A review of investigations on the effect of drag-reducing agents
in curved pipe flows is presented in this work. Proposed mechanisms of
drag reduction, as well as factors that influence their effectiveness
also received attention. In addition, this review outlined proposed
friction factor and fluid flux models for flow of drag-reducing agents
in curved pipes. It was shown in this report that significant drag
reduction in curved pipes can be achieved using drag-reducing agents.
Drag reduction by additives in curved pipes are generally lower than the
corresponding drag reduction in straight pipes. It decreases with
increase in curvature ratio and is more pronounced in the transition and
turbulent flow regimes. Drag reduction depends strongly on the
concentration of polymers and surfactants as well as the bubble fraction
of micro-bubbles. It is also reported that drag reduction in curved
pipes depends on other factors such as temperature and presence of
dissolved salts. Maximum drag reduction asymptote differed between
straight and curved pipes and between polymer and surfactant. Due to the
limited studies in the area of drag reduction for gas-liquid flow in
curved pipes no definite conclusion could be drawn on the effect of
drag-reducing agents on such flows. A number of questions remain such as
the mechanism of drag reduction in curved pipes and how drag-reducing
agents interact with secondary flows. Hence, some research gaps have
been identified with recommendations for areas of future researches.
Key words: drag reduction; drag reducing agents; curved pipes; single
phase flow; two phase flow