Abstract:
A method of sealing a gap between an aerofoil component and a further component. The method comprises placing the aerofoil component in close proximity with the further component to define a gap therebetween, applying a thermoplastic material to the gap in a molten phase and cooling the thermoplastic material to set the thermoplastic material.

Description:
[0001]    This application is a divisional application of U.S. patent application Ser. No. 13/740,483 filed on Jan. 14, 2013, which claims priority to GB 1200845.4 filed on Jan. 19, 2012. The prior applications are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method of sealing a gap between an aerofoil component and a further component. In particular, the invention relates to sealing a gap between a nozzle guide vane (NGV) and a fixture used for flow checking purposes. 
         [0003]    Hollow aerofoil components such as turbine blades and NGVs provided in gas turbine engines often comprise internal passages, which extend from the hollow interior of the blade to the exterior to provide a cooling air film in use, and thereby cool the surface of the component. 
         [0004]    It is sometimes necessary to test the performance of such passages, for example to validate a new component design, or to diagnose blockages during engine overhaul or repair. A previous method of testing NGVs comprises placing the NGV to be tested in a tight fitting rubber or silicone housing, and clamping the housing against the NGV. However, such a method requires a relatively close fit between the housing and the NGV to be tested. Gas turbine engines typically comprise a large number of NGVs having slightly different dimensions, and so a large number of housings must be provided to test each NGV. 
         [0005]    The present invention provides an improved method of sealing an aerofoil component against a further component that addresses some or all of the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention there is provided a method of sealing a gap between an aerofoil component and a further component, the method comprising:
       placing the aerofoil component in close proximity with the further component   to define a gap therebetween;   applying a thermoplastic material in a molten phase to the gap; and   cooling the thermoplastic material to set the thermoplastic material.       
 
         [0011]    It has been found that by applying a thermoplastic material to the gap between an aerofoil component and a further component, a relatively large gap can be effectively sealed. As a result, a less precise alignment between the aerofoil and the component can be provided, which can still be sealed. This in turn has the effect that a single housing can be used for differently shaped NGVs. It has also been found that this method is capable of forming a robust seal, which cures in a relatively short time and can be melted and reformed to permit repair of a defective seal. It has been found that the seal is typically effective up to a pressure ratio of 2:1, which is the pressure typically required for NGV testing. 
         [0012]    The method may comprise heating at least one of a surface of the aerofoil component and the further component to a temperature above 20° C. while the thermoplastic material is applied to facilitate adhesion. The surface of the aerofoil component may be heated to between 50° C. and 60° C. The heated surface may be heated by an air gun, and the air provided by the air gun may have a temperature of between 140° C. and 170° C. 
         [0013]    By heating the surface of the aerofoil component or the further component prior to applying the thermoplastic material, the time taken for the thermoplastic to solidify is increased. As a result, the thermoplastic material remains in its molten state for a longer duration, and so provides a larger wetted surface in contact with the surface of the aerofoil component and the further component, thereby leading to improved sealing between the two components. 
         [0014]    The thermoplastic material may comprise Ethylene-vinyl acetate (EVA). The thermoplastic material may comprise Tec-Bond 240™, or may comprise Tec-Bond 260™. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1A  is a plan view of an aerofoil and a further component sealed using a first method in accordance with the present invention; 
           [0017]      FIG. 1B  is a side view along the line A-A of the method of  FIG. 1A ; 
           [0018]      FIG. 2  is a side view of a second method in accordance with the present invention; 
           [0019]      FIG. 3A  is a side view of an adhesive bead following solidification where the preheating step is employed; and 
           [0020]      FIG. 3B  is a side view of an adhesive bead following solidification where the preheating step is not employed; and 
           [0021]      FIG. 4  is a perspective view of an aerofoil component. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIGS. 1A and 1B  show a first method of sealing an aerofoil component such as a nozzle guide vane (NGV)  30  for a gas turbine engine (not shown), to a further component in the form of a housing  32 . The NGV is shown in detail in  FIG. 4 . The NGV  30  comprises Nickel Alloy, with a surface coating of ceramic thermal barrier material. The NGV may alternatively be uncoated. The housing  32  is formed of a plastics material, and includes a cavity  34 , shaped to correspond to an external profile of the NGV  30 , and an aperture  36  for introducing compressed air to the cavity  34 . In use, the NGV  30  is placed in the cavity  34  for testing. 
         [0023]    The shape of the cavity  34  does not necessarily precisely correspond to the profile of the NGV  30 , such that a gap  38  is generally defined between the edges of the cavity  34  and NGV  30 . The gap  38  is generally 0.25 to 0.5 mm wide when the NGV  30  is placed in the cavity  34 . Such a gap  38  enables slightly different shaped NGVs to be tested in the same housing  32 . 
         [0024]    The gap  38  between the NGV  30  and the housing  32  is sealed using the following method. One or both of the NGV  30  and the housing  32  is heated to a temperature above 20° C., and preferably to a temperature of around 50° C. to 60° C. The heating step could be carried out using any suitable process. For example, hot air could be applied to the NGV  30  using an air gun. Where an air gun is used, the hot air supplied by the air gun is supplied at a temperature of between 140° C. and 170° C. This has been found to be sufficient to provide a surface temperature of 50° C. to 60° C. Alternatively, the whole assembly (i.e. both the NGV  30  and the housing  32 ) could be placed in an oven (not shown) during the heating step. In a still further alternative, an electrical current could be conducted or induced in the NGV  30  to provide resistive or inductive heating. 
         [0025]    Subsequent to the heating step, a bead  20   a  of thermoplastic material in the form of Tec-Bond 240™, or Tec-Bond 260™ thermoplastic hot melt adhesive in a molten phase (i.e. above the melting point of the material) is applied to the gap  38 . The particular type of adhesive used will depend on a number of factors, including the required strength of the bond, and the properties of the NGV. Where the NGV comprises Nickel alloy having a ceramic thermal barrier coating, Tec-Bond 240™, or Tec-Bond 260™ have been found to be suitable. Applying the adhesive at a temperature of approximately 200° C. has been found to result in the adhesive having the correct viscosity to cover the gap  38  without an excessive amount running into the cavity  36 . The adhesive bead  20   a  is then allowed to cool to a temperature of around 20° C. and set, i.e. solidify, to form a seal between the NGV  30  and housing  32  across the gap  38 . The cooling time may be controlled by, for instance, controlling the ambient temperature or air flow around the NGV  30  and housing  32 . 
         [0026]    Alternatively, the heating step could be omitted, and a bead  20   b  of thermoplastic material could be applied with both of the NGV  30  and housing  32  at ambient temperature, i.e. around 20° C., and allowed to cool. 
         [0027]      FIGS. 3A and 3B  show the beads  20   a,    20   b  applied with and without the heating step respectively. The heating step results in one or both of the NGV  30  and housing  32  being at a higher temperature (i.e. around 50° C. to 60° C.) when the adhesive bead  20   a  is applied, relative to when the heating step is omitted, where the NGV  30  and housing  32  would be at room temperature (i.e. around 20° C.). As a result of the heating step, the bead  20   a  cools more slowly and more evenly than when the heating step is omitted, resulting in the bead  20   a  remaining in the molten phase for a longer period in comparison to the bead  20   b.  As a result, the bead can spread further into the gap  38  before solidifying, thereby resulting in a larger surface area of the bead  20   a  in contact with the edges of the NGV  30  and housing  32 , thereby forming an improved seal. However, such a heating step increases the time taken to seal the gap  38  due to both the time taken to heat the NGV  30  and or housing  32 , and the longer cooling time required for the adhesive to set. Generally, the NGV  30  requires heating to ensure good adhesion, though it has been found that where the housing  32  comprises a plastics material, heating of the housing  32  is not necessary to provide good adhesion. 
         [0028]    In contrast, where the heating step is omitted, part of the bead  20   b  in contact with the edges of the NGV  30  and housing  32  cools very quickly when applied. As a result, the part of the bead  20   b  in contact with the edges of the NGV  30  and housing  32  solidifies very quickly and contracts, resulting in a more spherical, less flattened shape relative to bead  20   a,  and thus a lower area in contact with the surface  22 . The rounded shape of the bead  20   b  is also easier to peel off relative to the flattened shape of the bead  20   a.    
         [0029]      FIG. 2  shows a second method of sealing an aerofoil component such as a nozzle guide vane (NGV)  30 , to a housing  32 . The NGV  30  and housing  32  are the same as used in the method described in relation to  FIGS. 1A and 1B , but the adhesive is applied with the base of the housing  32  at an angle to the horizontal, such that the adhesive runs into the gap  38  on the lower side of the housing  32 . This method has been found to provide an improved seal relative to when the adhesive is applied with the base parallel to the horizontal, since a thicker bead  40  can be provided. 
         [0030]    Once the adhesive bead  20  has solidified, the NGV  30  can be tested by passing compressed air into the housing  32  through the aperture  36 . If the seal is found to be defective however, i.e. some air is able to pass through the gap  38  during testing, the seal can be repaired. This can be done either by adding further material as above, or by applying localised heat (for example using a soldering iron or glue gun tip) to the defective bead  20 , such that it is heated above its melting point. The melted bead  20  is then allowed to flow across the gap  38  to thereby cover the defect. The repaired bead  20  is then allowed to cool, and the NGV  30  can be tested again. The present invention therefore permits the seal to be repaired, without removing or necessarily adding further material to the blade. This method thereby saves time in comparison to prior sealing methods, in which the sealant has to be removed and reapplied where a defective seal is found. 
         [0031]    Once testing is complete, the adhesive bead  20  can be removed by hand, by peeling the solidified adhesive from the gap  38 . The NGV  30  may be heated to soften or partially melt the adhesive to facilitate removal. Once removed, very little residue remains. The residue has been found not to gas turbine engine components, and is generally burned off during operation of the gas turbine engine. 
         [0032]    While the invention has been described in conjunction with the examples described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the examples of the invention set forth above are considered to be illustrative and not limiting, Various changes to the described embodiment may be made without departing from the spirit and scope of the invention. 
         [0033]    For example, where the NGV includes film cooling holes, the thermoplastic material could be applied to one or more of the cooling holes to seal these for testing. Different thermoplastics could be used for sealing the holes, depending on the required adhesion properties. In particular, this will be dependent on the pressures used during testing, as higher pressures will require a stronger adhesion. The method could also be used to join other components such as turbine blades. The component for testing and the housing could be made from substantially any materials, and different adhesives and preheating steps may be required for different materials. However the invention has been found to work particularly well for Nickel alloy components and plastics material housings.