Ultrafast X-ray computed tomography for phase distribution and velocity measurements in multiphase flows

Various flow measurement and visualization techniques are based on optical and laser-based methods. However, in many multiphase flow situations, e.g. at higher interfacial density or in flows with internals, the optical access is no longer given. Radiation based methods are in principle able to penetrate most of these systems, but are normally too slow to capture the dynamics of the flow. With ultrafast X-ray tomography a flow visualization and measurement technique has been developed, which is able to recover the dynamic phase distributions in various multiphase flow scenarios. The high imaging rate is achieved by deflecting an electron beam along a circular target, where a moving X-ray spot is generated. Tomographic projections are gathered simultaneously by a static detector ring with fast read-out. Thus, no components of the X-ray tomography system have to be rotated mechanically. The reconstructed tomography slices represent the non-superimposed phase distribution within a cross-section as a function of time. Up to 8,000 fps can be achieved in single plane mode. For velocity measurements, a second set of X-ray target and detector ring arranged at a small axial distance can be included to form the so-called dual plane mode. Although the alternating scanning of both planes reduces the frame rate by a factor of two, the benefit of combining the information from both planes to retrieve velocity information arises directly. There are different ways to extract velocity information from the two stacks of slice image data. Cross-correlation techniques offer the opportunity to retrieve time averaged as well as time resolved local or global velocities of the disperse phase. Some systems also allow the determination of single bubble or particle velocities, provided that they can be identified as the same object in both planes.