Large Eddy Simulation of a premixed jet flame stabilized by a vitiated co-flow: Evaluation of auto-ignition tabulated chemistry

A tabulation technique assuming an auto-ignition dominated reaction pathway for highly turbulent premixed combustion is presented, implemented in an LES framework and evaluated. The tabulation method enables the reduction of the chemical system dimension to two scalars, allowing a computationally efficient model implementation, yet still retaining a sufficiently accurate representation of the chemical kinetics. The sensitivity of the LES model to the grid, inflow conditions, subgrid model, tabulation method assumptions and the chemical mechanism used in the tabulation process is evaluated with reference to detailed experimental measurements. The particular chemical mechanism utilized for the tabulation is shown to have a significant effect on the CO and OH concentrations, whilst only a small influence on the temperature and mixing fields. Comparisons with laminar flame based tabulation explain the misprediction of CO concentration. However, both the auto-ignition and laminar flame based tabulations fail to capture the OH concentration. The ability of the two tabulation techniques to capture the non-flamelet structure is discussed and the predictive capability of the two approaches is established. The general utility of a global Karlovitz number for describing the combustion regime and hence the selection of an applicable combustion model is brought into question considering that the variation of the local Karlovitz number in the simulations varies by up to 2 orders of magnitude, indicating a broad range of accessed flame structures.