CUED Publications database

Evolution of flame-kernel in laser-induced spark ignited mixtures: A parametric study

Mulla, IA and Chakravarthy, SR and Swaminathan, N and Balachandran, R (2016) Evolution of flame-kernel in laser-induced spark ignited mixtures: A parametric study. Combustion and Flame, 164. pp. 303-318. ISSN 0010-2180

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© 2015. The present work focuses on the early stages of flame-kernel development in laser-induced spark ignited mixtures issuing out of a Bunsen burner. The time-scale of 3 μs to 1 ms associated with the flame-kernel evolution stage of an ignition event is targeted in this work. A CH 4 /air mixture (equivalence ratio φ = 0.6) is studied as a base case, and compared with CH 4 /CO 2 /air (mole fractions = 0.059/0.029/0.912, respectively) and CH 4 /H 2 /air (mole fractions = 0.053/0.016/0.931, respectively) mixtures for nearly the same adiabatic flame temperature of 1649 K. The spatio-temporal flame-kernel evolution is imaged using planar laser induced fluorescence of the OH radical (OH-PLIF), simultaneously with H-alpha emission from the plasma. The H-alpha emission suggests that the plasma time-scale is well below 1 μs. The PLIF images indicate all the stages of kernel development from the elongated kernel to the toroidal formations and the subsequent appearance of a front-lobe. The different time-scales associated with these stages are identified from the rate of change of the kernel perimeter. The plasma is followed by a supersonic kernel-perimeter growth. Larger flame-kernel spread is found in the case of CH 4 /H 2 mixtures. A distinct shift in the trends of evolution of LIF intensity and kernel perimeter is observed as the fuel concentration is varied near the lean flammability limit in CH 4 /air (φ = 0.35-0.65) and H 2 /air (φ = 0.05-0.31) mixtures. The flow velocity (Reynolds number, Re) effect in both laminar and turbulent flow regimes (Re = ~600-6000) indicates that the shape of the flame-kernel changes at higher velocities, but the size of the kernel does not change significantly for a given time from the moment of ignition. This could be due to a balance between two competing effects, namely, inc rease in the strain rate that causes local extinction and thus decreases the flame-kernel growth, and increase in the turbulence levels that facilitates increased flame-kernel surface area through wrinkling, which in turn increases the flame-kernel growth.

Item Type: Article
Divisions: Div A > Energy
Depositing User: Cron Job
Date Deposited: 17 Jul 2017 19:08
Last Modified: 14 Sep 2017 01:27