Laser investigations on a plasma assisted flame

Abstract

Sustainable and low emission combustion requires new combustion paradigms and solutions to increase efficiency, comply with more stringent regulations on pollutants, and cope with the varying qualities of renewable fuels. Plasma Assisted Combustion (PAC) could be one of the tools to achieve these goals in practical combustion systems. Previous studies showed that PAC can be used in a variety of applications: to improve ignition in difficult environments, to extend the operating range of burners to leaner conditions, to contrast thermoacoustic instability, to allow flame-holding in extreme conditions, and more. While applications keep being proposed, there are efforts to model and understand the coupling between flames and plasma discharges. This work contributes to the unraveling of the action of plasma discharges on flames by performing a number of investigations on a simple PAC burner. Trends and temporal evolution of key chemical species and electric fields are measured during plasma actuation of the flame. Experimental datasets resulting from this work are meant to be used in cross-validating numerical simulations. The considered PAC burner generates a lean methane-air stagnation flame, across which discharges are applied, developing partially in the fresh and partially in the burned gases. Time-resolved 2D imaging of atomic hydrogen and oxygen is obtained by using two-photon absorption planar laser induced fluorescence (TALIF) while OH and CH radicals are measured by using planar laser induced fluorescence (PLIF). To measure the electric field, the Electric Field Induced Second Harmonic generation (EFISH) technique is used. A novel deconvolution-like post-processing procedure is proposed and used to calibrate the measurements and improve the spatial resolution, overcoming limitations and distortions typical of EFISH measurements. Presented results quantify the effect of the plasma actuation on the flame and lend themselves to the validation of numerical models.