Equilibrium evaporation coefficients quantified as transmission probabilities for monatomic fluids

Equilibrium molecular dynamics (MD) simulations are used to investigate the liquid/vapor interface where particleexchange between the liquid and vapor phase is quantified in terms of the evaporation and condensation coefficient.The coefficients are extracted from MD simulations via a particle counting procedure. This requires defining avapor boundary position for which we introduce an accurate and robust method and present a comparative studywith existing methods from the literature. This novel method relies on the behavior of the flux coefficient withinthe interphase region by scanning the position of a particle sink boundary from the liquid towards the vaporphase. We find a distinct local maxima is attained on the vapor side of the interphase that is identified as thevapor boundary position based on an interpretation of transmission probability theory and the Kullback-Leiblerdivergence. The ratio of the evaporation flux to the outgoing flux at this location is defined as the evaporationcoefficient. This method retains the simplicity of existing methods but eliminates several disadvantages. We applythis method to MD simulations of monatomic fluids neon, argon, krypton and xenon. We observe a correlationbetween the molecular transport parameter appearing in transmission probability theory and the characteristicinterface fluctuation length scale from capillary wave theory. This gives an expression for the evaporation coefficientthat agrees well with values extracted from MD using the particle counting procedure. Compared to existingmethods, the evaporation/condensation coefficient is determined more accurately for temperatures between thetriple and critical points.Equilibrium molecular dynamics (MD) simulations are used to investigate the liquid/vapor interface where particle exchange between the liquid and vapor phase is quantified in terms of the evaporation and condensation coefficient. The coefficients are extracted from MD simulations via a particle counting procedure. This requires defining a vapor boundary position for which we introduce an accurate and robust method and present a comparative study with existing methods from the literature. This novel method relies on the behavior of the flux coefficient within the interphase region by scanning the position of a particle sink boundary from the liquid toward the vapor phase. We find a distinct local maxima is attained on the vapor side of the interphase that is identified as the vapor boundary position based on an interpretation of transmission probability theory and the Kullback–Leibler divergence. The ratio of the evaporation flux to the outgoing flux at this location is defined as the evaporation coefficient. This method retains the simplicity of existing methods but eliminates several disadvantages. We apply this method to MD simulations of monatomic fluids neon, argon, krypton, and xenon. We observe a correlation between the molecular transport parameter appearing in the transmission probability theory and the characteristic interface fluctuation length scale from the capillary wave theory. This gives an expression for the evaporation coefficient that agrees well with values extracted from MD using the particle counting procedure. Compared to existing methods, the evaporation/condensation coefficient is determined more accurately for temperatures between the triple and critical points

Computation of accommodation coefficients and the use of velocity correlation profiles in molecular dynamics simulations

For understanding the behavior of a gas close to a channel wall it is important to model the gas-wall interactions as detailed as possible. When using molecular dynamics simulations these interactions can be modeled explicitly, but the computations are time consuming. Replacing the explicit wall with a wall model reduces the computational time but the same characteristics should still remain. Elaborate wall models, such as the Maxwell-Yamamoto model or the Cercignani-Lampis model need a phenomenological parameter (the accommodation coefficient) for the description of the gas-wall interaction as an input. Therefore, computing these accommodation coefficients in a reliable way is very important. In this paper, two systems (platinum walls with either argon or xenon gas confined between them) are investigated and are used for comparison of the accommodation coefficients for the wall models and the explicit molecular dynamics simulations. Velocity correlations between incoming and outgoing particles colliding with the wall have been used to compare explicit simulations and wall models even further. Furthermore, based on these velocity correlations, a method to compute the accommodation coefficients is presented, and these newly computed accommodation coefficients are used to show improved correlation behavior for the wall models

Exploring the Electronic Structure of New Doped Salt Hydrates, Mg1–xCaxCl2·nH2O, for Thermochemical Energy Storage

Chloride-based salt hydrates, MgCl2·nH2O and CaCl2·nH2O (n = 0,1,2,4,6), are promising materials for thermochemical heat storage systems due to their high sorption energy capacity. However, both salts have their own shortcoming characteristics within the operational temperature of the thermochemical heat storage applications. While the higher hydrates of CaCl2·nH2O (n = 4,6) have a low melting point, the lower hydrates of MgCl2·nH2O (n = 0,1,2) can form the highly toxic and corrosive HCl gas. Both shortcomings cap the individual use of these salts to a restricted range of the available hydrates. A combination of these two salts showed to have the potential to overcome these shortcomings. The present study focuses on finding stable configurations of potential superior salt hydrate combinations using the evolutionary algorithm USPEX as well as manual mutations of known pristine structures. The newly found structures are less stable than the pure salts, but stable enough to be combined. Extensive electronic density-derived tools, like the Density Derived Electronic and Chemical (DDEC6) bond orders and net atomic charges, as well as Bader topological analysis, are used to predict the HCl gas formation based on the chemical environment in the new metastable structures. We find that doping MgCl2·nH2O with calcium considerably reduces HCl formation compared to its pure form, caused by a combination of the stronger Ca-Cl interaction than Mg-Cl and a less polar H2O molecule in a calcium environment than in a magnesium environment. This provides the possibility to shift the p, T-equilibrium curve of HCl outside the thermal storage operational window

Modeling thermochemical reactions in thermal energy storage systems

In this chapter on simulation techniques for thermochemical reactions in thermal energy storage systems the focus is mainly on molecular modeling techniques for the hydration and dehydration (sorption and desorption) processes occurring in salt hydrates at the nanoscale. Modeling techniques such as density function theory, molecular dynamics, and Monte Carlo are briefly introduced. Some attention is also given to micro- and macroscale modeling techniques used at larger length scales, such as Mampel’s model and the continuum approach. Before introducing all the length (and time) scales involved when modeling a heat storage system, a qualitative description is given of the hydration and dehydration processes on the nano/microscale

Homogeneous nucleation of water in argon : nucleation rate computation from molecular simulations of TIP4P and TIP4P/2005 water model

Molecular dynamics (MD) simulations were conducted to study nucleation of water at 350 K in argon using TIP4P and TIP4P/2005 water models. We found that the stability of any cluster, even if large, strongly depends on the energetic interactions with its vicinity, while the stable clusters change their composition almost entirely during nucleation. Using the threshold method, direct nucleation rates are obtained. Our nucleation rates are found to be 1.08×1027 cm−3 s−1 for TIP4P and 2.30×1027 cm−3 s−1 for TIP4P/2005. The latter model prescribes a faster dynamics than the former, with a nucleation rate two times larger due to its higher electrostatic charges. The non-equilibrium water densities derived from simulations and state-of-art equilibrium parameters from Vega and de Miguel [J. Chem. Phys. 126, 154707 (2007)] are used for the classical nucleation theory (CNT) prediction. The CNT overestimates our results for both water models, where TIP4P/2005 shows largest discrepancy. Our results complement earlier data at high nucleation rates and supersaturations in the Hale plot [Phys. Rev. A 33, 4156 (1986)], and are consistent with MD data on the SPC/E and the TIP4P/2005 model

Velocity correlations and accommodation coefficients for gas-wall interactions in nanochannels

In order to understand the behavior of a gas close to a channel wall, it is important to model the gas-wall interactions correctly. When using Molecular Dynamics (MD) simulations these interactions are modeled explicitly, but the computations are time consuming. Replacing the explicit wall with an appropriate wall model reduces the computational time, but should still remain the same characteristics. In this paper the focus lies with an argon gas confined between two platinum walls at different temperature. Several wall models are investigated for their feasibility as a replacement of the MD simulations and are mainly compared using the velocity correlations between impinging and reflecting particles. Moreover, a new method to compute the accommodation coefficient using the velocity correlations is demonstrated

Development of a scattering model for diatomic gas-solid surface interactions by an unsupervised machine learning approach

This work proposes a new stochastic gas-solid scattering model for diatomic gas molecules constructed based on the collisional data obtained from Molecular Dynamics (MD) simulations. The Gaussian Mixture (GM) approach, which is an unsupervised machine learning approach, is applied to H2 and N2 gases interacting with Ni surfaces in a two parallel walls system under rarefied conditions. The main advantage of this approach is that the entire translational and rotational velocity components of the gas molecules before and after colliding with the surface can be utilized for training the GM model. This creates the possibility to study also highly nonequilibrium systems, and accurately capture the energy exchange between the different molecular modes that cannot be captured by the classical scattering kernels. Considering the MD results as the reference solutions, the performance of the GM-driven scattering model is assessed in comparison with the Cercignani-Lampis-Lord (CLL) scattering model in different benchmarking systems: the Fourier thermal problem, the Couette flow problem, and a combined Fourier-Couette flow problem. This assessment is performed in terms of the distribution of the velocity components and energy modes, as well as accommodation coefficients. It is shown that the predicted results by the GM model are in better agreement with the original MD data. Especially, for H2 gas the GM model outperforms the CLL model. The results for N2 molecules are relatively less affected by changing the thermal and flow properties of the system which is caused by the presence of a stronger adsorption layer

Water nucleation in helium, methane, and argon: a molecular dynamics study

Nucleation of highly supersaturated water vapor in helium, methane, and argon carrier gases at 350 K was investigated using molecular dynamics simulations. Nucleation rates obtained from the mean first passage time (MFPT) method are typically one order of magnitude lower than those from the Yasuoka and Matsumoto method, which can be attributed to the overestimation of the critical cluster size in the MFPT method. It was found that faster nucleation will occur in carrier gases that have better thermalization properties such that latent heat is removed more efficiently. These thermalization properties are shown to be strongly dependent on the molecular mass and Lennard-Jones (LJ) parameters. By varying the molecular mass, for unaltered LJ parameters, it was found that a heavier carrier gas removes less heat although it has a higher collision rate with water than a lighter carrier. Thus, it was shown that a clear distinction between water vapor-carrier gas collisions and water cluster-carrier gas collisions is indispensable for understanding the effect of collision rates on thermalization. It was also found that higher concentration of carrier gas leads to higher nucleation rate. The nucleation rates increased by a factor of 1.3 for a doubled concentration and by almost a factor of two for a tripled concentration