Instrumented flow passage of a turbine engine

Abstract

An annular air flow passage includes two radially internal and external annular walls. An element is elongated in a direction between the internal and external annular walls and a first of the internal or external ends of the element is fixed rigidly to a first of the internal or external walls. The center of gravity position of the element is variable.

Claims

1. An annular air flow passage comprising two radially internal and external annular walls, wherein an element is elongated in a direction between the internal and external annular walls and a first of the internal or external ends of the element is fixed rigidly to a first of the internal or external walls, wherein said annular flow passage comprises means of control of means of variation, along said direction, of the position of the element's center of gravity, wherein the means of variation of the center of gravity position comprise means of movement of a mass along said direction of elongation, wherein the means of movement comprise a rod mounted movably in translation, according to the direction of elongation, in a cavity of the element, wherein this rod bears the mass, and wherein a first end of the ends of the rod passes through said first wall and is connected to means of measurement of the position of the mass along the element elongated in the cavity.

2. The annular airflow passage of claim 1, wherein said mass is arranged on a second end of the rod opposite the first end.

3. The annular airflow passage of claim 1, wherein the rod comprises weights spaced along the rod.

4. The annular airflow passage of claim 3, wherein the second end of the element elongated opposite said first end is free.

5. A turbine engine comprising the annular air flow passage of claim 1 and an annular row of mobile blades driven in rotation by a rotor, wherein said row is arranged in the flow passage upstream from said elongated element.

6. A method for shifting the center of gravity of the element of the annular flow passage of claim 1, comprising the steps of: determining a first frequency f.sub.1 corresponding to the vibration frequency of said element; b) determining a second frequency fr.sub.1 corresponding to the first natural frequency of said element; c) calculating the absolute value |f.sub.1fr.sub.1|; d) performing a comparison of |f.sub.1fr.sub.1|in relation to a first threshold consisting of establishing a risk of resonance if the absolute value of said difference is less than or equal to the first threshold; e) if a risk of resonance has been determined at the previous stage, shifting the center of gravity of the element along its direction of elongation in order to reduce the natural frequency of the element when f.sub.1>fr.sub.1 or increase the natural frequency of the element when f.sub.1<fr.sub.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood and other details, characteristics and advantages thereof will become apparent in reading the following description, given by way of a non-restrictive example with reference to the appended drawings in which:

(2) FIG. 1 already described is a cross-sectional axial diagrammatic half-view of an aircraft turbofan of a known type;

(3) FIG. 2 already described is a perspective diagrammatic view of an element for measuring characteristics of a flow according to the known technology;

(4) FIG. 3 is a perspective diagrammatic view of an element for measuring characteristics of a flow according to a first embodiment of the invention;

(5) FIG. 4 is a larger scale view of the area IIIa of FIG. 3;

(6) FIG. 5 is a larger scale view of the area IIIb of FIG. 3;

(7) FIGS. 6 and 7 are perspective and isolated diagrammatic views of the measuring element;

(8) FIGS. 8 and 9 are diagrammatic representations of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) FIG. 3 represents a portion of a turbine engine annular flow passage such as an annular secondary air flow passage, comprising a measurement device 56 for measuring characteristics of the flow featuring a measuring element 58 extending in the air flow between internal 60 and external 62 revolution walls delimiting the air flow. FIGS. 4 to 7 show different views of the measuring element 58 and reference will be made to these figures in addition to FIG. 3 in that which follows.

(10) The measuring element 58 comprises a first radially external end 64 and a second end 66, radially internal. The radially external end is connected to a cylindrical portion 68 interdependent with a disc-shaped base 70 perforated by four holes 72 (FIGS. 5, 6 and 7). The measuring element 58 is inserted from outside the external wall 62 into an opening in the latter such that the cylindrical portion 68 and the disc engage in recesses of matching shape formed in the thickness of the external wall 62. Fixing screws are subsequently inserted into the holes 72 in the base 70 and into the holes opposite in the external wall 62. The first end 64 of the element is thus fixed rigidly in all directions to the external wall 62 (FIG. 5).

(11) According to the invention, the measurement device for measuring the characteristics of a flow comprises means of variation of the center of gravity position of the element 58.

(12) In the embodiments illustrated in the figures, these means comprise means of movement of a mass. Shifting the center of gravity of the element is favored by the movement of a mass arranged in the element.

(13) In the first embodiment of the invention, the means of movement of a mass comprise a rod 74 mounted movably in translation in a cavity 76 of the element 58. The element 58 comprises a first upstream portion 78 comprising the holes for passage of nozzles 54 for measuring the characteristics of a flow, wherein these nozzles 54 comprise an opening oriented in the upstream direction and a downstream portion 80 accommodating the tubular-shaped cavity 76 in which the rod 74 is able to slide. It will be noted that the external surface of the downstream 78 and upstream 80 portions have an aerodynamic profile adapted to passage of an air flow so as to limit the impact of the measuring element 58 in the air flow.

(14) The rod 74 comprises a first radially external end 82 and a second end 84, radially internal. The second end 84 bears a mass 86 which is an appreciably cylindrical-shaped solid. The first end 82 of the rod passes through the external wall 62 and comprises a cylindrical portion 87 of larger diameter serving as a gripping section on which a clip 88 is clamped. This clip 88 is connected interdependently to means of measurement comprising a graduated element 90. Movement of the rod 74 can be effected by a rack and pinion type system, for example.

(15) According to the invention, moving the mass 88 allows a shift in the center of gravity of the element 58, thereby allowing modification of the natural frequency of the element 58.

(16) Indeed, it is clear that the fixing method of the measuring element 58, with the first external end 64 fixed rigidly in all directions and the second end 66 devoid of any fixing in the three axial, radial and circumferential directions, resembles a single-embedded beam model and that shifting the center of gravity allows modification of the natural frequency of the element 58 (refer to equation 1 above).

(17) Weights 92 are evenly spaced along the rod 74 between its first end 64 and its second end 66. The weights 92 make it possible to avoid appearance of normal modes of the rod 74. The weights 92 may for example be evenly spaced along the rod 74. The cumulative mass of the weights 92 and that of the rod 74 must preferably not exceed 10% of the mass of the mass located at the second end of the rod 74. This mass condition may mean that the rod 74 is internally hollow.

(18) It is also noted that the portion 87 of larger diameter at the first end of the rod 74 never penetrates into the element 58 and remains outside the annular flow passage. This portion allows an increase in inertia of the rod 74 in movement.

(19) During operation, when the resonance frequency of the element 58 is close to its vibration frequency, the mass 86 needs to be moved in order to modify the resonance frequency. Moving the mass 86 towards the second, not rigidly fixed end 66 of the element 58 results in a reduction in the first natural frequency (and in the natural frequencies of higher orders) of the element 58. Moving the mass 86 towards the first rigidly fixed end 64 of the element 58 results in an increase in the first natural frequency (and in the natural frequencies of higher orders) of the element 58 (refer to equation 1).

(20) The mass 86 may be made from the same material as that of the rod 74 or from a different material.

(21) FIGS. 8 and 9 illustrate a second embodiment of the invention in which the center of gravity of the element 94 is shifted by means of a liquid mass 96 movable between a first position (FIG. 8) and a second position (FIG. 9). To this end, the element 94 comprises at least one duct 98 providing a leaktight connection between a first external tank 100 arranged outside the external wall 62, at the first end 102 of the element 94 and a second tank 104 arranged at the second end 106 of the element 94.

(22) The first tank 100 comprises a rigid jacket forming a body in which a piston 108 is slidably installed for transferring the liquid between the first 100 and second 104 tanks. The second tank 104 comprises a flexible jacket arranged at the second end 106 of the element 94.

(23) Use of a slidably installed piston 108 simplifies the liquid displacement device, generating a vacuum at the piston 108, to a mere movement of the piston 108. The piston 108 may be movable among several positions so as to displace a given quantity of liquid between the first 100 and second 104 tanks.

(24) The liquid is chosen so as to adapt itself to the temperature and pressure conditions at the position of the measurement plane. In this respect, it may consist of water at the level of the fan or oil at the level of the low-pressure turbine. The tank may have a capacity of 5 milliliters.

(25) Other means of moving the liquid may be used, such as for example a pump, located outside the flow passage.

(26) The flexible jacket of the second tank 104 may be manufactured from a material resistant to the temperature and pressure conditions at the position of the measurement plane, from elastomer for example.

(27) During operation, movement of the piston 108 towards the first end 102 of the element 94 results in displacement of the volume of fluid from the first tank 100 (FIG. 8) towards the second tank 104 (FIG. 9). This has the effect of reducing the natural frequencies of the element 94. When it is a matter of increasing the natural frequencies, the piston 108 is moved in an opposite direction, as a result of which the liquid is sucked towards the first tank 100, with the second flexible tank 104 undergoing deformation.

(28) Implementation of movement will be controlled by a system allowing comparison of the vibration frequency f.sub.1 with the resonance frequency of the element fr.sub.1. In this respect, the element may comprise a dynamic strain gauge or any other system for determining the vibration frequency connected to a data processing system that will analyse the adapt the position of the mass as a function of the absolute value of the difference between a first vibration frequency f.sub.1 of said element and the resonance frequency fr.sub.1 corresponding to the first natural frequency of said element in order to check that the behaviour of the element is consistent with that which is expected.

(29) Consequently, the method for shifting the center of gravity of the element comprises the stages involving: a) determining a first frequency f.sub.1 corresponding to the vibration frequency of said element; b) determining a second frequency fr.sub.1 corresponding to the first natural frequency of said element; c) calculating the absolute value |f.sub.1fr.sub.1|; d) performing a comparison of |f.sub.1fr.sub.1| in relation to a first threshold consisting of establishing a risk of resonance if the absolute value of said difference is less than or equal to the first threshold; e) if a risk of resonance has been determined at the previous stage, shifting the center of gravity of the element along its direction of elongation in order to reduce the natural frequency of the element when f.sub.1>fr.sub.1 or increase the natural frequency of the element when f.sub.1<fr.sub.1.

(30) During operation, the element will vibrate at a frequency f.sub.1 following mainly the frequential excitation f derived from rotation of the blades, without however excluding other sources of vibrations. When the frequencies f.sub.1 and fr.sub.1 are not sufficiently wide apart, two cases may arise: f.sub.1>fr.sub.1 corresponding to a situation in which the vibration frequency f.sub.1 of the element is greater than the natural frequency fr.sub.1 f.sub.1<fr.sub.1 corresponding to a situation in which the vibration frequency f.sub.1 of the element is greater than the natural frequency fr.sub.1.

(31) In practice, the difference |f.sub.1fr.sub.1| should be at least equal to 10% of the value of fr.sub.1 and in absolute terms should not be less than 5% of fr.sub.1. The method of movement of the center of gravity of the element 58, 94 makes it possible to safeguard the vibratory dynamics of the element by rendering the center of gravity position dependent on the difference |f.sub.1fr.sub.1| and by maintaining the frequency f.sub.1 at a maximum of 95% of fr.sub.1 or a minimum of 105% of fr.sub.1, hence excluding the interval [0.95 fr.sub.1; 1.05 fr.sub.1] and preferably the interval [0.9 fr.sub.1; 1.1 fr.sub.1].

(32) Consequently, fr.sub.1 needs to be moved apart from f.sub.1, avoiding equality between these two values. This is achieved through a reduction in fr.sub.1 when f.sub.1 is higher and an increase in fr.sub.1 when f.sub.1 is lower.

(33) Concretely, the reduction in fr.sub.1 is obtained by shifting the mass 86, 96 towards the end that is not rigidly fixed and the increase in fr.sub.1 is obtained by shifting the mass towards the rigidly fixed end.

(34) Such a method makes it possible to control the natural frequencies fr.sub.1 of the element taking account of the excitation frequency f.sub.1 induced by operation of the turbine engine.