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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.

The laminar flame speeds and structures of near-limit autoignition-assisted cool and warm n-heptane/air flames at different ignition Damkӧhler numbers which are the ratios between flow residence time and the ignition delay time, elevated temperatures and pressures are studied computationally and analytically over a broad range of equivalence ratios. The primary objective of this work is to understand the effects of the ignition Damkӧhler number, mixture temperature, equivalence ratio, and pressure on the dynamics and structures of cool and warm flame propagation near the flammability limit. Different transitions among near-limit cool, warm, and hot flames are examined. The results show that both cool and warm flame structures and propagation speeds change dramatically with the increase of the ignition Damkӧhler number. Moreover, the dependence of normalized cool and warm flame speeds on the ignition Damkӧhler number is affected by the equivalence ratio, pressure, and flame regimes. Furthermore, for equivalence ratios within the hot flame flammability limits, the results show that there exist two flame speeds, one for the hot flame and the other for the cool flame. It is shown that the cool flame speed has a non-monotonic dependence on the initial mixture temperature due to the negative temperature coefficient (NTC) effect. However, the warm flame speed has a much weaker NTC effect and the hot flame speed only increases monotonically with the increase of the initial temperature. The results also reveal that the lean cool flame speed can be much higher than the hot flame speed near the NTC region. Finally, a simple analytical model for predicting the flame speed of autoignition assisted flames is developed. The model implies that the reduced activation energy of autoignition strongly affects the flame speed dependence on the ignition Damkӧhler number. The present results significantly advanced the understanding of the near-limit low temperature flame dynamics.

The effects of low-temperature chemistry (LTC) and turbulent transport with thermal and fuel stratifications on knocking formation are numerically modeled for dimethyl ether/air mixtures with detailed chemistry. The critical conditions of knocking formation with thermal and fuel concentration gradients in both one-dimensional laminar and two-dimensional turbulent flow are examined with and without LTC and with different turbulent timescales, length scales, and Reynolds numbers. The results show that LTC has a significant effect on the ignition-to-detonation transition caused by both thermal and concentration gradients. The results also show that at the same dimensional gradient, LTC can accelerate, enable, or quench the detonation transition, depending on the magnitude of the gradient. These three different LTC effects are demonstrated in the detonation peninsula by using the same ignition delay time and a unified criterion for combined thermal and concentration gradients. Moreover, by using the ignition delay time with and without LTC, the results showed that the critical conditions for the detonation transition are not affected. This result clearly revealed that large heat release rate from hot ignition is required to initiate ignition and shockwave coupling for detonation transition. The results further show that turbulent transport can delay knocking/detonation transition and dramatically reduce detonation strength due to turbulent mixing. It is found that when the turbulent timescale is much shorter than the ignition delay time of a gradient field, the knocking formation will be inhibited. With a given turbulent timescale, a larger turbulent length scale or turbulent Reynolds number will induce a broader mixing in the preheated zone in front of detonation wave and thus reduce the knocking strength. Moreover, knocking formation in a smaller gradient kernel is weakened more by turbulent transport. The present research provides important insights of knocking formation and control of knocking using low-temperature fuel reactivity, thermal and fuel stratifications, and turbulence in advanced engines.

A novel inwardly off-center shearing jet-stirred reactor is proposed and examined computationally. The inwardly off-center shearing jet-stirred reactor is compared with two traditional jet-stirred ...