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