Modeling non-premixed flames in the presence of electric fields

The impingement of electric fields on flames is known to have potential for mitigating combustion instabilities, enhancing flame propagation and decreasing pollutant emissions. In this work, a computational analysis of counterflow methane-oxygen laminar diffusion flames impinged by electric fields is performed, using axisymmetric numerical simulations, complex transport and a detailed chemistry mechanism, with the final aim of studying in detail the effects of electric fields on the flow and the kinetics. The electric field steers the charged intermediate species, which exchange momentum with the rest of the gas, thereby changing the flow around the flame and creating an ionic wind whereby anions and cations flow towards the corresponding electrodes. As a result, the aerothermal field and scalar dissipation rate undergo variations that may be of significance for the subgrid-scale modeling of turbulent flames subject to electric fields. The results are found to agree well with previous experiments considering the state-of-the-art on this type of calculations. The same numerical configuration has also been studied with a newly developed flamelet model in the mixture fraction space able to account for the impinging electric field. The results of this model have been compared with those of the aforementioned detailed model showing a very good agreement between the two sets of data. Thanks to the lower dimensionality of the model, the computational cost of each simulation is very low and, therefore, it can be employed for computing enough flamelets to construct a complete electrified s-curve for a particular chemical configuration. This study determines the response of the reacting layer to the applied electric field in a wide range of reaction regimes regulated by the variation of the diffusion timescale. The last part of the present work describes an efficient flamelet progress-variable approach developed to model the fluid dynamics of flames immersed in an electric field. The main feature of this model is that it can use complex ionization mechanisms without increasing the computational cost of the simulation. The model is based on the assumption that the combustion process is not directly influenced by the electric field. It has been tested using two chemi-ionization mechanisms of different complexity, in order to examine its behavior with and without the presence of heavy anions in the mixture. Using one- and two-dimensional numerical test cases, the proposed flamelet progress-variable approach has been able to reproduce all the major aspects encountered when a flame is subject to an imposed electric field and the main effects of the different chemical mechanisms. Moreover, the proposed model is shown to produce a large reduction in the computational cost, being up to 40 times faster than the standard simulation methods.