Spectroscopic studies of low-pressure flames; temperature measurements in acetylene flames

Abstract

Flames at very low pressure haw a relatively thick reaction zone (or flame front) and are especially suitable for detailed study of the combustion processes and of the distribution of energy during the reaction. Temperature measurements have been made, by various spectroscopic methods, on flames of acetylene with air, oxygen and nitrous oxide, in some cases down to a pressure of 1.5 mm. Hg. The excitation temperature has been measured through the reaction zone by the spectrum-line reversal method using Fe, introduced as the carbonyl; the characteristics of the flame containing Fe(CO) 5 are described. The rotational temperature of the excited OH radicals has been determined from the emission spectrum; at pressures above 10 mm. it is fairly constant at around 5700° K, but at lower pressure rises to a higher value of nearly 9000° K. The results are explaihed in terms of the collision and radiative deactivation of the electronically excited OH radicals. These radicals are believed to be formed, as the result of chemical reaction, in the excited 2 Σ state and with a rotational energy equivalent to above 9000° K. Deactivation by collision appears to occur on the average after about forty collisions, if a normal collision diameter is assumed. Removal of electronic excitation occurs mainly by collisions with O 2 molecules, but CO 2 or CO molecules are most effective in removing rotational energy. The variation of concentration of OH through the reaction zone has been determined by its absorption spectrum; it is abnormally high just below the visible reaction zone. Calculations of flame temperature and composition are given. The lack of equipartition of energy is discussed.