Evolution of sensitivity directions during autoignition
Evolution of sensitivity directions during autoignition
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DOI：
10.1016/j.proci.2018.07.005
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发表时间：
2019
影响因子：
3.4
通讯作者：
Weiqi Ji;Zhuyin Ren;C. Law
中科院分区：
文献类型：
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作者：
Weiqi Ji;Zhuyin Ren;C. Law
点火延迟时间和物种分布对动力学参数的敏感性分析已被广泛用于识别自燃过程中的限速步骤，为反应机理的优化提供见解。本文研究了自燃过程中温度和组分浓度灵敏度方向随时间的变化规律。方向由沿模拟输出到动力学参数的梯度的单位向量沿着表示，并且两个方向之间的对准由对应单位向量之间的内积测量。我们使用的敏感性方向的演变，以揭示在限速步骤的变化和物种之间的相关性在不同阶段的点火延迟期。研究发现，在整个点火延迟期内，温度和主要中间产物浓度的敏感方向相似。特别地，当接近点火状态时，它们收敛到相同的方向，并且该方向与点火延迟时间的方向相同。该相关性在广泛的压力和温度范围内对各种燃料进行了验证，并适用于单级点火和两级点火。因此，点火延迟时间的灵敏度可以基于点火点处的温度灵敏度来有效地评估，对于点火点处的温度灵敏度，单次运行的仿真可以产生对所有参数的灵敏度，否则点火延迟时间的灵敏度必须通过有限差分来评估，其中运行的次数等于参数的数量。它还可以显着降低基于梯度的算法的计算成本，用于机构优化和不确定性量化。
Sensitivity analysis of the ignition delay time and species profiles to kinetic parameters has been widely used to identify the rate-limiting steps during the autoignition process, providing insights for the optimization of the reaction mechanism. This work studies the time evolution of the sensitivity directions of the temperature and species concentration during autoignition. The direction is represented by a unit vector along the gradient of the simulation output to the kinetic parameters, and the alignment between the two directions are measured by the inner product between the corresponding unit vectors. We use evolution of the sensitivity directions to reveal changes in the rate-limiting steps and the correlation among species at different phases of the ignition delay period. It is found that the sensitivity directions of temperature and the concentrations of the major intermediate species are similar to each other during the entire ignition delay period. In particular, they converge to the same direction when approaching the ignition state, and the direction is the same as the one for the ignition delay time. The correlation is validated for various fuels across a wide range of pressures and temperatures, and works for both single-stage ignition and two-stage ignition. Consequently the sensitivity of the ignition delay time can be efficiently evaluated based on the temperature sensitivity at the ignition point, for which a single run of the simulation can produce the sensitivity to all parameters, as otherwise the sensitivity of the ignition delay time has to be evaluated through finite difference, in which the number of runs equals to the number of parameters. It can also significantly reduce the computation cost of gradient-based algorithms for the purpose of mechanism optimization and uncertainty quantification.