The initiation, propagation, and transition of the autoignition assisted spherical cool flame and double flame are studied numerically and experimentally using n-heptane/air/He mixtures under shock-tube experimental conditions over a wide range of temperatures. The primary goal of the current study is to understand the effects of the ignition Damkohler number, ignition energy, flame curvature, and autoignition-induced flow compression on the propagation of spherical flames to ensure the proper interpretation of shock-tube flame speed measurements at engine-relevant conditions. The results show that at high ignition Damkohler number, there are three different flame regimes, cool flame, double flame, and hot flame. The cool flame speed accelerates dramatically with the increase of ignition Damkohler number. In addition, it is found that the change of flame regime, low-temperature autoignition, flame stretch, and autoignition-induced flow compression result in a complicated non-linear dependence of flame speed on stretch. The results also reveal that the spherical cool flame has much lower Markstein length compared to the hot flame at T > 600 K. Moreover, it is found that both the autoignition assisted cool flame and the trailing hot flame front in the double flame can propagate much faster that the hot flame alone at the same mixture conditions, leading to a nonlinear dependence of flame speed on the mixture initial temperature. The simulated flame trajectories and the flame speed dependence on temperature agree qualitatively well with the shock-tube experiments. A quantitative criterion to ensure the accurate speed measurement of the cool and hot flame is proposed. The present study provides important physical insight and guidance for the flame speed measurement using a shock-tube at engine relevant conditions.

使用正庚烷/空气/ He混合物，在冲击管实验条件下，在较宽的温度范围内，通过数值和实验研究了自燃辅助球形冷火焰和双火焰的引发，传播和过渡。当前研究的主要目的是了解点火达姆勒数，点火能量，火焰曲率和自燃引起的流动压缩对球形火焰传播的影响，以确保正确解释发动机上的冲击管火焰速度测量结果-相关条件。结果表明，在高点燃达姆勒数下，存在三种不同的火焰状态，即冷火焰，双火焰和热火焰。随着点火Damkohler数的增加，冷火焰速度急剧加快。此外，结果发现，火焰状态，低温自燃，火焰拉伸和自燃诱导的流动压缩的变化导致火焰速度对拉伸的复杂非线性依赖性。结果还表明，与热火焰相比，球形冷火焰的Markstein长度要低得多。T  > 600K。此外，发现在相同的混合条件下，自燃辅助的冷火焰和后火焰在双火焰中的传播都比单独的热火焰快得多，从而导致了火焰速度的非线性依赖性。根据混合物的初始温度。模拟的火焰轨迹和火焰速度对温度的依赖性与激波管实验定性吻合。提出了一种定量标准，以确保准确测量冷热火焰的速度。本研究为在发动机相关条件下使用减震管测量火焰速度提供了重要的物理见解和指导。