BE1024743B1 - LOW AXIAL TURBOMACHINE COMPRESSOR - Google Patents

Abstract

The invention relates to a low-pressure turbomachine compressor, such as a low-pressure turbojet turbojet compressor. The compressor comprises several rows of stator vanes (26) and several rows of rotor vanes. Each stator vane (26) shows a leading edge (36), a trailing edge (38), an extrados surface (40), an intrados surface. Several blades, or all the blades of the same annular row of stator vanes (26) have extrados surfaces (40) receiving one or more suction orifices (44) of flow (18) of the turbomachine. These orifices (44) aspire a boundary layer, and reduce the risk of pumping into the compressor.

Description

(30) Priority data:

(73) Holder (s):

SAFRAN AERO BOOSTERS S.A.

4041, HERSTAL (MILMORT)

Belgium (72) Inventor (s):

HIERNAUX Stéphane 4680 OUPEYE Belgium (54) LOW PRESSURE COMPRESSOR OF AXIAL TURBOMACHINE (57) The invention relates to a low pressure compressor of a turbomachine, such as a low pressure compressor of a turbofan engine. The compressor comprises several rows of stator vanes (26) and several rows of rotor vanes. Each stator vane (26) shows a leading edge (36), a trailing edge (38), an upper surface (40), a lower surface.

Several blades, or all the blades of the same annular row of stator blades (26) have upper surfaces (40) receiving one or more suction orifices (44) for flow (18) of the turbomachine. These orifices (44) suck in a boundary layer, and reduce the risk of pumping in the compressor.

FIG. 3

BELGIAN INVENTION PATENT

FPS Economy, SMEs, Middle Classes & Energy

Publication number: 1024743 Deposit number: BE2016 / 5867

Intellectual Property Office International Classification: F04D 29/68 F04D 29/54 Date of issue: 06/21/2018

The Minister of the Economy,

Having regard to the Paris Convention of March 20, 1883 for the Protection of Industrial Property;

Considering the law of March 28, 1984 on patents for invention, article 22, for patent applications introduced before September 22, 2014;

Given Title 1 “Patents for invention” of Book XI of the Code of Economic Law, article XI.24, for patent applications introduced from September 22, 2014;

Having regard to the Royal Decree of 2 December 1986 relating to the request, the issue and the maintenance in force of invention patents, article 28;

Given the patent application received by the Intellectual Property Office on November 21, 2016.

Whereas for patent applications falling within the scope of Title 1, Book XI of the Code of Economic Law (hereinafter CDE), in accordance with article XI. 19, §4, paragraph 2, of the CDE, if the patent application has been the subject of a search report mentioning a lack of unity of invention within the meaning of the §ler of article XI.19 cited above and in the event that the applicant does not limit or file a divisional application in accordance with the results of the search report, the granted patent will be limited to the claims for which the search report has been drawn up.

Stopped :

First article. - It is issued to

SAFRAN AERO BOOSTERS S.A., Route de Liers 121, 4041 HERSTAL (MILMORT) Belgium;

represented by

LECOMTE Didier, P.O. Box 1623, 1016, LUXEMBOURG;

a Belgian invention patent with a duration of 20 years, subject to the payment of the annual fees referred to in article XI.48, §1 of the Code of Economic Law, for: LOW PRESSURE COMPRESSOR OF AXIAL TURBOMACHINE.

INVENTOR (S):

HIERNAUX Stéphane, Rue des Hirondelles 3, 4680, OUPEYE;

PRIORITY (S):

DIVISION:

divided from the basic application: filing date of the basic application:

Article 2. - This patent is granted without prior examination of the patentability of the invention, without guarantee of the merit of the invention or of the accuracy of the description thereof and at the risk and peril of the applicant (s) ( s).

Brussels, 06/21/2018, By special delegation:

BE2016 / 5867

Description

LOW PRESSURE COMPRESSOR OF AXIAL TURBOMACHINE

Technical area

The invention relates to pumping in a turbomachine. More specifically, the invention relates to an axial turbomachine compressor. The invention also relates to an axial turbomachine, in particular an aircraft turbojet or an aircraft turboprop. The invention also relates to a method of controlling pumping in a turbomachine compressor.

Prior art

During the operation of a turbojet compressor, stalling phenomena can be observed at the blades of the rectifiers. These stalls restrict the compressor capacities and can give rise to occasional pumping. The latter lead in particular to a flow reversal which can damage the compressor in question, in addition to limiting the maximum thrust that the turbojet engine can offer.

Document US2006 / 0051199 A1 discloses a turbomachine with a system for removing part of the flow from the turbomachine. To this end, a wall of the turbomachine in line with a row of vanes has openings for removing fluid between said vanes. But the presence of these openings disrupts the flow as well as their lengths. Also, their efficiency of removal of fluid is limited in certain configurations of veins, in particular on the internal walls of veins having a reduction in diameter.

Summary of the Invention

The object of the invention is to solve at least one of the problems posed by the prior art. More specifically, the invention aims to improve the efficiency of a turbomachine while increasing the pumping margin. The invention also aims to provide a simple solution,

BE2016 / 5867 resistant, light, economical, reliable, easy to produce, convenient to maintain, easy to inspect, and independent of the geometry of the vein.

Technical solution

The invention relates to an axial turbomachine compressor, in particular a low-pressure turbomachine compressor, the compressor comprising at least one annular row of compressor blades, each blade comprising a leading edge, a trailing edge, a surface upper surface, a lower surface, said surfaces extending from the leading edge to the trailing edge; remarkable in that the upper surface of at least one or each blade comprises a flow suction orifice of the turbomachine, in particular of a flow unhooked from the associated blade.

According to advantageous modes of the invention, the compressor can include one or more of the following characteristics, taken in isolation or according to all possible technical combinations:

- The annular row of blades has an upstream half and a downstream half, the suction orifice being arranged in the downstream half of the annular row of blades.

- Each blade includes a stack of aerodynamic profiles, each aerodynamic profile having a chord and surface surface points distant by a distance D from the associated chord, each blade has an extreme point with a maximum distance D which is upstream of the suction port.

- Each blade has a setting angle ß whose inclination relative to the axis of rotation of the compressor is greater than: 20 °, or 30 °.

- The annular row of blades has axially a central section in which the orifice is arranged, said central section representing one half, or one third, or one quarter, or one fifth axial of the annular row of blades.

- The orifice is opposite the lower surface of the next blade of the annular row, preferably the orifice is formed on a portion of the upper surface perpendicular to the circumferential direction of the corresponding annular row of blades.

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- Each blade comprises a fixing platform with a surface intended to radially delimit the flow of the turbomachine, the suction orifice being in contact with said surface.

- Each upper surface has a radial end, the suction orifice being arranged in a radial end, each radial end of the upper surface represents at most 20%, or at most 10%, or at most 5% of the radial height of the corresponding upper surface.

- Each blade is made of composite material with an organic matrix, the suction port being made of said composite material.

- Each blade is a variable pitch blade, in particular with a disk delimiting the upper surface, the orifice passing through the disk of the corresponding blade.

- Each blade comprises a monobloc body forming the lower surface and the upper surface, the suction orifice being formed in said monobSoc body.

- The rows of blades comprise rows of rotor blades and rows of stator blades, the orifice being formed in a blade of a row of stator blades.

- The suction port includes a suction pipe, each suction pipe being dedicated to a suction port so that each suction port independently sucks the flow of the turbomachine.

- The axial length of the orifice represents between 0.1% and 20% of the average thickness of the blade, preferably between 1% and 5%.

- The suction opening being a first opening, the upper surface of the blade further comprising a second suction opening downstream from the first suction opening.

- The compressor includes a suction device in communication with each suction port.

- Each blade upper surface of the annular row of blades comprises a turbomachine flow suction port, said blades and / or the suction ports of said blades being identical.

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The invention also relates to a turbomachine, in particular a turbojet engine, comprising a compressor, remarkable in that the compressor is in accordance with the invention.

Another object of the invention is a method of controlling pumping in a turbomachine compressor, in particular of a turbojet engine, the method comprising a step (a) compression of a flow of the turbomachine by the compressor, and a step (b) suction of a flow of the turbomachine by a blade of the compressor, said blade having a suction surface, remarkable in that in step (b) suction, ie flow of the turbomachine is sucked from the suction surface of said blade, possibly , the compressor is in accordance with the invention.

According to an advantageous embodiment of the invention, during step (a) compression, the compressor has a pumping phenomenon and / or a flow stall phenomenon, in step (b) suction the upper surface sucks a unhooked flow from a blade, for example so as to limit or eliminate the pumping phenomenon and / or a flow stall phenomenon.

In general, the advantageous modes of each object of the invention are also applicable to the other objects of the invention. Each object of the invention can be combined with the other objects, and the objects of the invention can also be combined with the embodiments of the description, which in addition can be combined with one another, according to all possible technical combinations.

Benefits

The invention makes it possible to evacuate the parasitic flows which appear downstream of the upper surfaces of the blades due to the breakage which they cause in the flow. Thus, the homogeneity of the flow passing through the blade grid considered improves. The remaining flow presents less eddies and undergoes a lesser pressure drop. The grid ie blocks less and the idle less, which increases the air flow available for the thermal cycle of the turbomachine. In addition, positioning the suction ports on the upper surface makes it possible to get as close as possible to the place of birth of the vortices. What is more, this implantation makes it possible to move away from the radial limits, and therefore to treat

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BE2016 / 5867 the boundary layers in their radial thicknesses. This solution offers efficiency while operating compact means. The intrusiveness remains limited and respects the mechanical resistance of the equipped blades.

Brief description of the drawings

FIG. 1 represents an axial turbomachine according to the invention.

FIG. 2 is a diagram of a turbomachine compressor according to the invention.

FIG. 3 illustrates a blade of an annular row of blades according to the invention.

FIG. 4 is a section through a blade of FIG. 3, the section being taken along the axis 4-4.

ίο FIG. 5 is a diagram of the pumping control process in a turbomachine compressor according to the invention.

Description of the embodiments

In the following description, the terms internal and external refer to a positioning relative to the axis of rotation of an axial turbomachine. The axial direction corresponds to the direction along the axis of rotation of the turbomachine. The radial direction is perpendicular to the axis of rotation.

Upstream and downstream are in reference to the main flow direction of the flow in the turbomachine.

Figure 1 shows in a simplified manner an axiaie turbomachine. In this specific case, it is a double-flow turbojet engine. The turbojet engine 2 comprises a first level of compression, called a low-pressure compressor 5, a second level of compression, called a high-pressure compressor 6, a combustion chamber 8 and one or more levels of turbines 10. In operation, the mechanical power from the turbine 10 transmitted via the central shaft to the rotor 12 sets in motion the two compressors 5 and 6. The latter comprise several rows of rotor blades associated with rows of stator blades. The rotation of the rotor 12 around its axis of rotation 14 thus makes it possible to generate an air flow ef to gradually compress the latter until the inlet of the combustion chamber 8.

An inlet fan commonly designated as a fan or blower 16 is coupled to the rotor 12 and generates an air flow which is divided into a primary flow 18 passing through

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BE2016 / 5867 your various levels mentioned above of the turbomachine, and in a secondary flow 20 passing through an annular duct (partially shown) along the machine and then joining the primary flow at the outlet of the turbine. The blower can be of the non-faired type, for example with double counter-rotating rotor, possibly downstream.

The secondary flow 20 can be accelerated so as to generate a thrust reaction. The primary 18 and secondary 20 flows are coaxial annular flows which are fitted one inside the other. They are channeled through the casing of the turbomachine and / or of the ferrules.

ίο Figure 2 is a sectional view of a compressor of an axial turbomachine such as that of Figure 1. The compressor can be a low pressure compressor 5. We can observe part of the blower 16 and the separation nozzle 22 of the primary flow 18 and the secondary flow 20. The rotor 12 comprises several rows of rotor blades 24, in this case three.

The low pressure compressor 5 comprises several rectifiers, in this case four, which each contain a row of stator vanes 26.

The rectifiers are associated with the fan 16 or with a row of rotor blades to straighten the air flow, so as to convert the speed of the flow into pressure, in particular into static pressure.

The stator vanes 26 extend between an internal ferrule 28 and an external ferrule 30 to which they can be screwed or welded. The outer shell 30 can be, at least partially, formed by the outer casing of the compressor 5. The rotor vanes 24 extend radially from their internal support, such as a drum 32 or a disc. In order to reduce your risk of stalling or pumping, the turbomachine comprises a suction system 34. This system makes it possible to suck a flow from the turbomachine, such as part of the primary flow.

Within a row, the blades are regularly spaced from one another. Optionally, the spacing between the blades can vary locally as can their angular orientations. Some vanes may differ from the rest of the vanes in their row. The housing can be formed of several rings, or half-shells.

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The blades (24; 26) can be made of metal, for example a titanium alloy. They can also be made of composite material with an organic matrix; with reinforcing fibers. They can be mixed: with a metallic leading edge and a composite body as mentioned above.

The invention can also be applied to a high pressure compressor such as that mentioned in FIG. 1.

FIG. 3 sketches a blade 26 representative of an annular row of blades of the compressor shown in FIG. 2. The blades of the row can be identical, so that the characteristics detailed in relation ίο with ia this blade can be applied to all the blades in its row.

In the present case, the vane shown is a stator vane 26, but the present teaching can be applied to a rotor vane.

The blade 26 includes a leading edge 36, a trailing edge 38, an upper surface 40, a lower surface (hidden side). The lower surface and the upper surface extend from the leading edge 36 to the trailing edge 38 along the radial height of the primary vein 42 traversed by the primary flow 18. In order to manage the cases of stalling and pumping, the suction system 34 comprises at least one suction port 44, possibly several suction ports 44. In this case two ports 44 are shown, however any other number is possible per upper surface.

Each suction port 44 is formed on the upper surface 40 of its blade 26. Each suction port can open circumferentially, either in a direction perpendicular to the axis of rotation 14 and ia radial direction. A location on the upper surface makes it possible to partially suck in the flow 45 which drops off there, and possibly forming vortices. In operation, the unhooked flow 45 obstructs the passage between two consecutive blades 26, and limits the maximum flow rate of the compressor. Moreover, this off-hook flow 45 can be at the origin of pumping.

Each blade 26 of the annular row can be provided with at least one or more suction orifices. Optionally, some blades in this row are free of suction ports.

Each suction port 44 can be placed towards the trailing edge 38 of the blade 26. Each suction port 44 can be placed half downstream. Half

BE2016 / 5867 upstream of the upper surface and / or the blade can be free of suction port.

Each suction port 44 may be facing the lower surface of the next blade of the annular row, which is opposite this blade 26 along the circumference. Each suction port 44 of the same blade 26 can be formed on a portion of the upper surface 40. This upper surface portion can be orthogonal to the circumferential direction of the corresponding annular row of blades.

Each suction port 44 can be made of the composite material, for example organic of the blade. Each suction port 44 can in particular be produced in a monobioc portion, and / or made of material from the blade for better resistance.

According to one embodiment, the blade is in fixed orientation, in particular with respect to the external shell 30 and / or with respect to the internal shell 28. This orientation can be determined with respect to the rotation tax 14, possibly by considering a medium chord in Taube space. The blade can comprise a body welded to the external shell 30. Or, the blade can comprise a fixing platform 46 which is pressed and fixed to the external shell 30. The platform 46 shows an internal surface 48 which delimits the primary vein 42 This surface 48 may be radially at the level of each orifice 44, and may touch each associated orifice 44. Optionally, this surface 48 surrounds Taube 26, and extends upstream and downstream of the leading edge 36 and the trailing edge 38 respectively.

Each upper surface 40 has a radial end, possibly two radial ends. Each suction orifice 44 is advantageously disposed in the same radial end of the upper surface 40, or the suction orifices of the blade are distributed in the radial ends of the upper surface 40. The orifices 44 may be radially the outer shell 30 and / or the inner shell 28. Each radial end of the upper surface 40 represents at most 20%, or at most 10%, or at most 5% of the radial height of the corresponding upper surface 40. The upper surface 40 comprises radially a central section between its radial ends. This central section can extend over at least: 60%, or 80%, or

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90% of the radial height of the upper surface. This radial section is free of suction opening.

Each suction port 44 comprises a suction duct 50. Each suction duct 50 extends radially its orifice 44 towards the outside of the primary vein 42. Each duct 50 is connected independently to a suction device 52 of the suction system 34. This suction can be ensured by an auxiliary pump, or by means of a vacuum around the external shell 30.

Optionally, each suction duct 50 is dedicated to a clean suction port 44. The suction conduits 50 can pass radially through their platform 46. Thus, each suction port 44 sucks independently ie flow 18 of the turbomachine. Such a system avoids the circulation from one orifice 44 to the other, which would be disadvantageous in terms of evacuation of unhooked flow 45.

According to another alternative, the dawn is of variable orientation; also referred to as a variable pitch vane. Such a dawn is known by the acronym VSV corresponding to the Anglo-Saxon expression "Variable Stator Vane".

Such a blade then forms part of a row of blades forming a stator with variable geometry. Such a blade generally has a disc delimiting the upper surface. The orifice then crosses the disc of the corresponding dawn.

The disc can be the equivalent of the platform for fixing a vane with variable orientation. Optionally, a row of blades includes blades with variable orientations and others with fixed orientations.

FIG. 4 represents an aerodynamic profile 54 of the blade 26 illustrated in relation to FIG. 3. This aerodynamic profile 54 comes from the cutting plane traced along the axis 4-4 of FIG. 3. It belongs to a stack of profiles aerodynamics of the blade 26 concerned.

The aerodynamic profile 54 includes a curve of the upper surface 40 and a curve of the lower surface 56 which meet at a point on the leading edge

36 and a point on the trailing edge 38. These latter points make it possible to define a cord 58. For each point of the upper surface curve 40, a distance D relative to the cord 58 is defined. Each blade 26 has an end point 60 for which the distance D is the maximum of the blade considered. The

BE2016 / 5867 if necessary, Taube has several extreme points 60 whose distance D is the same maximum. These points can describe an alignment or a curve in space. At least one or each end point 60 is upstream of at least one or each suction port 44 of the blade 26.

The suction ports 44 can be grouped axially, and / or radially. They approach the beginning of the off-hook flow 45. For example, the blade 26 has axially a central section in which is disposed at least one or each orifice 44 of the upper surface 40. The axial center of the blade 26 is in this section center. This central section can be delimited by an upstream section and by a downstream section of Taube, these last two sections possibly being free of suction orifices. This central section represents a half, or a third, or a quarter, or a fifth axial of the corresponding upper surface.

The axial length of the orifice 44 represents between 0.1% and 20% of the average thickness of Taube 26, preferably between 1% and 5%. Furthermore, the width along the circumference of each orifice 44 can represent between 0.1% and 3% of the axial length of the mole 26, and / or of the cord 58.

The pitch angle ß of Taube 26 can be greater than or equal to 35 °. This setting angle ß is representative of the deviation that Taube brings. It can be measured between Rotation tax 14 and an average chord 58 of Taube 26. The invention offers a substantial gain for ies with high deviation.

FIG. 5 is a diagram of the pumping control method in a turbomachine compressor. The compressor may correspond to that described in relation to FIGS. 1 to 4.

The process can comprise the following stages, possibly carried out as follows:

- (a) compression 100 of a flow of ia turbomachine by the compressor during its operation;

detection 102 of a pumping phenomenon and / or a phenomenon of stalling of flow in the compressor, said defection can be a predictor; and

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- (b) aspiration 104 of a flow of ia turbomachine by a vane of the compressor, the aspiration of the flow taking place from the upper surface of at least one or more blades of the row.

The aspiration can be an aspiration of a boundary layer, in particular a 5 boundary layer radially. The suction step (b) 104 can be continuous, or discontinuous. It can be ordered.

Thus, step (b) suction makes it possible to limit or eliminate the phenomenon of pumping and / or a phenomenon of stalling of flow in the compressor.

BE2016 / 5867

Claims (5)

Claims 1. Compressor (4; 6) of axial turbomachine (2), in particular a low pressure compressor (4) of axial turbomachine (2), the compressor (4; 6) comprising at least one annular row of blades (24; 26 ) of compressor (4; 6), each blade (24; 26) comprising a leading edge (36), a trailing edge (38), an upper surface (40), a lower surface (56), said surfaces (40; 56) extending from the leading edge (36) to the trailing edge (38); characterized in that the upper surface (40) of at least one or each blade (24; 26) comprises a suction orifice (44) for flow from the turbomachine (2), in particular from an off-hook flow (45 ) of the associated dawn (24; 26). 2. Compressor (4; 6) according to claim 1, characterized in that the annular row of vanes (24; 26) has an upstream half and a downstream half, the suction orifice (44) being arranged in the downstream half of the annular row of vanes (24; 26). 3. Compressor (4; 6) according to one of claims 1 to 2, characterized in that each blade (24; 26) comprises a stack of aerodynamic profiles (54), each aerodynamic profile (54) having a cord (58 ) and upper surface points (40) distant by a distance D from the associated cord, each blade (24; 26) has an end point (60) with a maximum distance D which is upstream of the orifice suction (44). 4. Compressor (4; 6) according to one of claims 1 to 3, characterized in that each blade (24; 26) comprises a setting angle β whose inclination relative to the axis of rotation (14) of the compressor (4; 6) is greater than: 20 °, or 30 °. 5. Compressor (4; 6) according to one of claims 1 to 4, characterized in that the annular row of blades (24; 26) has axially a central section in which the orifice (44) is disposed, said central section! 2016/5867 BE2016 / 5867 representing a half, or a third, or an axial quarter of the annular row of vanes (24; 26). 6. Compressor (4; 6) according to one of claims 1 to 5, characterized in that the orifice (44) is opposite the lower surface (56) of the blade (24; 26) 5 following the annular row, preferably the orifice is formed on a portion of the upper surface (40) perpendicular to the circumferential direction of the corresponding annular row of blades (24; 26). 7. Compressor (4; 6) according to one of claims 1 to 6, characterized in that each blade (24; 26) comprises a fixing platform (46) with ίο a surface (48) intended to delimit radially ie flow ia turbomachine (2), the suction orifice (44) being in contact with said surface. 8. Compressor (4; 6) according to one of claims 1 to 7, characterized in that each upper surface (40) has a radial end, the suction orifice (44) being arranged in a radial end, each end 15 radial upper surface represents at most 20%, or at most 10%, or at most 5% of the radial height of the corresponding upper surface (40). 9. Compressor (4; 6) according to one of claims 1 to 8, characterized in that each blade (24; 26) is made of composite material with organic matrix, the suction orifice (44) being made in said material 20 composite. 10. Compressor (4; 6) according to one of claims 1 to 9, characterized in that each blade (24; 26) is a variable pitch blade, in particular with a disc delimiting the upper surface (40), the orifice passing through (44) the disk of the corresponding blade. 25 11. Compressor (4; 6) according to one of claims 1 to 10, characterized in that each blade (24; 26) comprises a monobloc body forming the lower surface (56) and ia upper surface (40), the suction port (44) being formed in said monobioc body. BE2016 / 5867 12. Compressor (4; 6) according to one of claims 1 to 11, characterized in that the rows of blades comprise rows of rotor blades (24) and rows of stator blades (28), l orifice (44) being formed in a blade of a row of stator vanes (26). 13. Compressor (4; 6) according to one of claims 1 to 12, characterized in that the suction orifice (44) comprises a suction duct (50), each suction duct being dedicated to a suction port so that each suction port independently sucks the flow of the turbomachine (2). 14. Compressor (4; 6) according to one of claims 1 to 13, characterized in that the axial length of the orifice (44) represents between 0.1% and 20% of the average thickness of the blade (24; 26), preferably between 1% and 5%. 15. Compressor (4; 6) according to one of claims 1 to 14, characterized in that the suction orifice (44) being a first orifice, the upper surface (40) of the blade (24; 26 ) further comprising a second suction port (44) downstream of the first suction port. 16. Compressor (4; 6) according to claim 15, characterized in that it comprises a suction device (52) in communication with each suction port (44). 17. Compressor (4; 6) according to one of Claims 1 to 16, characterized in that each upper surface (40) of the blade (24; 26) of the annular row of blades comprises one comprises an orifice suction (44) of the turbomachine flow (2), said vanes and / or the suction orifices (44) of said vanes being identical. 18. Turbomachine (2), in particular a turbojet, comprising a compressor, characterized in that the compressor (4; 6) conforms to one of claims 1 to 17. 2016/5867 BE2016 / 5867 19. Method for controlling pumping in a compressor (4; 6) of a turbomachine (2), in particular of a turbojet engine, the method comprising a step (a) compression (100) of a flow of the turbomachine (2) by the compressor (4; 6), and a step (b) aspiration (104) of a flow of the 5 turbomachine (2) by a blade (24; 26) of the compressor (4; 6), said blade having an upper surface (40), characterized in that in step (b) suction (104), ie flow ia turbomachine (2) is drawn from the upper surface of said blade (24; 26), optionally, the compressor (4; 6) is in accordance with one of claims 1 to 17. io 20. Method according to claim 19, characterized in that during step (a) compression (100), the compressor (4; 6) has a pumping phenomenon and / or a flow stall phenomenon, step (b) suction (104) the upper surface sucks an off-hook flow (45) from a blade (24; 26). BE2016 / 5867 BE2016 / 5867