Dynamics of the removal of viscoplastic fluids from inclined pipes

Abstract

The dynamics of the removal of a viscoplastic fluid by a Newtonian fluid is investigated experimentally and theoretically in an inclined pipe based on our previous studies on near-horizontal and highly inclined configurations. The fluids are miscible. The displacing Newtonian fluid is heavier than the displaced viscoplastic one i.e. the configuration is density-unstable. In our earlier work it was found that two major flow regimes, namely center-type and slump-type, might occur depending on the density difference. These flows are explored in great details through measurements of the displacement speeds and hydraulic Reynolds numbers. The residual viscoplastic layer unevenness is characterized revealing that the flows are in the range of large roughness regimes. Through an integrated experimental-theoretical approach, estimates of the interfacial and wall shear stresses are given which is of great importance in designing the displacement and cleaning processes involving fluids with yield stress. Accompanied by Ultrasonic Doppler Velocimetry (UDV) data, the dynamics of the removal of the viscoplastic fluid from a pipe is elucidated suggesting three distinct phases in the displacement process namely a plug flow, inertial multi-dimensional flow at the displacing front and steady multi-layer developed flow. Finally, the viscoplastic displacement flow results are compared against the predictions of the closure model, previously proved successful for Newtonian and shear-thinning fluids displacement in pipe. It is found that in the case of viscoplastic fluids the closure model always over predicts the displacing front velocity due to the inertial stresses present at the front not captured in the model.