Rheological characterization of viscoplastic fluid flow in a pipe with wall slip using in situ particle image velocimetry

The current experimental study investigates the rheological characterization of Carbopol gel solutions into a pipe flow using in situ visualization. The shear rate and shear stress profiles for different pressure-driven values are presented and correlated to obtain new steady-state flow curves compared with rheometrical data performed with cross-hatched parallel plate (CHPP) and smooth concentric cylindrical (SCC) geometries at a rotational rheometer. The rheological behavior for the test performed by the in situ visualization was well fitted by the generalized Herschel-Bulkley model, and different values for the coefficient of consistency (K), flow behavior index (n), and yield stress (τ₀) were fitted for the three gel solutions due to the presence of wall slip behavior. The discrepancies between the values of the rheological parameters suggest that conventional rheometrical measurements, which avoid the slippage of the fluid, lead to an overestimation of these parameters, and as a consequence, these discrepancies are extended to the dimensionless numbers calculated for the hydrodynamic flow description. Also, the experimental plug core velocity was compared with the analytical value obtained by the rotational rheometer tests to calculate an equivalent slip velocity, and such velocity depicts a quasi-linear trend with the wall shear stress. This is supported by the apparent viscosity profiles along the pipe diameter, suggesting that the slippage is an inherent characteristic of polymer gel solutions, and it is disseminated by the presence of layers near the pipe wall where the Newtonian like-behavior is presented. Finally, applying in situ visualization technique assures a better rheological characterization and accurate description of the flow conditions for fluids with complex behavior.