Optimisation of the behaviour of the fan in an aeronautical propulsion system

Abstract

The invention relates to an assembly (11a, 11b, 11c) of an aeronautical propulsion system (1) comprising: - a movable element (35) rotatable about an axis (X) of the assembly (11a, 11b, 11c); and - a mode damper (25) comprising an inner ring (30), an outer ring (32) and a fluid (29) confined between the inner ring (30) and the outer ring (32). The surface viscance of the mode damper (25) is between 10^8 m-1 and 2 x 10^9 m-1: 1.0 x 10^8<sp / > ≤ surface viscance = L^2 / 2πC^3 ≤ 2.0 x 10^9 (formula I), where L is the axial length of the mode damper (25) and is expressed in metres (m); and C is the maximum radial clearance of the mode damper (25) and is expressed in metres (m).

Description

[0001] DESCRIPTION

[0002] Optimization of fan behavior in an aeronautical propulsion system

[0003] TECHNICAL FIELD

[0004] This application relates generally to the field of propulsion systems, and more particularly to mode dampers comprising a pressurized fluid ("squeeze film" in English) which can be mounted between the outer ring of a bearing and a rigid support fixedly attached to a stator part of the propulsion system.

[0005] STATE OF THE ART

[0006] A propulsion system generally comprises, from upstream to downstream in the direction of gas flow, a fan section, a compressor section which may include a low-pressure compressor and a high-pressure compressor, a combustion chamber, and a turbine section which may include, in particular, a high-pressure turbine and a low-pressure turbine. The high-pressure compressor is driven in rotation by the high-pressure turbine via a high-pressure shaft. The fan, and where applicable the low-pressure compressor, are driven in rotation by the low-pressure turbine via a low-pressure shaft.

[0007] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into account the impactful factors in all phases of design and development to obtain aeronautical components and products that are less energy-intensive, more environmentally friendly, and whose integration and use in civil aviation have moderate environmental consequences, with the aim of improving the energy efficiency of aircraft.

[0008] To minimize mechanical loads, vibrations, and displacements between a moving part, such as a rotor, and a stator part of the propulsion system, it has been proposed to interpose a mode damper (or "squeeze-film") and, where appropriate, a flexible cage (also known as a squirrel cage) between the moving and stator parts. For this purpose, a mode damper comprises an inner ring and an outer ring, which are fixed relative to the stator part, extending radially outward from the moving part, and a pressurized fluid confined between the inner and outer rings.

[0009] The performance and noise requirements of modern propulsion systems are constantly increasing. To improve propulsion system performance, one approach is to reduce the dynamic backlash in the engine. This could be achieved by increasing the radius of the mode dampers (the damping capacity of mode dampers being dependent on their radius). However, the available space between the rotors and their respective drive shafts is increasingly limited in order to improve propulsion system efficiency and reduce specific fuel consumption.

[0010] In the more specific case of engines intended for business jets, which have a moderate fan diameter (generally less than or equal to 60 inches (1.524 meters)) and a small high-pressure core, integration constraints are severe, limiting the bearing radius. However, the vibration and cabin noise thresholds required by aircraft manufacturers for these engines are very low compared to commercial aviation, and performance requirements are high, necessitating the minimization of dynamic clearances within the engine. Since the damping of mode dampers depends on the radius, achieving the necessary damping to meet these vibration thresholds is challenging.

[0011] EXPOSED

[0012] One aim of this application is to optimize the performance of the aeronautical propulsion system, particularly in terms of specific consumption, while limiting the noise and vibrations generated by the propulsion system.

[0013] To this end, a first aspect of an aeronautical propulsion system is proposed, comprising:

[0014] - a movable element rotating around an axis of the assembly; and

[0015] - a mode damper comprising an inner ring, an outer ring and a fluid confined between the inner ring and the outer ring.

[0016] The inner and outer rings are coaxial with the moving element, and the inner ring extends radially between the moving element and the outer ring. Furthermore, the mode damper has an axial length and a maximum radial clearance such that its surface viscosity, which is the ratio of the square of the axial length to the product of the cube of the maximum radial clearance multiplied by twice the value of Pi, is between 10 A 8 rrr 1 and 2 x 10 A 9 rrr 1 :

[0017] 10 8 10 9 Or :

[0018] L is the axial length of the damper and is expressed in meters; and

[0019] This is the maximum radial play of the damper mode and is expressed in meters.

[0020] Some preferred but not exhaustive characteristics of the set according to the first aspect are the following, taken individually or in combination:

[0021] - the surface viscosity is greater than or equal to 2 x 10 A 8 rrr 1 ;

[0022] - the mode damper also exhibits a surface clearance, which corresponds to the ratio between the maximum radial clearance and the product of twice the value of Pi multiplied by the mounting radius of the mode damper and the axial length of the mode damper, and is between 5 x 10 A -4 rrr 1 and 7.5 x 10 A -3 rrr 1 :

[0023] 5 x 10 -4 < surface area 7.5 x 10 -3 where R is the radius of implantation of the mode damper and is expressed in meters;

[0024] - the surface clearance is less than or equal to 5.8 x 10 A -3 rrr 1 ;

[0025] - the moving element comprises a moving part of a bearing of the propulsion system; and / or

[0026] - The mode damper further comprises at least one sealing segment, preferably two sealing segments, axially delimiting the fluid between the inner and outer rings. According to a second aspect, an aeronautical propulsion system is proposed comprising an assembly according to the first aspect and a rotor coaxial with the moving element.

[0027] The assembly may include a bearing, the bearing further comprising an additional ring mounted on the rotor.

[0028] For example, the aeronautical propulsion system according to the second aspect further includes:

[0029] - a blower section comprising a rotor, the diameter of the blower rotor being less than or equal to 1.524 meters, the blower section being able to be enclosed; and

[0030] - a drive turbine configured to directly drive the blower shaft in rotation around the axis of rotation.

[0031] According to a third aspect, it is proposed an aircraft comprising at least one propulsion system according to the second aspect fixed to the aircraft by means of a mast.

[0032] According to a fourth aspect, a manufacturing process for an aeronautical propulsion system assembly is proposed, comprising the following steps:

[0033] - determine an axial length and a maximum radial clearance of a mode damper such that a surface viscosity of the mode damper, which corresponds to the ratio between the square of the axial length and the product between the maximum radial clearance cubed multiplied by twice the value of Pi, is between 10 A 8 rrr 1 and 2 x 10 A 9 rrr 1 :

[0034] L 2

[0035] 10 8< - - 3 < 2 x 10

[0036] 2nC 9 Or :

[0037] L is the axial length of the damper and is expressed in meters; and

[0038] C is the maximum radial play of the mode damper and is expressed in meters;

[0039] - manufacture an inner ring and an outer ring of the mode damper according to the axial length and radial clearance thus determined;

[0040] - assemble the inner and outer rings with a moving element; and

[0041] - inject a fluid between the inner ring and the outer ring.

[0042] Some preferred but not limiting characteristics of the manufacturing process according to the fourth aspect are as follows, taken individually or in combination:

[0043] - the axial length and radial clearance are determined so that the surface viscosity is greater than or equal to 2 x 10 A 8 rrr1 ;

[0044] - the axial length, radial clearance and an installation radius are further determined so that a surface clearance of the mode damper is between 5*10 A -4 rrr 1 and 7.5*10 A -3 rrr 1 , where the surface clearance is defined as follows: where R is the radius of the mode damper's installation and is expressed in meters; and / or

[0045] - the axial length, radial clearance and installation radius are determined so that the surface clearance is less than or equal to 5.8 x 10 A -3 rrr 1 .

[0046] DESCRIPTION OF THE FIGURES

[0047] Other features, purposes, and advantages will become apparent from the following description, which is purely illustrative and not exhaustive, and should be read in conjunction with the attached drawings on which:

[0048] Figure 1 is a schematic, partial, cross-sectional view of an example of a propulsion system conforming to a first embodiment;

[0049] Figure 2 is a schematic, partial, cross-sectional view of an example of a propulsion system conforming to a second embodiment;

[0050] Figure 3 is an enlarged schematic cross-sectional view of part of an example assembly for a propulsion system according to a first variant;

[0051] Figure 4 is an enlarged schematic cross-sectional view of part of an example assembly for a propulsion system according to a second variant;

[0052] Figure 5 is an example of an aircraft that may include at least one propulsion system conforming to the first or second embodiment; and

[0053] Figure 6 is a flowchart illustrating examples of steps in a manufacturing process for an assembly for a propulsion system.

[0054] Across all figures, similar elements bear identical references.

[0055] DETAILED DESCRIPTION

[0056] A propulsion system 1 has a principal direction extending along a longitudinal axis X and comprises, from upstream to downstream in the direction of gas flow in the propulsion system 1 when it is in operation, a fan section 2 and a primary body 3, often called a "gas generator," including a compressor section 4, 5, a combustion chamber 6, and a turbine section 7, 8. The propulsion system 1 here is an aeronautical propulsion system 1 configured to be mounted on an aircraft 100 by means of a pylon (or mast). The engine may, in particular, be mounted at the rear of the fuselage (lateral attachment at the tail assembly).

[0057] The compressor section 4, 5 comprises a series of stages, each including a rotating blade wheel (rotor) 4a, 5a in front of a stationary blade wheel (stator) 4b, 5b. The turbine section 7, 8 also comprises a series of stages, each including a stationary blade wheel (stator) 7b, 8b behind which a rotating blade wheel (rotor) 7a, 8a rotates.

[0058] In this application, the axial direction corresponds to the direction of the longitudinal axis X, corresponding to the rotation of the gas generator shafts, and a radial direction is a direction perpendicular to and passing through this axis X. Furthermore, the circumferential (or lateral, or tangential) direction corresponds to a direction perpendicular to and not passing through the longitudinal axis X. Unless otherwise specified, internal (respectively, inside) and external (respectively, outside) are used with reference to a radial direction such that the inner part or face of an element is closer to the X axis than the outer part or face of the same element.

[0059] In operation, an airflow F entering the propulsion system 1 is divided between a primary airflow F1 and a secondary airflow F2, which flow from upstream to downstream in the propulsion system 1.

[0060] The secondary airflow F2 (also called the "bypass airflow") flows around the primary body 3. The secondary airflow F2 cools the periphery of the primary body 3 and is used to generate most of the thrust provided by the propulsion system 1.

[0061] The primary airflow F1 flows in a primary channel inside the primary body 3, passing successively through the compressor section 4, 5, the combustion chamber 6 where it is mixed with fuel to serve as an oxidizer, and the turbine section 7, 8. The passage of the primary airflow F1 through the turbine section 7, 8 receiving energy from the combustion chamber 6 causes a rotation of the rotor of the turbine section 7, 8, which in turn drives the rotation of the rotor of the compressor section 4, 5 as well as a rotor part 9 of the blower section 2.

[0062] In a twin-spool propulsion system 1, the compressor section 4, 5 may include a low-pressure compressor 4 and a high-pressure compressor 5. The turbine section 7, 8 may include a high-pressure turbine 7 and a low-pressure turbine 8. The rotor of the high-pressure compressor 5 is driven in rotation by the rotor of the high-pressure turbine 7 via a high-pressure shaft 10. The rotor of the low-pressure compressor 4 and the rotor portion 9 of the blower section 2 are driven in rotation by the rotor of the low-pressure turbine 8 via a low-pressure shaft 11. Thus, the primary body 3 comprises a high-pressure body including the high-pressure compressor 5, the high-pressure turbine 7, and the high-pressure shaft 10, and a low-pressure body including the blower section 2, the low-pressure compressor 4, the low-pressure turbine 8, and the low-pressure shaft 11.The rotational speed of the high-pressure body is greater than the rotational speed of the low-pressure body.

[0063] The low-pressure shaft 11 is generally housed, along a portion of its length, within the high-pressure shaft 10 and is coaxial with the high-pressure shaft 10. The low-pressure shaft 11 and the high-pressure shaft 10 may be co-rotating, i.e., driven in the same direction around the longitudinal axis X. Alternatively, the low-pressure shaft 11 and the high-pressure shaft may be counter-rotating, i.e., driven in opposite directions around the longitudinal axis X. If applicable, the intermediate shaft is housed between the high-pressure shaft 10 and the low-pressure shaft 11. The intermediate shaft and the low-pressure shaft 11 may be co-rotating or counter-rotating.

[0064] The fan section 2 includes at least the fan rotor 9, which is driven in rotation relative to a stator portion of the propulsion system 1 by the turbine section 7, 8. Each fan rotor 9 comprises a hub 13 and blades 14 extending radially from the hub 13. The blades 14 of each rotor 9 may be fixed relative to the hub 13 or may optionally have variable pitch. In this case, the root of the blades 14 of each rotor 9 is pivotally mounted about a pitch axis and is connected to a pitch-changing mechanism 15 mounted in the propulsion system 1, the pitch being adjusted according to the flight phases by the pitch-changing mechanism 15. The pitch-changing mechanism 15 is shown in dashed lines in Figure 1 to indicate that this feature is optional.

[0065] The fan section 2 may further include a fan stator 16, or rectifier, which comprises blades 17 mounted on a hub of the fan stator 16 and whose function is to rectify the secondary airflow F2 exiting the fan rotor 9. The blades 17 of the fan stator 18 may be fixed relative to the hub or have variable pitch. Similar to the rotor blades 14, the base of the stator blades 17 is pivotally mounted about a pitch axis X and is connected to a pitch-changing mechanism 15a, which is generally separate from that of the fan rotor 9, the pitch being adjusted according to the flight phases by the pitch-changing mechanism.

[0066] The fan rotor 9 also includes at least twelve blades 14 and at most twenty-four blades 14, for example at least sixteen blades 14 and at most twenty-two blades 14. The number of blades 16 in the fan stator 17 depends on the acoustic criteria defined for the propulsion system 1 and is at least equal to the number of blades 14.

[0067] The blower section 2 can be shrouded. In the case of a shrouded blower section 2, the blower section 2 includes a blower housing 12 and the blower rotor 9 is housed in the blower housing 12.

[0068] A shrouded fan section 2 comprises a fan rotor 9 extending upstream of a fan stator. The fan stator blades are then generally called outlet guide vanes (OGVs) and have a fixed pitch relative to the fan stator hub. Furthermore, the bypass ratio of the propulsion system 1 is, for example, greater than or equal to 4, for example, between 4 and 10 inclusive. The fan rotor blades 14 may be fixed or have a variable pitch. The fan pressure ratio may then be between 1.20 and 1.70, for example, less than or equal to 1.45.

[0069] To calculate the dilution ratio, the mass flow rate of the secondary airflow F2 and the mass flow rate of the primary airflow F1 are measured when the propulsion system 1 is stationary, uninstalled, in takeoff mode in a standard atmosphere (as defined by the International Civil Aviation Organization (ICAO) manual, Doc 7488 / 3, 3 eedition) and at sea level. These conditions are generally referred to by the acronym SLS, for Sea Level Static. It should be noted that, in this application, the parameters (pressure, flow rate, thrust, speed, etc.) are systematically determined under these conditions. "Not installed" here means that the measurements are taken when the propulsion system 1 is in a test bench (and not installed on an aircraft 100), as the measurements are then simpler to perform. The distances (length, radius, diameter, etc.) are, however, measured at ambient temperature (approximately 20°C) when the propulsion system 1 is cold, that is, when the propulsion system 1 has been at rest for a sufficient period for the components of the propulsion system 1 to reach ambient temperature, it being understood that these dimensions vary little compared to the conditions under which the propulsion system 1 is in takeoff mode.

[0070] In an unshod section 2 of a fan, the fan section 2 (which can also be referred to as the propeller) is not enclosed by a fan casing. Because the fan section 2 is unshod, the blades 14 of the fan rotor 9 have variable pitch. The low-pressure shaft 11 is supported by two, three, or four bearings 11a, 11b, and / or 11c, in order to control the deformation modes of the low-pressure shaft 1. The low-pressure shaft 11 can thus include one or two front bearings 11a, which extend upstream of the combustion chamber 6, and two rear bearings 11b, 11c, which extend downstream of the combustion chamber 6. The bearings 11a, 11b, 11c include a first ring mounted on the low-pressure shaft 11 and a second ring mounted on a stator part of the propulsion system 1, typically on a housing of the propulsion system 1 through which forces pass in the propulsion system 1.Thus, a first front bearing 11a can be mounted on the low-pressure shaft 11 and on the inlet housing 26 of the propulsion system 1, which extends between the blower rotor 9 and the low-pressure compressor 4. If necessary, a second front bearing can be mounted on the low-pressure shaft 11 and on the inter-compressor housing 23 (or intermediate housing) or on the inlet housing 26 of the propulsion system 1, i.e. between the low-pressure compressor 4 and the high-pressure compressor 5. A first rear bearing 11b can be mounted on the low-pressure shaft 11 and on the exhaust housing 27, which extends immediately downstream of the low-pressure turbine 8 (Figure 1).Alternatively, the first rear bearing 11b may include an inter-shaft bearing positioned between the low-pressure shaft 11 and the high-pressure shaft 10 (Figure 2), the second rear bearing 11c being associated with the exhaust housing as in Figure 1. The first rear bearing 11b extends downstream of the bearing 12b furthest downstream of the high-pressure shaft 10. If necessary, the first and second rear bearings 11b, 11c may be mounted on the same cylindrical ferrule, which is itself fixed to the exhaust housing 27.

[0071] In one embodiment, the low-pressure shaft 11 is supported by a front bearing 11a and two rear bearings 11b, 11c.

[0072] All or part of the bearings, particularly bearings 11a-11c of the low-pressure shaft 11, may also include a flexible cage mounted between the inner ring and a rigid support fixedly attached to the stator part of the propulsion system 1 in order to control the stiffness of the bearings and to fine-tune the position of the first mode of deformation. The cage comprises, for this purpose, a generally cylindrical wall mounted between the rigid support and the second ring of the bearing, and radially deformable columns to allow radial displacement of the generally cylindrical wall, and therefore of the bearing, relative to the rigid support. Examples of bearings with a flexible cage are described in documents WO 2021 / 001610 and WO 2022 / 195198 on behalf of the Applicant.

[0073] The dual-spool propulsion system 1 may include, in particular, a single- or two-stage high-pressure turbine 7, a high-pressure compressor 5 comprising at least four and at most ten stages, a low-pressure turbine 8 comprising at least three and at most five stages, and optionally, a low-pressure compressor 4 comprising at least one and at most four stages. For example, the propulsion system may include a high-pressure body comprising four axial compressor stages and one centrifugal stage driven by a single-stage high-pressure turbine, and a low-pressure compressor comprising four low-pressure compressor stages driven by a four-stage low-pressure turbine.The overall compression ratio of the propulsion system 1, which corresponds to the pressure ratio between the pressure at the outlet of the high-pressure compressor 5 and the pressure at the inlet of the blower rotor 9 (measured at the base of the blower rotor 9), shall be greater than or equal to 15 and less than or equal to 70, preferably greater than or equal to 25 and less than or equal to 55, for example greater than or equal to 35 and less than or equal to 48.

[0074] The propulsion system 1 further includes an assembly comprising a mode damper 25 (known in English as a "squeeze film") capable of sufficiently damping the vibrations generated by a moving element 35 of the propulsion system 1 while ensuring its integration below the flow path. The mode damper 25 can, in particular, be mounted between a fixed portion of a bearing of the propulsion system 1 and a stator portion of the propulsion system 1 (e.g., the inlet housing or intermediate housing). The propulsion system 1 may include a mode damper 25 on each bearing supporting the low-pressure shaft and / or on each bearing supporting the blower shaft.

[0075] The mode damper 25 comprises an inner ring 30 and an outer ring 32 which define a damping chamber 34, and a fluid 29 confined within the damping chamber 34. The fluid 29 is pressurized within the damping chamber 34 and is configured to dampen the mode change (viscous damping) of the stationary part on which the mode damper 25 is mounted. The inner ring 30 and the outer ring 32 are annular and coaxial with the moving element 35 (the bearing of the housing or the planetary gears of the reduction mechanism). The inner ring 30 therefore extends radially between the moving element and the outer ring 32.

[0076] Fluid 29 can include any type of viscous fluid 29, for example oil or fuel.

[0077] The supply parameters of the mode 25 damper can be conventional. For example, the supply pressure of the fluid 29 can be between 0.5 bar (50 kPa) and 30 bar (3000 kPa), preferably between 4 bar (400 kPa) and 18 bar (1800 kPa); the temperature of the fluid 29 can be between 10°C and 300°C, preferably between 50°C and 200°C; the supply flow rate of the fluid 29, which depends on the supply pressure and the sealing system of the mode 25 damper, can be between a few liters per hour (L / h) and a few hundred liters per hour.

[0078] The damping chamber 34 is radially delimited internally by the inner ring 30 and radially externally by the outer ring 32. The inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30 which delimit the damping chamber 34 may be smooth, in which case the inner ring 30 and the outer ring 32 are spaced radially apart from each other so as to define the damping chamber 34. Alternatively, a recess may be formed in at least one of the inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30, the damping chamber 34 then being delimited by the walls of the recess.

[0079] When the mode 25 damper is mounted on a bearing, the inner ring 30 of the mode 25 damper is aligned with the second ring of the bearing (or is integral with the second ring of the bearing). Similarly, when the mode 25 damper is mounted on a reduction mechanism, the inner ring 30 is integral with the crown gear or planet carrier, depending on the configuration of the reduction mechanism.

[0080] The inner ring 30 is fixed relative to the outer ring 32. For example, the assembly further includes an anti-rotation device configured to lock the inner ring 30 relative to the outer ring 32. The anti-rotation device may, for example, include a radial spacer mounted in notches formed in the inner ring 30 and in the stator part of the propulsion system 1 (or, alternatively, in the outer ring 32 of the mode damper 25) so as to circumferentially lock the inner ring 30 relative to the outer ring 32.

[0081] The damping chamber 34 is axially delimited by axial terminals 34a, 34b configured to prevent the fluid 29 from leaking during the operation of the propulsion system 1. The axial terminals 34a, 34b may correspond to the upstream and downstream axial limits of the damping chamber 34, typically the upstream and downstream faces of the recess formed in the outer ring 32 and / or the inner ring 30. Alternatively, at least one of the axial terminals 34a, 34b may be formed by a sealing segment mounted between the inner ring 30 and the outer ring 32 so as to axially delimit the damping chamber 34. Preferably, the mode damper 25 comprises two sealing segments, each forming one of the axial terminals 34a, 34b.

[0082] Each sealing segment 34a, 34b can be made of a metallic material, a composite material, a ceramic or an elastomeric material, preferably a metallic or elastomeric material.

[0083] In one embodiment, the mode damper 25 is continuous, that is to say, the damping chamber 34 is devoid of a separating means forming independent damping chamber sectors 34.

[0084] Optionally, the mode damper 25 includes a fluid supply groove 36 29 configured to distribute the fluid 29 circumferentially in the damping chamber 34. The supply groove 36 can, for example, be formed in the external surface 31 of the inner ring 30 and extend circumferentially and continuously over the entire circumference of the inner ring 30.

[0085] In order to improve the performance of propulsion system 1 and reduce the noise generated by propulsion system 1, the axial length and maximum radial clearance of mode 25 damper are configured such that the surface viscosity of mode 25 damper, which corresponds to the ratio of the square of the axial length to the product of the cube of the maximum radial clearance and twice the value of Pi, is between 10 A 8 rrr 1 and 2 x 10 A 9 rrr 1 :

[0086] L 2

[0087] 10 8 < surface viscosity = < 2 x 10 9 (1) where: L is the axial length of the mode 25 damper and is expressed in meters (m); and

[0088] C is the maximum radial play of the mode 25 damper and is expressed in meters (m).

[0089] The axial length L of the mode 25 damper corresponds to the distance along which the oil film operates (viscous damping). When the mode 25 damper lacks a feed groove, the axial length is therefore the axial distance L between the axial terminals 34a, 34b of the damping chamber 34, and, where applicable, between the sealing segments of the damping chamber 34 (FIG. 3). When the mode 25 damper includes a feed groove 36 (FIG. 4), the axial length does not include the length of the feed groove 34; it therefore corresponds to the sum of the axial distance L1 between the upstream axial terminal 34a and the upstream limit of the groove 36, and the axial distance L2 between the downstream limit of the groove 36 and the downstream axial terminal 34b.

[0090] The maximum radial play of the mode damper 25 corresponds to the maximum radial distance, outside the feed groove 36, between the inner surface 33 of the outer ring 32 and the outer surface 31 of the inner ring 30. The maximum radial play therefore corresponds to the maximum thickness of the part of the oil film which plays a role in the damping of the modes.

[0091] When the moving element 35 rotates at a given rotational speed n, the inner ring 30 is subjected to a precessional motion (i.e., an orbit without relative rotation of the inner ring 30 around its axis) of the same frequency, resulting from the presence of an (inevitable) imbalance on the moving element 35, with a relative eccentricity e with respect to the outer ring 32. The damping generated by the mode damper 25 then depends on the tangential force generated by the compression of the fluid film 29 between the inner and outer rings. The tangential force Ft can be estimated by the following formula:

[0092] Ft = —Cv xexa> where: Cv is the viscosity of the mode 25 damper.

[0093] Viscosity is a drag coefficient that reflects the ability of a mode 25 damper to dissipate energy. This viscosity can be estimated using the following formula:

[0094] Cv = A x |ix R where: p depends on the properties of the fluid 29 of the mode 25 damper;

[0095] R is the mounting radius of the mode 25 damper, i.e., the maximum radius between the X-axis and the external surface 31 of the inner ring 30, the mounting radius being measured in a plane normal to the X-axis and expressed in meters (m); and

[0096] A is a constant.

[0097] The surface viscosity defined above is therefore a parameter allowing us to approximate the viscosity of the mode 25 damper by taking into account the surface area of ​​the mode 25 damper:

[0098] Since R, L and C are distances, these parameters are determined when the propulsion system 1 (and therefore the assembly including the mode 25 damper) is cold, as specified above.

[0099] When the mode damper 25 is dimensioned to comply with formula (1), the damping provided by the mode damper 25 is improved while taking into account the mechanical integration constraints of the assembly including the damper. In particular, increasing the surface viscosity allows for an increase in the damping of the moving element 35. However, this increase does not need to be achieved at the expense of the available space in the propulsion system 1.

[0100] Preferably, the surface viscosity of the mode 25 damper is greater than or equal to 2 x 10 A 8 m -1 Preferably, the surface viscosity should also remain below 2 x 10 A9 because, beyond this value, the integration constraints become too significant without improving the damping of the deformation modes. However, improving the damping makes it possible to reduce the vibration levels transmitted to the aircraft 100, the consumption of dynamic clearances between the moving element 35 and the stator part of the propulsion system 1 (and therefore the performance of the propulsion system 1) as well as the long-term fatigue loads (and therefore the service life and vibration fatigue resistance of the propulsion system 1).

[0101] The dimensioning of the mode damper so that its surface viscosity complies with formula (1) reproduced above allows the suspension modes of the blower (which is associated with the engine structure) to be damped.

[0102] To further improve the damping provided by the mode 25 damper, the mode 25 damper has a surface clearance, which corresponds to the ratio between the maximum radial clearance (between the outer surface 31 of the inner ring 30 and the inner surface 33 of the outer ring 32) and the product of twice the value of Pi multiplied by the mounting radius of the mode 25 damper and the axial length of the mode 25 damper, which is between 5 x 10 A -4 rrr 1 and 7.5 x 10 A -3 rrr 1 : 7.5 x 10 -3 (2)

[0103] The surface clearance parameter takes into account, in particular, the mounting radius of the mode 25 damper, and therefore the available space in the propulsion system 1. Indeed, the smaller the mounting radius R, the greater the surface clearance, but the less effective the damping. A mode 25 damper exhibiting a surface viscosity between 10A 8 rrr 1 and 2 x 10 A 9 rrr 1 and a surface clearance between 5*10 A -4 rrr 1 and 7.5*10 A -3 rrr 1 is therefore capable of sufficiently reducing the vibration response of the moving element 35 around which it is mounted, while respecting strict space requirements, thanks to the adaptation of the axial length and radial play of the mode damper 25.

[0104] Preferably, the surface clearance of the mode 25 damper is less than or equal to 5.8 x 10 A -3 m- 1 .

[0105] The present exposition applies mainly to propulsion systems in which at least one of the shafts (low pressure shaft 11, blower shaft 20, high pressure shaft 10) has a single natural mode of deformation within the operating range of the propulsion system 1. In general, this will be more particularly the low pressure shaft 11 (in the case of direct drive propulsion systems or systems including a reduction mechanism 19).

[0106] A first example of a propulsion system 1 to which the present description applies is an engine for business jets, comprising a shrouded fan rotor 9 with a diameter Dg of 60 inches (1.524 meters (m) or less (and preferably 0.55 m or more) and driven directly (without a speed reduction mechanism) by a drive turbine with a limiting speed of between 5,000 and 12,000 revolutions per minute (rpm), preferably between

[0107] 6,000 and 10,000 rpm. The blower rotor 9 therefore rotates at the same speed as the drive turbine. The maximum speed of the high-pressure shaft, on the other hand, can be between 20,000 rpm and 35,000 rpm. The bypass ratio of motor 1 is less than or equal to

[0108] 7 and its overall compression ratio is less than or equal to 48. The engine 1 is further configured to provide a thrust of between 8 kN and 110 kN, preferably between 10 kN and 90 kN, for example between 25 kN and 85 kN. In this first example of a propulsion system 1, the mode damper 25 can be mounted on at least one bearing of the low-pressure shaft. For example, the mode damper 25 can be mounted on the forward bearing 11a and on the downstream bearing 11c, which is mounted on the exhaust housing; these two bearings can, for example, be roller or ball bearings.

[0109] The invention applies, however, to any type of bearing capable of including a mode 25 damper, and in particular to roller bearings, ball bearings or tapered roller bearings.

[0110] It should be noted here that, regardless of the configuration of the propulsion system 1, the diameter Dg of the fan rotor 9 is measured in a plane normal to the axis of rotation of the fan rotor, at the intersection between a vertex 21 and a leading edge 22 of the blades 14 of the fan rotor 9, and is expressed in meters (m). Note that since Figures 1 and 2 are partial views, the diameter Dg is only partially visible.

[0111] The limiting speed of the low-pressure shaft 11 corresponds to the absolute maximum speed that the low-pressure shaft 11 is likely to encounter during the entire flight (according to the European certification regulation EASA CS-E 740 (or according to the American certification regulation 14-CFR Part 33)). The limiting speed corresponds to the maximum rotational speed when the propulsion system 1 is functioning correctly. It is therefore the speed that the low-pressure shaft 11 is likely to reach under flight conditions. This limiting speed is part of the data declared in the engine certification (the "type certificate data sheet"). Indeed, this rotational speed is usually used as a reference speed for the sizing of propulsion systems 1 and in certain certification tests (such as blade loss or rotor integrity tests).

[0112] It should be noted that this example of a propulsion system presents severe mechanical integration constraints (related in particular to the diameters of the fan section and the high-pressure body, which are smaller than on conventional engines) leading to constrained bearing environments (bearings supporting the shafts close to the X axis).

[0113] Examples of sizing for mode 25 shock absorbers, particularly suitable for assemblies 1 to 14 including a bearing supporting the low-pressure shaft of a propulsion system 1, are as follows:

[0114] These examples of mode 25 dampers all reduce the vibration response of the shaft around which they are mounted and can be implemented in propulsion systems conforming to the first engine example described above.

[0115] In these examples, Mode 25 dampers Nos. 1 through 4 exhibit high surface viscosity and low surface clearance, which optimizes the damping of bearing deformation modes, particularly in propulsion systems such as the engines described in the example above. Their low surface clearance also helps to limit cavitation effects. Mode 25 dampers Nos. 3 and 8 are particularly optimized in terms of size (small mounting radius and axial length) and damping (high surface viscosity and low surface clearance).

[0116] Mode 25 dampers #1, 4-6, and 9, on the other hand, feature optimized surface clearance. However, their radial dimensions remain larger than those of the other Mode 25 damper examples listed in the table.

[0117] Manufacturing process

[0118] An assembly for a propulsion system conforming to this exposition can be manufactured in accordance with the following steps, after defining the damping requirement of the mode 25 damper (surface viscosity) and specifying the geometry of the mode 25 damper (in particular its axial length L and its maximum radial clearance C as a function of the defined surface viscosity).

[0119] A mode 25 damper comprising an inner ring 30 and an outer ring 32 is dimensioned such that the mode 25 damper has the maximum axial length and radial clearance such that the surface viscosity of the mode 25 damper is between 10 A 8 rrr 1 and 2 x 10 A 9 rrr 1

[0120] The mode damper 25 is then manufactured and assembled with a movable element 35, for example a bearing, so that the inner ring 30 and the outer ring 32 are coaxial with the bearing and the first ring (which are movable around the X axis) of the bearing and that the inner ring 30 extends radially between the bearing and the outer ring 32.

[0121] A fluid 29 is then injected under pressure into the damping chamber 34 of the mode damper 25, between the inner ring 30 and the outer ring 32.

[0122] Comparative example:

[0123] Engine 1 is a twin-spool, twin-flow propulsion system comprising a shrouded fan section 2 conforming to the current technical standard (at the date of filing of this application), which we seek to improve. Engine 2 is a twin-spool, twin-flow propulsion system 1 comprising a shrouded fan section 2 conforming to the teachings of this application, including a low-pressure shaft supported by three bearings.

[0124] Engines 1 and 2 have the following elements in common: - the architecture includes an inter-shaft bearing;

[0125] - the low pressure and high pressure shafts are counter-rotating;

[0126] - the engines do not have a low-pressure compressor (the low-pressure shaft only drives the blower rotor);

[0127] - the high-pressure compressor comprises four axial stages and a centrifugal impeller; - the high-pressure turbine is single-stage;

[0128] - the low pressure shaft is supported by three bearings in total, including two front bearings upstream of the combustion chamber and one rear bearing downstream of the combustion chamber;

[0129] - one of the front bearings of the low-pressure shaft and the rear bearing are roller bearings including a mode damper; and - the low-pressure shaft is supercritical with respect to the high-pressure speed range.

[0130] Engine 1 does not include a mode damper on the frontmost bearing of the low-pressure shaft (at the blower section). Engine 2 includes a mode damper on each bearing of the low-pressure shaft.

[0131] Compared to engine 1, the fan rotor diameter of engine 2 has been increased. The fan section compression ratio has been reduced and its bypass ratio increased. The overall compression ratio of engine 2 has been maintained, as has the high-pressure turbine inlet temperature. Engine 2 therefore offers an advantage in terms of specific fuel consumption through increased component efficiencies and propulsive efficiency, which allows for an increased aircraft range.

[0132] However, using a larger diameter fan and turbine results in increased vibration levels. Without managing the surface viscosity of the mode dampers, these vibration levels would be unacceptable in a business aviation context. Adding the mode damper to the forward bearing adjacent to the fan and adapting the mode damper on the rear bearing (at the low-pressure turbine) so that the surface viscosity of the mode dampers complies with formula (1) makes it possible to obtain vibration levels identical to those of engine 1 while allowing the integration of the mode dampers into the turbomachine.

Claims

DEMANDS 1. Assembly (11a, 11b, 11c) of an aeronautical propulsion system (1) comprising: - a movable element (35) rotating about an axis (X) of the assembly (11a, 11b, 11c); and - a mode damper (25) comprising an inner ring (30), an outer ring (32) and a fluid (29) confined between the inner ring (30) and the outer ring (32), the inner ring (30) and the outer ring (32) being coaxial with the moving element (35); the inner ring (30) extending radially between the moving element (35) and the outer ring (32); and the mode damper (25) having an axial length and a maximum radial clearance such that a surface viscosity of the mode damper (25), which corresponds to the ratio of the square of the axial length to the product of the cube of the maximum radial clearance multiplied by twice the value of Pi, is between 1.0 and 10 A 8 m 1 and 2.0 x 10 A 9 m 1 : 1.0 x 10 8 2.0 x 10 9 where: L is the axial length of the mode (25) damper and is expressed in meters (m); and C is the maximum radial play of the mode damper (25) and is expressed in meters (m).

2. Assembly (11a, 11b, 11c) according to claim 1, wherein the surface viscosity is greater than or equal to 2.0 x 10 A 8 m -1 .

3. Assembly (11a, 11b, 11c) according to any one of claims 1 and 2, wherein the mode damper (25) further has a surface clearance, which corresponds to the ratio between the maximum radial clearance and the product between twice the value of Pi multiplied by the mounting radius of the mode damper (25) and the axial length of the mode damper (25), is between 5.0 x 10 A -4 m -1 and 7.5 x 10 A - 3 m -1 : 5.0 x 10" 4 < surface area 7.5 x 10 -3 where R is the implantation radius of the mode (25) damper and is expressed in meters (m).

4. Assembly (11a, 11b, 11c) according to claim 3, wherein the surface clearance is less than or equal to 5.8 x 10 A -3 m -1 .

5. Assembly (1 1a, 11 b, 1 1c) according to any one of claims 1 to 4, wherein the moving element (35) comprises a moving part of a bearing of the propulsion system (1).

6. Assembly (11a, 11b, 11c) according to claim 5, wherein the mode damper (25) further comprises at least one sealing segment (34a, 34b), preferably two sealing segments (34a, 34b), axially delimiting the fluid (29) between the inner ring (30) and the outer ring (32).

7. Aeronautical propulsion system comprising an assembly (11a, 11b, 11c) according to any one of claims 1 to 6 and a rotor coaxial with the moving element (35).

8. Aeronautical propulsion system according to claim 7, wherein the assembly (11a, 11b, 11c) comprises a bearing, the bearing further comprising an additional ring mounted on the rotor.

9. Aeronautical propulsion system (1) according to any one of claims 7 and 8, further comprising: - a blower section comprising a rotor, a diameter (Dg) of the blower rotor (9) being less than or equal to 1.524 meters (m) (60 inches), the blower section being able to be enclosed; and - a drive turbine (8) configured to directly drive the blower shaft (20) in rotation around the axis (X) of rotation (X).

10. Aircraft (100) comprising at least one propulsion system (1) according to any one of claims 7 to 9 fixed to the aircraft by means of a mast.

11. Method for manufacturing an assembly (11a, 11b, 11c) of an aeronautical propulsion system (1) comprising the following steps: - determine an axial length and a maximum radial clearance of a mode damper (25) such that a surface viscosity of the mode damper (25), which corresponds to the ratio between the square of the axial length and the product between the maximum radial clearance cubed multiplied by twice the value of Pi, is between 10 A 8 m -1 and 2 x 10 A 9 m -1 : L 2 10 8 < - - < 2 x 10 2nC 3 9 where: L is the axial length of the mode (25) damper and is expressed in meters (m); and C is the maximum radial play of the mode damper (25) and is expressed in meters (m); - manufacture an inner ring (30) and an outer ring (32) of the mode damper (25) according to the axial length and radial clearance thus determined; - assemble the inner ring (30) and the outer ring (23) with a movable element (35); and - inject a fluid (29) between the inner ring (30) and the outer ring (32).

12. A manufacturing method according to claim 11, wherein the axial length and radial clearance are determined such that the surface viscosity is greater than or equal to 2 x 10 A 8 m -1 .

13. A manufacturing method according to any one of claims 11 and 12, wherein the axial length, radial clearance, and mounting radius are further determined such that a surface clearance of the mode damper (25) is between 5*10 A -4 m -1 and 7.5*10 A -3 m -1 , where the surface clearance is defined as follows: C AC surf set = - — — - TIKL where R is an implantation radius of the mode damper (25) and is expressed in meters (m).

14. A manufacturing method according to claim 13, wherein the axial length, radial clearance, and mounting radius are determined such that the surface clearance is less than or equal to 5.8 x 10 A -3 m- 1 .

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