UC Irvine

Electric fields can have several effects on the behavior of hydrocarbon flames: they may affect flame shape, burning velocity, temperature profile, speed of propagation, lift-off distance, species diffusion, stabilization, and extinction. The reason is that combustion of hydrocarbon fuels involves a chemi-ionization process, which generates electrically charged species, namely ions and electrons; external manipulation of these chemi-ions can potentially produce two major effects on the flame: (1) alteration of the chemical kinetics and (2) generation of a body force.

The former arises because the chemistry of the system is affected by the redistribution of charges due to their mobility and to the direction of the applied electric field; the latter includes physical effects: ion wind and Ohmic heating.

The applied electric field makes charged species acquire momentum, which is then lost during collisions with neutral molecules; these multiple collisions have two consequences. Chemi-ions gain a drift velocity, which depends on their mobility, that is what makes them travel toward the respective oppositely charged electrode; while neutral molecules gain a small net velocity in the same direction (known as ion wind effect), which produces a net force whose contribution is included in the momentum equation.

The Ohmic heating represents the work done by electrostatic forces; it includes both the work done by the electric field on the charged species (by pushing them toward the electrode of opposite charge) and the work done by the chemi-ions themselves (because they have also a diffusion velocity).

The aim of this thesis is to better understand, through numerical simulations, the effects of chemi-ionization and electric fields on a non-premixed coflow flame that impinges on a metallic plate; this configuration is very useful in order to investigate both how electric fields can be used to reduce carbon monoxide emissions and how they affect the heat flux on a solid surface. In order to analyze how chemi-ionization and electric fields influence the fluid dynamics and the chemistry of a flame in this configuration, numerical simulations have been performed using OpenFOAM. The validation of the numerical model has been performed by comparing numerical results to experimental result; these comparisons show that, taking into account some simplifying assumptions (e.g., axialsymmetric geometry, absence of radiation heat losses, unitary Lewis and Schmidt number) introduced in the model, results agree well with literature findings.