Kinetic Modeling of Plasma-Enhanced Vitiated Combustion

Plasma-enhanced combustion can improve the performance of combustion systems for which ignition and flameholding are issues through augmentation of radical species concentrations. Electron impact generates electronically excited N-2 and O-2, both of which participate in reactions that create atomic oxygen and nitrogen. OH and H concentrations are also altered, both through equilibration of the radical pool, and through direct production pathways from the excited N-2 and O-2. In the case of vitiated combustion, it has been demonstrated that the presence of NO in the oxidizer stream enhances ignition. However, the impact of the plasma enhanced radical pool on the effects of vitiation is unknown. It was the goal of this work to explore the impact of plasma generated species on vitiated kinetics, and the resulting effect on ignition. In this paper we describe the development and validation of a model for ethylene-air ignition by a nanosecond pulsed plasma. The model employs a zero-dimensional simulation of the plasma to predict the radical concentrations produced by the plasma. These concentrations are then used as input to an ignition delay time calculation using a kinetic mechanism for vitiated combustion that has been previously developed by the authors. The modeling results show that the O-atom, H-atom, N-atom and C2H3 production by the plasma are important in determining ignition delay. The ignition delay time calculations show that both vitiation and plasma effects are important in determining ignition delay, with the plasma effect becoming dominant as the plasma strength is increased, especially in the low-temperature oxidation regime. Thus in designing practical plasma-assisted combustion systems, an understanding of these effects is important in determining the plasma requirements to achieve the desired level of combustion enhancement.