Dynamics Of Supercritical Water Research Articles

Molecular Dynamics of Supercritical Water for Nuclear Data Development

Abstract The Canadian supercritical water-cooled reactor was selected as one of the Generation IV International Forum initiatives for reactor design. It uses supercritical light water as a coolant under operating conditions of 25 MPa (250 bar) and 623–898 K. However, the simulation codes used to assess the performance and safety of such a design depend upon the accuracy of available nuclear data parametrizations, which currently do not include models of light water in the supercritical regime. In this paper, we present a study of supercritical water (SCW) through molecular dynamics simulations. Flexible variants of the TIP4P/2005 and simple point charge models for H2O are assessed to determine their ability to reproduce experimental measurements of SCW properties, and their suitability for the future development of nuclear data parametrizations for thermal neutron scattering from SCW. Planned experiments measuring thermal neutron scattering from SCW to inform nuclear data development are also summarized.

Hydrogen Dynamics in Supercritical Water Probed by Neutron Scattering and Computer Simulations.

In this work, an investigation of supercritical water is presented combining inelastic and deep inelastic neutron scattering experiments and molecular dynamics simulations based on a machine-learned potential of ab initio quality. The local hydrogen dynamics is investigated at 250 bar and in the temperature range of 553-823 K, covering the evolution from subcritical liquid to supercritical gas-like water. The evolution of libration, bending, and stretching motions in the vibrational density of states is studied, analyzing the spectral features by a mode decomposition. Moreover, the hydrogen nuclear momentum distribution is measured, and its anisotropy is probed experimentally. It is shown that hydrogen bonds survive up to the higher temperatures investigated, and we discuss our results in the framework of the coupling between intramolecular modes and intermolecular librations. Results show that the local potential affecting hydrogen becomes less anisotropic within the molecular plane in the supercritical phase, and we attribute this result to the presence of more distorted hydrogen bonds.

Flow structure and bubble dynamics in supercritical water fluidized bed and gas fluidized bed: A comparative study

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.

Effect of the local hydrogen bonding network on the reorientational and translational dynamics in supercritical water

Molecular dynamics simulations have been performed in a wide range of densities along a near critical isotherm of supercritical water in order to reveal the interconnection between the local hydrogen bonding (HB) network and several related dynamic properties. The results obtained have revealed a significant slowing down of reorientational dynamics of the water molecules as the value of the number of hydrogen bond per molecule increases and this is reflected on the increase in the reorientational correlation times. The calculated reorientational times exhibit also an increasing trend by increasing the bulk density, and this effect is more pronounced in the case of the first-order Legendre reorientational correlation functions. A clear nonlinear dependence of the librational mode frequencies of the water molecules on the augmented local density around them has also been revealed. This result could be regarded as an additional support of experimental observations suggesting the use of a nonlinear relation when analyzing the density dependence of spectroscopic peak frequencies in order to extract information about local density augmentation in supercritical fluids. The HB dynamics have been also investigated, revealing a plateau in the calculated HB lifetimes at intermediate and higher liquidlike densities and a small increase at low, gaslike densities.

A large-scale molecular dynamics study of dynamic structure factor and dispersion relation of acoustic mode in liquid and supercritical water

Large-scale and long-time molecular dynamics calculations have been performed using a fluctuating charge model TIP4P-FQ in order to investigate structure and dynamics of supercritical water near the critical point. The calculated Ornstein–Zernike behavior of the structure factor S ( k ) in the small wave number region was in satisfactory agreement with experiment. The intermediate scattering function F ( k , t ) showed much slower decay in the supercritical water than that found in the ambient water. Further, the calculated dynamic structure factor S ( k , ω ) clearly showed the existence of acoustic mode for both systems. The calculated dispersion relation for the liquid water is in good agreement with the IXS data. In the supercritical water near the critical point, the acoustic excitation propagates with a speed of about 420 m/s.

高圧下ＮＭＲ 超臨界水の構造とダイナミクスのＮＭＲによる研究

A high-temperature and high-pressure NMR method to investigate the structure and dynamics of supercritical water is reviewed. In this method, a high-temperature condition is realized by introducing hot air directly into the sample portion of a specially designed probe, and a high pressure is achieved by sealing the material of interest into a quartz capillary. The method allows a high-resolution measurement up to 400°C and 0.6 g/cm3 (corresponding to 55 MPa) of water. It is found from the proton chemical shift measurement that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one in the supercritical densities. The measurement of the spin-lattice relaxation time also shows that while the reorientational relaxation proceeds on the order of picosecond in ambient water, it does on the order of several tens of femtoseconds in supercritical water.

Density dependence of reorientational dynamics in supercritical water

A series of molecular dynamics (MD) simulations have been carried out in order to examine the density dependence of reorientational motions in supercritical water. The SPC/E water model was assumed. The reorientational correlation times resulting from MD simulations are compared with recent dielectric relaxation and NMR spectroscopy data. Special interest is devoted to analyse the behaviour of the reorientational correlation times in the low density region. The degree of anisotropy of molecular reorientations in supercritical water is also discussed.

超臨界水の構造・ダイナミクス・反応

The hydrogen bonding structure and dynamics of supercritical water is studied by means of high-temperature and high-pressure NMR. The proton chemical shift is measured over a wide range of thermodynamic conditions and is related to the number of hydrogen bonds in supercritical water with the help of computer simulations. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one in the supercritical densities. The spin-lattice relaxation time is further measured to determine the reorientational correlation time. It is shown that while the reorientational relaxation proceeds on the order of picosecond in ambient water, it does on the order of several tens of femtoseconds in supercritical water. The role of water in noncatalytic reactions in hydrothermal conditions is also examined by focusing on the dehydration of 1, 4-butanediol into tetrahydrofuran, which proceeds under the presence of a strong acid at ambient conditions. The water-induced and acid-catalyzed rate constants are separately determined, and water is proven to promote the reaction in its undissociated form.

Proton dynamics in supercritical water

An inelastic neutron scattering experiment has been performed on supercritical water at high momentum transfer, up to 90 Å−1, in order to study single proton dynamics. The value of the proton mean kinetic energy has been extracted in the framework of the impulse approximation. The anisotropy of the proton momentum distribution inside a single water molecule is discussed. The extracted experimental mean kinetic energy is found in good agreement with the predictions of a harmonic model, under the assumptions of decoupling between translational, rotational and vibrational degrees of freedom. Differences emerge between our results and those obtained in a recent inelastic neutron scattering experiment on water in sub- and supercritical conditions. These differences are pointed out and examined in detail.

Structural study of supercritical water. III. Rotational dynamics

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O–D axis of a single water molecule. It is then found that while τ2R decreases rapidly with the temperature on the liquid branch of the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400 °C. The comparison of τ2R with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.

流体のダイナミクス 超臨界水の動的構造解析

The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T1 of heavy water (D2O). The experimentally determined T1 is shown to be governed by the quadrupolar mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time τ2R of the O-D axis. It is then found that while τ2R decreases rapidly with the temperature on the saturation curve, it remains on the order of several tens of femtoseconds when the density is varied at a temperature above the critical. The comparison of τ2R with the angular momentum correlation time shows that the inertial effect is operative in the rotational dynamics of supercritical water. The dependence of τ2R on the hydrogen bonding state is also examined in combination with computer simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale. Actually, although τ2R is divergent in the limit of zero density, it is observed to increase with the density when the density is above ∼1/3 of the critical.

流体のダイナミクス 超臨界水・水溶液の計算機シミュレーション

In the recent decade, many computational studies as well as physicochemical experiments such as neutron diffraction and NMR spectroscopic measurements have actively been done in order to clarify the structure and dynamics of supercritical water and its solutions. In particular, studies on hydrogen bonding in supercritical water have been attracting much interest from physical chemists. Long-ranged structure and the relevant collective motion have also been investigated in detail. For supercritical solutions, thermodynamic properties such as solubility and the microscopic structure of ion hydration have been studied. Here, recent progress in computational studies on the structure and dynamics of the fluids is reviewed.

Ion solvation dynamics in supercritical water

A theoretical study of ionic solvation dynamics in supercritical water is presented. The short-time decay of the calculated equilibrium solvation energy time correlation function ( S E ( t)) is found to be in good agreement with the recent computer simulation results. The simulated long-time decay is, however, somewhat slower than that which is predicted by the present theory. Two plausible molecular explanations for the slow long-time decay of S E ( t) are offered.