Modeling and Simulation of Charged Species in Lean Methane-Oxygen Flames
Charged species occur in all combustion systems. Chemical pathways which involve ions are known to contribute to the formation of air pollutants like soot and aerosols. The appearance of electrical charges, which are closely related to the combustion process itself, offers the opportunity of their use for sensing purposes in a variety of applications. Reliable models for the chemical reaction network and the transport processes are required for numerical simulations in these fields of research. Laminar flat flames have proven to be suitable systems for the development and validation of chemical reaction mechanisms. In this work, the concentrations of charged species along a flat, fuel-lean, and laminar methane-oxygen flame are calculated and compared to experimental results. For the first time, these simulations also include negative ions. Existing software was enhanced to enable the inclusion of these ions. The chemical reaction mechanism of the charged species is compiled from different sources found in according literature. Altogether, the model contains 65 reversible reactions involving 11 charged species. Also special emphasis is put on the diffusion processes of the ions. A model is developed and discussed which describes the mutual interactions of the charged species during diffusion. It allows an arbitrary fraction of negative ions, because it does not depend on the assumption that the electrons dominate this process. The simulations are used to validate the reaction mechanism. Reaction flow analyses show which chemical pathways are taken. Reactions which were suggested in the literature are discussed quantitatively. The influence of charged species diffusion on the simulation results as well as their sensitivities to uncertainties in the input data of the transport model are analyzed.
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