International Journal of Hydrogen Energy, Vol.45, No.19, 11350-11367, 2020
Study on effect of hydrogen addition on extinction dynamics of dimethyl ether spherical diffusion flame
In this paper, numerical calculations were conducted to investigate the effect of H-2 addition on dimethyl ether (DME) spherical diffusion flames in the hot- and cool-flame conditions, in terms of the S-curve flammability, near-limit oscillatory dynamics, and extinction mechanism. The mole fraction of H-2 in the fuel mixture was varied from 0% to 15% gradually, by 5% in increment. The results indicate that the flammability limit of DME spherical diffusion flame, in either hot- or cool-flame condition, was considerably extended due to H-2 addition to the fuel mixture. It is interesting to note that with increasing H-2 addition, the hot-flame extinction limit was decreased, while that for coolflame was increased. Additionally, the spherical diffusion flame exhibited a strong oscillation behavior near extinction, which made flame extinguishment happen prior to the extinction turning point of S-curve in either hot- or cool-flame regime. In either H-2 addition case, the oscillation dynamics of near-extinction hot flame featured a single oscillatory mode with a constant frequency which was uncorrelated with the ambient oxygen mole fraction (X-O2*), while the cool-flame oscillatory featured the dual oscillatory modes that had fairly distinct frequencies, and its high-frequency oscillatory period was slightly relevant with X-O2*. Furthermore, oscillatory period of the near-extinction hot flame slightly increased with increasing H-2 addition. While for the near-extinction cool flame, as H-2 addition increased, its high-frequency oscillatory period considerably decreased, and its low-frequency oscillatory period kept nearly unchanged, but its oscillation amplitude attenuated quickly and finally became rather indiscernible. Specially, when H-2 addition approached 15%, the dual oscillatory mode of cool flame was nearly disappeared, replaced by a single oscillatory mode. Besides, the sensitivity analysis was conducted to reveal the key reactions that controlled the oscillatory extinction in hot- and cool-flame regimes. It was found that in either hot- or cool-flame regime, the reactions involving H radical became increasingly more significant to the oscillatory extinction with the increment in H-2 addition. Additionally, it was further found that the hot-flame oscillatory extinction was governed by the competition between exothermicity/endothermicity and branching/termination involving small molecules, while the cool-flame oscillation extinction was primarily governed by the competition between low-temperature branching and termination reactions involving large molecules in the negative temperature coefficient (NTC) regime. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.