Non-Newtonian flow of highly-viscous oils in hydraulic components

Abstract

Viscous oils flowing in the geometrically-complex hydraulic circuits of earth-moving machines are associated with extensive friction losses, thus reducing the fuel efficiency of the vehicles and increasing emissions. The present investigation examines the performance effectiveness of different hydraulic oils, in terms of secondary-flow suppression and pressure-drop reduction. The flow of two non-Newtonian oil compounds, containing poly(alkylmethacrylate) (PMA) and poly(ethylene-co-propylene) (OCP) polymers, respectively, have been comparatively investigated against a base, monograde liquid through Particle Image Velocimetry. An 180° curved-tube layout and a check-valve replica have been selected as representative examples of the hydraulic components comprising the hydraulic circuit. The flow conditions prevailing in the experimental cases are characterized by Reynolds-number values in the range 76–1385. Precursor viscosity measurements with shear rate along with a theoretical analysis conducted using the FENE and PTT models have verified the influence of viscoelasticity and/or shear-thinning on the liquid flow behavior. PIV results have demonstrated that viscoelastic effects setting in due to the OCP additives tend to reduce the magnitude of the secondary flow pattern, commonly known as a Dean-vortex system, arising in the curved geometry by as much as 15% on average compared to the base liquid. A similar flow behavior was also demonstrated in the valve replica layout with reference to the geometry-induced coherent vortical motion in the constriction region, where a vorticity decrease up to 38% was observed for the OCP sample. On the contrary, the flow behavior of the primarily shear-thinning PMA oil was found to be comparable to that of the base oil, hence not presenting significant flow-enhancement characteristics.