Experimental investigation of non-equilibrium plasma-assisted ammonia flames using NH2* chemiluminescence and OH planar laser-induced fluorescence

Abstract

This study investigates the effect of nano-second pulsed non-equilibrium plasma on ammonia combustion in a model gas turbine combustor using NH2* chemiluminescence and OH planar laser-induced fluorescence (PLIF). Nano-second pulsed plasma discharge is generated at the exit of the premixed ammonia/air mixture injection tube. Broadband chemiluminescence observation shows that plasma can improve the stabilization of ammonia/air flame and extend the attached flame regime to a lower equivalence ratio. To understand the coupling effect between plasma kinetics and flame dynamics, NH2* chemiluminescence and OH PLIF of ammonia/air flames are measured at different conditions. With the increase of discharge voltage or discharge power, both NH2* chemiluminescence intensity and OH PLIF signal intensity increase, and this observation may explain the improved stabilization of ammonia flames. At very lean condition (equivalence ratio between 0.48 and 0.57, no visible flame existed), both NH2* chemiluminescence and OH PLIF signal are observed in the proximity of electrode region with plasma activation. If air is replaced by pure nitrogen (N2), NH2* chemiluminescence is measurable but extremely weak. As the oxygen (O2) concentration in the oxidizer stream gradually increases, NH2* chemiluminescence intensity increases linearly with O2 concentration under plasma activation. This finding indicates that the NH2* production in plasma-assisted ammonia oxidation process is possibly related to the production of OH or oxygen-related species. The direct electron impact on NH2* production might be secondary.