Tutorials Manual

Chemkin 4.1.1
Chapter 2: Combustion in Gas-phase Processes
2.7.1.2 Project Results
Two simulations, one with both H-abstraction-C 2 H 2 -addition (HACA) and PAH
condensation growth reactions and the other with HACA growth reactions only, are
performed so that contributions of either soot mass growth mechanism can be
identified.
Results of the T JSR = 1630K and Φ = 2.2 case are presented in Figure 2-64 and
Figure 2-66. As can be seen from these figures, predictions obtained by the Particle
Tracking Module are in good agreements with experimental data. The Particle
Tracking Module in general slightly underpredicts gas-phase species. There are many
factors that can contribute to the discrepancies shown in the figures. For example,
Marr did not provide details composition and temperature of the inlet gas mixture to
the JSR. Since the temperature of the JSR is maintained by adjusting the N 2 fraction
in the inlet gas stream, uncertainties in inlet condition, reactor heat loss, and reactor
residence time will surely affect the simulation results in the PFR section behind the
JSR.
Comparison of the predicted and measured soot mass concentration profiles in the
PFR is presented in Figure 2-65. While the HACA-only mechanism shows an
excellent agreement with the data at the PFR inlet, the slope of the soot mass profile
predicted by the HACA-only mechanism (dash-dot line) is much smaller than that of
the experimental profile. This is an indication that the HACA growth mechanism alone
gives a too-slow soot mass growth rate in the post-flame region. Since the present
soot model underpredicts C 2 H 2 mole fraction in the PFR (Figure 2-64), it is possible
that the lower C 2 H 2 concentration leads to lower HACA soot mass growth rate. It is
also possible that another growth mechanism, possibly PAH condensation, might
contribute equally to soot mass growth under this condition. The soot mass growth
rate predicted by the HACA + PAH mechanism (solid line), on the other hand, shows
a much better agreement to the experimental data that the HACA only mechanism
does. Since the sticking coefficients of all the PAH considered here are within the
range suggested by Marr 23 , the PAH contribution to soot mass growth should be
reasonably predicted by the model. However, the HACA + PAH mechanism does
overpredict the soot mass density at the PFR inlet. Note that experimental data
indicate that soot mass density increases by about 4 x 10 -8 gm/cm 3 for the first
5 mini-seconds in the PFR. Since the residence time in the JSR is about 5 miniseconds
and the temperature in JSR is only 10K higher than that of PFR, the soot
mass density at the PFR inlet should be higher than the measured value. Of course,
this assessment is based on the assumption that soot particles start to grow once
they are created inside the JSR.
© 2007 Reaction Design 82 RD0411-C20-000-001