Flame Stabilization Enhancement and NOx Production using Ultra Short Repetitively Pulsed Plasma Discharges

Abstract

*† ‡ § This paper examines the use of plasma discharges in flame stabilization. Three different types of plasma discharges are applied to a lifted jet diffusion flame in a coflow configuration, and evaluated for their abilities to enhance flame stabilization. A single electrode corona discharge (SECD) between a platinum electrode and the flame base is found to maintain the flame at a 20% higher coflow speed than that without the discharge. An asymmetric dielectric barrier discharge (DBD) results in flame stabilization at up to 50% higher coflow speed. The nonequilibrium properties of the DBD are characterized by a spectral line analysis and simulation of the nitrogen 2 nd positive system. Finally, an ultra short repetitively-pulsed discharge (USRD, pulse width of ~10ns) is used in an opposed platinum electrode configuration and found to increase in the stability limit by nearly tenfold. The degree of nonequilibrium of this pulsed discharge is found to be higher than that of the DBD. The stabilization process is sensitive to the positioning of the discharge in the flame flow field, and the optimal position of the discharge is mapped into mixture fraction space by comparing the emission spectra from the plasma-stabilized flame to that in a fully premixed reference flame. The result shows that the local mixture fraction at the optimal position is much leaner than that of a conventional lifted jet flame. In a second part of this study, the USRD is used to stabilize lean premixed methane flames. Nitric Oxide (NO) production is measured using probe sampling and chemiluminescence analysis. While the discharge is a potential source of NO, it is found that the flame partially consumes NO in a reburn mode. The NO production is modeled by use of PLASMAREACTOR followed by the standard PREMIX code. The modeling results show some promise in its ability to predict NO concentration. The flame structure of plasma assisted premixed combustion is also discussed. Under certain conditions, we observe a cold inner flame that has an abundance of OH radicals which have an unusually high vibrational temperature with low rotational temperature when compared to the OH found in a conventional lean premixed flame. While the role of the OH in this inner flame is not fully understood, we believe it may be important in igniting the surrounding combustible mixture.