Frank Barthel

Ultrafast three-dimensional x-ray computed tomography

X-ray computed tomography (CT) is a well established visualization technique in medicine and nondestructive testing. However, since CT scanning requires sampling of radiographic projections from different viewing angles, common CT systems with mechanically moving parts are too slow for dynamic imaging, for instance of multiphase flows or live animals. Here, we introduce an ultrafast three-dimensional x-ray CT method based on electron beam scanning, which achieves volume rates of 500 s−1. Primary experiments revealed the capability of this method to recover the structure of phase boundaries in gas-solid and gas-liquid two-phase flows, which undergo three-dimensional structural changes in the millisecond scale.

Scatter analysis and correction for ultrafast X-ray tomography

Ultrafast X-ray computed tomography (CT) is an imaging technique with high potential for the investigation of the hydrodynamics in multiphase flows. For correct determination of the phase distribution of such flows, a high accuracy of the reconstructed image data is essential. In X-ray CT, radiation scatter may cause disturbing artefacts. As the scattering is not considered in standard reconstruction algorithms, additional methods are necessary to correct the detector readings or to prevent the detection of scattered photons. In this paper, we present an analysis of the scattering background for the ultrafast X-ray CT imaging system ROFEX at the Helmholtz-Zentrum Dresden-Rossendorf and propose a correction technique based on collimation and deterministic simulation of first-order scattering.

Experimental facility for two- and three-dimensional ultrafast electron beam x-ray computed tomography

An experimental facility is described, which has been designed to perform ultrafast two-dimensional (2D) and three-dimensional (3D) electron beam computed tomographies. As a novelty, a specially designed transparent target enables tomography with no axial offset for 2D imaging and high axial resolution 3D imaging employing the cone-beam tomography principles. The imaging speed is 10 000 frames per second for planar scanning and more than 1000 frames per second for 3D imaging. The facility serves a broad spectrum of potential applications; primarily, the study of multiphase flows, but also in principle nondestructive testing or small animal imaging. In order to demonstrate the aptitude for these applications, static phantom experiments at a frame rate of 2000 frames per second were performed. Resulting spatial resolution was found to be 1.2 mm and better for a reduced temporal resolution.

Schnelle tomographische Bildgebungsverfahren für Mehrphasenströmungen

Zusammenfassung Tomographische Bildgebungsverfahren sind heute vor allem aus der Medizin und der zerstörungsfreien Prüfung bekannt. Sie werden aber auch als äußerst vielversprechend für die Analyse komplexer Strömungsvorgänge, etwa in chemischen Apparaten, erachtet. Besonderes Interesse besteht hierbei in der Aufklärung hydrodynamischer Phänomene in Mehrphasenapparaten, wie beispielsweise Blasensäulen, Kolonnen, Festbettreaktoren und Wirbelschichten, in denen die Hydrodynamik die Stoff- und Wärmetransportvorgänge sowie das makroskopische Reaktionsgeschehen entscheidend beeinflussen. Allerdings müssen tomographische Messverfahren für solche Anwendungen besonders schnell sein, da sich Strömungsstrukturen in einer räumlichen Skale von Millimetern im Allgemeinen im Millisekundenmaßstab ändern. In der jüngeren Vergangenheit wurden mit der Gittersensormesstechnik und der ultraschnellen Röntgentomographie zwei Messverfahren entwickelt, die diese Anforderungen erfüllen. Ihre Funktionsprinzipien und Anwendungen werden in diesem Artikel beschrieben. Abstract Tomographic methods are well known today from medicine and non-destructive testing. However, they are also considered as promising techniques for the analysis of complex flow phenomena, for instance in chemical reactors. The investigation of hydrodynamic phenomena is of special interest in multiphase reactors such as bubble columns, separation and distillation columns, fixed bed reactors and fluidized beds in which the hydrodynamics decisively determines the heat and mass transfer as well as the global course of chemical reactions. For such applications tomography methods must be very fast because flow structures change within milliseconds in the millimeter range. With the wire-mesh sensor technology and ultrafast X-ray tomography two new imaging methods have been developed in the recent past, which fulfil these requirements. Here their functional principles and applications are described.

High-resolution two-phase flow measurement techniques for the generation of experimental data for CFD code qualification

Abstract Computational fluid dynamics simulations for two-phase flows are important in different fields of engineering and science. Since two-phase flows are inherently complex, also CFD modeling development requires special attention. The validation of model implementation and the derivation of physics based models for momentum, heat, and mass transfer in two-phase flow require experiments with generation of high-resolution measurement data. This, however, is a great challenge, since most standard flow measurement tools used in single phase flow situations, are not suited for multiphase flows. In this article we report on advanced imaging and measuring methods for two-phase flow experiments, which have been extensively used in the recent past to conduct experiments for two-phase flows at the Helmholtz-Zentrum Dresden-Rossendorf. In particular the application of wire-mesh sensors, ultrafast X-ray tomography, gamma ray tomography and positron emission tomography will be introduced and discussed.

Experimental investigations of single and two-phase flow in a heated rod bundle

Abstract An experimental facility for the study of boiling flows in a 3 × 3 rod bundle geometry was setup. The bundle resembles in essential geometrical parts the geometry in a pressurized water reactor fuel element. The facility is operated with a refrigerant fluid. Beside standard instrumentation for temperature, pressure and flow rate we employed particle image velocimetry for single phase flow studies, gamma ray densitometry for integral gas fraction measurement sand ultrafast X-ray tomography for the study of the void dynamics in the cross-section. Moreover extensive thermo-instrumentation allows axial rod surface temperature measurements for the central heated rod. First results will be discussed in this article.

Multiphase flow investigations with ultrafast electron beam x-ray tomography

We introduce ultrafast electron beam X-ray tomography as an imaging modality for multiphase flow studies. A dedicated electron beam tomography scanner (ROFEX) has been developed which allows cross-sectional X-ray tomography with 1 mm spatial resolution and up to 7000 cross-sectional images per second recording speed. It is applicable to flow problems in vessels with up to 120 mm diameter and moderate X-ray attenuation. The tomography system has been applied in various flow studies, including gas-liquid two-phase flow in vertical pipes and channel structures.

Ultrafast electron beam X-ray computed tomography for 2D and 3D two-phase flow imaging

Imaging of complex and dynamic processes such as two- or multiphase flows with high structural as well as temporal resolution has always been a challenging task. In recent years, the electron beam X-ray computed tomography technique has been developed towards a powerful imaging tool, which reaches frame rates of 8000 fps in 2D and 1000 fps in 3D. In this paper, the latest developments as well as selected applications of ultrafast electron beam X-ray CT are presented.

Study of Flow Behavior of Granular Material Inside Cylindrical Silo Using Ultrafast X-Ray Imaging Technique

Abstract This paper presents an application of an ultrafast electron beam X-ray CT scanner for investigating the gravitational flow behavior of granulates through cylindrical silo model. The CT scanner allows obtaining crosssectional images of the granular material distribution with a spatial resolution of approximately 1 mm and a time resolution of 2 kHz. In order to conduct a deep analysis of the granular flow concentration changes, two image processing algorithm steps were applied. The first step deals with preprocessing and re-centering stacks of raw images. The second step divides the preprocessed image into several concentric rings and calculates the mean value to study radial concentration changes. Independent analysis of granular concentration in each ring provides useful knowledge to study the silo discharging during mass flow and funnel flow.