Metrological use of cavitation – Innovative flow generation of fluid flows by means of special cavitation nozzles

Initial investigations by means of a high-velocity camera in a special Plexiglas nozzle and with a resolution of up to 140,000 shots per second allow novel insights into the behavior of a cavitating liquid flow inside this nozzle. The results obtained are an important part of the current work in the "Liquids" Department which is dealing with the development of a completely new test procedure for water meters on the basis of so-called cavitation nozzles. The procedure is to be characterized in particular by a high reproducibility of the generated liquid flowrates and the possibility of strongly reduced testing times.

Besides the well-known damaging effects through so-called cavitation corrosion, cavitation definitely also offers a series of very useful potential technical applications, for example for the cleaning of surfaces, for fuel injection in diesel engines, or for the twisting of chemical fibers in the textile industry. Its use for concrete measurement purposes has so far, however, been unknown.
In analogy to the critical nozzles which are already widely used in gas metrology, the "Liquids" Department has made an attempt to develop a procedure with similar advantages also for liquids. The starting point for the corresponding investigations was the observation of liquid flows which are accelerated in such a way that the local liquid vapor pressure in the nozzle throat is undercut, through which cavitation is generated. Initial experiments with Venturi nozzle forms which have a toroidal nozzle throat (see Figure 1) and are known from gas measurement confirmed the expected flowrate constancy after the pressure had fallen below a specific pressure ratio. The determination of the reproducibility of the created flowrate also showed very satisfying results with relative standard deviations in the order of 10^-5 to 10^-6.

Figure 1: Originally used nozzle form: Venturi nozzle with a toroidal nozzle throat (here with a throat diameter of 0.54 mm for a flowrate of 20 l/h at an input pressure of 3000 mbar and a temperature of 20 °C) [1]

Within the scope of a scientific collaboration with the Institute of Fluid Mechanics of Duisburg-Essen University, the theoretical bases of cavitating nozzle flows have been elaborated and documented on the basis of simulations. One of the focal points of this work was the optimization of the nozzle form for applications in the field of liquids. On the basis of the corresponding calculations and simulations, the proposal was made to use so-called Herschel-Venturi tubes – whose nozzle throat is a cylinder – instead of  the toroidal Venturi nozzle. With the aid of a Herschel-Venturi tube made of Plexiglas, pictures of the flow were taken with a high-velocity camera on the water meter test rig of PTB. The resolution was between 50,000 shots per second and 140,000 shots per second. Figure 2 shows a single frame of a shot with 140,000 pictures per second.

Figure 2: Image of a cavitating flow inside a Herschel-Venturi tube (diameter: 11.2 mm, length of the cylindrical throat: 11.2 mm, flowrate 7.7 m³/h)

Clear structures can be seen which can also be tracked over time. These records allowed the 2D and 3D simulations performed to be verified and the determined nozzle form to be confirmed.
Detailed information can be found in [2] and [3].

Literature:

[1] ISO 9300, Measurement of gas flow by means of critical flow Venturi nozzles, 2005

[2] S. Brinkhorst, E. von Lavante, D. Güler, G. Wendt, „Experimental Investigation of Cavitating Herschel Venturi-Tube Configuration“, in FLOMEKO 2016, Sydney, 2016, submitted

[3] S. Brinkhorst, E. von Lavante, G. Wendt, (2015): “Numerical investigation of cavitating Herschel Venturi-Tubes applied to liquid flow metering”. In: Flow Measurement and Instrumentation 43, S. 23–33. DOI:  10.1016/j.flowmeasinst.2015.03.004

Contact person:

Daniel, Schumann, FB 1. 1.5, AG 1. 53, E-Mail: daniel.schumann(at)ptb.de