Laminar Flame Speeds and Strain Sensitivities of Mixtures of H_2 with CO, CO_2 and N_2 at Elevated Temperatures

摘要

Laminar flame speed and strain sensitivities have been measured for mixtures of H_2/CO/CO_2/N_2/O_2 with a wall stagnation flame technique at high preheat temperature (700 K) and lean conditions. The measurements are compared with numerical predictions based on two reaction mechanisms: GRI Mech 3.0 and a H_2/CO mechanism (Davis et al). For H_2:CO 50:50 fuel mixtures, both models tend to over predict the temperature dependence of the flame speed especially at very lean conditions, which confirms the trend found in an earlier study employing a Bunsen flame technique. The predicted strain sensitivities are in good agreement with the measurements. For 50:50 H_2:CO fuel mixtures diluted with 40% CO_2, the amount of over prediction by the models is about the same as in the undiluted case, which suggests that radiation effects associated with CO_2 addition are not important for this mixture at highly preheated lean condition. For low H_2 content (5 to 20%) H_2/CO fuel mixtures at 5 atm and fuel lean condition, the predicted unstrained flame speeds are in excellent agreement with the measurements, but the models fail to predicted the strain sensitivity as the amount of H_2 increases to 20%. Results are also presented for pure H_2 with N_2 diluted air (O_2:N_2 1:9) over a range of equivalence ratios. At lean conditions, the models over predict the measured flame speed by as much as 30%, and the amount of over prediction decreases as the equivalence ratio increases to stoichiometric and rich condition. The measured strain sensitivities are three times higher than the model predictions at lean conditions. More importantly, the predicted strain sensitivities do not change with equivalence ratio for both models, while the measurements reveal a clear trend (decreasing and then increasing) as the fuel-air ratio changes from lean to rich.