Ultra fast electron beam X-ray CT for two phase flow phenomena

Two-phase flows are of primary importance in the understanding of thermal hydraulic phenomena in nuclear light water reactors. The qualification of CFD codes for the simulation of stationary and even transient two-phase flows in complex three dimensional geometries requires extending our knowledge toward the details of the flow structure under various thermal hydraulic conditions. At Helmholtzzentrum Dresden-Rossendorf the thermal hydraulic test facility TOPFLOW is currently extensively used to conduct two-phase flow experiments which aim at the disclosure of fine flow structure details in generic and also more complex geometries. Especially for upward adiabatic two-phase pipe flows in the bubbly to churn turbulent flow regimes valuable experimental data of the flow structure was generated in the past using the wire-mesh sensor technology. A known drawback of the wire mesh sensor is its intrusiveness.
Consequently, we have extended our measurement technology for two-phase flow to high-speed X-ray tomography which offers non-intrusive flow measurement at a speed comparable to the wire-mesh sensor. X-ray CT as an imaging modality is highly advantageous due to its non-invasiveness and its ability to penetrate opaque wall materials. One essential disadvantage of classical CT systems is the requirement for a rotating object or source-detector setup. To measure multiphase flows in a flow velocity range of up to 1 meter per second frame rates of up to 1000 frames per second are required. To achieve this, mechanically rotating parts are to be avoided. Scanned electron beam technology provides a mean to circumvent this problem. Instead of mechanical rotation of scanner components an electron beam is rapidly swept across an X-ray target using deflection coils. This technology has been introduced in medicine more than two decades ago were it is mainly used for cardiovascular diagnostics. However, medical systems with frame rates of about 20 fps are still too slow for technical flow diagnostic problems. For that reason we have developed a scanned electron beam X-ray CT apparatus “ROFEX” which can today already perform cross-sectional imaging at high frame rates and will in the future be extended to other measurement features, such as multi-plane tomography, phase velocity measurement, higher scanning diameters and high energy X-rays. This paper introduce the scanner design, discuss major performance parameters along with the application example of air-water two-phase flow measurements in a DN50 vertical test section of the TOPFLOW facility. Furthermore preparations for steam-water-experiments @70bar/286°C will be discussed.