The use of carbon monoxide/hydrogen burning velocities to examine the rate of the CO+OH reaction

Abstract

New measurements of burning velocity are reported for selected CO/H 2 mixtures in air, to provide datafor examining the rate of the CO+OH reaction. Burning velocities were obtained from measurements on constant-pressure expanding spherical flames. Flame speeds over the first 35 mm of travel were measured by high-speed schlieren cine photography and extrapolated to infinite radius, using a simple phenomenological model, to yield one-dimensional (1D) values. Extensive computational testing shows that the burning velocities obtained are accurate to within 3%. Experimental burning velocities were then compared with 1D computed values, obtained using full kinetics. Fuel mixtures of 95% CO+5% H 2 and 50% CO+50% H 2 with air were studied across the stoichiometricrange, along with stoichiometric mixtures with varying H 2 /CO ratio. The 95+5% mixture was the prime object of study, since computation revealed that burning velocities for this mixture had the greatest sensitivity to the CO+OH reaction. For the 95+5% mixture, the maximum burning velocity was 0.65 ±0.02 m s −1 at 51.7% fuel; the stoichiometric value was 0.34 m s −1 . For the 50+50% mixture, the corresponding velocities were 1.82±0.06 (at 47.2% fuel) and 1.20 m s −1 . Computed 1D burning velocities agreed well with experiment when the 1976 recommendation of Baulchet al. for the rate of the CO+OH reaction was used. A more recent recommendation gave values more than 20% too high. Further computation revealed that the reaction exerts its influence on the burning velocity of these flames at low temperatures, in a relatively narrow region around 1165 K. Use of different (artificially derived) rate expressions, but having a common value at 1165 K, produced essentially the same burning velocities. Implications of this for flame chemistry are explored.