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Modelling of fire plumes remains a formidable challenge even with the large-eddy simulation (LES) approach due to the complex multi-scale, multi-physics interaction. LES-based gaseous fire modelling involves three main components: (a) a radiation model, (b) a chemistry model, and (c) a model for subfilter scale turbulence-chemistry interaction (TCI). The focus of this work is this latter aspect as several recent studies have shown that the description of the subfilter TCI is of primary importance in affecting predictability [1, 2]. To this end, a consistent LES/transported probability density function (TPDF) approach, which is combined with tabulated chemistry based on a radiation flamelet/progress variable (RFPV) approach, is employed to model the subfilter TCI and include detailed chemistry effects. The Sandia 1-m diameter CH4 fire plume is chosen to validate the LES/TPDF/RFPV model, investigate the fire plume structure, and examine the suitability of a widely used presumed PDF assumption in buoyancy-driven turbulence. It is found that while unsteady flamelet effects are taken into account in the tabulated chemistry, the fire plume presents a large amount of steady, radiative flamelet structures due to low scalar dissipation rate encountered and the presence of strong radiation. Furthermore, filtered scalar PDFs at select locations are reconstructed from Monte Carlo particle information to examine the suitability of the presumed β function for mixture fraction (Z) and δ function for progress variable (C) in the context of buoyancy-driven turbulence. The results show that while the subfilter Z-PDF can reasonably be approximated by a β-PDF, the subfilter C-PDF and its source term cannot easily be characterised by a δ-distribution for this fire plume, which suggests that accurate characterisation of subfilter buoyancy-driven TCI may need further consideration in flamelet-based fire modelling. The particular capability of TPDF models allows the LES/TPDF/RFPV method to be a promising approach for including detailed chemistry effects and capturing fire dynamics accurately. |
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