Ray-Tracing Based Reconstruction Of PIV Measurements In The Outlet Zone Of Gaseous Flow In Packed Beds

The flow characteristics of packed beds are essential for the optimization of heat and mass transfer in countless industrial processes. In this study a method is introduced to allow optical measurement techniques in a transparent bulk reactor, so that optical distortions are corrected by a ray tracing based reconstruction. The velocity of the gas flow is measured by a classical particle image velocimetry measurement approach, where the optically distorted raw particle images, obtained through the transparent spheres, are corrected by a ray tracing step before a common cross-correlation evaluation of the velocity fields. Special calibration procedures, linking the exact geometrical set-up of the measurement equipment with respect to the packed bed reactor to the numerical model used for distortion correction, are necessary for this approach, which is validated in this study at the outlet flow field of the bed. The main principles like the experimental setup, calibration and correction method are presented and discussed, as well as the precision of the initial flow field obtained in the experimental symmetric reactor. Results of flow fields at the surface of a 17layer transparent packed bed for particle Reynolds numbers in the range of 200 to 500 show the impact and advantages of the additional ray tracing step. A validation of the ray tracing based correction method is shown by comparing the flow of the freeboard, obtained from undistorted particle images, to the flow field behind transparent spheres, which initially cause strong optical distortions in the raw particle images. The continuously evolving velocity profiles, through regions of originally strong optical distortion, show the applicability of the method even for the reconstruction of vortical structures in the interstices of spherical packings.