Oil Residual Layers in Water-Oil Displacement Flow

This paper investigates on the water-oil displacement flow in downward inclined pipe, which is a practical application in chemical and oil industry. The experiments were conducted in a 3.5m long 15mm inner diameter downward transparent pipe. The pipe can be flexibly tilted to specific inclination and water inlet velocity was controlled by a needle valve. During the experiments, it was revealed that in a certain range of pipe inclination and water inlet velocity, the oil layers remained stationary in the upper section of the pipe and it lasted through the entire displacement procedure. Meanwhile the thickness of the oil layers was altered according to different displacing configurations. A dimensionless parameter λ is introduced to represent the relative magnitudes of buoyancy stress and viscous shear stress. Based on the two-fluid model the relationship between normalized water-oil interface height and the dimensionless parameter is established. It is found that the transition from stable interface to unstable interface will be induced by the dominated inertia force or interface instability. Two critical value of the dimensionless parameter are derived as two criterions for the transition to from stable interface to unstable interface. The model prediction and criterions are in good agreement with the experiment results, thus verifying the effectiveness of the model.