Troels Dyhr Pedersen.indd - Solid Mechanics

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Chemical reaction mechanism implementation
STAR-CD offers two choices of solvers for version 4.10: The “coupled complex
chemistry” solver and DARS-CFD. While DARS-CFD would be the preferred option,
licenses were not available and could not be acquired in time for this investigation.
The coupled complex chemistry solver allows up the 30 species in a mechanism. These
species are defined as scalars in the software. The reaction mechanism is implemented as
a list of forward reactions. Reverse reactions are calculated by STAR assuming chemical
equilibrium. The reduced mechanism for DME combustion developed earlier includes
only 27 species and is thus applicable to the solver.
Formatting of the input file is critical and must be done according to the rules dictated by
the manual. These are slightly different from the format used by CHEMKIN. It should be
mentioned that it is absolutely critical to check the contents of the cpl.inp01-echo file
created by STAR, which holds the information that STAR-CD uses. It was found that
minor deviations from the required procedure in making the input file resulted in severe
misinterpretations by STAR, such as several orders of magnitude in reaction parameters.
The transport properties are defined in the file tran.dat which is identical to the one used
by CHEMKIN.
12.4.2 Simulation results
Although a number of variations were made to the setup of the test case, not much
difference was found in the transient solution in those cases that successfully resulted in
detonations. The general tendency for development in temperature, pressure and gas
velocity is shown in figures 29-31. Each figure is composed of 10 frames recorded at 5
microsecond intervals, with the first frame at top. The scale for temperature is Kelvin, for
pressure it is Pascal and for gas velocity it is m/s.
To create a disturbance that will ultimately trigger the detonation, a small section at the
end of the volume was subjected to an elevated temperature of 1500 K. This establishes a
flame front (left of the pictures) as well as inducing a small pressure wave that travels
through the volume. This method is widely used to trigger detonation in experiments as
well, typically by igniting the mixture with a spark plug.
While the flame is travelling it is accelerated into the unburned mixture by the expansion
of the gas in the burned fraction behind the flame. This movement compresses the
mixture ahead, which is also starting to react. At some point, typically when that flame
front has moved less than a centimetre through the volume, a minor section of the
mixture auto ignites some distance ahead of the flame. When this happens, the rapid
expansion triggers the development of the detonation.