Gamma and X-Ray tomography for transient two-phase flows and other instrumentation developed by Rossendorf


Gamma and X-Ray tomography for transient two-phase flows and other instrumentation developed by Rossendorf

Prasser, H.-M.

First subject of the paper is a gamma-tomography setup for imaging a periodically changing density field. It is based on a time-resolved acquisition of the detector signals. The system consists in a 5 Ci source of Cs-137 and a detector arc with 64 BGO scintillation crystals coupled with photo-multipliers. 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. The detectors operated in pulse mode. The pulses were counted by several banks of 64 counters activated in a successive order - each for a period of 100 µs. The counting procedure is restarted in the first bank after each full rotation of the impeller. After a measuring time of typically 3 -6 min, projections of the density distribution inside the object are acquired in a rotation-angle resolved manner. This allowed to afterwards reconstruct the void fraction distribution inside the rotating impeller structure by applying filtered back-projection algorithms. In a second application, the tomography system was applied to a hydraulic clutch (coupling). These measurements showed the distribution of the hydraulic liquid inside both semi-filled working wheel of the clutch at different slip ratios. An advanced detector system with a total number of 320 crystals is under construction. This will allow to increase the resolution from now about 7 mm to about 2-3 mm inside the measuring plane.
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. For the experiments, an electron beam of a 150 kV gun with a current of 5 mA together with a line of 64 CdZnTe detectors working in current mode with a sampling frequency of 100 kHz for each detector (total data rate: 6.4 MHz) were used. It is planned to continue this development in direction towards an application in two-phase flow experiments.
A second application of X-rays concerns a cone-beam tomography of a stirred vessel reactor. In this case, time-averaged gas fraction distributions produced by a gassing stirrer were visualized using a standard X-ray tube and a 2D detector array. The rotation of the fluid inside the reactor was used to obtain the projections necessary for the 3D reconstruction.
The paper will furthermore present a summary of other kinds of two-phase instrumentation developed in the Research Center Rossendorf. Examples are: (1) local void probes based on impedance measurements, that are equipped with a micro-thermocouple substituting the traditional electrode applied to non-adiabatic gas-liquid flows, (2) the use of high-speed video imaging together with image processing techniques for bubble-column studies, (3) the developments of an optical tomograph and other optical sensors.

Keywords: two-phase flow; gamma tomography; X-ray tomography; local void probes; optical tomography; image processing

  • 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-6647
Publ.-Id: 6647