On the Formation of Nitrogen Oxides During the Combustion of ...

A
Chemical Mechanisms
All practical fuels are a mixture of a large number of different hydrocarbons.
The combustion of each single hydrocarbon in reality involves hundreds of
species and up to thousands of reaction steps (see Chap. 2.3). While Marchese
et al. [271] were able to simulate the combustion of spherically symmetric
droplets in combination with a mechanism involving 51 species, Sazhin [381]
argues that even a mechanism with no more than 13 species did not prove
applicable for realistic multi-dimensional models (cf. Chap. 5.6).
In numerical modeling, a scalar transport equation must be solved for each
species. However, the system of differential equations is commonly very stiff
due to discrepancies in the timescales of the reactions involved [341, 443]. Implicit
time-integration schemes have to be employed that are computationally
expensive [43, 157]. In order to overcome the issues of a stiff system of equations
and a large number of degrees of freedom, several strategies have been
suggested in combustion modeling, of which some are highlighted in the following
paragraphs. Still, the focus of NO x formation modeling is kept in mind.
A.1 Global Kinetics
Global reaction kinetics can be used for first estimates of temperature and the
major species in flames. These mechanisms consist of only a few species and
one or a few nonelementary reactions. Thus, computational cost remains low.
The conversion of fuel and oxidizer is described by either a single reaction or
a few reactions with some intermediate species. There is no explicit correlation
to the elementary reactions, and reaction equations as well as rate coefficients
for reactions are determined from measurements. This is typically done
by obtaining appropriate values for the adiabatic flame temperature T ad and
flame propagation velocity S L to fit the experimental data in a limited field of
application.
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