Abstract:
A device for measuring vibration amplitudes of the blade tips in a turbomachine is provided. The device includes a support mounted in an orifice of a casing of a turbomachine, in which are housed two optical guides for the emission and reception of a light signal exiting inside the casing across from the tips of the blades of a turbine wheel Each optical guide includes an optical fiber connected by a mechanical connector to a needle of which the core is made of a material able to transmit a light signal and which is resistant to temperatures less than or equal to 1100° C. and which exits at its distal end in the casing across from the blade tips.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a device for measuring vibration amplitudes of the blade tips in a turbomachine such as an aircraft turbojet or a turboprop engine for aircraft. 
     2. Description of the Related Art 
     In a known manner, a turbomachine comprises a turbine of which the rotor is driven in rotation by the flow of hot gases exiting from an annular combustion chamber arranged upstream. The rotor carries one or several wheels of blades arranged alternately with annular rows of fixed blades in a casing. 
     During the operation of the turbomachine, it is important to know the deformation of the mobile blades. For this purpose, it is known to mount on the casing sensors of which the sensitive element is arranged to the right of the mobile blades. The sensitive element of each sensor makes it possible to detect the passage of a blade tip (known as “tip timing”) in order to determine through comparison between the theoretical time of passage of a blade tip and the measured time of passing the deformation mode of the blade, in bending, in twisting, etc., as well as the intensity of the deformation. 
     The applicant has already proposed in application FR1155983 to use sensors of the capacitive type in a blower or a low-pressure compressor of a turbomachine. 
     However, these sensors cannot be mounted in a turbine due to the high temperature present therein and which is of a magnitude of 1000° C. In addition, in the current state of the art, these capacitive sensors have a low temporal resolution which does not make it possible to obtain deformation measurements less than 100 μm, which corresponds to a magnitude of deformations that the tips of the blades can potentially be subjected to when the latter are made of composite material having a ceramic matrix. 
     In order to overcome these difficulties, it has been proposed to use optical probes to detect the times of passage of the blades. Conventionally, these probes include at least two optical fibres of which one is connected to means for emitting a source of light and of which the other receives and transmits the light reflected by the blade tip to means for processing. However, these optical probes initially designed for an integration into a turbomachine compressor are poorly adapted for use in the hot environment of the turbine. It would be possible to cool them during operation by supplying for example cold air from the high-pressure compressor arranged upstream of the combustion chamber. However, such a solution would highly penalise the output and the thrust produced by the turbomachine by taking the compressed air which is used for the combustion. 
     BRIEF SUMMARY OF THE INVENTION 
     The purpose of the invention in particular is to provide a simple, economical and effective solution to these different problems. 
     For this purpose, it proposes a device for measuring vibration amplitudes of the blade tips in a turbomachine, comprising a support mounted in an orifice of a casing of a turbomachine and wherein are housed two optical guides exiting inside the casing across from the tips of the blades of a turbine wheel, with one of the optical guides being connected to a source of light for the propagation of a light signal to the blade tips and the other optical guide being connected to means for processing and analysing the light signal reflected by the blade tips, characterised in that each optical guide comprises an optical fibre connected by a mechanical connector to a rigid needle of which the core is made of a material able to transmit a light signal and which exits at its distal end inside the casing across from the blade tips. 
     In the device according to the invention, a rigid needle having a core made of a material able to transmit a light signal and resistant to temperatures equal to 1100° C. can easily be integrated into the hot and turbulent environment of the turbine in operation, and easily adjustable at a short distance from the ends of the blades. Each needle is connected to an optical fibre of which the flexibility allows it to be arranged in a complex environment such as that of a turbomachine. 
     The device according to the invention moreover comprises two needles and two optical fibres that are separate and not a single needle connected at its proximal end to two optical fibres as the interface between the needle and the optical fibre induces a strong reflection of the light signal which is stronger than the signal reflected by a blade tip, which would cause difficulties in detecting the signal reflected by the blade tips. 
     Preferentially, the support is mounted in an orifice of a boss of the casing and comprises a flange formed on its external surface whereon is mounted with radial abutment from the exterior a member for blocking the support on the boss, with this member being fixed in a detachable manner to the boss. 
     In a particular embodiment of the invention, the member for blocking is formed by a nut mounted around the support and screwed onto a threaded internal surface of the orifice of the boss. 
     According to another characteristic of the invention, each connector comprises an axially split tube wherein are maintained in contact the proximal end of the needle and the distal end of the optical fibre. 
     This tube provides an end-to-end maintaining by tightening the proximal end of the needle and the distal end of the optical fibre for the transmission of the light signal. 
     According to another characteristic of the invention, the support comprises two tubular housings converging towards the interior of the turbomachine on the blade tips and exiting inside the casing and wherein are inserted from the exterior the needles of the optical guides. 
     With this arrangement, it is possible to arrange the two needles so that their axes converge towards the blade tips, which makes it possible to guarantee that a light signal reflected by a blade tip and not by another portion of the blade will be detected. 
     According to another characteristic of the invention, each optical guide comprises a bushing mounted around the proximal end portion of the needle, with this bushing comprising an external annular edge coming to abut radially towards the interior on an internal radial shoulder of the housing of the support wherein is mounted the needle of the optical guide. 
     According to another characteristic of the invention, each needle is retained radially towards the exterior in its housing by a cylindrical part mounted around the distal end portion of the optical fibre and in axial abutment in the direction of the needle on an external annular edge of said distal end portion, with the cylindrical part comprising a threaded external surface which is screwed into a threaded internal surface of the housing. 
     The needle of each optical guide is as such maintained radially in its housing, by the external annular edge of the bushing coming to abut on the internal radial shoulder of the housing and by the cylindrical part screwed into the housing and applied on the annular edge of the distal end portion of the optical fibre to which the needle is connected. 
     The annular edge of the distal end portion of each optical fibre can be formed with an added part crimped around said distal end portion. 
     In a particular embodiment of the invention, the core of the needle is surrounded by a metal sheath, such as titanium. 
     Advantageously, the distal surface of the core of the needle is offset radially towards the interior in relation to the distal end of the sheath, which prevents the formation of metal oxide on the distal surface of the core of the needle. 
     In a particular embodiment of the invention, the core of the needle is made of a material that can resist a temperature equal to 1100° C. 
     The core is preferentially made of sapphire. The use of sapphire allows for the insertion of the needles of the device inside a turbine without the risk of thermal degradation due to the fact that sapphire resists high temperatures well. 
     The utilisation of a metal sheath having a low coefficient of thermal expansion makes it possible to limit the expansion of the needle in its housing. In addition, sapphire has a very low coefficient of thermal expansion, which makes it possible to limit the differences in thermal expansion between the metal sheath and the core made of sapphire. 
     According to the invention, the source of light emits a beam with a wavelength centred on a wavelength that is less than the wavelengths of the radiations emitted in the turbine. Advantageously, the means for processing and analysing include a low-pass filter for the filtering of wavelengths greater than the wavelength of the source of light. In a particular embodiment, the source of light emits a beam of wavelength centred on 405 nm. 
     The invention further relates to a turbomachine comprising a device for measuring such as described hereinabove. 
     The invention further proposes a system for measuring the vibration amplitudes of the blade tips in a turbomachine, characterised in that it comprises a device such as described hereinabove, with one of the optical guides being connected by its proximal end to a laser source and the other of the optical guides being connected at distal end to means for amplifying, sampling, converting and transferring data to means for processing. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Other advantages and characteristics of the invention shall appear when reading the following description given by way of a non-restricted example and in reference to the annexed drawings wherein: 
         FIG. 1  is a cross-section partial diagrammatical half-view of the device according to the invention arranged on a turbine casing of a turbomachine; 
         FIG. 2  is a diagrammatical view on a larger scale of the zone delimited with dotted line in  FIG. 1 ; 
         FIG. 3  is a diagrammatical view of a housing of the support of the device according to the invention; 
         FIG. 4  is a diagrammatical view in perspective from the exterior of the casing of the device of  FIG. 1 ; 
         FIG. 5  is a diagrammatical view of the distal end of an optical guide of the device according to the invention; 
         FIG. 6  is a cross-section diagrammatical view of an optical guide of the device according to the invention; 
         FIGS. 7 and 8  are diagrammatical side views of the optical guide of  FIG. 4 ; 
         FIG. 9  is a diagrammatical representation of a measurement chain comprising the device according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is first made to  FIG. 1  which shows a device  10  according to the invention mounted on a casing surrounding exteriorly annular rows of fixed blades and wheels of blades arranged axially alternately with one another.  FIG. 1  is a view according to a cut plane perpendicular to the axis of the turbine, passing through an impeller  14  and through the device according to the invention. 
     The device  10  according to the invention comprises a support  16  mounted in a radial orifice of a boss  18  of the casing  12 . Two optical guides  20 ,  22  are mounted in the support  16  and exiting at their distal ends inside the casing  12 , with one  20  of the optical guides being an optical guide for emitting  20  connected to a source of light for the propagation of a light signal to the blade tips  26  and the other being an optical guide for receiving  22  which receives the signal reflected by the blade tips  26  and transfers it to a processing unit. The measurement chain associated to the device  10  shall be described in more detail later. 
     The support  16  comprises three substantially cylindrical portions, a first proximal portion  28  of outer diameter greater than that of the second intermediary portion  30  itself with an outer diameter greater than that of the third distal portion  32 . The first proximal portion  28  and the second intermediary portion  30  of the support  16  are separated from one another by an annular flange  34  formed as a protrusion radially towards the exterior on the outside edge of the support  16 . 
     The second intermediary portion  30  and third distal portion  32  of the support  16  are mounted in an opening of a centring part  36  engaged from the exterior in the boss and blocked radially towards the interior by an external annular edge  38  coming to abut on an internal shoulder  40  of the boss  18 . The centring part  36  comprises a first portion  42  of which the inner diameter allows for the mounting of the second intermediary portion  30  of the support  16  and a second portion  44  of which the inner diameter is smaller and allows the passage only of the third distal portion  32  of the support  16 , in such a way as to block radially towards the interior the support  16  in the boss  18 . 
     An annular wedge  46  is inserted between the centring part  36  and the flange  34  of the support  16 . The modification of the radial thickness of the wedge makes it possible to adjust the radial position of the distal ends of the optical guides  20 ,  22  inside the casing  12 , in relation to the blade tips. 
     A nut  48  is mounted around the first proximal portion  28  of the support  16  and is screwed onto a threaded internal surface of the radially external end of the orifice of the boss  18 . This nut  48  comes to abut against the flange  34  of the support and provides a blocking radially towards the exterior of the support  16  in the boss  18 . 
     The support  16  comprises two separate tubular housings  50  (an isolated diagrammatical representation of a housing is shown in  FIG. 3 ) exiting inside the casing and of which the axes converge towards the interior of the turbomachine and are cut on a blade tip when a blade is aligned with the axis of the support  16 . Each housing is formed of three tubular portions  52 ,  54 ,  56  of which the internal sections decrease radially towards the interior and are separated from one another by radial shoulders  58 ,  60  of the internal surface of the housings  50  ( FIGS. 2 and 3 ). 
     The first  52  and second  54  tubular portions of each housing  50  extend in the first portion  28  of the support  16  and the third tubular portion  56  of each housing  50  extending partially in the first portion  28  of the support  16  and in the second and third portions  30 ,  32  of the support. 
     Each optical guide  20 ,  22  is formed by an optical fibre  61 ,  62  connected by a connector  64  to a needle  65 ,  66  inserted into a housing  50  of the support  16  and exiting radially towards the interior in the casing  12  across from the tips  26  of blades ( FIGS. 1 and 6 to 7 ). 
     The optical fibres  61 ,  62  of the optical guides  20 ,  22  are housed in a flexible protective sheath  68  (FIG. 4 ). 
     The description given hereinbelow in reference to the optical guide for receiving  22  applies identically to the optical guide for emitting  20 . 
     The needle  66  comprises a core  70  made of sapphire surrounded by a metal sheath  72  having a low coefficient of thermal expansion which is less than the coefficient of thermal expansion of the sapphire in such a way as to limit the expansion of the sapphire core in the support  16 . The sapphire core  70  is made integral with the metal sheath  72  by means of a ceramic glue  74  that resists high temperatures ( FIGS. 5 and 6 ). 
     In operation, the surface of the metal layer directly in contact with the hot gases is covered with a layer of metal oxide  76 , such as for example a layer of titanium oxide when the sheath is made of titanium ( FIG. 4 ). The distal surface  73  of the sapphire core exiting inside the casing is offset radially towards the interior in relation to the metal sheath, for example by approximately 0.5 mm, in such a way as to prevent the layer of oxide from partially covering the distal surface of the sapphire core. 
     A bushing  78  is mounted around the proximal end portion of the needle  66 . This bushing  78  comprises an annular edge  80  intended to abut radially towards the interior on the radial shoulder  58  separating the first and second  54  tubular portions of the housing  50  ( FIGS. 3 and 5 ). The proximal end portion of the needle extends beyond the bushing  78  on the side of the annular edge  80 . 
     An annular part  84  is crimped around the proximal end portion of the optical fibre  62  and includes an external annular edge  86 . 
     A tube  82  split according to its axis is mounted tight for a portion around the distal end of the optical fibre  62  and for the other opposite portion around the proximal end of the needle  66 . This split tube  82  provides an end-to-end maintaining of the proximal end of the needle  66  and of the distal end of the optical fibre for the optical transmission and a maintaining in alignment of the bushing  78  and of the annular part  84  for the mounting in the tubular housing  50 . 
     A cylindrical part  88  is mounted around the proximal end portion of the optical fibre  62 . This cylindrical part  88  comprises an annular edge  90  at its proximal end coming to abut on the annular edge  86  of the annular part crimped on the proximal end portion of the optical fibre  62 . The distal portion of the cylindrical part  88  comprises a threading  92  on its external surface for its screwing into a corresponding threading  94  of the internal surface of the housing ( FIGS. 2, 3, 5 and 6 ). 
     As such, when the needle  66  is inserted into its housing  50 , it is blocked radially towards the interior by the annular edge  80  of the bushing  78  pressing against the shoulder  58  of the housing  50  and radially towards the exterior by the annular edge  86  of the crimped part  84  pressing on the annular edge  90  of the cylindrical part  88  screwed into the housing  50 . 
     As shown in  FIG. 1 , the shoulders  58  of the support  16  are positioned along each housing and the lengths of the needles  66  are determined in such a way that the distal ends of the needles  66  are flush with the radially internal end of the third distal portion  32  of the support  16 . In this way, the needles  66  are protected by the support  16  and their heating during operation is limited. 
     The two needles  65 ,  66  can have different lengths ( FIG. 2 ) or be identical according to the space available around the turbomachine. 
     The optical fibre  62  comprises a central portion  94  made from a material providing the propagation of the light signal such as silica and is surrounded by a sheath  96  that resists high temperatures made from a metal material such as copper, aluminium or gold. 
       FIG. 7  shows a system  98  for measuring vibration amplitudes of the blade tips wherein is integrated the device  10  described hereinabove. 
     The optical fibre  61  of the optical guide for emitting  20  is connected to its end opposite the needle to a laser source  100  by the intermediary of a connector  102  comprising an optical lens and a centring sleeve. The light signal transmitted by the optical fibre  61  and the needle  65  of the optical guide for emitting  20  is reflected by a blade tip and transmitted to the needle  66  of the optical guide for receiving  22  then is propagated in the optical fibre  62  for reception. The light signal then passes through a lens  104  providing a focusing of the light signal on a photodiode  106  allowing for a conversion of the light signal into an electrical signal. A low-pass filter  108  is inserted between the focusing lens  104  and the photodiode  106  and is configured to eliminate the wavelengths greater than the wavelength of the laser radiation. The electrical signal is then transmitted to a preamplifier  110  and to an adjustable gain amplifier  112 , to a sampling board  114  then to a board  116  for converting the electrical signal into an optical signal for its transfer by optical cable  118  to the means for processing  120  such as for example full authority digital engine control, also known by its acronym FADEC. 
     According to the invention, the use of a sapphire core for each needle  65 ,  66  allows for a mounting of the needles  65   66  inside the turbine without the risk of thermal degradation of the needle  65 ,  66  since sapphire is a material that resists the high temperatures that are present in a turbine and which are of a magnitude of 1000° C. 
     In addition, the use of a needle connected to a flexible optical fibre makes it possible to guarantee an assembly of the device according to the invention in the complex three-dimensional environment of the casing of the turbine. 
     In a practical example, the source of light can emit a laser beam with a power of about 120 mW and with a wavelength centred on 405 nm. This wavelength is selected to be less than the wavelengths of the radiation emitted by the flow of hot gases exiting the combustion chamber. The low-pass filter is configured to eliminate all of the wavelengths greater than 450 nm. In this way, the wavelengths located in the red and infrared ranges, emitted by the hot gases coming from the combustion chamber and the hot environment of the turbine, are eliminated. 
     In a practical embodiment of the invention, the metal sheath surrounding the sapphire core is made of titanium which is covered during operation with a layer of titanium oxide. The bonding glue of the sapphire core with the metal sheath is for example Ceramabond 569®.