Strain Effects in Partially Premixed Methane-air Jet Flames

Abstract

A large-eddy simulation (LES) subgrid combustion model has been developed based on an innovative application of the linear-eddy model to the counter flow configuration (LEM-CF). This subgrid model is developed by parameterizing flame statistics from the LEM-CF sub-model in terms of a reduced set of variables. A database of closure statistics for LES transport equations for these variables is then generated. The run-time application of the model within LES retrieves the closure statistics from the database resulting in a computationally efficient formulation. One key feature of this formulation is that it includes the LEM-CF prediction of turbulent flame extinction limits to develop a localized extinction criterion for the LES. This extinction criterion is a function of the LES resolved scale strain rate which may be directly computed from the LES resolved scale velocity field. The model’s capabilities for capturing strain rate and local extinction effects were assessed through its application to the Sandia piloted partially premixed CH4/air-air jet flame series. A comparison of model predictions for scalar means and RMS fluctuations for Flame D and F of this series showed good overall agreement with the experimental data. This included good predictions of the temperature and product species in the neck region of Flame F which is dominated by local extinction effects. The model was also applied to determine the blow-out jet bulk velocity for this configuration which was found to be in reasonable agreement with what is available from the experiment. Through this blow-out assessment, three flame modes for this configuration are also identified beyond the Flame F condition. The flame dynamics of a highly oscillating flame mode just prior to blow-out were also investigated.