Non-premixed axisymmetric flames driven by ion currents

Abstract

Non-premixed flames under the influence of external electric fields can become complicated as ion winds change the transport/mixing dynamics of the flame environment. Burke and Schumman type flames are well studied non-premixed flames and provide a well-suited combustion environment for studying non-premixed flames in longitudinal electric fields. This work studies external electric field effects on a coflow burner stabilized flame. The flame’s spatial features and CH* chemiluminescence light emission are studied under various positive field strengths. The flame base modification was dependent on the geometry of the coflow burner. Despite this geometric dependence, CH* and chemi-ion production rates also increased in the flames that did not show modification to the flame base height indicating that the enhanced ion current is not due to partial premixing as previously suggested. Roper’s flame model for axially symmetric conical type flames was expanded upon to predict what type of electric field forcing is needed to create the observed changes in flame geometries, e.g. flame height and width. The core assumptions made from the classical model were all retained while introducing an ion wind acceleration term to the gas velocity. Ion current measurements from a long wire electrical probe positioned just below the downstream mesh electrode were deconvoluted to reveal the ion current density spatial distribution, and the electrical body force was derived. The calculated body force (200–400 N/m2) agreed well with the theoretical model. The probe measurements also revealed that for the coflow geometry the peak current density develops 2.5 mm from the center axis. This result shows that the plane-to-plane configuration produces a distinctly different current distribution from a point-to-plane configuration whose peak current density remains at the centerline.