Terms Of Pressure Dependence Research Articles

A model for assisted periodic breathing with degenerate permeability

The process of breathing is interpreted as forced periodic motion of a viscoelastic deformable porous medium with gas exchange between the interior and exterior, driven by forces acting on its boundary. Energy and mass are supplied to the system by muscle activity, and also possibly by external ventilation in case of assisted breathing for patients with lungs insufficiency. Hysteresis in the pressure–volume characteristic and possible degeneracy of the pressure-dependent terms are also taken into account. The main result consists in proving the existence of a periodic solution under arbitrary periodic forcing in suitable function spaces.

Using the Quasi-chemical formalism beyond the phase Diagram: Density and viscosity models for molten salt fuel systems

CALPHAD models to compute the density and viscosity of four keystone systems related to Molten Salt Reactor (MSR) technology have been optimized: NaCl-UCl3, LiF-ThF4, LiF-UF4, and LiF-ThF4-UF4. Revised thermodynamic assessments of all four systems, using the modified quasichemical formalism in the quadruplet approximation for the description of the liquid solutions, are reported. In the case of NaCl-UCl3, phase diagram and mixing enthalpy data available in the literature are taken into account. For the fluoride systems, recently published data on some solid phases are taken into account, while retaining the most recently published descriptions of the liquid solutions. The densities of the liquid solutions are modelled using pressure-dependent terms of the excess Gibbs energy, while the viscosities are then modelled using an Eyring equation. Both state functions are related to the thermodynamic assessments through the quadruplet distributions.

Contribution of Pressure to the Energy–Momentum Density in a Moving Perfect Fluid: A Physical Perspective

In the energy–momentum density expressions for a relativistic perfect fluid with a bulk motion, one comes across a couple of pressure-dependent terms, which though well known, are to an extent, lacking in their conceptual basis and the ensuing physical interpretation. In the expression for the energy density, the rest mass density along with the kinetic energy density of the fluid constituents due to their random motion, which contributes to the pressure as well, are already included. However, in a fluid with a bulk motion, there are, in addition, a couple of explicit, pressure-dependent terms in the energy–momentum density, whose presence to an extent, is shrouded in mystery, especially from a physical perspective. We show here that one such pressure-dependent term appearing in the energy density, represents the work done by the fluid pressure against the Lorentz contraction during transition from the rest frame of the fluid to another frame in which the fluid has a bulk motion. This applies equally to the electromagnetic energy density of electrically charged systems in motion and explains in a natural manner an apparently paradoxical result that the field energy of a charged capacitor system decreases with an increase in the system velocity. The momentum density includes another pressure-dependent term, that represents an energy flow across the system, due to the opposite signs of work being done by pressure on two opposite sides of the moving fluid. From Maxwell’s stress tensor we demonstrate that in the expression for electromagnetic momentum of an electric charged particle, it is the presence of a similar pressure term, arising from electrical self-repulsion forces in the charged sphere, that yields a natural solution for the notorious, more than a century old but thought by many as still unresolved, 4/3 problem in the electromagnetic mass.

Energy–momentum density and pressure relations for a relativistic ideal gas with a bulk motion

We derive here, from first principles, the energy–momentum density of a perfect fluid in the form of an ideal molecular gas, in an inertial frame where the fluid possesses a bulk motion. We begin from the simple expressions for the energy density and pressure of a perfect fluid in the rest frame of the fluid, where the fluid constituents (gas molecules) may possess a random motion, but no bulk motion. From a Lorentz transformation of the velocity vectors of the molecules moving in different directions in the rest frame of the fluid, we compute their energy–momentum vectors and number densities in an inertial frame moving with respect to the rest frame of the liquid. From that, we arrive at the energy–momentum density of the fluid in a frame where it has a bulk motion. This way, we explicitly demonstrate how a couple of curious pressure-dependent terms make an appearance in the energy–momentum density of a perfect fluid having a bulk motion. In addition to an ideal molecular gas, we compute the energy–momentum density for a photon gas also, which of course matches the energy–momentum density expression obtained for a molecular gas having an ultra-relativistic random motion.

Nonlinear power loss in the oscillations of coated and uncoated bubbles: Role of thermal, radiation and encapsulating shell damping at various excitation pressures.

This study presents the fundamental equations governing the pressure dependent disipation mechanisms in the oscillations of coated bubbles. A simple generalized model (GM) for coated bubbles accounting for the effect of compressibility of the liquid is presented. The GM was then coupled with nonlinear ODEs that account for the thermal effects. Starting with mass and momentum conservation equations for a bubbly liquid and using the GM, nonlinear pressure dependent terms were derived for power dissipation due to thermal damping (Td), radiation damping (Rd) and dissipation due to the viscosity of liquid (Ld) and coating (Cd). The pressure dependence of the dissipation mechanisms of the coated bubble have been analyzed. The dissipated energies were solved for uncoated and coated 2-20μm in bubbles over a frequency range of 0.25fr-2.5fr (fr is the bubble resonance) and for various acoustic pressures (1kPa-300kPa). Thermal effects were examined for air and C3F8 gas cores. In the case of air bubbles, as pressure increases, the linear thermal model looses accuracy and accurate modeling requires inclusion of the full thermal model. However, for coated C3F8 bubbles of diameter 1-8μm, which are typically used in medical ultrasound, thermal effects maybe neglected even at higher pressures. For uncoated bubbles, when pressure increases, the contributions of Rd grow faster and become the dominant damping mechanism for pressure dependent resonance frequencies (e.g. fundamental and super harmonic resonances). For coated bubbles, Cd is the strongest damping mechanism. As pressure increases, Rd contributes more to damping compared to Ld and Td. For coated bubbles, the often neglected compressibility of the liquid has a strong effect on the oscillations and should be incorporated in models. We show that the scattering to damping ratio (STDR), a measure of the effectiveness of the bubble as contrast agent, is pressure dependent and can be maximized for specific frequency ranges and pressures.

Constitutive modeling and experimental calibration of pressure effect for polyurea based on free volume concept

Abstract The combined effect of pressure and temperature on the mechanical behavior of polyurea is characterized and integrated into a constitutive model based on Williams-Landel-Ferry (WLF) equation. An increase in pressure is related to a decrease in temperature, both pressure and temperature affecting the polymer's molecular free volume and thereby introducing a pressure-dependent term into the WLF equation. The model is derived based on Fillers and Tschoegl's original work, regarding the free volume as a state variable. The model provides a closed-form expression for the time scale shift factor for equivalent temperature in terms of temperature and pressure and its three-dimensional surface representation. The prediction of the present model agrees well with pressure shift factor data obtained from confined compression tests of polyurea, reported elsewhere, confirming our constitutive model and its experimental calibration procedure. A simplified model of the pressure effect is also developed which yields good approximate results.

Validation of turbulent flame speed models for methane–air-mixtures at high pressure gas engine conditions

To describe the premixed combustion of natural gas in internal combustion engines, the flamelet approach is often assumed to be appropriate. It postulates a thin flame front which is wrinkled by turbulence but locally spreads like a laminar flame. Numerous correlations for the macroscopic turbulent flame speed to the local laminar flame speed exist. In this study seven correlations are validated at engine conditions, i.e., at high pressure (up to 7.0MPa) and high turbulence (turbulent Reynolds number up to 4.4·104), with the help of measurements from a single cylinder research gas engine. The stoichiometry, the exhaust gas recirculation rate, and the engine speed are varied. Simulations of the detailed reaction kinetics with the diffusive species and energy transport are performed to calculate the laminar flame speed. The flame propagation is described by a zonal cylinder model on the basis of the measured in-cylinder pressure traces, to determine the turbulent flame speed with respect to the thermal expansion of the reacting gas. Additionally, Computational Fluid Dynamics (CFD) simulations of the in-cylinder charge motion are performed to describe the turbulent flow field. It is found that four flame speed correlations including a pressure-dependent term predict flame speed ratios in quantitative accordance to the measurements. This is noteworthy, as these correlations are mostly developed for lower turbulence and at considerable lower pressure. It supports the hypothesis that flamelet-like combustion is still found above the Klimov–Williams criterion.

Pyrolysis and oxidation of decalin at elevated pressures: A shock-tube study

Ignition delay times and ethylene concentration time-histories were measured behind reflected shock waves during decalin oxidation and pyrolysis. Ignition delay measurements were conducted for gas-phase decalin/air mixtures over temperatures of 769–1202K, pressures of 11.7–51.2atm, and equivalence ratios of 0.5, 1.0, and 2.0. Negative-temperature-coefficient (NTC) behavior of decalin autoignition was observed, for the first time, at temperatures below 920K. Current ignition delay data are in good agreement with past shock tube data in terms of pressure dependence but not equivalence ratio dependence. Ethylene mole fraction and fuel absorbance time-histories were acquired using laser absorption at 10.6 and 3.39μm during decalin pyrolysis for mixtures of 2200–3586ppm decalin/argon at pressures of 18.2–20.2atm and temperatures of 1197–1511K. Detailed comparisons of these ignition delay and species time-history data with predictions based on currently available decalin reaction mechanisms are presented, and preliminary suggestions for the adjustment of some key rate parameters are made.

Improved Kinetic Model of Crystallization for Isotactic Polypropylene Induced by Supercritical CO2: Introducing Pressure and Temperature Dependence into the Avrami Equation

Supercritical carbon dioxide (SC-CO2) provides a highly tunable technique to induce changes in morphology and crystallization kinetics of various polymers. In this study, the effect of SC-CO2 treatment on the crystallinity of isotactic polypropylene (i-PP) was analyzed by differential scanning calorimetry (DSC). The Avrami equation is suitable for describing crystallization kinetics of various solids. However, the assumption that the crystallinity approaches 1.0 as the time approaches infinity is not valid for semicrystalline polymers, especially in the atmosphere of SC-CO2. An improved kinetic model for the CO2-induced crystallization of i-PP was purposed by introducing equilibrium crystallinity, as well as temperature- and pressure-dependent terms into the Avrami equation. The parameters of the crystallization kinetics model were obtained by least-squares fitting of the DSC data. The results show that the improved kinetic model provides a reasonable description of the crystallization behavior of i-PP induced by SC-CO2. The successful application of the improved kinetic model to the CO2-induced crystallization of i-PP suggests that this model may be adopted to other SC-CO2-semicrystalline polymer systems.

Theory of the Kinetics of Chemical Potentials in Heterogeneous Catalysis

Catalysis is of paramount importance in our daily life. In particular, heterogeneous catalysts which speed up reactions on their solid surfaces play a vital role in a wide range of industries, such as petroleum, energy, and environment-related industries. There is no doubt that rational design of new catalysts is a major endeavor in chemistry. A key to achieve this goal is reaction kinetics, bridging the gap between microscopic elementary chemical reactions and macroscopic performance of catalysts. Although great progress in understanding reaction kinetics has been made, rational design of new catalysts remains one of the profound challenges. Here, we present a new formulism of reaction kinetics at surfaces in terms of the involved chemical potentials, which simplifies the reaction kinetics significantly. Furthermore, within this formulism we propose a new approach of searching for new catalysts. The effectiveness and universality of this theory are discussed.

A non-invasive probe for online-monitoring of turgor pressure changes under field conditions

An advanced non-invasive, field-suitable and inexpensive leaf patch clamp pressure probe for online-monitoring of the water relations of intact leaves is described. The probe measures the attenuated output patch clamp pressure, P(p), of a clamped leaf in response to an externally applied input pressure, P(clamp). P(clamp) is generated magnetically. P(p) is sensed by a pressure sensor integrated into the magnetic clamp. The magnitude of P(p) depends on the transfer function, T(f), of the leaf cells. T(f) consists of a turgor pressure-independent (related to the compression of the cuticle, cell walls and other structural elements) and a turgor pressure-dependent term. T(f) is dimensionless and assumes values between 0 and 1. Theory shows that T(f) is a power function of cell turgor pressure P(c). Concomitant P(p) and P(c) measurements on grapevines confirmed the relationship between T(f) and P(c). P(p) peaked if P(c) approached zero and assumed low values if P(c) reached maximum values. The novel probe was successfully tested on leaves of irrigated and non-irrigated grapevines under field conditions. Data show that slight changes in the microclimate and/or water supply (by irrigation or rain) are reflected very sensitively in P(p).

A chemical kinetically based ignition delay correlation for iso-octane covering a wide range of conditions including the NTC region

An ignition delay correlation has been developed for iso-octane based on the functional behavior exhibited by a detailed chemical kinetic mechanism. The correlation employs a traditional Arrhenius-based, power law formulation, τ = A ϕ α p β χ O 2 γ × exp ( λ ) , including dependencies for equivalence ratio ( ϕ), pressure ( p) and oxygen percentage ( χ O 2 ). However the exponents for these parameters, α, β, and γ, respectively, are expressed as third-order polynomials with respect to temperature in order to capture changes in functionality seen across different regimes. At very low temperatures α, β, and γ are forced to a constant value, as seen within the mechanism. The activation energy term, λ, is written as a combination of two quadratic expressions so that the behavior in the negative temperature coefficient (NTC) region can be captured. A pressure-dependent term is also included in the expression for λ in order to reduce the activation energy at higher pressures in the NTC region due to increased low temperature reactivity, and the appearance of cool flame, or low temperature heat release (LTHR). The resulting expression contains 37 constants. The new correlation is applicable over a wide range of conditions and can be used for data comparisons and mechanism evaluation, as well as systems-level engineering simulations. In this work experimental data from rapid compression machines (RCM) and shock tubes (ST) are compared through normalizing features of the correlation, and the performance of a detailed kinetic mechanism is evaluated based on the functional behavior of the α, β, γ and λ parameters. Six hundred and sixty-one (661) data points have been used to fit the 37 constants of the expression where the experimental conditions cover ϕ = 0.2 – 2.0 , p = 1 – 60 atm , χ O 2 = 0.125 – 21 % and T = 650 – 2000 K . Data normalized through the correlation indicate a standard deviation of ±34%. Departures from the correlation can be attributed to an incomplete description of the functional dependencies, some inconsistencies with regard to diluent composition, experimental uncertainties, and facility-influenced phenomena. The experimental points have also been simulated using the LLNL detailed iso-octane mechanism, where the computed ignition delay times have been fit to the new correlation. Agreement and differences between the correlated experimental and simulation α, β, γ and λ functions are highlighted, where it is noted that there is a substantial discrepancy concerning the increased low temperature chemistry at high pressure. Implications for future engine design are discussed.

General Transfer Functions for Multiphase Flow in Fractured Reservoirs

Summary We propose a physically motivated formulation for the matrix/fracture transfer function in dual-porosity and dual-permeability reservoir simulation. The approach currently applied in commercial simulators (Barenblatt et al. 1960; Kazemi et al. 1976) uses a Darcy-like flux from matrix to fracture, assuming a quasisteady state between the two domains that does not correctly represent the average transfer rate in a dynamic displacement. On the basis of 1D analyses in the literature, we find expressions for the transfer rate accounting for both displacement and fluid expansion at early and late times. The resultant transfer function is a sum of two terms: a saturation-dependent term representing displacement and a pressure-dependent term to model fluid expansion. The transfer function is validated through comparison with 1D and 2D fine-grid simulations and is compared to predictions using the traditional Kazemi et al. (1976) formulation. Our method captures the dynamics of expansion and displacement more accurately.

PRESSURE-INDUCED TRANSITION OF 2DEG IN δ-DOPED GaAs TO INSULATING STATE

The tunneling and the lateral conductance of 2DEG formed in the GaAs with the δ-doped layer were measured simultaneously at hydrostatic pressures at helium temperatures. The resistivity of the δ-doped layer sharply increases by > 3 orders of magnitude at about 2 GPa, and its temperature coefficient changes from ~ - 0.04 to ~ - 4 K -1. The tunneling resistance shows only slight change of exponential behavior at about 1.5 GPa, while a drastic change of the Zero Bias Anomaly (ZBA) takes place just at the metal–insulator transition. Such a behavior is interpreted in terms of pressure dependence of the GaAs band structure and the DX-level position.

Compaction equations: a comparison of the Heckel and Kawakita equations

The background and theoretical and experimental requirements of powder compaction equations have been briefly reviewed. There have been many equations proposed. However, the equations most commonly used today are the Heckel and Kawakita ones, because they alone have claimed to be able to relate to the physical properties of the materials being compacted. There has been much discussion about their relative merits, and in their form, they appear very different. The two equations are discussed in detail and examples are given, and their relative merits are discussed. However, it is first shown for applied pressures, which are low compared to the yield strength of the particles, that they are identical in form. It is then shown mathematically that if a pressure dependent term is used in the Heckel equation then the resulting modified equation is very similar to the Kawakita one and, in fact, becomes identical in form for a particular, likely value of the pressure dependency term. It is concluded that the Kawakita equation is a special case of the more general, modified Heckel equation. It is further concluded that compaction equations need further development to take into account the anisotropy in compacts made by uniaxial compaction. It is shown that the Poisson's ratio of compacts will also increase with applied pressure, and this must be considered.

Velocities and quality factors of sedimentary rocks at low and high effective pressures

Techniques for measuring acoustic velocities and quality factors (Q) of sedimentary rocks in the laboratory are sufficiently accurate to establish closely fitting regression curves which represent the variation of these acoustic properties with pressure. Ultrasonic P- and S-wave measurements of velocity (Vp, Vs) and Q (Qp, Qs) were made on 16 different water-saturated sandstone samples at various effective pressures using an ultrasonic reflection technique, and the data were fitted using a simple regression equation containing a constant, plus linear and exponential pressure-dependent terms. The redundancy of the linear term in the regression of Q data suggests that microcracks alone govern the pressure variation of Q, whereas velocities are additionally dependent on another mechanism, consistent with the changes in elastic moduli caused by intergranular compression (causing small reductions in porosity). The implied rates of microcrack closure with increasing effective pressure in all samples were found to be comparable. The regression equations describe the data closely enough to enable the extrapolation of acoustic properties to both low and high effective pressures. This is of practical use, since laboratory techniques can produce uncertain results at low pressures, due to coupling problems and low signal strengths, whilst high pressures require large confining vessels. Low-pressure and high-pressure data are applicable to shallow seismic surveys and hydrocarbon exploration, respectively.

Activation volume (.DELTA. V.dbldag.q) for energy- and electron-transfer quenching of [Cu(dpp)2+]* (dpp = 2,9-diphenyl-1,10-phenanthroline)

Author(s): Crane, DR; Ford, PC | Abstract: The quenching of emission from the metal to ligand charge-transfer (MLCT) state of the copper(I) complex Cu-(dpp)2+ (dpp = 2,9-diphenyl-l, 10-phenanthroline) by the uncharged chromium(III) complexes CrL3 (L =acetylacetonate, 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, l,l,l-trifluoro-2,4-pentanedionate) and several nitrobenzene derivatives has been investigated in dichloromethane, chloroform, and tetrahydrofuran solutions as a function of hydrostatic pressure (0.1−250 MPa). The patterns in all three solvents were similar. For those quenchers having reduction potentials E1/2(Q) l −1.6 V (vs the ferrocenium/ferrocene, Fc+/Fc, couple in dichloromethane), the principal bimolecular deactivation mode is energy transfer. In such cases, there is little effect of pressure on the quenching rate constant kq (Δq = ∼0 cm3 mol−1). In contrast, substantially more positive E1/2(Q) values lead to kq's approaching diffusion limits and significantly positive ΔV‡q values (up to +8 cm3 mol−1), reflecting the pressure-induced increases in solution viscosity. For E1/2(Q) ∼ −1.6 V, the dominant pressure-dependent term is electron transfer within the outer-sphere precursor complex, and the resulting charge creation in the transition state leads to substantially negative ΔV‡q values approaching −20 cm3 mol−1. © 1993, American Chemical Society. All rights reserved.

The influence of void distribution on the yielding of an elastic-plastic porous solid

Abstract In a previous study, the authors proposed an anisotropic yield function for a porous metal. The present investigation extends the previous work by deriving a rate-type constitutive law based on the proposed yield function. A comparison is also made between yield loci derived from the yield function and those determined from an FE model. The FE calculations were performed for a flat plate, with a periodic array of cylindrical holes, under biaxial loading; and also for a porous solid with a periodic array of spherical voids under axial load and superimposed hydrostatic pressure. For the plate, the agreement between the yield function and numerical results was good, less so far the three-dimensional case. Good correspondence can be obtained by introducing a scalar multiplier in the pressure-dependent term in the yield function.

Plastic Deformation of a Material with Voids and an Anisotropic Yield Function. 2nd Report. Deformation Behavior of a Unit Cell and Yield Locus.

In a previous study, the authors proposed an anisotropic yield function for a porous solid. The FE calculations were performed in this investigation for a unit cell of a porous solid and a comparison was made between yield loci derived from the proposed yield function and those determined from the FE model. For two types of FE models, a flat plate with a periodic array of cylindrical holes under biaxial loading and a porous solid with a periodic array of spherical voids under axial load and superimposed hydrostatic pressures, the agreement between the yield function and numerical results were good, especially for the flat plate. Good correspondence can be obtained by introducing a scalar multiplier in the pressure-dependent term in the yield function.

Pressure dependence of zero-field splittings in organic triplets. I. Aromatics

Optically detected magnetic resonance experiments have been conducted at pressure up to 45 kbar for p-dichlorobenzene (DCB), s-tetrachlorobenzene (TCB) and quinoxaline (Q). The pressure dependence of their zero-field splitting parameters and, for DCB, the quadrupolar splitting are reported. For all three systems the D value decreases moderately with increasing pressure. The slope for TCB is approximately twice that for DCB. These results are interpreted in terms of pressure dependence of the mesomeric effect and the inductive effect of the substituents on the aromatic triplet state. For Q, the influence of pressure on spin-orbit interaction is also suggested.