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Daniele Tabacco and Claudio Bruno, University of Rome La Sapienza
- Department of Mechanics and Aeronautics, Rome, Italy; and Giorgio Calchetti
and Marco Rufoloni, Italian National Agency for New Technology, Energy,
and the Environment (ENEA), Rome, Italy
The 3D hybrid mesh used for all simulations
Researchers at the Combustion Technology Section of ENEA (Research Center
La Casaccia) in Rome recently validated FLUENT through a series of simulations
of the WS Rekumat C-150 B burner. Operation of the 40 kW burner is based
on flameless combustion technology1 , which gives rise to high process
efficiency with low pollutant emissions. While the burner is designed
to operate in either conventional flame or diluted combustion (flameless)
modes, only the latter was the subject of the present studies. Measurements
were made for three sets of operating conditions, corresponding to process
temperatures of 950, 1050, and 1150 K. These were compared to results
predicted by FLUENT for the corresponding conditions.
The measured temperatures for the 1050 K reference temperature case
The temperatures predicted by FLUENT for the 1050 K reference temperature
case, using the one-step Magnussen model
In FLUENT, two different combustion modeling approaches were tested:
the mixture fraction/pdf method, using an equilibrium assumption, and
the Magnussen, or finite rate/eddy dissipation method, using a one-step
reaction mechanism. For both sets of simulations, NOx prediction was performed.
The realizable k-e turbulence model was chosen to give the most accuracy
for the least amount of CPU effort, based on earlier benchmark tests performed
for similar conditions. Radiation was incorporated through the use of
the discrete ordinates (DO) model.
Both the pdf and Magnussen models gave good qualitative agreement with
the experimental data, with the Magnussen model outperforming the pdf
model in its prediction of centerline temperatures for the low and moderate
process temperature cases. This result suggests that at these temperatures,
the diluted combustion is controlled more by kinetics than by turbulent
mixing. The equilibrium assumption at the core of the pdf model fails
to accurately predict the ignition delay in this regime.
At the highest process temperature, the ignition delay is reduced. The
Magnussen model overpredicts the delay as well as the maximum temperature.
The pdf model, on the other hand, comes closer to predicting the overall
temperature field, even though the maximum temperature is again higher
than that suggested by the measurements. This result suggests that turbulent
fluctuations in the local temperature and mixture fraction, which are
better handled by the statistical methods of the pdf model, play a more
important role in this regime of operation. Temperature fluctuations were
found to play a significant role in thermal and prompt NOx production
at the higher temperature, as well.
Reference :
1 Wunning, J. A., and Wunning, J. G., Burners for Flameless Oxidation
with Low-Nox Formation Even at Maximum Preheat, Journal of the Institute
of Energy 65, 35-40, 1992.
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