Quenching of laminar iso-octane flames at cold walls

Quenching of laminar premixed iso-octane flames at cold walls is studied using detailed kinetics. Previous investigations of flame quenching used low-molecular-weight fuels such as methane, methanol, and acetylene. For these fuels postquench oxidation of hydrocarbons is very fast and the amount of intermediate hydrocarbons in the quench layer is low compared to the amount of unreacted fuel. However, this does not hold true for more complex, higher-molecular-weight fuels which exhibit different characteristics, leading to higher levels of intermediate hydrocarbons in the quench layer than for unreacted fuel. Oxidation is considerably slower, resulting in very high levels of unburned hydrocarbons in comparison to the simple, low-molecular-weight fuels. In this study calculations are performed with iso-octane for pressures of 1, 5, 10, and 20 atm, initial temperatures of 300, 400, and 500 K, and equivalence ratios of 0.9, 1.0, and 1.1. The oxidation of intermediate hydrocarbons predominantly controls the overall evolution of unburned hydrocarbons. Thus, the use of global chemistry appears to be inadequate to describe quenching of more complex fuels. The influence of the Soret effect which is often neglected in flame studies is investigated in terms of postquench oxidation. A short mechanism for iso-octane applied previously to flame propagation was found to be inadequate to describe the hydrocarbon evolution after quenching. Especially for low pressures, agreement is not satisfactory. It is shown that by adding a small number of species and reactions to the reduced mechanism, results are improved, leading to better agreement between the detailed and the short mechanism in its extended version. (C) 2000 by The Combustion Institute.