Ultra Fast Electron Beam X-ray Computed Tomography for Two-phase Flow Measurement

Electron beam X-ray CT is a new technique for a fast measurement of multiphase flows with frame rates of 1000 images per second and more. It gives, in principle, quantitatively accurate images of the flow at high spatial resolution and it is non-intrusive since moderately radiation absorbing vessel walls can be penetrated by X-rays. However, on the road to a technical realisation of such a technique within a computed tomography system many problems have to be solved. As a first prototype for scientific flow measurement studies we devised and built a fast scanned electron beam X-ray tomography scanner. The scanner consists of an electron beam unit that can be operated at up to 150 kV acceleration voltage and up to 65 mA electron beam current, with the required electron optics for beam adjustment, beam focussing and beam deflection unit and a fast circular CZT detector comprising 240 elements of 1.5 mm x 1.5 mm x 1.5 mm active pixel area. X-ray radiation is produced on a circular water cooled tungsten target The CT system achieves up to 7000 frames per second with a spatial resolution of 1 millimetre. First two-phase flow experiments have been carried out on gas-water flows in bubble columns. On the basis of these data we developed image processing algorithms which enable to extract information on bubble shapes, bubble size distributions and interfacial area density distribution. Further, a vertical test section made of titanium alloy has been installed at the TOPFLOW facility and will be used in the future to study the evolution of two-phase gas-water pipe flow at high pressures and temperatures.