This study fits within the context of turbulent combustion modelling in fires. Compared to jet flames, fire
configurations are typically characterized by low scalar dissipation rates and strong influence of
buoyancy. Furthermore, when it comes to extinction, particularly interesting is a low scalar dissipation
rate limit, where extinction occurs due to radiative heat losses. This motivates the present study where
different radiation approaches are tested at low scalar dissipation rates, as well as different chemical
mechanisms for methane combustion, since chemistry plays an important role when it comes to
extinction. The following chemical mechanisms are used: a one-step global mechanism [1], the
‘Smooke’ mechanism [2], the ‘ARM2’ [3] mechanism and the ‘GRI3’ [4] mechanism. Each of these
chemical mechanisms is further combined with two different radiation models, an optically thin model
with radiative properties based on the RADCAL model [5], and a model with a prescribed constant
radiative fraction as obtained from the experiments. The impact of radiation is quantified through
comparison to cases without radiation. The turbulent combustion model is the Conditional Moment
Closure (CMC) method, where no transport in physical space is considered and thus the flow field
simulations are not required. As the transport effects in physical space are not considered, an unsteady
laminar flamelet model [6] is effectively applied and referred to as “CMC-0D”, as it corresponds to a 0D
reactor flow computation. The settings for fuel and the oxidizer correspond to the UMD line burner [7],
for which a range of cases has been studied experimentally, with variable degrees of local extinction due
to an increase of the N2 concentration in the oxidizer. The fuel is pure methane. The values of O2 mass
fraction at extinction obtained with the GRI3, ARM2 and Smooke mechanism are close to each other,
regardless of the radiation model, for a range of scalar dissipation rates. Results obtained with the onestep
mechanism deviate more strongly, confirming that a one-step mechanism is not a good choice to
predict extinction due to dilution of air. An important finding of the study is that the constant radiative
fraction model fails to predict a low scalar dissipation rate extinction limit, which the RADCAL-based
model does predict. Considering the CO values, the results from the ARM2 and the Smooke mechanisms
differ quite strongly, with relatively low values with the Smooke mechanism for the range of scalar
dissipation rates. Results obtained from the GRI3 are very close to the ARM2 results (within 2%) and
therefore it is possible to reduce the computational costs, by relying on the ARM2 mechanism.
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