Two-Phase Flow Measurement Techniques
In these notes the most common measurement techniques for two-phase flows are reviewed. The working principles and the configurations of instruments for void fraction measurements, flow visualization and velocity measurements are presented; in detail: rad
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Abstract. In these notes the most common measurement techniques for two-phase flows are reviewed. The working principles and the configurations of instruments for void fraction measurements, flow visualization and velocity measurements are presented; in detail: radiation attenuation, optical and electrical impedance techniques for void fraction measurement; tomographic and time-average visualization techniques; velocity measurements from signal cross-correlation, hot film anemometry, particle image velocimetry.
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Introduction
Two-phase flow is one of the most common flows in nature as well as in industrial applications; it covers gas-solid, liquid-liquid, solid-liquid and gas-liquid flows. Among these, gas-liquid flow, which also includes the whole subject of boiling and condensation, is probably the most relevant topic, and can be encountered in a wide range of industrial applications including evaporators, boilers, distillation towers, chemical reactors, condensers, oil pipelines, nuclear reactors, etc. Thus, the early research on two-phase flow was carried out on gas-liquid flow; anyway, the theoretical framework developed for gas-liquid flow can be often extended to other two-phase systems, such as liquid-liquid flow. Advancements in the knowledge of two-phase flow need a close working relationship among experimentalists, theoreticians and numerical analysts. In particular, experiments are necessary to verify the reliability and the limitations of both mathematical models and numerical codes; furthermore, they allow capturing the physics of phenomena, and therefore provide a basis for their theoretical description. Measurement techniques in two-phase flows are quite different from those of single-phase flows: in fact, there exist peculiar quantities ofthis kind of flows, such as the void fraction and the interfacial area concentration, which require a specifically conceived instrumentation. On the other hand, measurement techniques which have been developed for single-phase flow measurements, such as hot wire anemometry or particle image velocimetry, generally cannot be used in two-phase flows as they are, but require some modifications.
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Void Fraction Measurements
Void fraction is a dimensionless quantity indicating the fraction of a geometric or temporal domain occupied by the gas phase, and it is probably the most significant quantity one can measure in two-phase flow. Although the first void fraction measurements are dated back to the 1940s, they still play the most important role in today's experiments: a recent statistical study of the experimental research on two-phase flow shows that almost any journal article on this subject V. Bertola (ed.), Modelling and Experimentation in Two-Phase Flow © Springer-Verlag Wien 2003
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presents void fraction data (Chanson, 2002). The importance is even greater in applications, where the void fraction is often a key parameter in design as well as in fmancial issues Gust think of the petroleum industry, where one of the main problems is the oil-wa
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