Abstract:
A method for maintaining efficiency and duty of a turbine engine compressor to reduce specific consumption, while guaranteeing a high enough pumping margin with partial load. The method adapts airflow to a variable demand for flow or mechanical or electric power in a centrifugal compressor of a turbine engine. The method includes diffusing the airflow through a first annular blade ring having variable pitch blades, radially bordered by a second annular blade ring having a same number of fixed pitch blades with an equivalent extension, guiding the radial extension diffusion by coupling the blades of the two blade rings. According to the method, each blade of the first blade ring is spun off-axis.

Description:
[0001]    The invention relates to an adaptation method for the air flow rate in a turbine engine, comprising a centrifugal compressor, in particular of a turbine engine of helicopters or auxiliary power units (in short APU) to a variable demand for flow rate or mechanical or electrical power. The invention also relates to a diffuser equipped with variable pitch blades for implementing such method. 
         [0002]    The scope of the invention is gas compression in turbine engines and more particularly the adaptation of the compressed air flow to take the engine performances into account, whatever these are turbine engines or APUs, in particular the specific consumption (in short Cs) thereof under partial load. 
         [0003]    With this end in view, a general problem is to meet the needs of the pumping margin and to obviate the compression rate reductions at the intermediate speeds of the turbine engines, as well as the demand variations in terms of compressed air flow rate and electrical power in the case of the APUs. 
         [0004]    It is known that a sufficient pumping margin can be obtained by reducing the operation line of the turbine engines. However, a reduction of the motive cycle rate leads to a degradation of the yield and such solution needs then to operate the compressor under the maximum yield thereof, including at high speed. 
         [0005]    It is also known to introduce, at the inlet of the compressor, a pre-rotation ring formed with inlet guide vanes (in short IGVs). But, in this case, the compression rate is substantially reduced for a given rotation speed. 
         [0006]    It is advisable in such conditions to try and operate a compressor with a quasi constant compressing rate while staying near the maximum yield, whatever the load variation. 
         [0007]    In the field of the mono-stage compressors, there exist radial diffusers with variable pitch blade assemblies. Such diffusers are described for example in the patent documents U.S. Pat. No. 5,207,559 or EP 0 589 745, this last one being filed on the name of the Applicant. These diffusers enable to deviate towards smaller flow rates the characteristics of the flow rate/pressure ratios of the compressor in an intermediate rating without significantly degrading either the compression rate or the yield. 
         [0008]    The variable pitch is obtained with appropriate controls in relationship with a control unit depending on the physical parameters at issue (rotation speed, pressures, temperatures). However, the pitch angle ranges that the control system must cover need a control jack of high power, lead to important variations of the inlet and outlet diameters of the diffuser, which can generate high mechanical biases between rotating parts (sheave) and static parts (radial variable pitch diffuser) and reduces yield under partial load (intermediate rating). 
       DESCRIPTION OF THE INVENTION 
       [0009]    The invention aims at obviating such drawbacks, in particular by maintaining the compressor yield to substantially reduce the Cs while providing a sufficient pumping margin with a better yield of the motive cycle under partial load. To do so, it proposes an optimized method for a variable diffusion of the air flow in a centrifugal compressor of turbine engines. 
         [0010]    More precisely, the invention has for an object a method for a variable diffusion of the air flow in a centrifugal compressor of turbine engines, consisting in providing a diffusion of the air through a first annular blade ring with variable pitch blades, radially bordered with a second annular blade ring having the same number of fixed pitch blades with an equivalent extension, guiding the diffusion in the radial direction by coupling the blades of the two blade rings, each blade of the first blade ring being driven into rotation at distance of the blade. The expression “turbine engines” means the turbine engines, in particular those of helicopters with a centrifugal mono- or bi-staged compressor, and the APUs equipped with a centrifugal mono- or bi-staged centrifugal compressor. 
         [0011]    In these conditions, on the one side, the radial extension of the variable pitch blades is substantially reduced by the presence of the fixed blade ring comprising real blades, thereby allowing to limit the stresses to vary their pitch as well as the clearances between the fixed blade ring and the support flange and, thus, the upstream/downstream re-circulations, thereby having as an effect to reduce the pumping line deteriorations and the load losses. On the other side, the off-axis implantation of the rotation axis of the variable pitch blades substantially reduces the radial extension variations for such iso-diffusion blades: the closing increase is lesser, thereby favouring the yield under partial load and the opening decrease also lesser, thereby limiting the mechanical biases due to the non stationary aerodynamic fluctuations by a sheave/diffuser interaction. 
         [0012]    A sufficient pumping margin then allows the turbine engine to operate with any appearance of pumping—providing a great acceleration capacity—, and the APUs to face important load variations without calling on a discharge valve, while maintaining the rotation speed of the turbine engine and the pressure rate thereof at levels close to their nominal values and providing a sufficient yield level. 
         [0013]    According to particular modes, the method applying to turbine engines equipped with a power turbine, the variable pitch radial diffusion on a centrifugal compressor, such as defined above, is coupled with a variable pitch power turbine distributor. The power production can be implemented according to several configurations: free or bound power turbine, of the axial or centripetal type, with or without downstream heat exchange. 
         [0014]    The coupling between the diffuser and the variable pitch distributor allows the operation line to be adapted to the flow rate reduction, thereby improving the motive cycle yield (through a better pressure rate) and thus the Cs of the turbine engines of helicopters and APUs. 
         [0015]    The invention also aims at providing a variable pitch turbine engine diffuser being able to implement the above mentioned method, as well as the turbine engine equipped with such a diffuser. The diffuser comprises a first annular blade ring with variable pitch blades being radially bordered by a second annular blade ring with fixed pitch blades of an equivalent extension. Moreover, each blade of the first blade ring is driven by control means adapted to exert a proper rotation of each blade being off-centered with respect to the rotation axis thereof. 
         [0016]    According to particular embodiments: 
         [0017]    each variable pitch blade extends between two facing cups and in a parallel and off-axis way with respect to the common axis of the cups coinciding with the rotation axis; 
         [0018]    each blade is coupled with a driving rod that presents at least one orifice in which a lock pin is introduced for an adjustment washer for the axial position of the cups; 
         [0019]    the rod is integral with a lever presenting a ball joint coupling housed within a cylindrical housing ( 38 ) of a control crown adapted to drive into rotation around the motive axis the lever being adapted to slide in the cylindrical housing; 
         [0020]    the cylindrical housings present a depth being a function of the stroke of the levers, itself a function of the predetermined rotation interval of the blades; 
         [0021]    the leading edge of each variable pitch blade is close to the peripheries of the cups, the distance from the blade to the rotation axis being higher than or equal to a mid-radius; 
         [0022]    the diffuser upstream is a smooth diffuser, i.e. with no vane; 
         [0023]    the inlet air stream of the diffuser located between the sheave and the variable pitch blade ring is convergent, thereby improving the performances; 
         [0024]    the fixed blades of the second blade ring present a thicker leading edge profile than the ones of the first blade ring so as to absorb the incidence variations; 
         [0025]    the fixed pitch blades present a sufficient thickness to be crossed by screws allowing the passage of structural stresses; 
         [0026]    the fixed blades present a skeleton angle evolution law between the leading and trailing edges, thereby allowing the diffusion in the fixed blade ring to be controlled and the aerodynamic efficiency to be optimized; 
         [0027]    the fixed blades are keyed in azimuth with respect to the blades of the first mobile blade ring so as to take the wake again on the suction side of the blades in the first blade ring so as to limit the load losses of the diffuser; 
         [0028]    the pitch angles of the variable blades are comprised between +12° and −5° with respect to the nominal keying, which could be the one of a fixed diffuser. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0029]    Other characteristics and advantages of the invention will be more evident on reading the following description referring to the accompanying figures, in which, respectively: 
           [0030]      FIG. 1  represents half a view in an axial partial section of a diffuser according to the invention; 
           [0031]      FIGS. 2   a  and  2   b  represents two views in perspective of a variable pitch blade coupled to the control rod thereof in rotation; 
           [0032]      FIG. 3  represents a front global view of the upstream annular flange of the diffuser equipped with blade rings according to the invention; 
           [0033]      FIGS. 4   a  to  4   c  represent a partial schematic view in the diffuser for three mobile blade pitches, the two extreme pitches around the nominal pitch, and 
           [0034]      FIG. 5  represents the clearance between a mobile blade and the annular flanges of the diffuser. 
       
    
    
     DETAILED DESCRIPTION ON ONE EXEMPLARY EMBODIMENT 
       [0035]    The terms “upstream” and “downstream” relate to the air flow direction in a turbine engine. 
         [0036]    Referring to the axial view in a partial section of  FIG. 1 , the centrifugal compressor  10  of a turbine engine, such as a turbomachine, a turbine reactor jet, a turbine propeller or an APU, comprises a casing  12  coupled with a cover  14  for radially covering the sheave  16 , the last centrifugal stage of the compressor, rotationally mounted on the motive shaft  18  according to the axis Y′Y. The airflow F circulates from the sheave  16  toward the annular diffuser  19  in an inlet stream converging by radial a narrowing. The diffuser  19  is defined between two upstream and downstream flanges  20  and  22 . The cover  14  is maintained by an attachment  23  fastened to the casing and to the upstream flange  20 . 
         [0037]    The blades  24  forming a first annular blade ring are mounted in the diffuser  19 . Centerings  25  and  26  are facing the flanges  20  and  22  accommodating the cups  17  and  27  on which the blades  24  are mounted on an off-axis way. The cups are centered in the flanges  20  and  22  with adapted clearances, from 0.03 to 0.05 mm in the illustrated examples on a washer  9  inserted into the centering  25  (see below referring to  FIG. 5 ). 
         [0038]    Blades  28  being integral with the flange  22 , forming a second annular blade ring externally bordering the first blade ring, are mounted on the annular flange  20  by screws  29  housed in through-holes  29   t.  These screws also admit the passage of structural stresses. 
         [0039]    The control of the variable blades  24  is made through rods  30  extending integrally the upstream cup  17 . These rods  30  with an axis X′X are mounted in a cylindrical boring  32  of the upstream flange  20  and centered with a quasi null clearance by joints  30   j  mounted in grooves  30   g.  At the end, each rod  30  presents a flat part  31  jointed on a driving lever  33  pinched by two screws  35  on such flat part  31 . The positions of the ends  31  of the rods  30  are adjusted with adapted clearance tolerances. The rod  30  also presents an orifice  30   t  into which a pin  36  is inserted so as to lock a washer  30   u —for adjusting the axial position of the cups  17  and  27 —in an locking ring  12   a  being formed in the casing  12 . For this purpose, the pin  36  makes the rod  30  and the locking ring  12   a  integral. 
         [0040]    In operation, the lever  33  is driven by a control crown  34  forming a cylindrical hole  38  for housing the ball joint coupling  37  of the lever  33  with an adapted axial position tolerance and a contact on a generatrix of the ball joint. To do so, the control crown  34  is centered on sectors presenting needle roller bearings  39 . The control crown  34  put into rotation around the motive axis Y′Y by a rocker bar (not shown) drives into rotation the levers  33  sliding in the cylindrical housings  38  thanks to their ball joint  37 . The depth of the housings  38  depends on the stroke of the lever  33 , itself being a function of the rotation interval of the blades  24 . Such architecture is particularly adapted for a blade rotation being able to go up to +12° with a section closure of 50% section and up to −5° with a section opening of 20%. The position angles of the rods and thus of the blades  24  are a function of the power rates to provide the adequate air compression to these ratings. 
         [0041]    Referring to  FIGS. 2   a  and  2   b , a mobile blade  24  is represented between the parallel cups  17 ,  27  and integral with the latter by welding  21 , so that the blade extends parallel to the axis X′X of the facing cups. The leading edge  24   c  of the blade  24  is flush with the external circumferences  17   c  and  27   c  of the cups, the thickness of the blade  24  being quite fine, about 2 mm in the illustrated example. Moreover, the distance between the blade  24  and the axis X′X of the rod  30  is equal to about 80% of the radius of the cups in the illustrated example. This confers to the blade  24  a strong off-centering with respect to the axis X′X of the rod coinciding with the rotation axis of the assembly. The rod  30  also presents the cylindrical centering grooves  30   g  and the locking orifice  30   t  for the adjustment washer of the axial position of the cups  17  and  27 . The flat part  31  thereof is crossed by reception holes  30   a  for the screws  35  used for mounting the control lever. 
         [0042]    The global view of  FIG. 3  illustrates the upstream annular flange  20  equipped with annular blade rings G 1  and G 2  being respectively mounted mobile and fixed and formed with the blades  24  and  28 . 
         [0043]    The blades  28  present a substantially thicker profile in the leading edge BA than the one of the blades  24 , respectively 0.5 and 2.5 mm, so as to keep a good behavior to the incidence variation upon the rotation of the mobile blades  24 . In addition, the skeleton angle law of the blades  28  between the leading and trailing edges BA and BF is evolutive, thereby allowing the aerodynamic efficiency of the fixed blade ring to be optimized by a maximum recovery of the static pressure. 
         [0044]    Moreover, the blades  28  of the fixed blade ring present a maximum thickness of 7 mm in the illustrated example, allowing the flange  20  of the diffuser to be fastened by screws housed in the holes  29   t,  while admitting the passage of structural stresses. 
         [0045]    The airflow F circulates along a fixed blade  28  in a radial extension of a mobile blade  24  and between two adjacent blades of the same nature, either mobile or fixed. Thanks to the off-centering of the mobile blades  24  with respect to the rotation axes X′X of their cups  17 , the variations of the radial extensions formed by such mobile blades  24  are limited relative to the extension variation that the centered blade should implement. This limitation enables to improve the performances of a centrifugal compressor: it allows the operation lines of the pumping line to be more distant by an offset towards lower flow rates, and this line of operation to be increased near the yield maximums at higher ratings. 
         [0046]    The radial extensions of the mobile blades  24  facing the fixed blades  28  are illustrated by the schemas of  FIGS. 4   a  to  4   c , on which also appear, in dot lines, the cups  17 ,  27  of the blades. Referring to  FIG. 4   b , the nominal keying of 0° corresponds to a reference airflow stream F for which the adjustment of the mobile blades  24  with respect to the fixed blades  28  is adapted to the stable intermediate ratings. 
         [0047]    At the small load demands, the keying of the mobile blades  24  may rise up to 12°, this keying corresponding to a passage section at the inlet of the collar Sa, between the blades  24  and  38 , closed at 50% with respect to the nominal keying corresponding to a section at the collar Sb.  FIG. 4   a  illustrates the case of a closure of 25% associated with a keying of 6°, the collar section being then 75% of the section b. At the high load demands, the keying adjustment may also go down to −5°.  FIG. 4   c  illustrates the case of an opening of 2.5°, the collar section Se presenting then a relative value of 110%. 
         [0048]    The fixed blades  28  are keyed in azimuth with respect to the blades  24  of the first mobile blade ring G 1  so as to take again the wake on the suction side Ex of the blades of this first blade ring G 1 . 
         [0049]    The radial extensions of the blades  24  limited by the presence of the fixed blades  28  allow a control to be kept on the clearances between the cups  17  and  27  of the blades  24  and the flanges  20  and  22 , as illustrated by  FIG. 5 . Thus, in this example, the clearance values stay lower than or equal respectively to 0.02 mm (for J 1  or J 2 ), 0.10 mm (for J 3 ) and 0.25 mm (for J 4 ). The clearance (J 1  and J 2  assembly) of the blade  24  on the washer  9  thus stays of about 0.03 mm of slightly more. 
         [0050]    The invention is not limited to the example being described and represented. It is for example possible to perform the keying of the mobile blades only by a mechanical, individual or centralized, adjustment or by an electrical or electronic control with or without digital regulation.