Yongliang Xie

Effect of H2O Addition on the Flame Front Evolution of Syngas Spherical Propagation Flames

ABSTRACT An experimental study on the cellular instability of spherical propagation syngas flames with H2O addition was performed in a constant volume combustion bomb over a wide range of CO/H2 ratios at elevated pressures. Schlieren images were recorded to observe and analyze the flame front evolution of spherical propagation flames. Results show that the cellular instability of syngas flames is promoted at atmospheric pressure and suppressed at elevated pressure with H2O addition for syngas mixtures with CO/H2 ratio of 80/20. At elevated pressure, critical radius, beyond which flame front starts to accelerate, increases with H2O addition for syngas mixtures with higher CO/H2 ratios of 65/35 and 50/50, while decreases for syngas mixtures with lower CO/H2 ratios of 20/80 and 0/100. Critical Peclet number decreases with H2O addition for all of the mixtures. The acceleration exponent increases with the flame radius and reaches a constant value at some points. This implies that both self-acceleration and self-similarity regions exist in spherical syngas flames. The acceleration exponent in a self-similar region changes little with H2O addition due to the minor change of intrinsic flame instability.

Effects of Initiation Radius Selection and Lewis Number on Extraction of Laminar Burning Velocities from Spherically Expanding Flames

ABSTRACT Experimental flame trajectories of hydrogen/air and n-butane/air spherically expanding flames were measured over a wide range of equivalence ratios at pressures of 0.1 MPa and 0.05 MPa. An experimental radius range selection method was developed to determine the critical initiation and end radii for extraction of laminar burning velocity. The results showed that both flame radius range selection and nonlinear flame stretch have significant effects on laminar burning velocity extraction. As the selected initiation radius increases, the flame radius range effect increases. Thus, the widest unaffected radius range should be processed using a proper method to reduce the extraction induced uncertainty. Additionally, the extraction method, which reflects a linear relationship between the flame propagation speed and the flame curvature, well fits the experimental data for a wide radius range when Lewis number, Le > 1. And the performances of all tested extraction methods are almost equal when Le < 1, especially for Le slightly smaller than unity. The nonlinear method is still recommended in extraction for very small Le. The critical initiation radius depends strongly on the Lewis number and initial pressure, and increases linearly with the absolute value of Markstein length. At large and small Le, the initiation radius used in measurements should be large enough to ensure reliable extraction results. Furthermore, the critical initiation radius is affected by transition and spark-assisted propagation regimes for Le > 1 and Le < 1, respectively. It is concluded that the proper flame radius range for extraction is narrowed primarily by the strong flame stretch effects, which increase the critical initiation radius for stable flames.