Abstract:
A flowmeter determines the flow direction of a fluid. The flowmeter has a measuring element, around which the fluid flows and which comprises at least one fiber-optic cable and at least two electrical heating elements that lie adjacent to the fiber-optic cable(s) by a respective heat stream emanating from the respective heating element and directed towards at least one fiber-optic cable, the directions of the heat streams being at least proportionately reversed. In addition, the values of the individual heat streams can be influenced to different extents, depending on the flow direction of the fluid. An electromagnetic wave that can be coupled into the fiber-optic cable(s) can also be influenced according to the temperature of the fiber-optic cable(s). Additionally, the flow meter has a control unit, which is used to feed electric energy to the two or more heating elements one after the other, and an evaluation unit, which is used to evaluate the temperature effect of the electromagnetic wave that emanates from the individual heat streams and to determine the flow directed of the fluid.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based on and hereby claims priority to German Application No. 10 2006 012 229.1 filed on Mar. 16, 2006 and PCT Application No. PCT/EP2007/050990 filed on Feb. 1, 2007, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a flowmeter for determining a flow direction of a fluid. The invention also relates to a method for determining a flow direction of a fluid and to an electrical machine having the flowmeter. 
     In all rating classes of electrical machines, but in particular relatively high-rating machines, a considerable amount of heat is developed which must be dissipated by cooling measures in order to achieve better machine efficiency and/or a longer life. By way of example, air-cooled machines such as generators or motors are known, in particular with ratings of less than 300 MVA, in which cooling is provided by a comparatively large air flow. This air flow may in particular be passed through a line system which comprises numerous flow channels (cf. for example DE 42 42 132 A1 or EP 0 853 370 A1). For example, the flow channels of the line system can be used to force air from the outside inward through the stator of the machine. At the same time, however, air is sucked in by the machine rotor and is forced from the inside outward through the stator, in the opposite direction. If the two airflows are superimposed disadvantageously, the flow ceases to flow within the line system, therefore possibly leading to local overheating of and damage to the machine. 
     WO 2004/042326 A2 specifies a flowmeter for determining a flow rate of a fluid which is flowing around a measurement element in the flowmeter, such as a gas flow, having an optical waveguide which has a plurality of fiber Bragg gratings and having at least one electrical heating element which is arranged adjacent to the waveguide. This allows the flow rate along the longitudinal extent of the measurement element to be determined from the influence of the temperature on the conductor on an electromagnetic wave which is fed into the optical waveguide. The optical waveguide can be heated via the electrical heating element with a constant amount of heat being applied, resulting in a temperature distribution in the longitudinal direction on the measurement element, corresponding to the local flow rate. This flowmeter is therefore suitable for determining a multiplicity of local flow rates using just one single measurement element. However, it is not possible to determine the direction into which the fluid is flowing relative to the measurement element. 
     SUMMARY 
     One potential object is therefore to provide a flowmeter and a method by which the flow direction of a fluid can be determined, and of specifying an electrical machine in which the flow direction of a cooling fluid can be monitored. 
     The inventors propose a flowmeter for determining a flow direction of a fluid, having
         a measurement element around which the fluid can flow and having at least two optical waveguides and having at least one electrical heating element, which is arranged adjacent to the optical waveguides,   in which   heat can be applied to the optical waveguides via a respective heat flow, which is directed from the at least one heating element to the respective optical waveguides,   at least a proportion of the directions of the heat flows is in opposite directions,   the individual heat flows are correlated to different extents with the flow direction of the fluid,   and
           at least one electromagnetic wave, which can be injected into the optical waveguides, can be influenced in accordance with the respective temperature of the optical waveguides,   
           a control unit by which electrical power can be supplied to the at least one heating element, and   an evaluation unit, by which it is possible to evaluate the temperature influence, originating from the individual heat flows, on the at least one electromagnetic wave, and to determine the flow direction of the fluid.       

     The measurement element, whose longitudinal extent is preferably arranged at right angles to the flow direction of the fluid in it has different local flow conditions over the circumference of its cross section, which, in particular, is circular. Heat is therefore not transported uniformly over the circumference of the cross section on the surface of the measurement element, because of the locally different flow rates of the fluid. For this reason, when a constant amount of power is supplied to the at least one heating element, a different heat flow respectively occurs in the direction of the optical waveguides in the measurement element, as a function of the position of the optical waveguides in the measurement element. Different temperatures can therefore occur at each of the locations on the optical waveguides as a function of the arrangement of the optical waveguides relative to the flow direction. Finally, the flow direction of the fluid which is flowing around the measurement element can be deduced by determining the corresponding temperature differences. 
     It is therefore advantageous if the optical waveguides each comprise at least one fiber Bragg grating and the at least one electromagnetic wave which can be injected into the optical waveguides can be influenced in accordance with the respective temperature of the optical waveguides at the location of the at least two fiber Bragg gratings. A sensor type such as this is distinguished by its particular multiplexing capability, so that a sensor network can be provided in a simple manner. A further advantage of the fiber Bragg grating technology is the capability to carry out a measurement virtually at a point, that is to say a locally very tightly constrained measurement. It is thus possible when a relatively high measurement accuracy is required, in particular a measurement accuracy with position resolution, along the measurement element, to arrange a plurality of fiber Bragg gratings close to one another and following one another in the respective optical waveguides. 
     In order to allow better distinction, the fiber Bragg gratings which are arranged in an optical waveguide preferably each use mutually different main wavelengths. In each fiber Bragg grating, a component of the at least one electromagnetic wave that is fed in, with this component being governed by the respective main wavelength, is reflected back. The main wavelength changes with the influencing variable prevailing at the measurement location, in this case in particular the temperature of the optical waveguide. This change in the wavelength range (or wavelength spectrum) of the respective portion reflected back of the at least one electromagnetic wave that is fed in will be used as a measure of the influencing variable to be detected. However, in principle, it is also possible to investigate the transmitted portion of the at least one electromagnetic wave that is fed in, for the change in the wavelength spectrum. In particular, a broadband light source can be used to check the fiber Bragg grating by the at least one electromagnetic wave, for example an LED (light-emitting diode) with a bandwidth of about 45 nm, an SLD (superluminescence diode) with a bandwidth of about 20 nm or a tunable laser with a bandwidth of about 100 nm. 
     It is proposed that the measurement element be in the form of a rod. The measurement element can advantageously be fitted easily and, for example, can be introduced into the flow channel through an opening. Furthermore, it is possible to achieve a situation in which the measurement element can be maintained with little fitting effort. For this purpose, the corresponding attachments are released and the measurement element is pulled out through the opening. In addition, of course, the measurement element may be in any other desired form. For example the measurement element may be circular or else in the form of an Archimedes screw. 
     A further refinement proposes that the measurement element be elastic. Depending on the purpose, the measurement element can therefore advantageously be pre-shaped quickly, thus making it possible to reduce the number of different measurement element shapes. Storage costs can be saved. 
     It is advantageous for the at least one heating element to be formed from metal. This ensures uniform heating along the heating elements. 
     It is also proposed that the at least one heating element be formed by a common electrically conductive coating on the optical waveguides, with the optical waveguides being in contact in the longitudinal direction. This makes it possible to further simplify the shape of the measurement element. The heating element can thus in each case be integrally connected in a simple manner to the waveguides which make contact with one another, thus allowing a protective function to be achieved for the waveguides by the heating elements, in addition to cost-effective production. By way of example, the conductive coating may be formed from a metal such as tungsten or else from an alloy such as steel or the like. 
     It is also proposed that the at least one heating element have a constant electrical resistivity. This advantageously makes it possible to apply heat uniformly to the measurement element over its longitudinal extent. For the purposes of this application, the electrical resistivity means the electrical resistance per length unit. 
     Furthermore, it is proposed that the resistivity be largely independent of temperature in the operating temperature range. This makes it possible for the respective heat supply, which is directed from the at least one heating element in the direction of the optical waveguides to be substantially independent of the instantaneous local temperature along the longitudinal extent of the measurement element. This makes it possible to increase the measurement accuracy as well as the reliability of the measurement. Furthermore, the at least one heating element may, for example, be formed from a material such as constantan. 
     One advantageous development proposes that the measurement element have a sheath. The measurement element can thus, for example, be protected against chemical attack. Furthermore, the sheath allows mechanical protection, for example during fitting. 
     It is also proposed that the sheath be composed of a ceramic material. A measurement element for a high temperature load can advantageously be formed with the ceramic sheath. 
     In addition, it is proposed that the sheath be formed by a metal sleeve. For example, the measurement element can then advantageously be protected against electrostatic charging, since the metal sleeve can be connected to a ground potential. 
     Furthermore, it is proposed that the sheath ( 8 ) at the same time have the at least one heating element ( 6 ,  7 ). Components and costs can be further reduced. 
     The inventors also propose a method for determining a flow direction of a fluid by a flowmeter, in which
         at least one electromagnetic wave is injected into at least two optical waveguides of a measurement element around which the fluid flows,   at least one heating element for the measurement element is supplied with electrical power such that.
           heat is applied by the heating elements to the optical waveguides, and   the at least one electromagnetic wave is influenced to a different extent as a function of the different local temperature in the at least two optical waveguides,   
           the different influences on the at least one electromagnetic wave are determined and the flow direction of the fluid at right angles to the longitudinal extent of the measurement element is determined from this.       

     The method results in the advantages that have been explained above for the flowmeter. 
     It is therefore also advantageous for the optical waveguides each to comprise at least one fiber Bragg grating and for the at least one electromagnetic wave to be influenced as a function of the different local temperatures at the location of the respective at least one fiber Bragg grating. 
     Furthermore, it is proposed that the at least one electromagnetic wave be formed by at least one electromagnetic pulse. Energy can advantageously be saved, and the measurement accuracy increased. The electromagnetic pulse may, for example be produced by a pulsed laser, which is injected into the optical waveguides by a suitable known coupling unit. 
     It is also proposed that the measurement element be heated in its longitudinal extent by the at least one heating element. The flow rate along the measurement element can also advantageously be determined from the temperature variation along the measurement element resulting from fluid flow. 
     It is advantageous for a constant electrical power to be applied to the at least one heating element. Particularly in the case of a resistance profile which is constant over the longitudinal extent of the measurement element, this makes it possible to ensure that a constant amount of heat is applied in each case, in accordance with Ohm&#39;s Law. This can be done by direct current or alternating current. In particular, the heating effect of the at least one heating element can be influenced by variation of the alternating-current frequency, if the frequency is varied in a range in which current displacement effects occur. 
     One advantageous development of the method proposes that a plurality of measurements be carried out with a different power applied. This allows the measurement accuracy to be increased further. 
     It is also proposed that a gas, in particular air, or a liquid, in particular water or liquid nitrogen be used as the fluid for cooling an electrical machine, in particular a generator or motor. The measurement element used in the flowmeter can in this case be matched cost-effectively to the physical and/or chemical requirements in the flow channel of a cooling device of the generator or of the motor. Furthermore, a flow distribution in the cross section of a flow channel can likewise be measured accurately. 
     Furthermore, the inventors propose an electrical machine having
         a rotor which is mounted such that it can rotate,   an associated fixed-position stator in a machine housing,   a device for cooling parts by a fluid within the machine housing, with the cooling device containing a line system,
 
and
   the flowmeter described above.       

     In this case, a measurement element, which is arranged in a flow channel of the line system of the flowmeter is provided in order to measure the flow direction of the fluid in the flow channel. 
     The electrical machine gains the advantages as explained above for the flowmeter. 
     The flowmeter makes it possible to achieve efficient cooling of the machine by monitoring the flow direction of the cooling fluid, for example air, in the flow channels of the cooling device. Any cessation of flow that occurs as a result of disadvantageously superimposed flows can in this case be identified sufficiently early that suitable measures can be taken in order to avoid local heating of and damage to the machine. The reliability of operation of the flow machine can therefore be increased. 
     It is proposed that the measurement element be arranged radially with respect to the cross section of the flow channel. In this case, the flow direction can advantageously be determined as a function of the radius of the flow channel cross section using a plurality of fiber Bragg gratings arranged one behind the other. A plurality of measurement elements may, of course, also be provided in the flow channel in order to make it possible to determine the flow direction at different circumferential positions of the flow channel. 
     It is also proposed that a plurality of measurement elements be arranged at a distance from one another axially in the flow channel. This advantageously allows axial changes in the flow direction to be recorded and evaluated. It is also possible to use a plurality of differently shaped measurement elements in order to obtain the desired information about the flow profile. For example, it is possible to combine radial measurement elements in the form of rods with measurement elements arranged along a circular line in the flow channel. In particular, it is proposed that the measurement elements be operated using the proposed method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  shows a side view of a measurement element of the proposed flowmeter in the form of a rod, with a connecting plug at one end, 
         FIG. 2  shows a section through one refinement of a measurement element with a heating conductor and two optical waveguides arranged parallel to it, 
         FIG. 3  shows a section through one refinement of a measurement element with two heating conductors and two optical waveguides arranged parallel to them, 
         FIG. 4  shows a section through one refinement of a measurement element with a heating conductor and four optical waveguides arranged parallel to it, 
         FIG. 5  shows a section through a further refinement of a measurement element with a heating element surrounding two optical waveguides, 
         FIG. 6  shows a section through a refinement of a measurement element with two optical waveguides which are arranged parallel and are each surrounded by a heating element, 
         FIG. 7  shows a section through a further refinement of a measurement element with heating element fitted directly to the surfaces of two touching optical waveguides, 
         FIG. 8  shows an outline circuit diagram of one embodiment of the flowmeter with the measurement element shown in  FIG. 2 , 
         FIG. 9  shows an outline circuit diagram of one embodiment of the flowmeter with the measurement element shown in  FIG. 3 , 
         FIG. 10  shows an outline circuit diagram of one embodiment of the flowmeter with the measurement element shown in  FIG. 4 , 
         FIG. 11  shows an outline circuit diagram of one embodiment of the flowmeter with the measurement element shown in  FIG. 6 , 
         FIG. 12  shows an outline circuit diagram of one embodiment of the flowmeter with the measurement element shown in  FIG. 5  or  7 , 
         FIG. 13  shows a cross section through a flow channel of a cooling device with a measurement element of the flowmeter, and 
         FIG. 14  shows a section through a generator with a plurality of measurement elements of the flowmeter. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  shows a side view of a measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  of the proposed flowmeter, with a plug connection  15  fitted to one end of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  for connection of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  to a control unit  20  and an evaluation unit  23  (see  FIGS. 8 to 12  and  FIG. 14 ). The measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  is in the form of a rod. Furthermore, the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  may be elastic, such that the geometric shape can be matched to the specified requirements. 
     In  FIGS. 2 to 13 , a coordinate system  80  is in each case associated with an x, y and a z axis in order to assist clarity. For the sake of simplicity, and without any restriction, it is assumed that the fluid  22  to be investigated is flowing in the x direction. The fluid  22  which is flowing in the x direction is in this case indicated by arrows pointing in the x direction. The fluid  22 , which is flowing in the x direction and arrives at the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  which extends in the y direction, flows around the latter. In particular, the fluid flow is a turbulent flow. Different flow rates occur on the surface  9  of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 . The length of the arrows in this case reflects the magnitude of the fluid velocity at the indicated location. While the velocities are highest on that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction, they are lowest on that part of the measurement element surface  9  which points substantially in the flow direction. In this case, heat is transported through the measurement element surface  9  inhomogeneously, as a function of the local flow rate. The heat transport on that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction, is therefore greater than on that part of the measurement element surface  9  which points substantially in the flow direction. If the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , which has at least one heating element  5 ,  6  or  7  for example at the center of the cross-sectional area of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , is in thermal equilibrium at least with respect to its cross section while heat is being applied to it by the at least one heating element  5 ,  6  or  7 , an optical waveguide  4   a  which is arranged at or relatively close to that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction will be at a lower temperature than an optical waveguide  4   b  which is arranged at or relatively close to that part of the measurement element surface  9  which points substantially in the flow direction. Optical waveguide  4   a  which is arranged at or relatively close to that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction is subjected to a lesser heat flow  10   a  from the direction of the at least one heating element  5 ,  6  or  7  than an optical waveguide  4   b  which is arranged at or relatively close to that part of the measurement element surface  9  which points substantially in the flow direction. The heat flow associated with this optical waveguide  4   b  is annotated  10   b . The arrows which point in the direction of the respective optical waveguide  4   a ,  4   b  starting from the at least one heating element  5 ,  6  or  7  in this case indicate the corresponding heat flow  10   a ,  10   b , whose magnitude is reflected in the respective arrow length. 
       FIG. 2 ,  FIG. 3  and  FIG. 4  show three refinements of a respective measurement element  1   a ,  1   b ,  1   c  of the flowmeter. According to the exemplary embodiment shown in  FIG. 2 , two optical waveguides  4   a ,  4   b  and a heating element  5  arranged between them are contained in the measurement element  1   a , embedded in a ceramic material. According to the exemplary embodiment in  FIG. 3 , two optical waveguides  4   a ,  4   b  and two heating elements  5  arranged in between them are contained, embedded in the ceramic material, in the measurement element  1   b . In each of the  FIGS. 2 and 3 , an optical waveguide  4   a  is arranged close to that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction, while the other optical waveguide  4   b  is positioned close to that part of the measurement element surface  9  which is directed substantially in the flow direction. The single heating element  5  in  FIG. 2  and the two heating elements  5  in  FIG. 3  are arranged on an axis of symmetry  30  of the respective measurement element  1   a  or  1   b , which at the same time represents the mirror axis with respect to the two optical waveguides  4   a ,  4   b , such that their respective distances from the two optical waveguides  4   a ,  4   b  correspond to one another. According to the exemplary embodiment in  FIG. 4 , four optical waveguides  4   a ,  4   b  and a heating element  5  arranged between them are contained, embedded in the ceramic material, in the measurement element  1   c . The four optical waveguides  4   a ,  4   b  are arranged in pairs close to that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction, and respectively close to that part of the measurement element surface  9  which is directed substantially in the flow direction. The heating element  5  is arranged on an axis of symmetry  30  of the measurement element  1   c , which at the same time represents the mirror axis with respect to the optical waveguide pairs  4   a ,  4   b , such that their distances from the respect optical waveguides  4   a ,  4   b  correspond to one another. By way of example, the optical waveguides  4   a ,  4   b  are glass or plastic fibers. The at least one heating element  5  and the optical waveguides  4   a ,  4   b  are embedded in a body  16  which is composed of ceramic material, in particular is cylindrical and is itself surrounded by a passivating sheath  8 . The one (see  FIGS. 2 and 4 ) or the two (see  FIG. 3 ) heating elements  5  is or are, for example, in the form of heating wires. The sheath  8  in one embodiment can also be formed from a metal, such that it is electrically conductive (see  FIGS. 8 and 10 ). 
       FIG. 5  shows a further refinement of a measurement element  2  of the flowmeter according to the invention with two optical waveguides  4   a  and  4   b  which are surrounded by a body  16  which is composed of ceramic material and in particular is cylindrical. One optical waveguide  4   a  is arranged close to that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction, while the other optical waveguide  4   b  is positioned close to that part of the measurement element surface  9  which is directed substantially in the flow direction. A heating element  6  is arranged around the ceramic body  16  such that it surrounds the measurement element  2 . In particular, the heating element  6  at the same time forms a sheath  8 , in the form of a sleeve for the measurement element  2 . 
       FIG. 6  shows a further refinement of a measurement element  31  of the flowmeter according to the invention with two optical waveguides  4   a  and  4   b . Each optical waveguide  4   a ,  4   b  is surrounded by a corresponding heating element  6   a ,  6   b  or  7   a ,  7   b  in the form of a sleeve  6   a ,  6   b  or a coating  7   a ,  7   b . The heating elements  6   a ,  6   b  or  7   a ,  7   b  are in turn surrounded by a body  16 , which is composed of ceramic material and in particular is cylindrical. One optical waveguide  4   a  with an associated heating element  6   a  or  7   a  is arranged close to that part of the measurement element surface  9  which is directed substantially in the opposite direction to the flow direction, while the other optical waveguide  4   b  is positioned, with the associated heating element  6   b  or  7   b , close to that part of the measurement element surface  9  which is directed substantially in the flow direction. The ceramic body  16  itself is in turn surrounded by a passivating sheath  8 , in the form of a sleeve, on the measurement element  31 . 
       FIG. 7  shows a section through a measurement element  3  of the flowmeter according to the invention, with two optical waveguides  4   a ,  4   b  lying one on top of the other having metal layers  7   a ,  7   b  vapor-deposited on them, at the same time representing a heating element  7  for the measurement element  3 . The metal layer  7  forms a common sheath  8  for the optical waveguides  4   a ,  4   b . This refinement is distinguished by elasticity such that the three-dimensional extent of the measurement element  3  can be adapted as required. Furthermore, the measurement element  3  is distinguished by a particularly simple production process, in which the pair of optical waveguides  4   a ,  4   b  are coated in a coating process of a conventional, known type, with a suitable electrically conductive material. The refinement is furthermore distinguished in that it has a particularly low heat capacity in comparison to the exemplary embodiments of the measurement element  1   a ,  1   b ,  1   c ,  2  or  31  shown in  FIGS. 1 to 6 , and therefore reacts more quickly to changing flow conditions. 
     The heating elements  5 ,  6  and  7  used in the abovementioned refinements are preferably formed from a metal or a metal alloy. By way of example, steel, copper, aluminum, bronze constantan or the like can be used, depending on the physical and/or chemical load. For high-temperature applications, for example in the flow channel of a gas turbine, a coating with a metal such as tungsten or the like is preferable. For applications at low temperatures in a chemically aggressive environment, for example, it is also possible to use conductive polymers. In the exemplary embodiments described here, the material of the heating elements  5 ,  6  and  7  in each case has a constant electrical resistance. In particular, the resistance in the operating temperature range is largely independent of the temperature. Application of a constant current or an alternating current with a constant root mean square value to the heating element  5 ,  6 ,  7  therefore leads to a power supply which is uniform over the length of the heating elements  5 ,  6 , and  7 , such that heat is applied uniformly to the corresponding heating element  5 ,  6  or  7  over the longitudinal extent of the respective measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 . 
       FIGS. 8 to 12  show exemplary embodiments of the flowmeter, in the form of outline circuit diagrams. The flowmeter in  FIG. 8  in this case comprises the measurement element  1   a  shown in  FIG. 2 , the flowmeter in  FIG. 9  comprises the measurement element  1   b  shown in  FIG. 3 , the flowmeter in  FIG. 10  comprises the measurement element  1   c  shown in  FIG. 4 , the flowmeter in  FIG. 11  comprises the measurement element  31  shown in  FIG. 6 , and the flowmeter in  FIG. 12  comprises the measurement element  2  or  3  shown in  FIG. 5  or  FIG. 7 , respectively. All the exemplary embodiments mentioned of the flowmeter also have a control unit  20  and evaluation unit  23 . The respective measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  extends with its longitudinal axis in the y direction. The measurement element  2  or  3  of the flowmeter shown in  FIG. 12  is electrically connected at its respective ends by heating element  6  or  7  to the control unit  20 , and is optically connected to the evaluation unit  23  at one of the two ends. In this case, the two optical waveguides  4   a  and  4   b  are jointly connected via a respective optical connecting fiber  25   a ,  25   b  to the evaluation unit  23 . The measurement element  1   a ,  1   b ,  1   c  or  31  of the flowmeter shown in  FIGS. 8 to 11  is electrically connected at one end to the control unit  20 , and is optically connected to the evaluation unit  23 , while the other end of the measurement element  1  is freely available. This allows the measurement element  1   a ,  1   b ,  1   c  or  31  to be fitted and/or handled particularly easily. One of the optical waveguides  4   a ,  4   b  of the measurement element  1   a ,  1   b ,  1   c  or  31  is connected via an optical connecting fiber  25  to the evaluation unit  23 , with the optical waveguides  4   a ,  4   b  being connected to one another in series. The individual optical waveguides  4   a ,  4   b  may, however, also be connected individually to the evaluation unit  23 , analogously to  FIG. 12 , without there necessarily being any connection between them. 
     The control unit  20  has an electrical power source  21 . The power source  21 , which has two connections, is connected to the heating elements  5 ,  6  or  7  in accordance with the exemplary embodiments, such that electrical power is applied to the heating element  5 ,  6  or  7 , and heat is produced. The electrical power source  21  is, in particular, a current source by which a constant direct current can be preset. 
     The fluid  22  flows around the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , in which case the fluid flow along the longitudinal extent of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  may have a different flow rate, indicated by the arrows of different length. The flow direction of the fluid  22  points in the x direction, for the sake of simplicity, as already mentioned above. In order to measure the flow direction of the fluid  22 , electrical power is applied to the heating element  5 ,  6  or  7  of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , as a result of which it is heated. The heating process should in this case continue at least until a thermal equilibrium has been reached in the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 . However, it can also be chosen to be shorter. 
     By the evaluation unit  23 , which has a light source, a detector and an analyzer, light in the form of a continuous laser beam or in the form of laser pulses is injected via the optical connecting fiber  25  into the optical waveguides  4   a ,  4   b  of the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , and light that is scattered back is analyzed by the analyzer. The measurement makes use of the effect that an electromagnetic wave which is injected into an optical waveguide  4   a ,  4   b  is scattered as it passes through the optical waveguides  4   a ,  4   b . A portion of the scattered light is scattered in the opposite direction, as a result of which it can be detected at the input of the optical waveguide  4   a ,  4   b . The temperature of the optical waveguide  4   a ,  4   b  can be deduced because this scattering effect is temperature-dependent. The light signal that is scattered back comprises different components which are of different suitability for the measurement requirements. For example, the signal that is scattered back contains a Raman-scattered component. In comparison to Raman technology, fiber Bragg grating technology makes it possible to achieve higher spatial resolution, which is preferable in particular for use of the temperature measurement in machines. 
     The laser light is produced in a known manner using equipment from the related art. Depending on the temperature, a portion of the laser light in the optical waveguides  4   a ,  4   b  is scattered back from fiber Bragg gratings  13 . This light signal which is scattered back is passed via the optical connecting fiber  25  to the evaluation unit  23 , which uses this signal to determine the temperature at the location of the fiber Bragg grating  13  in the optical waveguide  4 . 
     The evaluation unit  23  determines the corresponding temperatures, associated with the individual optical waveguides  4   a ,  4   b , within the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 . Different temperatures occur in a flowing fluid  22  with a directed flow at the location of the optical waveguides  4   a ,  4   b  in the measurement element  1 ,  2 , or  3 , depending on the relative position of the respective optical waveguide  4   a ,  4   b . The evaluation unit  23  compares the different temperatures with one another, for example by subtraction in a computer unit associated with the evaluation unit  23 , and from this determines the flow direction of the fluid  22 . 
     If the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  has a plurality of fiber Bragg gratings  13  along the optical waveguides  4   a ,  4   b , as is indicated in the exemplary embodiments in  FIG. 8  to  FIG. 12 , the flow rate with the flow rate distribution of the fluid  22  can be determined from the temperature distribution along the measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 . 
     In the exemplary embodiment of the flowmeter shown in  FIG. 8 , the measurement element  1   a  has a heating element  5  which, by way of example, is in the form of a heating wire. One connection of the power source  21  is connected to the heating element  5  and the other connection is connected to the electrically conductive sheath  8  on the measurement element  1   a . In this case, the heating element  5  is likewise connected at the opposite end of the measurement element  1   a  to the electrically conductive sheath  8 . In the exemplary embodiment of the flowmeter shown in  FIG. 9 , the measurement element  1   b  has two parallel heating elements  5 , which, for example, are in the form of heating wires, with the two heating wires being jointly connected at one end of the measurement element  1   b  to an electrical connecting conductor. At the other end of the measurement element  1   b , one of the two heating elements  5  is in this case connected to one connection of the power source  21 , and the other heating element  5  is connected to the other connection of the power source  21 . At one end of the measurement element  1   b , the two optical waveguides  4   a ,  4   b  which run parallel are likewise jointly connected to an optical connecting fiber, while at the other end of the measurement element  1   b , only one of the two optical waveguides  4   a ,  4   b  is connected via an optical connecting fiber  25  to the evaluation unit  23 . 
     In the exemplary embodiment of the flowmeter shown in  FIG. 10 , the measurement element  1  has a total of four parallel optical waveguides  4   a ,  4   b . A heating element  5  in the form of a heating wire is arranged between the optical waveguides. One connection of the power source  21  is connected to the heating element  5 , and the other connection is connected to the electrically conductive sheath  8  on the measurement element  1   c . In this case, the heating element  5  is likewise connected at the opposite end of the measurement element  1   c  to the electrically conductive sheath  8 . The four optical waveguides  4   a ,  4   b  are connected to one another in series by optical connecting fibers at the measurement element ends, such that only one of the optical waveguides  4   a ,  4   b  is directly connected to the evaluation unit  23 . The use of a measurement element  1   c  such as this with numerous optical waveguides  4   a ,  4   b  which, for example, are arranged in a circular shape around the heating element  5  that is arranged at the center of the cross section of the measurement element  1   c , allows the flow direction of the fluid  22  to be determined even more accurately. In the exemplary embodiment of the flowmeter shown in  FIG. 11 , the measurement element  31  has two parallel heating elements  5  which, for example, are in the form of electrically conductive sleeves or coatings and are both connected together at one end of the measurement element  31 . In this case, at the other end of the measurement element  31 , one of the two heating elements  5  is connected to one connection of the power source  21 , and the other heating element  5  is connected to the other connection of the power source  21 . The two optical waveguides  4   a ,  4   b , which run parallel are likewise connected to one another by an optical connecting fiber at one end of the measurement element  1   b , while at the other end of the measurement element  1   b , only one of the two optical waveguides  4   a ,  4   b  is connected via an optical connecting fiber  25  to the evaluation unit  23 . 
     However, analogously to  FIG. 8  and  FIG. 10 , it is also feasible for the two heating elements  5  from  FIG. 9  and  FIG. 11  to be jointly connected together at one end to the sheath  8 , which in this case is designed to be electrically conductive, in such a way that the power source  21  can be likewise be connected to the sheath  8  at the other end of the respective measurement element  1   b  or  31 . In this refinement example, which is not illustrated, it must be possible for both heating elements  5  to be jointly connected to one and the same connection of the power source  21 . 
     In the exemplary embodiment of the flowmeter shown in  FIG. 12 , one connection of the power source  21  is connected at one end of the measurement element  2  or  3  to the heating element  6  or  7 , which is in the form of an electrically conductive sleeve  8  ( FIG. 5 ) or an electrically conductive coating ( FIG. 7 ). The second connection of the power source  21  is connected at the other end of the measurement element  2  or  3  by an electrical line to the heating element  6  or  7 . 
       FIG. 9  shows a round cross section of a flow channel  14  through which a fluid  22  flows in the x direction. In this case, the flow channel  14  is, as an example, provided with two measurement elements  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , which are arranged radially with respect to the flow channel cross section. The two measurement elements  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  are connected via an electrical connecting line  26  to the control unit  20 , and via an optical connecting fiber  25  to the evaluation unit  23 . 
       FIG. 10  schematically illustrates a generator as an electrical machine. The generator has a stationary stator  19 , which is firmly connected to a housing  28 , and a rotor  18  which is mounted such that it can move on a rotor shaft  17 . The generator is cooled, for example with air as a cooling fluid  22 , by a cooling device. For this purpose, the cooling device has two fans  27  which pass the cooling air  22  through the generator by a line system. For this purpose the line system has numerous flow channels, in particular in the stator  19  as well. In the illustrated exemplary embodiment, the cooling air  22  is passed from the outside inward in the direction of the rotor  18  through the stator  19 , and is then transported to the outside through a gap arranged between the stator  19  and the rotor  18 . At the same time, however, air  22  can be sucked in by the rotor of the generator, and can be forced through the stator  19  in the opposite direction, from the inside outward. If the two air flows are disadvantageously superimposed, this results in the flow being stationary within the line system and therefore, possibly, in local overheating of and damage to the generator. In order to avoid this, the flow direction in the flow channels is monitored by the flowmeter. In this exemplary embodiment, two flow channels each having one measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  of the flowmeter are provided, as an example, at two points in the generator. The two measurement elements  1   a ,  1   b ,  1   c ,  2 ,  3  or  31  are in this case connected to the associated control unit  20  and evaluation unit  23 . In the event of irregularities in the cooling air flow, it is therefore possible to react and to initiate suitable protective measures in good time. 
     The use of the flowmeter in an air-cooled generator in this case serves only as an example. It is also possible to use the flowmeter in electrical machines which are cooled by hydrogen gas, a noble gas or any other desired gas as the fluid  22 . A cooling liquid, such as water or else in the case of cryogenic cooling a liquid noble gas or liquid nitrogen, can also be provided as the cooling fluid  22 . 
     The flowmeter can also be used in a turbine, for example in a steam turbine or a gas turbine. The flowmeter thus makes it possible to measure flow directions in particular in turbulent flow areas in the associated cooling air system, in the associated compressor, at the associated compressor inlet and/or in the corresponding exhaust gas flow. 
     The exemplary embodiments illustrated in the figures serve only to explain the method and device and have no restrictive effect on it. In particular, the type of measurement element  1   a ,  1   b ,  1   c ,  2 ,  3  or  31 , in particular its geometric shape, may thus be varied without departing from the scope of protection. Furthermore, of course, a plurality of elements may also be interconnected in order to make it possible to investigate specific changes in the flow direction more accurately. 
     The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide  v.  DIRECTV , 69 USPQ2d 1865 (Fed. Cir. 2004).