Four-dimensional flow imaging by ultrafast X-ray computed tomography

Ultrafast X-ray computed tomography has been developed as a flow imaging technique [1][2] that is able to visualize the phase distribution inside a cross-section of a flow vessel with a spatial resolution of about 1 mm and frame rates of up to 10,000 s-1. The temporal resolution is achieved by fast deflection of an electron beam across an elongated target to produce a moving X-ray source spot. The performance and applicability of this technique has been demonstrated in different two-phase flow experiments such as water-air pipe flows [3], mixing processes or fluidized beds [4] which are of great relevance for the understanding and controlling of various industrial processes. Especially in applications with dense phases, which are opaque to common imaging techniques, ultrafast X-ray tomography can provide important additional information.
Recently, this technique has been advanced towards time-resolved imaging of three-dimensional flow structures. This is achieved by repeated consecutive scanning of a number of tomography planes, which are realised using a stepped target. Thus, several vertically distributed X-ray source paths can be generated. With the reconstructed time-resolved volume data it is possible not only to determine phase distributions but also phase velocities and bubble volumes which are characteristic parameters of the flow. This extended data basis provides the opportunity for an improved validation of computational fluid dynamics codes. The method and first experimental results will be presented.