An Overview of Highly Viscous Fluid Flows in Straight and Elbow Pipes: II Gas–Liquid Flows

This review summarizes the latest research concerning the horizontal flow of two-phase mixtures with viscosities ranging from 0.2 Pa·s to 6.4 × 104 Pa·s. Although our survey is concerned with Newtonian fluids, a short section is included to briefly discuss certain rheological aspects that should be generally considered. In contrast with previous work reporting on the progress in specific domains (e.g., in the oil and gas, chemical, or geophysical contexts), we seek to provide a comprehensive overview of the methods and results used in different contexts. Accordingly, the scope is widened to encompass a broader range of industrial applications and naturally occurring flows. The interest in high-viscosity flows is motivated by the operational challenges occurring in certain systems, most notably in the oil and gas industry, where the production of heavy and extra-heavy crude oils reduces the margins for a safe and efficient operation. Furthermore, this review underlines the cross-field analogies appearing in a broad range of scales and applications. It emphasizes the fundamental role of viscosity in determining the flow patterns, as experimental evidence suggests that the transition boundaries are largely altered at higher viscosities. Some gaps that could be addressed in future work are briefly discussed.