Wire-mesh sensors and tomography methods developed by FZR

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. 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. 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. Some other prominent examples of the application of wire-mesh sensors were given, like (1) boiling water reactor stability studies, (2) the visualization of cavitation at fast-acting cut-off valves, (3) the visualization of the flow structure behind a closing globe valve, 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.

Second subject of the paper is radiation tomography. A gamma-tomography setup for imaging a periodically changing density field is described. It is based on a time-resolved acquisition of the detector signals. It was used to visualize the gas fraction distribution within the impeller of an axial turbo-pump operating at about 1500 rpm, that delivered a gas-liquid mixture, as well as to a hydraulic clutch (coupling). In the field of X-ray tomography, the status of the development of an ultra-fast system based on a scanning electron beam is presented. An electron beam is linearly deflected over a tungsten target with a frequency of 1 kHz. X-rays generated by the traveling focus penetrate the object and arrive at a detector line placed behind the object. The detectors are read-out with a sufficiently high speed in order to obtain projections of the density distribution in different projecting directions, which change thanks to the scanning. First results showing tomographic image sequences of a phantom consisting of small spheres kept in arbitrary motion in a cylindrical test box will be presented. Moving spheres of 3 mm diameter with cylindrical holes of 1 mm diameter were resolved at a framing rate of 1 kHz.