4.0 Knowledge gap

The review revealed key findings that go a long way in answering questions regarding the effects of drag-reducing polymers, surfactants and micro-bubbles for flows in curved pipes. However, certain research gaps were identified particularly in the area of two-phase liquid-liquid drag reduction in curved pipe flows.

4.1 Single phase flow in curved pipes

Several studies have been carried out to investigate the non-Newtonian effects of drag-reducing agents in single phase liquid flow in curved pipes. However, some research gaps remain. Some of the areas where further works are needed are outlined below:
  1. Further studies are required to investigate the effect of drag-reducing agents on secondary flows. Understanding of the mechanism of interaction of polymer and surfactant macromolecules with secondary flow streamlines could provide answers to the reported trends in the laminar flow regime.
  2. For flow in bends (the flow is not hydrodynamically developed) the effect of drag-reducing agents on flow separation and reattachment is not yet understood. The reduced drag reduction in bends (compared to coiled pipes) has been linked to flow separation and reattachment. Thus, insight into the effect of DRAs on flow separation and reattachment could provide answers to this observation.
  3. Most of the existing researches on drag reduction in curved pipes are centred on coils. Limited studies have been carried out to interrogate the effect of drag-reducing agents for flows in bends. In particular, research is needed to investigate the effect of bend angles on drag reduction.
  4. Detailed study of the effect of pipe diameter on the effectiveness of polymer, surfactant and micro-bubble drag reduction is required.
  5. The effect of micro-bubble size on drag reduction for curved pipe flows is another area where research is required. The exiting studies are limited to straight channel flow and reports on the effect of bubble size on drag reduction are conflicting.
  6. Proper understanding of mechanism of polymer and surfactant drag reduction in curved pipes could provide a means of quantitatively linking polymer and surfactant properties to the reported drag reduction.
  7. The maximum drag reduction for polymer and surfactant DRAs in curved pipes is an area that needs to be explored further. There is the need to further investigate the effects of pipe geometry such as diameter and curvature on DRAs in curved pipes.
  8. The synergistic effect of polymer-surfactant combination may also be explored.
  9. The effects of temperature, dissolved salts and silt on drag reduction by additives in pipe bends and curves needs to be further explored. Reports on these are either scanty or conflicting.
  10. Research into the effect of drag-reducing agents on velocity profile distribution in curved pipes is not available in open literature. Studies in this area could provide more explanation to this behaviour observed in straight pipe flow.

4.2 Two-phase flows in curved pipes

Studies into the effects of drag-reducing agents on liquid-liquid flows in curved pipes are lacking and those pertaining to gas-liquid flow are scanty. Experience from single phase flow shows that straight pipe data cannot be extended to account for observations in curved pipes. Some of the areas where there are need for further research include the following.
  1. Effects of DRAs on phase distribution, pressure drop among others for two-phase liquid-liquid flows through curved pipes is yet to receive proper scholarly attention.
  2. The effect of pipe orientation on pressure drop, phase distribution and effectiveness of drag-reducing agents for two-phase flows in curved pipes needs to be investigated.
  3. There is limited study that investigates the effects of temperature, dissolved salts and silt on drag reduction for liquid-liquid flows in curved pipes. Research in this area could provide practical solutions for field scale operations.
  4. Researches in the area of drag reduction in two-phase flows in bends are lacking. The few existing literatures have focused on coiled pipe flows. Insight into the effect of drag-reducing agent on phase distribution in pipe bends could go long way in improving process management and safety.