Effect of unsteady stretching on the flame local dynamics
Effect of unsteady stretching on the flame local dynamics
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DOI：
10.1016/j.combustflame.2016.05.028
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发表时间：
2017
影响因子：
4.4
通讯作者：
Feichi Zhang;T. Zirwes;P. Habisreuther;H. Bockhorn
中科院分区：
文献类型：
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作者：
Feichi Zhang;T. Zirwes;P. Habisreuther;H. Bockhorn
采用详细的反应动力学和运输的数值模拟已被应用到计算稳态和谐波激发层流氢-空气火焰当量比范围从贫燃料到富燃料的混合物。其目的是获得一个基本的了解的影响，非定常流拉伸火焰的响应。计算了火焰位移速度和火焰消耗速度，以及拉伸、弯曲和应变。在稳态模拟中，已经获得了一个准线性相关的火焰速度与拉伸为中等范围内，这证实了小拉伸的渐近关系的适用性。在不稳定流动条件的情况下，观察到的流体流和火焰表面位置的振荡之间的相移，指示火焰的延迟响应变化的流动条件。相应的延迟时间或弛豫时间被发现是取决于流动振荡的周转时间。在较高的激励频率下，对于具有较大火焰过渡时间τ L0的火焰，从其稳定状态位置的位移方面的火焰响应被抑制。由于延迟响应，不同相位角的局部消耗速度对火焰拉伸的依赖性不同。这种差异是很大的火焰流激发频率较低，以及当量比较小的火焰渡越时间。此外，对于具有正拉伸的区域，观察到火焰拉伸过程中不同相位角的消耗速度变化较大，因为火焰的内部结构发生了改变，导致局部火焰过渡时间较小。这解释了湍流燃烧中火焰传播统计数据的大散射，其中流动的特征在于具有不同翻转频率的相互作用的涡流的级联。通过比较火焰和流动的特征时间，定量分析了非定常效应。通过这种方法，在模拟结果的基础上，对Markstein回归进行了推广，给出了Ma与Damköhler数的函数关系式：Da= τ flo w/τ L0 = SL 0/δ L0/f.在振荡的周转期间评估不同阶段的消耗速度并在该期间平均，导致平均马克斯坦数与达姆科勒数的相关性。
Numerical simulations employing detailed reaction kinetics and transport have been applied to calculate steady-state and harmonically excited laminar hydrogen–air flames with equivalence ratios ranging from fuel-lean to fuel-rich mixtures. The objective is to gain a basic understanding of the influence of unsteady flow stretching on the flame’s response. The flame displacement speed and consumption speed have been evaluated, together with stretch, curvature and strain. In the steady-state simulations, a quasi-linear correlation has been obtained for the flame speeds with stretch for a moderate range, which confirms the applicability of the asymptotic relation for small stretch. In the case of unsteady flow conditions, a phase shift is observed between the oscillations of the fluid flow and the flame surface position, indicating a delayed response of the flame to changing flow conditions. The corresponding delay time or relaxation time is found to be depending on the turnover time of the flow oscillations. The flame response in terms of displacement from its steady state location is shown to be dampened at higher excitation frequencies and for flames with large flame transit times τ L0. Due to the delayed response, the local consumption speeds at different phase angles show different dependences on flame stretch. This discrepancy is large for flames in flows excited with lower frequencies as well as equivalence ratios with smaller flame transit times. Also, a larger variation of consumption speeds from different phase angles over flame stretch has been observed for regions with positive stretch, because the internal structure of the flame is altered, leading locally to smaller flame transit times. This explains the large scattering of flame propagation statistics in turbulent combustion, where the flow is characterized by a cascade of interacting vortices with different turnover frequencies. The unsteady effect is quantitatively analysed by comparing the characteristic times of the flame and the flow. In this way, an extension of the Markstein regression has been provided based on the simulation results, giving Ma as a function of the Damköhler number defined by Da= τ f l o w/τ L 0= S L 0/δ L 0/f. Evaluating the consumption speed for different phases during a turnover period of the oscillations and averaging over the period results in a correlation of the averaged Markstein number with the Damköhler number.