Local Flame And Flow Properties Of Propagating Premixed Turbulent Flames During Light-Round Process In A Micca-Type Annular Combustor

In this paper, local flame and flow properties of propagating premixed turbulent propane/air flames were derived by two-dimensional time-resolved Mie scattering and PIV methodology in a MICCA-type annular combustor. The curvature of flame front, local flame speed and the evolution of leading points during the light-round process were analyzed in detail. All experiments were performed under a constant equivalence ratio of 0.67, with the bulk flow velocity set from 1.95 to 5.85 m/s in five cases to achieve different turbulent intensities. The instantaneous flame fronts and velocity fields of unburned gases were measured to investigate the flame-flow interactions corresponding to four regions of the annular combustor. The results indicate that the slight asymmetry of the propagation in two flame branches is mainly due to the competition between the local swirling injection and the overall circumferential flow along the inner or outer parts of the combustor. Based on the statistical analysis of the local radius of flame curvature, the whole flame front tends to be convex to the unburned mixture. As a characteristic length scale, the inner-cutoff scale epsilon(i) of the flame front is about 2-3 times the flame thickness delta(L), which decreases with the increase of turbulent intensity. The value of fractal dimension increases with the increase of turbulence intensity, and its value is about 2.15-2.25 for all cases. Besides, in order to analyze the flame properties during the ignition process, the local flame speed w(n) was calculated using statistical method as well as the velocity of unburned mixture gas. Moreover, the experimental results (U-b =3.90 m/s) of the leading points' trajectory were consistent with the large-eddy simulations at the height of h =25 mm, and the leading points of the flame fronts were generally concentrated near (about 5 mm away) the inner tube for both flame branches. The researches of ignition mechanisms in annular combustors have been carried out for almost ten years, but the quantitative measurements of flow and flame fronts are barely reported. Motivated by this, it will provide new insights into the ignition process in annular combustors from this work. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.