Young Investigators Group on Measurement Techniques for Liquid Metal Flows
Liquid metals play an important role in metallurgy (e.g. continuous casting of steel), crystal growth, in cooling of power plants and also in fundamental research on magnetohydrodynamic instabilities or the dynamo effect. In all these applications the most interesting properties are local and integral velocities, pressure and temperature and void fractions, local bubble size and velocity distributions in case of two-phase flows.In contrast to the vast deployment of optical measurement techniques for transparent liquids like water or air, almost nothing comparable is commercially available when it comes to the corresponding measurement tasks in liquid metals, like liquid steel, silicon or sodium. Not only the high temperatures of liquid metals, (for instance the melting point of steel is at about 1500 °C) but also their opaqueness poses a huge challenge for measurement techniques. The restricted access to powerful measuring techniques for liquid metal flows has to be considered as a main handicap for any practical investigation of liquid metal flows.
The main focus is the development and application of measurement techniques for liquid metals, because the availability of appropriate measurement techniques is a necessity for any experimental work on liquid metal technologies. As part of the Helmholtz LIMTECH Alliance, we bundle the experience about different measurement techniques of the partners of the alliance, in order to have a broad tool set for measuring flow properties of liquid metals. This leads to the following three main tasks of the work:
- Enhancement of the existing techniques towards a multidimensional flow mapping with high frame acquisition rates and excellent spatial resolutions for examinations of turbulent, three-dimensional flows
- Characterization of dispersed liquid metal two-phase flows, using ultrasound and X-ray techniques. Imaging techniques become more and more important for detailed explorations of complex flows as well as for an efficient validation of numerical simulations.
- Development of benchmark experiments in order to bring together and compare the various measurement techniques.
The following three measurement techniques are the main focus of the development:
- The Contactless Inductive Flow Tomography (CIFT), developed at the HZDR, allows for a fully contactless inference of the velocity field in a volume of liquid metal by externally applying a magnetic field and measuring the flow induced perturbation of the magnetic field outside the melt. It was shown in an demonstration experiment that the mean three-dimensional flow structure in a cylindrical vessel can be reconstructed. This technique was used to visualize the two-dimensional flow field in a model of a continuous caster of the LIMMCAST facility, which is available at the HZDR.
- The Lorentz Force Velocimetry (LFV), developed at the University of Ilmenau, allows to measure the flow induced force on a permanent magnet close to the channel containing the liquid metal. The flow rate of the melt can be determined from this force measurement. This technique is developed to the state where its application in aluminum production and steel making is imminent. Recently, it has been successfully demonstrated that LFV can also be used for local measurements of the flow close to the wall by measuring velocities in a turbulent liquid metal duct flow.
- The Ultrasound Transit-Time Technique (UTTT), developed at the University of Dresden, is used to determine the size and the velocity of gas bubbles in liquid metals. This is achieved by measuring the ultrasound transit time between the ultrasound sender/receiver and the reflecting bubbles. From the transit time the position of the bubble in the direction of the ultrasound pulse can be deduced immediately. By using differently arranged transducer arrays, both the velocity and the size of the gas bubbles in connection with their spatial distribution can be determined.
Dr. Thomas Wondrak (HZDR) head
Matthias Ratajczak (HZDR) Contactless Inductive Flow Tomography
Thomas Richter (TU Dresden) Ultrasound Transit-Time Technique
Daniel Hernandez (TU Ilmenau) Lorentz Force Velocimetry
Dennis Otte (KIT) inductive bubble detection
Dr. Christiane Heinicke