The unsteady response of radiating laminar diffusion flames exposed to decreasing mixing rate conditions representative of fires
Résumé
The objective of this study is to bring fundamental information on the coupling between combustion and thermal radiation occurring at flame scale. The study considers a simplified configuration corresponding to one-dimensional counterflow planar laminar diffusion flames subjected to time-evolving moderate-to-slow mixing conditions that are representative of fires. The flame simulations are performed assuming methane–air combustion and using two formulations: a simplified formulation featuring single-step chemistry and an optically thin flame radiation model; and a high-fidelity formulation featuring detailed chemistry and different radiation models based on solving the Radiative Transfer Equation combined with grey or non-grey descriptions of gas radiation properties – the non-grey description is based on a Weighted-Sum-of-Gray-Gases model. The simulations describe the gradual decrease in heat release rate resulting from the evolution towards slower mixing conditions, as well as the corresponding increase in the flame optical depth and decrease in peak flame temperature. Radiation extinction is observed at critically low values of the rate of mixing. The analysis demonstrates that for conditions before the extinction limit is reached, the flame belongs to the semi-unsteady regime in which mixing processes occurring in the outer diffusive layers of the flame are unsteady whereas heat release processes occurring in the inner reactive layer remain quasi-steady. In the semi-unsteady regime, the flame structure can be parametrised by an effective strain rate (or scalar dissipation rate) and remains close to the structure of radiating flames considered at steady state and subjected to the effective value of the strain rate.