TABLE OF CONTENTS Multiphase flow is a complex phenomenon that is difficult to understand, predict, and model. Common single-phase flow characteristics such as velocity profile, turbulence, and boundary layer are thus inappropriate for describing the nature of such flows. The flow structures are rather classified in flow regimes, whose precise characteristics depend on a number of parameters. Flow regimes vary depending on operating conditions, fluid properties, flow rates, and the orientation and geometry of the pipe through which the fluids flow. The transition between different flow regimes may be a gradual process. Due to the highly nonlinear nature of the forces that rule the flow regime transitions, the prediction is near impossible. In the laboratory, the flow regime may be studied by direct visual observation using a length of transparent piping (Wallis, 1969). However, the most utilized approach is to identify the actual flow regime from signal analysis of sensors whose fluctuations are related to the flow regime structure. This approach is generally based on average cross-sectional quantities, such as pressure drop or cross-sectional liquid holdup. Many studies have been documented using different sensors and different analysis techniques (Dukler and Hubbard, 1966; Jones and Zuber, 1975; Lin and Hanratty, 1987; Rajkovic et al., 1996; Soldati et al., 1996).
In order to obtain optimal design parameters and operating conditions, it is necessary to clearly understand two- and three-phase flow regimes and the boundaries between them, where the hydrodynamics of the flow, as well as the flow mechanisms, change...
