Turbulent flame speeds and NOx kinetics of HHC fuels with contaminants and high dilution levels
This progress report documents the second year of the project, from October 1, 2011 through September 30, 2012. Characterization of the new turbulent flame speed vessel design was completed. Turbulence statistics of three impellers with different geometric features were measured using particle image velocimetry inside a Plexiglas model (~1:1 scale) of a cylindrical flame speed vessel (30.5 cm ID × 35.6 cm L). With four impellers arranged in a central-symmetric configuration, turbulence intensities between 1.2 and 1.7 m/s with negligible mean flow (0.1u´) were attained at the lowest fan speeds. Acceptable ranges for homogeneity and isotropy ratios of the velocity fields were set within a narrow bandwidth near unity (0.9-1.1). Homogeneity ratios were unaffected by changes to the impeller geometry, and the prototype with the higher number of blades caused the flow to become anisotropic. The integral length scale of the flow fields varied between 27 and 20 mm, which correlates well with those typically observed inside a gas turbine combustor. The mechanism to independently vary the intensity level and the integral length scale was established, where turbulence intensity level was dependent on the rotational speed of the fan, and the integral length scale decreased with increasing blade pitch angle. Ignition delay times of H₂/O₂ mixtures highly diluted with Ar and doped with various amounts of N₂O (100, 400, 1600, 3200 ppm) were measured in a shock tube behind reflected shock waves over a wide range of temperatures (940-1675 K). The pressure range investigated during this work (around 1.6, 13, and 30 atm) allows studying the effect of N₂O on hydrogen ignition at pressure conditions that have never been heretofore investigated. Ignition delay times were decreased when N₂O was added to the mixture only for the higher nitrous oxide concentrations, and some changes in the activation energy were also observed at 1.5 and 30 atm. When it occurred, the decrease in the ignition delay time was proportional to the amount of N₂O added and depended on pressure and temperature conditions. A detailed chemical kinetics model was developed using kinetic mechanisms from the literature. This model predicts well the experimental data obtained during this study and from the literature. The chemical analysis using this model showed that the decrease in the ignition delay time was mainly due to the reaction N₂O +M ⇌ N₂ + O +M which provides O atoms to strengthen the channel O + H₂ ⇌ OH + H. Ignition delay times have been measured behind reflected shock waves at 1.5, 12 and 30 atm for a mixture representative of a syngas produced from biomass (0.29659% CO / 0.29659% H₂ / 0.15748% CO₂ / 0.08924% CH₄ / 0.20997% H₂O / 0.95013% O₂ in 98% Ar (mol.%)) and for the same biomass-derived syngas mixture doped with 200 ppm of NH₃. The importance of the various constituents on the ignition delay time was investigated by comparing the results with data from various baseline mixtures (H₂/O₂/Ar, H₂/CO/O₂/Ar and H₂/CO/O₂/Ar with one of the other constituent of the syngas (i.e. CO₂, H₂O, CH₄ or NH₃)). The equivalence ratio was set to 0.5 during this study. Several recent detailed kinetics mechanisms from the literature were computed against these data, with fair agreement. Results showed that the mixture composition can have an important effect on the ignition delay time, with most of the effect being due to CH₄ addition through the reaction CH₄+OH⇌CH₃+H₂O. The ammonia impurity had very little effect on the ignition delay time over the range of conditions studied.
- Research Organization:
- Texas A & M Univ., College Station, TX (United States). Texas A & M Engineering Experiment Station
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FE0004679
- OSTI ID:
- 1212668
- Country of Publication:
- United States
- Language:
- English
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