Laminar flame propagation in mixtures with compositional stratification at small length scales

Stratified Combustion;Flame Instability;Wrinkled Flame;Flame Stretch;Combustion Kinetics

Schmidt, David P. ; Kyritsis ; Dimitrios C.

关键词: Stratified Combustion;    Flame Instability;    Wrinkled Flame;    Flame Stretch;    Combustion Kinetics;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/24146/Schmidt_David.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

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.