Tutorials Manual

Chapter 2: Combustion in Gas-phase Processes
Tutorials Manual
O 2 /N 2 ratio matches the composition of air, and that the fuel/air ratio is stoichiometric
(two H 2 per one O 2 ). The Normalize button will set the mole fractions so that they
sum to one, although this is optional, since the normalization will also occur
automatically within the program.
On the Basic tab of the Solver panel, the end time of the simulation is set to
0.0002 sec. The Time Interval for Printing is set to 0.0001 sec, which will minimize the
size of the text output file. This is only an issue because all reaction sensitivities are
being printed, which would result in a very large text file. In contrast, the Time Interval
for Saving Data is set to 1E-6 sec, which will provide the time-resolution needed to
study ignition phenomena using the Graphical Post-Processor. The text output file
lists a value for the ignition time near the end of the file.
On the Output Control panel, the criterion for Ignition Delay is specified using a
Temperature Delta of 400 K. This means that ignition will be registered when the
temperature reaches a value of 400 K above the initial temperature.
On the Output Control panel, the checkbox for All A-factor Sensitivity has been
marked. This results in sensitivities being calculated for all species and all reactions,
saved in the XML Solution File, and printed in the output file. This option should be
used with care, as the computation time and solution-file sizes increase as a higher
power of the size of the reaction mechanism. Except in the case of a very small
reaction mechanism, such as the one being used in the sample problem, it is better to
use the Species Sensitivity and ROP panel to request that these quantities be output
only for a few species of highest interest. Including a value for the Threshold for
Species Sensitivity that is higher than the default value of 0.001 will also help keep the
amount of information to a manageable level.
There are no Continuations used for this problem.
2.3.2.3 Project Results
Figure 2-4 shows the temperature profile as a function of time for this problem. At the
end of this simulation, the temperature is still rising; if it is run much longer, the
temperature increases another ~300 K, nearing the adiabatic flame temperature.
Although not shown, the volume in this constant-pressure system shows a
corresponding increase at ignition. The text output file lists a value for the ignition time
of 1.7263E-04 sec, where ignition is defined as the time at which the gas reaches a
temperature of 1400 K. Figure 2-5 shows a close-up of species mole fractions as a
function of time. Note that zooming in on the x-axis shows the expected increase in
radical species and the product at ignition, along with a decrease in hydrogen and
oxygen reactants.
RD0411-C20-000-001 25 © 2007 Reaction Design