Effects of NO on the ignition of hydrogen and hydrocarbons by seated counterflowing air

Experiments were carried: out to study the influence of NO in air on the ig nition temperature of hydrogen and hydrocarbons in a nonpremixed counterflo wing system. These experiments were performed from 0.5 to 6 atm, with the N O concentration varying from 100 ppm to 15,000 ppm. It is shown that additi on of a small amount of NO in air significantly reduces the ignition temper ature of all fuels. For hydrogen, under certain pressures, NO eventually be comes an inhibitor at higher levels of addition. Thus there appears to exis t an optimal NO concentration at which the catalytic effect is the most pro nounced, and this optimal concentration was found to also depend on the sys tem pressure. Numerical simulation was performed in the hydrogen case to be tter understand the kinetics of the observed phenomenon. It was found that at low NO concentrations, the ignition temperature was determined by the in teraction of the H-2-O-2-NO subsystem, whereas at high NO concentrations th e ignition temperature was mostly affected by the NOx chemistry. For hydroc arbons, the minimum temperature was much less pronounced and in most cases nonexistent. Furthermore, the extent of temperature decrease depended on th e nature of the fuel. (C) 1999 by The Combustion Institute.