Advanced imaging techniques for multiphase flows

Multiphase flows are widely found in different fields of science, engineering, and industry. Examples are mineral oil processing, chemical reaction engineering, and nuclear thermal hydraulics. Measurement and visualisation of multiphase flows is therefore of high scientific and engineering relevance. In particular the development of multiphase CFD codes requires measurement data from flow scenarios with high spatial and temporal resolution, in order to develop and validate physics based models for momentum, heat and mass transfer. If information about phase distributions, interfacial area structure, velocity and species concentration fields in flows are required, high-resolution imaging modalities are needed as measurement tools. However, yet there is no universal measurement or imaging modality for multiphase flow. Traditional visualisation tools, such as high-speed camera imaging and ultrasound measurements fail in multiphase systems due to the nonlinear propagation and attenuation of the signal carriers, e.g. light and sound waves. Especially radiation based tomographic methods are therefore being considered as the key technology for multiphase flow visualisation. However, to date only few methods are suited because of the stringent requirements for high spatial and temporal resolution. In particular, methods are sought, which can visualize multiphase flow in complex geometries, often with opaque walls and inserts, such as chemical reactor vessels, heat exchangers or nuclear fuel rod assemblies, but also porous media of fixed bed reactors or rock samples. The presentation introduces two novel imaging modalities, namely wire mesh sensors and ultrafast X-ray tomography and their use in two-phase flow measurement in complex flow domain geometries. It will be shown how gas-liquid and gas-solid flows can be visualized in in pipes, columns, fixed bed packings and flow channels with inserts. The presentation shows results of dedicated flow studies and introduces data analysis methods for extraction of quantitative flow parameters.