Computational Investigation Of Highly-Turbulent Counterflow Flames Using Large-Eddy Simulation/Probability Density Function (Les/Pdf) Methods

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The understanding of the complex interactions between turbulence and chemical reactions is of fundamental importance for the development of practical combustion devices with improved efficiency and low emissions. In this work, our focus is to understand these interactions in the experimentallystudied Yale turbulent counterflow flame (TCF) burner using large-eddy simulation/probability density function (LES/PDF) methods. The TCF configuration consists of two coaxial opposed nozzles which emit highly-turbulent streams of varying species compositions depending on the mode considered. In the LES/PDF computational methodology, LES is used to represent the flow and turbulence, and the PDF method is used to represent the turbulencechemistry interactions. The high-fidelity LES/PDF simulations of the TCF configuration are performed on a small cylindrical computational domain that encompasses the volume between the two nozzle exit planes. A new methodology is developed to specify inflow boundary conditions for the non-trivial velocity field at the nozzle exit planes in these simulations so as to match the turbulent Reynolds number Ret in the simulations to that of the experiments. The method is found to be successful when implemented in the coupled LES/PDF simulations on the small domain for three different modes of the counterflow configuration: N2 vs. N2 ; N2 vs. hot stoichiometric combustion products; and CH4 /N2 vs. O2 . Subsequently, the premixed mode of the Yale TCF burner - CH4 /O2 /N2 vs. hot stoichiometric combustion products - is investigated using a detailed analysis of conditional statistics. The LES/PDF simulations predict the experimentally-observed trends of the effects of four critical parameters on the interactions of turbulent premixed counterflow flame with the counterflowing combustion products very well. In a companion study, we investigated the behavior of the LES/PDF models in the direct numerical simulation (DNS) limit using two different premixed laminar flames. The 3D LES/PDF simulations of the turbulent premixed flames are appraised using this companion study to determine if the simulations can be considered to be in the DNS limit. Finally, we investigated extinction/reignition events in two contrasting turbulent premixed flames of the Yale TCF burner, namely, high-burning (HB) and low-burning (LB) flames, that have different probabilities of extinction to determine how the compositional structure of the two turbulent flames are related to their laminar counterparts. The cell-mean profiles resemble the laminar profiles at different strain rates even though the LES/PDF are non-trivially 3D and unsteady. Further, the scatter plots from the particle data are quite different for the two flames with more samples close to the extinguished laminar profile for the LB flame than for the HB flame.

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Turbulent counterflow flames; Large eddy simulations; Probability density function methods


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Pope,Stephen Bailey

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Mechanical Engineering

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Ph. D., Mechanical Engineering

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Doctor of Philosophy

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dissertation or thesis

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