Laminar flame speeds of H2/CO mixtures: Effect of CO2 dilution, preheat temperature, and pressure

Laminar flame speeds of lean H2/CO/CO2 (syngas) fuel mixtures have been measured over a range of fuel compositions (5–95% for H2 and CO and up to 40% for CO2 by volume), reactant preheat temperatures (up to 700 K), and pressures (1–5 atm). Two measurement approaches were employed: one using flame area images of a conical Bunsen flame and the other based on velocity profile measurements in a one-dimensional stagnation flame. The Bunsen flame approach, based on imaging measurements of the reaction zone area, is shown to be quite accurate for a wide range of H2/CO compositions. These data were compared to numerical flame speed predictions based on two established chemical mechanisms: GRI Mech 3.0 and the Davis H2/CO mechanism with detailed transport properties. For room temperature reactants, the Davis mechanism predicts the measured flame speeds for the H2/CO mixtures with and without CO2 dilution more accurately than the GRI mechanism, especially for high H2 content compositions. The stagnation flame measurements for medium levels of H2 at both 1 and 5 atm, however, show lower than predicted strain sensitivities, by almost a factor of two at lean conditions (Φ=0.6–0.8Φ=0.6–0.8). At preheat temperatures comparable to those found in gas turbine combustors, the accuracy of the flame speed predictions worsens. For example in fuels with low levels of H2, both models underpredict the measurements. In contrast, for medium H2 content fuels, both measurement techniques show that the models tend to overpredict flame speed, with the discrepancy increasing as Φ decreases and temperature increases. In general, the Davis mechanism predictions are in good agreement with the measurements for medium and high H2 fuels for preheat temperatures up to 500 K but overpredict the measurements at higher temperatures.