For a gas-liquid two-phase flow, the interface changes its position with respect to time and space. Flow and thermal field in each phase in the vicinity of the interface influence the interface location. The movement of interface in turn influences the flow field. Various two-phase flow regimes are thus observed as the flow parameters of the two phases are varied. The phenomena of bubble coalescence, growth and breakup, and the mass, momentum and energy exchange between the two phases add up to the numerical complexities towards capturing these flow patterns using Computational Fluid Dynamics (CFD) models. The numerical models for the two-phase flow are thus still in the developmental phase. A comprehensive comparison with experiment can go a long way in establishing the strength and reliability of the existing CFD models. Towards this objective, an experimental setup is developed to visualize the temporal and spatial organization of air-water two-phase flow patterns. The flow patterns captured with a high speed camera are compared with the numerically visualized flow patterns obtained using the Volume of Fluid (VOF) model for the two-phase flow. The development of stratified, wavy, plug, slug and annular flow patterns are discussed in detail, and interfacial and wall shear stresses for all these patterns are compared.

Multiphase flow involves flow of more than one phase or component. In multiphase flow, a phase can be defined as an identifiable class of material that has a particular inertial response to and interaction with the flow and the potential field in which it is immersed. For example, different-sized solid particles of the same material can be treated as different phases because each collection of particles with the same size will have a similar dynamical response to the flow field. Two-phase flow is a case of multiphase flow in which the two phases may exist in the four combinations of gas-liquid, gas-solid, liquid-solid and liquid-liquid. Among these, analysis of gas-liquid two-phase flow is complex due to the fact that the interface between the two phases changes its shape with respect to time and space.

Mandhane et al. (1974), and Taitel and Dukler (1976) experimentally captured various air-water type two-phase flow patterns. They developed the flow pattern map by varying the mass flow rate of the gas and liquid phase. The flow pattern map showed various flow regimes and their transitions. Kaminsky (1999), and Ghajar and Tang (2007) reestablished these flow pattern maps using sophisticated instrumentation and highly accurate measurement facility. They could capture various intermittent regimes which were not observed previously. In addition, the pressure drop and heat transfer characteristics for different flow patterns were measured.

Due to the advancement in computational techniques and numerical algorithms, the practical experimentation is being replaced by numerical experimentation (Lun et al., 1996). The numerical analysis involved in the two-phase flow includes algorithms to locate the interface, drift velocity, and the momentum and energy exchange at the interface (Hetsroni, 1982). Various numerical models have been proposed in literature. These include Euler-Lagrange and Euler-Euler approaches. Efforts have been made to model various flow patterns independently by various researchers (Neti and Mohamed, 1990; Ghorai and Nigam, 2006; Schepper et al., 2008; and Vallee et al., 2008).

Mechanical Engineering Journal, Computational Fluid Dynamics Models, Multiphase Flow, Numerical Experimentation, Computational Techniques, Numerical Algorithms, Visualization Section, Experimental Visualization, Air Reynolds Number, Water Reynolds Number, Numerical Simulations, Geo-Reconstruction Scheme.