Drag reduction in the flow of dilute polymer solutions through a rectangular microchannel

Experiments are performed to investigate the reduction in turbulent drag for the flow of dilute polymer solutions through high-aspect-ratio rectangular microchannels with a height of 200 μm and a width of 2000 μm, which closely approximate plane Poiseuille flow and differ from conventional pipe flow drag-reduction studies. Pressure drop measurements are performed across the fully developed section of the microchannel, and the corresponding friction factor charts are presented. As the flow undergoes transition, the friction factor of the polymer solution is lower than that of its Newtonian counterpart, indicating drag reduction. A collapse of the percentage drag-reduction data is observed when plotted as a function of the non-dimensional parameter Wi(1−β), regardless of polymer concentration and Reynolds number Re, indicating the universal nature of drag reduction. Using microchannels, we access large Weissenberg numbers even with dilute polymer solutions. The nature of the transition is further examined by tracking the evolution of the normalized friction factor with increasing Re in the transition regime. At very low polymer concentrations (0.5–2 ppm), the transition exhibits a subcritical bifurcation, whereas at higher concentrations (2–50 ppm) a supercritical bifurcation is observed. When the Reynolds number at the onset of transition, Ret, is expressed in terms of the non-dimensional elasticity parameter E(1−β), the data collapse, yielding the scaling relation Ret∼(E(1−β))−3/5.