1. In this experiment, the composition of partially premixed oxidizer stream was fixed at CH4/O2/Ar/He ratio of 0.02/0.26/0.32/0.4. The fuel stream was CH4/Ar mixture and the CH4concentrations varied from0.2 to 0.4. During the measurements, the fuel mole fraction was fixed, and the flow velocity increased gradually. With the increase of flow velocity, the strain rate increased and the flame had less residence time to complete the fuel oxidation and extinguished when the strain rate was above a critical value. This critical strain rate at flame extinction was recorded as the global extinction strain rate.Figure 11 shows the relationship of extinction strainrates and fuel mole fractions without plasma and with plasma at pulse repetition frequency of f=4 and 10 kHz (with oxidizer temperature To= 423-4 K and 613-5 K, respectively). The experimental results were further compared with numerical simulations. The simulation was computed by using OPPDIFF of the CHEMKIN package[34]with a modified arc-length continuation method[35]for plug flow. The mechanism used in the calculation was USC-Mech II[24]with addition of plasma related reactions discussed in section II. All simulations were performed by setting the boundary conditions using the measured temperatures, flow velocity, and species concentrations by FTIR, GC and TALIF. It is seen that with the increase of the pulse repetition frequency, there was a significant increase of extinction limit enhancement and the experiments agree well with the simulations. The present results indicate that the plasma generated fuel oxidationplaysan importantrolefortheextensionofflameextinction limit.
2. The extinction strain rates measurements were also conducted at f = 20, 30 and 40 kHz (with oxidizer temperature To=678-5K,775-5Kand889-8K,respectivelyandwithmeasuredboundaryconditionsasdescribed above) by fixing the fuel mole fraction and are shown in Fig. 13. At f=10 kHz, the experimental measurement agree with the simulation, but with the increase in f, the experiments and simulations begin to deviate, which may indicate that there are additional reaction paths to enhance the flame extinction. The extinction strain rates at constant enthalpy condition with the original 2% CH4inthepartiallypremixedoxidizerstreamarealsoshowninFig.13asa reference. With the further oxidization of CH4(increasedf)in thepartiallypremixed oxidizerstream,theextinction strainrates increased significantly.
3. As indicated in Fig. 3, when f was larger than 10 kHz, a visible plasma jet appeared. Emission spectroscopy (Ocean Optics, USB2000+ spectrometer) was employed to identify the possible products of the discharge excitation. Apertures and collimating lens were used to collect the emission in the afterglow region beneath the nozzle exit. The strongest emission was from Ar* at 750 nm, together with emissions from He*, O*, OH*, HCO* and CH*. The emission intensity from Ar* at 750 nm is shown at Fig. 14 versus pulse repetition frequency. With the increase of pulse repetition, the Ar* emission in the afterglow region increased, indicating the increased concentration of Ar*. It is possible that the "missing" reaction path(s) affecting flame extinction involve Ar*. A preliminary test on the effect Ar* addition on extinction was performed by simulations and is shown in Fig. 14. In order to mitigate the deviation of extinction strain rates between the experiments and simulations, different amounts of Ar* were tried at different values of f. With the increase in f, more Ar* was required and the trend was similar to the emission intensity curve. Up to 7000 ppm of Ar* was required for the simulation to match with the experiments with f=40 kHz. Optical emission spectra also identified other species like He*, O*, H*, OH*, HCO* and CH*. However, quantitative measurements ofAr* andotherspecies are challenging, and thus measurement of these species will be a focusforfutureresearch.
4. Kinetic Ignition Enhancement of Diffusion Flames by Nonequilibrium Magnetic Gliding Arc Plasma