Plasma assisted combustion: Dynamics and chemistry

Soot formation in laminar counterflow flames

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

Plasma-Discharge Stabilization of Jet Diffusion Flames

The effects of electric fields on the reattachment of lifted flames have been investigated experimentally in laminar coflow jets with propane fuel by applying high voltages to the fuel nozzle. In case of AC, the frequency has also been varied. Results showed that reattachment occurred at higher jet velocity when applying the AC voltages, thus the stabilization limit of attached flames was extended by the AC electric field. Higher voltage and lower frequency of the AC were found to be more effective. On the contrary, the effect of DC was found to be minimal. To understand the early onset of the reattachment with the AC, occurring at higher jet velocity, the influence of AC electric fields on the propagation speed of tribrachial flame edge was investigated during the transient reattachment processes. The propagation speed increased reasonably linearly with the applied AC voltage and decreased inversely to the distance between the flame edge and the nozzle electrode. Consequently, the enhancement in the propagation speed of tribrachial flame edge was correlated well with the electric field intensity, defined as the applied AC voltage divided by the distance.

K.T. Kim

Papers

Soot suppression by nonthermal plasma in coflow jet diffusion flames using a dielectric barrier discharge