Pressure and preheat dependence of laminar flame speeds of H2/CO/CO2/O2/He mixtures

Laminar flame speeds of lean H-2/CO/CO2 (syngas) fuel mixtures have been measured for a range of H-2 levels (20-90% of the fuel) at pressures and reactant preheat temperatures relevant to gas turbine combustors (15 atm and up to 600 K). A conical flame stabilized on a contoured nozzle is used for the flame speed measurement, which is based on the reaction zone area calculated from chemiluminescence imaging of the flame. An O-2:He mixture (1:9 by volume) is used as the oxidizer in order to suppress the hydrodynamic and thermo-diffusive instabilities that become prominent at elevated pressure conditions for lean H-2/CO fuel mixtures. All the measurements are compared with numerical predictions based on two leading kinetic mechanisms: a H-2/CO mechanism from Davis et al. and a C-1 mechanism from Li et al. The results generally agree with the findings of an earlier study at atmospheric pressure: (1) for low H-2 content (<40%) fuels, the model predictions are in good agreement with measurements at both 300 and 600 K preheat temperature; but (2) the models tend to over predict the temperature dependence of the flame speed for medium (similar to 40-60%) and high (>60%) H-2 content fuels, especially at very-lean conditions. At elevated pressure, however, the effect is less pronounced than at atmospheric conditions. The exaggerated temperature dependence of the current models may be due to errors in the temperature dependence used for so-called "low temperature" reactions that become more important as the preheat temperature is increased. There is also evidence of slight radiative heat transfer effects on the laminar flame speed for lean syngas Mixtures associated with CO2 addition to the fuel (up to 40%) at elevated pressure and preheat temperature. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.