FLOWMETER AND METHOD FOR MEASURING THE FLOW OF A FLUID

Description

[0001] The invention relates to a flow meter and a method for measuring the flow of a fluid based on ultrasound.

[0002] Different measuring principles are known for determining the flow velocity or flow rate of a fluid using ultrasound.

[0003] In a differential transit time method, a pair of ultrasonic transducers are mounted on the outer circumference of the pipeline with a mutual longitudinal offset. These transducers alternately transmit and record ultrasonic signals perpendicular to the flow along a measuring path spanned between the ultrasonic transducers. The ultrasonic signals transported through the fluid are accelerated or decelerated by the flow depending on the direction of travel. The resulting transit time difference is calculated with geometric variables to determine the average flow velocity of the fluid. With the cross-sectional area, this yields the volume flow or flow rate. For more precise measurements, multiple measuring paths, each with a pair of ultrasonic transducers, can be provided to record a flow cross-section at more than one point.To achieve high measurement accuracy with asymmetric velocity distributions across the flow cross-section, multiple measurement paths are required that do not run through the pipe axis or the centerline of the pipeline, so-called non-diametrical measurement paths or secant paths. In particular, secant paths located far off-center are desirable for high insensitivity of the volume flow measurement to inhomogeneous flow distributions.

[0004] The ultrasonic transducers used to generate ultrasound have a vibrating body, often made of ceramic. This body converts an electrical signal into ultrasound and vice versa, for example based on the piezoelectric effect. Depending on the application, the ultrasonic transducer functions as a sound source, sound detector, or both. A coupling between the fluid and the ultrasonic transducer must be ensured. A common solution is to have the ultrasonic transducers protrude into the pipe with direct contact to the fluid. Such intrusive probes can make accurate measurements difficult by disrupting the flow. Conversely, immersed ultrasonic transducers are exposed to the fluid and its pressure and temperature, and may therefore be damaged or lose their function due to deposits.

[0005] Techniques are also known in which the inner wall remains completely closed. One example is the so-called clamp-on mounting, as described in US Pat. No. 4,467,659, which attaches the ultrasonic transducers to the outside of the pipe. However, this only allows for diametrically opposed measurement paths through the pipe axis, which creates additional errors in non-axisymmetric flow profiles.

[0006] Another design is presented in DE 10 2013 101 950 A1, in which the ultrasonic units themselves consist of groups of several individual transducers. In the case of multi-layer pipe walls, e.g. made of fiber composite materials, these can be integrated directly into the pipe wall. The operating principle uses the transducer groups to specifically emit or receive ultrasound using structure-borne sound waves, as in classic clamp-on arrangements. This has the advantage, as is known from so-called clamp-on designs in which the ultrasonic transducers are mounted externally on the duct wall, that the transducer unit does not protrude into the flow channel, thus preventing flow disruption and preventing contamination.

[0007] Another disadvantage of known intrusive probes occurs at high flow velocities. This is clearly described in EP 2 103 912 A1 in Figuren 3 und 4 Due to the drift effect, the ultrasonic package hits different points on the opposite pipe wall depending on the flow velocity and may no longer hit the ultrasonic transducer unit located there.

[0008] From Kang et al.'s "Two-dimensional flexural ultrasonic phased array for flow measurement" in the 2017 IEEE International Ultrasonics Symposium (IUS), Washington, DC, USA, September 6-9, 2017. Published in: 2017 IEEE International: Ultrasonics Symposium (IUS) ISSN 1948-5727, it is known that the aforementioned drift effect can be counteracted by so-called "phased-array beam steering." A "phased array" consists of individual ultrasonic transducers that emit superimposed ultrasonic signals. The beam direction can be changed by changing the individual phases of the individual signals. These "phased-array" ultrasonic transducer units are installed in the openings of a flow channel.

[0009] A disadvantage of the differential transit time methods known from the prior art is that at least two ultrasonic transducer units are required for each measurement path. Furthermore, reciprocal electronics or complete symmetry—i.e., exactly the same behavior of the ultrasonic transducer units and the connected electronics for the forward and return directions—is required, which further increases the complexity of the device.

[0010] A Doppler method is also known for determining flow velocity. This method evaluates the frequency shift of an ultrasonic signal reflected within the flowing fluid, which varies depending on the flow velocity. Only one ultrasonic transducer is used to transmit and receive the ultrasonic signals. However, a measurement is only possible if there are enough suitable scattering particles in the fluid that reflect the ultrasonic signal.

[0011] Document US 2015 / 0020608 A1 describes a flowmeter with an array of ultrasonic transducer elements configured to activate a first subarray of the array of ultrasonic transducer elements to direct at least two outgoing ultrasonic beams through a fluid, and to activate a second subarray of the array of ultrasonic transducer elements to detect the ultrasonic beams after passing through a measuring path. However, only diametrical measuring paths extending through a pipe center axis are disclosed, which span a measuring plane in which the pipe center axis also lies. For non-axisymmetric flow profiles, such diametrical measuring paths produce inaccurate measurement results because the flow profile is only inadequately captured across the cross-section.

[0012] Based on this prior art, it is an object of the invention to provide an improved device for measuring flow velocity which is suitable for measuring fluids which contain no or only a small number of scattering particles, wherein the device has a reduced technical complexity and can provide improved measurement accuracy in the case of non-axisymmetric flow profiles.

[0013] This object is achieved by a flow meter having the features of claim 1 and a method for measuring the flow of a fluid having the features of claim 12.

[0014] The flow meter according to the invention comprises a measuring sensor having a pipeline for the fluid with a pipe wall, at least one phased-array ultrasonic transducer unit, wherein a phased-array ultrasonic transducer unit in the context of this application comprises ultrasonic transducer units that can emit ultrasonic signals at different angles and receive ultrasonic signals from different angles, in particular also arrangements of only two ultrasonic transducers, a control and evaluation unit that is designed to control the ultrasonic transducer unit to emit the ultrasonic signals along a measuring path, to evaluate the received ultrasonic signals, and to determine a flow rate using transit times of the ultrasonic signals, wherein the measuring sensor has at least one reflector that is designed to reflect the ultrasonic signals emitted by the ultrasonic transducer unit back to the ultrasonic transducer unit,wherein the ultrasonic signals pass through the measuring path from the ultrasonic transducer unit to the reflector and back to the ultrasonic transducer unit on at least three different path sections, and the measuring path is a secant path that does not run diametrically through a central axis of the pipeline.

[0015] The particular advantage of the invention is that the flowmeter according to the invention requires only one ultrasonic transducer unit for determining the flow of a fluid using the differential transit time method and provides improved measurement accuracy even with non-axisymmetric flow profiles. By eliminating the usually required second ultrasonic transducer unit, the complexity of the flowmeter is significantly reduced.

[0016] In one embodiment, the flowmeter according to the invention can be designed such that the ultrasonic transducer unit is a one-dimensional ultrasonic transducer unit comprising a one-dimensional, linear array of ultrasonic transducers. Since the radiation angle of the ultrasonic signal can only be changed in one plane with a one-dimensional ultrasonic transducer unit, and the ultrasonic signal can be emitted and received again in this plane, the ultrasonic transducer unit and the reflector are aligned such that the ultrasonic signals, after being reflected by the reflector and the pipe wall, re-enter the ultrasonic transducer essentially in the plane in which they were emitted."Substantially" in this case means that the one-dimensional ultrasonic transducer unit can have an acceptance angle at which ultrasonic signals can also be received that do not directly impinge on the ultrasonic transducer unit in the plane of the emitted ultrasonic signals. Such an acceptance angle is typically in the range of + / - 10 degrees to a nominal transmitting and receiving plane of a one-dimensional ultrasonic transducer unit, with the nominal transmitting and receiving plane being the plane into which the ultrasonic signals are emitted and in which the efficiency for receiving ultrasonic signals is highest. For a one-dimensional ultrasonic transducer unit with a linear arrangement of ultrasonic transducers, this is usually a plane that encompasses the ultrasonic transducer row and the radiation direction of the ultrasonic signals.

[0017] In a first measurement, the ultrasonic signals are first reflected by a first reflector after being emitted by the one-dimensional ultrasonic transducer unit and then impinge on a second reflector, which reflects the ultrasonic signals back to the ultrasonic transducer unit. The ultrasonic signals thus travel along a measurement path that has at least three different path segments: from the ultrasonic transducer unit to the first reflector, from the first reflector to the pipe wall, and from the pipe wall back to the ultrasonic transducer unit. The reflector and the ultrasonic transducer unit are coordinated and aligned such that the measurement path is a secant path, meaning that the pipe centerline does not lie in a plane spanned by the measurement path, but merely intersects it at a single point.As described above, a received ultrasonic signal can lie in a nominal transmission and reception plane of the ultrasonic transducer unit or be at an angle to the nominal transmission and reception plane that is no larger than the acceptance angle of the ultrasonic transducer unit. The measurement path can also have additional path sections, whereby the measurement signal can be reflected by additional reflectors and / or the pipe wall. It is essential that the ultrasonic signals transmitted and received again after passing through the measurement path lie essentially in one plane.

[0018] For differential measurement, the ultrasonic transducer unit is further configured to emit ultrasonic signals in a second measurement such that they travel the measurement path in the opposite direction, i.e., first from the ultrasonic transducer unit to the second reflector, from the second reflector to the first reflector, and from the first reflector back to the ultrasonic transducer unit. From the difference between the transit times of the ultrasonic signals determined in the two measurements, the evaluation unit can calculate an average flow velocity of the fluid in a known manner.

[0019] In a further development of this embodiment, the flowmeter according to the invention can be designed such that a plurality of measuring paths are realized within the measuring plane, whereby the ultrasonic signals can be transmitted and received at different angles within the measuring plane. Preferably, a reflector can then be provided for each measuring path.

[0020] In an alternative embodiment, the flowmeter according to the invention can be designed such that the ultrasonic transducer unit is a two-dimensional ultrasonic transducer unit having a two-dimensional array of ultrasonic transducers, wherein the individual ultrasonic transducers of the ultrasonic transducer unit can preferably be arranged in rows and columns. This provides greater flexibility with regard to the possible measuring paths. In particular, with a two-dimensional array, measuring paths designed as secant paths can be realized in different measuring planes. Since the measuring paths are designed as secant paths, in this embodiment too, the raw center axis does not lie in the measuring planes spanned by the measuring paths, but merely intersects them at one point in each case.

[0021] In a first measurement, the ultrasonic signals are first reflected at least once by the pipe wall after being emitted by the ultrasonic transducer unit. After one or more reflections from the pipe wall, the ultrasonic signals hit a reflector, which reflects the ultrasonic signals back to the ultrasonic transducer unit. The ultrasonic signals thus travel along a measurement path that has at least three different path segments: from the ultrasonic transducer unit to the pipe wall, from the pipe wall to the reflector, and from the reflector back to the ultrasonic transducer unit. If there is more than one reflection from the pipe wall, the measurement path also has path segments from pipe wall to pipe wall.

[0022] For differential measurement, the ultrasonic transducer unit is further configured to emit ultrasonic signals in a second measurement such that they travel the measurement path in the opposite direction, i.e., first from the ultrasonic transducer unit to the reflector, from the reflector to the pipe wall, and after one or more reflections from the pipe wall, back to the ultrasonic transducer unit. From the difference between the transit times of the ultrasonic signals determined in the two measurements, the evaluation unit can calculate an average flow velocity of the fluid in a known manner.

[0023] If the ultrasonic transducer unit is designed as a two-dimensional ultrasonic transducer unit, it can transmit and receive ultrasonic signals in different measuring planes. For this purpose, the measuring sensor can have a plurality of reflectors arranged on or in the pipe wall. Alternatively, the measuring sensor can have an arc-shaped reflector arranged in or on the pipe wall, and the ultrasonic transducer unit can be controlled so that the ultrasonic signals hit the arc-shaped reflector at different locations. This allows for the flexible use of different measuring paths. The reflector can also be circular, i.e., cover the entire inner circumference of the pipe wall, further increasing the number of possible measuring paths.

[0024] What is particularly advantageous in both of the described embodiments is if the ratio r / R lies between 0.3 and 0.65 for at least one path section, where R is the radius of the pipeline and r is the shortest distance of the path section to the central axis of the pipeline. These path sections are particularly well positioned for effectively sampling the flow. They are off-center with respect to the pipe axis but not too close to the edge. The paths then also lie approximately on Gaussian nodes. This is advantageous because at the Gaussian node the flow profile does not change with the fluid velocity. Overall, this results in greater measurement accuracy. To further improve measurement accuracy, at least two path sections can have different values for the ratio r / R.

[0025] In a preferred embodiment of the flowmeter according to the invention, the path section between the ultrasonic transducer unit and the reflector runs at a path angle of less than 20 degrees, particularly preferably less than 15 degrees, to the central axis of the pipeline. This has the advantage that the ultrasonic signal runs in a region close to the pipe wall and is less influenced by the fluid flow in this path section than in the region of the central axis of the pipeline, since flow profiles of fluid flows in pipelines generally have a significantly lower flow velocity in the region of the pipe wall than in the region of the central axis of the pipeline.

[0026] The ultrasonic transducer unit can be advantageously integrated into the pipe wall. This ensures that the fluid flow is not affected and unwanted disturbances, such as turbulence, are prevented.

[0027] The reflector(s) can preferably be arranged downstream of the ultrasonic transducer unit in the flow direction so that it does not influence the fluid flow in the area between the ultrasonic transducer unit and the reflectors.

[0028] In one embodiment of the invention, the ultrasonic transducer unit and / or the reflector can be arranged in a recess in the pipe wall to reduce disturbances to the fluid flow. In this embodiment, the recess can preferably be at least partially concealed. Particularly preferably, only openings for the inlet and outlet of the ultrasonic signals are provided.

[0029] Because the ultrasonic transducer unit is designed as a phased array, it can transmit ultrasonic signals at a first angle and receive them at a second angle that differs from the first angle. The ultrasonic transducer unit can be aligned so that ultrasonic signals are transmitted at the same angle and received again after passing through the measurement path. This symmetry simplifies the further processing of the received data.

[0030] In one embodiment of the invention, the ultrasonic transducer unit can be designed as a linear array consisting of a row of at least two ultrasonic transducers aligned parallel to the measurement path. This makes it possible to counteract the drift effect by controlling the phase of the ultrasonic transducers and thus adjusting the radiation angle accordingly. This allows for better measurement results to be acquired over a wide range of flow velocities. The phased-array ultrasonic transducer unit can then account for the drift effect online and adapt the direction of the radiation of the ultrasonic packets to the flow velocity.

[0031] The ultrasonic transducer unit can also be designed to simultaneously emit ultrasonic signals with different radiation angles and to simultaneously receive the reflected ultrasonic signals at different reception angles, wherein the received ultrasonic signals can either be separated from one another by digital post-processing and thus the differential propagation time can be determined or the interference of the received ultrasonic signals can be evaluated and the differential propagation time can be determined from the signal image.

[0032] If a sufficient number of particles is present in the fluid to carry out the Doppler measurement mentioned above to determine the flow velocity, the flow meter according to the invention can also be designed to carry out both methods in order to achieve a higher accuracy in determining the flow velocity.

[0033] The method according to the invention can be developed in a similar manner and thereby exhibits similar advantages. Such advantageous features are described by way of example, but not exhaustively, in the dependent claims following the independent claims.

[0034] The invention will be explained in detail below using exemplary embodiments with reference to the drawings. The drawings show: Fig. 1 is a schematic representation of a flow meter; Fig. 2a is a schematic top view of an ultrasonic transducer unit designed as a two-dimensional array; Fig. 2b is a schematic side view of an ultrasonic transducer unit designed as a two-dimensional array; Fig. 3 is a schematic representation of a flow meter according to the invention; Fig. 4 is a schematic perspective representation of a flow meter according to the invention; Fig. 5 is a schematic representation of an alternative embodiment of a flow meter according to the invention for multi-path measurement; Figs. 6a - 6c are schematic representations of shields of a measuring path in a flow meter according to the invention; Fig. 7 is a schematic representation of a further embodiment of the flow meter according to the invention; Fig. 8 is a schematic representation of a flow meter according to the prior art;

[0035] In Fig. 8 A flowmeter 110 according to the prior art is shown for the general explanation of the function of a generic flowmeter. The flowmeter 110 comprises a sensor 112, which has a pipe 114 for the fluid 118 with a pipe wall 116. The fluid flowing through the pipe 114, a gas or a liquid, is in Fig. 8 represented by a wide arrow and flows in the z-direction along a central axis 126 of the pipe 114.

[0036] The flowmeter 110 further comprises two ultrasonic transducers 120 and 122, which define a measuring path 124 between them in the pipeline 114. The ultrasonic transducers 120 and 122 are arranged offset in the flow direction z, i.e., spaced apart longitudinally along the center axis 126 of the pipeline 114. As a result, the measuring path 124 is not orthogonal to the flow direction z, but rather at a path angle α. Each of the ultrasonic transducer units 120 and 122 can operate as a transmitter or receiver and is controlled by a control and evaluation unit 128.

[0037] The path angle α and the pipe diameter D determine the length L of the measuring path 124 in the fluid medium. Ultrasonic signals, which are transmitted and received as ultrasonic wave packets on the measuring path 124 in opposite directions, have a component in the direction of flow z and another component opposite to the flow z and are thus accelerated with the flow of the fluid 118 or decelerated against the flow. The average flow velocity v of the fluid is calculated in this time-of-flight method according to v = t 2 − t 1 2 ∗ t 2 ∗ t 1 ∗ L cos α where t 2 and t 1 denote the sound propagation times required by the emitted ultrasonic wave signals to travel upstream and downstream of the measuring path 124, respectively, and are recorded in the control and evaluation unit 128. The flow rate can then be calculated using the pipe cross-section and the average flow velocity v of the fluid 118.

[0038] The flow meter 10, which is located in Fig. 1 is shown very schematically. It also has a measuring sensor 12 with a pipe 14 and pipe wall 16, as well as a control and evaluation unit 28. The fluid 18, a gas or a liquid, flowing through the pipe 14 is shown with a broad arrow and flows in the z-direction along a central axis 26 of the pipe 14. The flowing fluid 18 has a flow profile 32 which only slightly influences ultrasonic signals propagating along the pipe wall 16, for example, due to a lower flow velocity of the fluid 18 in the region of the pipe wall 16 compared to a flow velocity in the region of the central axis 26.

[0039] In contrast to the flow meter 110, made of Fig. 8 The device for measuring a flow of a fluid in Fig. 1 only one ultrasonic transducer unit 20 in the pipe wall 16. The ultrasonic transducer unit 20 is also not a "simple" ultrasonic transducer, but is designed as a phased-array ultrasonic transducer unit 20. It can be designed as a one-dimensional, linear array consisting of a row of at least two individually controllable ultrasonic transducers, whose alignment is parallel to the measuring path 24 or as in the schematic plan view of the Fig. 2a shown as a two-dimensional array of individually controllable ultrasonic transducers 22. The individual ultrasonic transducers 22 are controlled by the control and evaluation unit 28 to emit an ultrasonic signal in such a way that they each have a phase offset from one another, wherein the phase offset is selected such that the superposition of the resulting ultrasonic waves leads to an ultrasonic wave signal which leaves the ultrasonic transducer unit 20 at a radiation angle γ perpendicular to a surface normal 40 of the ultrasonic transducer unit 20, as in Figur 2b , in which the emitted ultrasonic wave signal is represented by a solid line 42. The ultrasonic transducer unit 20 can further be controlled so that ultrasonic wave signals which strike the ultrasonic transducer unit 20 at an angle of incidence ϕ (in Figur 2b (represented by the dot-dash line 44). The radiation angle γ and the angle of incidence ϕ can differ both in magnitude and direction. The operation of such phased-array ultrasonic transducer units is known from the prior art.

[0040] In the Fig. 2a An array of four by four ultrasonic transducers is shown as an example. This limitation is primarily due to the need to keep the drawing simple and clear. If 16 such individual ultrasonic transducers provide too low a signal level, the array can also have more ultrasonic transducers. Therefore, the array is preferably designed with more ultrasonic transducers, as not shown. The number of ultrasonic transducers is a compromise between signal strength, complexity, and cost.

[0041] The ultrasonic transducer unit 20 transmits and receives ultrasonic signals that move along a measuring path 24 through the pipeline 14. As shown in Fig. 1 As shown, the measuring path 24 has several path sections 24a, 24b, 24c.

[0042] In a first measurement, the ultrasonic transducer unit 20 transmits the ultrasonic signals along a first path section 24a of the measuring path 24 from the ultrasonic transducer unit 20 to the pipe wall 16, wherein the direction of travel of the ultrasonic signals in the first measurement in the Fig. 1 is marked by solid arrows 24.1. After being reflected at the pipe wall 16, the ultrasonic signals travel along a second path section 24b from the pipe wall 16 to a reflector 30, which reflects the ultrasonic signals back along a third path section 24c to the ultrasonic transducer unit 20. The reflector 30 is arranged downstream in the flow direction 18 of the fluid, i.e. after the ultrasonic transducer unit 20, on or in the pipe wall, so that the flow of the fluid in the area between the ultrasonic transducer unit 20 and the reflector 30 is only slightly or not at all disturbed, in particular when the ultrasonic transducer unit 20 is integrated flush into the pipe wall 16 (not shown).

[0043] In a second measurement, the ultrasonic transducer unit 20 transmits the ultrasonic signals in the opposite direction, indicated by the dashed arrows 24.2, along the third path section 24c of the measurement path 24 toward the reflector 30. After reflection by the reflector 30, the ultrasonic signals travel along the second path section 24b to the pipe wall 16, from which they are reflected back to the ultrasonic transducer unit 20 along the first path section 24a.

[0044] The ultrasonic transducer unit 20 and the reflector 30 are arranged such that the third path section 24c of the measuring path 24 between the ultrasonic transducer unit 20 and the reflector 30 runs at a path angle β of less than 20 degrees, preferably less than 15 degrees, to the central axis 26, wherein the path angle β between the central axis 26 and the third path section 24c is specified here in relation to a parallel 26.1 of the central axis 26. The third path section 24c therefore runs in an area as close as possible to the pipe wall 16. Due to the flow profile 32 in the pipeline 14, the ultrasonic signal on the third path section 24c between the ultrasonic transducer unit 20 and the reflector 30 is only slightly influenced by the fluid flow.

[0045] On the first path section 24a between the ultrasonic transducer unit 20 and the pipe wall 16, and on the second path section 24b between the pipe wall 16 and the reflector 30, the ultrasonic signal is strongly influenced by the flow profile 32 and the velocity of the fluid in the pipeline. As a result, the propagation time of the ultrasonic signals against the flow direction, which is measured with the second measurement (measurement path represented by dashed arrows), is longer than the propagation time with the flow direction, which is measured with the first measurement (measurement path represented by solid arrows). This makes it possible to calculate the average flow velocity of the medium by evaluating the differential propagation time of both measurements.

[0046] The average flow velocity v of the fluid is calculated in this transit time method according to v = L 24 a + L 24 b + L 24 c 2 2 ⋅ L 24 a cos α 24 a + L 24 b cos α 24 b + C v L 24 c cos β ⋅ t 24.2 − t 24.1 t 24.1 t 24.2

[0047] In this case, t 24.1 and t 24.2 are the sound travel times required by the emitted ultrasonic signals to travel the measuring path 24 in the first direction 24.1 and in the reverse direction 24.2; L 24a , L 24b , L 24c are the lengths of the path sections 24a, 24b, 24c, α 24a , α 24b are the path angles of the first path section 24a and the second path section 24b to the central axis 26; β is the path angle of the third path section 24c to the central axis 26; C v is a correction factor dependent on the flow profile and thus on the flow velocity, which can be determined by measurement, calibration or simulation;

[0048] The flow rate can then be calculated using the pipe cross-section and the average flow velocity v of the fluid 18.

[0049] The correction factor C v can be determined, for example, by measuring the sound travel times t 24.1 and t 24.2 in a calibration process at one or more different predetermined average flow velocities v and calculating the correction factor C v by rearranging the above equation. Measuring several different flow velocities is preferred because the flow profile 32 can also be dependent on the flow velocity. Alternatively, the correction factor C v can also be calculated by conventional simulation of the sound travel times t 24.1 and t 24.2 of the ultrasonic signals, taking into account a flow velocity-dependent flow profile that is also simulated in a conventional manner. The correction factor C v, which depends on the flow profile and thus also on the flow velocity, can therefore be specified as a function of the sound travel times t 24.1 and t 24.2.

[0050] Since the ultrasonic transducer unit 20 is designed as a phased array, the radiation angle γ can be varied by controlling the individual ultrasonic transducers 22 using the control and evaluation unit 28. This counteracts a drift effect, particularly at high flow velocities. The radiation angle γ can be adjusted so that, regardless of the flow velocity, the reflector 30 is always hit, and the emitted ultrasonic signals are reflected back to the ultrasonic transducer unit 20.

[0051] Due to the design of the ultrasonic transducer unit 20 as a phased array, the radiation angle γ depends on the set phase shift of the individual signals and on the speed of sound in the fluid. The speed of sound itself depends on ambient conditions such as temperature and pressure. It is therefore advantageous that by controlling the individual ultrasonic transducers 22 using the control and evaluation unit 28, the phase difference can be adjusted depending on the ambient conditions so that the radiation angle γ remains the same, even if the speed of sound changes. To determine the ambient conditions, an ambient detection unit (not shown) can be provided which, for example, detects the temperature and / or pressure in the pipeline 14 and transmits it to the control and evaluation unit 28 in order to monitor the fluid properties and thus be able to calculate the speed of sound and density.With this knowledge, the ultrasonic transducers 22 can be better controlled and evaluated. Density is necessary to calculate the mass flow and can be calculated from the properties of the medium, such as temperature and pressure. The speed of sound itself can initially be measured under known ambient conditions, a stationary fluid, and a known length of the measurement path by measuring a transit time for both directions of the ultrasonic signal along the measurement path, determining an average transit time from this, and dividing the length of the measurement path by the average transit time: . c = L t 1 + t 2 2 With: c = speed of sound L = length of the measuring path t 1 = travel time of the ultrasonic signal along the measuring path in the first direction t 2 = travel time of the ultrasonic signal along the measuring path in the second direction

[0052] The measuring path 24 is in Fig. 1 as a diametrical measuring path that runs through a central axis 26 of the pipeline 14. According to the invention, it can be designed as a secant path, as shown in Fig. 3 is shown.

[0053] Fig. 3 shows an embodiment of a flowmeter 310 according to the invention, wherein the pipeline 14 is shown in the flow direction. The ultrasonic transducer unit 20 emits ultrasonic signals on a measuring path 34, which now does not run diametrically through the central axis 26 of the pipeline 14, but as a so-called secant path with the path sections 34a, 34b, 34c. In addition, the measuring path 34 runs as in the example from Fig. 1 not orthogonal to the flow direction 18, i.e. out of the drawing plane or into the drawing plane. The use of secant paths is advantageous for asymmetrical velocity distributions. Since the path sections 34a, 34b, 34c of the measuring path 34 do not run through the central axis 26 of the pipeline 14, the central axis 26 of the pipeline 14 does not lie in a measuring plane spanned by the path sections 34a, 34b, 34c of the measuring path 34, but only intersects it at one point. This enables greater accuracy in determining the average flow velocity. In order to sample the flow profile effectively, the ratio r / R can be between 0.3 and 0.65 for at least one path section, where R is the radius of the pipeline and r is the shortest distance of the path section to the central axis of the pipeline, shown here for path section 34a.Preferably, a multi-path measurement can take place, wherein the ultrasonic transducer unit 20 emits ultrasonic signals at different angles, so that the ultrasonic signals pass through the pipeline 14 on different measuring paths within a measuring plane, wherein a reflector 30 can be arranged on or in the pipe wall for each measuring path.

[0054] Fig. 4 shows a perspective view of an embodiment of a flowmeter 410 according to the invention with an ultrasonic transducer unit 20, which has a two-dimensional array of individually controllable ultrasonic transducers and can transmit and receive ultrasonic signals in different measuring planes. Three different measuring paths 432, 434, 436 with path sections 432a-c, 434a-c, 436a-c are shown as examples. The measuring paths 432, 434, 436 are secant paths that do not run diametrically through the central axis 26 of the pipeline 14. The central axis 26 of the pipeline 14 thus does not lie in the measuring planes spanned by the measuring paths 432, 434, 436, but intersects them only at one point.For the sake of clarity, the direction of travel of the measurement signals along the measurement paths 432, 434, 436 is not shown, but here too, as in the previously shown examples, the measurement signals pass through the measurement paths 432, 434, 436 in both directions, i.e., for example, for the measurement path 432 in a first measurement, first along path section 432a from the ultrasonic transducer unit 20 to the pipe wall 16, then along path section 432b to the reflector 430 and from the reflector 430 along path section 432c back to the ultrasonic transducer unit 20. In a second measurement, the measurement path 432 is then passed through in the opposite direction, i.e., first along path section 432c from the ultrasonic transducer unit 20 to the reflector 430, then along path section 432b to the pipe wall 16 and along path section 432a from the pipe wall 16 back to the Ultrasonic transducer unit 20.

[0055] Instead of individual reflectors, this embodiment features a reflector 430 arranged in an arc on or in the pipe wall. Since the ultrasonic transducer unit 20 is designed as a two-dimensional ultrasonic transducer unit, it can transmit and receive ultrasonic signals in different measurement planes and be controlled so that the ultrasonic signals impinge on the reflector 430 at different locations. This allows for flexible use of different measurement paths and / or measurement planes. The reflector 430 can also be circular, i.e., cover the entire inner circumference of the pipe wall, further increasing the number of possible measurement paths.

[0056] In principle, the use of several ultrasonic transducer units is also possible for a multi-path measurement as in Fig. 5 shown. In addition to a first ultrasonic transducer unit 20 / 1, which transmits and receives ultrasonic signals along a measuring path 24 / 1, wherein the ultrasonic signals are reflected by a first reflector 30 / 1, the flow meter 10 has a second ultrasonic transducer unit 20 / 2, which transmits and receives ultrasonic signals along a second measuring path 24 / 2, wherein the ultrasonic signals are reflected by a second reflector 30 / 2. Depending on the accuracy required for the flow measurement, a plurality of N ultrasonic transducer units can be used, spanning N measuring paths. Instead of N reflectors, one reflector, for example, can then also be provided, which is designed as a circumferential elevation or groove in the pipe wall 16.

[0057] In the Fig. 3 und Fig. 4 In the embodiments shown, a simple reflection of the ultrasonic signals occurs at the pipe wall 16. To further increase the measurement accuracy, the measuring paths 34, 432, 434, 436 can also be designed such that the ultrasonic signals between the ultrasonic transducer unit 20 and the reflector 30, 430 are reflected multiple times at the pipe wall 16. Preferably, the path sections 34c, 423c, 434c, 436c, along which the ultrasonic signals travel directly, i.e., without reflection at the pipe wall 16, from the reflector 30, 430 to the ultrasonic transducer unit 20 (or in the opposite direction, from the ultrasonic transducer unit 20 directly to the reflector), as explained above, run in a region close to the pipe wall 16.

[0058] To further reduce the influence of the fluid flow on the ultrasonic signals passing directly between the ultrasonic transducer unit 20 and the reflector 30, these can be shielded from the fluid flow by various designs, which are described in the following Figuren 6a - 6c are shown.

[0059] Fig. 6a shows a mechanical shield 40, 42, which essentially encloses the area between the ultrasonic transducer unit 20 and the reflector 30 and has only openings for the entry and exit of the ultrasonic signals.

[0060] Fig. 6b shows an alternative embodiment in which the ultrasonic transducer unit 20 and the reflector 30 are arranged in a recess 44 of the pipe wall.

[0061] For further shielding, the recess 44 can be closed, as shown in Figure 5c, except for openings for the inlet and outlet of the ultrasonic signals, comparable to the embodiment in Figure 5a. The recess 44 can also be designed as a measuring module that can be flanged to an opening in the pipe wall 16 and contains the ultrasonic transducer unit 20 and the reflector 30.

[0062] A further alternative embodiment of the invention shows Fig. 7 The ultrasonic transducer unit 20 transmits and receives ultrasonic signals that travel along a measurement path 64 through the pipeline 54. As in the previous embodiments, the measurement path 64 has a plurality of path sections 64a, 64b, 64c.

[0063] In contrast to the examples in Fig. 3The reflector 60 for reflecting the ultrasonic signals is formed by the pipe wall 56 of the pipe 54 itself. The pipe 54 has a U-shaped winding.

[0064] In a first measurement, the ultrasonic transducer unit 20 transmits the ultrasonic signals along a first path section 64a of the measurement path 64, with the direction of travel of the ultrasonic signals during the first measurement being indicated by solid arrows 64.1. After being reflected by the pipe wall 56, the ultrasonic signals travel along a second path section 64b to a reflector 60 formed by the pipe wall 56, which reflects the ultrasonic signals back to the ultrasonic transducer unit 20 along a third path section 64c.

[0065] In a second measurement, the ultrasonic transducer unit 20 transmits the ultrasonic signals in the opposite direction, indicated by the dashed arrows 64.2, along the third path section 64c of the measurement path 64 toward the reflector 60. After reflection by the reflector 60, the ultrasonic signals travel along the second path section 64b to the pipe wall 56, from which they are reflected back to the ultrasonic transducer unit 20 along the first path section 64a.

[0066] Due to the U-shaped geometry of the pipeline 54, the measuring path 64 runs through the fluid 18 such that the third path section 64c runs essentially parallel to the flow of the fluid 18, while the other two path sections 64a, 64b run essentially perpendicular to the flow of the fluid 18. Thus, the propagation velocity of the ultrasonic signals on the first and second path sections 64a, 64b is only slightly influenced by the fluid flow.

[0067] On the third path section 64c between the ultrasonic transducer unit 20 and the reflector 60, the ultrasonic signal is strongly influenced by the fluid flow and the velocity of the fluid in the pipeline 54. As a result, the propagation time of the ultrasonic signals against the flow direction, which is measured with the second measurement (measurement path represented by dashed arrows), is longer than the propagation time with the flow direction, which is measured with the first measurement (measurement path represented by solid arrows). Thus, with this embodiment of the invention, it is also possible to calculate the average flow velocity of the fluid 18 by evaluating the differential propagation time of both measurements.

Claims

1. A device for measuring a flow of a fluid (18), comprising - a sensing element (12) which has a pipeline (14) for the fluid (18) having a pipe wall (16), - at least one phased array ultrasonic transducer unit (20) which can emit ultrasonic signals into different emission angles (γ) and can receive ultrasonic signals from different reception angles (ϕ), - a control and evaluation unit (28) which is configured for controlling the ultrasonic transducer unit (20) for emitting the ultrasonic signals along a measurement path (34, 432, 434, 436, 64) and for evaluating the received ultrasonic signals and determining a flow using transit times of the ultrasonic signals, - wherein the sensing element (12) has at least one reflector (30, 430, 50) which is configured to reflect the ultrasonic signals emitted by the ultrasonic transducer unit (20) back to the same ultrasonic transducer unit (20), wherein the ultrasonic signals pass through the measurement path (34, 432, 434, 436, 64) from the ultrasonic transducer unit (20) to the reflector (30, 50) and back to the ultrasonic transducer unit (20) on at least three different path sections (34a-c, 432a-c, 434a-c, 436a-c, 64a-c), characterized in that the measurement path (34, 432, 434, 436, 64) is a secant path which does not extend diametrically through a center axis (26) of the pipeline (14).

2. A device according to claim 1, characterized in that the control and evaluation unit (28) is configured to control the ultrasonic transducer unit (20) such that the ultrasonic signals in a first transit time measurement pass through the measurement path (34, 64) in a first direction (34.1, 64.1) and in a second transit time measurement in a second direction (34.2, 64.2) opposite to the first direction, and to determine a mean flow rate (v) of the fluid (18) from a difference in the transit time measurements.

3. A device according to one of the preceding claims, characterized in that the ultrasonic transducer unit (20) is integrated in the pipe wall (16).

4. A device according to any one of the preceding claims, characterized in that the ultrasonic transducer unit (20) is configured as a two-dimensional array of ultrasonic transducers (22).

5. A device according to any one of the claims 1 to 3, characterized in that the ultrasonic transducer unit (20) is configured as a one-dimensional array, wherein a received ultrasonic signal has an angle with respect to a nominal transmitting and receiving plane of the ultrasonic transducer unit (20) that is at most as large as an acceptance angle of the ultrasonic transducer unit (20).

6. A device according to claim 5, wherein a magnitude of the acceptance angle is less than 10 degrees.

7. A device according to any one of the preceding claims, characterized in that a path section (34c, 432c, 434c, 436c) of the measurement path (34, 432, 434, 436) lying between the reflector (30) and the ultrasonic transducer unit (20) extends at a path angle (β) of less than 20 degrees, preferably less than 15 degrees, to the center axis (26) of the pipeline (14).

8. A device according to any one of the preceding claims, characterized in that the ultrasonic transducer unit (20) is oriented such that the ultrasonic signals are emitted at emission angles (γ) and incidence angles (ϕ) that are equal in magnitude and are received again after passing through the measurement path (34, 432, 434, 436).

9. A device according to any one of the preceding claims, characterized in that the control and evaluation unit (28) is configured to control the ultrasonic transducer unit for tracking the emission angle (γ) as a function of the mean flow rate (v) of the fluid (18).

10. A device according to any one of the preceding claims, characterized in that, for at least one path section (34a-c), it applies that a ratio r / R is between 0.3 and 0.65, wherein R is the radius (R) of the pipeline (14) and r is the shortest distance (r) of the path section (34a-c) to the center axis (26) of the pipeline (14).

11. A device according to claim 10, characterized in that at least two path sections (34a-c) have a different ratio r / R.

12. A method for measuring a flow of a fluid (18) flowing in a pipeline (14), comprising the steps: - emitting ultrasonic signals along a measurement path (24, 34, 64) in the pipeline (14) with a phased array ultrasonic transducer unit (20) controlled by a control and evaluation unit (28), - receiving the emitted ultrasonic signals with the same ultrasonic transducer unit (20) after passing through the measurement path (24, 34, 64), - evaluating the received ultrasonic signals and determining a flow of the fluid (18) using transit times of the ultrasonic signals with the control and evaluation unit (28), - wherein the ultrasonic signals emitted by the ultrasonic transducer unit (20) are reflected back by at least one reflector (30) to the ultrasonic transducer unit (20), wherein the ultrasonic signals pass through the measurement path (34, 432, 434, 436, 64) from the ultrasonic transducer unit (20) to the reflector (30, 430, 50) and back to the ultrasonic transducer unit (20) on at least three different path sections (34a-c, 432a-c, 434a-c, 436a-c, 64a-cc), characterized in that the measurement path (34, 432, 434, 436) is a secant path which does not extend diametrically through a center axis (26) of the pipeline (14).

13. A device according to claim 12, characterized by the further steps: - emitting the ultrasonic signals in a first measurement such that the ultrasonic signals pass through the measurement path (34, 64) in a first transit time measurement time in a first direction (34.1, 64.1) and in a second transit time measurement in a second direction (34.2, 64.2) opposite to the first direction, and - determining a mean flow rate (v) of the fluid (18) from a difference of the first and second transit time measurement.