High-Lewis number premixed flame instabilities

Lean premixtures of high-molecular weight practical fuels typically have an associated Lewis number (Le) in excess of one and are susceptible to diffusive-thermal instabilities depending on the heat loss and hydrodynamic strain. These instabilities may lead to incomplete combustion, an increase in undesirable emissions and complex pressure oscillations. While a significant amount of work has focused on low-Lewis number cellular instabilities, the objective of this study was to explore high-Lewis number pulsating and traveling wave instabilities in burner-stabilized premixed gas flames and to improve our understanding of the role of the mixture Le, mixture composition, heat loss and ambient pressure on the onset and dynamics of the flame instabilities. To meet this objective, premixtures of n-C4H10, n-C7H16 and n-C8H18 mixed with O2 diluted by N2 or He were considered at different pressures for which high speed videos of different flame instability modes were acquired. The instability regimes included: (1) stable planar flames, (2) complex spiral or target patterns, (3) spiral waves and (4) axisymmetric target patterns, and they generally occurred in this order as the equivalence ratio was decreased for a fixed burner exit speeds. Stability diagrams were then developed as a function of equivalence ratio and burner exit flow speed for the mixtures considered. The effects of Le, heat loss and ambient pressure on the dynamics of the flame instability including the oscillation frequency in the case of target patterns and rotation rate for the rotating spiral waves. Note that the radial propagation speed of the flamelets ranged from 2 to 5 m/s which is significantly faster than the laminar flame speed for premixed flames. Numerical studies were also conducted using the 2-step Sal'nikov model developed by Scott-Wang-Showalter and Matkowsky with a focus on improving our understanding of the spatio-temporal structure of these unstable flames. The spatio-temporal temperature and species concentration were computed in a two-dimensional domain. The computed results were used to clarify the roles of Le and heat loss on the radial flame speed, the spacing between adjacent waves and the rotation frequency associated with the spiral waves. The numerical computations agreed favorably with the experimental observations.