Measurements of hydrogen syngas flame speeds at elevated pressures

Laminar flame speeds for syngas mixtures of various compositions up to 20 atm are measured with outwardly propagating spherical flames at constant pressure using more restrictive measures for data reduction that consider deviation from the ideal flow field caused by a non-spherical confinement. In the present paper, as in earlier high pressure study of flame speeds, a recently developed constant-pressure approach that utilizes a cylindrical test chamber is applied. For the first time, the effect of flow field on flame speed measurements is addressed in detail for these non-spherical constant-pressure chambers. The results from experiments and simplified analysis indicate that deviation from the assumed flow field causes significant errors in instantaneous flame speed measurement, which are amplified in the extrapolation to zero stretch rate. The relative deviation of the apparent flame speed from the true flame speed is found to scale with (σ-1) times the relative flow field deviation, where σ is the unburned to burned gas density ratio. This result demonstrates the significance of the flow field on the measured flame speed, since the density ratio typically assumes values of ∼5-9 for hydrogen and hydrocarbon fuels in air. A simple model is developed to study the effect of flow disturbance in cylindrical confinements. In cylindrical chambers, where the flow is typically most constrained in the plane of measurement (radial direction), failure to consider this effect results in lower values for the measured flame speed. Laminar flame speed measurements are reported for H₂/CO/CO₂ mixtures varying in equivalence ratio from 0.6 to 4.0, pressure from 1 to 20 atm, and CO₂ dilution from 0 to 25%. The corrected data range (0.6 cm < r < 0.30Rw where Rw is the chamber wall radius) is seen to raise the extrapolated burning velocity by as much as 6% from the burning velocity found from extrapolation over a wider range (0.6 cm < r < 0.54Rw), which is more strongly influenced by flow field deviations. The experimental measurements are compared with experiments from McLean et al., Sun et al., and Hassan et al. (where applicable) and planar calculations using the kinetic mechanisms of Li et al., Davis et al., and Sun et al. While the experimental data and predictions for burning velocity agree reasonably well at lean conditions, large discrepancies occur at rich conditions. The substantial variation in the available stretch-corrected flame speed data indicate that significantly more data and more rigorous calculations of uncertainty are necessary before quantitative conclusions can be made and the performance of kinetic mechanisms can be properly assessed.