Laminar-turbulent transition for the flow of dilute polyacrylamide solutions through rectangular microchannel

Experiments are performed to investigate the laminar-turbulent transition for the flow of dilute polyacrylamide solutions (PAM) through rectangular microchannels of height 300μm and width 3000μm. Pressure drop is measured across the developed region of the microchannel to obtain friction factor and pressure fluctuation data. The departure of the friction factor data from its laminar value is considered the beginning of a transition in the fluid flow. The transition onset occurs at a much lower Re for the flow of dilute PAM solutions compared to its Newtonian counterpart. Here, Re is the Reynolds number defined as Re=Dh×v×ρ/η, where Dh is the hydraulic diameter, v is the cross-sectional average velocity, ρ is the fluid density, and η is the dynamic fluid viscosity. The hydraulic diameter, Dh, is defined as Dh=4(W×H)/(2(W+H)), where W is the channel width and H is the channel height. Pressure fluctuations and dye stream visualization experiments also show that the flow of dilute PAM solutions undergoes a laminar-turbulent transition at a much lower Re compared to a Newtonian fluid. The PAM concentration is fixed at 20 ppm, and the solvent is made of glycerin solutions with a glycerin concentration in water ranging from 1% to 40%. For the highest glycerin concentration of 40%, the transition onset occurs at Rec∼90. The critical Reynolds number, Rec, when plotted with the elasticity number, E(1−β), for different glycerin concentrations, shows a scaling relationship of Rec∼(E(1−β))−2/5.