Experimental study of the structure of several fuel-rich premixed flames of methane, oxygen, and argon

A detailed experimental investigation has been performed on the structures of five low-pressure CH4/O-2/Ar flames at equivalence ratios of 0.92-1.94 (20-60 Torr). Stable compounds, atoms and radicals have been monitored using molecular beam mass spectrometry (MBMS). The maximum mole fraction of CH3 radicals is roughly similar in all the flames investigated, whereas those of C2H2, C2H4, C3H3, C3H4 and C4H2 increase strongly with the equivalence ratio. For these species there is a dependence on the equivalence ratio (Phi) in the form (X(i))(max) a(i) Phi(ni) over the equivalence ratios investigated. These results have been supported by a simple kinetic mechanism involving these species. The exponent n(i) on Phi depends on the species considered and varies as follows: C4H2 > C2H2 > C3H3 > C3H4 > C2H4. This means that the peak mole fraction of C4H2 increases faster than that of C3H4 with the equivalence ratio. From the net reaction rate of C2H4, the rate coefficient has been determined for the reaction CH3 + CH3 --> C2H6, which is the main process leading to the first C-2 compound. Moreover, it has been established that the formation of C3H4 must proceed through the formation of the propenyl radical C3H5 by reaction between C2H2 and CH3. The experimental rate coefficient of this reaction is 5.5 +/- 2.5 x 10(10) cm(3) mol(-1) s(-1) at 1670 K. In addition, disappearance of C2H2, C3H4 and C4H2 by reaction with H atoms has been examined. The deduced rate coefficients at 1650 K are 3 +/- 1 x 10(11), 1.75 +/- 0.5 x 10(12), 8.1 +/- 3 x 10(11) cm(3) mol(-1) s(-1), respectively. This work provides experimental information for further comparison with modeling