AmericanChemicalSociety.com

New methodologies for flame speed measurement were developed and flame speeds of syngas flames across a wide range of conditions were determined using these methodologies.  In the present study, as in earlier high pressure study of flame speeds, a recently developed constant-pressure approach that utilizes a cylindrical test chamber was applied.  The effect of cylindrical confinement on flame speed measurements was addressed in detail for these non-spherical constant-pressure chambers.   The results from experiments and analysis indicated 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.  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.  A flow-correction factor to account for the actual flow velocities in the chamber was developed and implemented for a test case.  The results revealed an improvement of accuracy especially over data ranges that include flame radii near the wall where the confinement effect is strongest.

The effects of flow compression and flame stretch on the determination of flame speeds using spherical bombs under constant-pressure and constant-volume conditions were studied theoretically and numerically. A time-accurate and front-adaptive numerical algorithm was developed to simulate the outwardly propagating spherical flame in a closed chamber for a broad range of pressures and equivalence ratios. The results showed that both flow compression and flame stretch have significant impacts on the accuracy of measured flame speeds.  For the constant-pressure method, a new expression was presented to calculate a compression corrected flame speed.  Likewise, for the constant-volume method, a new expression was presented to calculate a stretch corrected flame speed.  The results demonstrate that the present corrective methods not only greatly improve the accuracy of the flame speed measurements but also extend the valid parameter range of experimental conditions.

The trajectories of outwardly propagating spherical flames initiated by an external energy deposition were studied theoretically, numerically, and experimentally by using hydrogen/air mixtures. Emphasis was placed on how to accurately determine the laminar flame speeds experimentally from the time history of the flame fronts for mixtures with different Lewis numbers and ignition energies. The results showed that there is a critical flame radius only above which is the linear and non-linear extrapolation for flame speeds valid. It was found that the critical radius depends strongly on the Lewis number. At large Lewis numbers, the critical radius is larger than the minimum flame radius used in the experimental measurements, leading to invalid flame speed extrapolation. The results also showed that there is a maximum Karlovitz number beyond which propagating spherical flame does not exist. The maximum Karlovitz number decreases dramatically with the increase of Lewis number. Furthermore, the results showed that the ignition energy has a significant impact on the flame trajectories. It is found that the unsteady flame transition causes a flame speed reverse phenomenon near the maximum Karlovitz number with different ignition energies. The occurrence of flame speed reverse greatly narrows the experimental data range for flame speed extrapolation. The strong dependence of flame trajectory on ignition energy and the existence of the flame speed reverse phenomenon were also confirmed by experimental results.

Flame speeds and burning rates were measured across a range of pressures from 1 to 25 atm, flame temperatures from 1500 to 1800K, equivalence ratios from 0.85 to 2.5, CO fuel fractions from 0 to 0.9, CO₂ dilution ratios from 0 to 0.4.  Negative pressure dependence was observed for burning rates in H₂ flames at high pressure, low flame temperature conditions.  The pressure dependence of H₂/CO flames was observed to be similar to pure H₂ flames with CO fuel fractions as high as 0.5.  Dilution with CO₂ was observed to strengthen the pressure dependence.  Based on the numerical results, dilution with H₂O is expected to strengthen the pressure dependence to an even larger extent than CO₂.

While for low pressures or high flame temperatures, the various models employed in this study all yield predictions in good agreement with experimental data, none predicts the observed burning rate pressure dependence across all conditions. Large variations, up to a factor of three, are observed among the model predictions.  Key kinetic pathways in high-pressure, low-flame temperature flames, potential for previously unconsidered pathways, and possible areas for improvement in kinetic modeling were identified.  Now, efforts to increase model performance are underway with our collaborators in the elementary reaction kinetics field.            The ACS grant contributed significantly for us to further establish my research program for the study of combustion chemistry, experimental methodologies, and burning properties of alternative fuels at high pressures. Through the ACS support, we developed improved experimental techniques, published six peer reviewed journal papers, and educated two graduate students for Ph.D. in the area of energy and environment. I cannot imagine that I could have succeeded in my research without the ACS grant support.