Publications (86)163.95 Total impact

Article: Intermolecular potential energy surface and thermophysical properties of the CH4N2 system.
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ABSTRACT: A fivedimensional potential energy surface (PES) for the interaction of a rigid methane molecule with a rigid nitrogen molecule was determined from quantumchemical ab initio calculations. The counterpoisecorrected supermolecular approach at the CCSD(T) level of theory was utilized to compute a total of 743 points on the PES. The interaction energies were calculated using basis sets of up to quadruplezeta quality with bond functions and were extrapolated to the complete basis set limit. An analytical sitesite potential function with nine sites for methane and five sites for nitrogen was fitted to the interaction energies. The PES was validated by calculating the cross second virial coefficient as well as the shear viscosity and binary diffusion coefficient in the dilutegas limit for CH4N2 mixtures. An improved PES was obtained by adjusting a single parameter of the analytical potential function in such a way that quantitative agreement with the most accurate experimental values of the cross second virial coefficient was achieved. The transport property values obtained with the adjusted PES are in good agreement with the best experimental data.The Journal of Chemical Physics 12/2014; 141(22):224301. · 3.12 Impact Factor  Journal of Physical and Chemical Reference Data 09/2014; 43(3):033101. · 3.11 Impact Factor

Article: Viscosity of liquids—Enskog2σ model
Fluid Phase Equilibria 06/2014; 372:34–42. · 2.24 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: An extended hardsphere model is reported that may be applied to correlate and predict the viscosity of gases, liquids and supercritical fluids. The method is based on the hardsphere model of Dymond and Assael and uses their roughness factors and molar core volumes to relate reduced viscosity to a universal function of reduced volume. The extended model behaves correctly in the limit of low densities and offers improved accuracy at high densities. The new universal reference function was determined from a large database of experimental viscosities for alkanes extending up to reduced densities of 0.84. It has been tested by correlating the viscosity of two highviscosity liquids not used in the development of the universal function and has shown to perform satisfactorily up to reduced densities of approximately 0.9.Fluid Phase Equilibria 02/2014; 363:239–247. · 2.24 Impact Factor 
Article: Calculation of the relaxation properties of a dilute gas consisting of Lennard–Jones chains
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ABSTRACT: The relaxation properties in the dilutegas limit have been calculated by the classical trajectory (CT) method for a gas consisting of chainlike molecules that are rigid and interact through site–site Lennard–Jones 12–6 potentials. Results are reported for volume viscosity ηVηV, rotational collision number ζrotζrot and the ratio of the rotational to selfdiffusion coefficient Drot/DDrot/D. The results indicate that the volume viscosity increases with temperature and decreases with chain length. The rotational relaxation of chains is efficient, as it takes of the order of 1.75–2.7 collisions to attain equilibrium. The rotational collision number is only weakly temperature dependent.Chemical Physics Letters 06/2013; 574:37–41. · 1.99 Impact Factor 
Article: Calculation of the transport properties of a dilute gas consisting of LennardJones chains.
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ABSTRACT: The transport properties in the dilute gas limit have been calculated by the classicaltrajectory method for a gas consisting of chainlike molecules. The molecules were modelled as rigid chains consisting of spherical segments that interact through a combination of sitesite LennardJones 126 potentials. Results are reported for shear viscosity, selfdiffusion, and thermal conductivity for chains consisting of 1, 2, 3, 4, 5, 6, 7, 8, 10, 13, and 16 segments in the reduced temperature range of 0.3  50. The results indicate that the transport properties increase with temperature and decrease with chain length. At high temperatures the dependence of the transport properties is governed effectively by the repulsive part of the potential. No simple scaling with chain length has been observed. The higher order correction factors are larger than observed for real molecules so far, reaching asymptotic values of 1.019  1.033 and 1.060  1.072 for viscosity and thermal conductivity, respectively. The dominant contribution comes from the angular momentum coupling. The agreement with molecular dynamics calculations for viscosity is within the estimated accuracy of the two methods for shorter chains. However, for longer chains differences of up to 7% are observed.The Journal of Chemical Physics 02/2013; 138(8):084309. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper contains new, representative reference equations for the thermal conductivity of nheptane. The equations are based in part upon a body of experimental data that have been critically assessed for internal consistency and for agreement with theory whenever possible. In the case of the dilutegas thermal conductivity, a theoretically based correlation was adopted in order to extend the temperature range of the experimental data. Moreover, in the critical region, the experimentally observed enhancement of the thermal conductivity is well represented by theoretically based equations containing just one adjustable parameter. The correlations are applicable for the temperature range from the triple point to 600 K and pressures up to 250 MPa. The overall uncertainty (considered to be estimates of a combined expanded uncertainty with a coverage factor of 2) of the proposed correlation is estimated, for pressures less than 250 MPa and temperatures less than 600 K, to be less than 4%.Journal of Physical and Chemical Reference Data 01/2013; 42:023101. · 3.50 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In this work, we propose a new kinetic theory model (Enskog2σ model) to analyze and predict the viscosity of simple fluids. The model is based on the Enskog formulation and makes use of two effective diameters to represent two aspects of molecular interactions in Enskog's treatment; namely, the effective diameter representing the excluded volume of a molecule and the effective diameter accounting for the increased probability of collision in comparison to the dilute gas. We have tested the model by analyzing the viscosity of five simple fluids (Ar, CH4, N2, CO2 and SF6) of increasing complexity at supercritical temperatures. The new Enskog2σ model outperforms previous approaches based on Enskog theory in terms of its correlative ability. We demonstrate that the effective diameter associated with the excluded volume exhibits a universal behaviour as a function of reduced temperature. The conformal behaviour can be achieved by means of a single fluiddependent length scaling parameter. We make use of this finding to develop a general model that allows the prediction of the viscosity of one fluid from the knowledge of the viscosity of a reference fluid, which in this work was chosen to be Ar. The accuracy of the predicted viscosity depends on the way in which the length scaling parameter was estimated. If the length scaling parameter is obtained from the knowledge of viscosity along a single isotherm, the accuracy of the predicted viscosity is similar to the uncertainty of the original correlation over its entire supercritical range. If a single viscosity value is used to estimate the length scaling parameter, the viscosities of supercritical CH4, N2, CO2 and SF6 are predicted with the maximum deviation of 3.3%, 4.2%, 9.9% and 10.5% respectively over the entire range of validity of the Enskog2σ model.Fluid Phase Equilibria 11/2012; 334:89–96. · 2.24 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: New, simple, and practical correlations for shear viscosity, selfdiffusion coefficient, bulk viscosity, and thermal conductivity of hydrogen sulfide in the limit of zero density are provided, together with a correlation for the second pressure virial coefficient. The correlations are based on the values of thermophysical properties generated from a highly accurate, fully quantummechanical, ab initio potential energy surface. The validation of the computed values of thermophysical properties against the rather scarce experimental data demonstrates an excellent agreement with the most accurate data sets. The analysis undertaken indicates that the correlated values provide the most reliable, accurate, and internally consistent representation of thermophysical properties of hydrogen sulfide. The correlations extend over the temperature range (180 to 2000) K. The behavior of each transport property is represented by an independent correlation of the appropriate effective collision cross section as a function of temperature, while the behavior of the second pressure virial coefficient is directly represented as a function of temperature. The uncertainty of the proposed transport property correlations varies from ± 0.4 % for the shear viscosity in the temperature range (300 to 700) K to ± 5.0 % for the bulk viscosity. The uncertainty of the second pressure virial coefficient correlation is estimated to be of the order of ± 1 cm3·mol–1 at temperatures above 400 K, decreasing to ± 30 cm3·mol–1 at 180 K.Journal of Chemical & Engineering Data 03/2012; 57(4):1312–1317. · 2.05 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: New expressions for the viscosity of liquid mixtures, consisting of chainlike molecules, are derived by means of Enskogtype analysis. The molecules of the fluid are modelled as chains of equally sized, tangentially joined, and rigid spheres. It is assumed that the collision dynamics in such a fluid can be approximated by instantaneous collisions. We determine the molecular size parameters from the viscosity of each pure species and show how the different effective parameters can be evaluated by extending the VesovicWakeham (VW) method. We propose and implement a number of thermodynamically consistent mixing rules, taking advantage of SAFTtype analysis, in order to develop the VW method for chain molecules. The predictions of the VWchain model have been compared in the first instance with experimental viscosity data for octanedodecane and methanedecane mixtures, thus, illustrating that the resulting VWchain model is capable of accurately representing the viscosity of real liquid mixtures.The Journal of Chemical Physics 02/2012; 136(7):074514. · 3.12 Impact Factor 
Article: Ab initio intermolecular potential energy surface and thermophysical properties of hydrogen sulfide.
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ABSTRACT: A sixdimensional potential energy hypersurface (PES) for two interacting rigid hydrogen sulfide molecules was determined from highlevel quantummechanical ab initio computations. A total of 4016 points for 405 different angular orientations of two molecules were calculated utilizing the counterpoisecorrected supermolecular approach at the CCSD(T) level of theory and extrapolating the calculated interaction energies to the complete basis set limit. An analytical sitesite potential function with eleven sites per hydrogen sulfide molecule was fitted to the interaction energies. The PES has been validated by computing the second pressure virial coefficient, shear viscosity, thermal conductivity and comparing with the available experimental data. The calculated values of volume viscosity were not used to validate the potential as the low accuracy of the available data precluded such an approach. The second pressure virial coefficient was evaluated by means of the Takahashi and Imada approach, while the transport properties, in the dilute limit, were evaluated by utilizing the classical trajectory method. In general, the agreement with the primary experimental data is within the experimental error for temperatures higher than 300 K. For lower temperatures the lack of reliable data indicates that the values of the second pressure virial coefficient and of the transport properties calculated in this work are currently the most accurate estimates for the thermophysical properties of hydrogen sulfide.Physical Chemistry Chemical Physics 06/2011; 13(30):1374958. · 4.20 Impact Factor 
Article: What happens inside a fuel cell? Developing an experimental functional map of fuel cell performance.
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ABSTRACT: Fuel cell performance is determined by the complex interplay of mass transport, energy transfer and electrochemical processes. The convolution of these processes leads to spatial heterogeneity in the way that fuel cells perform, particularly due to reactant consumption, water management and the design of fluidflow plates. It is therefore unlikely that any bulk measurement made on a fuel cell will accurately represent performance at all parts of the cell. The ability to make spatially resolved measurements in a fuel cell provides one of the most useful ways in which to monitor and optimise performance. This Minireview explores a range of in situ techniques being used to study fuel cells and describes the use of novel experimental techniques that the authors have used to develop an 'experimental functional map' of fuel cell performance. These techniques include the mapping of current density, electrochemical impedance, electrolyte conductivity, contact resistance and CO poisoning distribution within working PEFCs, as well as mapping the flow of reactant in gas channels using laser Doppler anemometry (LDA). For the hightemperature solid oxide fuel cell (SOFC), temperature mapping, reference electrode placement and the use of Raman spectroscopy are described along with methods to map the microstructural features of electrodes. The combination of these techniques, applied across a range of fuel cell operating conditions, allows a unique picture of the internal workings of fuel cells to be obtained and have been used to validate both numerical and analytical models.ChemPhysChem 09/2010; 11(13):271431. · 3.36 Impact Factor 
Article: What happens inside a fuel cell? Developing an experimental functional map of fuel cell performance.
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ABSTRACT: Fuel cell performance is determined by the complex interplay of mass transport, energy transfer and electrochemical processes. The convolution of these processes leads to spatial heterogeneity in the way that fuel cells perform, particularly due to reactant consumption, water management and the design of fluidflow plates. It is therefore unlikely that any bulk measurement made on a fuel cell will accurately represent performance at all parts of the cell. The ability to make spatially resolved measurements in a fuel cell provides one of the most useful ways in which to monitor and optimise performance. This Minireview explores a range of in situ techniques being used to study fuel cells and describes the use of novel experimental techniques that the authors have used to develop an 'experimental functional map' of fuel cell performance. These techniques include the mapping of current density, electrochemical impedance, electrolyte conductivity, contact resistance and CO poisoning distribution within working PEFCs, as well as mapping the flow of reactant in gas channels using laser Doppler anemometry (LDA). For the hightemperature solid oxide fuel cell (SOFC), temperature mapping, reference electrode placement and the use of Raman spectroscopy are described along with methods to map the microstructural features of electrodes. The combination of these techniques, applied across a range of fuel cell operating conditions, allows a unique picture of the internal workings of fuel cells to be obtained and have been used to validate both numerical and analytical models.ChemPhysChem 09/2010; 11(13):271431. · 3.36 Impact Factor 
Article: Viscosity and Density of Carbon Dioxide + 2,6,10,15,19,23Hexamethyltetracosane (Squalane)†
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ABSTRACT: The viscosity and density of mixtures of carbon dioxide and 2,6,10,15,19,23hexamethyltetracosane (squalane) are reported. The measurements were carried out using a vibrating wire instrument over a range of temperatures from (303.15 to 448.15) K and at pressures ranging from approximately the minimum miscibility pressure at a given composition to 170 MPa. Pure squalane and three different mixtures, with mole fractions of CO2 of 0.423, 0.604, and 0.788, were investigated. The estimated expanded relative uncertainties of the measurements were ± 2 % for viscosity and ± 0.2 % for density with a coverage factor of 2. The data for each composition were correlated by simple expressions with an absolute average relative deviation less than 2 % for viscosity and less than 0.2 % for density. The results show that the addition of CO2 to squalane at a given pressure and temperature reduces greatly the viscosity and increases slightly the density. A hardsphere model, adjusted to fit the viscosity data of the pure substances but containing no adjustable binary parameters, was tested against the experimental data. Relative deviations bounded by approximately ± 60 % were found.Journal of Chemical & Engineering Data 09/2009; 54(9). · 2.05 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Transport properties of dilute water vapor have been calculated in the rigidrotor approximation using four different potential energy hypersurfaces and the classicaltrajectory method. Results are reported for shear viscosity, selfdiffusion, thermal conductivity, and volume viscosity in the dilutegas limit for the temperature range of 2502500 K. Of these four surfaces the CCpol surface of Bukowski et al. [J. Chem. Phys. 128, 094314 (2008)] is in best accord with the available measurements. Very good agreement is found with the most accurate results for viscosity in the whole temperature range of the experiments. For thermal conductivity the deviations of the calculated values from the experimental data increase systematically with increasing temperature to around 5% at 1100 K. For both selfdiffusion and volume viscosity, the much more limited number of available measurements are generally consistent with the calculated values, apart from the lower temperature isotopically labeled diffusion measurements.The Journal of Chemical Physics 08/2009; 131(1):014303. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Transport properties of pure methane have been calculated in the rigidrotor approximation using the recently proposed intermolecular potential energy hypersurface [R. Hellmann et al., J. Chem. Phys. 128, 214303 (2008)] and the classicaltrajectory method. Results are reported in the dilutegas limit for the temperature range of 801500 K. The calculated thermal conductivity values are in very good agreement with the measured data and correlations. In the temperature range of 310480 K the calculated values underestimate the best experimental data by 0.5%1.0%. We suggest that the calculated values are more accurate, especially at low and high temperatures, than the currently available correlations based on the experimental data. Our results also agree well with measurements of thermal transpiration and of the thermomagnetic coefficients. We have shown that although the dominant contribution to the thermomagnetic coefficients comes from the Wjj polarization in the spherical approximation, the contribution of a second polarization, Wj, cannot be neglected nor can a full description of the Wjj polarization. The majority of the volume viscosity measurements around room temperature are consistent with the calculated values but this is not the case at high and low temperatures. However, for nuclearspin relaxation the calculated values consistently exceed the measurements, which are mutually consistent within a few percent.The Journal of Chemical Physics 04/2009; 130(12):124309. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report the results of simultaneous measurements of the viscosity and density of five pure hydrocarbon liquids (octane, decane, 1,3dimethylbenzene, 1,2,3,4tetrahydronaphthalene, and 1methylnaphthalene) at temperatures between (298.15 and 473.15) K and at pressures ranging from 0.1 MPa to approximately 200 MPa. The measurements were made with a vibratingwire instrument, and the estimated expanded relative uncertainties are ± 2 % for viscosity and ± 0.2 % for density with a coverage factor of 2. The densities were correlated by means of a modified Tait equation, while the viscosities were correlated both with the theory of Dymond and Assael and in terms of an empirical function of temperature and pressure. We also present correlations of the viscosity of dodecane and octadecane based on results that we published previously [Caudwell et al. Int. J. Thermophys. 2004, 25, 1340−1352]. Extensive comparisons with literature data are presented.Journal of Chemical & Engineering Data 02/2009; · 2.05 Impact Factor  01/2009;
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ABSTRACT: Transport properties of pure methane gas have been calculated in the rigidrotor approximation using the recently proposed intermolecular potential energy hypersurface [R. Hellmann et al., J. Chem. Phys. 128, 214303 (2008)] and the classicaltrajectory method. Results are reported in the dilutegas limit for shear viscosity, viscomagnetic coefficients, and selfdiffusion in the temperature range of 801500 K. Compared with the best measurements, the calculated viscosity values are about 0.5% too high at room temperature, although the temperature dependence of the calculated values is in very good agreement with experiment between 210 and 390 K. For the shear viscosity, the calculations indicate that the corrections in the secondorder approximation and those due to the angularmomentum polarization are small, less than 0.7%, in the temperature range considered. The very good agreement of the calculated values with the experimental viscosity data suggests that the rigidrotor approximation should be very reasonable for the three properties considered. In general, the agreement for the other measured properties is within the experimental error.The Journal of Chemical Physics 09/2008; 129(6):064302. · 3.12 Impact Factor 
Article: A kinetic theory description of the viscosity of dense fluids consisting of chain molecules.
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ABSTRACT: An expression for the viscosity of a dense fluid is presented that includes the effect of molecular shape. The molecules of the fluid are approximated by chains of equalsized, tangentially jointed, rigid spheres. It is assumed that the collision dynamics in such a fluid can be approximated by instantaneous collisions between two rigid spheres belonging to different chains. The approach is thus analogous to that of Enskog for a fluid consisting of rigid spheres. The description is developed in terms of two molecular parameters, the diameter sigma of the spherical segment and the chain length (number of segments) m. It is demonstrated that an analysis of viscosity data of a particular pure fluid alone cannot be used to obtain independently effective values of both sigma and m. Nevertheless, the chain lengths of nalkanes are determined by assuming that the diameter of each rigid sphere making up the chain can be represented by the diameter of a methane molecule. The effective chain lengths of nalkanes are found to increase linearly with the number C of carbon atoms present. The dependence can be approximated by a simple relationship m=1+(C1)3. The same relationship was reported within the context of a statistical associating fluid theory equation of state treatment of the fluid, indicating that both the equilibrium thermodynamic properties and viscosity yield the same value for the chain lengths of nalkanes.The Journal of Chemical Physics 06/2008; 128(20):204901. · 3.12 Impact Factor
Publication Stats
1k  Citations  
163.95  Total Impact Points  
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Institutions

1981–2014

Imperial College London
 • Department of Earth Science and Engineering
 • Department of Chemical Engineering
Londinium, England, United Kingdom


2004–2009

University of Rostock
 Institut für Chemie
Rostock, MecklenburgVorpommern, Germany


2001

Imperial Valley College
Imperial, California, United States


2000

Aristotle University of Thessaloniki
 Department of Chemical Engineering
Saloníki, Central Macedonia, Greece


1986

Institute of Chemistry, Technology and Metallurgy
Beograd, Central Serbia, Serbia
