Evaluation of wall slip effects in the lubricating grease/air two-phase flow along pipelines

Evaluation of wall slip phenomena during the horizontal pipeline flow of air/lubricating grease mixtures was investigated. With this aim, pressure drop measurements have been carried out along pipelines with different diameter and roughness. A modified Jastrzebski's equation for the slip velocity, based on the introduction of the relative roughness, has been used to correct wall slip effects for a lithium lubricating grease/air system. This expression has been introduced in the classical Rabinowitsch–Mooney treatment and applied to the superficial liquid velocity instead of the single-phase average velocity following a single-phase treatment analogy. Thus, the non-slip flow curve data for the two-phase mixture were obtained from roughened pipes and compared with those obtained from pipes with smooth internal surfaces. The effect of air on the extension of wall slip has been established as a function of air flow rate. Thus, the consideration of the reduction of the wetted pipe surface as air is injected allows an adequate explanation of this phenomenon, confirmed by the reduction of the effective slip contribution on the observed apparent shear rate. A power-law relationship between the slip velocity and the wall shear stress has been deduced, although this tends asymptotically to linearity as air flow rate is increased.