Quantitative data from ultra-fast electron beam X-ray computer tomography measurements

The ultra-fast electron beam X-ray computer tomography was developed during last years at HZDR and turned out to be a suitable measuring technique to get new insights into two-phase flow structures. The aim of new experiments done at a vertical pipe with an inner diameter of about 54 mm and a length of 4 m (usable for experiments) was to obtain quantitative data suitable for CFD-code development and validation. The pipe was built up using Titanium. This allows a wall thickness of only 1.6 mm which is enough for steam-water experi¬ments which will be carried out at a pressure up to 6.5 MPa. To generate the X-rays an electron beam is focused on a circular two-step target. This electron beam is deflected very fast and moves over the target resulting in a fast moving X-ray source. The X-rays pass through the object to be investigated and are registered by an arc of detectors. A tomographic reconstruction algorithm provides images of the attenuation distribution in the cross section of the object which is interpreted as density distribution. This allows the determination of the gas- liquid interfacial structure inside the considered object. Measurements were done with a measuring frequency of 2500 images/s and measuring time was 10 s. Air-water and steam-water flows were investigated for different combinations of gas and liquid flow rates and different distances from the gas injection.
The tomographic reconstruction provides 2 3D matrices of gray-scale values since the two-step target allows measurement in two planes with an axial distance of about 11 mm. Cross-correlation algorithms are applied to obtain the information on local gas velocities. Due to noise and artifacts a binarisation of the data seems to be necessary. Using a simple threshold method may cause small bubble to go missing. For this reason another method basing on gradients was developed. The presentations discusses this method and the present status of this new measuring technology in respect to quality and uncertainties of the quantitative data obtained as e.g. local void fraction and bubbles sizes.