Stephan Kabelac

Transport coefficients of the Lennard-Jones model fluid. III. Bulk viscosity

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This first of a series of four papers presents the results for the viscosity, and discusses and interprets the behavior of this transport coefficient in the fluid region of the phase diagram. Moreover, the kinetic-kinetic, kinetic-potential, and potential-potential viscosity contributions are resolved over the whole range of fluid states, and their characteristic dependence on temperature and density is described. Finally, an additional analysis of the shear-stress correlation functions reveals aspects of the momentum-transport mechanisms on the molecular scale.

Thermodynamik : Grundlagen und technische Anwendungen

Das seit Jahrzehnten bewAhrte und immer wieder aktualisierte Lehrbuch bietet eine moderne und gut verstAndliche Darstellung der technischen Thermodynamik einschlieAlich einer Thermodynamik der Gemische und der chemischen Reaktionen. Schwerpunkt ist die ausfA1/4hrliche und dem AnfAnger verstAndliche Darstellung der Grundlagen mit der sorgfAltigen EinfA1/4hrung der thermodynamischen Begriffe und der fundamentalen Bilanzgleichungen fA1/4r Energie, Entropie und Exergie. Die thermodynamischen Eigenschaften reiner Fluide und fluider Gemische werden eingehend erlAutert. Darauf aufbauend wird die Thermodynamik der Gemische und der chemischen Reaktionen entwickelt. Auch die thermodynamischen Aspekte wichtiger energie- und verfahrenstechnischer Anwendungen werden praxisnah behandelt: StrAmungs- und Arbeitsprozesse, thermische Stofftrennverfahren, Phasen- und Reaktionsgleichgewichte, Verbrennungsprozesse und Verbrennungskraftanlagen einschlieAlich Brennstoffzellen, thermische Kraftwerke, Heizsysteme und KAlteanlagen. FA1/4r die 13. Auflage wurde das umfangreiche Kapitel A1/4ber Verbrennungsprozesse und Verbrennungskraftanlagen grundlegend bearbeitet; dabei wurden neue Abschnitte A1/4ber Verbrennungsrechnung, Brennwertkessel und Gasturbinen eingefA1/4gt. Das Werk ist bestens geeignet als Lehrbuch fA1/4r Studierende des Maschinenbaus und der Verfahrenstechnik sowie als Nachschlagewerk fA1/4r den Ingenieur in Forschung und Praxis.

Transport coefficients of the Lennard-Jones model fluid. II Self-diffusion

In an extensive computer simulation study, the transport coefficients of the Lennard-Jones model fluid were determined with high accuracy from equilibrium molecular-dynamics simulations. In the frame of time-correlation function theory, the generalized Einstein relations were employed to evaluate the transport coefficients. This second of a series of four papers presents the results for the self-diffusion coefficient, and discusses and interprets the behavior of this transport coefficient in the fluid region of the phase diagram. The uncertainty of the self-diffusion data is estimated to be 1% in the gas region and 0.5% at high-density liquid states. With the very accurate data, even fine details in the shape of the self-diffusion isotherms are resolved, and the previously little-investigated behavior of the self-diffusion coefficient at low-density gaseous states is analyzed in detail. Finally, aspects of the mass transport mechanisms on the molecular scale are explored by an analysis of the velocity autocorrelation functions.

Speed of sound instrument for fluids with pressures up to 100 MPa

An instrument for highly accurate measurements of the speed of sound in fluids in the temperature range between 240 and 420 K with pressures up to 100 MPa is described. The measurement principle of the speed of sound sensor is based on a double path length pulse-echo technique. The achieved measurement uncertainties are 3 mK for the temperature, 0.01% for the pressure below 10 MPa and 0.005% for the pressure between 10 and 100 MPa, and 0.014% for the speed of sound. The high accuracy of the instrument is demonstrated by measurements in liquid water and compressed argon. The results for argon prove that our pulse-echo technique agrees with the highly accurate spherical resonator technique, which is commonly employed for speed of sound measurements in gases, in the pressure range where both methods overlap within our measurement uncertainty.

Pressure derivatives in the classical molecular-dynamics ensemble.

The calculation of thermodynamic state variables, particularly derivatives of the pressure with respect to density and temperature, in conventional molecular-dynamics simulations is considered in the frame of the comprehensive treatment of the molecular-dynamics ensemble by Lustig [J. Chem. Phys. 100, 3048 (1994)]. This paper improves the work of Lustig in two aspects. In the first place, a general expression for the basic phase-space functions in the molecular-dynamics ensemble is derived, which takes into account that a mechanical quantity G is, in addition to the number of particles, the volume, the energy, and the total momentum of the system, a constant of motion. G is related to the initial position of the center of mass of the system. Secondly, the correct general expression for volume derivatives of the potential energy is derived. This latter result solves a problem reported by Lustig [J. Chem. Phys. 109, 8816 (1998)] and Meier [Computer Simulation and Interpretation of the Transport Coefficients of the Lennard-Jones Model Fluid (Shaker, Aachen, 2002)] and enables the correct calculation of the isentropic and isothermal compressibilities, the speed of sound, and, in principle, all higher pressure derivatives. The derived equations are verified by calculations of several state variables and pressure derivatives up to second order by molecular-dynamics simulations with 256 particles at two state points of the Lennard-Jones fluid in the gas and liquid regions. It is also found that it is impossible for systems of this size to calculate third- and higher-order pressure derivatives due to the limited accuracy of the algorithm employed to integrate the equations of motion.

The entropy of terrestrial solar radiation

Complete thermodynamic evaluation of energy conversion devices calls for energy and entropy balance equations. While the subject of radiation energy has been well taken care of, the same is not true for the entropy of radiation. This has become obvious from the discussion about the maximum work that can be drawn from solar radiation. This article collects and reflects the basic equations needed to calculate the radiation entropy, and it discusses the influence of the three major input functions on the entropy, namely the spectral and hemispherical distribution of radiation intensity, and its degree of polarization. Results of realistic calculations using atmospheric models are given in the last section in form of an energy-entropy diagram.

A new look at the maximum conversion efficiency of black-body radiation

Abstract A new derivation of the maximum conversion efficiency for a continuous, steady-flow radiation conversion process is given. Using basic thermodynamic considerations only, this theoretical efficiency is shown to be the Carnot efficienty. Other existing conversion efficiencies give smaller values, because they suffer from constraints which are discussed in detail.

Boiling of R134a in a Plate-Fin Heat Exchanger Having Offset Fins

This paper presents experimental results on boiling heat transfer of R134a in a compact plate fin heat exchanger. The exchanger is made of aluminum and has high density offset fins (30 fins/in.). Such heat exchangers are widely used in air separation industry and aerospace applications because of their high compactness and low weight. The test heat exchanger is attached to a vapor cycle refrigeration basic module to study the effects of boiling phenomena and its influence on performance as there is limited information available for this type of fins. This in turn allows for discussion on boiling mechanism of R134a inside the fins using the water circuit on the other side of the test heat exchanger. The water side single phase heat transfer coefficient (Colburn j factor) is calculated using the cfd tool fluent and validated with available open literature. The results are presented for heat fluxes up to 5500 W/m2 and mass fluxes up to 20 kg/(m2s) with water side flow rate varying from 0.033 to 0.17 kg/s for water temperatures of 10, 15, 20, 25, and 30 °C.

Local Entropy Production Rates in a Polymer Electrolyte Membrane Fuel Cell

Abstract A modeling study on a polymer electrolyte membrane fuel cell by means of non-equilibrium thermodynamics is presented. The developed model considers a one-dimensional cell in steady-state operation. The temperature, concentration and electric potential profiles are calculated for every domain of the cell. While the gas diffusion and the catalyst layers are calculated with established classical modeling approaches, the transport processes in the membrane are calculated with the phenomenological equations as dictated by the non-equilibrium thermodynamics. This approach is especially instructive for the membrane as the coupled transport mechanisms are dominant. The needed phenomenological coefficients are approximated on the base of conventional transport coefficients. Knowing the fluxes and their intrinsic corresponding forces, the local entropy production rate is calculated. Accordingly, the different loss mechanisms can be detected and quantified, which is important for cell and stack optimization.

Monte Carlo Simulations of Binary Lennard–Jones Mixtures: A Test of the van der Waals One-Fluid Model

Monte Carlo simulations in the canonical ensemble have been performed in the liquid and supercritical regions of a binary Lennard–Jones mixture with differences in size parameters of 6.4% and energy parameters of 37%. The results are compared with a recent fundamental equation of state employing the van der Waals one-fluid model and new simulation data at the corresponding state conditions of the pure Lennard–Jones fluid. The van der Waals one-fluid model describes the mixture properties well at high densities, while at low densities the predicted internal energies and isochoric heat capacities are too low.