Experimental investigation of flow fields in the interstices of bulk particles using optical measurement techniques

The flow behaviour of the gas phase in a packed bed has important effects on mass and energy transport processes that are taking place in the bed. It is hence also a central parameter for process optimisation of such systems. Currently, however, only very limited data on the gas flow in packed beds exists, since the access to the particle interstices is very challenging with both probe-based and optical measurement methods. Furthermore, the existing results were typically obtained using refractive index matching, and are hence limited to liquids. For gaseous flows, mainly conclusions obtained using similarity theory are available, which limits the potential range of application.

In the first funding period of this project, we extended optical particle image velocimetry (PIV) of the velocity fields in the gas phase within packed beds by ray tracing reconstructions. For this, we used beds consisting of transparent bulk material so that the velocity field determination can be aided with a numerical simulation of light propagation through the bed. The simulation was performed with ray tracing, and the resulting information was used to correct the raw PIV particle images of the flow. This technique then allowed for the direct measurement of velocity fields in the gas phase of transparent packed beds.

The main emphasis in the second funding period will be on extending the optical measurements to other quantities, such as temperature and dispersion. Also a new experimental configuration will be used that consists of parallel transparent bars arranged in rotatable layers modelling a polyhedral packing. It will be far less regular than the reference configuration of FP1, while still providing direct optical access without considerable optical distortion. Simultaneous measurements of gas temperature and velocity will be performed using thermographic phosphor particles. Further, laser induced fluorescence (LIF) of Anisole will be used for the determination of gas dispersion in the packed bed. The multi-camera set-up from FP1 will be further extended to a matrix arrangement of the cameras for the acquisition of three dimensional three component (3D3C) velocity and simultaneous temperature fields in the interstices of the new reference configuration. A systematic evaluation of refractive index matching for the use in complex, also moving transparent packed beds will be done on the basis of dimensional analysis to compare results from liquid and gaseous flows. These results will be of particular interest for FP3 when moving beds will be considered.
After the development of the aforementioned measurement methods, the proposed approach will enable an effective determination of concentration/dispersion, temperature and gas flow velocities in the packed bed. A wide range of parameters, such as packing density, packing size, inflow type and direction can be explored in this way. The measured data will then be used in other projects, e.g. to validate numerical calculations or for model development, especially concerning dispersion and heat transfer. Furthermore, the data provided is spatially and temporally highly resolved, allowing for the derivation of turbulence quantities for further processing and use in the partner projects.