Influence of electric fields on premixed laminar flames: Visualization of perturbations and potential for suppression of thermoacoustic oscillations

Abstract

In this experimental study the effects of weak DC electric fields on laminar flames are studied. In the first part transient DC electric fields were applied with different frequencies (1–200Hz) to a laminar Bunsen type flame for the investigation of the changes occurring in the flow field and the flame structure using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV). For various low frequencies similar response behavior of the flame was observed. After the activation of the electric field the flow field changes inducing a disturbance at the flame root which travels as a wave-like oscillation towards the flame tip until stationary conditions are reached. For higher frequencies different flame behavior was revealed since the process cannot establish a steady state anymore. For 100Hz the detected flame response time and the pulse duration of the electric field are similar leading to the largest flame oscillation amplitude. For 200Hz the time between activation and deactivation of the electric field is too short as compared to the flame response time. Therefore, the influence of the electric field is very weak. From this changed response behavior a limiting frequency of the ionic wind for technical application in the range of 100Hz was derived.In the second part the potential of electric fields to suppress acoustic flame instabilities was studied on a flat flame using the high-speed detection of the flame chemiluminescence. Acoustic oscillations were generated by an injector with defined injection frequencies resulting in a periodically lifted flame. Depending on the electric field strength the duration of flame oscillation could be reduced by up to 54%, so the flame returns quicker to its initial condition.