Realistic Temperature Conditions Research Articles

Dissociation of the Watson-Crick base pairs in vacuum and in aqueous solution: a first-principles molecular dynamics study

The damage of the DNA structure can affect the correct functioning of the cellular processes. This work investigates the required forces to dissociate the Watson-Crick (WC) base pairs AT into A and T, and GC into G and C. The WC base pairs are immersed in water under realistic conditions of temperature, volume, and density that reproduce the main characteristics of a biological system. The simulations are based on first-principles molecular dynamics combined with steering atomic forces. In addition to the force intensities, the charge transfers between the nucleic acid bases, energy variations, and temperature fluctuations in the cleavage moments are reported. With the purpose of evaluating the effects of the aqueous medium, simulations of the WC base pairs in vacuum are included. The results considering the solvated medium are consistent with the experimental measurements, and show the importance of the aqueous solution to regulate the structural modifications of the nucleic acid bases. The investigation contributes with a novel molecular model in molecular simulations, and to better understand the biological processes where the DNA compounds play an active role in life forms. Communicated by Ramaswamy H. Sarma

The neutrino emission from thermal processes in very massive stars in the local universe

ABSTRACT We present a new overview of the life of very massive stars (VMS) in terms of neutrino emission from thermal processes: pair annihilation, plasmon decay, photoneutrino process, bremsstrahlung, and recombination processes in burning stages of selected VMS models. We use the realistic conditions of temperature, density, electron fraction, and nuclear isotropic composition of the VMS. Results are presented for a set of progenitor stars with mass of 150, 200, and 300 M⊙Z = 0.002 and 500 M⊙Z = 0.006 rotating models which are expected to explode as a pair instability supernova at the end of their life except the 300 M⊙ would end up as a black hole. It is found that for VMS, thermal neutrino emission occurs as early as towards the end of hydrogen burning stage due to the high initial temperature and density of these VMS. We calculate the total neutrino emissivity, Qν and luminosity, Lν using the structure profile of each burning stages of the models and observed the contribution of photoneutrino at early burning stages (H and He) and pair annihilation at the advanced stages. Pair annihilation and photoneutrino processes are the most dominant neutrino energy loss mechanisms throughout the evolutionary track of the VMS. At the O-burning stage, the neutrino luminosity ∼1047−48 erg s−1 depending on their initial mass and metallicity are slightly higher than the neutrino luminosity from massive stars. This could shed light on the possibility of using detection of neutrinos to locate the candidates for pair instability supernova in our local universe.

Droplet vaporization for conventional and alternative jet fuels at realistic temperature conditions: Systematic measurements and numerical modeling

The desired reduction of fossil fuel consumption of the aviation sector requires the introduction of alternative jet fuels from renewable sources. A major hurdle for their introduction is the cost-intensive assessment of fuel effects on combustion performance, which relies on fuel-dependent processes such as atomization, vaporization and chemical reaction. The present work describes results from a newly designed experiment that provides accurate measurements of vaporization of free-falling droplets of realistic size (D ≈ 80 µm) in a vertical laminar flow with temperatures typical of technical combustors. Measurements are performed for a set of systematically chosen conventional and alternative multi-component jet fuels for which detailed compositions are available, and for three single species. The results show that the differences of their vaporization can be accurately resolved and related to physical properties. In particular, it is found that the effect of fuel boiling point on vaporization is largest for ambient gas temperatures T∞ below ≈ 1000 K, whereas for higher T∞ the influence of heat of vaporization is dominant. The well-defined boundary conditions of the experiment further enable a numerical simulation of the ambient flow and the droplet vaporization, where the latter uses a multi-component vaporization model based on a continuous thermodynamic representation of chemical species. Comparisons to measurements show that the present model accurately predicts the temporal evolution of droplet diameters and fuel-dependent effects on vaporization.

The effect of curing temperature on autogenous deformation of fly ash concretes

For concrete subjected to restrained conditions, autogenous deformation (AD) and thermal dilation (TD) are the main driving forces behind stress development in the hardening phase. Generally, AD in concrete is modelled by model codes or measured in the laboratory under 20 °C isothermal conditions, and then used as basis for stress calculations under realistic temperature conditions. To evaluate this common simplification, the current study investigated the effect of curing temperature on the AD of a series of fly ash concretes. A comprehensive test program was performed for fly ash concretes subjected to various temperature curing conditions. The model codes and measurements under 20 °C isothermal conditions provided AD developments in the same order of magnitude. However, when subjected to a realistic temperature history during curing, major changes in the AD developments were observed, emphasising that AD must be determined under relevant temperature conditions.

Structure–property relationships of the thermal gelation of partially hydrolyzed polyacrylamide/polyethylenimine mixtures in a semidilute regime

In this work, the structure–property relationships of the thermal gelation of partially hydrolyzed polyacrylamide (PHPA) and polyethylenimine (PEI) mixtures were investigated under realistic conditions of temperature (80 °C) and salinity (total dissolved solids = 3.4 g/l) of the Algerian reservoir (Tin Fouye Tabankort) prior to a conformance control application. The reactants were characterized with regard to their hydrolysis degree or branching degree using 13C-nuclear magnetic resonance, and viscosity–average molecular weights ($$ \bar{M}_{\text{v}} $$) were estimated using the Mark–Houwink equation and intrinsic viscosities measurements. The polymers had molecular weights that varied from 5 to 10 × 106 g/mol for PHPAs with initial hydrolysis degrees between 6 and 20 mol%, while the molecular weights of the PEI were between 2 and 67 × 104 g/mol with a constant branching degree of 57–59. Consequently, the effect of steady shear on the gelation time was investigated followed by the effect of reactant concentrations, the polymer and cross-linker molecular weights, the polymer’s hydrolysis degree, the temperature and the initial pH. All experiments were conducted in a semidilute concentration regime while maintaining practical initial gelant viscosities. As a result, the gelation time was found to decrease with reactant concentrations, molecular weights and temperature (Ea = 62 kJ/mol) and to increase with hydrolysis degree.

The effect of realistic curing temperature on the strength and E-modulus of concrete

The strength and E-modulus of concrete are decisive parameters when it comes to ultimate limit state design, serviceability limit state design, and early age crack assessment. The properties of concrete are generally determined in the laboratory under 20 °C isothermal conditions and then used as the basis for calculations under realistic temperature conditions. It is well-known, however, that the curing temperature affects both the rate of property development in concrete and the “final value” of a given property. The current study investigated the effect of a realistic temperature history on the compressive cube strength, the tensile strength, and the tensile E-modulus for two concretes, a reference concrete and a fly ash concrete. Concrete specimens were subjected to either (1) 20 °C isothermal curing conditions, or (2) realistic temperature curing conditions for 14 days and then 20 °C isothermal conditions, until they were tested after 28 and 91 days. Parallel tests performed in a Temperature-Stress Testing Machine were also used to evaluate the results. The reference concrete showed a general reduction in strength and E-modulus when subjected to a realistic curing temperature, whereas the fly ash concrete showed an 11% increase in the 28-day E-modulus when cured under realistic temperature conditions. Furthermore, in both isothermal and realistic curing temperature conditions, the fly ash concrete showed a pronounced property development beyond 28 days, which could not be described by the material model currently used.

Heating of Low and High Temperature PEM Fuel Cells with Alternating Current

AbstractAn innovative method for heating proton exchange membrane fuel cell (PEMFC) stacks during start‐up by direct heating of the cells is presented and investigated. By imposing an alternating current on the stack, heat is generated locally depending on the internal cell resistance. It is shown, that an alternating current (AC) perturbation with a suitable high frequency mainly heats the ohmic resistors which are membrane and contact interfaces. The electrodes are protected from voltage cycling, due to (de)charging currents of the double layer capacitances at high frequency. This AC heating technique is applied on a 6‐cell low temperature (LT) PEMFC stack and a 30‐cell high temperature (HT) PEMFC stack, both with an active area of approximately 30 cm2. Both stacks are heated under realistic temperature conditions, the LTPEMFC stack from sub‐zero temperatures to 40 °C and the HTPEMFC stack from room temperature to 120 °C. Heating times are shown and discussed for different stack geometries and voltages. In both cases, this strategy leads to a short and efficient heating, as the cells are heated directly and not externally by coolant or other stack components.

Presupernova Neutrinos: Realistic Emissivities from Stellar Evolution

Abstract We present a new calculation of neutrino emissivities and energy spectra from a massive star going through the advanced stages of nuclear burning (presupernova) in the months before becoming a supernova. The contributions from β decay and electron capture, pair annihilation, plasmon decay, and the photoneutrino process are modeled in detail, using updated tabulated nuclear rates. We also use realistic conditions of temperature, density, electron fraction, and nuclear isotopic composition of the star from the state-of-the-art stellar evolution code MESA. Results are presented for a set of progenitor stars with mass between and . It is found that β processes contribute substantially to the neutrino emissivity above realistic detection thresholds of few MeV, at selected positions and times in the evolution of the star.

Ab Initio Simulation of the Effects of Hydrogen Concentration on Anatase TiO2

We have performed first-principles calculations of hydrogen doping in anatase TiO2. Neutral and charged defects in interstitial and substitutional (for oxygen) positions have been considered, at concentrations between 0.125 and 0.03125 nH/nTi. A region of stability has been found for positively charged interstitial hydrogen, at realistic conditions of temperature and pressure. For example, at a partial pressure of hydrogen of 0.01 atm and a Fermi energy 2.3 eV above the top of the valence band, this defect is stable up to ∼500 K. Remarkably, at the highest concentration, metastable ordered substitutional neutral hydrogen leads to the appearance of bandlike states at the bottom of the conduction band, which lead to a band gap narrowing by 1 eV. On the contrary, in the presence of disorder or at lower concentration the neutral defects yield only localized defect states, located 0.7–0.9 eV below the bottom of the conduction band. Finally, the electronic structure of charged defects is very similar to that of...

Evaluation of the Potential of Osmotic Energy as Renewable Energy Source in Realistic Conditions

The possibility of using osmotic energy as renewable energy source is discussed here, and some experimental results at realistic operating conditions are presented. It is shown how a membrane-based PRO system could transform osmotic energy of seawater into electricity aided by river or wastewater. This process mainly requires novel membranes, currently in development: some of these membranes are tested in realistic conditions of temperature, salinity and different flows values which makes possible to predict expected power productions

An experimental approach for the analysis of early-age behaviour of high-performance concrete structures under restrained shrinkage

This paper presents a systematic approach for the experimental testing and analysis of the early-age thermo-mechanical behaviour of large prismatic high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint in order to enable the characterisation of high-performance concrete specimens without premature termination of the test in case of significant restrained shrinkage. The instrumentation system can measure the strains and other parameters from the setting time with high accuracy and reliability. The analysis method takes into account the temperature effects on the measured properties and provides equations to determine the time-evolution of shrinkage, thermal expansion, stiffness and creep of the concrete. Results from the testing of 200 × 200 × 1000 mm specimens made with a 0.34 water–cement ratio concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.

Theoretical insight of adsorption thermodynamics of multifunctional molecules on metal surfaces

Adsorption thermodynamics based on density functional theory (DFT) calculations are exposed for the interaction of several multifunctional molecules with Pt and Au(1 1 0)-(1 × 2) surfaces. The Gibbs free adsorption energy explicitly depends on the adsorption internal energy, which is derived from DFT adsorption energy, and the vibrational entropy change during the chemisorption process. Zero-point energy (ZPE) corrections have been systematically applied to the adsorption energy. Moreover the vibrational entropy change has been computed on the basis of DFT harmonic frequencies (gas and adsorbed phases, clean surfaces), which have been extended to all the adsorbate vibrations and the metallic surface phonons. The phase diagrams plotted in realistic conditions of temperature (from 100 to 400 K) and pressure (0.15 atm) show that the ZPE corrected adsorption energy is the main contribution. When strong chemisorption is considered on the Pt surface, the multifunctional molecules are adsorbed on the surface in the considered temperature range. In contrast for weak chemisorption on the Au surface, the thermodynamic results should be held cautiously. The systematic errors of the model (choice of the functional, configurational entropy and vibrational entropy) make difficult the prediction of the adsorption–desorption phase boundaries.

Improving the Reliability of Cement-Setting-Time Tests by Taking Into Account the Influence of Shear

Summary During a laboratory API thickening-time test of a cement slurry, temperature and pressure are simulated according to downhole conditions. However, the shear rate applied to a slurry in the API test generally differs considerably from the shear rate in actual field cementations. This leads to inaccurate prediction of slurry setting time. Cement setting times should be determined in a laboratory test under realistic conditions of temperature, pressure, and shear.

Laboratory experiments in the study of the chemistry of the outer planets

The investigation of chemical evolution of bodies in our solar system has, in the past, included observations, theoretical modeling, and laboratory simulations. Of these programs, the last one has been the most criticized due to the inherent difficulties in accurately recreating alien environments in the laboratory. Processes such as wall reactions and changes in chemistry due to difficulties in achieving realistic conditions of temperature, pressure, composition, and energy flux may yield results which are not truly representative of the systems being modeled. However, many laboratory studies have been done which have yielded data useful in planetary science. Gross simulations of atmospheric chemistry have placed constraints on the nature of complex molecules expected in planetary atmospheres. More precise studies of specific chemical processes have provided information about the sources and properties of product gases and aerosols. Determinations of basic properties such as spectral features and reaction rate constants yield data useful in the interpretation of observations and in computational modeling. Alone, and in conjunction with modeling, laboratory experiments will continue to be used to further our understanding of the outer solar system, and some experiments that need to be done are listed.

Aspects of nitrogen fixation in Lough Neagh. II. Competition between Aphanizomenon flos‐aquae, Oscillatoria redekei and Oscillatoria agardhii

SUMMARY. The effect of nutrient availability on the growth of natural samples of Lough Neagh plankton (Aphanizomenon flos‐aquae, initial innoculum 0.98 mm3 l‐1, Oscillatoria redekei, 2.52 mm3 l‐1 and Oscillatoria agardhii, 4.96 mm3 l‐1) was studied, using ecologically realistic conditions of temperature (15°C), nitrate‐N (<0.35g m‐3) and orthophosphate‐P (<0.55g m‐3), in which the combinations –N–P, –N+P, +N–P, and +N+P were used. After 8 days A. flos‐aquae had become dominant in treatments lacking N. After 12 days with N available the mean yields of O. redekei (37.7 mm3 l‐1) and O. agardhii (10.82 mm3 l‐1) were significantly greater (P=5%) than their respective yields (3.6 and 2.6 mm3 l‐1) in the absence of N. Somewhat surprisingly the mean yield of A. flos‐aquae when N was present (0.15 mm3 l‐1), was significantly less (P=5%) than in the absence of N (13.26 mm3l‐1). The relative rates and duration of growth, the availability of P and N, and heterocyst frequency as an index of nitrogen fixation are considered in relation to the seasonal succession and dominance of these three principal cyanophyte members of the Lough Neagh phytoplankton.

An approach to in situ fourier-transform infrared spectroscopic studies of solid/gas interfaces under realistic conditions of temperature and pressure

In situ infrared spectroscopic studies of the solid/gas interface (adsorbates, intermediates) are difficult to perform under realistic conditions of temperature and pressure since the desired information can be lost in the spectral information of the solid and the gas phases. With the aid of Fourier transform IR spectroscopy it is possible to filter out those solid-phase and gas-phase contributions and to produce detailed spectra of the monomolecular interface.