Forum on Measurement Techniques in Multiphase Rows. 1995 ASME IMEC&E. November 1995. San Francisco. CA DEVELOPMENT OF AN ELECTRICAL IMPEDANCE TOMOGRAPHY SYSTEM FOR AN AIR-WATER VERTICAL BUBBLE COLUMN* T. J. OHem and J. R. Torczynski Sandia National Laboratories Albuquerque, New Mexico 871 85 S. L Ceccio and A. L. Tassin University of Michigan Ann Arbor, Michigan, 481 09 G. L. Chahine, R. Duraiswami, and K. Sarkar Dynaflow, Inc. Fulton, Maryland 20759 ABSTRACT Because the components of a multiphase flow often exhibit dif- ferent electrical properties. a variety of probes have been devel- oped to study such flows by measuring impedancein the region of interest. Researchers are now using electric fields to reconstruct the impedance distribution within a measurement volume via Electrical Impedance Tomography @IT). ElT systems employ voltage and current measurements on the boundary of a domain to create a representation of the impedance distribution within the domain. The development of the Sandia ElT system (S-W is reviewed. The construction of the projection acquisition system is discussed and two specific €ET inversion algorithms are detailed. The first reconstruction algorithm employs boundary element methodstand the second utilizes finite elements. The benefits and limitations of ElT systems are also discussed. Preliminary results are provided. INTRODUCTION The problem of measuring the spatial distribution of the sepa- rate phases in multiphase flows is one of great interest in a number of research and industrial applications (Plaskowski et al., 1995). Diagnostic techniques typically applied to such measurements include radiation densitometry and tomography such as gamma tomography, positron emission tomography (PET). and magnetic resonance imaging (MRI) (Kumar et al.. 1994; Shollenberger et al., 1995; Simons, 1995).These techniques offer high spatial reso- lution. However, radiation tomography often requires significant data collection times, which are usually much longer than the characteristic time scales of a temporally evolving multiphase flow. Electrical impedance measurements can be acquired rela- ?his work was performed at Sandia National Laboratories, sup- ported by the US. Department of Energy, under contract DE- AC04-94AL85000. tively fast and have been used for some time to measure bulk and local void fractions (Ceccio and George, 1995). However. the spa- tial resolution of electricalprobes is limited. Electrical impedance tomography (EIT) may offer a method to quickly acquire images of the spatial distribution of phases within a multiphase flow. ElT is the process by which electrical measurements. acquired at the boundary of a domain, can be used to reconstruct the electrical impedance distribution within the domain (Webster, 1990; Plaskowski et al.. 1995; Jones et al.. 1993). In this paper, we will detail an EIT system being constructed at Sandia National Labora- tories for the purpose of investigating multiphase bubble column reactors, such as Fischer-Tropsch reactors. Slurry-phase bubble- column Fischer-Tropsch reactors are recognized as one of the more promising technologies for indirect liquefaction. i.e.. con- verting synthesis gas from coal into liquid fuel products (see, e.g.. Bukur et al.. 1987). However, hydrodynamic effects must be con- sidered when attempting to scale these reactors to sizes of indus- trial interest. Development and application of noninvasive tomographic diagnostics capable of measuring void fraction (pas holdup) spatial distributions in these reactors would greatly facili- tate characterization of reactor hydrodynamics. We will discuss the experimental setup, including design aspects of the EIT hardware, reconstruction algorithms under development, and some preliminary results. BASICS OF ELECTRICALIMPEDANCE TOMOGRAPHY FLIT is a technique by which the impedance distribution within a domain may be determined via voltage and current measurements performed on the boundary of the domain. For AC electrical con- duction with field frequencies on the order of tens of megahertz or lower, the electrical potential within a conducting domain. Q is given by v*ovl#l=o (1)