04 Fakultät Energie-, Verfahrens- und Biotechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5

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Institute: search.filters.UBSInstitut.Institut für Technische Thermodynamik und Thermische VerfahrenstechnikStart date: 2020

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Now showing 1 - 10 of 21
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    Development of hydrodynamic density functional theory for mixtures and application to droplet coalescence
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2021) Stierle, Rolf; Groß, Joachim (Prof. Dr.-Ing.)
    Predicting accurately coalescence phenomena is critical to the accurate description of the hydrodynamics of fluids and their mixtures. A promising framework for the development of models for such phenomena is dynamic density functional theory. Dynamic density functional theory enables the analysis of dynamical processes in inhomogeneous systems of pure fluids and fluid mixtures at the molecular level. In this work, a hydrodynamic density functional theory model for mixtures in conjunction with Helmholtz energy functionals based on the PC-SAFT equation of state is proposed, that obeys the first and second law of thermodynamics and simplifies to the isothermal Navier-Stokes equation for homogeneous systems. The hydrodynamic density functional theory model is derived from a variational principle and accounts for both viscous forces and diffusive molecular transport. A Maxwell-Stefan model is applied for molecular transport. This work identifies a suitable expression for the driving force for molecular diffusion of inhomogeneous systems that captures the effect of interfacial tension. The proposed hydrodynamic density functional theory is a non-local theory that requires the computation of weighted (spatial averaged) densities around each considered spatial coordinate by convolution, which is computationally expensive. This work uses Fourier-type transforms to determine the weighted densities. A pedagogical derivation is presented for the efficient computation of the convolution integrals occurring in the Helmholtz energy functionals in Cartesian, cylindrical, and spherical coordinates on equidistant grids using fast Fourier and similar transforms. The applied off-the-shelf algorithms allow to reduce dimensionality and complexity of many practical problems. Furthermore, an algorithm for a fast first-order Hankel transform is proposed, allowing fast and easy density functional theory calculations in rotationally symmetric systems. Application of the hydrodynamic density functional theory model using a well-balanced finite-volume scheme to one-dimensional droplet and bubble coalescence of pure fluids and binary mixtures is presented. The required transport coefficients, shear viscosity and Maxwell-Stefan diffusion coefficients, are obtained by applying entropy scaling to inhomogeneous fluids. The considered systems show a qualitative difference in the coalescence characteristics of droplets compared to bubbles. This constitutes a first step towards predicting the phase rupture leading to coalescence using dynamic density functional theory.
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    Correlating and predicting thermal conductivity and self-diffusion from entropy scaling using PCP-SAFT
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2022) Hopp, Madlen; Groß, Joachim (Prof. Dr.-Ing.)
    For a complete description and design of thermodynamic processes, knowledge of the properties of all substances involved is absolutely necessary. While the equilibrium properties are already well understood, there is still a lack of a handy description of the transport properties. Entropy scaling is an intriguingly simple approach for correlating and predicting transport properties of real substances and mixtures. As convincingly documented in the literature entropy scaling is indeed a firm concept for the shear viscosity of real substances, including hydrogen-bonding species and strongly non-spherical species and for mixtures. In this thesis, we investigate whether the entropy scaling approach is applicable for the thermal conductivity as well as the self-diffusion coefficients of pure substances. In accordance with the entropy scaling approach proposed by Y. Rosenfeld [Phys. Rev. A 1977, 15, 2545-2549], we observe that the thermal conductivity and the self-diffusion coefficient of real substances, once made dimensionless with an appropriate reference expression, only depend on residual entropy. We propose suitable reference expressions for both properties, to calculate the coefficients of pure substances from entropy scaling using the Perturbed-Chain Polar Statistical Associating Fluid Theory (PCP-SAFT) equation of state. Good entropy scaling behavior is found for the entire fluid region for water and more than 130 organic substances from various chemical families: linear and branched alkanes, alkenes, aldehydes, aromatics, ethers, esters, ketones, alcohols and acids. Models for both, thermal conductivity and self-diffusion coefficient, show satisfying robustness for extrapolating the coefficients to conditions rather distant from state points where experimental data is available. Additionally, a predictive group-contribution method for thermal conductivity based on entropy scaling is derived. The excess entropy for this approach is calculated using the group-contribution PCP-SAFT equation of state. The model is applicable for gaseous phases and for liquid-phase conditions covering wide ranges of temperature and pressure.
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    On the prediction of thermodynamic properties by atomistic simulation : from vapor-liquid equilibrium of alcohols to self-assembly in mixed solvents
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2020) Baz, Jörg; Hansen, Niels (apl. Prof. Dr.-Ing. habil.)
    This dissertation presents the results of atomistic molecular simulations. Therefor systems of varying complexity with relevance in materials science, biotechnology and chemical engineering have been considered.
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    PC-SAFT density functional theory in 3 dimensions : adsorption in ordered porous media and solvation free energies in non-polar solvents
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2023) Eller, Johannes; Groß, Joachim (Prof. Dr.-Ing.)
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    Biomolecular force fields probed by free energies of binding and solvation
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2021) Gebhardt, Julia; Hansen, Niels (apl. Prof. Dr.-Ing.)
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    On the use of side‐chain NMR relaxation data to derive structural and dynamical information on proteins : a case study using hen lysozyme
    (2020) Smith, Lorna J.; Gunsteren, Wilfred F. van; Hansen, Niels
    Values of S2CH and S2NH order parameters derived from NMR relaxation measurements on proteins cannot be used straightforwardly to determine protein structure because they cannot be related to a single protein structure, but are defined in terms of an average over a conformational ensemble. Molecular dynamics simulation can generate a conformational ensemble and thus can be used to restrain S2CH and S2NH order parameters towards experimentally derived target values S2CH(exp) and S2NH(exp). Application of S2CH and S2NH order‐parameter restraining MD simulation to bond vectors in 63 side chains of the protein hen egg white lysozyme using 51 S2CH(exp) target values and 28 S2NH(exp) target values shows that a conformational ensemble compatible with the experimentally derived data can be obtained by using this technique. It is observed that S2CH order‐parameter restraining of C-H bonds in methyl groups is less reliable than S2NH order‐parameter restraining because of the possibly less valid assumptions and approximations used to derive experimental S2CH(exp) values from NMR relaxation measurements and the necessity to adopt the assumption of uniform rotational motion of methyl C-H bonds around their symmetry axis and of the independence of these motions from each other. The restrained simulations demonstrate that side chains on the protein surface are highly dynamic. Any hydrogen bonds they form and that appear in any of four different crystal structures, are fluctuating with short lifetimes in solution.
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    Dynamic properties of fluids from molecular simulations and entropy scaling
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2022) Fischer, Matthias; Groß, Joachim (Prof. Dr.-Ing.)
    The design of most processes in chemical industry depends on reliable estimates of the transport properties of fluids. Various approaches exist for the prediction of these quantities, which can be used to compensate for insufficient experimental data. The present work deals with two of the approaches: Molecular simulations and entropy scaling. According to the latter approach, transport coefficients, such as shear viscosity, thermal conductivity or self diffusion coefficients, defined as dimensionless quantities using a suitable reference, are univariant functions of only the residual entropy of the fluid. The two methods, molecular simulations and entropy scaling are used jointly in order to achieve synergistic effects. A suitable mixture-model for entropy scaling models was investigated in molecular simulations as part of this work. Mixtures of simple model fluids, namely Lennard-Jones mixtures, are regarded and it is found that the principle of entropy scaling holds also for mixtures, to excellent approximation. Entropy scaling, in turn, is used to more efficiently design and evaluate molecular simulations. In this context, the TAMie force field developed in Stuttgart is assessed with respect to the accuracy of predicted transport coefficients. The TAMie model, like many other force fields developed for thermodynamic properties, uses rigid bond lengths between interaction sites within a molecule. In order to ensure a meaningful assessment of transport coefficients in Molecular Dynamic simulations, an analysis of bond-length models is conducted: what is the influence of the model for intramolecular atomic bonds on the predicted static and dynamic fluid properties? It is shown that it is possible to obtain the same results for transport coefficients with flexible atomic bonds, within statistical accuracy, as with the same force field but using a rigid description of the bonds. Within the context of the simulation studies carried out in this thesis, a workflow has been developed that enables efficient evaluation of simulations for determining transport properties. In combination with entropy scaling, this work presents a methodology that can be used to efficiently determine transport quantities from molecular simulations, thus enabling extensive simulation studies for either predicting fluid properties or to enable force field development where transport coefficients are considered in the objective function.
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    Calculation of pure substance and mixture viscosities using PCP-SAFT and entropy scaling
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2020) Lötgering-Lin, Oliver; Gross, Joachim (Prof. Dr.-Ing.)
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    A new approach to optimize the transferable anisotropic Mie force field (TAMie) for mixtures
    (Stuttgart : Universität Stuttgart, Institut für Technische Thermodynamik und Thermische Verfahrenstechnik, 2021) Weidler, Dominik; Groß, Joachim (Prof. Dr.-Ing.)
    In this thesis the development of a molecular force field is presented, which allows an application focus on the calculation of phase equilibria. Based on the ”Transferable Anisotropic Mie (TAMie)” force field by Hemmen et al. the parameter set of the force field is extended to small cyclic molecules and polar groups of substances such as esters and ketones. It is a classical atomistic force field, but hydrogen atoms are often effectively considered together with neighbouring larger atoms. The force field parameters are transferable, i.e. they can be used for all substances within a group of substances. Although the phase equilibrium results obtained are very good, the transferable model with simple point charges reaches some limitations. This is shown in deviations of the saturation vapor pressure from the simulations compared to experimental data. However, it is desirable to describe the vapor pressure as accurately as possible in order to be able to predict mixture properties with good agreement to experimental data. In order not to destroy the transferable character of the force field and at the same time ensure the accuracy of the vapor pressure for individual substances, the individualized TAMie force field is introduced. With the help of a correction parameter ψ all energetic interactions of a pure substance are scaled in order to increase the accuracy for experimentally well measured substances. It is shown that this concept leads to significantly improved correlations and predictions of mixture properties. Using various binary mixtures, the transferability of cross-interaction parameters that correct van der Waals interactions between two pure substances is also demonstrated. Further investigations and experiments are recommended for validation.
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    Predicting and rationalizing the Soret coefficient of binary Lennard‐Jones mixtures in the liquid state
    (2022) Zimmermann, Nils E. R.; Guevara‐Carrion, Gabriela; Vrabec, Jadran; Hansen, Niels
    The thermodiffusion behavior of binary Lennard‐Jones mixtures in the liquid state is investigated by combining the individual strengths of non‐equilibrium molecular dynamics (NEMD) and equilibrium molecular dynamics (EMD) simulations. On the one hand, boundary‐driven NEMD simulations are useful to quickly predict Soret coefficients because they are easy to set up and straightforward to analyze. However, careful interpolation is required because the mean temperature in the measurement region does not exactly reach the target temperature. On the other hand, EMD simulations attain the target temperature precisely and yield a multitude of properties that clarify the microscopic origins of Soret coefficient trends. An analysis of the Soret coefficient suggests a straightforward dependence on the thermodynamic properties, whereas its dependence on dynamic properties is far more complex. Furthermore, a limit of applicability of a popular theoretical model, which mainly relies on thermodynamic data, was identified by virtue of an uncertainty analysis in conjunction with efficient empirical Soret coefficient predictions, which rely on model parameters instead of simulation output. Finally, the present study underscores that a combination of predictive models and EMD and NEMD simulations is a powerful approach to shed light onto the thermodiffusion behavior of liquid mixtures.