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

5 Results
In total, it includes 99 species and 693 reactions, and nitrogen (N 2 ) is treated as
the species in excess. The emission index of NO x (Eqs. (4.43) and (4.44)) is calculated
from the production rates ˙ω m of the respective species m (Eq. (4.50)).
5.1 Droplets in Exhaust Gas Atmosphere
Droplet combustion in a hot exhaust gas atmosphere is investigated in this
first configuration by a mere numerical study. Generally, an atmosphere of
hot exhaust gas can be assumed for the frequent case of fuel droplets being
injected into a combustion chamber where combustion has already been taking
place. The composition of the atmosphere depends on the combustionrelated
parameters and the progress of the whole combustion process under
consideration. On the other hand, atmospheres of pure (i.e. unburned),
hot oxidizer around a droplet are interesting when studying the influence of
atmosphere composition. Thus, presuming an elevated temperature level,
an exhaust gas atmosphere features a higher conformity with engine-like
conditions than regular air. The latter case was studied in the pioneering
work of Bracco [54, 55], Altenkirch et al. [17], and Kesten [204]. They performed
numerical studies on the nitric oxide (NO) generation in single droplet
flames in the early 1970s. However, strongly simplifying assumptions limit
the significance of those references regarding realistic NO x emissions (see also
Chap. 2.2.3).
Some supplementary work is required before proceeding to the actual single
droplet study here. Its purpose is to determine the exhaust gas composition
used as input data in the main simulations. A one-dimensional laminar premixed
flame with an inlet temperature of 300 K is employed for this purpose
by utilizing the software package Cantera. The pressure is kept constant at
1bar. The fuel is C 10 H 22 again, and the composition of air is set according to
reference conditions (Eqs. (2.22) through (2.24)). The equivalence ratio φ is
varied in the range of 0.5875 to 0.9375. The resulting exhaust gas composition
is taken from a constant axial position within the calculation domain at
x = 0.02 m. 1 This position is chosen only slightly downstream of the flame
front as the exhaust gas approaches equilibrium state farther downstream.
1 Typical profiles of temperature T and the NO x mass fractions were presented beforehand in Figure 2.11 for a
laminar premixed flame and an equivalence ratio of φ= 0.8.
156