Impact of electric body force on lifted flame displacement speed: Numerical insights

Abstract

Recent advancements in experimental capabilities have unveiled that externally applied electric fields primarily affect flames through flow modification induced by an electric body force. Specifically, the displacement speed of a flame could be significantly increased (decreased) by a parallel (orthogonal) electric field to the flame's propagation direction, while the flame's propagation speed remained negligibly influenced by an electric field. Here we present numerical findings attributing a modified upstream velocity to an electric body force created by redistributed space charges and a resulted electric field. Our investigation involved both simulation and experiment on propagating edge-flames from a stationary lifted flame exposed to the parallel external electric field. Nitrogen diluted methane jet in a coflow air was employed, and seven charged species (electron, H3O+, O2–, O–, OH–, CHO3–, and CO3–) were included in a reduced methane kinetic mechanism. Both the experiment and simulation consistently demonstrated that enhanced flame displacement speeds resulted from a reverse flow in the upstream, created by the electric body force. A stronger reverse flow, and thus more enhanced flame displacement, was achieved when the electric field direction was aligned with the flame propagation direction compared to the opposite direction. This difference in the reverse flow was attributed to variations in the electric field, caused by the different electric field screening zone within the flame. Surprisingly, space charge densities in the upstream were similar in both polarities, challenging previous hypothesis. The rapid transformation of electrons into O2–, CHO3–, and CO3– imposed a substantial number of charges in the upstream. This insight enhanced our understanding of how flame displacement can be controlled by applying electric fields. Importantly, we suggested that the use of a simple ionic kinetic model may fail to predict the flow field and, consequently, cannot adequately investigate modified flame characteristics.