Ohio State University has scanned mixed gas-liquid-solid flow in 3D using a technique previously employed only for 2D scans.

"Capacitive tomography has been practiced for many years, but not in 3D," Professor Liang-Shih Fan at the University told EW.

Fan and his team study were looking for a technique to study flow without the sensor causing disturbance and developed the capacitive technique, called electrical capacitance volume tomography (ECVT), when they found no alternatives.

They have settled on 12 thin sheet electrodes which can be positioned around the volume of interest in various ways.

Providing the array has a presence in three dimensions, that it is not just a 2D array, said Fan, 3D results can be extracted. A favoured arrangement is two rings of six electrodes around an 80mm pipe.

The capacitance, which is in the femtofarad range, between every possible pair of electrodes is measured at 100 or 200Hz sampling rate and recorded.

This recording is then processed to reconstruct a 3D flow movie using a modelling technique developed by the team, taking several hours to process each data set.

"The image construction is unique," said Fan. "Mathematicians like to use iterative techniques, but these do not give good images. We use a neural network-based technique. Image reconstruction is established by introducing a 3Dsensitivity matrix."

Called neural-network multi-criterion optimisation (NNMOIRT), Fan describes this as the key to his team’s development of 3D capacitive tomography.

The result is 20 x 20 x 20 relative permittivity voxels.

Charged objects in the flow have until recently cause problem. "We now have a technique to compensate for free-charges," said Fan, who’s interest is in gas-solid fluidised beds, and the hydrodynamics of evaporative liquid jets in gas-solid media. Being non-contact, ECVT can be used in high-temperature environments including fluidised-bed furnaces.