Typical Applications - Mixing Process
Flow visualisation is one of most effective tools for use in understanding and characterising of complex, unsteady mixing processes. Over recent years, flow visualisation technologies have been greatly developed, which has changed our concept of vision from photographic-based “seeing is believing” (Mann et al 1995) to computer-based seeing “without eyes” (Mann 2005). Typically, this progress can be demonstrated representatively by the developments and applications of Particle Image Velocimetry and Process Tomography.
Electrical process tomography is a relatively new technology, which is based on measurements of electrical properties of materials by applying a low frequency (from DC up to few MHz) electric field (e.g. current or voltage) or a magnetic field. A low frequency electromagnetic field can penetrate most process materials which are opaque to the light. The electrical techniques also avoid the hazards of ionising radiation generated from nuclear emission techniques, e.g. x-ray or g–ray based techniques.
They are inexpensive and relatively straightforward to implement with the potential for sub-millisecond temporal resolution.
Figure 1 shows a typical set of resistivity contours interpolated from a stack of 8-plane 2D images, rendered as a solid body isometric image. Results were presented from times of 1, 2, 3 and 4 s following the surface addition of 10 dm3 of concentrated brine (conductivity of 13.5 mS cm-1) into a background conductivity of 0.1 mS cm-1 at t = 0. The stirrer speed is 100 rpm generating an estimated internal flow of 0.686 m3 s-1. The mixing time has been estimated using a colorimeter probe and is approximately 14 s. High conductivity is presented by red regions and low conductivity as blue regions (cut off by the isosurface). The conductivity at the isosurface, as a mixing index, is 0.16 mS cm-1, which was adopted from the final conductivity of the liquids after mixing was completed.
Further studies of a gas-liquid system (air-water) have allowed tomographic gas distribution to be compared with established characteristic flow patterns. A multi-isosurface 3D gas concentration distribution was produced from stack of 8-plane 2D images, which presented gas equal-concentration contours (Figure 2). In the experiment, gas was sparged from a pipe beneath a six-blade Rushton turbine at 3.5 litre sec-1 with agitation at 73 rpm.
|Figure 1: Monitoring a dynamic miscible liquid mixing in a baffled 2.7 m3 mixing tank ||
Figure 2: Pseudo-stationary gas-liquid mixing 
 M. Wang, R. Mann, F.J. Dickin, T. Dyakowski, R.A. Williams and R.B. Edwards, Stirred vessel mixing in 3-D using electrical resistance tomography, in proceedings of the 3rd International conference on electronic measurement & instruments, Electronic Measurement and Instrument Society of CIE, Vol.11, ISSN 1000-7105, 650-653 (1997).
 R. Mann and M. Wang, Electrical process tomography: Simple and inexpensive techniques for process imaging, measurement + Control, 30, 206-211 (1997).
 M. Wang, A. Dorwood, D. Vlaev, and R. Mann, Measurements of gas-liquid mixing in a stirred vessel using electrical resistance tomography, Chem. Eng. Sci., 77, 93-98 (2000).