Downloaded By: [Simões-Moreira, J. R.] At: 13:23 17 July 2008 Heat Transfer Engineering, 29(11):924–935, 2008 Copyright C  Taylor and Francis Group, LLC ISSN: 0145-7632 print / 1521-0537 online DOI: 10.1080/01457630802186015 Void Fraction Measurement and Signal Analysis from Multiple-Electrode Impedance Sensors M. S. ROCHA and J. R. SIM ˜ OES-MOREIRA Escola Polit´ ecnica, Universidade de S˜ ao Paulo, Mechanical Engineering Department, S˜ ao Paulo, Brazil Void fraction sensors are important instruments not only for monitoring two-phase flow, but for furnishing an important parameter for obtaining flow map pattern and two-phase flow heat transfer coefficient as well. This work presents the experimental results obtained with the analysis of two axially spaced multiple-electrode impedance sensors tested in an upward air-water two-phase flow in a vertical tube for void fraction measurements. An electronic circuit was developed for signal generation and post-treatment of each sensor signal. By phase shifting the electrodes supplying the signal, it was possible to establish a rotating electric field sweeping across the test section. The fundamental principle of using a multiple- electrode configuration is based on reducing signal sensitivity to the non-uniform cross-section void fraction distribution problem. Static calibration curves were obtained for both sensors, and dynamic signal analyses for bubbly, slug, and turbulent churn flows were carried out. Flow parameters such as Taylor bubble velocity and length were obtained by using cross- correlation techniques. As an application of the void fraction tested, vertical flow pattern identification could be established by using the probability density function technique for void fractions ranging from 0% to nearly 70%. INTRODUCTION Two-phase heat transfer process design, safety, and perfor- mance improvement necessarily require the knowledge of two important parameters: the heat transfer coefficient and the void fraction. Most predicting methods for two-phase heat transfer are based on experimental studies. Visualization and quantita- tive measurement revealed that the heat transfer coefficient is strongly dependent on the void fraction distribution and flow regime. One of the imposed heat transfer problems, the flow boiling, is affected by the influence of flow direction on the heat trans- fer coefficient and void fraction during fully developed nucleate boiling in vertical channels. Studies on void fraction measure- ment have been performed by means of many techniques in heated tubes with subcooled liquids, and show that, at low flow rates, the direction of the flow affect the void fraction consider- ably. Furthermore, where counter-flow can occur, the value of Address correspondence to Professor J. R. Sim˜ oes-Moreira, Escola Polit´ ecnica - Universidade de S˜ ao Paulo Mechanical Engineering Department, SISEA – Alternative Energy Systems Laboratory, Av. Prof. Mello Moraes, 2231, 05508-900, S˜ ao Paulo, SP, Brazil. E-mail: jrsimoes@usp.br actual void fraction governs the upward or the downward flow boiling heat transfer at identical flow conditions. Many void fraction measuring techniques have been exten- sively studied lately in connection with not only determining the void fraction itself but also with characterizing the two-phase flow structure and regime. Due to its fluctuating nature, flow pa- rameter identification relies mostly on measurement techniques, and a specific instrumentation development is needed. Evidently, two-phase flow instrumentation must be quite reliable for labo- ratory and industrial application and, therefore, the development of instrumentation for the void fraction measurement is the key- stone for the multidimensional multiphase flow modeling as well as for flow monitoring purposes [1]. In this context, many tech- niques for measuring the void fraction have been developed, and their particular success depends on a specific application. Gener- ally, their signal response is two-phase flow structure-dependent and can be designed to indicate void fraction values that are in- stantaneous or time-averaged, local or global. One widely used technique for void fraction measure- ment is based on the measurement of the two-phase electri- cal impedance, the working principle of which relies on taking the advantage of the difference in electrical impedance of each 924