WO1994024522A1

WO1994024522A1 - Process, arrangement and device for determining the flow of an open-surfaced liquid

Info

Publication number: WO1994024522A1
Application number: PCT/DE1994/000445
Authority: WO
Inventors: Wolfgang Frey
Original assignee: Wolfgang Frey
Priority date: 1993-04-22
Filing date: 1994-04-20
Publication date: 1994-10-27

Abstract

A process for detecting the flow of an open-surfaced liquid flowing in a channel using gas bubbles as the measuring medium, and an arrangement and device for implementing the process. It is possible with the process and arrangement of the invention to measure flows directly, without losses and with a high degree of accuracy, especially at low flow rates. To this end, a time sequence of individual gas bubbles is generated by means of diffusers (1) spatially distributed over a cross-sectional region of the bottom (8) of the channel (14), in which the size of the gas bubbles is such that they rise from the bottom (9) of the channel (14) to its surface at an approximately constant rate (us). At least one image of the horizontal drift of the gas bubbles at the surface (13) of the channel (14) is taken by at least one optical imaging device outside the liquid and the at least one image is evaluated by means of an image evaluation unit to determine the flow of the channel.

Description

Method, arrangement and device for determining the outflow of a liquid flowing with a free mirror

The invention relates to a method for determining the outflow of a liquid flowing with a free mirror in a channel using gas bubbles as the measuring medium, and to an arrangement and a device for carrying out the method.

A basic prerequisite for all current tasks in hydrology is adequate information about the temporally and spatially very variable water supply. In this respect, an exact determination of the outflow is very important for all users of water, especially in the case of free-level outflows with fluctuating volume flows. The problem of being able to precisely detect drains and possible backflows arises in all of the areas of application presented below, which are subject to the legal requirements to be met with regard to environmental protection. So are all eligible companies Discharging the untreated or treated wastewater into the waters (receiving water) is obliged to set up and maintain wastewater control stations in order to control their wastewater discharge and document it accordingly.

So far it has not been possible to measure the flow directly and continuously in a natural or artificial channel (Dyck, Siegfried "Fundamentals of Hydrology", Ernst & Sohn Verlag, Berlin, Munich, 1983), especially not without extensive, costly installations or conversions in the channel.

As a rule, velocity-area methods are used, which measure the velocity field in a channel flow at great expense for various outflows in order to derive a water level-outflow relationship, i.e. to construct a discharge curve and ultimately to be able to draw conclusions about the discharge curve.

It is known that the logarithmic speed distribution law applies in stationary flowed, fully developed, prismatic flow sections. In these cases, measuring the flow velocity at any point in the channel can provide the velocity distribution over the entire cross section. In contrast, in the case of non-prismatic open channels, the speed is in most cases distributed unevenly over the depth and the width. Time-consuming flow velocity measurements over the flow depth in various verticals are to be carried out. In order to determine the flow rate, the flow depth or the flow cross-section for the respective flow state must also be known. For the local measurement of the flow velocity, for example, hydrometric blades, mobile ultrasonic or magnetic-inductive measuring devices and miniaturized LDA systems (for example DFLDA diode-fiber-laser-Doppler anemometer) are known. The discharge determined by means of flow velocity measurements also becomes inaccurate if the discharge does not remain constant during the measurement time or if the measurement is disturbed by shipping traffic.

It is also known to determine the discharge indirectly using inductive flow measuring devices via calibration functions or by means of installations in the channel, for example by means of a venturi channel. However, these internals lead to energy losses in the channel flow and partially obstruct the drain so significantly that the potential energy required for the drain must be artificially generated with pumps. In addition, temperature and density fluctuations as well as changes e.g. Suspended matter concentrations are recorded with temperature, pH and conductivity measuring probes and the discharge values are then / subsequently corrected. Backflow phenomena and temporary backflows cannot be detected with all the methods of discharge determination that have been used up to now.

The invention is therefore based on the object of creating a method for determining the outflow with a high temporal resolution of the type mentioned at the outset, with which outflows can be carried out directly and without loss with a high degree of accuracy, in particular at low flow rates, in a clear mirror flowing liquids can be determined. Furthermore, the invention is directed to an arrangement for carrying out the method and a device for generating defined gas bubbles as a measuring medium for the method.

The solution according to the invention in the method mentioned at the outset is that a time sequence of individual gas bubbles is generated by means of diffusers which are spatially distributed over a cross-sectional area of the bottom of the channel, the gas bubbles being of such a size that they have an almost constant rate of climb of ascending the bottom of the channel up to its surface, at least one image of the horizontal drift of the gas bubbles on the surface of the channel is recorded by means of at least one optical image sensor arranged outside of the liquid, and that the at least one image taken to determine the outflow of the channel is evaluated by means of an image processing unit.

As will be explained in more detail below, an essential element of the invention for the precise determination of the outflow over an entire cross-sectional area having a two-dimensional flow profile of a liquid flowing with a free mirror by means of the rapid measuring method according to the invention is the suitable size of the gas bubbles , which is to be chosen in such a way that the gas bubbles integrate the height profile of the flow velocity in the flume in the vertical line during the ascent. The drift measured on the surface serves as the measured value for the path integral of a gas bubble. The outflow is determined from the measured transverse distribution of the drift of gas bubbles from many diffusers.

With a correspondingly complex image processing unit, the spatial arrangement of the diffusers on the bottom of the channel can be made as desired. The Image processing and evaluation for determining the outflow is advantageously simplified in that the diffusers are arranged next to one another in a row running transversely to the flow direction of the channel.

In principle, the method according to the invention can be used to measure the outflow of any liquids. The most important practical application is that for measuring the runoff of channels, the majority of which consists of water, the other components being admixtures, impurities or foreign substances. In these cases, it is generally the cheapest for cost reasons if the gas bubbles are air bubbles. In special applications, however, other gases, such as Helium or nitrogen can be used.

In general, any gas can be used, the bubbles of which meet the requirements with regard to the rate of ascent and the compatibility with the liquid. Insofar as air bubbles are mentioned in the following, this applies to any gas bubbles without restriction of generality with corresponding modifications. Exemplary applications of air bubbles in water are mentioned. The size of any gas bubbles in any liquids can be measured in a corresponding manner. If necessary, experimental investigations of the rate of ascent can also be carried out for this purpose.

For the application of a discharge measurement in a liquid which is predominantly water, an advantageous embodiment of the invention is that the air bubbles have an equivalent bubble diameter of between 3 mm and bubble volumes of the same volume and 10 mm, particularly advantageously about 4.5 mm. These air bubbles meet in a particularly good way the requirement of an approximately constant rate of climb.

In applications with less stringent requirements, it may be sufficient to record the image of the horizontal drift of the gas bubbles on the surface of the channel using a camera and to evaluate the photo to determine the runoff. In most cases, however, it will be desirable to carry out a continuous measurement. In these cases, a video camera, for example a CCD camera, can be used as the optical image recorder, which can have the advantage that the image can be processed and evaluated digitally.

Further advantageous developments of the method according to the invention are explained in the dependent claims and the description.

The advantages achieved by the invention are, in particular, that with the method according to the invention and the arrangement according to the invention described below, backflow phenomena and backflows, such as occur regularly in sewer systems, can also be detected, and the direction of flow can be determined and the volume flow in the relevant direction can be determined at any time. The measuring method according to the invention allows the detection of unsteady flows due to the very short measuring time. It measures the discharge directly, without the knowledge of the flow velocity and the flow cross-section being required. Depending on the spatial arrangement of the diffusers, the entire flow cross section can be recorded at the same time without impairing any shipping traffic. However, it is also possible not to arrange the diffusers over the entire cross-section of the channel or not to arrange them directly on the bottom of the channel and to take account of the cross-sectional components not recorded during the measurement in the calculation of the discharge. The arrangement of the diffusers at a distance from the base can be particularly advantageous if the gas bubble formation or its ascent would be disturbed by special features of the base of the channel, such as, for example, existing silt or vegetation.

A further advantage of the invention is that influences such as the density and the suspended matter content of the liquid and the temperature of the measuring medium can be taken into account.

The outflow at the respective measuring location, the direction of flow and thus also occurring transient flow processes can thus be recorded over any period of time. With the method according to the invention or the arrangement according to the invention, outflows can be measured and recorded directly in natural as well as in artificial channels, regardless of their flow cross-sectional geometry. The proposed invention can thus be used in rectangular, triangular, trapezoidal and, with corresponding adaptations, also in circular, egg-shaped and channel-shaped channel cross sections. In addition, the discharge can also be determined and measured in graded, structured discharge cross-sections across the width of the channel. Further advantages are that individual measurements do not have to be carried out one after the other, but the entire flow cross section is recorded simultaneously, and that the outflow of the channel on the water surface area is visible in the form of a bubble trace and can thus be checked visually at any time. The invention is particularly suitable for applications in which the liquid flows slowly, that is to say for channels in which the flow velocity of the liquid is less than 1 m / s, in particular less than 0.05 m / s.

The advantages and possible uses of the method according to the invention in artificial channels are described below, which apply in the same way to the arrangement according to the invention.

With the method, it is possible to measure turbulent, flowing, low-vortex outflows in free-flowing channels directly, with a high temporal resolution, with an unprecedented accuracy and without obstructing the outflow, even with changing flow directions.

It enables all operators of wastewater treatment plants to determine the inflows and outflows of treated wastewater with high temporal resolution at the inlet to the sewage treatment plant and at the outlet into the receiving water.

The operating areas of municipalities, municipalities and cities responsible for the wastewater can not only use the measuring method to measure the outflows at any place in the collecting channels of the city drainage system, but also quantify the proportion of untreated wastewater behind rain retention basins and rain overflow basins after heavy rainfall, the receiving water must be fed directly. Industrial companies can use the measuring method not only to document the wastewater discharges into the receiving water that have been approved for them, but also to control the outflows in internal water cycles.

When recovering valuable ingredients from wastewater, the volume flow determines the quality and efficiency of the recovery process. It can be determined with the measuring method in the free-flowing channels of such companies with high measuring accuracy.

Industrial companies, in particular energy producers, have to meet requirements regarding the consumption of cooling water. This is aimed inter alia at according to the current temperature and the water flow of the receiving water. The companies are therefore interested in an exact measurement of the cooling water withdrawal and return taking place with a high temporal resolution. The discharge measurement according to the invention offers a previously unknown possibility for this purpose at cooling water inlets and cooling water outlets.

Outflows from agricultural holdings - especially those from holdings with factory farming if they cannot store their pre-treated wastewater on their own farmland - are difficult to record. These can also be determined with the measuring method directly where it is discharged into the receiving water.

Outflows from built-up and paved areas, for example from streets outside of built-up areas, motorways, railway systems or airports, are fed to the sewage treatment plants or directly to the receiving waters in artificial free-flowing channels. They can be determined with the measuring method at the point of introduction. In the case of silt treatments, the water drainage from rinsing fields depends on the type of silt rinsing and the duration of storage of the silt. This unsteady outflow can be determined in the collecting channels which follow the flushing springs using the measuring method. The same applies to the drainage drains from landfill leachate and the wastewater drains arising from the dewatering of sewage sludge.

Precise discharge measurements are required not only in the field of wastewater technology. Another area of application for the discharge measurement method is in the discharge measurement of additional water to increase the low water level of the receiving water. The aim here is to optimize the proportion of high-quality feed water that is often of drinking water quality. The inflow into the receiving water can be measured directly in front of the discharge structures.

A further area of application of the discharge measurement method according to the invention is the discharge measurement in supply channels in the case of overflow irrigation (wild trickling, regulated surface flooding, etc.), strip and furrow irrigation in cultivated agriculture. Here the watering is to be controlled in such a way that the plant water requirements are met. Of particular importance is the discharge measurement during melioration - both during "leaching" and when washing out with irrigation water - from salted soils in order not to lose unnecessary parts of the irrigation water, which is usually scarce anyway.

The features of an arrangement according to the invention for carrying out the method according to the invention for determining the outflow of a liquid flowing in a channel with a free mirror using gas bubbles as Measuring medium can be seen in the fact that, over a cross-sectional area of the bottom of the channel, there are provided diffusers which are spatially distributed and are connected to a compressed gas supply and with which a chronological sequence of individual gas bubbles can be generated, gas bubbles being of such a size that they also have an almost constant rate of climb from the bottom of the channel to the surface thereof, at least one optical image sensor being arranged outside the liquid being provided, with which at least one image of the horizontal drift of the gas bubbles on the surface of the channel can be taken, and one of which the at least one image processing unit that can be connected is provided, with which the at least one recorded image can be evaluated to determine the outflow of the channel.

Further advantageous special features can be that the diffusers are arranged next to one another in a row running transversely to the flow direction of the channel or that an illumination device is provided for illuminating the surface of the channel in the area of the ascending gas bubbles.

A further advantageous embodiment can be that a support structure for holding at least one of the at least one image sensor is provided in a position from which an image of the ascending gas bubbles can be taken. The position of the image sensor is preferably in the air space above the surface of the liquid. If this is not possible, the image sensor can also be arranged on the side of the channel, for example on the bank. Furthermore, it can be provided that instead of the image processing unit, a storage medium can be connected to the at least one image recorder, with which the at least one recorded image can be stored for determining the outflow of the channel and can be supplied to an image processing unit.

An essential prerequisite is the generation of a sufficient number of individual bubbles per unit of time in order to ensure clear recognition of a bubble trace on the surface of the channel. On the other hand, however, only so many bubbles may be created that they do not influence each other in the swarm, but behave like single bubbles with regard to their rate of climb.

The formation of air bubbles using an air supply system with the following devices is known from the prior art:

Holes in thin sheets from 0.06 mm diameter,

Bores in thick sheets from 0.4 mm in diameter,

Glass tubes with extended tips from 0.025 mm diameter,

Glass capillaries from 0.045 mm diameter,

Injection cannulas from 0.2 mm diameter.

With the exception of the glass tubes with extended tip, which, however, cannot be reproduced with sufficient accuracy, all other nozzles have the disadvantages that swarms of bubbles form even at very low pressures or bubbles of the same size can be produced at constant pressure, but the frequency does not remain constant. The invention is therefore based on the further problem of creating a device for carrying out the method according to the invention with a diffuser for generating gas bubbles of defined and constant size in a liquid, to which the bubbles required to generate a clear bubble trace are generated in sufficient numbers per unit of time form, whereby it is ensured that the bubble formation is absolutely uniform in size and frequency.

This object is achieved by a diffuser which has a gas inlet with an inlet opening for the supply of the gas, a gas outlet with an outlet opening for the outlet of the gas bubbles and a gas channel connecting the gas inlet and the gas outlet, in the gas channel one in the flow direction of the gas a throttle located downstream of the inlet opening for throttling the gas flow is arranged with a throttle opening, part of the gas channel being formed by a blind hole which extends from its bottom towards its open end in the direction of the gas outlet extends, the inlet point of the gas channel, at which the gas flowing from the throttle downstream into the blind hole enters the blind hole, being arranged at a point in the blind hole from which the gas at least a portion of the blind hole downstream in the direction of flows through the gas outlet, and wherein the blind hole at the inlet point an S acklochquerschnitt which is larger than the opening area of the throttle opening.

An advantageous embodiment is that the opening area of the throttle opening is in the range from 0.002 mm ² to 0.785 mm ² . The blind hole advantageously has a diameter in the range from 1 mm to 8 mm, preferably approx. 2 mm, and a length in the range from 2 mm to 80 mm, preferably approx. 14 mm.

In a preferred embodiment, the gas outlet comprises an expansion recess arranged in the diffuser, which has an inner end upstream of the gas flow and a downstream outer end, the open end of the blind hole being connected to the inner end of the expansion recess and the diameter of the expansion recess at the outlet opening of the gas outlet is larger than the diameter of the open end of the blind hole. The expansion recess advantageously has a diameter between 2 mm and 10 mm, preferably approximately 3 mm, and a length in the flow direction of the gas between 1 mm and 15 mm.

The expansion recess can be cylindrical. In a preferred embodiment, however, it can also have a course, for example conical, which widens from its inner end to its outer end. It can be advantageous if the diameter of the inner end of the expansion recess corresponds to that of the open end of the blind hole.

The expansion recess advantageously has a diameter in the range between 1 mm and 8 mm, preferably approximately 2 mm, at its inner end and a diameter in the range between 2 mm and 10 mm, preferably approximately 4 mm, at its outer end. In the case of the diameters designated above as preferred, the length of the expansion recess is advantageously approximately 1 mm.

According to a further advantageous feature, it is proposed that the throttle and the blind hole and, if appropriate, the expansion recess be embedded in a carrier part. are worked. This has the advantage that the diffuser can easily be inserted into a diffuser holder and exchanged. It can further advantageously be provided that the carrier part is shaped like a screw and has a thread for screwing in. The carrier part can be made of steel, stainless steel, plastic or another material suitable for use in the respective liquid and with the respective gas, for example made of non-ferrous metal, glass or ceramic.

Another advantageous special feature may be that the throttle has a cannula. Another advantageous training feature can be that a wire is arranged in the opening of the cannula. The wire reduces the opening cross section of the cannula, which is advantageously in the range from 0.007 mm ² to 0.013 mm ² , preferably from approximately 0.01 mm ² , the wire having a cross section in the range from 0.0070 mm ² to 0.0086 mm ² , preferably about 0.0078 mm ² . In a modified embodiment, the cannula can have an opening cross section in the range from 0.002 mm ² to 0.003 mm ² , preferably from approximately 0.0025 mm ² .

The cannula can advantageously consist of stainless steel, plastic or glass. Stainless steel, spring steel or glass can preferably be provided for the material of the wire.

Further advantageous embodiments of the diffuser according to the invention are specified in the dependent claims and in the description.

The diffuser according to the invention has the advantage that it is necessary to produce a clear bubble trace A sufficient number of gas bubbles can be formed per unit of time, ensuring that the size and frequency of the bubble formation is uniform so that it can supply the gas bubbles required as the measuring medium in the method and the arrangement according to the invention as the measuring medium. For this purpose, it is preferably designed such that the gas bubbles generated have an equivalent bubble diameter, based on the volume of the same bubbles, of between 1 mm and 20 mm, preferably between 3 mm and 10 mm and particularly preferably about 4.5 mm. The measurement can be carried out on the basis of the preferred dimensions described or by experimental investigations.

The following exemplary embodiments of the invention reveal further advantageous features and special features which are described and explained in more detail below with reference to the illustration in the drawings.

Show it:

Fig. 1 is a graphical representation of the vertical

Flow velocity profile in a channel,

2 shows a graphical representation of the characteristic ascent path of air bubbles in a water channel according to the invention,

3 shows an enlarged detail "X" according to FIG. 2,

4 shows a graphical representation of the measurement results of measurements of the rate of rise of air bubbles in distilled water and in tap water,

5 an air bubble depicted as a flattened rotational ellipsoid, 6 shows a schematic perspective illustration of the arrangement according to the invention for carrying out the method,

7 is a schematic sectional drawing through a modified arrangement according to the invention,

8 shows a basic illustration of a diffuser according to the invention in section,

9 shows a schematic diagram of a modified diffuser according to the invention in section and

10 shows a basic illustration of a further modified diffuser according to the invention in section.

FIG. 1 shows a graphical representation of the vertical flow velocity profile VP in a channel as a function of the height y between the sole 8 and the surface 13. In FIG. 2, the characteristic ascent path AB of air bubbles 12 in a water channel is shown in FIG Invention shown. The air bubbles 12 are driven away by the drift S by the liquid flow on their way from the sole 8 to the surface 13. A detail at X in FIG. 2, which is explained in more detail below, is shown in FIG. 3.

The measuring method according to the invention is based on the principle of the so-called "integrating Schwi measurement". According to this principle, gas bubbles of a suitable size rise from the bottom 8 in a channel 14, being driven off at every point during the ascent by the amount which corresponds to the local speed (cf. FIG. 2). The horizontal drift which the bubbles 12 have covered from the starting point on the channel bottom 8 to the thawing on the surface 13 (see FIG. 2) multiplied by the rate of rise u _{s of} the gas bubbles (see FIG. 4) then exactly the outflow in a strip of the blister track Flow cross-section. To determine the total discharge, according to the invention, at least one image of the ascending gas bubbles floating on the surface 13 is recorded, for example with the aid of a video camera 5, and the image is then digitally processed. In this way, the method measures the discharge directly, without knowing the flow rate and the flow cross-section.

The method according to the invention is based on physical considerations from the field of fluid dynamics, which are defined by the following equations and illustrated using the graphical representations according to FIGS. 1 to 5:

If v (y) means the speed of a liquid particle and y its height above the channel bottom 8, the outflow is in an infinitely narrow strip of the flow cross section

y = h

= / v (y) dy (1) y = 0

If u _{s is} a constant rate of rise of the gas bubble, and t is the time since the start of its ascent, the following applies

dy = u _s • dt ( ₂ )

The drift s of the gas bubble at time t at height y corresponds to the local speed ds = v (y) • dt (3)

If the rate of climb is assumed to be constant, then by inserting Eq. (2) and (3) in Eq. (1) for the special drain

s = S ds u «dt

^■ / dt (4) s = 0

s = S q = u _s • f ds ₍₅₎ s = 0

The integral f ds corresponds to the distance S, and represents the drift of an ascending gas bubble reaching the surface (FIG. 2). The specific outflow q is therefore directly proportional to the drift S.

q = u _s ^• s ₍₆₎

The total volume Q is then

B

Q - u _s ^• / s (b) ^• db (7)

0

and is therefore proportional to the area which arises from the fact that the drifts S of the various verticals are plotted over the cross-sectional width B. An essential prerequisite for the validity of the basic equation of the total volume flow Q derived above is the guarantee of a constant rate of rise u _{s of} the gas bubble from the sole 8 to the surface 13. In order to be able to meet this requirement, the size of the individual gas bubbles. This is explained in detail below for air bubbles 12.

4 shows measurement results of the rate of rise u _g of air bubbles 12 in pure and contaminated water (cf. Clift, Grace and Weber, 1978, Bubbles, Drops and Particles. ACADEMIC PRESS, New York, San Francisco, London). 4 shows the rate of climb u _s as a function of the equivalent bubble diameter d _orf . The entire area shown is divided into three sub-areas, namely in the area Bl of the spherical bubbles, area B2 of the ellipsoid-like bubbles and area B3 of the screen bubbles. The designation DW stands for "distilled water" and the designation LW for "tap water".

With increasing equivalent bubble diameter d _o e _r q _r , ie with a bubble diameter related to spheres of equal volume, the behavior of the bubble depends solely on its size. Very small bubbles have a spherical shape due to the dominating influence of the surface tension, and the bubble has the properties of a rigid ball from a fluidic point of view. The movement sol¬ cher bubbles takes place in the Stokes region, said Be¬ range limit Re - u _r d / v <0.1 is indicated by d bubble diameter and u _r relative speed between bubble and water (cf. GLASE and WAIREGEL, 1986. Properties and characteristics of drops and bubbles, in Encyclopedia of Fluid Mechanics, Volume 3, Gas-Liquid Flows, ed. NP Cheremisinoff, Gulf Publishing Company, Houston, London, Paris, Tokyo).

The equivalent diameter d _orr that an air bubble 12 rising in water may assume to a maximum, so that the bubble still behaves like a rigid ball, is approximately 0.2 mm (cf. BAUER 1971, Basics of Single-Phase and Multi-Phase Flows, Sauerländer AG, Aaran).

As the bubble diameter increases, the shear stress at the phase interface inside the bubble causes a circulation flow. Since the velocity gradient at the movable phase interface is smaller than at the rigid one, the shear stress at the interface is reduced and thereby the resistance is reduced. Accordingly, the rate of climb u _s of large bubbles is higher than that of rigid balls of the same shape.

Regardless of the internal circulation, the bubble deforms with a further increase in the equivalent bubble diameter d _or to a flattened ellipsoid of revolution (FIG. 5). For the rate of climb u _{s of} this bubble, two influences acting in the opposite direction must be taken into account. Although the mobility of the phase surface contributes to increasing the rate of climb u _s , the vortices which occur periodically behind the flattened ellipsoid of rotation cause a tumbling movement, so that the bubble rises on a screw-like path. However, since only the subdued in the vertical direction height difference is used to calculate the speed Steigge¬ u _s, the rate of climb is u _s thus smaller than the actual web speed (see FIG. BAUER, 1971, Fundamentals of Single-phase and multi-phase flows, Sauerländer AG, Aaran).

A further increase in the equivalent bubble diameter d__ leads to the conversion of the bubble shape from the ellipsoid of rotation to the so-called "umbrella bubble" and an increase in the rate of climb.

For an equivalent bubble diameter d _e _> 1.3 mm, the rate of climb u _s in pure water can be approximately described by equation (8), where σ is the surface tension between water and air (cf. CLIFT, GRACE, WEBER, 1978 , Bubbles, Drops and Particles; ACADEMIC PRESS, New York, San Francisco, London).

The actual dimensions of the ellipsoid of revolution (Fig. 5) can be calculated using Eq. (9) and that of the equivalent bubble diameter d _orτ with Eq. (10) can be calculated:

_k v _β = π a ₂ ^■ (bi + b ₂ ) (9)

* eq (10)

The experimental results in Fig. 4 also show the influence of the impurities in the water (surface-active substances). They collect at the phase interface and form an adsorbing film there. It remains at rest relative to the bubble, so that the air bubble behaves like a rigid ball with respect to the outside flow. When taking a closer look, however, the transport of the contaminants in the water and the influence of these substances on the surface tension must also be taken into account (cf. PRANDTL, 1989: "Guide through fluid dynamics", Vieweg, Braunschweig).

As already mentioned at the beginning, the suitable choice of the bubble size is decisive for the successful implementation of the method according to the invention.

The bubble trace on the surface 13 required for the discharge measurement by means of gas bubbles requires the formation of individual bubbles in sufficient numbers per unit of time. From Fig. 4 it can be seen that for equivalent bubble diameters d ^e _4_ between 3 mm and 10 mm for air bubbles 12 in water, the rate of climb u _s remains approximately constant. With regard to the generation of individual bubbles, this area offers the advantage that a change in the rate of climb u _s due to expansion of the air bubbles 12 during the ascent is prevented in the case of bubbles with an equivalent bubble diameter d -, - of approximately 4.5 m - Can do so, even with a change in volume, the rate of climb u _s remains almost constant.

The method of determining the discharge according to the invention has been successfully investigated for the measurement of small and at the same time fluctuating discharges with changing water levels in the following example. The measurements simulate a typical use of the discharge measurement method, as it would result in the different areas of application mentioned above.

In the application under investigation, the arrangement for carrying out the is in an open test channel with a width of 0.60 m, with a variable water depth between 0.10 m and 1.00 m and a controllable outflow between 0 and 350 1 / s Drainage measuring method according to FIG. 6 has been installed.

In these tests, different operating states were set and the discharge values determined by the discharge measurement method were checked. In these investigations, discharge values with a high measurement accuracy were delivered according to the previously described measurement principle of the integrated float measurement in connection with the image acquisition by video technology and the digital image processing by means of the measurement method claimed.

An advantageous arrangement for carrying out the method according to the invention is described below with reference to FIG. 6. A channel 14 of flowing water with the surface 13 is shown. On the bottom 8 of the channel 14, a plurality of diffusers 1 are arranged side by side by means of a diffuser holder 2. The diffusers 1 are connected to a compressed air supply unit 4 by means of a compressed air line 3. Furthermore, a video camera 5 and a microprocessor 6 connected to it and an illumination device 7 are provided. The arrangement can be arranged, for example, in the area of a bridge. In addition to the microprocessor 6 and the compressed air supply unit 4, a digital image processing unit 10 is also accommodated in the equipment cabinet 9. The video camera 5 is advantageously on an already existing above the channel 14 Bridge girders attached. However, it can also be attached to the side of the channel, for example on the bank, or, as shown in FIG. 6, on a support structure 11 constructed for this purpose. The supporting structure 11 also carries the lighting device 7, with which the surface 13 can be illuminated with artificial lighting at night or in difficult lighting conditions, so that uninterrupted operation of the method according to the invention is ensured. The measurement of the discharge using the method according to the invention is carried out as follows.

From the diffusers 1, the ascending air bubbles 12 generated by the compressed air supply unit 4 reach the surface 13 of the fluid. The air bubbles 12 floating on the surface are detected by means of the video camera 5. The video images are evaluated in the microprocessor 6 with digital image processing in the equipment cabinet 9 on site. The video images can also be optionally stored digitally on mass storage or analogously on video recorders, in order to be evaluated at a later point in time in an image processing unit 12 with a microprocessor 6 and digital image processing.

The considerations on which the measurement method is based are described below with regard to the digital determination of the resulting current rate of climb u _s according to FIG. 4 and the digital detection of the bubble drift.

The current rate of rise u _{s of} the air bubbles can be determined by an intermittent operation of the compressed air supply 4 in order to determine the influence of the temperature, to take into account the density and the contents of the measuring medium. For this purpose, the compressed air supply 4 to the bubble-generating diffusers 1 is briefly interrupted. The result is that no new air bubbles 12 form at the moment. To calculate the current bubble rate u _s according to Eq. (11) it is necessary to determine the period of time t ₂ -t- _j _ between the detachment of the last bubble before the interruption or optionally the first bubble after restarting from the bubble-generating diffusers 1 and reaching the water surface 13 as well as the water depth w.

In Eq. (11) mean u _s - the current rate of climb of the bubbles, ie the value to be determined, w - the vertical distance between the separation point on the diffuser 1 and the water surface 13 in the defined partial section of the electronic camera 5; depending on the place of use, this quantity is measured by a suitable measuring device (eg precision level, ultrasound probe, etc.), t ₁ ~ the point in time at which the compressed air supply is interrupted or, optionally, the compressed air supply is restarted; this size is registered by the electronic evaluation unit, t ₂ - the time of the appearance of the last or optionally the first bubble in a variable calibration section of the camera image; this size is determined with the aid of the digital image processing unit 10 and corresponding software. With the digital image processing unit 10, the drift of the air bubbles 12 recorded with a camera 5 can be further processed in such a way that the outflow through the flow cross section can be determined directly.

The light reflected at the bubble interface provides brightness information on the surface 13, over the gray value range of which the position of the air bubbles 12 is detected and the coordinates of the bubbles in the image section are determined in a computer-aided manner according to predetermined categories.

A complex processing sequence is required to find the air bubbles 12 in an image of the surface 13. In addition to functions of system initialization, look-up table operations, image acquisition and image storage functions, the algorithm for recognizing air bubbles 12 on surface 13 is composed primarily of filter operations.

After this preprocessing, the features of the image (area and coordinates of the center of gravity of the air bubbles 12) are extracted, statistically processed and evaluated. All coordinates of the detected bubbles fall within the limited value range that is determined by the image section of the camera 5. This range of values is divided into a number of constant intervals. Each interval forms a class, the class width determining the resolution of the drift. All drift measurement results (x coordinates of the bubbles) are assigned to the individual classes and the relative frequency is determined.

Statistical measures of the frequency distribution are the mean and the skew, since the bubbles flow through the surface 13 or through can distribute a wind-induced surface flow asymmetrically around the mean. A distinction can be made between positive and negative skewness. Since the statistical measures depend on the number of bubbles, the scattering range is reduced by overlaying several images.

After filtering out a background noise (threshold value) which is caused by the flooding or by reflections on the water surface, the drift of the bubbles can now be clearly determined on the basis of the mean value and the skewness of the frequency distribution.

If the drift of the bubbles and thus the bubble exit trace is determined in the observation window of the video camera 5, the total discharge is calculated according to Eq. (7). The measurement according to the invention is possible with a high temporal resolution.

7 shows a further example of application of the arrangement according to the invention or of the method according to the invention in the region of a bridge. A partial cross-section is shown in which the bridge girder 15 held on a foundation 17 by means of a pillar 16 can be seen. The camera 5, for example a CCD camera, is arranged on the bridge support 15. The channel 14 with the surface 13 and the sole 8 has a low and at the same time fluctuating flow rate. Conventional measuring methods for determining the flow velocity or the outflow, such as, for example, wing or float measurements, cause considerable difficulties in channels with very low flow velocities. If, for example, the discharge in a channel decreases extremely, it can happen that the mean flow velocity falls below 10 mm / s drops. An example of this is the outflow of the Spree in the summer months. With a flow cross-section with a width of approx. 60 m and a water depth of approx. 2.3 m, an average flow speed of 3.625 mm / s is established. This speed value cannot be recorded with conventional measuring methods, just like the runoff present.

As has been found in the example shown in FIG. 7, the discharge measurement by means of air bubbles 12 is a suitable method for determining the discharge directly and precisely. The measurement was made in the flow cross section under a bridge. The uniformity of the flow cross-section allowed it to be limited to the discharge measurement on the bridge pillars 16. In order to avoid falsification of the measurement results by boundary layer development on the bridge pier 16 or its foundation 17, rows of the diffusers 1 according to the invention were installed in a diffuser holder 2 on the base 8, which protrude 3.0 m into the flow cross-section (FIG 7). The diffuser holder 2 was fastened to the pillar 16 by means of a holding device 19. The compressed air line 3 and the compressed air supply unit 4 connected to it and the digital image processing unit 10 are not shown in FIG. 7. The camera 5 is connected to the digital image processing unit 10 by means of the signal derivative 18.

When an air bubble 12 rises from the sole 8, it is driven off at each point in the flow direction by an amount which corresponds to the local flow velocity at this point. The horizontal drift, which it has covered from the starting point on the sole 8 to the surface 13, multiplied by the rate of ascent u _{s of} the air bubbles 12 corresponds exactly to the outflow in a strip of the flow cross section corresponding to the bubble trace. To determine the total outflow, the image of the air bubbles 12 ascending to the surface 13 was recorded with the aid of a camera 5, in which the image data are converted into electrical signals with semiconductor image sensors and subsequently in a microprocessor connected to the camera 5, which is not shown in the drawing is shown, digitally processed. The combination of the integration of the flow rate by means of rising air bubbles 12 and the digital image processing thus enabled a continuous, direct discharge measurement with high temporal resolution and accuracy.

FIGS. 8 to 10 show diffusers 1, with which gas bubbles, in particular air bubbles 12 in water, can be produced absolutely uniformly with regard to their size and chronological order in order to generate gas bubbles as a measuring medium for the method according to the invention for measuring the discharge .

The diffuser 1 according to FIG. 8 has a gas inlet 30 with an inlet opening 31 for the supply of the gas. The gas exits at the gas outlet 45 with the outlet opening 46 to form gas bubbles (not shown) into the liquid. The gas inlet 30 is connected to the gas outlet 45 via the gas channel 47. A throttle 32 designed as a cannula 43 with a wire 44 inserted therein is arranged in the gas channel 47. The diameter of the wire 44 is smaller than the diameter of the cannula 43, so that the throttle opening 33 which throttles the gas flow is formed in the space between the wire 44 and the cannula 43 and which sem case extends along the gap. The opening area of the throttle opening 33 is equal to the cross-sectional area of the intermediate space.

The throttle 32 opens downstream in the region of the bottom 35 of the blind hole 34 into the blind hole 34 formed as a cylindrical bore. The inlet point 48 at which the gas from the throttle 32 enters the blind hole 34 is not found in this example directly at the bottom 35 of the blind hole 34, since the cannula 43 protrudes somewhat into the blind hole 34. However, the inlet point 48 could also be in the bottom 35 of the blind hole 34. The blind hole 34 extends from its bottom 35 towards its open end 36 in the direction of the gas outlet 45. The cross section of the blind hole 34 is larger than the opening area of the throttle opening 33, so that at this point the available for the flow of the gas Cross section enlarged.

The last fluidic construction element of the diffuser 1 is the open end 36 of the blind hole 34, which forms the outlet opening 46 of the gas outlet 45. As a rule, the diffuser 1 is oriented in the liquid such that the air bubbles 12 emerge vertically upward. However, other orientations are also conceivable, including those in which the cannula 43 and the blind hole 34 have a different angle than shown in FIG. 8 to form each other. The blind hole 34 and the cannula 43 lie in a polyamide support part 41, which is designed as a hexagon screw and has a thread 42 on the outside.

The cannula 43 is made of stainless steel with an inside diameter of 0.115 mm and an outside diameter of 0.68 mm. The wire 44 guided in it also exists made of stainless steel with a diameter of 0.10 mm and is secured to the cannula 43 by soldering against displacement. The cannula 43 is 20 mm long. The cannula 43 and the wire 44 guided in it protrude 2 mm into the blind hole 34, measured from the bottom 35 of the blind hole 34. The length of the cannula 43 inside and outside of the carrier part 41 is 9 mm in each case.

The carrier part 41 designed as a hexagon screw has a total length of 24 mm, the shaft being 20 mm and the head being 4 mm long. The blind hole 34 is 14 mm deep, its diameter is 2 mm. The air or the gas enters the gas channel 47 of the diffuser 1 from a compressed air supply system through the gas inlet 30. Due to the operating pressure present at the gas inlet 30 and the lower external hydrostatic pressure of the liquid, a pressure difference results, due to which gas bubbles form which exit into the liquid at the gas outlet 45.

A modified diffuser 1 is shown in FIG. It differs from the diffuser 1 according to FIG. 8 by an additional cylindrical expansion recess 38 formed in the diffuser 1. The inner end 39 and the outer end 40 of the expansion recess 38 have a diameter of 3 mm. The expansion recess 38 is 4 mm long. The blind hole 34 is therefore 4 mm shorter than in the diffuser 1 according to FIG. 8; the other dimensions are the same. The outer end 40 of the expansion recess 38 forms the outlet opening 46.

FIG. 10 shows a further modified embodiment of the diffuser 1. It differs from the diffuser according to FIG. 8 in that it is additionally formed in the diffuser 1. dete, conical expansion recess 38. The conical expansion recess 38 has a diameter of 3 mm at the outer end 40 and a diameter of 2 mm at the inner end 39, which corresponds to the diameter of the blind hole 34. The conical expansion recess 38 is 1 mm long. The blind hole 34 is thereby shortened by 1 mm compared to the embodiment in FIG. 8; the other dimensions correspond to those of the diffuser 1 according to FIG. 8. The outer end 40 of the expansion recess 38 forms the outlet opening 46.

Claims

Expectations

1. A method for determining the outflow of a liquid with a free mirror in a channel (14) flowing liquid using gas bubbles as the measuring medium, characterized in that by means of a cross-sectional area of the bottom (8) of the channel (14) spatially distributed Dif ¬ fusors (1) a time sequence of individual gas bubbles is generated, the gas bubbles being of such a size that they have an almost constant rate of climb (u _s ) from the bottom (8) of the channel (14) to its surface ( 13) rise, at least one image of the horizontal drift of the gas bubbles on the surface (13) of the channel (14) is recorded by means of at least one optical image recorder arranged outside the liquid, and that the at least one recorded image is used for the determination the outflow of the channel (14) is evaluated by means of an image processing unit (10).

2. The method according to claim 1, characterized in that the diffusers (1) are arranged side by side in a row running transversely to the flow direction of the channel (14).

3. The method according to claim 1, characterized in that the liquid is predominantly water and the gas bubbles are air bubbles (12).

4. The method according to claim 3, characterized in that the air bubbles (12) with an equivalent bubble-based equivalent bubble diameter (d _e _) between 3 mm and 10 mm, preferably about 4.5 mm, are generated.

5. The method according to claim 1, characterized in that the image processing unit (10) digitally processes and evaluates the at least one image.

6. The method according to claim 1, characterized in that to determine a discharge value, images of the surface (13) are taken in succession at different times and are evaluated by means of the image processing unit (10) using statistical evaluation methods.

7. The method according to claim 1, characterized in that the surface (13) of the channel (14) in the area of the ascending gas bubbles is illuminated by means of a lighting device (7).

8. The method according to claim 1, characterized in that it includes a step in which to determine the rate of climb (u _s ) of the gas bubbles, the generation of the gas bubbles stopped at a first point in time and the time difference up to a second point in time, from no more gas bubbles appear on the surface (13) of the channel (14), measurement is carried out.

9. The method according to claim 1, characterized in that it includes a step in which to determine the rate of climb (u) of the gas blow the generation of the gas bubbles switched on at a first point in time and the time difference up to a second point in time at which the first gas bubbles appear on the surface (13) of the channel (14) is measured.

10. Arrangement for carrying out the method according to claim 1 for determining the outflow of a liquid flowing with a free mirror in a channel (14) using gas bubbles as the measuring medium, characterized in that over a cross-sectional area of the sole (8 ) of the channel (14) spatially distributed diffusers (1) are provided, which are connected to a pressurized gas supply and with which a chronological sequence of individual gas bubbles can be generated, gas bubbles being of such a size that they have an approximately constant Rise speed (u _s ) from the bottom of the channel (14) to its surface (13), at least one optical image sensor arranged outside the liquid is provided, with which at least one image of the horizontal drift of the gas bubbles the surface (13) of the channel (14) can be picked up and an image processor that can be connected to the at least one image pickup Processing unit (10) is provided, with which the at least one recorded image can be evaluated to determine the outflow of the channel (14).

11. The arrangement according to claim 10, characterized in that the diffusers (1) are arranged side by side in a row running transversely to the flow direction of the channel (14).

12. The arrangement according to claim 10, characterized in that an illumination device (7) for illuminating the surface (13) of the channel (14) is provided in the region of the ascending gas bubbles.

13. The arrangement according to claim 10, characterized in that a supporting structure (11) for holding at least one of the at least one image sensor is provided in a position from which it can take an image of the ascending gas bubbles.

14. Arrangement according to claim 10, characterized in that instead of the image processing unit (10), a storage medium can be connected to the at least one image sensor, with which the at least one recorded image for determining the outflow of the channel (14) can be stored and one Image processing unit (10) can be fed.

15. Device for carrying out the method according to claim 1 with a diffuser (1) for generating gas bubbles of defined and constant size in a liquid, the diffuser (1) having a gas inlet (30) with an inlet opening (31) for the supply of the gas, a gas outlet (45) with an outlet opening (46) for the outlet of the gas bubbles and one the gas inlet

(30) and the gas outlet (45) connecting gas channel

(47), in the gas channel (47) a throttle (32) lying downstream of the inlet opening (31) in the flow direction of the gas for throttling the gas flow with a throttle opening (33) is arranged, a part of the gas channel (47) is formed by a blind hole (34) which extends from its bottom (35) to its open end (36) towards the gas outlet (45), the inlet point (48) of the Gas channel (47), at which the gas flowing from the throttle (32) downstream into the blind hole (34) enters the blind hole (34), is arranged at a point in the blind hole (34) from which the gas at least one Part of the blind hole (32) flows downstream in the direction of the gas outlet (45), and the blind hole (32) at the inlet point (48) has a blind hole cross section which is larger than the opening area of the throttle opening (33).

16. The apparatus according to claim 15, characterized in that the opening area of the throttle opening (33) is in the range from 0.002 mm ² to 0.785 mm ² .

17. The apparatus according to claim 15, characterized in that the blind hole (34) has a diameter in the range of 1 mm to 8 mm, preferably approximately 2 mm, and a length in the range of 2 mm to 80 mm, preferably approximately 14 mm having.

18. The apparatus according to claim 15, characterized in that the gas outlet (45) comprises an expansion recess (38) arranged in the diffuser (1) and having an inner end (39) and an upstream end of the gas stream (39) has a downstream outer end (40), the open end (36) of the blind hole (34) being connected to the inner end of the expansion recess (38) and the diameter of the expansion recess (38) at the outlet opening (46) of the gas outlet (45) is larger than the diameter of the open end (36) of the blind hole (34).

19. The apparatus according to claim 18, characterized in that the expansion recess (38) has a diameter between 2 mm and 10 mm, preferably about 3 mm, and in the direction of flow of the gas has a length between 1 mm and 15 mm.

20. The apparatus according to claim 18, characterized in that the expansion recess (38) is cylindrical aus¬.

21. The apparatus according to claim 18, characterized in that the diameter of the expansion recess (38) increases from its inner end (39) to its outer end (40).

22. The apparatus according to claim 21, characterized in that the expansion recess (38) is conical.

23. The device according to claim 22, characterized in that the diameter of the inner end (39) of the expansion recess (38) corresponds to that of the open end (36) of the blind hole (34).

24. The device according to claim 21, characterized in that the inner end (39) has a diameter in the range between 1 mm and 8 mm, preferably about 2 mm, and the outer end (40) has a diameter in the range between 2 mm and 10 mm, preferably about 4 mm.

25. The device according to claim 24, characterized in that the length of the expansion recess (38) is approximately 1 mm.

26. The apparatus according to claim 15, characterized in that the throttle (32) and the blind hole (34) are incorporated in a carrier part (41).

27. The apparatus according to claim 18, characterized in that the throttle (32), the blind hole (34) and the expansion recess (38) are incorporated into a carrier part (41).

28. The apparatus according to claim 26 or 27, characterized gekenn¬ characterized in that the carrier part (41) is shaped like a screw and has a thread (42) for screwing into a diffuser holder (2).

29. The device according to claim 26 or 27, characterized gekenn¬ characterized in that the carrier part (41) consists of steel.

30. The device according to claim 29, characterized in that the carrier part (41) consists of stainless steel.

31. The device according to claim 26 or 27, characterized gekenn¬ characterized in that the carrier part (41) is made of plastic be¬.

32. Apparatus according to claim 15, characterized in that the throttle (32) has a cannula (43).

33. Apparatus according to claim 32, characterized in that a wire (44) is arranged in the opening of the cannula (43).

34. Apparatus according to claim 33, characterized in that the cannula (43) has an opening cross section in the range from 0.007 mm ² to 0.013 mm ² , preferably from approximately 0.01 mm ² , and the wire (44) has a cross section in the Has range from 0.0070 mm ² to 0.0086 mm ² , preferably about 0.0078 mm ² .

35. Device according to claim 32 or 33, characterized in that the cannula (43) has an opening cross section in the range from 0.0020 mm ² to 0.0030 mm ² , preferably of approximately 0.0025 mm ² having.

36. Apparatus according to claim 32, characterized in that the cannula (43) consists of stainless steel.

37. Apparatus according to claim 32, characterized in that the cannula (43) consists of plastic.

38. Apparatus according to claim 32, characterized in that the cannula (43) consists of glass.

39. Apparatus according to claim 33, characterized in that the wire (44) consists of stainless steel.

40. Apparatus according to claim 33, characterized in that the wire (44) consists of spring steel.

41. Apparatus according to claim 33, characterized in that the wire (44) consists of glass.

42. Apparatus according to at least one of claims 15 to 41, characterized in that it is designed such that the gas bubbles generated have an equivalent bubble diameter (deq) between 1 mm and 20 mm, preferably between, with respect to the same volume bubbles 3 mm and 10 mm and particularly preferably about 4.5 mm.

43. Use of the device according to at least one of claims 15 to 42 for generating gas bubbles as a measuring medium in a method for determining the outflow of a liquid flowing with a free mirror, in particular in a method according to claim 1.