Taylor vortices, i.e., toroidal rings, are formed in an annulus when one pipe is rotated inside another. These rings form when the critical rotation rate (rpm) is reached. Equations are presented to calculate the critical rpm above which Taylor vortices form for both Newtonian and power law fluids. The theoretical computations show that under typical drilling rotation rates Taylor vortices do form.
Experiments were conducted with a wide range of Newtonian and power law (shear thinning) fluids in a transparent annular geometry. The experimental values of critical rpm are in agreement with computed ones. Taylor vortices were clearly observed to form and did contribute to the lift of simulated drill cuttings.
In Newtonian fluids, the lifting capability increased as the viscosity was increased. For power law (shear thinning) fluids, contrary to expectation, the lifting capability decreased as the apparent viscosity was increased. This was because increasing the polymer concentration in order to increase apparent viscosity also lowered the ‘n’ value of the fluid. Fluids with low ‘n’ values exhibit lower velocities close to the stationary walls. It can be inferred, therefore, that with regard to the transport of cuttings (by vortices or otherwise), high ‘n’ value fluids are more efficient.