Abstract:
A flow evacuation system for an aircraft engine including an engine nozzle that conveys the engine exhaust flow and an eductor receiving the engine exhaust flow and engine ventilation flow is disclosed. The engine nozzle has a final section in contact by its outer surface with the ventilation flow. The engine exhaust flow has a low swirl when the engine operates under design conditions and a high swirl when the engine operates out of design conditions. The engine nozzle includes a plurality of local flow conditioners placed in its inner surface in the final section for reducing the swirl of the engine exhaust flow at the exit of the engine nozzle.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the flow evacuation system of an aircraft gas turbine engine and, more in particular, of a turboprop or a turboshaft engine. 
       BACKGROUND OF THE INVENTION 
       [0002]    A typical turboprop engine comprises a core engine that includes a compressor section, a combustor and a first turbine in serial flow relationship, and a power turbine located aft the first turbine. Pressurized air from the compressor section is mixed with fuel and burned in the combustor to produce a high energy gas stream. The power turbine extracts energy from the gas stream to power the propeller. The engine also includes a nozzle which drives this flow outwards the engine. 
         [0003]    In the prior art are known aircraft engine flow evacuation systems for evacuating the engine exhaust gases flow and the ventilation flow that ventilates the engine nacelle, including an eductor where both flows are mixed and channeled to the eductor outlet. 
         [0004]    U.S. 2007/0089398 disclose a flow evacuation system with an eductor having a plurality of flow straighteners configured to reduce swirl motion of the gas that flows therethrough in order to avoid that a portion of exhaust gases may swirl within the eductor but may not flow out the eductor outlet. 
         [0005]    One problem of these known flow evacuation systems is that the ventilation capacity decrease in certain operating conditions causing an overheating of the engine nacelle. 
         [0006]    The present invention is intended to the solution of this problem. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide an efficient flow evacuation system for an aircraft engine, particularly for a turboprop or a turboshaft engine, in a variety of operating conditions. 
         [0008]    Another object of the present invention is to provide a flow evacuation system for an aircraft engine, particularly for a turboprop or a turboshaft engine, with a high ventilation capacity. 
         [0009]    In one aspect, these and other objects are met by a flow evacuation system for an aircraft engine comprising an engine nozzle that conveys the engine exhaust flow and an eductor receiving said engine exhaust flow and the engine ventilation flow, the engine nozzle having a final section in contact by its outer surface with said ventilation flow, the engine exhaust flow having a low swirl when the engine operates under design conditions and a high swirl when the engine operates out of design conditions, the engine nozzle comprising a plurality of local flow conditioners placed in its inner surface in said final section for reducing the swirl of the engine exhaust flow at the exit of the engine nozzle, for producing further depression to improve the suction effects over the ventilation flow and for facilitating the mixture of the engine exhaust flow and the ventilation flow, enhancing its evacuation along the eductor. 
         [0010]    In embodiments of the present invention said local flow conditioners are fins oriented radially, fins oriented with predetermined angular deviation with respect to a radial orientation or fins oriented with a variable angular deviation with respect to a radial orientation. Therefore some fins orientation choices are provided for a better accommodation to the flow evacuation needs of each engine. 
         [0011]    In embodiments of the present invention said local flow conditioners are fins distributed along the full final section of the engine nozzle or fins distributed along a sector of the final section of the engine nozzle, preferably, in both cases, in an equally spaced distribution. Therefore some fin distribution choices are provided for a better accommodation to the flow evacuation needs of each engine. 
         [0012]    In embodiments of the present invention said fins are flat plates, curved plates or airfoil-shaped bodies. Therefore some fin configuration choices are provided for a better accommodation to the flow evacuation needs of each engine. 
         [0013]    In embodiments of the present invention, said flat plates have a rectangular or a trapezoidal shape, being the inclined side its leading edge with respect to the engine exhaust flow. These specific shapes are suitable shapes for a great number of aircraft engines. 
         [0014]    In embodiments of the present invention the relevant dimensions of said flat pates for obtaining good results are the following:
       the height H of said fins is comprised between D/20 and D/10, being D the diameter of the engine nozzle in said final section;   the length L of said fins is comprised between 2 H and 4 H;   the distance S of said fins to the engine nozzle outlet is comprised between H and 3 H.       
 
         [0018]    In another aspect the above mentioned objects are met by an aircraft engine comprising a flow evacuation system having the abovementioned features, being said engine a turboshaft engine or a turboprop engine. 
         [0019]    Other features and advantages of the present invention will be understood from the following detailed description in relation with the enclosed drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIGS. 1 and 2  show schematically the flow evacuation system of an aircraft engine. 
           [0021]      FIG. 3  shows schematically a flow evacuation system where the engine exhaust flow has a significant swirl at the exit of an engine straight nozzle. 
           [0022]      FIG. 4  shows schematically a flow evacuation system where the engine exhaust flow has a significant swirl at the exit of an engine curved nozzle. 
           [0023]      FIG. 5  shows the total pressure distribution of an engine exhaust flow in the engine nozzle final section. 
           [0024]      FIG. 6  shows schematically a case of failure of a flow evacuation system due to an excessive swirl of the engine exhaust flow in the engine nozzle. 
           [0025]      FIG. 7  shows schematically a flow evacuation system with local flow conditioners according to the present invention. 
           [0026]      FIG. 8  shows schematically the operation of a flow evacuation system with local flow conditioners according to the present invention in a typical case of engine exhaust flow with low swirl. 
           [0027]      FIG. 9  shows schematically the operation of a flow evacuation system with local flow conditioners according to the present invention in a typical case of engine exhaust flow with high swirl. 
           [0028]      FIG. 10  is a transversal section of a nozzle with local flow conditioners distributed radially. 
           [0029]      FIGS. 11 and 12  are transversal sections of a nozzle with local flow conditioners distributed, respectively, with a predetermined angular deviation and with a variable angular deviation with respect to the radial orientation of  FIG. 10 . 
           [0030]      FIG. 13  is a transversal section of the final section of a nozzle with local flow conditioners distributed along a sector of the nozzle. 
           [0031]      FIGS. 14 ,  15  and  16  show schematically local flow conditioners configured respectively as a flat plate, as a curved plate and as an airfoil-shaped body. 
           [0032]      FIGS. 17 and 18  are partial schematic lateral views of a nozzle with two embodiments of local flow conditioners configured as flat plates according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    For a better understanding of the present invention, a definition of several terms or expressions used in this application follows:
       Design condition: Operating condition in which the engine exhaust flow presents a low swirl angle, i.e. a swirl angle sufficiently small in the exit of the engine nozzle so that it does not significantly deteriorate the engine performance in terms of fuel consumption, residual thrust or ventilation. It is typically comprised between −15° and +15°.   Eductor. Duct where the mixture between the engine exhaust flow and the ventilation flow takes place. A part of the energy of the engine exhaust flow is transmitted to the ventilation flow allowing a better ventilation of the engine nacelle.   Swirl (α): Measurement in degrees (°) of the angular rate of rotation of the engine exhaust flow in the exit cross section of the engine nozzle. α=atan (V t /V axial ).   Turboprop: Type of turboengine which has a propeller moved by a power turbine.   Turboshaft: Type of turboengine which has a shaft moved by a power turbine.       
 
         [0039]    An aircraft engine, housed in a nacelle, requires a cooling and ventilation system to reduce the high temperatures generated within the engine bay under acceptable limits. 
         [0040]    One of the procedures used nowadays to ensure ventilation in aircraft engines is the use of an eductor that mixes (see  FIG. 1 ) the engine exhaust flow  11  with the ventilation flow  13 . The cooling air is supplied through inlets  15  in the nacelle  17  into the engine ventilation bay  19  around the engine  21 , and the engine exhaust flow  11  conveyed by the engine nozzle  31  induces the ventilation flow  13  from the engine bay  19  and mixes it with the engine exhaust flow  11  in the eductor  33 . 
         [0041]    The operation of this flow evacuation system is based on the fact that the higher energy or primary flow (the engine exhaust flow  11 ) suctions the lower energy or secondary flow (the ventilation flow  13 ) to help ventilation through the engine bay  19 . 
         [0042]    To ensure an effective suction of said ventilation flow  13 , the parameters which define the engine exhaust flow  11  must lie between certain limits. 
         [0043]    An ideal condition for the engine exhaust flow  11  along the engine nozzle  31  is, as shown in  FIG. 2 , zero swirl. The velocity of the engine exhaust flow  11  in the exit of the engine nozzle  31  has then only an axial component V axial . 
         [0044]    However the engine exhaust flow  11  has usually a certain swirl as shown in  FIGS. 3 and 4  for straight and curved engine nozzles  31 . Therefore the velocity of the engine exhaust flow  11  in the exit of the engine nozzle  31  has an axial component V axial  and a tangential component V t , which involves, for a high swirl, the flow pressure distribution shown in  FIG. 5  with high total pressure areas  38  near the engine nozzle wall. 
         [0045]    In particular, an excessive swirl of the engine exhaust flow  11  at the engine nozzle exit can prevent the correct suction of the ventilation flow  13  by the engine exhaust flow  11 , blocking the nacelle ventilation flow  13  and possibly causing the reingestion of exhaust engine gases  11  towards the ventilation bay of the aircraft engine as shown in  FIG. 6 . 
         [0046]    A design objective of an aircraft engine is the avoidance of said swirl in those engine operating conditions which imply greater fuel consumption. Typical aircraft engine configuration restrictions (for example a propeller/power turbine group rotating at a constant speed) can lead to significant swirl values lying outside design conditions, especially when the design conditions are cruise points or high performance points in terms of power. When the swirl of the engine exhaust flow  11  through the engine nozzle  31  increases, the performance of the engine decreases in terms of residual thrust, ventilation capacity and/or increase in fuel consumption. 
         [0047]    It is therefore desirable to mitigate the effects of an engine exhaust flow  11  with a high swirl over the ventilation flow  13 , permitting the correct ventilation of the nacelle with a minimum cost in terms of consumption and residual thrust. 
         [0048]    According to this invention, said mitigation is achieved as shown in  FIG. 7  by means of a plurality of local flow conditioners  41  installed in the wetted surface of the engine nozzle  31  in its final section  32  to condition the engine exhaust flow  11  leaving the engine nozzle  31  in the area  35  in which the ventilation flow  13  is discharged into the eductor  33 . 
         [0049]    In embodiments of the present invention said local flow conditioners are, as shown in  FIGS. 8-10  a plurality of small fins  41  fixed to the wetted surface of the exhaust nozzle  31 , oriented according to the pattern of an engine exhaust flow  11  without swirl, i.e. a radial orientation. 
         [0050]    In design conditions as shown in  FIG. 8 , said small fins  41  do not interfere with the engine exhaust flow  11  so energy losses in said flow are minimized. 
         [0051]    In operating conditions out of design conditions, as shown in  FIG. 12 , the effect of said fins  41  is the local conditioning of the engine exhaust flow  11  leaving the engine nozzle  31  in the area  35  enhancing the ventilation flow  13  due to:
       The local vortices  27  induced by the viscous effects of the engine swirling exhaust flow  11  over the wet walls of the fins  41  help the mixture between the engine exhaust flow  11  and the ventilation flow  13  and produces a further depression to improve the suction effects over this last flow.   The local reduction of swirl due to the effect of solidity in the space between fins  41 .       
 
         [0054]    In embodiments of the invention said small fins  41  have an orientation deviated with respect to the radial orientation of  FIG. 10 , whether a predetermined angular deviation as shown in  FIG. 11  or a variable angular deviation as shown in  FIG. 12 , for a better accommodation to the operating conditions expected for each engine. 
         [0055]    Said fins  41  are normally distributed along the full final section of the nozzle  31  as shown in  FIGS. 10-12 , preferably in an equally spaced distribution, but they can be distributed only in one sector of the nozzle  31 , as shown in  FIG. 13 , if the ventilation needs in the area not covered by said fins  41  do not require a mitigation of the effects of an engine exhaust flow  11  with a high swirl. 
         [0056]    In embodiments of the invention said fins  41  are configured as flat plates as shown in  FIG. 14 , as curved plates as shown in  FIG. 15  or as airfoil-shaped bodies as shown in  FIG. 16 . A suitable election of the fins shape for a particular engine may achieve an optimization of the evacuation of the engine exhaust flow  11  and/or of the ventilation of the engine nacelle. 
         [0057]    In a preferred embodiment said fins  41  are flat plates of a rectangular or a trapezoidal shape (see  FIGS. 17 and 18 ) with a height H comprised between D/20 and D/10, being D the diameter of the engine nozzle  31  in said final section  32 , and a length L comprised between 2 H and 4 H, that are placed a distance S to the engine nozzle outlet comprised between H and 3 H. 
         [0058]    An advantage of the present invention is that it provides passive means that enable a significant reduction in the swirl of the engine exhaust flow in operating conditions outside the design conditions, thus increasing the flow evacuation system capacity to boost the ventilation flow. 
         [0059]    Another advantage of the present invention is that said local flow conditioners also contribute to an efficient mixing of the engine exhaust flow and the ventilation flow due to the local vorticity and local suction effects produced in the engine exhaust flow by said local flow conditioners. 
         [0060]    Further advantages of the present invention are:
       The small height H of said local flow conditioners relative to the nozzle diameter D, thereby exposing a small surface to the engine exhaust flow, results in very low pressure losses, compared to other known systems using fins as flow straighteners.   The positioning of said local flow conditioners inside the engine nozzle prevents pressure losses on the ventilation flow.   Location of said local flow conditioners in the engine nozzle reduces its structural impact.   The weight increase due to said local flow conditioners is very low given its small dimensions relative to other elements of the engine.       
 
         [0065]    Although the present invention has been fully described in connection with preferred embodiments, it is evident that modifications may be introduced within the scope thereof, not considering this as limited by these embodiments, but by the contents of the following claims.