Flow pattern transitions in two-phase liquid-liquid flow in horizontal tubes

As the various concepts and results experienced in gas-liquid two-phase flows cannot readily be translated to liquid-liquid systems, an attempt is made to form a basis for constructing a general two-fluid flow pattern map. The departure from a steady stratified configuration to other bounding flow patterns is analysed in view of the relationships between the instability criterion and the conditions for reality of characteristics, which evolve from exploring the stability and well-posedness of the governing equations. The transitional boundaries between the other flow patterns encountered in liquid-liquid systems are obtained based on mechanistic models. A parametric study made for wide ranges of geometry and physical properties, as encountered in liquid-liquid systems, is also included. Comparisons of the proposed transitional criteria with (limited) available data in liquid-liquid systems show reasonable agreement. The convergence of the general criteria to the extremes of gas-liquid data, on the one hand, and the data of highly viscous core flows, on the other hand, is satisfactory.