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Effects of H_2 enrichment on the propagation characteristics of CH_4-air triple flames

机译：H_2富集对CH_4-空气三重火焰传播特性的影响

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摘要

The effects of H_2 enrichment on the propagation of laminar CH_4-air triple flames in axisymmetric coflow-ing jets are numerically investigated. A comprehensive, time-dependent computational model, which employs a detailed description of chemistry and transport, is used to simulate the transient ignition and flame propagation phenomena. Flames are ignited in a jet-mixing layer far downstream of the burner. Following ignition, a well-defined triple flame is formed that propagates upstream along the stoichiometric mixture fraction line with a nearly constant displacement velocity. As the flame approaches the burner, it transitions to a double flame, and subsequently to a burner-stabilized nonpremixed flame. Predictions are validated using measurements of the displacement flame velocity. As the H_2 concentration in the fuel blend is increased, the displacement flame velocity and local triple flame speed increase progressively due to the enhanced chemical reactivity, diffusivity, and preferential diffusion caused by H_2 addition. In addition, the flammability limits associated with the triple flames are progressively extended with the increase in H_2 concentration. The flame structure and flame dynamics are also markedly modified by H2 enrichment, which substantially increases the flame curvature and mixture fraction gradient, as well as the hydrodynamic and curvature-induced stretch near the triple point. For all the H2-enriched methane-air flames investigated in this study, there is a negative correlation between flame speed and stretch, with the flame speed decreasing almost linearly with stretch, consistent with previous studies. The H2 addition also modifies the flame sensitivity to stretch, as it decreases the Markstein number (Ma), implying an increased tendency toward diffusive-thermal instability (i.e. Ma → 0). These results are consistent with the previously reported experimental results for outwardly propagating spherical flames burning a mixture of natural gas and hydrogen.
机译：数值研究了H_2富集对层状CH_4-空气三重火焰在轴对称同流射流中传播的影响。一个综合的，与时间有关的计算模型，使用了化学和运输的详细描述，用于模拟瞬态点火和火焰传播现象。火焰在燃烧器下游的射流混合层中点燃。点火后，形成了清晰的三重火焰，该火焰沿化学计量混合物分数线向上游传播，位移速度几乎恒定。当火焰接近燃烧器时，它过渡为双重火焰，然后过渡为燃烧器稳定的非预混火焰。使用位移火焰速度的测量来验证预测。随着燃料混合物中H_2浓度的增加，由于增加的H_2引起的化学反应性，扩散性和优先扩散，置换火焰速度和局部三重火焰速度逐渐增加。此外，随着H_2浓度的增加，与三重火焰相关的可燃性极限逐渐提高。氢气的富集也显着地改变了火焰的结构和火焰动力学，这大大增加了火焰的曲率和混合物分数梯度，以及三点附近的流体动力学和曲率引起的拉伸。对于本研究中研究的所有富含H2的甲烷-空气火焰，火焰速度与拉伸之间存在负相关关系，火焰速度几乎随拉伸呈线性下降，这与以前的研究一致。 H 2的添加还改变了火焰敏感性以拉伸，因为它降低了Markstein数（Ma），这意味着扩散热不稳定性增加的趋势（即Ma→0）。这些结果与先前报道的向外传播燃烧天然气和氢气混合物的球形火焰的实验结果一致。

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