Pressure Dependence Of Density Research Articles

Toward a continuum description of lubrication in highly pressurized nanometer-wide constrictions: The importance of accurate slip laws.

The Reynolds lubrication equation (RLE) is widely used to design sliding contacts in mechanical machinery. While providing an excellent description of hydrodynamic lubrication, friction in boundary lubrication regions is usually considered by empirical laws, because continuum theories are expected to fail for lubricant film heights h0 ≪ 10 nm, especially in highly loaded tribosystems with normal pressures pn ≫ 0.1 GPa. Here, the performance of RLEs is validated by molecular dynamics simulations of pressurized (with pn = 0.2 to 1 GPa) hexadecane in a gold converging-diverging channel with minimum gap heights h0 = 1.4 to 9.7 nm. For pn ≤ 0.4 GPa and h0 ≥ 5 nm, agreement with the RLE requires accurate constitutive laws for pressure-dependent density and viscosity. An additional nonlinear wall slip law relating wall slip velocities to local shear stresses extends the RLE's validity to even the most severe loading condition pn = 1 GPa and h0 = 1.4 nm. Our results demonstrate an innovative route for continuum modeling of highly loaded tribological contacts under boundary lubrication.

A Computation Fluid Dynamics Methodology for the Analysis of the Slipper–Swash Plate Dynamic Interaction in Axial Piston Pumps

This paper proposes a CFD methodology for the simulation of the slipper’s dynamics of a swash-plate axial piston unit under actual operating conditions. The study considers a typical slipper design, including a vented groove at the swash-plate interface. The dynamic fluid–body interaction (DFBI) model is exploited to find the instantaneous position of the slipper, while the morphing approach is adopted to cope with the corresponding mesh distortion. A modular approach is adopted to ensure high-quality mesh on the entire slipper surface and sliding interfaces provide the fluid dynamic connection between neighboring regions. The external forces acting on the slipper are included by means of user-defined lookup tables with the simulation estimating the lift force induced by fluid compression. Moreover, the force produced by the metal-to-metal contact between the slipper and the swash plate is modeled through a specific tool of the software. The pressure signal over an entire revolution of the pump is taken as an input of the simulation and a variable time step is used to manage the high-pressure gradients occurring in the regions of inner and outer dead points of the piston. The weakly compressible characteristic of the fluid is considered by a specific pressure-dependent density approach, and the two-equation eddy-viscosity k-ω SST (shear stress transport) model is used to assess the turbulent behavior of the flow. Furthermore, the transitional model predicts the onset of transition, thus solving different equations depending on whether the flow enters a laminar or turbulent regime. In conclusion, the proposed methodology investigates the motion of the slipper in response to several external forces acting on the component. The numerical results are discussed in terms of variable clearance height, pressure distribution within the gap, and lift forces acting on the slipper under specific pump operations.

Simple measurement of the pressure dependence of density for fatty acid esters.

A simple variable-volume method using a polytetrafluoroethylene capsule as a sample cell was applied to measure the pressure dependence of liquid density (ρ). The method was applied to 2-ethylhexyl benzoate at 313, 333, and 353K, and the reproducibility of its volume change was confirmed at pressures of up to 400 MPa. A correction term for the apparatus was also estimated. The uncertainty in the high-pressure ρ measurements was estimated to be ∼±1%. Using the developed method, the ρ values of methyl laurate at pressures of up to 400 MPa at 313 and 333K, as well as those of methyl linoleate at pressures of up to 400 MPa at 313, 333, and 353K, were measured. The results were in good agreement with the literature values in the low-pressure range. The proposed method enables the determination of the high-pressure ρ of liquids with greater ease than previously possible.

Precision measurements of the zero-temperature dielectric constant and density of liquid He4

The resonant frequencies of three-dimensional (3D) microwave cavities are explicitly dependent on the dielectric constant of the material filling the cavity, making them an ideal system for probing material properties. In particular, dielectric constant measurements allow one to extract the helium density through the Clausius-Mossotti relation. By filling a cylindrical aluminum cavity with superfluid helium, we make precision measurements of the dielectric constant of liquid $^{4}\mathrm{He}$ at saturated vapor pressure for range of temperatures 30 to 300 mK and at pressures of 0 to 25.0 bar at 30 mK, essentially the zero temperature limit for the properties of $^{4}\mathrm{He}$. After reviewing previous measurements, we find systematic discrepancy between the low and high frequency determination of the dielectric constant in the zero-temperature limit and moderate discrepancy with previously reported values of pressure-dependent density. Our precision measurements suggest 3D microwave cavities are a promising choice for refining previously measured values in helium, with potential applications in metrology.

An improved prediction of frictional pressure drop using an accurate shear rate equation for high-yield stress drilling fluids at the surface and downhole conditions

This paper evaluates a more accurate and relatively new shear rate equation based on the phenomenological model for the high yield stress drilling fluids to estimate frictional pressure drop in laminar and turbulent flow regimes under surface and downhole conditions. Fann viscometer readings are used to determine shear rates using the above phenomenological model that differ from the conventional shear rates, i.e., γ˙(s−1)=1.7N, where N = Fann viscometer rotor rpm. The new shear rates are used to predict model parameters of Bingham plastic (BP), power-law (PL), Herschel–Bulkley (HB), and Robertson-Stiff (RS) models, and corresponding frictional pressure drops. The efficacy of the above rheology models with improved model parameters is tested by validating the literature-reported experimental pressure drop data of low to high yield stress water-based drilling fluids. The validation results are consistent and accurately predict frictional pressure drops using HB and RS models with the lowest MSE-values. The above methodology is applied to predict frictional pressure drops in a pipe for ten test high-pressure, high-temperature (HPHT) drilling fluids of varying base fluids at the surface and HPHT conditions in laminar and turbulent flow regimes. For this, isothermal compressibility and thermal decomposition temperature of drilling fluids are measured to forecast the temperature- and pressure-dependent densities and shear viscosities. The proposed density model is experimentally validated and found to predict density at downhole conditions adequately with mean absolute- and standard-deviation, i.e., 1.67 and 1.36%, respectively. Temperature- and pressure-dependent viscosity and density have been used to predict pressure drop under downhole conditions using HB and RS models with improved model parameters from the phenomenological model. The HB model is found to be more accurate than the RS model for predicting pressure drop. The proposed frictional pressure drop estimation method in the present study is more general and accurate under the surface and downhole conditions than the conventional practice.

Thermodynamic properties of water from SAFT and CPA equations of state: A comprehensive assessment

The performance of eight versions of the representative and most commonly used Statistical-Association-Fluid-Theory (SAFT) equations of state for water along with the Cubic-Plus-Association (CPA) equation are examined in detail both throughout the entire liquid phase region and at supercritical conditions. In addition to the temperature–pressure dependence of density, the fundamental response functions, namely, isothermal compressibility, isobaric expansivity, and isobaric heat capacity, have been evaluated and compared with experimental data along five isobars from P=0.1 MPa up to 1000 MPa, and along four isotherms within the range from 300 K to 750 K. It turns out that to draw a general conclusion on the quality and accuracy of these equations (for pure water) is practically impossible with different equations reproducing reasonably well different properties and at different thermodynamic conditions but failing in other instances. In general, the equations seem to be able to capture simple pressure dependence at isothermal conditions but fail primarily in estimating isobaric properties. All the equations (i) tend to perform reasonably well at pressures above the critical one where the excluded volume begins to predominate and the effect of hydrogen bonding becomes insignificant, (ii) but fail again at very high pressures, and (iii) have the most serious problems with predicting the residual isobaric heat capacity. The most successful results are obtained from the recently developed Association Dependent PC-SAFT equation, which can be mainly attributed to its fitting using a broader set of experimental data compared to other equations; nonetheless, its performance still cannot be considered as overall reliable.

Dual-capillary dilatometer for density measurements of supercooled water

An apparatus tailored to accurate density measurements of supercooled water, i.e. liquid water in a metastable state below the freezing point temperature, was recently developed at the Institute of Thermomechanics of the Czech Academy of Sciences. The apparatus, dual-capillary dilatometer, is described, together with the measurement procedure and the evaluation methodology. The primary result of the dual-capillary method are relative densities with respect to the density at a reference temperature and given pressure. In order to calculate absolute densities, densities at the reference temperature as a function of pressure are needed. For calculation of such pressure dependence of density, so called thermodynamic integration involving literature thermodynamic data and our experimental results is used. The dual-capillary dilatometer was successfully employed in density measurements of ordinary water, heavy water and seawater. The data in the temperature range from 238.15 to 303.15 K at pressures from atmospheric up to 200 MPa are presented and compared with respective IAPWS formulations of thermodynamic properties. The data for ordinary water are also compared with an accurate equation of state for supercooled water of Holten et al. (2014) revealing good mutual agreement. The expanded uncertainty of relative densities acquired by means of the dual-capillary method is estimated to be lower than 50 ppm.

Elastic Simulation of Joints with Particle-Based Fluid

Skinning, which is used in skeletal simulations to express the human body, has been weighted between bones to enable muscle-like motions. Weighting is not a form of calculating the pressure and density of muscle fibers in the human body. Therefore, it is not possible to express physical changes when external forces are applied. To express a similar behavior, an animator arbitrarily customizes the weight values. In this study, we apply the kernel and pressure-dependent density variations used in particle-based fluid simulations to skinning simulations. As a result, surface tension and elasticity between particles are applied to muscles, indicating realistic human motion. We also propose a tension yield condition that reflects Tresca’s yield condition, which can be easily approximated using the difference between the maximum and minimum values of the principal stress to simulate the tension limit of the muscle fiber. The density received by particles in the kernel is assumed to be the principal stress. The difference is calculated by approximating the moment of greatest force to the maximum principal stress and the moment of least force to the minimum principal stress. When the density of a particle increases beyond the yield condition, the object is no longer subjected to force. As a result, one can express realistic muscles.

Influence of Realistic Lubricant Density-Pressure Dependence on the Stiffness of Elastohydrodynamic Lubricated Contacts

Abstract The analysis of the dynamic response of elastohydrodynamic lubricated contacts has often invoked a universal law for lubricant density–pressure dependence, even though the densities of many lubricants exhibit a substantial deviation from this widely adopted law. The current work investigates the influence of real lubricant density–pressure behavior on the stiffness of elastohydrodynamic lubricated contacts. It is shown that accounting for the real lubricant density–pressure dependence is crucial for an accurate estimation of the oil film stiffness, under steady-state considerations. The influence on the overall stiffness of the contact is found to be negligible though. Finally, an analytical correction procedure is provided, allowing a correction of oil film stiffness predictions that are based on the universal law for lubricant density–pressure dependence (or any other unrealistic law), to account for the real lubricant density–pressure response.

Cold Sintering of Fe–Ag and Fe–Cu Nanocomposites by Consolidation in the High-Pressure Gradient

The results of fabricating dense Fe–Ag and Fe–Cu nanocomposites from mixtures of powders consolidated by high-pressure cold sintering and from nanosized powders of silver (Ag), iron (Fe), and copper (Cu) are reported. The results of mechanical tests of Fe–Ag and Fe–Cu nanocomposites are presented. Nanocomposite powders were prepared by milling the micron powder of carbonyl iron (Fe) and nanosized silver oxide (Ag2O) powder, as well as iron and cuprous oxide (Cu2O) nanopowders in a high-energy attritor. The microstructure was investigated using a high-resolution scanning electron microscope. Compacts with a density of about 70% of the theoretical density were annealed in hydrogen to reduce silver oxide and cuprous oxide to metals and remove oxide films from the surface of iron powder particles. Then cold sintering followed—high-pressure consolidation at room temperature. The data on the pressure dependence of density of samples are found in a range of 0.25–3.0 GPa. Densities higher than 95% of the theoretical density are attained for all nanocomposites at a pressure of 3.0 GPa, while a density of about 100% is attained for Ag and Cu powders. High mechanical properties are found for all compositions in experiments for the three-point bending and for compression. It is established that mechanical properties of nanocomposites are noticeably higher than for composites formed from micron-sized powders. The higher plasticity was observed for Fe‒Ag and Fe–Cu nanocomposites when compared with the samples formed from nanostructured Fe.

A model for foamed cementing of oil and gas wells

We present a two-dimensional model of the primary cementing process for foamed cement slurries. Foamed cement slurries have a number of claimed advantages, but also have a pressure-dependent density and rheology. The rheology is hard to quantify fully over all ranges of foam quality, which compromises the accuracy of models. The density variation is due to expansion/compression of the gas phase along the well, caused by variations in the static pressure. We show that in the absence of careful control, buoyancy-driven instabilities can result in the annulus, as the foamed slurry expands and the density drops below that of the displaced drilling mud. These instabilities appear to be of a classic porous media/Hele-Shaw cell fingering type, triggered by a threshold unstable density difference. We show that these instabilities are amplified by wellbore eccentricity, occurring lower in the well than in a concentric annulus. Our results question the safe usage of foamed cements in primary cementing.

Temperature and pressure dependence of densities and viscosities for binary mixtures of methyl decanoate plus n-heptane

New experimental densities and viscosities data for binary mixtures of methyl decanoate plus n-heptane were reported at temperatures of (298.15, 308.15 and 318.15) K and pressures of (0.1–15) MPa. The expanded uncertainty for density measurement is estimated to be 2 kg m−3 and the relative expanded uncertainty for viscosity measurement is estimated to be 0.03. A fitting equation was proposed to correlate the density of the binary mixture and Grunberg-Nissan equation was used to predict the viscosity of the binary mixture, respectively. The maximum absolute relative deviations between the experimental results and calculation values for densities and viscosities were 0.11% and 5.94%, respectively.

Trisubstituted Imidazolium-Based Ionic Liquids as Innovative Heat Transfer Media in Sustainable Energy Systems

One of the possible research directions for developing the modern energy sector is searching for innovative working fluids with exceptional properties, which leads to reducing energy consumption and the costs of the operation of the system. As a continuation of our investigations of ionic liquids (ILs) for their use as heat transfer fluids (HTFs), we present new thermophysical properties of 1-ethyl-2,3-dimethyl-1H-imidazol-3-ium bis[(trifluoromethyl)sulfonyl]azanide ([C2MMIm][TFSI]) and 1-butyl-2,3-dimethyl-1H-imidazol-3-ium bis[(trifluoromethyl)sulfonyl]azanide ([C4MMIm][TFSI]). Speeds of sound were conducted in the temperature range 298–323 K and in the pressure range 0.1–76 MPa. The temperature and pressure dependence of densities, isobaric heat capacities, and related quantities were obtained by acoustic method. The viscosities, the TGA, and the cytotoxicities were also conducted. The thermal conductivities, energy storage densities, Prandtl numbers, isobaric heat capacities, viscosities, densities, i...

Temperature and Pressure Dependence of Density of a Shale Oil and Derived Thermodynamic Properties

The temperature and pressure dependence of density was measured experimentally from 293 to 473 K and 0.1 to 12 MPa for a shale oil produced from Kukersite oil shale in Estonia. The shale oil sample was a fuel oil fraction of a whole oil produced in a commercial plant that uses solid heat carrier retorting technology. The fraction had a boiling range of approximately 460 to 780 K and contained significant quantities of polar phenolic compounds (hydroxyl group content of 5.3 wt %). The effect of these compounds on the properties of the oil was investigated by removing most of the phenolic compounds via extraction to create the second sample (dephenolated sample with hydroxyl group content of 1.1 wt %). The dephenolation resulted in a shale oil with a composition being more similar to that of other shale oils from well explored deposits. On the basis of a review of the literature, these are the first experimental data on the pressure dependence of density for this shale oil, and shale oils generally. Thermal...

Impact of non-linear transport properties on low permeability measurements

The results of modeling nonlinear transport properties on a plug scale for steady state, unsteady state, pulse decay and sinusoidal pressure measurements are presented. We include the gas slippage (Klinkenberg corrections) and pressure dependent density effects.To validate the modeling, we compare it to analytical calculations based on an assumption of constant mass flow once the transients have dissipated for steady state models. Validation of the other measurement protocols is performed by comparison to finite difference calculations.For low permeability samples, significant pressure drops must be modeled to obtain large enough flow rates to allow accurate measurement in the laboratory. We limited ourselves to 100 psi pressure differences because of the large influence of effective stress on gas density that would be present for larger pressure differences. Flowing pressures of this magnitude imply significant pressure dependent density effects, which reduce the flow rates. In contrast, gas slippage increases the transport of gas. The result is that nearly identical fits are obtained for widely varying magnitudes of permeability and gas slippage. This result is obtained for all of the modeled measurement protocols. We introduce a new technique to interpret the modeled data, the ”k0-b plot”. The method allows the values of k0 and b to be extracted if multiple measurements are performed at different mean pressures. The technique is also compared to laboratory data to demonstrate that the effects occur in the measurements.The influence of the non-linearities predicted on the measured chamber pressures are presented for each of the measurement protocols (other than steady state in which there are no chambers). In particular, sinusoidal pressure variation of the upstream chamber results in an increased average downstream chamber pressure and the introduction of harmonic distortion. These effects have not been discussed in the literature and give a motivation for development of this measurement for low permeability samples.The modeling indicates that plug scale measurements are practical, but multiple mean pressures must be used to separate the competing effects of permeability and gas slippage. We recommend performing unsteady state measurements at reservoir stress at multiple mean pressures, supplemented with sinusoidal pressure and pulse decay to calibrate the nonlinear and adsorption effects.

Pressure dependence of density and structural relaxation of glass near the glass transition region

AbstractA simplified and effective pressure cell together with an experimental procedure has been developed and applied to compress samples of SCHOTT N‐BK7® glass under static high pressures in a piston‐cylinder apparatus. Results from the density and volume recovery measurements show that, the glass samples were effectively densified in piston‐cylinder apparatus with the density at room temperature increasing linearly with frozen‐in pressure. To explain the experimental data, we developed a mathematical model based on a suggestion by Gupta (1988) with two internal parameters, named fictive temperature (Tf) and fictive pressure (Pf), which fits the experimental data well.

Developing coarse-grained potentials for the prediction of multi-properties of trans-1,4-polybutadiene melt

In this paper, we combined structure-based and thermodynamic quantities-based coarse-graining (CG) approaches and adopted the bulk density, local conformational distributions and radial distribution function of the underlying atomistic model system as the target properties to derive the CG force field (FF) for the polymer system of trans-1,4-polybutadiene. The resultant CG FF exhibits good observable representability. In addition to the overall chain size, the extracted bulk modulus, isothermal compressibility, storage and loss moduli from CG model systems are of the same level of magnitude as the atomistic results, suggesting that this hybrid CG approach improves the reproduction capability of the CG model on mechanical property, pressure-dependent density fluctuations, and viscoelasticity. Upon the introduction of a standard dissipative particle dynamics (DPD) thermostat to remedy the removed degrees of freedom and reduced friction during CG, the CG model can completely mimic the dynamics of the atomistic model. Furthermore, we found that the standard DPD thermostat approach is applicable to the steady shear flow simulation at low and moderate rates and the relevant dissipative factor derived by matching diffusion coefficients can only quantitatively reproduce the shear viscosity at low shear rates. Overall, these findings demonstrate the ability of combined CG methods in optimizing CG FF to reproduce multi-properties of polymers.

Elastic and mechanical properties of hexagonal diamond under pressure

Hexagonal diamond is the harder and stiffer alternative of traditional cubic diamond for today’s technology. Although several theoretical attempts have been performed to understand the ground-state elastic properties of hexagonal diamond, little is known about the high-pressure elastic properties of this key material. Unlike previous theoretical methods, we report the application of second-generation reactive bond order potential for the first time to elaborate the pressure-dependent properties of hexagonal diamond in conjunction with geometry optimization calculations up to 500 GPa. Pressure dependency of density, five independent elastic constants, bulk, shear and Young moduli, Poisson ratio, elastic wave velocities, anisotropy parameter, Kleinman parameter, and stability conditions of hexagonal diamond were evaluated. Overall, considered properties of hexagonal diamond display evident increments under pressure, and their ground-state values are in reasonable agreement with available theoretical data.

Simulation of the load rejection transient process of a francis turbine by using a 1-D-3-D coupling approach

This paper presents the simulation and the analysis of the transient process of a Francis turbine during the load rejection by employing a one-dimensional and three-dimensional (1-D-3-D) coupling approach. The coupling is realized by partly overlapping the 1-D and 3-D parts, the water hammer wave is modeled by defining the pressure dependent density, and the guide vane closure is treated by a dynamic mesh method. To verify the results of the coupling approach, the transient parameters for both typical models and a real power station are compared with the data obtained by the 1-D approach, and good agreements are found. To investigate the differences between the transient and steady states at the corresponding operating parameters, the flow characteristics inside a turbine of the real power station are simulated by both transient and steady methods, and the results are analyzed in details. Our analysis suggests that there are just a little differences in the turbine outer characteristics, thus the traditional 1-D method is in general acceptable. However, the flow patterns in the spiral casing, the draft tube, and the runner passages are quite different: the transient situation has obvious water hammer waves, the water inertia, and some other effects. These may be crucial for the draft tube pulsation and need further studies.

Phase transition and elasticity of gallium arsenide under pressure

Geometry optimization calculations were performed for some structural, elastic and mechanical properties of gallium arsenide (GaAs) under pressures up to 25 GPa. In contrast to previous works, a recent Stillinger-Weber type potential was used for the first time to elaborate the pressure dependence aspects of GaAs. B3→B1 phase transition pressure was determined as 17 GPa. Pressure dependence of density, typical cubic elastic constants, bulk, shear, and Young moduli, Poisson ratio, elastic velocities, anisotropy parameter, Kleinman parameter, elastic anisotropy degree, and stability conditions of GaAs were also evaluated. Overall, our results are satisfactory and can be helpful for future investigations of GaAs under pressure.