Quasi-equilibrium Conditions Research Articles

Shear-rate dependence of thermodynamic properties of the Lennard-Jones truncated and shifted fluid by molecular dynamics simulations

It was shown recently that using the two-gradient method, thermal, caloric, and transport properties of fluids under quasi-equilibrium conditions can be determined simultaneously from nonequilibrium molecular dynamics simulations. It is shown here that the influence of shear stresses on these properties can also be studied using the same method. The studied fluid is described by the Lennard-Jones truncated and shifted potential with the cut-off radius rc* = 2.5σ. For a given temperature T and density ρ, the influence of the shear rate on the following fluid properties is determined: pressure p, internal energy u, enthalpy h, isobaric heat capacity cp, thermal expansion coefficient αp, shear viscosity η, and self-diffusion coefficient D. Data for 27 state points in the range of T ∈ [0.7, 8.0] and ρ ∈ [0.3, 1.0] are reported for five different shear rates (γ̇ ∈ [0.1,1.0]). Correlations for all properties are provided and compared with literature data. An influence of the shear stress on the fluid properties was found only for states with low temperature and high density. The shear-rate dependence is caused by changes in the local structure of the fluid which were also investigated in the present work. A criterion for identifying the regions in which a given shear stress has an influence on the fluid properties was developed. It is based on information on the local structure of the fluid. For the self-diffusivity, shear-induced anisotropic effects were observed and are discussed.

Impact of conformational structures on primary decomposition of cis-1,2-dimethylcyclohexyl isomers: A theoretical study

The different orientations of two methyl groups in “strain-free” cyclic structure generate multiple conformational structures for dimethyl-substituted cyclohexanes. These conformational structures are most likely to affect the radical stabilities, activation energies, and rate coefficients of key types of reactions in dimethyl-substituted cyclohexane combustion. The conformational inversion-topomerization mechanism among various conformers for cis-1,2-dimethylcyclohexyl isomers has been explored by applying high-level quantum electronic-structure methods and transition state theory (TST). Intramolecular H-transfers and β-scissions were also investigated to fundamentally unravel the way how the conformational structures impact their initial decomposition. The present kinetic predictions show that conformational changes are much more rapid compared with the primary decomposition of cis-1,2-dimethylcyclohexyl isomers. It contributes to the establishment of quasi-equilibrium condition for various conformers retained in each radical and ensures the coexistence of all conformers over 300–2500 K. For the primary decomposition, the intramolecular H-transfers are greatly influenced by the conformational structures. Of particular interest is to observe that 1,4 and 1,5 H-transfers that shift the radical site between side chain and ring are only feasible for chair and twist-boat conformers with the radical site locating in axial side chain. Additionally, the β-scissions of cis-1,2-dimethylcyclohexyl isomers also exhibit the dependence on the conformational structures in aspect of steric energy and substituent effect. Furthermore, facilitated by the speedy equilibration among distinct conformers for each isomer, the contribution of each conformer to kinetic predictions for the initial decomposition was systemically evaluated in terms of the temperature-dependent population for diverse conformers obtained by Boltzmann distribution, and then the appropriate rate parameters for each decomposition type were finally recommended.

A Novel Reentry Trajectory Generation Method Using Improved Particle Swarm Optimization

This paper proposes a new reentry trajectory generation method, which mainly comprises two parts: reentry profile design and trajectory optimization. Different from conventional methods that design angle-of-attack (AOA) and bank-angle profiles, a height-range profile is proposed in this paper. Using this profile, the AOA can be directly derived and the quasi-equilibrium glide condition is easily satisfied. In addition, the bank angle is analytically derived with the heading error, with only one reversal. Finally, the original problem is transformed into a parameter optimization problem and solved by an improved particle swarm optimization (PSO) method. In PSO, a comparison principle is proposed to tackle constraints and three modifications are applied to the swarm evolution to enhance the searching capacity. To conquer disturbances, an online profile updating strategy is developed based on the nominal profile, so complicated work of designing a trajectory tracker is avoided. Simulation validates the effectiveness of the proposed method and demonstrates that the improved PSO is comparable to the pseudospectral method and is better than the basic PSO.

Identifying Climate-Induced Groundwater Depletion in GRACE Observations

Depletion of groundwater resources has been identified in numerous global aquifers, suggesting that extractions have exceeded natural recharge rates in critically important global freshwater supplies. Groundwater depletion has been ascribed to groundwater pumping, often ignoring influences of direct and indirect consequences of climate variability. Here, we explore relations between natural and human drivers and spatiotemporal changes in groundwater storage derived from the Gravity Recovery and Climate Experiment (GRACE) satellites using regression procedures and dominance analysis. Changes in groundwater storage are found to be influenced by direct climate variability, whereby groundwater recharge and precipitation exhibited greater influence as compared to groundwater pumping. Weak influence of groundwater pumping may be explained, in part, by quasi-equilibrium aquifer conditions that occur after “long-time” pumping, while precipitation and groundwater recharge records capture groundwater responses linked to climate-induced groundwater depletion. Evaluating groundwater response to climate variability is critical given the reliance of groundwater resources to satisfy water demands and impending changes in climate variability that may threaten future water availability.

Rapid trajectory planning of a reusable launch vehicle for airdrop with geographic constraints

Online feasible trajectory generation for an airdrop unpowered reusable launch vehicle is addressed in this article. A rapid trajectory planning algorithm is proposed to satisfy not only the multiple path and terminal constraints but also the complex geographic constraints of waypoints and no-fly zones. Firstly, the lower and upper boundaries of the bank angle that implement all the path constraints are obtained based on the quasi-equilibrium glide condition. To determine the bank angle directly, a weighted interpolation of the boundaries is then developed, which provides an effective approach to simplify the planning process as a one-parameter search problem. Subsequently, three types of lateral planning algorithms are designed to determine the sign of the bank angle according to the requirements of waypoints passage, no-fly-zones avoidance, and terminal constraints in the airdrop process, and the convergence of these methods for passing over the waypoints and meeting the terminal conditions has been clarified and formally demonstrated. Considering the constraints in the actual airdrop flight missions, the planning trajectory is divided into several subphases to facilitate the application of corresponding algorithms. Finally, the performance of the proposed algorithm is assessed through three airdrop missions of reusable launch vehicle with different geographic constraints. Besides, the effectiveness of the algorithm is demonstrated by the Monte Carlo simulation results.

Specific Features of the Electron Structure of ZnTPP Aggregated Forms: Data of Optical Measurements and Quantum-Chemical Calculations

Zinc tetraphenylporphyrin (ZnTPP) films and ZnTPP-based composites are promising materials of organic photonics. Porphyrins are inclined towards self-assembly and the formation of molecular ensembles or differently structured aggregates. The energies of the formation of aggregates and ordered structures and modifications of their electron structures compared to that of a free ZnTPP molecule have not been adequately explored. In the study, the first comprehensive investigation of the structure, absorption and luminescence spectra, and photoluminescence kinetics of structurally perfect ZnTPP thin films produced under quasi-equilibrium conditions in vacuum is conducted. It is shown that changes in the absorption spectra and the red shift of the luminescence spectra of films by 0.15 eV from the spectra observed for ZnTPP solutions in toluene can be interpreted as a result of the formation of an ordered thin-film structure through the dimerization of porphyrin planar molecules under nearly equilibrium conditions. The optimal geometric structure, the energy of the ground state, and the electronic spectrum of excitations of the dimerized (ZnTPP)2 state are calculated in the context of density functional theory and time-dependent density functional theory. The energy gain on the formation of a dimer of symmetry Ci compared to two separate molecules is 0.23 eV per dimer; the HOMO–LUMO gap for the dimer is decreased by 70 meV. The radiative emission time in the ordered solid phase is an order of magnitude shorter than that in the solution in toluene and corresponds to 277 ps, which is typical of J aggregates of porphyrins.

Quantification of Mutual Mass Transfer of CO2/N2–Light Oil Systems by Dynamic Volume Analysis

A novel and pragmatic method has been developed to quantify mutual mass transfer between a gas (e.g., CO2 and N2) and light oil by dynamic volume analysis. Experimentally, diffusion experiments for pure CO2/N2-light oil systems are conducted at a constant pressure and temperature with a pressure/volume/temperature (PVT) system. During the diffusion experiments, the diluted volume of oil phase is continuously monitored and recorded, while gas samples are collected at the end of the diffusion tests to measure gas compositions by performing gas chromatography (GC) analysis. Theoretically, the mass transfer from light oil to gas phase and the solubility of a gas into the light oil is quantified by combining the GC analysis and Fick’s second law, while the Peng–Robinson equation of state is employed to determine the concentration of each gas component in the oil phase by imposing the quasi-equilibrium boundary condition at the gas–oil interface. The diffusion coefficient of each component can be determined onc...

Nanostructure Growth in the Ga(In)AsP–GaAs System under Quasi-Equilibrium Conditions

The formation of nanostructures on the surface of GaAs under quasi-equilibrium conditions in a quasi-closed volume from saturated phosphor and indium vapors in the presence of a Au catalyst with growth according to the “vapor–liquid–crystal” mechanism is considered for the first time. The influence of the growth temperature and size of Au drops on the morphology and composition of the fabricated nanostructures is studied. Experimental data on the formation of Ga(In)AsP nanocrystals on GaAs substrates with various orientations are presented. It is established that the temperature growth range of the nanostructures when using this method is 540–640°C with a drop size from 30 to 120 nm. It is shown that the size of the catalyst drops substantially affects the morphology and growth rate of the fabricated nanostructures while their composition weakly depends on both the drop size and the substrate orientation.

Thermodynamic modeling of processes in carbide-forming systems Ti–C–H–N and Ti–O–C–H–N

Thermodynamic analysis of high-temperature interactions in Ti–C–H–N, Ti–O–C–H–N systems was carried out, corresponding to the processes of titanium carbide synthesis in the conditions of plasma flow using various titanium-containing raw materials. Modeling processes in these carbide-forming systems showed that the formation of titanium carbide is possible in equilibrium and quasi-equilibrium conditions, at temperatures below 3500 K, with stoichiometric and excess carbon according to gas-phase reaction involving titanium and cyanide vapour. The possibility of achieving a high yield of titanium carbide in real synthesis conditions is predicted.

Localized transition models in luminescence: A reappraisal

Localized energy levels within the forbidden energy band are the source of various stimulated luminescence phenomena. The present study deals with the case of electrons stimulated from localized levels and recombining with a hole at a luminescence center, without the mediation of the conduction band. Previous research was based on three different assumptions as follows. Firstly, it was assumed that the recombination rate is independent of the concentration of recombination centers, and is proportional only to the concentration of electrons in the excited state of the trap. Secondly, it is assumed that the principle of detailed balance holds for these localized transitions. A third common assumption is that the system is in quasi-equilibrium condition. When these three conditions are applied to the system of differential equations describing the localized transitions, it was shown that the resultant thermoluminescence (TL) signals follow first order kinetics. This paper examines the assumptions used in these previous studies, and extensive simulations are carried out for a wide range of parameters in the localized transitions model. The results of the simulations show that the TL peaks in the localized model have very similar characteristics with TL peaks derived from delocalized models, including non-first order kinetic characteristics. The differential equations describing the localized transition model are solved analytically using the Lambert W function, and the resulting analytical master equation can describe a variety of optically and thermally stimulated phenomena.

Study on the Ability of PCMSR to Produce Valuable Isotopes as a By Product of Energy Generation

PCMSR (Passive Compact Molten Salt Reactor) is a variant of MSR (Molten Salt Reactor) type reactors. The MSR is one type of the Advanced Nuclear Reactor types. PCMSR uses mixtures of fluoride salt if the liquid form is in a high temperature operation. The use of liquid salt fuel allows the application of on line fuel processing system. The on line fuel processing system allows extraction of several valuable fission product isotopes such as Mo-99, Cs-137, Sr-89 etc. The capability of MSR to produce several valuable isotopes has been studied. This study is based on a denaturized breeder MSR design with 920 MWth of thermal power and 500 MWe of electrical output power with the thermal efficiency of 55 %. The initial composition of fuel salt is 70 % of a mole of LiF, 24 % of a mole of 232ThF4, 6 % of a mole of UF4. The enrichment level of U is 20 % of a mole of U-235. The study is performed by a numerical calculation to solve a set of differential equations of fission product balance. This calculation calculates fission product generation due to fission reaction, precursor decay, and fission product annihilation due to decay, neutron absorption, and extraction. The calculation result shows that in quasi equilibrium conditions, the reactor can produce several valuable isotopes in substantially sufficient quantities, those are Sr-89 (0.3 kCi/MWth/day, Sr-90 (1,91 Ci/MWth/day), Mo-99 (1.7 kCi/MWth/day), I-131 (0.42 kCi/MWth/day), I-132 (0.782 kCi/MWth/day), I-133 (1.12 kCi/MWth/day), Xe-133 (11.8 Ci/MWth/day), Cs-134 (39.3 mCi/MWth/day), Cs-137 (2.32 Ci/MWth/day) and La-140 (1.05 kCi/MWth/day).

Trapping/detrapping kinetic rates of hydrogen around a vacancy in nickel and some consequences on the hydrogen-vacancy clusters thermodynamic equilibrium

The jump rates of H in the interstitial sites of a perfect crystal and in the vicinity of a vacancy in nickel have been determined accurately from ab initio calculations up to 1200 K. The migration free energy of H implemented in the Wert and Zener theory for interstitial diffusion is expressed as a sum of static energy, thermal expansion, vibration and electronic contributions. We show that the thermal expansion and the vibrations of the lattice are the major corrections to the migration energy at finite temperature. The computation of the jump rates indicates that the ratio between the trapping and the detrapping frequencies decrease from ∼104 at 300 K to ∼100 at 1200 K. The effects of the elastic displacement field induced by the vacancy on the jump kinetic rates are minor and the solute diffuses similarly to the perfect crystal out of the vacancy core. Finally, we implement a macroscopic rate equation model to evaluate the concentration of mobile and trapped hydrogen atoms as function of time in a one-dimension sample of 1 μm. The comparison of the H-vacancy cluster concentrations formed during H diffusion with the concentrations given by the quasi-equilibrium condition indicates that the system can develop internal stresses at low temperature due to an oversaturation of H-poor defects. These internal stress fields may be associated with the formation of superabundant vacancies and voids implied on the H embrittlement processes.

Investigating the Scale Adaptivity of a Size-Filtered Mass Flux Parameterization in the Gray Zone of Shallow Cumulus Convection

Abstract In this study, the scale adaptivity of a new parameterization scheme for shallow cumulus clouds in the gray zone is investigated. The eddy diffusivity/multiple mass flux [ED(MF)n] scheme is a bin-macrophysics scheme in which subgrid transport is formulated in terms of discretized size densities. While scale adaptivity in the ED component is achieved using a pragmatic blending approach, the MF component is filtered such that only the transport by plumes smaller than the grid size is maintained. For testing, ED(MF)n is implemented into a large-eddy simulation (LES) model, replacing the original subgrid scheme for turbulent transport. LES thus plays the role of a nonhydrostatic testing ground, which can be run at different resolutions to study the behavior of the parameterization scheme in the boundary layer gray zone. In this range, convective cumulus clouds are partially resolved. The authors find that for quasi-equilibrium marine subtropical conditions at high resolutions, the clouds and the turbulent transport are predominantly resolved by the LES. This partitioning changes toward coarser resolutions, with the representation of shallow cumulus clouds gradually becoming completely carried by the ED(MF)n. The way the partitioning changes with grid spacing matches the behavior diagnosed in coarse-grained LES fields, suggesting that some scale adaptivity is captured. Sensitivity studies show that the scale adaptivity of the ED closure is important and that the location of the gray zone is found to be moderately sensitive to some model constants.

A high precision extrapolation method in multiphase-field model for simulating dendrite growth

The phase-field method coupling with thermodynamic data has become a trend for predicting the microstructure formation in technical alloys. Nevertheless, the frequent access to thermodynamic database and calculation of local equilibrium conditions can be time intensive. The extrapolation methods, which are derived based on Taylor expansion, can provide approximation results with a high computational efficiency, and have been proven successful in applications. This paper presents a high precision second order extrapolation method for calculating the driving force in phase transformation. To obtain the phase compositions, different methods in solving the quasi-equilibrium condition are tested, and the M-slope approach is chosen for its best accuracy. The developed second order extrapolation method along with the M-slope approach and the first order extrapolation method are applied to simulate dendrite growth in a Ni-Al-Cr ternary alloy. The results of the extrapolation methods are compared with the exact solution with respect to the composition profile and dendrite tip position, which demonstrate the high precision and efficiency of the newly developed algorithm. To accelerate the phase-field and extrapolation computation, the graphic processing unit (GPU) based parallel computing scheme is developed. The application to large-scale simulation of multi-dendrite growth in an isothermal cross-section has demonstrated the ability of the developed GPU-accelerated second order extrapolation approach for multiphase-field model.

New topology of levyne B under quasi-equilibrium conditions: A temperature-dependent in situ single crystal X-ray diffraction study

The dehydration behavior of natural levyne-Ca was investigated from 25 to 400 °C by in-situ single-crystal X-ray diffraction under dry conditions. The dehydration started at 50 °C. From 125 to 250 °C the structure changed to levyne B topology, characterized by statistical breaking of T1-O-T1 bonds in the original D6R units. At 275 °C, additional rupture of T2-O-T2 bonds in 6mR units started. This second rupture associated with the release of remaining water was not reported before. At 300 °C the structure is assumed to be anhydrous. The new levyne B topology, referred to levyne B′, is characterized by 37% T1B and 15% T2B that are new tetrahedral sites arising as a consequence of the T-O-T breaking process. In contrast to the T1B site, T2B is at general position, originating a system of T2B face sharing tetrahedra, mutually exclusive. Complete migration of T sites to the new positions would lead to different LEV topologies with different kind of cages and, as a consequence, of stacking sequences depending on the position occupied by the T2B tetrahedra.The different results obtained in this study compared to previous ones, confirmed that the applied experimental set-up is fundamental in tracking the transformations upon dehydration in zeolitic materials. Understanding these differences is crucial for subsequent applications of these microporous compounds.

Рост наноструктур в системе Ga(In)AsP-GaAs в квазиравновесных условиях

AbstractThe formation of nanostructures on the surface of GaAs under quasi-equilibrium conditions in a quasi-closed volume from saturated phosphor and indium vapors in the presence of a Au catalyst with growth according to the “vapor–liquid–crystal” mechanism is considered for the first time. The influence of the growth temperature and size of Au drops on the morphology and composition of the fabricated nanostructures is studied. Experimental data on the formation of Ga(In)AsP nanocrystals on GaAs substrates with various orientations are presented. It is established that the temperature growth range of the nanostructures when using this method is 540–640°C with a drop size from 30 to 120 nm. It is shown that the size of the catalyst drops substantially affects the morphology and growth rate of the fabricated nanostructures while their composition weakly depends on both the drop size and the substrate orientation.

Weakly turbulent plasma processes in the presence of inverse power-law velocity tail population

Observations show that plasma particles in the solar wind frequently display power-law velocity distributions, which can be isotropic or anisotropic. Particularly, the velocity distribution functions of solar wind electrons are frequently modeled as a combination of a background Maxwellian distribution and a non-thermal distribution which is known as the “halo” distribution. For fast solar wind conditions, highly anisotropic field-aligned electrons, denominated as the “strahl” distribution, are also present. Motivated by these observations, the present paper considers a tenuous plasma with Maxwellian ions, and electrons described by a summation of an isotropic Maxwellian distribution and an isotropic Kappa distribution. The formalism of weak turbulence theory is utilized in order to discuss the spectra of electrostatic waves that must be present in such a plasma, satisfying the conditions of quasi-equilibrium between the processes of spontaneous fluctuations and of induced emission. The kappa index and relative density of the Kappa electron distribution are varied. By taking into account the effects due to electromagnetic waves into the weak turbulence formalism, we investigate the electromagnetic spectra that satisfy the conditions of “turbulent equilibrium,” and also the time evolution of the wave spectra and of the electron distribution, which occurs in the case of the presence of an electron beam in the electron distribution.

Entry trajectory generation without reversal of bank angle

A rapid entry trajectory planning algorithm for hypersonic glide vehicles is presented in this paper, and the avoidance of no-fly zones with large cross-range is achieved. The highly-constrained entry trajectory generation mission is accomplished using an adjustable angle of attack and bank, instead of a nominal angle of attack and reversal logic of bank angle. In this way, the maneuvering capability of the vehicle is greatly improved. Moreover, as no reversal of bank angle exists in the whole flight, the control command is easier to follow. Firstly, longitudinal entry corridor with an adjustable angle of attack is built up. The drag acceleration profile is represented as five piecewise affine functions of the energy, which are adjusted to meet the requirements of the trajectory length and lateral maneuvering capability. Secondly, based on the quasi-equilibrium glide condition (QEGC), the maximum and minimum boundary of the lateral lift-to-drag corridor corresponding to the current drag acceleration profile is approximated, and the lateral profile is represented as an interpolation result of the boundaries. Finally, the parameters of longitudinal and lateral profile are found by the Newton iteration scheme with the help of a reduced order system. A combined proportional integral derivative (PID) tracker is also designed to follow the drag and heading angle profile. The approach is tested using the Common Aero Vehicle model. Simulation results demonstrate that the generated entry trajectories can avoid no-fly zone with a large cross-range and achieve the terminal conditions while satisfying all the complex path constraints.

Feasible footprint generation with uncertainty effects

Landing footprint is critical in generating feasible entry trajectories for hypersonic glide vehicles. In this paper, a new landing footprint generation algorithm that considers multiple uncertainty effects is proposed, based on the improved 3D acceleration profile planning method. First, a new entry corridor with uncertainty effects is derived, in which the angle of attack profile is adjustable at any time during the entire flight. Second, the longitudinal drag profile is designed as the interpolation results of the upper and lower fitting safe boundaries. The corresponding lateral lift-to-drag corridor is obtained using the quasi-equilibrium glide condition. A combined Proportion Integration Differentiation (PID) tracker is used to follow the planned profiles in the longitudinal and lateral corridors, and the feasible entry trajectories are completed. Finally, feasible footprint is generated by repeatedly computing the reachable boundaries for all the profiles in the new safe corridor, as well as the analytical calculation of the maximum range point. The approach is tested using the Common Aero Vehicle-H model. Simulation results demonstrate that the proposed algorithm can rapidly generate a feasible footprint of entry for vehicles while satisfying all the path and terminal constraints.

The nature of plateau on time-of-flight curves in molecularly doped polymers

Transient current curves in a polar molecularly doped polymer for surface and bulk generations of charge carriers are compared. No transformation of the horizontal plateau on the former to the curve predicted by the diffusion and drift theory was found, contrary to expectations for quasi-equilibrium transport conditions. This behavior indicates a nonequilibrium character of the transport of charge carriers. The plateau appearance is explained in our case by the depleted subsurface layer effect, as we already pointed out earlier.