Investigation of NO production and flame structure in plasma enhanced premixed combustion

Abstract

A pulsed plasma source 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 observed that the flame also consumes some NO. The NO production in the flame is modeled by use of a modified opposed diffusion flame simulation (OPPDIF), augmented by the initial radical pool created by the discharge. The simulations are in qualitative and quantitative agreement with the measurements, despite the simplified modeled geometry. The structure of this plasma assisted premixed flame is discussed within the framework of two interacting premixed streams, as simulated by the OPPDIF configuration. Under certain conditions, we observe a cold pre-flame associated with the plasma-excited premixed stream, the products of which (OH radicals) have an unusually high vibrational temperature and low rotational temperature in electronically excited states, when compared to the OH characteristics found in a conventional lean premixed flames. While the role of the OH radicals in this cold pre-flame stream is not fully understood, we believe it plays an important role in igniting the surrounding combustible mixture, as confirmed by the simulations.