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:
- 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.
- 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.
- 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.
- Detailed study of the effect of pipe diameter on the effectiveness of
polymer, surfactant and micro-bubble drag reduction is required.
- 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.
- 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.
- 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.
- The synergistic effect of polymer-surfactant combination may also be
explored.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.