The influence of chemical mechanisms on PDF calculations of nonpremixed piloted jet flames

Seven different chemical mechanisms for methane are used in PDF model calculations of the Barlow and Frank flames D, E, and F in order to investigate the ability of these mechanisms to represent the local extinction, reignition, and other chemical phenomena observed in these nonpremixed piloted jet flames. The mechanisms studied range from a 5-step reduced mechanism to the GRI3.0 mechanism which involves 53 species. As in several other recent studies, we use the PDF method based on the joint probability density function of velocity, turbulence frequency, and composition. Extensive tests are performed to ensure the numerical accuracy of the calculations, to relate them to previous calculations based on the same model, and to reexamine the sensitivity of the calculations (especially of flame F) to uncertainties in the pilot temperature and the treatment of radiation. As has been observed in other studies of laminar and turbulent nonpremixed flames, we find that the GRI3.0 mechanism overpredicts the levels of NO, typically by a factor of 2. Apart from this, the GRI3.0 and GRI2.11 mechanisms yield comparably good agreement with the experimental data for all three flames, including the level of local extinction and the conditional means of major and other minor species. Two augmented reduce mechanism (ARM1 and ARM2) based on GRI2.11 and containing 16 and 19 species are slightly less accurate; while the 5-step reduced mechanism and two C1 skeletal mechanisms containing 16 species display significant inaccuracies. An examination of the autoignition and laminar-flame behavior of the different mechanisms confirms (with some exceptions) expected trends: there is an association between long ignition delay times, small extinction strain rates, and high levels of local extinction. This study again demonstrates the ability of the joint PDF method to represent accurately the strong turbulence–chemistry interactions in these flames, and it clarifies the necessary level of description of the chemical kinetics.