A diagnostic quasi-dimensional model of heat transfer and combustion in compression-ignition engines.
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Date
1989
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Abstract
Investigations into the combustion of alternative fuels in
compression-ignition engines in South Africa have underlined the
inadequacies of existing zero-dimensional combustion models. The
major aspect of concern in these models was the computation of
heat transfer which had been singled out by a number of
researchers as the leading cause of inaccuracies in heat release
computations.
The main objective of this research was to develop a combustion
model that was less empirically based than the existing zerodimensional
models for use in evaluating the combustion and
resulting thermal stresses generated by alternative fuels. in
diesel engines. Particular attention was paid to the development
of a spatial and temporal model of convective heat transfer that
was based on gas flow characteristics and to the introduction of
a radiation heat transfer model that made use of fuel properties
and fuel-air ratio. The combustion process was divided into two
zones representing burnt and unburnt constituents and the
resulting temperatures in each zone were used in the calculations
of convective and radiative heat transfer. The complete model
was formulated in such a way that it could be applied with the
aid of a micro-computer.
Calibration and verification of the gas flow sub-models which
involved the squish, swirl and turbulence components necessitated
the use of published data. Good agreement for the squish and
swirl components was obtained between the present model and the
experimental data from three engines, two with a bowl-in-piston
and the other with a flat piston. These gas flow components
dominated the gas velocities in the combustion chamber and
provided a reliable foundation for the calculation of convective
heat transfer. In spite of the well documented difficulties of
characterising turbulence, after calibration the model generated
turbulence levels with acceptable trends and magnitudes. Tests were carried out on a naturally aspirated ADE 236 engine
involving the measurement of cylinder pressure and heat flux at
a single point. Motored engine data were used to verify the
convective heat transfer rates and to ascertain the effects of
soot deposition on the heat flux probe. Close correlation
between predicted and measured heat flux was achieved after
accounting for the effects of chamber geometry at the probe site.
Soot deposition on the probe caused a significant attenuation of
the heat flux within a short period of the engine running under
fired conditions.
The results from fired engine tests showed that the two zone
combustion model was providing plausible trends in the burnt and
unburnt zone temperatures and that the model generated combined
heat transfer rates which were credible not only on a global
basis but also in terms of point predictions in the combustion
chamber. The results also highlighted the considerable variation
in heat transfer that could occur from one point in the chamber
to another. Such variations added considerable weight to the
objective of moving away from a zero-dimensional model to a
quasi-dimensional type where predictions could be made on a more
localised rather than global basis. It was concluded that the
model was a definite improvement over zero-dimensional models and
competed favourably with existing quasi-dimensional models with
advantages in both simplicity and accuracy.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1989.
Keywords
Diesel motor--Combustion--Mathematical models., Theses--Agricultural engineering.