The Enskog-2σ model, a new viscosity model for simple fluids and alkanes
Author(s)
Umla, Rudolf
Type
Thesis
Abstract
This thesis describes the development of a new model for viscosity, the
Enskog-2σ model. The model is based on Enskog theory in which the viscosity
of a hard sphere fluid is computed from the hard sphere interactions on
the molecular level. The idea of the Enskog-2σ model is to introduce two effective
weakly temperature dependent diameters in Enskog's approach. One
of the diameters is linked to the collision rate between the fluid molecules,
the other diameter to the molecule size.
The optimisation of the two effective temperature dependent diameters allows
the Enskog-2σ model to reproduce the viscosity data of simple fluids,
i.e. fluids with non-polar, fairly spherical molecules, very well over a wide
range of pressures and temperatures. For argon, for example, the model
covers a pressure range from 0 to 400 MPa and temperatures from 0.6Tc to
4:6Tc (Tc = critical temperature of argon) and correlates the experimental
reference correlation within ±10%. Making use of the universal behavior of
the diameters for various simple fluids, the number of free parameters can
be reduced to one or two constant scaling parameters that can be predicted
well from viscosity data along one isotherm.
The Enskog-2σ approach has been extended to model n-alkanes from ethane
to octane. The molecules of n-alkanes are described as chains of equally
sized hard spheres and a collision between two chains is modelled as collision
between two spherical segments of the colliding chains. The Enskog-2σ
model for n-alkanes contains two effective weakly temperature dependent
chain lengths. The number of free parameters can be reduced by relating
the chain lengths to the carbon number or to the chain lengths of a reference
n-alkane. The remaining free parameters can usually be determined
satisfactorily from viscosity data along one isotherm.
Enskog-2σ model. The model is based on Enskog theory in which the viscosity
of a hard sphere fluid is computed from the hard sphere interactions on
the molecular level. The idea of the Enskog-2σ model is to introduce two effective
weakly temperature dependent diameters in Enskog's approach. One
of the diameters is linked to the collision rate between the fluid molecules,
the other diameter to the molecule size.
The optimisation of the two effective temperature dependent diameters allows
the Enskog-2σ model to reproduce the viscosity data of simple fluids,
i.e. fluids with non-polar, fairly spherical molecules, very well over a wide
range of pressures and temperatures. For argon, for example, the model
covers a pressure range from 0 to 400 MPa and temperatures from 0.6Tc to
4:6Tc (Tc = critical temperature of argon) and correlates the experimental
reference correlation within ±10%. Making use of the universal behavior of
the diameters for various simple fluids, the number of free parameters can
be reduced to one or two constant scaling parameters that can be predicted
well from viscosity data along one isotherm.
The Enskog-2σ approach has been extended to model n-alkanes from ethane
to octane. The molecules of n-alkanes are described as chains of equally
sized hard spheres and a collision between two chains is modelled as collision
between two spherical segments of the colliding chains. The Enskog-2σ
model for n-alkanes contains two effective weakly temperature dependent
chain lengths. The number of free parameters can be reduced by relating
the chain lengths to the carbon number or to the chain lengths of a reference
n-alkane. The remaining free parameters can usually be determined
satisfactorily from viscosity data along one isotherm.
Date Issued
2013-02
Date Awarded
2013-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Vesovic, Velisa
Sponsor
Qatar Petroleum ; Shell International Ltd ; Qatar Science & Technology Park.
Publisher Department
Earth Science and Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)