. The two damper wires  58  are together helically wrapped about, circumscribe and are brazed to the composite structure  64 . It should be appreciated that the damper wires  58  may also be brazed to each other and are to be disposed parallel to each other and not intersect. It should be further appreciated that although one or more damper wires  58  may be disposed in the gap  62  between the outer surface of the sheath  54  and the inner surface of the external conduit  60 , other damping materials may be substituted for the damper wires  58  in the gap  62 . 
         [0036]      FIG. 7  shows yet another exemplary embodiment wherein the vibration damping element includes an alternative visco-elastic material with thermal shrink properties, such as shrink tubing  66 , and disposing it in the gap  62 . Shrink tubing  66  is a Teflon type material which is disposed about the composite structure  64  such that the Teflon material shrinks in size upon heating. As the shrink tubing material  66  is heated it shrinks in size, compressing the composite structure  64 , and it becomes possible to slip the composite structure  64  with the shrink tubing  66  into the external conduit  60 . The shrink tubing material  66  immobilizes the composite structure  64  within the external conduit  60 . However, the shrink tubing material  66  may provide less structural support or reinforcement to the external conduit  60  because it is not as stiff as the damper wire  58  disposed in a helical configuration about composite structure  64 . Shrink tubing  66  is used in low temperature applications where the maximum temperature is less than three hundred degrees Fahrenheit (149 degrees Celsius). 
         [0037]      FIG. 8  shows yet another exemplary embodiment wherein the vibration damping element includes a wire mesh material  68  disposed in the gap  62 . The wire mesh  68  is wrapped around the composite structure  64  so that the wire mesh  68  circumscribes the composite structure  64  at least one time. The wire mesh  68  is brazed to the composite structure  64 . Further, it should be appreciated that in other various exemplary embodiments where the wire mesh  68  circumscribes the composite structure  64  a plurality of times, a plurality of layers of the wire mesh  68  are disposed about and circumscribe the composite structure  64 . These layers of wire mesh  68  may also be brazed to each other. The wire mesh  68  acts as a damper as well as tolerates a wide range of extreme vibration frequencies and temperatures. However, the wire mesh material  68  provides less structural support or reinforcement to the external conduit  60 . 
         [0038]    In the exemplary embodiment, the vibration damping element effectively controls adverse vibration responses through friction damping over a wide forcing frequency range and a wide temperature range. The frequency range is generally from about 30 Hertz to about 3 Kilohertz and corresponds to passing frequencies of compressor rotor blades  32 ,  34  and turbine rotor blades  36 ,  38 . This frequency range contains an ensemble of pure tone, random and harmonic-forcing frequencies. The temperature range extends from a minimum of minus forty degrees Fahrenheit (−40 degrees Celsius) to seven hundred fifty degrees Fahrenheit (399 degrees Celsius). 
         [0039]    The exemplary embodiment of the vibration damping element described herein is designed to attenuate all vibratory activity, without the need to tune to a specific frequency. In the exemplary embodiment, the vibration damping element controls adverse vibration by dissipation of friction energy induced by resonance and relative movement between the internal sensing element  52  and the external conduit  60 . Additionally, the vibration damping element flexibility allows the combined conducting, or sensing element  52 , vibration damping element and external conduit  60  to be assembled while in an un-bent straight condition and then formed to meet configuration routing requirements. Furthermore, the vibration damping element provides thermal protection in high temperature applications while maintaining thermal conductivity and transient response to the external conduit  60  member into the internal temperature sensing element  52 . In addition, the vibration damping element provides vibration protection for sensitive electrical circuit elements when exposed to high energy acoustic or mechanical vibratory inputs at elevated temperatures. 
         [0040]    It should be appreciated that various other exemplary embodiments may be used with any conducting element requiring protection from elevated external temperatures or excessive mechanical vibration, or both. The various other exemplary embodiments may be applied in any environment that vibrates, such as, but not limited to, lawnmower engines, motor boat engines, and cruise line ship engines. 
         [0041]    While the invention has been described with reference to a specific embodiment, the description of the specific embodiment is illustrative only and is not to be construed as limiting the scope of the invention. Various other modifications and changes may occur to those skilled in the art without departing from the spirit and scope of the invention.