An experimental and computational study on the propagation and kinetic structure of laminar premixed flames.

摘要

The steady propagation and chemical kinetic structure of adiabatic, planar, laminar premixed flames were experimentally and numerically studied with systematic and extensive variations of the effects of fuel, oxidizer, pressure, stoichiometry, and flame temperature. Experimentally, the laminar flame speeds were determined by using the counterflow twin-flame configuration, with laser Doppler velocimetry probing and systematic elimination of flame stretch effects. Numerical simulation was conducted by using validated hydro and transport codes together with various detailed chemical kinetic mechanisms. The parameter matrix of investigation included H{dollar}sb2{dollar}, CO, CH{dollar}sb4{dollar}, all C{dollar}sb2{dollar}-hydrocarbons, and C{dollar}sb3{dollar}H{dollar}sb8{dollar} as fuels, He, Ar, N{dollar}sb2{dollar}, and CO{dollar}sb2{dollar} as inert diluents, pressure variations from 0.2 to 4.5 atmospheres, stoichiometry from very fuel lean to very fuel rich, and flame temperature from 1550 to 2250 K varied independently of stoichiometry.; Results on methane flames clearly demonstrate the importance of branching-termination chain mechanisms on flame propagation. The pressure exponent n for the overall mass burning rate of the flame is shown to continuously decrease with pressure while the corresponding overall activtion energy continuously increases. For weakly-burning flames n can even assume negative values, indicating decreasing mass burning rates with increasing pressure.; Results on mixtures of ethane, ethylene, acetylene, and propane with oxygen and nitrogen show the inadequacies of all literature kinetic schemes in predicting the experimental flame speed. Consequently, a new scheme was compiled and was found to yield close agreement with the majority of the experimental data.; The study of hydrogen flames emphasizes on the role of high-, intermediate-, and low-temperature regime kinetics on the flame response. Results show that while literature kinetic schemes accurately predict the propagation rates of high temperature flames, they seriously underpredict those of low-temperature flames.; An independent study of the phenomena of flammability limits was conducted. Experimentally, limits of propagation of unstretched flames were determined by first measuring the extinction limits of stretched, counterflow flames and extrapolating the results to zero stretch. Consequently, lean and rich flammability limits were determined for a large variety of mixtures. By further hypothesizing that the limit phenomena are primarily controlled by the kinetic processes of chain branching versus termination, a predictive theory was advanced for the a priori determination of flammability limits. Calculated limits largely agree with the experimentally-determined values.