Experimental investigation on drag reduction in turbulent pipe flow with polymer injection

An experimental investigation is conducted to characterize the drag reduction of Polyethylene Oxide solutions and its influencing on turbulent pipe flow with polymer injection. The study systematically examines the effects of Reynolds number, injection angle (seven angles), molecular weight (seven molecular weights), and streamwise direction development on drag reduction. Similar to the dimensionless polymer flux K in the flat plate boundary layer flow with polymer injection, the normalized polymer flux K_p applicable to the pipe (i.e., the relative mass ratio between the injecting polymer and the main flow) is defined to further collapse experimental data. The K_p-scaling laws indicate that DR firstly grows linearly, and then saturates or even declines with increasing log₁₀ K_p, similar to the observed DR variation of the plate boundary layer flow with polymer injection. The DR vs K_p relationship could provide guidance to optimize the use of polymer injection for the pipeline flow. Notably, the variation law of drag reduction with molecular weight conforms to S-shaped curve, which can provide guidance to optimize the use of molecular weight in polymers drag reduction. The change in drag reduction with Reynolds number varying from 15,952 to 79,761 initially increases and then decreases; there exists a critical Reynolds number for achieving optimal drag reduction effect. The variation of drag reduction rate with injection angle and streamwise direction distance is influenced by both effective concentration and advection of the injected solution. Smaller injection angle does not bring more significant drag reduction effect. Meanwhile, the counter-stream injection exhibits a superior drag reduction effect compared to streamwise injection under certain conditions, primarily due to the influence of more suitable mixing rate.