Abstract:
A device for determining distance between a rotor blade and a wall of a gas turbine surrounding the rotor blade is provided. A waveguide guides and emits electromagnetic waves in the direction of the rotor blade through a waveguide opening facing the rotor blade. The electromagnetic waves are injected into the waveguide and reflected portions of the electromagnetic waves are received. An evaluation unit compares the phase of the electromagnetic waves to be injected with the phase of the reflected portions of the electromagnetic waves and determines phase comparison values for every frequency and the distance is determined based on the phase comparison values. The waveguide includes two waveguide segments made from different materials having temperature stability and damping capacity increasing in the direction from the segment connected to the unit for injecting the waves to the segment having the waveguide opening.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the US National Stage of International Application No. PCT/EP2007/059034, filed Aug. 30, 2007 and claims the benefit thereof. The International Application claims the benefits of German application No. 10 2006 046 695.0 filed Sep. 29, 2006, both of the applications are incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a device for determining the distance between at least one rotor blade and a wall of a gas turbine, said wall surrounding the at least one rotor blade, and to a use of the method. 
       BACKGROUND OF THE INVENTION 
       [0003]    Turbine engines such as steam or gas turbines, for example, are utilized as thermal power engines in engineering for the purpose of converting energy which is stored in a flow of gas into a mechanical energy, in particular into a rotational movement. In order to ensure the reliable operation of turbine engines of said kind, efforts are made to monitor the rotor blades of the blade wheel continuously, particularly during the operation and hence during the rotation of a blade wheel which is arranged in the turbine engine. In this case it is very important to exactly maintain the distance of the rotor blade tips, i.e. the radially outermost edges of the rotor blades, from the wall surrounding the rotor blades (radial clearance). A minimum radial clearance must be satisfied for safety reasons, although too great a radial clearance results in unnecessarily low efficiency. In addition to the radial clearance, the axial distance to wall sections is also important, particularly in the case of blade wheels in which the blade rows are covered by a shroud. Because these variables change due to various dynamic influencing factors, continuous monitoring of the radial clearance and the axial clearance is important during operation. The size of the radial clearance can be monitored e.g. by means of capacitive sensors which are positioned close to and almost touching the blade tips. However, these sensors are limited in terms of accuracy, spatial resolution and service life. 
         [0004]    DE 197 05 769 A1 discloses a device for monitoring radial and axial clearance in a turbine engine. In this case use is made of a radar system comprising a transmit and receive unit from which electromagnetic waves having a fixed frequency are directed through a waveguide onto a blade wheel of the turbine engine. In this case the waveguide is ducted through the housing which surrounds the blade wheel and is fixed there. The outlet of the waveguide is arranged very closely over the rotor blade tips of the blade wheel, such that it is possible to determine from the reflection of the emitted electromagnetic waves the distance of a rotor blade tip from the waveguide end and hence from the wall surrounding the rotor blade. The determining is done by means of an evaluation of the phases of the reflected electromagnetic waves. The distance is determined by determining the phase difference between emitted and reflected microwaves. 
         [0005]    Extreme thermal conditions can prevail in the region of the blade wheel within the housing during operation, particularly in gas turbines. In the case of gas turbines, temperatures of approximately 1200° C. usually occur in the flow channel. These extreme temperatures place particular demands on the nature of the waveguide, which must be embodied such that it exhibits temperature stability at the same time as having a low damping capacity for the electromagnetic waves to be guided. Materials having high temperature stability, e.g. superalloys, are generally characterized by a very high damping capacity for the electromagnetic waves to be guided, while materials having a low damping capacity, e.g. copper, are unstable at extreme temperatures of the specified level. 
       SUMMARY OF THE INVENTION 
       [0006]    The object underlying the present invention is to disclose an appropriate device and use of the device, wherein the waveguide can guide electromagnetic waves with the least possible damping while remaining stable at high temperatures. 
         [0007]    The object is achieved by a device according to the features recited in the independent claim. 
         [0008]    Accordingly, the device for determining the distance between at least one rotor blade and a wall of a gas turbine, said wall surrounding the at least one rotor blade, comprises the following parts:
       a waveguide for guiding electromagnetic waves and emitting electromagnetic waves in the direction of the rotor blade through at least one waveguide opening which faces the rotor blade,   at least one means, this being connected to the waveguide, for injecting the electromagnetic waves into the waveguide,   at least one means, this being connected to the waveguide, for receiving reflected portions of the electromagnetic waves to be injected into the waveguide, and   an evaluation unit for evaluating the reflected portions to be received of the electromagnetic waves to be injected, comprising means for comparing the phases of the electromagnetic waves to be injected with the phases of the reflected portions of the electromagnetic waves to be injected, wherein a phase comparison value can be ascertained for each frequency by means of the evaluation unit and the distance can be determined from a comparison of the phase comparison values,
 
and is embodied such that
   the waveguide is configured from at least two waveguide segments which are made from different materials, wherein the temperature stability and the electromagnetic wave damping capacity of the materials increase, starting from the segment which is connected to the means for injecting and receiving, in the direction of the segment having the waveguide opening.       
 
         [0014]    The invention is based on the insight that the temperature of the waveguide decreases from the waveguide opening in the direction of the means for injecting and receiving. In order to ensure that the electromagnetic waves are guided as effectively as possible in the waveguide, the present invention maps this temperature decrease profile onto the waveguide in first approximation, such that the waveguide meets the requirements relating to temperature stability and damping capacity, being inventively composed of segments of different materials, each having a different damping capacity and a different temperature stability. 
         [0015]    Advantageous embodiments of the device according to the invention are derived from the dependent claims of the independent claim. In this case the embodiment according to claim  1  can be combined with the features of one of the associated dependent claims or preferably also with the features of a plurality of dependent claims. Accordingly, the inventive device can additionally have further features as follows: 
         [0016]    The waveguide can be configured from three segments. In this way it is possible to achieve a better adaptation to the temperature profile which is prevalent in the waveguide during operation of the gas turbine. 
         [0017]    The segment which is connected to the means for injecting and receiving can advantageously be configured from a metal having a low damping capacity, in particular a group-11 element or platinum, and the segment featuring the waveguide opening from a superalloy having high temperature stability. 
         [0018]    Copper, silver and gold, which are options as group-11 elements, offer excellent electrical conductivity, and this is exhibited in a very low damping capacity when guiding electromagnetic waves. It is therefore possible to extend the waveguide segment, which is configured from a group-11 element or platinum or at least has an inner coating of a group-11 element or platinum, so far that the means which are connected to this segment for injecting and receiving can be arranged at a safe distance from the wall of the gas turbine. 
         [0019]    A superalloy designates alloys of complex composition for high-temperature applications. Suitable candidates in this case are alloys based on iron, nickel, or cobalt with additives of the elements cobalt, nickel, iron, chromium, molybdenum, tungsten, rhenium, ruthenium, tantalum, niobium, aluminum, titanium, manganese, zirconium, carbon and/or boron. With a temperature stability of more than 1200° C., such a waveguide segment resists high temperatures which are prevalent in the flow channel and hence at the side of the wall facing the flow channel during operation of the gas turbine. 
         [0020]    A central segment can advantageously be configured from a special steel. This ensures that such a segment is corrosion-resistant. The temperature stability and the damping capacity of special steel lie between superalloy and group-11 element. 
         [0021]    It can be favorable if the segment connected to the means for injecting and receiving is configured such that it can be cooled by a liquid or air. It is therefore possible to embed this segment deeper into the wall in the direction of the flow channel. Temperature damage is prevented by the cooling in this case. Water can be used as a cooling liquid, for example. 
         [0022]    In the transition zone of two consecutive segments, the waveguide can advantageously have a coating of one of the two segment materials. In this way reflections of the electromagnetic waves at the boundary surfaces of the transition zone between two segments are avoided and the guiding properties of the waveguide are improved. 
         [0023]    It can be advantageous if the segment having the waveguide opening is embodied as a horn. This ensures that the electromagnetic waves leave the waveguide with a radiation characteristic which is defined by the horn and the reflected portions of the emitted electromagnetic waves can be received again with a higher yield, since the receive yield is determined by the horn diameter at the waveguide end. In this case the segment having the waveguide opening terminates flush with the inner surface of the wall facing the flow channel. However, it can also be set back in the wall opening, so that the segment is not exposed to the direct gas stream in the flow channel. 
         [0024]    The electromagnetic waves can favorably be millimeter waves, in particular in the frequency range from 70 GHz to 150 GHz. Since the wavelengths in these frequencies are approximately 4 millimeters and less, it is possible to deploy very compact waveguides whose cross-sectional dimensions are typically selected to match the dimensions of the wavelengths to be guided. 
         [0025]    The invention also relates to a use of the inventive device for determining the distance between at least one rotor blade and a wall, surrounding the at least one rotor blade, of a gas turbine. 
         [0026]    The waveguide can advantageously be arranged in a cooling channel of the wall in this case. Consequently, one of many cooling channels which are already provided in the wall for cooling purposes can be used for installing the device according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Preferred but by no means restrictive exemplary embodiments of the device are explained below with reference to the drawing. For the sake of clarity, the drawing is not to scale and some features are illustrated schematically. 
           [0028]      FIG. 1  shows a gas turbine according to the prior art in a partially sectioned perspective view, 
           [0029]      FIG. 2  shows a rotor blade of the gas turbine from  FIG. 1 , 
           [0030]      FIG. 3  shows a schematic illustration of the inventive device comprising three waveguide segments, and 
           [0031]      FIG. 4  shows a schematic illustration of the inventive device comprising two waveguide segments. 
       
    
    
       [0032]    Corresponding parts are labeled with the same reference signs in  FIGS. 1 to 4 . 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0033]      FIG. 1  shows a gas turbine  10  according to the prior art which is designed for a high gas inlet temperature of approximately 1200° C. In a housing  11  comprising an inner wall  111 , the gas turbine  10  has a rotatably mounted rotor shaft  12  on which rotor blades  14  are arranged in a flow channel  13 . 
         [0034]      FIG. 2  shows a rotor blade  14  of said type in detail in an uninstalled state. In the installed state the upper end of the rotor blade  14 , the so-called rotor blade tip  141 , faces the inner wall  111  of the gas turbine housing  11 . 
         [0035]      FIG. 3  shows a schematic illustration of the inventive device in a first exemplary embodiment. For the sake of simplicity, only part of a rotor blade  14  is shown in outline. The arrow  142  indicates that during a distance measurement the rotor blade  14  moves in the direction of the arrow  142  during operation of the gas turbine  10 . The movement in the direction of the arrow can be considered approximately as a linear movement in the lateral direction z. The rotor blade tip  141  is located at a radial distance Δx from the inner wall  111  of the gas turbine housing  11 . In order to ensure optimal efficiency of the gas turbine  10 , the distance Δx between the ends of the rotor blades  14 , i.e. the rotor blade tips  141 , and the inner wall  111  of the gas turbine housing  11  is as small as possible and typically lies within a range of a few millimeters, in particular between 1 mm and 20 mm. The wall  111  has at least one opening in which a waveguide  40  for guiding electromagnetic waves  31 ,  32  is arranged with positive fit. The waveguide  40  is advantageously installed in one of numerous cooling channels which are already present in the wall  111 . The waveguide  40  is embodied as tubular and has e.g. a round or rectangular cross-section having an internal diameter d in the range from 2 mm to 10 mm. 
         [0036]    During operation of the gas turbine  10 , a temperature of approximately 1200° C. is prevalent in the flow channel  13 . The surface  112  of the wall  111  facing the flow channel  13  therefore likewise has this high temperature, though this decreases through the wall  111  in the direction of the opposite surface  113  of the wall  111 . The temperature profile  91  is represented in the diagram  90  by way of example. The temperature T is plotted on the ordinate, while the abscissa represents the section through the wall  111  in the x direction. According to the diagram  90 , the temperature decreases steadily from 1200° C. at the inner surface  112  of the wall  111  to approximately 200° C. at the outer surface  113  of the wall  111 , a temperature of approximately 600° C. prevailing midway between both surfaces  112 ,  113 . 
         [0037]    The waveguide  40  is configured from a plurality of segments  42   a ,  42   b ,  42   c , e.g. three according to  FIG. 3 , along its longitudinal axis which here points in the x direction. In this case its materials are selected according to the invention as a function of the temperature profile  91 , in order to ensure high temperature stability at the same time as optimal waveguide properties over the entire length of the waveguide  40 . 
         [0038]    Thus, the segment  42   a  having the waveguide opening  41  in the region of the inner surface  112  of the wall  111  is produced from a superalloy. Suitable materials for this segment  42   a  are in particular “Inconel” (a brand name of the company “Special Metals Corporation”, USA) or “PM 1000” (a brand name of the company “Plansee GmbH”, Germany). In this case the main consideration for the segment  42   a  coming closest to the flow channel  13  is good temperature stability at extreme temperatures in the region of 1200° C. The damping properties are less important in this region of the waveguide. For the purpose of improved radiation and reception characteristics of the waveguide  40 , the segment  42   a  having the waveguide opening  41  is also configured as a horn. 
         [0039]    The segment  42   c  of the waveguide  40 , which segment is located in the region of the outer surface  113  of the wall  111  and is connected to a transmit/receive unit  50 , is exposed to a relatively low temperature. Consequently, the temperature stability in this region of the waveguide  40  is of lesser importance. The priority here is to ensure a good wave conductivity of the waveguide  40  and hence a low damping capacity of the segment  42   c  for the electromagnetic waves  31 ,  32  to be guided in the waveguide  40 . This is inventively achieved in that the segment  42   c  which is connected to the transmit/receive unit is configured from a group-11 element or platinum. Alternatively, this segment  42   c  can also be produced from special steel, wherein the inner surface  43  of the waveguide  40 , said inner surface being responsible for guiding the electromagnetic waves  31 ,  32 , is provided with a coating of a group-11 element or platinum. In this case it is possible to extend the segment  42   c  and hence the waveguide  40  so far that the transmit/receive unit  50  which is attached to this segment  42   c  can be arranged at a safe distance from the wall  111  of the gas turbine  10 . 
         [0040]    The intermediate segment  42   b  which is arranged between both aforementioned segments  42   a  and  42   c  is advantageously configured from special steel. Consequently, the segment  42   b  is corrosion-resistant and has adequate temperature stability in the temperature range in the proximity of 600° C. The temperature stability and the damping capacity of special steel lie between superalloy and group-11 element. If the segment  42   a  having the waveguide opening  41  is manufactured from “PM 1000”, for example, the intermediate segment  42   b  can also be made from “Inconel” as an alternative. 
         [0041]    The transition zone of two consecutive segments  42   a ,  42   b  or  42   b ,  42   c  in the inner region of the waveguide  40 , said inner region guiding the electromagnetic waves  31 ,  32 , can be coated with a material from which one of the two segments  42   a ,  42   b  or  42   b ,  42   c  is made. By means of such a coating of the transition zone of the inner surface  43 , reflections of the electromagnetic waves  31 ,  32  at the boundary surfaces of the transition zone between two segments  42   a ,  42   b  or  42   b ,  42   c  is avoided, thereby improving the overall guiding properties of the waveguide  40 . 
         [0042]    An operation for determining the distance takes place as described in detail below: 
         [0043]    The transmit/receive unit  50 , which comprises means for injecting  51  and receiving 52 electromagnetic waves  31 ,  32 , in particular microwaves in the frequency range from 70 GHz to 150 GHz, injects electromagnetic waves  31  having e.g. a frequency a into the waveguide  40  using the injection means  51  which is connected to the waveguide  40 . The electromagnetic waves  31  are then emitted through the waveguide opening  41  in the direction of the rotor blade  14 . After traversing the distance Δx, at least a portion  32  of the emitted electromagnetic waves  31  is reflected by the rotor blade tips  141  to the waveguide  40  and then supplied from the waveguide  40  to the transmit/receive unit  50 . The reflected portion  32  of the emitted electromagnetic waves  31  is detected using e.g. a receive diode as a means  52  for receiving electromagnetic waves, and converted into corresponding electrical signals which are supplied to an evaluation unit  60 . The phase value φ r a of the electromagnetic waves  32  that are assigned to the frequency a is initially determined from the electrical signals by means of the evaluation unit  60 . The phases φ 0 a of the emitted electromagnetic waves  31  are then compared with the phases φ r a of the reflected portions  32  of the emitted electromagnetic waves  31  using a phase comparison means  61 . The phase comparison value Δφa, which is determined e.g. by means of a phase difference value Δφa=φ r a−φ 0 a, is directly dependent in this case on the distance traversed by the electromagnetic waves  31  that were injected by the transmit means  51 . The comparison value Δφa thus obtained is then assigned by an assignment means  62  to a measured value M for the distance Δx between rotor blade tip  141  and wall  111 . The assignment can be done e.g. on the basis of a value table or also a suitable algorithm. 
         [0044]    The measured value M which is determined for the distance Δx of the at least one rotor blade  14  is reported to a monitoring point or forwarded to a central office via display or reporting means which are not represented in greater detail. 
         [0045]    The evaluation unit  60  can also be equipped with a comparison function by means of which it is possible to detect that a predefinable distance threshold has not been met. If the threshold value is not met, a message can be output automatically, for example, in order to initiate a suitable protective measure such as the shutting-down of the gas turbine  10 , for example. 
         [0046]    A further exemplary embodiment of the inventive device is schematically illustrated in  FIG. 4 . It largely corresponds to the exemplary embodiment according to  FIG. 3 . Only the differences are discussed in the following: 
         [0047]    According to  FIG. 4 , the waveguide  40  has only two segments  42   a ,  42   c . The intermediate segment  42   b  indicated in  FIG. 3  is omitted. In this case the segment  42   a  having the waveguide opening  41  does not terminate flush with the inner surface  112  of the wall  111 , but is set back in the x direction in order that it is not directly exposed to the temperatures in the flow channel  13 . A superalloy, in particular “Inconel”, is also particularly suitable as a material for the segment  42   a . As described above, the segment  42   c  which is connected to the transmit/receive unit  50  is produced from a group-11 element or platinum or alternatively from special steel, the inner surface  43  of the waveguide  40  being provided with a coating of a group-11 element or platinum. Because the segment  42   c  which is connected to the transmit/receive unit  50  extends as far as midway between both surfaces  112 ,  113  of the wall  111 , at least this segment  42   c  is provided with a cooling device. For this purpose the segment  42   c  has channels (not shown in  FIG. 4 ) for carrying a liquid or gaseous cooling agent such as water or air, for example. The cooling agent can be carried through the cooling channels via inlet connection pieces  80  and outlet connection pieces  81  which are arranged at the segment  42   c . In this case the arrows  802 ,  811  indicate the inlet and outlet of the cooling agent, respectively. This ensures that the segment  42   c  which is connected to the transmit/receive unit  50  is protected against thermal damage in the central region of the wall  111 . 
         [0048]    The present invention is not restricted to the exemplary embodiments shown. The scope of protection also covers the provision of a plurality of waveguides  40  for emitting and/or receiving, in order, for example, to achieve measurement redundancy or also greater precision.