Modelling of heavy fuel oil spray combustion using continuous thermodynamics
Garaniya, VB (2009) Modelling of heavy fuel oil spray combustion using continuous thermodynamics. PhD thesis, University of Tasmania (Australian Maritime College).
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Commercial liquid petroleum fuels are complex mixtures of various hydrocarbons. In
multicomponent fuel modelling, these liquid fuels are represented typically with two
components or up to ten discrete components. Even with ten components, there are
limitations on the representation of real commercial fuels such as heavy fuel oil (HFO),
which contains large numbers of hydrocarbons with a wide range of molecular weights
and dissimilar structures. Continuous thermodynamics and pyrolysis chemical kinetics are
used to model the behaviour of HFO in diesel spray combustion.
The evaporation model is developed using the principle of continuous thermodynamics
rather than discrete component modelling, to accurately cover the entire range of
composition. Continuous thermodynamics reduces the computational simulation load
compared to conventional discrete thermodynamics, without degrading the quality of
prediction of the complex behaviours of such multicomponent fuels. In continuous
thermodynamics modelling, liquid mixture compositions are simply represented by
probability density functions (PDF). In the present study, HFO is represented by four fuel
fractions: n-paraffins, aromatics, naphthenes and heavy residue. Each of these fractions is
assigned a separate distribution function. In the evaporation model, both low-pressure and
high-pressure formulations for the calculation of vapour-liquid equilibrium (VLE) at the
droplet surface are provided. The formulations for high-pressure VLE are developed for a
semicontinuous mixture and a generic approach to the equation of state (EOS) is used.
Therefore, depending on the mixture compositions (continuous or semicontinuous) these
formulations can be applied with any EOS. It is identified that in the high-pressure model,
interaction coefficients between the liquid-liquid components and between the liquid
component and air plays an important role during evaporation. Interaction coefficients
help to improve the evaporation rate of heavy molecules.
HFO is primarily composed of high molecular weight hydrocarbons which cannot
evaporate but are pyrolised at high temperatures. Pyrolysis produces volatile gases and
polymers through thermal cracking and polymerisation respectively. Baert’s pyrolysis
model based on chemical kinetics for thermal cracking and polymerisation rate is
developed. Results of this pyrolysis model show that polymer formation within a droplet
is dependent on droplet heating rate and composition. Moreover, it is observed in Baert’s
pyrolysis model results that the process of polymerisation starts prior to the thermal
cracking. This order of thermal cracking and polymerisation is contradictory to the
experimental evidence. Subsequently, Baert’s pyrolysis model parameters are modified.
Results of the modified pyrolysis model did not show any significant dependency of
polymer formation on droplet heating rate and in addition it showed thermal cracking
beginning earlier than the polymerisation.
The low and high-pressure evaporation models along with the modified pyrolysis model
are applied to a single HFO droplet in a high pressure environment, showing good
agreement with experimental results obtained by other researchers. A comparison of the
low-pressure model with the high-pressure model for 100 micron and 30 micron droplets
at high pressure and temperature show that evaporation of the volatile hydrocarbons (nparaffins,
aromatics, naphthenes) from HFO occurs at a faster rate for the high-pressure
model. However, this faster evaporation does not significantly affect the droplet lifetime
because modelled HFO contains only 30% volatile hydrocarbons (cutter stock) by mass.
Therefore, droplet lifetime is found to be similar for both models. Thus in sprays where
droplets are generally small, the VLE calculation can be obtained with sufficient accuracy
by the low-pressure model avoiding the use of the complex high-pressure EOS model.
The low-pressure evaporation and modified pyrolysis models along with a heterogeneous
liquid phase soot burnout model are implemented via subroutines in a diesel spray
simulation in the CFD package StarCD. This simulation is applied to two different
constant volume spray combustion chambers which are used to examine the combustion
characteristics of HFO. The models are tested for two representative fuel samples; one
with the good combustion quality and the other poor. Good qualitative agreement is
shown between the computer simulations and the published experimental data.
A sample of HFO is characterised in the laboratory using chemical characterisation
procedures such as; sequential elution solvent chromatography (SESC), gaschromatography
(GC), mass spectrometry (MS) and elemental analysis, to obtain the
composition and mean molecular weights of HFO fractions required for continuous
thermodynamics modelling. A CFD simulation of the characterised HFO is performed
using the developed low-pressure evaporation and modified pyrolysis models.
|Item Type:||Thesis (PhD)|
|Additional Information:||Copyright 2009 the Author|
|Deposited By:||Mr Ian Bollard|
|Deposited On:||23 Jul 2012 09:32|
|Last Modified:||23 Jul 2012 09:32|
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