A comparison of magnetic resonance imaging and electrical capacitance tomography: An air jet through a bed of particles

Magnetic resonance imaging (MRI) and electrical capacitance volume tomography (ECVT) have been compared for the visualisation of a jet of air issuing from a distributor provided with a single orifice and supporting a bed of poppy seeds contained in a vertical tube of 50 mm diameter. The mean diameter of the seeds was 1.2 mm: the orifice was 4 mm in diameter. MRI and ECVT images were acquired in three dimensions for flow rates of air such that, when divided by the cross-sectional area of the tube, they gave superficial velocities below that required for minimum fluidisation. The ECVT images were found to be of lower contrast (as well as resolution) than the MRI, owing to smoothing inherent to the ECVT reconstruction process. An adaptive threshold method is developed to recover the 3D structure of the jet from the ECVT results. This method describes the smoothing in the ECVT images by a point-spread function that can be objectively defined by comparison with the MRI results. Using the new adaptive threshold method, the maximum discrepancy in the measured jet length by ECVT was an overestimation by 7 mm in comparison with the MRI results. The discrepancies in the measured maximum jet widths appeared to be ca. 2 mm.