Performance and calibration of two subgrid extinction models for turbulent diffusion combustion in an under-ventilated enclosure fire

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The fast-chemistry eddy dissipation combustion model fails to predict abrupt flame self-extinguishment when the oxygen content falls below the critical threshold. To overcome this limitation without recourse to the highly resolved simulations with the detailed chemical mechanisms, the bespoke subgrid extinction model is required. In this work, two subgrid extinction models are implemented in ANSYS Fluent and applied. The first extinction model utilizes conventional critical flame temperature concept, and the second one is the Damköhler number-based approach. The model formulation is used as derived earlier from the perfectly stirred reactor model, with the finite-rate reaction kinetics and radiative heat losses from the unresolved reaction zone.To evaluate model performance and to select optimum model parameters, the burner fire with the constant fuel supply rate (corresponding to the nominal heat release rate 400 kW in complete combustion) is considered in 4x6x4.5 m compartment with the 0.3 m diameter lower air inlet connected to the ambient atmosphere. Conjugate heat transfer at the compartment walls and transient thermal conductivity in the compartment structures are accounted for.Provided that the model parameters are optimized, both extinction models enable prediction of flame self-extinguishing, and the predicted times for flame to extinguish are similar to those recorded in the experiments. For the CFT model, best agreement with the experimental time to flame extinguishment is obtained if the critical flame temperature is set to 1900 K. For the Damköhler number-based approach, the optimum value of 0.25 is found for the model constant in the proportionality relationship between the resolved strain rate and the residence time in the reaction zone, which is not resolved in the simulations.In spite of the similar performance of the two extinction models in the experimental scenario with weakly strained flame, the Damköhler number-based model is expected to be applicable in a wider range of scenarios including highly strained flames such as those affected by strong jets, sprays, and cross-winds.