Experimental studies of inhibited counterflow flames

An experimental and numerical study was performed to investigate the fundamental mechanisms of chemical inhibition. The first part of this work examined the structure of non-premixed counterflow methane-air flames. Gas samples were taken with a quartz microprobe and analyzed using a gas chromatograph with a thermal conductivity detector. Experimental and detailed numerical results were obtained for an uninhibited flame and flames inhibited by 1.5% CF₃Br and 1.5% CF₃I added to the oxidizer, all with a strain rate of 150s−1. The experimental data showed a slight shift toward the oxidizer duct above the flame, but showed excellent agreement with the numerical results below the flame in all cases.; The inhibiting effect of CF₃Br on a non-premixed diluted hydrogen-air flame was also investigated in the counterflowing configuration. Extinction results were obtained for 15%H₂/85%N₂ and 16%H ₂/84%N₂ in the fuel stream. The experimental results supported the theory that carbon chemistry does not play a significant role in inhibition by CF₃Br. These results were compared with two different numerical models with different inhibition mechanisms.; The effect of partially premixing a methane-air counterflow flame on the extinction strain rate was also examined. Premixing the oxidizer flow had a stabilizing effect, premixing the fuel flow had a weak inhibiting effect, and premixing in both flows had a very weak stabilizing effect that was basically the average of the two individual cases. These results were compared with detailed calculations, asymptotic and one-step analysis. The detailed numerical calculations had excellent agreement with the experiments but the asymptotic and one-step analysis predicted incorrect trends for all cases.; Tests were also performed to examine the inhibiting effectiveness of alkali metal salts. Experiments were performed with NaHCO₃ and KHCO₃ with particle sizes of <30 microns and <20 microns, NaBr and KBr with a particle size of 5–25 microns, and silica (SiO ₂) with a particle size of 1–3 microns. Inhibiting effectiveness increased as the particle sized decreased for all powders. The KHCO₃ was approximately twice as effective as the NaHCO₃, NaBr and KBr for similar particle sizes. These powders were approximately 10 times more efficient than silica and CF₃Br on a mass basis.