Gravity effects on triple flames: Flame structure and flow instability

Abstract

A fundamental difference between a partially premixed flame and an equivalent premixed (or nonpremixed) flame pertains to the existence of multiple synergistically coupled reaction zones. A “triple flame” is a type of partially premixed flame that contains a fuel-rich premixed reaction zone, a fuel-lean premixed reaction zone, and a nonpremixed reaction zone. The objective of this investigation is to examine gravity effects on the flame structure and flow instabilities related to partially premixed triple flames. (An earlier investigation by us dealing with gravitational effects on partially premixed double flames essentially considered steady 0- and 1-g flames.) A detailed numerical model is employed to simulate a methane-air triple flame established on a slot burner. A relatively detailed mechanism involving both C1- and C2-containing species and 81 elementary reaction steps is used to represent the CH4-air chemistry. Validation of the computational model is provided through a comparison of predictions with nonintrusive measurements. The results indicate that the overall triple flame structure is determined by interactions between the three reaction zones, and can be controlled by changing the mixture velocity, equivalence ratio, and gravitational acceleration. While the inner rich premixed reaction zone is weakly affected by gravity, the central nonpremixed and outer lean premixed reaction zones exhibit significant differences at 0 and 1 g. For 0 g flames, these two reaction zones move away from the centerline compared to the corresponding 1 g flames, since the entrainment of the lean outer flow is reduced in the absence of buoyant advection. Velocity vectors outside the lean premixed zone are directed away from the centerline due to flow dilatation in a 0 g flame, whereas they are directed towards the centerline due to the buoyancy-induced entrainment that occurs in a corresponding 1 g flame. Consequently, there is an increased physical separation and reduced heat and mass transport between the three reaction zones of the 0 g flame. The nonpremixed reaction zone height decreases due to the increase in residence time at 0 g. The reduced advection and (transport) at 0 g results in a flame that is less compact and has thicker reaction zones and which, therefore, is more sensitive to flow or stoichiometry perturbations. The flame structure and the interactions between the three reaction zones are found to be well-represented in terms of a modified conserved scalar ξ. A fundamental difference between the 0 and 1 g triple flames is due to their transient behavior and markedly different response to changes in the coflow velocity. Our simulations indicate the presence of a shear-induced convective instability and a buoyancy-induced global instability in laminar triple flames that depend upon the magnitude of coflow velocity and gravitational acceleration. The outer premixed reaction zone of 0 g flames exhibits a hitherto unreported weak oscillatory behavior at higher coflow rates that is related to a Kelvin–Helmholtz instability of the momentum shear layer. The instability in the 0 g flame is confined to the outer premixed zone. In contrast, decreasing the coflow velocity at 1 g causes a well-organized flickering of the outer reaction zone that can affect all three reaction zones. The flickering frequency is relatively insensitive to changes in the coflow velocity, as both computed and measured frequencies are found to be in a narrow range of 9–12 Hz. However, the flickering amplitude exhibits strong sensitivity to the coflow velocity as it is reduced to a value smaller than the inner jet velocity, and becomes noticeably large causing oscillations in all three reaction zones. There is a good agreement between the predicted and measured dynamics of the flickering 1 g flames.