Evolution of flame-kernel in laser-induced spark ignited mixtures: A parametric study
Evolution of flame-kernel in laser-induced spark ignited mixtures: A parametric study
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DOI：
10.1016/j.combustflame.2015.11.029
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发表时间：
2016-02
影响因子：
4.4
通讯作者：
I. Mulla;S. Chakravarthy;N. Swaminathan;R. Balachandran
中科院分区：
文献类型：
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作者：
I. Mulla;S. Chakravarthy;N. Swaminathan;R. Balachandran
本文研究了本生灯中激光诱导火花点火混合气体的火焰核发展过程。在这项工作中，目标是与点火事件的火焰核演化阶段相关的3 µs至1 ms的时间尺度。以甲烷/空气（当量比为0.6）为基础，在绝热火焰温度为1649 K的条件下，与甲烷/二氧化碳/空气（摩尔分数分别为0.059/0.029/0.912）和甲烷/氢气/空气（摩尔分数分别为0.053/0.016/0.931）进行了比较。的时空火焰内核的演变成像使用平面激光诱导荧光的OH自由基（OH-PLIF），同时从等离子体的H-α发射。H-α发射表明等离子体时间尺度远低于1 μs。PLIF图像显示了从细长内核到环形结构以及随后出现的前叶的内核发展的所有阶段。与这些阶段相关联的不同的时间尺度从内核周长的变化率来识别。等离子体之后是超音速核周生长。在CH 4/H2混合物中发现较大的火焰核扩散。在甲烷/空气（CH_4 = 0.35-0.65）和氢气/空气（H_2 = 0.05-0.31）混合物中，当燃料浓度在贫燃极限附近变化时，LIF强度和核周长的演化趋势发生明显的变化。层流和湍流状态（Re= 1000 -6000）中的流速（雷诺数，Re）效应表明，火焰核的形状在较高的速度下发生变化，但核的大小在给定的时间内从点火时刻起没有显著变化。这可能是由于两种竞争效应之间的平衡，即应变率的增加导致局部灭绝，从而减少火焰核的生长，以及湍流水平的增加，通过起皱促进火焰核表面积的增加，这反过来又增加了火焰核的生长。
The present work focuses on the early stages of flame-kernel development in laser-induced spark ignited mixtures issuing out of a Bunsen burner. The time-scale of 3 µs to 1 ms associated with the flame-kernel evolution stage of an ignition event is targeted in this work. A CH4/air mixture (equivalence ratioϕ= 0.6) is studied as a base case, and compared with CH4/CO2/air (mole fractions = 0.059/0.029/0.912, respectively) and CH4/H2/air (mole fractions = 0.053/0.016/0.931, respectively) mixtures for nearly the same adiabatic flame temperature of 1649 K. The spatio-temporal flame-kernel evolution is imaged using planar laser induced fluorescence of the OH radical (OH-PLIF), simultaneously with H-alpha emission from the plasma. The H-alpha emission suggests that the plasma time-scale is well below 1 µs. The PLIF images indicate all the stages of kernel development from the elongated kernel to the toroidal formations and the subsequent appearance of a front-lobe. The different time-scales associated with these stages are identified from the rate of change of the kernel perimeter. The plasma is followed by a supersonic kernel-perimeter growth. Larger flame-kernel spread is found in the case of CH4/H2mixtures. A distinct shift in the trends of evolution of LIF intensity and kernel perimeter is observed as the fuel concentration is varied near the lean flammability limit in CH4/air (ϕ= 0.35–0.65) and H2/air (ϕ= 0.05–0.31) mixtures. The flow velocity (Reynolds number,Re) effect in both laminar and turbulent flow regimes (Re= ∼600–6000) indicates that the shape of the flame-kernel changes at higher velocities, but the size of the kernel does not change significantly for a given time from the moment of ignition. This could be due to a balance between two competing effects, namely, increase in the strain rate that causes local extinction and thus decreases the flame-kernel growth, and increase in the turbulence levels that facilitates increased flame-kernel surface area through wrinkling, which in turn increases the flame-kernel growth.