WO2011128587A1 - Method for adapting the air flow of a turbine engine having a centrifugal compressor and diffuser for implementing same - Google Patents

Abstract

The invention relates to a method for maintaining the efficiency and duty of a turbine engine compressor in order substantially to reduce the specific consumption Cs, while guaranteeing a high enough pumping margin with partial load. For this purpose, the invention proposes an optimised method for adapting the 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 (F) through a first annular blade ring (G1) having variable-pitch blades (24), radially bordered by a second annular blade ring (G2) having the same number of fixed-pitch blades (28) with an equivalent extension, guiding the radial extension diffusion by coupling the blades (24, 28) of the two blade rings. According to said method, each blade (24) of the first blade ring (G1) is spun off-axis.

Description

 METHOD FOR ADJUSTING TURBOMACHINE AIR FLOW WITH CENTRIFUGAL COMPRESSOR AND DIFFUSER FOR IMPLEMENTING THE SAME

The invention relates to a method of adapting the air flow rate of a turbomachine comprising a centrifugal compressor, in particular helicopter turbine engines or auxiliary power units (abbreviated as APU) to a request variable flow or mechanical or electrical power. The invention also relates to a diffuser equipped with variable pitch blades capable of implementing this method. The field of the invention is the compression of gases in turbomachinery engines and, more particularly, the adaptation of the compressed air flow to respect the performance of the engines, whether turboshaft engines or APUs, particular its specific consumption (abbreviated Cs) at partial load. In this context, a general problem is to meet the needs of pumping margin and to offset compression rate reductions at the intermediate speeds of the turboshaft engines, as well as variations in demand for compressed air flow and power. in the case of APUs.

It is known that a sufficient pumping margin can be obtained by lowering the operating line of the turboshaft engines. However, a lowering of the engine cycle rate leads to a degradation of the efficiency and this solution then requires operating the compressor below its maximum efficiency, especially at high speed.

It is also known to introduce, at the compressor inlet, a pre-rotation grid, formed of inlet guide vanes (abbreviated IGV, initials of "Inlet Guide Vanes" in English). . But in this case, the compression ratio is substantially lowered for a given rotational speed. It is appropriate in these conditions to seek to operate a compressor with a nearly constant compression ratio while remaining close to its maximum efficiency, regardless of the variation of the load.

In the field of single-stage compressors, there are radial diffusers having blade assemblies variable pitch. Such diffusers are described for example in patent documents US 5 207 559 or EP 0 589 745, the latter filed in the name of the applicant. These diffusers make it possible to shift towards lower flow rates the characteristics of the flow / pressure rates of the compressor in intermediate mode, without significantly degrading the compression ratio or efficiency.

The variable setting is achieved by appropriate commands in connection with a control unit according to the physical parameters involved (rotational speed, pressures, temperatures). However, the ranges of wedges that the control system must cover require a control cylinder of high power, cause significant variations in the inlet and outlet diameters of the diffuser, which can generate high mechanical stresses between rotating parts (wheel) and static (radial variable valve) and decreases the efficiency at partial load (intermediate regime). SUMMARY OF THE INVENTION

The invention aims to overcome these disadvantages, in particular by maintaining the efficiency of the compressor to substantially reduce the Cs while ensuring a sufficient pumping margin with a better efficiency of the engine cycle at partial load. To do this, it proposes an optimized method of variable diffusion of the air flow in a centrifugal compressor of turbomachines.

More specifically, the subject of the invention is a method for diffusing variable airflow in a centrifugal compressor of turbomachine engines, consisting in providing a diffusion of air through a first annular grid of paddle blades. variable radially bordered by a second annular grid of same number of fixed-pitch blades of equivalent extension, directing the diffusion in the radial direction by coupling the blades of the two blades, each blade of the first blade being rotated away from the blade. Turbomachines should be understood as turbine engines, in particular helicopter turboshaft engines with a single-stage or two-stage centrifugal compressor, and APUs equipped with a centrifugal compressor of single or two-stage power.

Under these conditions, on the one hand, the radial extension of the variable pitch blades is substantially reduced by the presence of the fixed blade comprising true blades, which limits the efforts to vary their rigging and the clearance between the mobile blade and the support flange and thus the upstream / downstream recirculation, which has the effect of reducing deterioration of the pumping line and pressure losses. On the other hand, the decentralized implantation of the axis of rotation of the variable-pitch blades substantially reduces the variations in radial extension of these iso-diffusion blades: the increase in closing is less, thus favoring the efficiency, partial load and decreasing opening less also, which limits the mechanical stresses due to unsteady aerodynamic fluctuations by wheel / diffuser interaction.

A sufficient pumping margin then allows the turbomachine to operate without pumping appearance - offering a large acceleration capacity - and the APU to cope with significant load variations without resorting to a valve while maintaining the speed of rotation of the turbomachine and its pressure ratio to levels close to their nominal values and providing a sufficient level of efficiency. According to particular embodiments, the method applying to turbomachines equipped with a power turbine, the variable-pitch radial diffusion on a centrifugal compressor, as defined above, is coupled to a wedge power turbine distributor. variable. Power production can be carried out according to several configurations: free power turbine - or linked, of axial or centripetal type, with or without downstream heat exchange. The coupling between the diffuser and the variable valve distributor makes it possible to adapt the operating line to the flow reduction, which improves the efficiency of the engine cycle (by a better pressure ratio) and therefore the Cs of the turboshaft engines. helicopters and APUs. The invention also relates to a turbomachine variable valve diffuser capable of implementing the method defined above, and the turbomachine equipped with such a diffuser. The diffuser comprises a first annular grid of variable pitch blades radially bordered by a second annular grid of fixed pitch blades of equivalent extension, forming successive diffusion channels by coupling the blades of the two grids in radial extension. In addition, each blade of the first gate is driven by control means capable of exerting a proper rotation of each blade off-center with respect to its axis of rotation.

According to particular embodiments: - each variable-pitch blade extends between two cups opposite and parallel and off-center with respect to the common axis of the cups coinciding with the axis of rotation;

each blade is coupled to a drive rod which has at least one orifice in which is introduced a locking pin of a washer for adjusting the axial position of the cups;

- The rod is secured to a lever having a spherical ball received in a cylindrical housing (38) of a control ring adapted to drive in rotation about the motor axis the lever adapted to slide in the cylindrical housing; - The cylindrical housing has a depth which is a function of the stroke of the levers, itself a function of the predetermined rotation interval of the blades; the leading edge of each variable-pitch blade is close to the peripheries of the cups, the distance from the blade to the axis of rotation being greater than or equal to half a radius;

- The diffuser upstream is a smooth diffuser, that is to say, not aubé; the inlet air duct of the diffuser located between the impeller and the variable-pitch grille is convergent, which improves the performances;

the fixed blades of the second grid have a thicker leading edge profile than those of the first grid in order to absorb the variations of incidence; the blades with fixed wedging have a thickness sufficient to be traversed by screws allowing the passage of structural forces;

the fixed blades have an evolutionary law of skeleton angle between the leading and trailing edges, which makes it possible to control the diffusion in the fixed grid and to optimize its aerodynamic efficiency; the fixed blades are wedged in azimuth with respect to the blades of the first movable grid so as to resume the wake on the extrados of the blades of this first gate to limit the pressure drops of the diffuser;

- The pitch angles of the variable blades are between +12 and - 5 ° compared to the nominal setting, which would be that of a fixed diffuser.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will emerge on reading the description which follows, with reference to the appended figures which represent, respectively: - Figure 1, a half-view in partial axial section of a diffuser according to the invention; - Figures 2a and 2b, two perspective views of a variable-pitch blade coupled to its control rod in rotation;

FIG. 3, an overall frontal view of the upstream annular flange of the diffuser equipped with the blade grids according to the invention, FIGS. 4a to 4c, a partial schematic view in the diffuser for three wedges of movable blades, the two extreme shims. around the nominal setting, and

- Figure 5, the games between a movable blade and the annular flanges of the diffuser. DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

 The terms "upstream" and "downstream" refer to the direction of flow of the air flow in a turbine engine.

With reference to the axial view in partial section of FIG. 1, the centrifugal compressor 10 of a turbomachine, such as a turbine engine, turbojet engine, turboprop engine or an APU, comprises a casing 12 coupled to a covering cover 14. radial wheel 16, last centrifugal stage of the compressor, rotatably mounted on the drive shaft 18 along the axis ΥΎ. The flow of air F flows from the impeller 16 to the annular diffuser 19, in a converging inlet vein by radial shrinkage. The diffuser 19 is defined between two upstream and downstream flanges 20 and 22. The cover 14 is held by a fastener 23 fixed to the casing and to the upstream flange 20.

The blades 24, forming a first annular grid, are mounted in the diffuser 19. Centerings 25 and 26, formed opposite in the flanges 20 and 22, accommodate the cups 17 and 27 on which the blades 24 are mounted. off-center way. The cups are centered in the flanges 20 and 22 with appropriate clearances, from 0.03 to 0.05 mm in the example illustrated, on a washer 9 inserted in the centering 25 (see below with reference to FIG. ). Blades 28 secured to the flange 22, forming a second annular grid externally bordering the first gate, are mounted on the annular flange 20 by through screws 29 housed in holes 29t. These screws also allow the passage of structural forces. The control of the variable blades 24 is performed by means of rods 30 integrally extending the upstream cup 17. These rods 30 X'X axis are mounted in a cylindrical bore 32 of the upstream flange 20 and centered with a set almost zero by 30j joints mounted in grooves 30g. At the end, each rod 30 has a flat portion 31 articulated on a drive lever 33 clamped by two screws 35 on this flat portion 31. The positions of the ends 31 of the rods 30 are adjusted with appropriate play tolerances. The rod 30 also has a hole 30t in which a pin 36 is inserted which makes it possible to lock a washer 30u - for adjusting the axial position of the cups 17 and 27 - in a locking ring 12a formed in the casing 12. For this purpose , the pin 36 secures the rod 30 and the locking ring 12a.

In operation, the lever 33 is driven by a control ring 34 forming a cylindrical hole 38 for housing the spherical ball 37 of the lever 33 with an adapted axial position tolerance and a contact on a generatrix of the ball joint. To do this, the control ring 34 is centered on sectors having needle bearings 39. The control ring 34, rotated about the motor axis ΥΎ by a rod (not shown), rotates the levers 33 which slide in the cylindrical housing 38 through their ball joint 37. The depth of the housing 38 is a function of the stroke of the levers 33, itself a function of the rotation interval of the blades 24. This architecture is particularly suitable for rotation blades up to + 12 ° with a 50% section closure, and up to -5 ° with a 20% section opening. The position angles of the rods and thus of the blades 24 as a function of the power regimes to provide the appropriate air compression to these regimes. Referring to Figures 2a and 2b, a movable blade 24 is shown between the parallel cups 17, 27 and secured by welding 21 to them, so that the blade extends parallel to the axis X'X cups opposite. The leading edge 24c of the blade 24 is flush with the outer circumferences 17c and 27c of the cups, the thickness of the blade 24 being relatively thin, 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 on the blade 24 a strong off-centering with respect to the axis X'X of the rod which coincides with the axis of rotation of the assembly. The rod 30 also has the cylindrical centering grooves 30g and the locking hole 30t of the adjusting washer of the axial position of the cups 17 and 27. Its flat portion 31 is traversed by holes 30a for receiving the mounting screws 35. to the control lever.

The overall view of Figure 3 illustrates the upstream annular flange 20 equipped with annular gates G1 and G2, mounted respectively mobile and fixed and composed of blades 24 and 28.

The blades 28 have a substantially thicker profile in the leading edge Ba than that of the blades 24, respectively 0.5 and 2.5 mm, to preserve a good resistance to variations in incidence during the rotation of the movable blades 24. In addition, the skeleton angle law of the blades 28 between the leading edges BA and LF leakage is scalable, to optimize the aerodynamic efficiency of the fixed gate by a maximum recovery of static pressure.

In addition, the blades 28 of the fixed gate have a maximum thickness of 7 mm in the example shown, for fixing the flange 20 of the diffuser by screws housed in the holes 29t, while allowing the passage structural efforts.

The air flow F flows along a fixed blade 28 in radial extension of a movable blade 24 and between two adjacent blades of the same type, mobile or fixed. Thanks to the off-centering of the mobile blades 24 with respect to the axes of rotation X'X of their cups 17, the variations of the radial extensions formed by these movable blades 24 are limited compared to variations of extensions that should be made of centered blades. This limitation makes it possible to improve the performance of a centrifugal compressor: it makes it possible to move the operating line away from the pumping line, by shifting towards lower flow rates, and to raise this operating line close to the yield maxima. higher diets.

The radial extensions of the movable blades 24 with respect to the fixed blades 28 are illustrated by the diagrams of Figures 4a to 4c, which also appear, in dashed lines, the cups 17, 27 of the blades. With reference to FIG. 4b, the nominal setting of 0 ° corresponds to a flow of the reference air flow F for which the adjustment of the mobile blades 24 with respect to the fixed blades 28 is adapted to stable intermediate speeds.

At low load demands, the wedging of the movable blades 24 can go up to + 12 °, this wedging corresponding to a passage section at the entrance of the neck Sa, between the blades 24 and 28, closed 50 % compared to the nominal setting corresponding to a Sb neck section. Figure 4a illustrates the case of a closure of 25% associated with a setting of 6 °, the neck section then being 75% of the section Sb. At high load demands, the calibration setting can also go down to -5 °. FIG. 4c illustrates the case of an opening of 2.5 °, the section at the neck being then a relative value of 1 10%.

The fixed blades 28 are wedged in azimuth with respect to the blades 24 of the first movable gate G1 so as to resume the wake on the extrados Ex of the blades of this first gate G1. The radial extensions of the blades 24, limited by the presence of the fixed blades 28, allow to maintain control games between the cups 17 and 27 of the blades 24 and the flanges 20 and 22, as shown in Figure 5. Thus in this example, the values of the clearances remain less than or equal to 0.02 mm (for J1 or J2), 0.10 mm (for J3) and 0.25 mm (for J4). The clearance (set J1 and J2) of the blade 24 on the washer 9 remains about 0.03 mm or slightly higher.

The invention is not limited to the examples described and shown. It is for example possible to set the pitch of the mobile blades by only mechanical adjustment, individual or centralized, or by electrical, electronic control with or without digital control.

Claims

A method of adjusting air flow in a centrifugal compressor (10) of a turbomachine, comprising providing a diffusion of air through a first annular blading (G1) of variable pitch blades (24) radially flanked by a second annular blade (G2) of the same number of fixed-pitch blades (28) of equivalent extension, orienting the diffusion in radial extension by coupling of the blades (24, 28) of the two blades, this method being characterized in that each blade (24) of the first vane (G1) is rotated axis (X'X) away from the blade (24). An adaptation method according to claim 1, wherein the variable-pitch radial diffusion on a centrifugal compressor is coupled to a variable-pitch power distribution, the power distribution being free with downstream exchange or connected, axially or centripetally, with or without downstream exchange. Variable valve turbomachine diffuser adapted to implement the method according to claim 1 or 2, comprising a first annular grid (G1) of variable pitch blades (24) radially bordered by a second annular grid (G2) of paddle blades fixed (28) of equivalent extension and the same number of blades, forming successive diffusion channels by coupling the blades (24, 28) of the two grids (G1, G2) in radial extension, characterized in that each blade (24 ) of the first gate (G1) is driven by drive means (30, 33, 34) capable of exerting a self-rotation of blade (24) off-center with respect to its axis of rotation (X'X). Diffuser according to the preceding claim, in which each variable-pitch blade (24) extends between two cups (17, 27) facing each other and parallel and off-center with respect to the common axis of the cups coinciding with the axis of rotation (X'X). Diffuser according to one of claims 3 or 4, wherein each blade (24) is coupled to a drive rod (30) having at least one orifice (30t) into which is introduced a pin (36) for blocking a washer (30u) for adjusting the axial position of the cups (17, 27). Diffuser according to the preceding claim, wherein the rod (30) is secured to a lever (33) having a spherical ball (37) housed in a cylindrical housing (38) of a control ring (34) adapted to drive in rotation around the motor axis (ΥΎ) the lever (33) slidable in the cylindrical housing 38. Diffuser according to the preceding claim, wherein the depth of the cylindrical housings (38) is a function of the stroke of the levers (33), itself a function of the predetermined rotation interval of the blades (24). Diffuser according to any one of claims 4 to 7, wherein the leading edge (24c) of each variable-pitch blade (24) is close to the peripheries (17c, 27c) of the cups (17, 27), the distance from the blade (24) to the axis of rotation (X'X) being greater than or equal to half a radius. Diffuser according to any one of claims 3 to 8, wherein the fixed blades (28) of the second gate (G2) have a leading edge profile (Ba) thicker than that (24c) of the blades (24) of the first grid (G1). Diffuser according to any one of claims 3 to 9, wherein k wedge angles of the movable blades (24) are between +12 and -5 corresponding to a neck section (Sa, Se) respectively of 50% and 120% with respect to the section corresponding to the nominal setting.