LOW PRESSURE TURBINE WITH ACTIVE AXIAL PLAY CONTROL SYSTEM VIA HYDRAULIC CYLINDER AND METHOD FOR ACTIVE AXIAL PLAY CONTROL OF A LOW PRESSURE TURBINE

Abstract

LOW PRESSURE TURBINE WITH ACTIVE AXIAL CLEARANCE CONTROL SYSTEM BY HYDRAULIC CYLINDER AND METHOD OF ACTIVE AXIAL CLEARANCE CONTROL OF A LOW PRESSURE TURBINE Low pressure turbine of an aircraft turbomachine comprising: a turbine shaft (40) including a recess (44); a compressor shaft (41), including a tooth (43) extending radially from the compressor shaft (41) and housed at least partially in the recess (44); an axial clearance control system between the turbine shaft (40) and the compressor shaft (41), said control system comprising: an upstream hydraulic cylinder (46); a downstream hydraulic cylinder (47); a fluidic circuit (48) connecting the upstream hydraulic cylinder and the downstream hydraulic cylinder, Each cylinder (46, 47) having a cylinder body (46a, 47a), each cylinder body (46a, 47a) comprising a first chamber (46a.1, 47a.1) and a second chamber (46a.2, 47a.2), the fluidic circuit (48) connecting the second chamber of the upstream hydraulic cylinder (46) to the second chamber of the downstream hydraulic cylinder (47). [FIG. 5].

Description

Title of the invention: LOW PRESSURE TURBINE WITH ACTIVE AXIAL CLEARANCE CONTROL SYSTEM BY HYDRAULIC CYLINDER AND METHOD FOR ACTIVELY CONTROLLING THE AXIAL CLEARANCES OF A LOW PRESSURE TURBINE TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the field of aeronautics and particularly to the field of active control systems for the axial clearances of a low pressure turbine of an aircraft turbomachine.

[0002] The invention relates to a low pressure turbine with an active axial clearance control system using a hydraulic cylinder. Another subject of the invention is a method for controlling axial clearances of a low pressure turbine. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] An aeronautical turbomachine conventionally comprises a compressor, a combustion chamber and a turbine. The role of the turbine is to ensure the rotational drive of the compressor by taking part of the pressure energy from the hot gases leaving the combustion chamber and transforming it into mechanical energy.

[0004] A compressor consists of a rotating part, a rotor comprising an axis of rotation, a fixed part, the stator, and a casing, the casing and the stator being integral with each other. The rotor comprises a drum consisting of an assembly of several discs on which rotating blades are circumferentially fixed. The stator consists of a plurality of fixed blades (because they do not rotate around the axis of rotation of the rotor but can be mobile in rotation along their own axis) fixed circumferentially on the casing or on shells. Each row of fixed blades of the stator, called rectifiers, constitutes a rectifier or distributor. A row of moving blades and a row of fixed blades form a compressor stage.

[0005] In operation, the thermal inertia and the forces applied to the stator line imply a greater expansion of the stator line than of the rotor line of the low pressure turbine. It is therefore necessary to size the clearances between the rotor line and the stator line in order to maintain positive clearances at all times during a mission and to prevent the two assemblies from coming into contact.

[0006] On the other hand, these games involve leaks and therefore a loss of engine efficiency which can lead to a drop in performance and an increase in fuel requirements during a flight mission.

[0007] [Fig.l] illustrates the axial displacements y as a function of time t during a flight mission. Curve 10 shows the displacements of the stator line, curve 11 shows the displacements of the rotor line. Curve 12 illustrates the differential displacement (stator - rotor). A positive displacement means a shift of the assembly considered downstream of the axial flow. The positive values ​​of the differential displacement 12 illustrate that the displacement of the stator part is greater than that of the rotor part, which implies a closing of the clearances where the stator and upstream of the rotor.

[0008] In order to avoid contact on this type of clearance, it is therefore necessary to provide a sufficient margin to absorb the relative displacement of the stator towards the downstream side of the flow. [Fig.2] illustrates an example of clearances 20 of non-contact between the rotor part 21 and the stator part 22. Such a type of clearance, however, involves air losses from the main stream to the outside. To limit them, the designer ensures axial overlap between the rotor and stator parts during operation. As illustrated in [Fig.3], in order to achieve axial overlap 30; spoilers 31 of varying length are added to the low-pressure distributors. The length of these spoilers will be limited by the risk of contact between the rotor and the stator.

[0009] The timing between the rotor part and the stator part is currently ensured by a rotor / stator adjustment shim, for example a D48 type shim. As illustrated in [Fig.4], the rotor / stator adjustment shim, in this case a D48 shim, is mounted upstream of the rotor part and makes it possible to maintain the same timing between the low-pressure turbine shaft 40 and the low-pressure compressor shaft 41, the inter-shaft timing being ensured by a tightening nut 42.

[0010] This solution is however not satisfactory because it does not allow compensation, during the different phases of a flight mission, for the differential displacement between the rotor part and the stator part. Summary of the invention

[0011] The invention provides a solution to the problems mentioned above by proposing a low pressure turbine with an active axial clearance control system.

[0012] A first aspect of the invention relates to a low pressure turbine comprising: • a turbine shaft having an axis of rotation X and comprising a recess delimited by an upstream axial stop element and a downstream axial stop element; • a compressor shaft, said compressor shaft being radially external to the turbine shaft and comprising a tooth extending radially from the compressor shaft towards the turbine shaft and housed at least partially in the recess; • an axial clearance control system between the turbine shaft and the compressor, said control system comprising: - an upstream hydraulic cylinder housed at least partially in the recess and positioned upstream of the tooth of the compressor shaft; - a downstream hydraulic cylinder housed at least partially in the recess and positioned downstream of the tooth of the compressor shaft; - a fluid circuit (48) connecting the upstream hydraulic cylinder and the downstream hydraulic cylinder;

[0013] Each cylinder having a cylinder body and a rod, the rod sliding in a direction parallel to the X axis, each cylinder body comprising a first chamber and a second chamber, the fluid circuit connecting the second chamber of the upstream hydraulic cylinder to the second chamber of the downstream hydraulic cylinder.

[0014] In the description, the terms “upstream” and “downstream” are defined in relation to the direction of air flow from the air inlet into the turbomachine to the air outlet from the turbomachine.

[0015] In the description, the low-pressure turbine comprises the compressor shaft integral in rotation with the turbine shaft. One aspect of the invention therefore relates to this set of shafts comprising the turbine shaft, the compressor shaft, and the axial clearance control system as defined above.

[0016] The term "first and second cylinder body chamber" means the two parts of the cylinder body separated by a piston fixed, in a known manner, to the end of the rod sliding inside the cylinder body. The piston present inside the cylinder body makes it possible to obtain sealing between the two chambers and to hydraulically actuate the cylinder rod.

[0017] The term “second chamber of the cylinder body” means the chamber comprising a portion of rod sliding inside the cylinder body. The term “first chamber” means the chamber opposite the second chamber and separated from it by the piston.

[0018] In other words, an injection of hydraulic fluid into the first cylinder chamber allows the cylinder to expand by increasing its length. An injection of hydraulic fluid into the second cylinder chamber allows the cylinder to shrink by decreasing its length.

[0019] Each hydraulic cylinder is in contact with an axial stop element and the tooth of the compressor shaft respectively. The two upstream and downstream hydraulic cylinders may be identical and their cylinder bodies may have the same dimensions.

[0020] The system for controlling the axial clearances of the low pressure turbine according to one aspect of the invention makes it possible to wedge the rotor relative to the stator "when hot", namely during operation of the low-pressure turbine, for example during a flight mission. This is achieved by installing two shims instead of just one as in the prior art. These two shims are here two hydraulic cylinders which can therefore be controlled using the fluid circuit. In other words, the two shims can expand or contract controlled by the fluid circuit.

[0021] By positioning the compressor shaft tooth on either side, the upstream hydraulic cylinder and the downstream hydraulic cylinder cooperate to shift the compressor shaft upstream or downstream relative to the turbine shaft. This amounts to shifting the rotor relative to the stator upstream or downstream and makes it possible to maintain a constant distance between the rotor part and the stator part.

[0022] It is important to note that, thanks to the fluid circuit connecting the second cylinder chambers together, an expansion of the downstream hydraulic cylinder by a length l corresponds to a narrowing of the upstream hydraulic cylinder by the same length l. Indeed, the hydraulic fluid expelled from the second chamber of the downstream hydraulic cylinder during its expansion is injected into the second chamber of the upstream cylinder, causing it to narrow.

[0023] In the same way, a narrowing of the downstream hydraulic cylinder by a length l corresponds to an expansion of the upstream hydraulic cylinder by the same length l. Thanks to this arrangement of the fluid circuit, the sum of the lengths of the two hydraulic cylinders remains constant, which ensures inter-shaft clamping for any offset of the stator relative to the rotor.

[0024] For example, the axial length of the hydraulic cylinder is understood to mean the distance separating the end of the rod outside the cylinder body and the wall of the first chamber opposite the piston, measured in the sliding direction of the rod. Alternatively, it is possible to define the axial length of the cylinder as the sum of the axial length of the cylinder body and the length of the portion of rod outside the cylinder body.

[0025] In one or more embodiments, the upstream axial stop element is an inter-shaft tightening nut; this makes it possible to use an element already in place in any low-pressure turbine using a static shim.

[0026] In one or more embodiments, the downstream axial stop element is a flange formed in the turbine shaft.

[0027] In one or more embodiments, the upstream cylinder body and the downstream cylinder body are shrunk to the low pressure turbine shaft; this allows axial movement of the cylinder bodies along the turbine shaft.

[0028] In one or more embodiments: • each hydraulic cylinder comprises a piston fixed to one end of the rod sliding inside the cylinder body and separating the cylinder body into a first and second cylinder chambers positioned respectively on either side of the piston, the second cylinder chamber comprising a portion of cylinder rod; • each cylinder chamber includes an opening allowing the passage of a hydraulic liquid circulating in the fluid circuit, each cylinder chamber being connected to the fluid circuit.

[0029] This arrangement makes it possible to connect the second chambers of the cylinder bodies together so as to keep the sum of the lengths of the two cylinders constant. Furthermore, this arrangement also makes it possible to inject hydraulic fluid into the first chambers so as to expand the hydraulic cylinders if necessary.

[0030] In one or more embodiments, the fluid circuit comprises a valve for each cylinder chamber to be controlled; each valve makes it possible to independently actuate a different cylinder chamber and therefore to offset the stator line relative to the rotor line;

[0031] In one or more embodiments, each valve is a solenoid valve;

[0032] In one or more embodiments, each solenoid valve is connected to a digital flight parameter control system; this feature allows the use of already deployed automatic flight parameter management systems such as FADEC or "Full Authority Digital Engine Control" in English terminology.

[0033] Another aspect of the invention relates to a method for controlling axial clearances between a turbine shaft and a compressor shaft in a low pressure turbine according to the first aspect of the invention, said control method comprising the following steps: • Determination of a target distance between a stator element and a rotor; • Measurement of the distance between the stator element and the rotor element; • Determination of the difference between the target distance and the measured distance; • Actuation, based on the determined deviation, of the fluid circuit of the low pressure turbine to modify the axial lengths of the upstream hydraulic cylinder and the downstream hydraulic cylinder so as to reduce the difference determined during the determination step, the sum of the axial lengths of the cylinders being constant.

[0034] Thanks to the method according to the second aspect of the invention, it is possible to actively control the distance between the rotor elements and the stator elements, so as to compensate for the differential displacement between these two series of elements. The method according to the second aspect of the invention makes it possible to adapt the axial clearances between the shafts so as to avoid contact between the stator elements and the rotor elements, while avoiding the deterioration of engine performance.

[0035] Thanks to the arrangement of the fluid circuit, an expansion of one of the two cylinders corresponds to a simultaneous contraction of the other cylinder, so as to keep the sum of the axial lengths of the cylinders constant and ensure inter-shaft clamping for any offset of the rotor part relative to the stator part.

[0036] In one or more embodiments, the distance between the rotor element and the stator element is measured using a distance sensor or a contact sensor.

[0037] In one or more embodiments, the step of actuating the fluid circuit comprises the actuation of one or more valves allowing the injection of hydraulic fluid into one of the first chambers of the hydraulic cylinders and the passage of hydraulic fluid between the second chambers of the hydraulic cylinders.

[0038] In one or more embodiments, the actuation step is performed using an automatic flight parameter control system.

[0039] The invention and its various applications will be better understood upon reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0040] Other advantages and characteristics of the invention will appear on reading the following description, illustrated by the figures among which:

[0041] [Fig. 1] illustrates a graph representing the axial displacements of the rotor part and the stator part of a low pressure turbine according to the state of the art;

[0042] [Fig.2] schematically illustrates the axial clearances between the rotor part and the stator part in a turbomachine according to the state of the art;

[0043] [Fig. 3] illustrates an example of axial overlap between the rotor part and the stator part in a low pressure turbine according to the state of the art;

[0044] [Fig.4] schematically illustrates an example of inter-shaft timing in a low-pressure turbine according to the state of the art;

[0045] [Fig.5] illustrates an embodiment of a low pressure turbine with an active axial clearance control system according to the first aspect of the invention;

[0046] [Fig.6] schematically illustrates a sectional view of an upstream hydraulic cylinder and a downstream hydraulic cylinder used in the example illustrated in [Fig.5];

[0047] [Fig.7] schematically illustrates a flowchart of an embodiment of a method according to the second aspect of the invention;

[0048] [Fig.8] illustrates a graph representing the axial displacements of the rotor part and the stator part of a low pressure turbomachine according to the first aspect of the invention. DETAILED DESCRIPTION

[0049] Figures 1, 2, 3 and 4 have been described in relation to the state of the art.

[0050] Unless otherwise specified, the same element appearing in different figures has a single reference.

[0051] For the understanding of the invention, the radial R, tangential T and axial A orientations will be adopted according to the reference RTA indicated in the figures, the tangent T and axial A axes of which extend in a horizontal plane according to the orientation in the figures. The axial axis A is parallel to an axis of rotation X of an aircraft turbomachine comprising the axial clearance control system according to one aspect of the invention.

[0052] [Fig.5] illustrates an embodiment of a low pressure turbine according to one aspect of the invention.

[0053] According to the example illustrated in [Fig.5], the low-pressure turbine comprises a turbine shaft 40 and a compressor shaft 4L. The compressor shaft 41 is radially external to the low-pressure turbine shaft 40. The two shafts are coaxial. The turbine shaft 40 and the compressor shaft 41 are secured in rotation by splines 40.1.

[0054] The low pressure turbine shaft 40 comprises a recess 44 delimited axially by an upstream axial stop element 42 and a downstream axial stop element 45.

[0055] According to the embodiment illustrated in [Fig. 5], the upstream axial stop element 42 is an inter-shaft tightening nut and the downstream axial stop element is a rim of the turbine shaft 40.

[0056] The recess 44 is sized to accommodate: • a tooth 43 of the compressor shaft extending radially from the low pressure compressor shaft and being at least partially housed in the recess 44; • an upstream hydraulic cylinder 46 positioned upstream of the tooth 43 and at least partially housed in the recess 44, the upstream hydraulic cylinder 46 is in contact respectively with the upstream axial stop element 42 and an upstream face of the tooth 43; • a downstream hydraulic cylinder 47 positioned downstream of the tooth 43 and at least partially housed in the recess 44; the downstream hydraulic cylinder 47 is in contact respectively with the downstream axial stop element 45 and a downstream face of the tooth 43.

[0057] The two hydraulic cylinders 46 and 47 form, with a fluidic system 48, an active control system for the axial clearances between the rotor part and the stator part of the low pressure turbine.

[0058] As illustrated in [Fig.5], the fluid circuit 48 comprises 4 conduits 48.1, 48.2, 48.3 and 48.4. The conduits 48.1 and 48.2 are connected respectively to a first chamber and a second chamber of the upstream hydraulic cylinder 46. The conduits 48.3 and 48.4 are connected respectively to a first chamber and a second chamber of the downstream hydraulic cylinder 47. Each conduit comprises a valve B, C, D or E controlling the flow of hydraulic fluid respectively in the conduits 48.1, 48.2, 48.3 and 48.4. The conduits 48.1, 48.2, 48.3 and 48.4 extend from a single hydraulic fluid inlet conduit, said single conduit being controlled by a valve A. Alternatively, according to an embodiment not illustrated in [Fig.5], the fluid circuit may comprise only 2 alternative valves to connect the primary chamber of one cylinder with the secondary chamber of the other. This fluid circuit makes it possible to maintain the sum of the lengths of the upstream hydraulic cylinder and the downstream hydraulic cylinder.

[0059] [Fig.6] illustrates an example of an embodiment of the two upstream 46 and downstream 47 hydraulic cylinders.

[0060] The upstream hydraulic cylinder 46 comprises a cylinder body 46a and a rod 46b. The sliding direction of the rod 46b is parallel to the axis of rotation X. A piston 46c fixed to one end of the rod 46b delimits a first chamber 46a. 1 and a second chamber 46a.2. The second chamber 46a.2 is defined as being the chamber comprising a portion of rod 46b sliding inside the cylinder body 46a. Each chamber 46a. 1 and 46a.2 comprises an opening, respectively 46d. 1 and 46d.2. These two openings allow the connection of respectively two conduits 48.1 and 48.2 of the fluid circuit 48.

[0061] In the example illustrated in [Fig.6], the downstream hydraulic cylinder 47 comprises a cylinder body 47a and a rod 47b. The sliding direction of the rod 47b is parallel to the axis of rotation X. A piston 47c fixed to one end of the rod 47b delimits a first chamber 47a.1 and a second chamber 47a.2. The second chamber 47a.2 is defined as being the chamber comprising a portion of rod 47b sliding inside the cylinder body 47a. Each chamber 47a.1 and 47a.2 comprises an opening, respectively 47d.1 and 47d.2. These two openings allow the connection of respectively two conduits 48.3 and 48.4 of the fluid circuit 48.

[0062] It is important to note that the fluid circuit 48 is arranged so as to connect the second chambers 46d.2 and 47.2 together. Thanks to this arrangement, each expansion of one of the two cylinders corresponds to a narrowing of the other cylinder, so as to keep the sum of the lengths of the two cylinders constant.

[0063] This mechanism makes it possible to control the axial offset of the rotor part relative to the stator part while maintaining inter-shaft clamping.

[0064] It is important that the mounting direction of the two cylinders shown in [Fig.6] is not essential as long as the connections between the chambers of the two cylinders are made as explained above. For example, the second chamber 46a.2 of the upstream hydraulic cylinder could be placed upstream - therefore in contact of the upstream axial stop element - without the operation of the device according to the first aspect of the invention being altered.

[0065] Indeed, the connection between the second chambers of the two cylinders ensures that the sum of the lengths of the cylinders remains constant for any elongation of the two cylinders.

[0066] Intentional shift of the turbine rotor downstream

[0067] Hydraulic fluid is injected under pressure into the inlet 47d.l of the downstream hydraulic cylinder 47 in order to lengthen it. This will have the effect of causing hydraulic fluid to exit the second chamber of the downstream hydraulic cylinder. The second chamber of the downstream hydraulic cylinder being connected to the second chamber of the upstream hydraulic cylinder, the hydraulic fluid will be injected into the second chamber of the upstream hydraulic cylinder while reducing its length. This injection is carried out by opening, for example, the valve C. Thanks to the fact that the sum of the lengths of the two hydraulic cylinders remains constant, the tightening of the nuts is ensured.

[0068] Intentional shift of the turbine rotor upstream

[0069] Hydraulic fluid is injected under pressure into the first chamber of the upstream hydraulic cylinder in order to lengthen it. This will have the effect of causing hydraulic fluid to exit from the second chamber of the upstream hydraulic cylinder. This hydraulic fluid will therefore be injected, for example by opening the valve E) in the second chamber of the downstream hydraulic cylinder so as to reduce its length.

[0070] According to one embodiment, the valves of the fluid circuit 48 are solenoid valves.

[0071] According to one embodiment, the expansions and contractions of the two hydraulic cylinders are managed by a flight parameter control system such as the FADEC system.

[0072] [Fig.7] illustrates an example of an embodiment of a method 90 for controlling axial clearances between a turbine shaft and a compressor shaft in a low pressure turbine according to another aspect of the invention.

[0073] As illustrated in [Fig.7], the method 90 according to one aspect of the invention comprises a step of determining a target distance between an element of the stator and an element of the rotor.

[0074] The target distance can be determined for example as a function of the phase of the flight mission or as a function of other engine operating parameters.

[0075] The method 90 further comprises a step of measuring the distance between the stator element and a rotor element. The distance is for example measured using a sensor, such as an optical sensor or a contact sensor.

[0076] In step 93, the difference between the target distance and the measured distance is determined. Comparing the two distances makes it possible to determine whether an offset of the part rotor relative to the stator part is necessary in order to maintain the target distance.

[0077] The actuation step 94 of the fluid circuit is carried out on the basis of the difference determined during step 93. During the actuation step 94 of the fluid circuit the axial lengths of the upstream hydraulic cylinder and the downstream hydraulic cylinder are modified simultaneously so as to move the rotor part relative to the stator part while maintaining the sum of the axial lengths of the cylinders constant.

[0078] The actuation step 94 may for example comprise the injection of hydraulic fluid into one of the first chambers of the cylinders and the passage of hydraulic fluid between the second chambers of the cylinders.

[0079] [Fig.8] shows a graph representing the difference between the axial displacements y of the stator part and the rotor part as a function of time t during a flight mission, namely the gap between an element of the stator line and an element of the rotor line as a function of time. Curve 80 shows the difference in axial displacements in the absence of an active control system according to one aspect of the invention. The offset between the stator part and the rotor part varies over time and is always positive. Curve 81 shows the same quantity in the presence of the active axial clearance control system according to the invention. It is obvious that in a low pressure turbine according to the invention the distance between the rotor part and the stator part is constant thanks to the dynamic compensation of the axial offset.

Claims

Claims

1. Low pressure turbine of an aircraft turbomachine, the low pressure turbine comprising: • a turbine shaft (40) having an axis of rotation X and comprising a recess (44) delimited by an upstream axial stop element (42) and a downstream axial stop element (45); • a compressor shaft (41), said compressor shaft (41) being radially external to the turbine shaft (40) and comprising a tooth (43) extending radially from the compressor shaft (41) towards the turbine shaft (40) and housed at least partially in the recess (44); • a system for controlling axial clearances between the turbine shaft (40) and the compressor shaft (41), said control system comprising:

1. an upstream hydraulic cylinder (46) housed at least partially in the recess (44) and positioned upstream of the tooth (43) of the compressor shaft (41); 2. a downstream hydraulic cylinder (47) housed at least partially in the recess (44) and positioned downstream of the tooth (43) of the compressor shaft (41); 3. a fluid circuit (48) connecting the upstream hydraulic cylinder and the downstream hydraulic cylinder; In which each cylinder (46, 47) has a cylinder body (46a, 47a) and a rod (46b, 47b), the rod sliding (46b, 47b) in a direction parallel to the X axis, each cylinder body (46a, 47a) comprising a first chamber (46a.1, 47a.1) and a second chamber (46a.2, 47a.2), the fluid circuit (48) connecting the second chamber of the upstream hydraulic cylinder (46) to the second chamber of the downstream hydraulic cylinder (47).

2. Low pressure turbine according to the preceding claim in which the upstream axial stop element (42) is an inter-shaft tightening nut.

3. Low pressure turbine according to the preceding claim in which the downstream axial stop element (45) is a rim formed in the turbine shaft (40).

4. Low pressure turbine according to one of the preceding claims in which the upstream cylinder body (46a) and the downstream cylinder body (47a) are shrunk to the low pressure turbine shaft (40).

5. Low pressure turbine according to one of the preceding claims wherein - each hydraulic cylinder (46, 47) comprises a piston (46c, 47c) fixed to one end of the rod (46b, 47b) sliding inside the cylinder body (46a, 47a) and separating the cylinder body (46a, 47a) into a first (46a. 1, 47a. 1) and a second cylinder chamber (46a. 2, 47a. 2) positioned respectively on either side of the piston (46c, 47c), the second cylinder chamber (46a. 2, 47a. 2) comprising a portion of cylinder rod (46b, 47b); - each cylinder chamber (46a. 1, 47a. 1, 46a. 2, 47a. 2) comprises an opening (46d. 1, 47d. 1, 46d. 2, 47d. 2) allowing the passage of a hydraulic liquid circulating in the fluid circuit (48), each cylinder chamber being connected to the fluid circuit.

6. Low pressure turbine according to the preceding claim in which the fluid circuit comprises a valve (B, C, D, E) for each cylinder chamber.

7. Low pressure turbine according to the preceding claim in which each valve (B, C, D, E) is a solenoid valve.

8. Low pressure turbine according to the preceding claim in which each solenoid valve is connected to a digital flight parameter control system.

9. Method (90) for controlling axial clearances between a turbine shaft and a compressor shaft in a low pressure turbine according to one of the preceding claims comprising the following steps: - Determining (91) a target distance between an element of the stator and an element of the rotor; - Measuring (92) the distance between the stator element and the rotor element; - Determining (93) the difference between the target distance and the measured distance; - Actuating (94), on the basis of the determined difference, the fluid circuit of the low-pressure turbine according to one of the preceding claims to modify the axial lengths of the upstream hydraulic cylinder and the downstream hydraulic cylinder so as to reduce the difference determined during the determination step (93), the sum of the axial lengths of the cylinders being constant.

10. Method (90) according to the preceding claim in which the step of actuating (94) the fluid circuit comprises the actuation of one or more valves allowing the injection of hydraulic fluid into one of the first chambers of the hydraulic cylinders and the passage of hydraulic fluid between the second chambers of the hydraulic cylinders.

11. Method (91) according to the preceding claim in which the actuation step (94) is carried out using an automatic flight parameter control system.

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