Pressure-viscosity Coefficient Research Articles

Rheology of an Ionic Liquid with Variable Carreau Exponent: A Full Picture by Molecular Simulation with Experimental Contribution

The rheological behavior of an ionic liquid was investigated by means of molecular dynamics simulations with experimental contribution, under conditions close to those found in the elastohydrodynamic and the very-thin film lubrication regimes. The molecular model was applied to nearly 200 temperature–pressure–shear rate cases, without any parameter adjustment. Experiments were conducted on a rheometer and a high-pressure falling-body viscometer. This unique combination of numerical and experimental tools has enabled the full description of the ionic liquid rheological response to extreme conditions of temperature, pressure and shear rate. In the linear domain, a very good consistency between the two computational approaches (nonequilibrium molecular dynamics, equilibrium molecular dynamics via the Green–Kubo formalism) and the experiments was obtained on the Newtonian viscosity. Reliable values of the pressure–viscosity coefficient, another rheological characteristic necessary for predicting film thickness in the regimes of interest in this work, were inferred. Compared with a conventional lubricant of almost identical Newtonian viscosity, the pressure–viscosity coefficient of the ionic fluid is much lower, its variations with temperature remaining, however, very similar. The application of the time–temperature–pressure superposition principle and the regression to the Carreau equation for describing the nonlinear domain have revealed, for the first time, significant variations in the exponent of the Carreau model which have been correlated with the changes in temperature and pressure.

ВЛИЯНИЕ ПРОТИВОИЗНОСНЫХ ПРИСАДОК НА ПАРАМЕТРЫ ГИДРОДИНАМИЧЕСКОГО РЕЖИМА ТРЕНИЯ В ПОДШИПНИКАХ ДВС

An essential condition to ensure an engine life having 300,000 – 1 000,000 km according to race consists in assurance of a minimum wear rate of crankshaft bearings. For such units as connectingrod bearings a maximum allowable value of bearing brass wear makes 100 mkm which corresponds to an integral linear wear intensity of 10-14 …10-13 that is possible only at a hydrodynamic mode of friction actually at all modes of engines operations. Ensuring liquid friction in all range of contact pressures is achieved both through design methods, and technological ones. To the latter belongs first of all the application of lubricants with high wearresistant properties. In this connection arises a problem to reveal a mechanism and degree of the effect of lubricant components upon a range of contact pressure where liquid friction is realized. This paper reports the experimental evidences of the increase of contact pressures at which is achieved a minimum value of a constant of friction at the introduction in lubricant a standard wear-resistant additive – dialkyldithiophosphate of zinc (ZDDP). So, at the introduction in coal oil without additives I-20A (viscosity type ISO 32) 1.25 and 2.5% ZGGP maxi-mum contact pressure at which is realized a hydrody-namic mode of friction increases by 1.8 and 2 times accordingly. Because of impossibility to explain experimental data by a chemical interaction of ZDDP with surface material of tribo-coupling there is offered and substantiated a mechanism of ZDDP influence upon rheological parameters of lubricant in a separation layer. A rheological model allowing the computation of the wearresistant additives influence upon a value of hydrodynamic pressures in a lubrication layer is of-fered. The idea of the initiation by a wearresistant additive of multimolecular adsorption of lubricant on the surface of tribocoupling may be used for the optimization of ZDDP structure and also for the prognostication of changes in pressure coefficient of viscosity in a lubrication layer.

New equations for enhanced film thickness in line contacts under pressurized ambient conditions

An elastohydrodynamic lubrication model is proposed for line contacts under pressurized ambient conditions often encountered in hydraulic pumps, submarine machinery and many other submerged systems. It has been demonstrated that the film forming behavior under such conditions is essentially different from that in conventional elastohydrodynamic lubrication contacts. The numerical simulation results are regressed to develop new central and minimum film thickness equations for Newtonian fluids as functions of ambient pressure, speed, load, and material parameters. An alternative approach is also discussed which involves the use of existing film thickness formulas with ambient viscosity and pressure–viscosity coefficient pertaining to the desired pressure range. A film thickness enhancement of more than 100% over conventional elastohydrodynamic lubrication case is observed. This enhancement is shown to be highly sensitive to the pressure–viscosity coefficient. Besides, the effect of shear-thinning behavior is also investigated and it is found to lower the film thickness enhancement, especially at high ambient pressures.

A New High-Pressure Viscometer for Oil/Refrigerant Solutions and Preliminary Results

ABSTRACTThe reliability and efficiency of refrigeration compressors is dependent on the lubricating effectiveness of the refrigeration oil, which is necessarily a solution of oil and refrigerant. In this article, the development of a high-pressure viscometer that can operate at elastohydrodynamic lubrication (EHL) inlet pressures for oil/refrigerant solutions is explained in detail. New viscosity measurements are presented for two compressor oils to 1.2 GPa. Preliminary results are given for a polyol ester with two concentrations of R134a refrigerant to 0.35 GPa. These are the first measurements of oil/refrigerant solutions to such high pressure. Temperature–pressure–viscosity correlations are applied to all materials for use in modeling. Correlations are provided for the temperature and pressure dependencies of the viscosity of the oils, refrigerant, and mixtures. The definition of pressure–viscosity coefficient is a pressing problem for elastohydrodynamics.

High-pressure rheology of alumina-silicone oil nanofluids

The effect of high-pressure on the rheological characteristics of alumina-silicone oil nanofluids at room temperature is investigated in this paper. Nanofluids are engineered colloidal suspensions of nano-sized particles dispersed in a basefluid. The rheological characteristics of nanofluids are studied with pressures up to 100MPa and particle concentrations up to 8% by mass using a high-pressure viscometer. The addition of nanoparticles is observed to increase the apparent viscosity of the nanofluids. A non-Newtonian, shear-thinning behavior is observed for both the basefluid and the nanofluids at all pressures at the higher shear rates. The critical shear rate at which shear thinning occurs is observed to be strongly influenced by particle addition. An increase in the pressure is observed to have an equal effect on the basefluid and on the nanofluids, which results in an average pressure-viscosity coefficient for both to be approximately 0.015mm2/N. The relative increase in the viscosity of the nanofluid to that of the basefluid was not affected by the pressure and was several orders higher than that predicted by the classical effective medium theory.

What is regularization, its role and place in solution of isothermal problems for heavily loaded elastohydrodynamically lubricated contacts?

A typical defect of numerical solutions obtained for isothermal heavily loaded elastohydrodynamically lubricated (EHL) contacts such as solution instability is considered. A detailed analysis and numerical examples of solutions for dry and lubricated contacts with instability are presented. The main roots of numerical instability are discussed, and a regularization process, that is, the process that makes the numerical solution stable, is revealed and analyzed. A regularization of EHL problems slightly modifies the problem equations to make their numerical solution stable and convergent. The level of this problem modification is controlled by just one parameter. The effect of the level of regularization on parameters of EHL contacts is considered. The idea of such a regularization comes from the solution of a thermal EHL problem with low generation of energy in the contact. Some examples of regularized numerical solutions for classic EHL problem and an EHL problem that takes into account tangential surface displacements caused by frictional stresses are presented. The approach showed its effectiveness and application simplicity for very high values of pressure viscosity coefficients. Copyright © 2016 John Wiley & Sons, Ltd.

Flow measurements of a polyphenyl ether oil in an elastohydrodynamic contact

A novel methodology, based on the use of phosphorescence imaging, is applied to determine the local through-thickness velocity profile of lubricant in an elastohydrodynamic contact. The technique has spatial and temporal resolutions of 40 μm and 340 μs respectively and thus allows lubricant rheology to be investigated at conditions close to service conditions. The capability of the newly-developed method is verified by examining the flow of 5P4E polyphenyl ether, a lubricant base fluid used in very high temperature applications and is well-known for its high viscosity-pressure coefficient. Experimental results highlight the effect of the contact pressure on the velocity profile of this fluid in lubricated contacts. At low pressures, the velocity profile of 5P4E is close to linear, characteristic of Couette flow. As the local pressure increases, its velocity profile progressively deviates from a Couette profile and shear banding is evident at high pressure.

ライスオイルの高圧物性に関する研究

In order to estimate the lubricating performance of rice bran oil, the experiments of high pressure viscosity and high pressure density have been carried out, and such high pressure physical properties as pressure viscosity coefficient and bulk modulus were grasped. Based on the experimental results including the measurement value of wax crystallization temperature at atmospheric pressure, phase transition diagram of test oil was made, and the phase transition behavior of test oil was compared with other vegetable oils. As a result, the pressure viscosity coefficient of the rice bran oil at the test temperature range from 293K to 333K showed higher value in comparison with other vegetable oils. The relation between bulk modulus and pressure was different depending on the test temperature. In addition, it was found that the rice bran oil has a similar solid-liquid phase transition properties to such vegetable oil as soybean oil in which the composition ratio of unsaturated fatty acids resembles.

Friction Reduction in Lubricated Rough Contacts: Numerical and Experimental Studies

Combining the contact model of elastic-layered solid with the concept of asperity contact in elastohydrodynamic lubrication (EHL), a mixed-lubrication model is presented to predict friction coefficient over rough surfaces with/without an elastic-layered medium under entire lubrication regimes. Solution of contact problems for elastic-layered solids is presented based upon the classical model of Greenwood and Williamson (GW) in conjunction with Chen and Engel's analysis. The effects of the Young's modulus ratio of the layer to substrate and the thickness of the layer on the elastic real area of contact and contact load for a fixed dimensionless separation are studied using the proposed method, which is used for the asperities having contact with an elastic coating. Coefficient of friction with elastic-layered solids in boundary lubrication is calculated in terms of Rabinowicz's findings and elastic-layered solutions of Gupta and Walowit. The effect of rough contacts with an elastic layer on friction coefficient in lubrication regimes has been analyzed. Variations in plasticity index ψ significantly affect friction coefficients in boundary and mixed lubrications. For a large value of ψ, the degree of plastic contact exhibits a stronger dependence of the mean separation or film thickness than the roughness, and for a small value of ψ, the opposite result is true. The effect of governing parameters, such as inlet oil viscosity at ambient pressure, pressure–viscosity coefficient, combined surface roughness, and El/E2 on friction coefficient, has been investigated. Simulations are shown to be in good agreement with the experimental friction data.

Pressure–Viscosity Coefficient of Hydrocarbon Base Oil through Molecular Dynamics Simulations

The pressure–viscosity coefficient (the α value), which represents the variation of viscosity as a function of pressure, is an important parameter for elastohydrodynamic lubrication analyses. The properties of hydrocarbons in the C20–C40 mass range are of fundamental importance as they are basic constituents of synthetic- and mineral-based lubricant stocks. The conventional acquisition of the α value requires preparation of lubricant samples and experimental testing by means of a high-pressure viscometer. In this paper, we present a method to obtain the α value of a typical base oil (1-Decene trimer) based solely on the molecular dynamics simulations. Non-equilibrium molecular dynamics (NEMD) simulations were performed to calculate the shear viscosity of the lubricant at various temperatures and pressures up to 1 GPa. Elevated temperatures and time–temperature superposition (TTS)-based extrapolations were applied to further extend the ability of the NEMD simulations, and the rotational relaxation time was calculated and used to determine the validity of the NEMD calculations. The α value at 100 °C was calculated and compared with experimental results. Effectiveness of the extrapolation was evaluated with a 95 % confidence interval.

Effects of Grease Characteristics on Sound and Vibration of a Linear-Guideway Type Recirculating Ball Bearing

This paper deals with the effects of grease characteristics on sound and vibration of a linear-guideway type ball bearing. First, sound, vibration, and temperature of a linear-guideway type ball bearing were measured by changing 16 types of greases. Next, in order to explain the effects of grease characteristics (absolute viscosity η0 of the base oil in the grease at atmospheric pressure, pressure–viscosity coefficient ξ of the base oil, and penetration P of grease) and linear velocity V on the sound pressure p and vibratory acceleration a of the linear bearing, dimensional analyses were carried out. The dimensional analyses derived dimensionless products Pp/(η0V) and η0ξV/P for the sound, and dimensionless products Pa/V2 and η0ξV/P for the vibration. The plot of the measured data using dimensionless products showed that both sound pressure p and vibratory acceleration a (under a certain linear velocity) decreased as the absolute viscosity η0 of the base oil in the grease at atmospheric pressure or the pressure–viscosity coefficient ξ increased. Whereas the sound pressure p decreased as the penetration P decreased, the vibratory acceleration a decreased as P increased.

Global existence of weak solution for the compressible Navier–Stokes–Poisson system for gaseous stars

We study the multi-dimensional compressible Navier–Stokes–Poisson system with γ-law pressure and density-dependent viscosity coefficients in the simulation of the motion of gaseous stars. The global existence of spherically symmetric weak solutions to the free boundary value problem for the Navier–Stokes–Poisson system for γ∈(65,43] is shown for arbitrarily large initial data with compact support.

An exact analytical solution for viscoelastic fluids with pressure-dependent viscosity

A linear relationship between the shear viscosity and the total pressure, a constant single relaxation time for a Maxwell-type viscoelastic fluid, and a unidirectional velocity profile are the major assumptions made in the present work in order to study the steady-state isothermal and pressure-driven flows in straight channels and circular tubes. Despite their non-linearity the final partial differential equations that govern the flows are solved analytically, and the dependence of all the primary flow variables on the geometrical aspect ratio, the dimensionless pressure-viscosity coefficient and the Weissenberg number is revealed explicitly. It is demonstrated that the pressure-dependent viscosity slightly affects the velocity profile, changes substantially the pressure gradient along the main flow direction, generates another normal to the wall, and it is responsible for significant variations of the extra-stresses along both spatial directions. An exponential increase of the viscosity, relative to its reference value, is predicted as the distance from the exit of the channel/tube increases. As a consequence, the average pressure difference, required to drive the flow and the shear stress at the wall increase substantially compared to that predicted by the classic Hagen–Poiseuille law. Last, it is revealed that the solution of the governing equations ceases to exist when the Weissenberg number reaches a threshold.

The effect of an oscillatory entrainment velocity on the film thickness in thermal EHL point contact

In this study, the mathematical model for transient thermal elastohydrodynamic lubrication (EHL) was established and the effect of tangential vibration on thermal EHL was investigated. The pressure calculation was carried out by a multi-grid technique, the elastic deformation was evaluated by a multilevel multi-intergration method, and the temperature was calculated by a single-direction sequential column sweeping process. The present study aims to explain the multi-dimple phenomenon observed experimentally by Ehret and Bauget [Proc Inst Mech Eng Part J: J Eng Tribol 2001; 215: 289–300]. The results show that a tangential vibration produces significant change of pressure and film thickness and it seems to be the most possible mechanism of the movement of multi-dimples observed by Ehret and Bauget [Proc Inst Mech Eng Part J: J Eng Tribol 2001; 215: 289–300]. Furthermore, the effect of the curvature radius, viscosity–pressure coefficient, load and the parameters of vibration was also investigated.

Internal viscoelastic flows for fluids with exponential type pressure-dependent viscosity and relaxation time

The isothermal steady-state and pressure-driven flows in a straight channel and a circular tube, of an incompressible viscoelastic fluid which follows the Maxwell constitutive model, are considered. Under the assumption that both the shear viscosity and the single relaxation time of the fluid vary exponentially with pressure, the governing equations are solved analytically using a regular perturbation scheme with small parameter the dimensionless pressure-viscosity coefficient. The solution is found up to sixth order in the small parameter, revealing a two-dimensional (2D) flow field and the dependence of the primary flow variables on the geometrical aspect ratio, the pressure-viscosity coefficient, and the Weissenberg and Reynolds numbers. It is demonstrated that the pressure-dependent viscosity and relaxation time enhance the pressure gradient along the main flow direction, generate another along the wall-normal direction, and cause vertical motion of the fluid. Viscoelastic extra-stresses, which affect significantly the average pressure difference, required to drive the flow and the shear stress at the wall, are also predicted. Moreover, the mean Darcy friction factor shows a substantial deviation from the average pressure difference, as the fluid elasticity increases. For the Newtonian fluid, the effect of the pressure-dependent viscosity on the velocity components is minor, but substantial on the pressure and shear-stress profiles. Most of these features are predicted for the first time, and they are due to the fact that the flow field is fully 2D, indicating the complex nature of fluids with pressure-dependent viscosity and relaxation time.

Investigation of the difference in liquid superlubricity between water- and oil-based lubricants

The difference in superlubricity behavior between water- and oil-based lubricants is investigated and the liquid superlubricity region dependent on pressure and the pressure–viscosity coefficient is established.

A critical evaluation of film thickness‐derived pressure–viscosity coefficients

AbstractA single parameter, the pressure–viscosity coefficient, α, quantifies the pressure dependence of the viscosity of the liquid in elastohydrodynamic lubrication (EHL). Most published values of α have not been obtained from measurements of viscosity as a function of pressure. Rather, these effective pressure–viscosity coefficients have been derived from the measurement of the EHL film thickness, a more difficult procedure. In this article, five well‐characterized liquids that should be Newtonian in the EHL inlet are identified for which film‐derived coefficients have been reported. These coefficients are compared with coefficients derived from published viscosity correlations and new viscosity measurements. The film‐derived coefficients are found to not be an accurate representation of the piezoviscous response. The procedure of deriving a pressure–viscosity coefficient from a film thickness measurement does not offer an alternative to the simpler and easier viscometer measurement. Copyright © 2014 John Wiley & Sons, Ltd.

Elastohydrodynamic Properties of Biobased Heat-Bodied Oils

Heat-bodied oils were prepared by thermal treatment of soybean oil under inert atmosphere. Different viscosity grades of heat-bodied oils synthesized by varying the reaction time were investigated for various properties including viscosity, viscosity index, elastohydrodynamic film thickness, and pressure–viscosity coefficient. Heat-bodied oils displayed elastohydrodynamic film thickness characteristics typical of lubricating oils. The film thickness of heat-bodied oils increased with increasing entrainment speed and viscosity, decreased with increasing temperature, and was unchanged with varying load. Pressure–viscosity coefficients of heat-bodied oils were estimated from film thickness as well as from physical property data. The pressure–viscosity coefficient values of heat-bodied oils increased with increasing viscosity and decreasing temperature and were in the range displayed by such oils as polyol esters, poly-α-olefins, and petroleum-based base oils. Heat-bodied oils provide access to a wide viscosity range of biobased oils not attainable with vegetable oils, without serious negative impact on critical lubricant properties such as viscosity index, film thickness, and pressure–viscosity coefficient.

Analysis on the Side Leakage Amount of the Friction between Piston and Cylinder Block in Axial Piston Pump

The spherical distribution pairs of the plunger and the cylinder friction, has an important influence on the performance of spherical port plate axial piston pump. Based on the analysis of fluid viscosity change with pressure and temperature, considering friction differential pressure flow and shear flow, establishes the mathematics model of the friction pair of leakage. The simulation analysis using MATLAB software, the leakage flow rate is not proportional to pressure, but with the increase of pressure leakage flow was increased, and with the increase of pressure viscosity coefficient and temperature coefficient of viscosity, the leakage flow rate correction coefficient increases obviously, so in the choice of the hydraulic oil cylinder hole, should choose a relatively moving average leakage rate had no effect the piston ring slot.

New Central Film Thickness Equation for Shear Thinning Lubricants in Elastohydrodynamic Lubricated Rolling/Sliding Point Contact Conditions

This paper offers central film thickness formula pertaining to shear-thinning lubricants under rolling/sliding point contact conditions. The shear-thinning behavior of the lubricants is modeled using Carreau viscosity equation and the piezo-viscous response employed herein is the free-volume based Doolittle equation in conjunction with Tait's equation of state for lubricant compressibility. The present formulation is based on reciprocal asymptotic isoviscous piezo-viscous coefficient as it is a more accurate measure of the high pressure piezo-viscous response of elastohydrodynamic lubricated (EHL) lubricants compared to the conventional pressure–viscosity coefficient. Comparisons between simulated, curve-fitted values, and experimental results validate both the employed numerical approach and rheological model.