Flame speed data reported in most literature are acquired in conventional apparatus such as the spherical combusion bomb and counter flow burner, and are limited to atmospheric pressure and ambient or slightly elevated unburnt temperatures. As such, these data bear little relevance to internal combustion engines and gas turbines, which operate under typical pressures of 10-50 bar and unburnt temperature up to 900K or higher. These elevated temperatures and pressures not only modify dominant flame chemistry, but more importantly, they inevitably facilitate pre-ignition reactions and hence can change the upstream thermodynamic and chemical conditions of a regular hot flame leading to modified flame properties. This study focuses on how auto-ignition chemistry affects flame propagation, especially in the negative-temperature coefficient (NTC) regime, where dimethyl ether (DME), n-heptane and iso-octane are chosen for study as typical fuels exhibiting low temperature chemistry (LTC). The structure of this thesis consists of the introduction of the combustion, the governing equations in thermodynamics and chemical reactions as well as the general structure of the flame. Then, the typicl experimental configuration exploited in the measurement of laminar flame speed is introduced, which is followed by the manifestation of the low temperature chemistry and the gap between the reality and the experimental understandings. Finally, the simulation results of laminar flame speed at constant pressure condition and HCCI engine condition are presented and discussed respectively. The computation of laminar flame speed of lean and stoichiometric mixtures of fuel/air was performed at different ignition reaction progress, by selecting the thermal chemical states corresponding to different residence times during auto-ignition as the flame upstream condition. Using scaling and budget analysis, it is shown that a well-defined flame speed for such a partially reactive mixture in the classical diffusion-reaction limit could still be feasible in the appropriate computational domain, especially with a sufficiently reduced induction length. The comparison of flame speed against different types of progress variables indicates a nearly linear relationship between the flame speed and progress variables based on the fuel mass fraction and temperature. The overall effect of the cool-flame reformation has been studied by comparing the flame speed of the initial mixture and that of the instantaneous mixture under the same thermodynamic conditions. It is found that the enhanced propagation is shown to be largely a thermodynamic effect, while chemistry nevertheless plays an overall retarding role. Sensitivity analysis has been performed to identify the key species which most influence flame propagation at different reaction progress. A general scheme of simplified mixture was constructed to describe flame propagation in a partially reactive mixture, for both lean and stoichiometric, as well as high pressures conditions. The findings and general simplified mixture scheme are validated in HCCI engine conditions.

The behavior and structure of laminar hydrocarbon flames which propagate parallel to the direction of a periodic gradient of mixture composition was studied both experimentally, using a specially designed burner, and computationally, using a planar numerical model. The variations in local mixture composition led to the formation of wrinkled flames, the structure of which were dependent on both chemical and physical parameters of the particular flame configuration. A qualitative study using chemiluminescence imaging of stratified methane and propane flames was conducted to characterize their response to variations in overall mixture composition, strength and length scale of the stratification, and flow field velocity. A two-dimensional numerical study was performed to assess the abilities of reduced global kinetic models to predict the behavior of stratified flames in comparison to computations performed using detailed mechanisms. It was observed that the reduced kinetic models display a lower limit of stratification length scale, which is on the order of the laminar flame thickness, below which accurate prediction of flame behavior is no longer possible. Under these conditions, the computed flames were observed to undergo a deformation which was much larger than the wrinkling imposed by the compositional variation, and was unsteady in time. Further analysis of these deformed flames suggested that a potential cause of this behavior was a failure of the reduced kinetics to capture the increase in local reaction rate attributed to stratification, and the inability of the driving reactions to counterbalance the incoming mass flux of fuel led to destabilization of the flame front. A preliminary analysis of the local stretch rates of the wrinkled flames was also conducted, and it was observed that even with a uniform incoming flow field, variations in mixture composition were capable of stretching the flames. The stretch behavior observed was one of alternating flame stretch and flame compression, which reached very large magnitudes over short distances.

A work on turbulent premixed combustion is important because of increased concern about the environmental impact of combustion and the search for new combustion concepts and technologies. An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts. Lean premixed flames have the potential to offer ultra-low emission levels, but they are notoriously susceptible to combustion oscillations. Thus, sophisticated control measures are inevitably required. The editors' intent is to set out the modeling aspects in the field of turbulent premixed combustion. Good progress has been made on this topic, and this cohesive volume contains contributions from international experts on various subtopics of the lean premixed flame problem.