Spectroscopic studies of C 2 , CH and OH radicals in low pressure acetylene+oxygen flames

Abstract

Temperatures and concentrations of OH(X 2 II and A 2 ∑+), CH(X 2 II and A 2 ∆) and C 2 (X 3 II u and A 3 II g ) have been further studied in low-pressure C 2 H 2 + O 2 flames. For any given state (with the possible but unlikely exception of upper state OH), vibrational and rotational temperatures are in close agreement. The upper state temperature of a given species always exceeds the corresponding lower state temperature. The highest upper state temperatures normally recorded are 7000 K, for C 2 (A 3 II g ) and OH(A 2 ∑+), but, quite exceptionally, values up to 9000 K are found for OH(A 2 ∑+) at the very earliest stages of combustion. No lower state temperature exceeds 3000 K. The temperature of CH( 2 II) is quite normal, equalling within 100 K the measured translational flame temperatures. The temperatures of CH( 2 ∆), OH(X 2 II) and C 2 (X 3 II u ) are all similar, lying in the range 2500 to 3000 K. C 2 (A 3 II g ) and OH(A 2 ∑+) temperatures are broadly similar too, falling between 5000 and 7000 K. Firm experimental evidence in support of Gaydon’s reaction C 2 (X 2 II 0 ) + OH(X 2 II) → CH(A 2 ∆) + CO(X 1 ∑+) Is provided for the first time; its rate constant at 2200 K is estimated to be 8 ± 4 x 10 -12 ml molecules -1 s -1 . This reaction is not, however, responsible for formation of CH(X 2 H). Neither C 2 (X 3 II u ) nor C 2 (A 3 II g ) are formed in the reaction CH + CH → C 2 + H 2. At the pressures of these experiments, OH(A 2 Ʃ -) is strongly quenehed, but both CH(A 2 ∆) and C 2 (A 3 II z ) decay predominantly through radiative emission.