Study on effect of hydrogen addition on extinction dynamics of dimethyl ether spherical diffusion flame

Abstract

In this paper, numerical calculations were conducted to investigate the effect of H2 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 H2 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 H2 addition to the fuel mixture. It is interesting to note that with increasing H2 addition, the hot-flame extinction limit was decreased, while that for cool-flame 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 H2 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 (XO2∗), 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 XO2∗. Furthermore, oscillatory period of the near-extinction hot flame slightly increased with increasing H2 addition. While for the near-extinction cool flame, as H2 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 H2 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 H2 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.