Wire-mesh sensors: an experimental tool for two-phase CFD model development and code validation


Wire-mesh sensors: an experimental tool for two-phase CFD model development and code validation

Prasser, H.-M.

The Institute of Safety Research of the Forschungszentrum Rossendorf, Germany, has developed electrode-mesh sensors, which allow the measurement of the electrical conductivity distribution in a flow duct. This can be used either for the detection of the gaseous phase in a gas-liquid flow or for mixing studies in single phase flow, when the components have different electric conductivities. Two grids of crossing wires are placed into the flow closely behind each other. The wires of the first plane (transmitter plane) are supplied with pulses of a driving voltage in a successive order. The data acquisition is done by measuring the electrical currents arriving at the second grid (receiver wires). After the last transmitter electrode has been activated, a two-dimensional matrix is available that reflects the conductivities at crossing points of the electrodes of the two grids. Sequences of these 2D distributions are recorded with a rate of up to 10 kHz.
Due to the high measuring rate each bubble is mapped in several successive instantaneous frames. This allows to obtain bubble size distributions as well as bubble-size resolved gas fraction profiles beside the visualisation and the calculation of profiles of the time-averaged void fraction. Two sensors placed behind each other can furthermore be used for bubble velocity measurements using cross-correlation techniques. Sensors with three layers of electrode grids can be used for the measurement of the velocity of individual bubbles.
The sensor is widely used to study the evolution of the flow pattern in an upwards air-water flow. The experiments aim at closure equations describing forces acting on bubbles as well as coalescence and fragmentation frequencies for the implementation in CFD-codes. The largest sensor used until now has a circular measuring cross-section of about 200 mm diameter and is equipped with two grids of 64 wires. Therefore, the spatial resolution is 3 mm, the measuring frequency is 2.5 kHz. In the meanwhile, a sensor of this kind has been constructed and successfully used in a hot steam-water flow at 70 bar and 286 °C. Experiments were carried out at a vertical test channel of 195 mm inner diameter, in which the distance between gas/steam injection and sensor can be varied in a wide range. Results will be presented.
Some other prominent examples of the application of wire-mesh sensors will be given, like (1) the use of two wire-mesh sensors at the CIRCUS test facility of the University of Delft in the Netherlands for boiling water reactor stability studies, (2) the visualization of cavitation at fast-acting cut-off valves at the Pilot Plant Pipework test facility of Fraunhofer UMSICHT, Oberhausen, (3) the visualization of the flow structure behind a closing globe valve at TU Munich, and finally (4) mixing studies in single-phase flow at the ROCOM test facility in Rossendorf, which are aimed at the mixing of deborated slugs during boron dilution transients. Results will be discussed on basis of animated data visualizations for all examples.
The accuracy and the effect of the wire grids to the flow were investigated using a sensor built into a transparent channel. The comparison with the frames of a high-speed video camera have shown that the sensor acts as a bubble fragmenting obstacle. Nevertheless it was be proved, that the sensor signal represents the bubble geometry present in the upstream flow.

Keywords: two-phase flow; wire-mesh sensor; gas fraction; gas velocity; bubble size measurement; coolant mixing; boron dilution

  • Lecture (Conference)
    Advances in the Modeling Methodologies of Two-Phase Flows S, 24.-26.11.2004, Lyons, France

Permalink: https://www.hzdr.de/publications/Publ-6646
Publ.-Id: 6646