Experimental study of grease flow in pipelines: wall slip and air entrainment effects

Prediction of pressure drop gradient and evaluation of wall slip and air bubbles entrainment effects observed during the piping flow of lubricating greases were investigated. With this aim, viscous flow tests in rotational rheometers and pressure drop measurements in pipelines were carried out using different geometries with both smooth and rough surfaces. The Sisko model was applied in the experimental range of flow rates for predicting pressure drop gradient. Air entrainment occurring when the pumping system was primed with a highly viscous material as lubricating greases significantly decreases pressure drop gradient. This air entrainment effect can be corrected using a modified expression to evaluate the drag ratio defined for non-Newtonian liquid/air intermittent flows. On the other hand, a new expression based on the internal relative roughness of pipelines was proposed to correct wall slip effect. Eliminating these two effects, the classical definition of the friction factor for a non-Newtonian fluid, f = 16/Re′, can be applied to predict pressure drop of grease flow in pipelines.