Electrical Impedance Sensors for Volume Fraction Measurements of Two-Phase Flows

Abstract

Two-phase flows are frequently encountered phenomena in various engineering fields such as chemical, oil and nuclear industries. In particular, the volume fraction in two-phase flow systems has very important roles in determining several variables associated with system analyses and design like the two-phase mixture density and viscosity, average velocity of two phases, pressure drop, and heat transfer. For this reason, a very wide variety of techniques including the quick-closing valve, gamma or X-ray absorption, optical probe, and electrical impedance have been proposed for volume fraction measurement. Among these, in particular, the electrical impedance technique has various favorable characteristics in terms of easy implementation, relatively low construction cost, fast data acquisition speed, no intrusiveness of flow fields, and convenient mobility. The electrical impedance technique is based on the fact that two phases have different electrical properties. In the electrical impedance technique, in general, voltages or currents are applied to electrode pairs installed in a test section and resultant electrical resistance and/or capacitance measurement is directly used to estimate the volume fraction. In volume fraction measurement based on the electrical impedance, electrode and gap sizes of an impedance sensor are the most important design parameters because the electric field distribution having significant influences on the electrical impedance directly depends on them. The non-uniformity of the electric field due to an improper sensor configuration causes undesirable impedance sensor characteristics such as a nonlinear response for the volume fraction. The present work considers this dependence of electrical signals on the sensor configuration and focuses on designing the impedance sensor providing favorable characteristics for volume fraction or film thickness measurement. For this, two conventional sensor configurations (plate- and ring-type sensors) and separated flow patterns (stratified and annular flow) are taken into account. For an annular flow application, gas core fluctuations which can distort output signals are considered as a key design parameter and the optimal sensor gap sizes, which give stable void fraction measurement without being affected by the gas core fluctuations, are determined for both plate- and ring-sensor types. Similarly, in a stratified flow example the electrode and gap sizes of a ring-type impedance sensor are optimized in view of the sensor linearity for liquid film thickness changes. In addition, as another application, a conductance sensor is applied for flow regime classification in an inclined pipe and its signal characteristics for various flow rate conditions which cover stratified, intermittent and annular flows are investigated.