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Abstract
Many application-relevant fluids exhibit shear thinning, where viscosity decreases with shear rate above some critical shear rate. For hydraulic fluids formulated with polymeric additives, the critical shear rate is a function of the molecular weight and concentration of the polymers. Here we present a model for predicting the critical shear rate and Newtonian viscosity of fluids, with the goal of identifying a fluid that shear thins in a specific range relevant to hydraulic pumps. The model is applied to predict the properties of fluids comprising polyisobutene polymer and polyalphaolefin base oil. The theoretical predictions are validated by comparison to viscosities obtained from experimental measurements and molecular dynamics simulations across many decades of shear rates. Results demonstrate that the molecular weight of the polymer plays a key role in determining the critical shear rate, whereas the concentration of polymer primarily affects the Newtonian viscosity. The simulations are further used to show the molecular origins of shear thinning and critical shear rate. The atomistic simulations and simple model developed in this work can ultimately be used to formulate polymer-enhanced fluids with ideal shear thinning profiles that maximize the efficiency of hydraulic systems.
Highlights
Hydraulic systems function and perform tasks through a pressurized fluid, which is controlled directly or automatically by control valves, distributed through hoses, displaced by pumps, and actuated by cylinders and motors
The PAOs used here are representative of the viscosity grades of base stocks used in formulating hydraulic fluids that usually range from 2–8 mm2 /s at 100 ◦ C, depending on the additives
The Newtonian viscosity η0 and density ρ of the blend were predicted by the Kendall–Monroe relation [55] and the sum of the mass fractional density of each component, respectively, using the viscosities (η0p and η0s ), densities (ρ p and ρs ), and concentrations (c p and cs ) of the polymer p and base oil solvent s
Summary
Hydraulic systems function and perform tasks through a pressurized fluid, which is controlled directly or automatically by control valves, distributed through hoses, displaced by pumps, and actuated by cylinders and motors. The efficiency of hydraulic power transmission is primarily affected by internal leakage flow in pumps and motors and by friction and viscous drag in pumps, motors, and cylinders. These losses generate heat and reduce the power available to engage the payload [1,2]. The product of the volumetric and mechanical efficiencies is the overall efficiency of a hydraulic system or component such as a pump or motor [2]. Both volumetric and mechanical efficiency depend on the viscosity of the hydraulic fluid, since the fluid functions as both lubricant and power transmission medium. Volumetric efficiency increases with increasing viscosity because thicker fluids leak less; in contrast, mechanical efficiency decreases
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