Various developments in the use of electrical impedance methods in multiphase flow are reviewed. Because the components of a multiphase flow often exhibit different electrical properties, a variety of probes have been developed to study such flows by measuring impedance in the region of interest. Nonintrusive devices are used to measure spatially averaged flow properties, such as void fraction. Local probes have been developed to measure a variety of pointwise flow quantities, including film thickness in annular flow, local void fraction in dispersed flows, bubble and particle sizes, and flow velocities. Such impedance probes are usually easy to build and use, and can have a high frequency response. However, the spatial resolution of the probes may be limited, calibration may be difficult, and the accuracy of some probes may be limited to specific phase distributions. Researchers are now using electric fields to reconstruct the impedance distribution within a measurement volume via Electrical Impedance Tomography (EIT). EIT systems employ voltage and current measurements on the boundary of a domain to create a representation of the impedance distribution within the domain. EIT inversion algorithms are discussed, and the application of EIT to multiphase flows is reviewed. The benefits and limitations of EIT systems are also discussed.

---A series of studies is presented in which an electrical-impedance tomography (EX") system is validated for two-phase flow measurements. The EIT system, developed at Sandia National Laboratories, is described along with the computer algorithm used for reconstructing phase volume fraction profiles. The algorithm is first tested using numerical data and experimental phantom 1

---Experiments are presented in which electrical-impedance tomography (EIT) and gammadensitometry tomography (GDT) measurements were combined to simultaneously measure the solid, liqui~and gas radial distributions in a vertical three-phase flow. The experimental testbed was a 19.05-cm diameter bubble column in which gas is injected at the bottom and exits out the top while the liquid and solid phases recirculate. The gas phase was air and the liquid phase was deionized water with added electrolytes. Four different particle classes were investigated for the solid phase: 40-100 pm and 120-200 pm glass beads (2.41 g/cm3), and 170-260pm and 200-700pm polystyrene beads (1.04 g/cm3), Superficial gas velocities of 3 to 30 cmk and solid volume fractions up to 0.30 were examined. For all experimental conditions investigated, the gas distribution showed only a weak dependence on both . particle size and density. Average gas volume fraction as a fimction of supert3cial gas velocity can be described to within &O.04by a curve passing through the center of the data. For most cases the solid particles appeared to be radially uniformly dispersed in the liquid.

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Surface electrical impedance probes are used to examine the bubbly flow beneath and in the closure region of partial attached cavities. A series of electrodes were mounted flush to the surface of a cavitating hydrofoil to detect the presence of liquid or vapor through changes in the impedance of the local fluid medium. Signals from the electrical probes were used to determine the near-surface gas-phase velocity and the shedding frequency of the gas phase. The impedance technique is shown to have the potential to measure advection velocities within a partial cavity, and the frequency content of the signal is used to determine the dynamics of the vapor shedding process. [S0098-2202(00)00501-0]