5.0 Conclusion

The importance of pipe fittings (e.g. bends) and curves in pipeline transport has necessitated the need for more understanding of the hydrodynamics of flow in them. Core among the areas of research interest is drag reduction by additives for single- and two-phase flows in curved pipes. It was shown in this review that significant drag reduction in curved pipes can be achieved using polymer, surfactant and micro-bubbles. However, the drag reductions reported in curved pipes are generally lower than the corresponding drag reduction in straight pipes. In general drag reduction decreased with increase in curvature due to higher centrifugal forces. Similar to straight channel flows, drag reduction by additives is predominantly in the turbulent flow regime, but a few studies reported DR in the laminar flow regime. Beyond certain critical Reynolds number, in the turbulent regime, drag reduction by additives reduced with increase in Reynolds number but prior to this the reverse is the case. This has been attributed to uneven distribution of air bubbles or degradation of polymer or surfactant as the case may be. It was also seen that the effectiveness of polymer and surfactant DRAs increased with concentration until a threshold concentration is attained. Likewise, the effectiveness of micro-bubble as drag-reducing agent in curved channel flow generally increased with air fraction. It was also reported that drag reduction depends on other factors such as temperature and the presence of dissolved salts. This review examined correlations for maximum drag reduction asymptote for flow of polymer and surfactants in curved pipes. It was seen that the maximum drag reduction asymptote differed between curved and straight pipes and between polymers and surfactants. Single phase drag reduction correlations for flows in curves of polymers and surfactants were presented in this review. A brief review of drag reduction for two-phase gas-liquid flows in coils was also provided. Though drag reduction was reported in the limited research in this area, no definite conclusion could be drawn on the effect of drag-reducing agents on gas-liquid flows in curved pipes. With the prospect of reduced pumping cost, improved operational flexibility and transport safety, research is expected to continue in this area in the future. Extension of hydrodynamic and drag reduction studies to three-phase gas-liquid-liquid flows is also essential owing to the occurrence of this flow in petroleum installations.
CONFLI CT OF INTEREST
The Authors wish to state that there are no known conflict of interest arising from the publication of this manuscript