DE102015100213B3 - Arrangement for detecting the flow rate field of a fluid flow in a flow cross-section - Google Patents

Abstract

The invention relates to an arrangement for detecting the flow velocity of a fluid flow in a flow cross-section, comprising a sensor element, a voltage application device and an evaluation device, wherein the sensor element has a plurality of formation of crossing points lattice-like electrodes and counter electrodes with interposed vibratable bodies, wherein the voltage application Device for applying an electrical voltage between the electrode and the counter electrode of the individual intersection points is formed, so that the electric field caused thereby is varied by the flow-induced vibration of the oscillatory body when introduced into the fluid flow sensor element, and wherein the evaluation device for determining the local flow velocities is formed at the individual intersection points by evaluating the associated field variation.

Description

The invention relates to an arrangement for detecting the flow velocity field of a fluid flow across a flow cross-section. The arrangement may, for. B. are used for the investigation of gas and liquid flows in flow channels.

In many areas, knowledge of the spatial flow velocity distribution of fluid flows across the flow area is relevant. The measurement of velocity fields in flows can, for. B. with laser-optical measurement methods such as particle image velocimetry or laser Doppler anemometry, but also with sound-based methods, such as the ultrasonic Doppler velocimetry, are made. However, these measurement methods require an optical or acoustic access to the flow as well as clear fluids, which must be loaded with a sufficient number of small buoyancy-neutral foreign particles. This is not always possible, especially not in many real process applications.

The pamphlets DE 10 2007 019 927 B3 . US 4,339,661 A . DE 10 2009 043 345 A1 . DE 10 2010 018 494 A1 . US Pat. No. 5,922,970 A . US 4,397,192 A. . DE 10 2010 060 131 A1 and AT 505 522 A1 concern measurements in flow fields with grids, the pamphlets US Pat. No. 4,592,240 . US 5 109 704 A . DE 690 02 249 T2 . FR 2 587 481 A1 . US 2012/0 325 014 A1 and US 2002/0 020 225 A1 pertain to flow velocity detection due to eddies generated in a flow field by means of a vibratory body.

The invention provides an uncomplicated sensor arrangement by means of which the flow velocity field (ie the spatial flow velocity distribution) of a fluid flow over a flow cross-section of the fluid flow can be detected, wherein the arrangement can also be used to characterize turbid fluids and without the use of foreign particles gets along. The sensor arrangement can also be designed robust against hot and aggressive fluids and can be realized with an efficient, uncomplicated electronic scheme. The sensor arrangement is suitable for the investigation of fluid flows, for. B. liquid flows, gas flows, but also multi-phase flows.

According to the invention, there is provided an arrangement for detecting the flow velocity field of fluid flow present in a flow section (also referred to as "sensor array" or "sensor device").

The sensor arrangement has a sensor element with a plurality of electrodes, a plurality of counterelectrodes and a plurality of oscillatable bodies. The electrodes are arranged running in a first plane, the counter electrodes are arranged to extend in a second plane, wherein the first and the second plane are parallel to each other and are arranged at a distance from one another. The electrodes and the counterelectrodes are arranged like a lattice with the formation of crossing points (therefore, the lattice-like structure formed of the electrodes and the counterelectrodes is also referred to as "electrode lattice" and the sensor element also as "lattice sensor"), whereby at each lattice crossing point one of the electrodes and Cross one of the counter electrodes of the electrode grid.

Preferably, both the electrodes and the counterelectrodes are rectilinear or rod-shaped, with all the electrodes running parallel to one another in the first plane along a first longitudinal direction, all counterelectrodes being parallel to each other in the second plane along a second longitudinal direction, and the first longitudinal direction not parallel to the second longitudinal direction. Preferably, the first and the second longitudinal direction form an angle of 90 °. The first and second levels are also referred to as the first and second lattice planes.

The electrodes and the counter electrodes can, for. B. be mounted in a sensor frame such that they are electrically isolated from each other and against the ground potential. The sensor frame can, for. B. in the form of a flange which can be mounted in a serving to guide the fluid flow pipe.

Each of the vibratable bodies is disposed at one of the crossing points of the electrode grid between the electrode and the counter electrode of the crossing point, so that the vibratable bodies are arranged between the first and second lattice planes. In this case, the electrode and the counterelectrode of a crossing point denote that electrode and counterelectrode which intersect at this point of intersection. The electrode and the counterelectrode of each intersection point form an electrode pair associated with this intersection point.

In particular, provision may be made for such a vibratable body to be arranged at each of the intersection points of the grating sensor. The oscillatory bodies are also referred to as vibrating bodies.

The sensor element acts as a sensor and is provided for introduction into the fluid flow to be examined such that the Electrodes are arranged upstream of the counter electrodes. Thus, the electrodes are also referred to as upstream electrodes, the counter electrodes also as downstream electrodes. Unless the context dictates otherwise, the term "electrodes" hereafter refers to the upstream electrodes and the term "counter electrodes" to the downstream electrodes.

In such a sensor element introduced into the fluid flow, a periodic formation and detachment of fluid vortices takes place at the electrodes, so that a Karman vortex street is formed on each of the electrodes and extends in the direction of flow from the electrodes to the counterelectrodes.

For a given flow velocity ν, the vortex shedding frequency f is proportional to the flow velocity of the fluid, with the relationship between the flow velocity ν, the characteristic dimension d of the vortex inducing obstruction, and the disengagement frequency f of the fluid vortex f = Sr / dν (1) with the Strouhalzahl Sr consists. In the present case, the characteristic extent d z. B. given by the diameter d of the electrodes.

Thus, based on the present at a crossing point local vortex shedding frequency present at this intersection point local flow velocity of the fluid flow can be determined. From the knowledge of the local flow velocities at a plurality of crossing points distributed over the electrode grid or the flow cross-section, the flow velocity field present across the flow cross-section can again be determined.

The fluid vortices detaching from the electrodes cause them downstream of them, i. H. in the region between the electrodes and the counter electrodes, periodic pressure fluctuations in the fluid. These pressure fluctuations, in turn, cause oscillation of the vibratable body disposed between the first and second lattice planes, and since the vortex shedding frequency is proportional to the flow velocity, the oscillation frequency is also proportional to the flow velocity. Thus, based on the present at a crossing point local vibration (in particular the oscillation frequency) present at this intersection point local flow velocity of the fluid can be determined.

For this purpose, the sensor arrangement has a device (also referred to as a voltage application device) which is designed to apply an electrical voltage between the electrode and the counter electrode of each of the points of intersection with the oscillating bodies. The Spannungsbeaufschlagungs device may in particular be designed such that from her the electrode pairs of (ie all) crossing points at which an oscillating body is provided, are applied successively with an electrical voltage, so that in each case only one of these crossing points an electric field is present ,

When a voltage is applied between the electrode and the counterelectrode of one of the crossing points, a (local) electric field forms at this point of intersection between the electrode and the counterelectrode. This electric field is periodically varied periodically by the oscillation of the oscillatory body arranged between the electrode and the counterelectrode-and thus in the region of the electric field-as a function of the local flow velocity present at the crossing point, in particular the variation frequency of the electric field due to the oscillation frequency of the vibrating body is determined. This variation of the electric field in turn is detected by means of an electrical measuring method and evaluated by determining the flow velocity.

For this purpose, the sensor arrangement has an evaluation device. The evaluation device is connected to the sensor element and designed such that the temporal variation of the electric field can be detected by it for the individual points of intersection provided with the oscillating bodies and the flow velocity can be determined on the basis of the detected temporal variation. The evaluation device is thus designed to detect the variation of the (local) electric field at each of these intersection points and to determine the (local) flow velocity at each of these intersection points based on the detected associated field variation, so that the flow velocity field can be determined from the flow velocities determined for the individual intersection points can be determined.

The evaluation can z. B. be formed such that from her present at the different intersection points local variation frequencies of the electric field can be detected and based on the present at the intersection points local flow velocities can be determined.

Thus, the sensor arrangement z. B. be designed such that by means of Voltage application means, the electrode pairs of the individual crossing points are sequentially applied with an electrical voltage, and detected by the evaluation device, the variation (eg, the variation frequency) of the electric field at the respective voltage applied to the pair of electrodes and based on that at the associated crossing point present local flow rate is determined so that the present at the different intersection points local flow velocities can be determined and thus also the associated flow velocity field.

For detecting the temporal variation of the electric field at the individual intersection points, the evaluation z. B. for detecting an electrical measurement variable, which varies with the variation of the electric field, so that the local flow velocities present at the intersection points with the oscillatory bodies can be determined by the evaluation device on the basis of the variation of the electrical measured variable.

For example, the evaluation device can be designed such that the electrical current flowing therefrom when a voltage is applied between the electrode and the counterelectrode of a crossover point is detected as a measured variable, and that from it the oscillation-induced modulation of the current signal is present at the crossover point local flow velocity is determined.

As another example, the evaluation device may be designed in such a way that it detects the capacitance of the capacitor formed by the electrode and the counterelectrode of an intersection point as a measured variable, and that it uses it to determine the local flow velocity present at the intersection point on the basis of the vibration-induced temporal modulation of the capacitance signal is determined.

The evaluation device can in particular be designed such that it determines the oscillation frequency of the oscillating body at the associated intersection point on the basis of the detected variation of the electrical measured variable and determines the local flow velocity present at the intersection point on the basis of the determined oscillation frequency.

By means of the sensor arrangement, the local flow velocity can be determined at a plurality of measuring positions distributed over the electrode grid and thus over the flow cross section (namely the intersection points with the oscillatable bodies), the flow velocity field present along this two-dimensional cross section can be detected. By detecting the flow velocity field on the basis of the temporal variation of the local electric fields, the arrangement is also suitable for investigating turbid flows, wherein in addition no foreign particles have to be added to the flowing medium. By an appropriate choice of material, the sensor arrangement can be robust against hot and aggressive fluids. In addition, the sensor arrangement can be operated with an uncomplicated electrical wiring, as explained in more detail below.

According to one embodiment, each of the oscillating bodies is attached to the electrode and / or to the counter electrode of the associated crossing point of the electrode grid. According to this embodiment, the lattice sensor can be designed uncomplicated, wherein in addition the influence of the flow to be characterized by the lattice sensor can be kept low.

It can, for. B. be provided that each of the vibrating body is attached to the electrode of the associated crossing point and is separated by a free gap of the counter electrode of the associated crossing point. Analogously, it can be provided that each of the oscillatory bodies is attached to the counter electrode of the associated crossing point and is separated by means of a free gap of the electrode of the associated crossing point. In other words, it can thus generally be provided that each of the oscillating bodies is fastened to one of the two electrodes of the electrode pair of the associated crossing point and is separated from the respective other electrode of the electrode pair by means of a free gap.

It can be provided that each of the oscillating body consists of an electrically insulating material, for. B. a plastic.

According to a further embodiment, each of the vibrating bodies has a surface layer made of an electrically conductive material or consists of an electrically conductive material (such vibrating bodies are also referred to below as "electrically conductive vibrating bodies"). According to this embodiment, a particularly effective vibration-induced variation of the electric field can be made possible due to the electrical conductivity of the oscillating body, and thus the sensitivity of the sensor arrangement can be increased.

It can, for. B. be provided that the vibrating body are formed as electrically conductive vibrating body, wherein each of the vibrating body so on one of the two electrodes of the Pair of electrodes of the associated intersection point is attached, that it is electrically conductively connected to this electrode, and is separated by means of a free gap of the respective other electrode of the electrode pair. By the electrically conductive vibrating body being electrically conductively connected to one of the two electrodes, so to speak a two-electrode system with a vibrating electrode is formed, whereby the oscillation-induced variation of the electric field can be detected even better.

Thus, it can be provided that the oscillating bodies are designed as electrically conductive vibrating bodies, wherein each of the oscillating bodies is fixed in an electrically conductive manner to the electrode of the associated crossover point and is separated from the counterelectrode of the crossing point by means of a gap. Analog can be provided that the vibrating body are formed as electrically conductive oscillating body, each of the vibrating body is electrically conductively attached to the counter electrode of the associated crossing point and is separated by a gap from the electrode of the crossing point.

According to a further embodiment, the sensor element for bundling the electric field lines at each of the points of intersection with the oscillatory bodies comprises an element (also referred to as a "field focusing body") which has a surface layer of an electrically conductive material or consists of an electrically conductive material. By focusing the field lines of the respective local electric fields by means of the Feldfokussierkörper, the vibration-induced variation of these electric fields can be detected even more reliable.

In the configuration described above, such that each of the vibrating bodies is fixed to the first of the two electrodes of the pair of electrodes of the associated crossing point and is separated by a gap from the second electrode of the pair of electrodes, z. B. be provided that the Feldfokussierkörper is attached to the gap-side second electrode of the electrode pair and is electrically conductively connected to the same, wherein the Feldfokussierkörper is arranged such that it protrudes toward the vibrating body in the gap.

So z. B. in a configuration such that each of the vibrating body is attached to the electrode of the associated crossing point and is separated by a gap from the counter electrode, provided that the Feldfokussierkörper is electrically conductively attached to the counter electrode and protrudes into the gap. Similarly, in a configuration such that each of the vibrating bodies is fixed to the counterelectrode of the associated crossing point and is separated from the electrode by means of a gap, it can be provided that the field focusing body is electrically conductively attached to the electrode and protrudes into the gap.

According to one embodiment, the vibratable bodies are vibratable by being elastically deformable (eg by being made of an elastically deformable material). As a result, the vibration capability is made possible in a straightforward manner.

According to another embodiment, the vibratable bodies are oscillatable in that they are oscillatably mounted (eg as explained above on the electrode and / or the counterelectrode of the associated crossing point), wherein the vibratable bodies according to this embodiment are also rigid bodies can be (for example, by being made of a rigid or inelastic material). As a result, the ability to vibrate can be ensured even when using solid and therefore durable materials.

To characterize the fluid flow, the sensor element is introduced into the same in such a way that the electrodes arranged in the first lattice plane are located upstream of the counterelectrodes arranged in the second lattice plane, the flow direction of the fluid flow being perpendicular to the first (and thus also to the second) lattice plane ,

The vibrating body can z. Flat platelets (eg of an elastic material) arranged such that the platelet plane is parallel to the longitudinal direction of the (upstream) electrodes and perpendicular to the first lattice plane (i.e., parallel to the normal direction of the first lattice plane). Since the fluid vortices always form alternately on both sides of the electrodes, this configuration enables a particularly effective vibration excitation by the fluid vortices.

According to one embodiment, the electrodes have a cross-section that tapers in the direction of the counterelectrodes. It can, for. B. be provided that each of the electrodes has a flat side which is parallel to the first lattice plane, said flat side facing upstream and thus is flowed perpendicular to the fluid flow. Furthermore, the cross-section of the electrodes, starting from this flat side, may have a section which tapers symmetrically in the direction towards the counterelectrodes, this section being, for. B. may be triangular or trapezoidal (for example, may have the shape of an isosceles triangle or isosceles trapezoid). In this case, the vortex shedding zone is largely independent of the Flow rate always at the same point of the electrode cross-section, so that the Strouhalzahl changes only slightly with the Reynolds number of flow and thus can be regarded as constant, which simplifies the process of recalculation of vortex frequency flow velocity.

The voltage application device can be designed to apply a DC voltage or an AC voltage to the electrode pairs (formed from the electrode and the counterelectrode of a respective intersection point).

When using the sensor arrangement for the investigation of electrically conductive fluid flows or flow fluids, the excitation can be effected by means of a DC voltage. However, in the case of electrically conductive fluids, it may also be contemplated to alternatively use bipolar pulse excitation or AC excitation.

When using the sensor arrangement to investigate flows of non-electrically conductive (i.e., electrically insulating) fluids, the excitation may be by means of an AC voltage. In this case, the flow rate z. B. by means of the capacitive measurement described above, z. B. by the electrode pair of a respective crossing point in a short time interval (pulse time) is applied with an AC signal.

The invention will be explained below with reference to exemplary embodiments with reference to the accompanying figures, wherein the same or similar features are provided with the same reference numerals, which show schematically:

1 a sensor element with an electrode grid,

2 a representation of two grid intersection points with oscillating bodies,

3 a sectional view of a portion of the sensor element, and

4 an exemplary electronic circuit of the sensor element.

1 shows a sensor element 1 a sensor arrangement according to an embodiment. The sensor element 1 has an electrode grid 3 with several rod-shaped electrodes 5 and a plurality of rod-shaped counter electrodes 7 on, according to 1 the electrodes 5 perpendicular and the counter electrodes 7 horizontally, so that the longitudinal direction of the electrodes 5 an angle of 90 ° to the longitudinal direction of the counter electrodes 7 having. Thus, of the electrodes 5 and the counter electrodes 7 several crossing points 9 formed, with each crossing point 9 by crossing one of the electrodes 5 and one of the counter electrodes 7 is defined. In 1 is an example of one of the crossing points 9 characterized.

The sensor element 1 also has a sensor frame 11 on, with the electrodes 5 and the counter electrodes 7 in the sensor frame 11 are secured against each other and against the ground potential electrically isolated.

2 shows in detail the embodiment of two grid crossing points 9 of the electrode grid 3 , The electrodes 5 are arranged extending in a first plane, the counter electrodes 7 are arranged running in a second plane, wherein the first and the second lattice plane are parallel to each other and have a (small) distance from each other. The sensor element 1 also has several oscillatory bodies 13 on (also as oscillating body 13 designated). Each of the vibrating bodies 13 is at one of the crossing points 9 between the electrode 5 and the counter electrode 7 arranged the crossing point. The present example is at each of the crossing points 9 of the electrode grid 3 such a vibrating body 13 intended.

Each of the vibrating bodies 13 is as a flat tile 13 formed on the electrode 5 the associated crossing point 9 is attached and towards the counter electrode 7 of the crossing point 9 extends. The dimensions of the plate 13 are dimensioned such that a free gap between the free end of the plate 13 and the counter electrode 7 remains. The tiles 13 can be made of a rigid material and vibrate on the electrodes 5 be stored. Here are the tiles 13 however as elastic platelets 13 formed (for example, by being made of an elastically deformable material). In a vibration, the oscillatory motion of the platelets resembles 13 hence the movement of a swinging flag, so the tiles 13 in the following also as flag-like platelets, swinging platelets or flags 13 be designated. The flat tiles 13 are so on the electrodes 5 attached that platelet plane parallel to the longitudinal direction of the electrodes 5 and perpendicular to the first lattice plane. The vibrating plates 13 may consist of an electrically insulating material or be formed as electrically conductive vibrating plates (ie have an electrically conductive surface layer or an electrically conductive material).

Furthermore, the sensor element 1 at each grid crossing point 9 an element called field focuser 15 from an electric conductive material that is electrically conductive with the counter electrode 7 is connected and the vibrating plate 13 is arranged opposite. Each field focuser 15 is present a flat plate 15 whose plate plane is parallel to the longitudinal direction of the electrodes 5 and is perpendicular to the first lattice plane, so that the plate plane of Feldfokussierkörper 15 in the same plane as the platelet plane of the associated vibrating plate 13 , Thus, the Feldfokussierkörper protrudes 15 with its narrow side into the free gap, between the swinging plate 13 and the counter electrode 7 is formed so that upon application of an electric field between the electrode 5 and the counter electrode 9 the electric field lines on the narrow side of Feldfokussierkörpers 15 be bundled, whereby the sensitivity of the measuring arrangement is increased.

The sensor element 1 is provided for introduction into the fluid flow to be examined such that the electrodes 5 upstream of the counter electrodes 7 are arranged and the fluid flow thus from the electrodes 5 towards the counter electrodes 7 goes. With such incorporated in the fluid flow sensor element 1 form at the (upstream) electrodes 5 fluid vortex 17 which peel off and one towards the counter electrodes 7 forming a Kárman vortex street.

Adjacent to the (upstream) electrodes 5 detaching fluid vortex 17 cause behind these periodic pressure fluctuations. These should be in every crossing point of the electrode grid 3 by means of the vibrating plates 13 be measured. The vibrating plates 13 consist of an elastic material, with material and thickness of the vibrating plates 13 are chosen so that they can swing freely back and forth in the flow and this vibration is minimized as little as possible by their material stiffness.

By the flow of one of the upstream electrodes 5 at a crossroads 9 dissolve at this fluid vortex 17 with a frequency which is proportional to the local flow velocity. Through the fluid vortex 17 form in the flow direction behind the electrode 5 Pressure fluctuations in the flowing fluid, which at the crossing point 9 arranged vibrating plates 13 to vibrate synchronously, as in 3 by means of the double arrow 19 indicated. 3 illustrates a sectional view of a portion of the sensor element 1 in plan view, wherein the flow direction of the fluid flow to be examined by the arrow 21 is illustrated.

The sensor arrangement has in addition to the sensor element 1 a voltage application device 23 and an evaluation device 25 on.

The voltage application device 23 is formed such that from it the electrode pairs of the different crossing points 9 successively or sequentially be acted upon by an electrical voltage (wherein the electrode pair of a crossing point of the electrode 7 and the counter electrode 9 exists that intersect at this crossing point). Depending on the purpose, the voltage application device 23 be designed for applying the electrode pairs with a DC voltage or an AC voltage, as explained in more detail below.

At each crossing point form the associated electrode 5 and counter electrode 7 a two-electrode system. When applying an electrical voltage between the upstream electrode 5 and downstream counter electrode 7 forms a concentrated local electric field between the two 27 out. Through the vortex-induced vibration of the vibrating plate 13 the electric field changes 27 between the electrode 5 and the counter electrode 7 periodically.

The evaluation device 25 is to detect this change in the electric field 27 formed by an electrical measuring method and determining the flow velocity based on the detected field change. In the present case this is done by measuring the electric current between the electrode 5 and the counter electrode 7 of each voltage-loaded electrode pair flows. By the periodic change of the local electric field 27 at a crossroads 9 is detected, the local oscillation frequency of the vibrating plate 13 are detected at this crossing point, which in turn - since the local oscillation frequency is proportional to the local flow velocity - the local flow velocity at the crossing point can be determined.

The evaluation device 25 So is to measure the between the electrode 5 and the counter electrode 7 formed of the respective voltage-loaded electrode pair flowing electric current, wherein the electric current is depending on the applied voltage to a DC or AC. The evaluation device 25 is also designed to detect the temporal modulation of the detected current signal and determine the present at the associated intersection point local flow velocity based on the detected modulation of the current signal.

The modulation of the current signal takes place by changing the electric field according to the following possible variants:
Case 1: The flowing fluid is electrically conductive (conductivity σ _fluid ). The vibrating plates 13 are electrically conductive with a conductivity σ _vane , where σ _vane »σ _fluid . Lie upstream electrode 5 and downstream counter electrode 7 on to the voltage U different electrical potentials, then there is the Ohm's law, the electric current to I (t) = U / R (t) = Uσ _fluid / K (t). Here, K (t) is due to the movement of the vibrating plates 13 time-varying geometry factor of the electrode arrangement.

Case 2: The flowing fluid is electrically conductive. The vibrating plates 13 are electrically non-conductive. In this case, the geometry factor remains unchanged in time, since the geometry of the conductive electrode components does not change. However, by the movement of the vibrating plates 13 at the crossing point, the total resistance of the flowing medium between the electrode 5 and the counter electrode 7 changed, since electrically conductive volumes (fluid) and electrically non-conductive volumes (vibrating plates 13 ) in their shares change over time. It therefore applies I (t) = UR / (t).

Case 3: The flowing fluid is electrically non-conductive. The vibrating plates 13 are electrically conductive. For non-conductive fluids, use an alternating current measurement method by applying an alternating voltage. The electrode 5 , the counter electrode 7 and the fluid form a capacitor with the capacity C = ε ₀ ε _{r, fluid} K. Here, K is again a geometry factor, the area and distance between the electrode 5 and counter electrode 7 ε ₀ is the vacuum permittivity and ε _{r, fluid is} the relative permittivity of the fluid (for example ε _{r, fluid} ≈ 1 for gases or ε _{r, fluid} ≈ 2 for oil). As a result of the oscillatory movement of the vibrating plates 13 the geometry factor changes in time. Is between upstream electrode 5 and downstream counter electrode 7 a high-frequency alternating voltage of the angular frequency ω applied, the result is an AC signal I (ω) = jωC (t) U (ω) = jωε ₀ ε _{r, fluid} K (t) U (ω). During a measurement, the electrical excitation frequency ω must be chosen clearly above the highest expected flow-induced oscillation frequency (ω »2πf).

Case 4: The flowing fluid is not electrically conductive. The vibrating plates 13 are electrically non-conductive. The measurement method corresponds to case 3. Differs the relative permittivity ε _{r of} the material of the vibrating plates 13 from the fluid (εr, fluid ≠ ε _{r, flag} ), then causes a flow-induced movement of the vibrating plates 13 also a change in the alternating current. Here, however, not by a change in the geometry factor of the electrode assembly, but by an analogous to Case 2, time-varying effective permittivity of the volume between the electrode 5 and the counter electrodes 7 , as the volume fractions of fluid and vibrating plates 13 in the area between electrode 5 and counter electrode 7 change over time.

4 shows an exemplary embodiment of the voltage application device 23 and the evaluation device 25 ,

According to 4 has the voltage application device 23 a voltage signal source 29 and a switching device with a plurality of analog switches 31 on. The quasi-synchronous and rapid measurement of the fluid velocity in the entire cross section is carried out by a special multiplex measuring method. These are the electrodes 5 the first grid level in succession via the analogue switches 31 with a voltage signal from the voltage signal source 29 applied. With conductive fluid (case 1 and 2), the excitation can be done by a DC voltage signal or a DC pulse. To avoid electrochemical reactions at the electrodes 5 and counter electrodes 7 when measured in low conductivity liquids, alternatively, a bipolar pulse excitation scheme or an AC excitation scheme may be employed. For capacitive measurements (case 3 and 4) the electrodes become 5 the first lattice plane in a short time interval (pulse time) applied sequentially with an AC signal. The necessary designs of the electronics are assumed to be familiar to the person skilled in the art.

According to 4 has the evaluation device 25 several current-voltage converters 33 , several demodulators 35 , several analog-to-digital converters 37 as well as a measuring computer 39 on; wherein each counter electrode 7 successively a current-voltage converter 33 , a demodulator 35 and an analog-to-digital converter 37 is downstream.

By means of the evaluation device 25 becomes simultaneously the excitation of the corresponding electrodes 5 at each counter electrode 7 the second lattice plane of the current through the currently voltage-loaded intersection point flowing direct or alternating electric current with the aid of the counter electrode 7 downstream current-voltage converter 33 converted into a voltage signal. From the analog voltage signal is with the help of the downstream demodulator 35 a characteristic size determined. This can be at AC voltage excitation z. B. the RMS value, the peak value or the phase position of the AC signal in the excitation time window. For DC excitation, the signal amplitude is after a determined settling time of the current-voltage converter by means of a sample and hold circuit generated. That through the demodulator 35 Generated analog signal is then connected to the downstream analog-to-digital converter 37 digitized and the measuring computer 39 supplied for evaluation. For fast and continuous measurement, the signals can be converted in parallel on the receiver side and fed to the measured value processing. Furthermore, it is assumed to be known that the signal demodulation, which in this embodiment is performed by a demodulator circuit, also by software in the measuring computer 39 can be accomplished. In this case, the output signal is the current-to-voltage converter 33 with a correspondingly high rate through the analog-to-digital converters 37 to scan directly.

Thus, when using a DC voltage, the sensor arrangement (by means of appropriate design of the voltage application device 23 and the evaluation device 25 ) be designed such that the voltage source 29 a DC voltage to the analogue switches 31 applies; the analog switches 31 the DC signal successively to the individual upstream electrodes 5 switch; the downstream counterelectrodes 7 downstream current-voltage converter 33 in the crossing points 9 of the electrode grid 3 convert flowing DC into a voltage; the current-voltage converters 33 downstream demodulators 35 provide the amplitude of the DC signal as a signal at a predetermined sampling instant in the excitation window; and the demodulators 35 downstream analog-to-digital converter 37 Digitize the signal voltage and the digitized signal voltage to the measuring computer 39 passed as output signal.

When using an AC voltage, the sensor arrangement (by means of appropriate design of the voltage application device 23 and the evaluation device 25 ) be designed such that the voltage source 29 generates an AC voltage whose frequency is at least half the sampling frequency of the analog-to-digital converter 37 is; the analog switches 31 an integer number of periods of the AC voltage of the voltage source 29 successively on the upstream electrodes 5 switch; the downstream counterelectrodes 7 downstream current-voltage converter 33 in the crossing points 9 of the electrode grid 3 convert alternating current into an alternating voltage; and the current-voltage converters 33 downstream demodulators 35 provide by analog signal processing a characteristic signal magnitude, such as RMS value, peak value or phase position of the AC signal as an output signal.

The vibrating plates 13 are due to the vortex-induced pressure fluctuations in a lateral vibration whose frequency is proportional to the flow velocity of the fluid, wherein the electric field 27 in the crossing points 9 of the electrode grid 3 temporally with the oscillation frequency of the vibrating plates 13 is changed. By means of the measuring computer 39 the evaluation device 25 By time series analysis of the respective output signals for each crossing point, the oscillation frequency of the vibrating plate 13 determines and determines the associated local flow rate.

According to the embodiments explained above, the voltage excitation occurs at the upstream electrodes 5 and the signal detection at the downstream counter electrodes 7 , Similarly, it can be provided that the voltage excitation at the downstream counter electrodes 7 takes place and the signal detection at the upstream electrodes 5 he follows.

According to the embodiments explained above, the vibrating plates are 13 at the upstream electrodes 5 attached and extend from these toward the downstream counter electrodes 7 out, with the vibrating plates 13 by means of a free gap from the counterelectrodes 7 are separated. Similarly, it can be provided that the vibrating plates 13 at the downstream counter electrodes 7 are attached and extending from these towards the upstream electrodes 5 extend, wherein the vibrating plates 13 by means of a free gap from the electrodes 5 are separated.

By the described successive detection of the changes in the electric field in the crossing points 9 of the electrode grid 3 the result is a data stream of digital measured values in the form of time series u _{i, j} (k) for each crossing point (indices i, j, temporal index k). As can be seen from the above explanations, the flow-induced vibration of the vibrating plates is formed 13 in the crossing points 9 as modulation of the measured value amplitude of the detected measured variable. The frequency of the modulation is proportional to the local flow velocity (ν _{i, j} ~ f _{i, j} ). The frequency of the modulation can be z. B. by a frequency analysis for each time series u _{i, j} (k) by means of the measuring computer 39 be recorded. In general, a partial sequence of the series u _{i, j} (k) is selected by means of a sliding window and subjected to a Fourier transformation. This shows at the prevailing modulation frequency, a maximum of the power spectrum, which is determined by maximum value determination. Based on the detected local modulation frequency at a crossing point 9 is by means of the measuring computer 39 calculates the local flow velocity present at this intersection.

LIST OF REFERENCE NUMBERS

1

sensor element

3

electrode grid

5

(upstream) electrode

7

(downstream) counter electrode

9

Intersection / grid intersection

11

sensor frame

13

oscillating body / vibrating plate

15

Feldfokussierkörper

17

fluid vortex

19

oscillating movement

21

Flow direction of the fluid flow

23

Spannungsbeaufschlagungs facility

25

evaluation

27

local electric field

29

Power source / voltage source

31

analog switches

33

Current-voltage converter

35

demodulator

37

Analog to digital converter

39

measuring computer

Claims (10)

Arrangement for detecting the flow rate field of a fluid flow in a flow cross section, comprising: a sensor element ( 1 ) with an electrode grid ( 3 ) and several oscillatory bodies ( 13 ), wherein the electrode grid ( 3 ) a plurality of electrodes extending in a first plane ( 5 ) and a plurality of counter electrodes extending in a second plane ( 7 ), which, with the formation of crossing points ( 9 ) are arranged like a lattice, each of the oscillatable bodies ( 13 ) at one of the crossing points ( 9 ) between the electrode ( 5 ) and the counter electrode ( 7 ) of the crossing point, and wherein the sensor element ( 1 ) is designed for introduction into the fluid flow such that the electrodes are arranged upstream of the counter electrodes, so that at the electrodes depending on the local flow velocity fluid vortices ( 17 ) that form a vibration ( 19 ) of the oscillatory body ( 13 ), - a facility ( 23 ) for applying an electrical voltage between the electrode ( 5 ) and the counterelectrode ( 7 ) each of the crossing points ( 9 ) with the oscillatory bodies ( 13 ), so that there is an electric field ( 27 ) formed by the vibration ( 19 ) of the oscillatory body ( 13 ) is varied over time, and - an evaluation device ( 25 ) connected to the sensor element ( 1 ) and is designed in such a way that, for each of the crossing points ( 9 ) with the oscillatory bodies ( 13 ) the temporal variation of the electric field ( 27 ) and based thereon the local flow velocity at the crossing point ( 9 ) can be determined. Arrangement according to claim 1, wherein each of the oscillatable bodies ( 13 ) on the electrode ( 5 ) and / or the counterelectrode ( 7 ) of the associated crossing point ( 9 ) is attached. Arrangement according to claim 1 or 2, wherein each of the oscillatable bodies ( 13 ) on the electrode ( 5 ) of the associated crossing point ( 9 ) is attached and by means of a free gap from the counter electrode ( 7 ) of the associated crossing point is separated, or wherein each of the oscillatory bodies ( 13 ) at the counter electrode ( 7 ) of the associated crossing point ( 9 ) and by means of a free gap from the electrode ( 5 ) of the associated crossing point is separated. Arrangement according to claim 3, wherein the sensor element ( 1 ) at each of the crossing points ( 9 ) with the oscillatory bodies ( 13 ) for bundling the electric field lines on the gap side a Feldfokussierkörper ( 15 ), which in the direction of the oscillatory body ( 13 ) projects into the gap, wherein the Feldfokussierkörper has a surface layer of an electrically conductive material or consists of an electrically conductive material. Arrangement according to one of claims 1 to 4, wherein the oscillatory body ( 13 ) consist of an electrically insulating material. Arrangement according to one of claims 1 to 4, wherein the oscillatory body ( 13 ) have a surface layer of an electrically conductive material or consist of an electrically conductive material. Arrangement according to one of claims 1 to 6, wherein the oscillatory bodies ( 13 ) are elastically deformable. Arrangement according to one of claims 1 to 6, wherein the oscillatory bodies are rigid bodies and are mounted oscillatably. Arrangement according to one of claims 1 to 8, wherein the electrical voltage is a DC voltage. Arrangement according to one of claims 1 to 8, wherein the electrical voltage is an AC voltage.

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