An Investigation on the Application of Plasma-Activated Gas to Laminar Lifted Nonpremixed Jet Flames

This paper is to experimentally investigate the effect of plasma-activated gas on the characteristics of a laminar lifted nonpremixed jet flame in a coflow stream. A coaxial dielectric-barrier discharge is employed to activate fuel (C₃H₈) and oxidizer, respectively. It is shown that an enhancement of 12% and 50% in the liftoff velocity is observed for oxidizer and fuel activation, respectively. With the addition of O₃ (1650 ppm) by oxidizer activation, the flame liftoff height is continuously decreased and the blowout velocity is extended. In contrast, a larger liftoff height, accompanied with a smaller blowout velocity, exhibits for flames with fuel activation. The inconsistent effect of fuel activation on the flame liftoff is presumably a contribution of the gas expansion, the conversion rate, and the corresponding product selectivity. The gas expansion is due to the fuel activation that produces lighter hydrocarbons than propane. The conversion rate and the H₂ and CH₄ selectivities are found to be associated with the plasma energy density. The flame propagation can be greatly enhanced to produce a nozzle-attached flame when the plasma energy density is high enough to effectively decompose C₃H₈ into H₂. Nevertheless, a decrease in plasma energy is more sufficient for the cleavage of C-C bonds, resulting in a significant decrease in H₂ selectivity, but a nearly unaltered CH₄ selectivity. The reduced conversion due to the lower plasma energy, along with the gas expansion, produces a lifted flame with a larger liftoff height than the nonactivated flame.