Numerical Study of Non-Premixed Methane/Air Flames Behavior under Different Body Forces: Buoyancy and Electric Field
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Numerical Study of Non-Premixed Methane/Air Flames Behavior under Different Body Forces: Buoyancy and Electric Field

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Active control of combustion has always been important given the potential practical applications to improve efficiency and stability, and to reduce unwanted emissions. Body force effects, such as applied electric fields and buoyancy, have been objects of attention given their capability to change small flame behavior. However, distinguishing between different body force effects and then controlling those body forces to affect combustion performance in predictable ways is a challenge that has not yet been solved. In addition to the inherent difficulties defining the flame chemistry in thermally driven buoyant flows, there is the further complication of capturing the relationship between flame charged species chemistry and the ultimate physical influences produced by electromagnetic forces on them. This work presents the development of numerical simulation tools to provide more insights into the coupling between flame behavior, flame chemistry, and different body force effects. Both buoyancy and electric field effects are explored. Also, the comparison of the resultant flame behavior when these body forces are applied to the flame is analyzed to provide possible equivalences in body force control, as well as to examine what might occur during combustion in alternative gravitational environments. There are four major parts in this work. The first one relates to the chemical kinetic model for the flame chemistry with charged and excited species; the second looks into the effects that two different jet burner geometries have on a flame that is exposed to different gravity environments; the third section explores the implementation of applied electric fields in the simulations and how this affects a non-premixed flame at 1g; and the last section carries out a comparison between the effects on flame behavior when body forces of different nature, buoyancy and electric field force, are present. This last section also provides insight into the possible similarities and regimes when both forces could be equivalent. This work starts with the development of a comprehensive chemical kinetic model including excited and charged species. Excited species are rarely included in combustion chemical kinetic models due to their low concentrations and low impact on major chemical pathways in the flame, yet they are crucial for the visual characterization of the flame. Chemiluminescent species CH* and OH* are added to the model since they are well-known markers for flame location and heat release. Additionally, naturally produced charged species in the flame are also included in the chemical kinetic model developed in this work since they will be interacting with the electric field when this is applied to the flame. Considered as the first reaction that naturally produces charged species in the flame, the chemi-ionization reaction -- as well as the subsequent reactions for charged species -- is also included in the chemistry model. Then, the model is validated against detailed chemistry and experimental literature results employing one- and two-dimensional burner geometries. The final reduced model, named Model 1, contains a total of 45 species and 216 reactions. The second part of this work reproduces how buoyancy forces affect the flame. For that purpose, simulations under gravities that vary from 0g to 2g are performed employing two-dimensional CFD calculations. The results are then contrasted with previous experimental and numerical literature. A comparison between both different gravities and different burners is performed, as well as a non-dimensional analysis of the behavior of the flame. Next, an implementation of the electric field solver to the 1g two-dimensional flame geometry is explored. The results are compared with experimental literature, showing similarities for the major characteristics, such as for the I-V curves and the downstream ion current spatial distribution. However, a detailed investigation of the electric field characteristics reveals that a non-physical behavior is obtained for the physical distribution of the charged species when employing the solving method proposed for the subsaturation regime, which is also discussed. Finally, an analogy between body forces and the possible similarities among them is analyzed. This comparison corroborates previous literature mentioning that, even though electric and buoyancy forces are from a different nature, they might be considered equivalent when applying an electric field in a 1g flame to achieve an equivalent supergravity flame. It is concluded that the 2g flame resembles the 1g flame with 0.5kV/cm applied. However, a large regime of supergravity conditions where both forces are equivalent is yet to be explored.

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