Incorporating reduced kinetic mechanisms in numerical simulations of nonpremixed flames.

摘要

In this study, numerical investigations are conducted to understand the limits of applicability and influence of progressively more accurate levels of reduction of fuel oxidation kinetics on nonpremixed flame structure, extinction characteristics, and turbulent flow structure under both steady and unsteady flow conditions. One-, three- and four-step reduced kinetic mechanisms for nonpremixed methane-air flames are employed to represent the combustion processes. First, steady flame calculations are performed in a one-dimensional Tsuji burner configuration. Next, unsteady Direct Numerical Simulations (DNS) is carried out for a two-dimensional flame interacting with a pair of vortices initialized on the fuel side to understand the dynamics of flame-vortex interaction. Lastly, interaction between a two-dimensional field of homogeneous decaying turbulence and a nonpremixed flame is investigated using the same DNS technique.; Results from the one-dimensional flame study indicate that flames respond differently to increasing strain rate in mixture fraction space. Although the flame structures are more accurately predicted using three- and four-step chemistries, this study shows the correlation between the accuracy of extinction limits prediction and increasing complexity of the combustion schemes is not straightforward. The two-dimensional unsteady flame-vortex interaction study shows consistent behavior with respect to the shifting of respective reaction zones and CH₄ species as in the one-dimensional steady Tsuji study. DNS data shows that the concentrations of H₂ and H species within the flame zone play a significant role in the unsteady flame-vortex dynamics. Key transient features observed in the three- and four-step mechanisms are not captured by the global one-step mechanism, thereby making a three-step model a minimum requirement in investigating flame structure modification. A model for the equivalent quasi-steady strain rate is proposed and validated. Results from the turbulent flame study with three- and four-step kinetic mechanisms show flame wrinkling due to unsteady effects. Localized extinction of the flame is not observed for the range of parameters investigated. Modification of the flame structure due to turbulence is consistent with that under the continuous burning conditions in the flame-vortex study. (Abstract shortened by UMI.)