Velocity Measurement Using ROFEX Ultrafast Dual Plane X-ray CT Imaging

Ultrafast electron beam X-ray computed tomography was developed at HZDR as a powerful imaging tool to observe two phase flows in several industrial applications, such as pipe flow that cannot be accessed by optical techniques. The ROssendorf Fast Electron beam X-ray Tomograph (ROFEX) combines advantages of non-invasive conventional CT systems, with a very high temporal and spatial resolution. This CT system is specially designed for investigations at a vertical test section at TOPFLOW (Transient two phase flow test facility) at HZDR. As a step forward recently ultrafast dual plane X-ray CT imaging was implemented, using new fast double ring X-ray detector electronics and a dual source path target in the ROFEX scanner. For the first time measurements of velocities in gas/water and steam/water flow are possible without any influence to the flow itself. The system provides pairs of slice images with an axial distance of 11mm at frame rates of up to 8000 kHz in time multiplex mode on both CT imaging planes. Thus one can assess the velocity of the interfacial area in the flow using cross correlation algorithm in two corresponding slice images. Beside parameters like phase fraction distribution, bubble sizes and interfacial areas, measurement of gas fraction velocity is possible for the first time non-invasively. From these data one can estimate slip between gas and liquid phase and calculate volumetric gas fraction. Furthermore CFD (Computational Fluid Dynamics) models can be enhanced providing parameters for calculations of bubble forces such as drag force or lift force, which influence shape and flow of bubbles. In chemical engineering applications like catalytic packaging or monolith ceramics it is possible to assess the residence time as well.