Non-premixed flame dynamics excited by flow fluctuations generated from Dielectric-Barrier-Discharge plasma

Abstract

Plasma-induced flow perturbation and its subsequent dynamic effects on a flame are decoupled, both experimentally and theoretically, from the plasma assisted combustion system. A coaxial Dielectric-Barrier-Discharge (DBD) plasma generator is designed, and the discharge is characterized using probes and cameras. Particle tracing method is elaborately demonstrated to map the velocity of flow fluctuation induced by the plasma, with uncertainty estimated to be less than 5%. Then the plasma generator is coupled with the fuel nozzle of the non-premixed counterflow burner. By using multiple diagnostics, several turbulent-like features are observed from the upstream laminar flow (Re ≈ 300) and the downstream non-premixed flat flame, including the distorted velocity profile, fluctuation intensity above 50%, wrinkled flame sheet, and near −5/3 slope of frequency spectra for both fluctuation velocity and CH* chemiluminescence intensity. The aerodynamic effect on the flame is resolved by more than 90% over frequency spectra and then, characterized using flame transfer functions (FTFs). The experimental results show a negative linear correlation between the FTFs’ gain and perturbation frequency on the logarithmic plot, which is then verified by a theoretical model derived from the Z-equation of non-premixed counterflow flame. Further, the model indicates that for small perturbations, the influence of the global stretch rate on the FTFs is linear, while the effect of the imposed amplitude is negligible, and the dimensionless perturbation frequency (St) scaled by the global stretch rate becomes the only variable.