Propagation Of Laminar Syngas Flames With Dilutions At Elevated Temperatures

Synthesis gas(syngas)is considered as one of the attractive alternative clean fuels.The complicated combustion properties and high amount H2 in syngas contributes to the fire and explosion accidents caused by the gas leakage.A systematical study of laminar flame speed of syngas/air flames with dilutions at elevated initial temperatures has been conducted experimentally and numerically in the present study.The experimentation was conducted using the Bunsen flame method incorporating the Schlieren technique,and the corresponding cases were computed using Chemkin-Pro.Such research is significant for directing the safe and efficiency usage of syngas in industry and preventing fire and explosion accidents due to the flash back and blow-off.For Bunsen flame method,the method-dependent flames propagation speed may deviate from theoretical laminar flame speed due to the flame stretch.To eliminate the flame stretch effect on the laminar flame speed measurement and improve the accuracy of the experimental results based on the Bunsen flame method,a non-linear extrapolation approach which described the relationship between Karlovitz and method-dependent flames propagation speed was proposed.In addition,this modified Bunsen flame data processing method was proved to give comparable accuracy with the referenced flame speed data.The Bunsen modified data processing method was applied to determining the laminar flame speed of the syngas/air mixtures with N2 and CO2 dilution in the fuels under atmospheric condition.Numerical simulations showed that computed laminar flame speeds using Davis-Mech and Li-Mech exhibited satisfactory agreement with the experimental data.CO2 had a stronger dilution effect than N2.The total dilution effect of N2 and CO2 was dominated by the thermal effect.The contribution of the chemical effect of CO2 to the total dilution effect on the reduction of the laminar flame speeds of syngas/air flames was decreased with the increase of dilution ratio.Detailed analysis of the computation results indicated that the laminar flame speed increased linearly with the maximum(H + OH)mole fraction for lean mixtures and increased linearly with the maximum H mole fraction for rich mixtures.The dilution effect of N2 and CO2 presented declining influence on reducing laminar flame speed of syngas/air mixtures with the increase of initial temperature.The estimation of laminar flame speed of H2/CO/air mixtures calculated by all four employing mechanisms was over-predicted when the initial temperature was more than 500K.The further growth of initial temperature increased the discrepancy between the measurements and numerical calculation.Davis-Mech and Li-Mech presented better performance in predicting the laminar flame speeds of syngas/air mixtures with H2O at 40OK.When the equivalence ratio was minor,the direct reaction effect caused by H2O on the laminar flame speed was changed from promotion into inhibition with the increase of H2 fraction in the fuels.When the amount of H20 was limited in the fuels(?20 vol%),the third body effect caused by H2O on the laminar flame speed was altered following the trend by promotion-inhibition-promotion with the increase of H2 fraction in the fuels.GRI 3.0-Mech presented better performance in predicting the laminar flame speeds of syngas/air mixtures with CH4 on the fuel-lean side at 300K.When there was a small amount of CH4 in the fuels(?30 vol%),the laminar flame speed decreased rapidly due to the significant reductions of H radicals for rich mixtures.With the further increase of CH4 ratio(>30 vol%),the major decline of laminar flame speed was suppressed due to the deficient of OH radicals and reduction of CH3 radicals.For the lower-cost of the engineering computation of laminar flame speed of syngas/air mixtures without calculating detailed chemistry and transport data,a mixing model for predicting the laminar flame speed of syngas/air mixtures over a wide condition was proposed based on the modification of the mass fraction of reacted H2 and the modified correlation of thermal related item.The present mixing model predicted well against the experimental data over a wide range of equivalence ratios(0.6-4.0),H2 fraction(25 vol%-100 vol%),initial temperatures(300K-500K),N2 and CO2 dilution ratios(0-20 vol%).In addition,its estimation also agreed well with the experimental data under pressure was 2atm.The present mixing model presented the practical significance to engineering calculations.