Effects of equivalence ratio variation on lean stratified methane-air laminar flames

Abstract

Flame propagation and structure in stratified combustion, a key technology for advanced low emission combustors, has been investigated using lean methane-air laminar counterflow flames. Configurations with steady spatial equivalence ratio gradients and with unsteady, time varying equivalence ratio profiles were examined in order to isolate the effects of equivalence ratio gradient from unsteady history effects. Stratified flames were compared with perfectly premixed flames where the equivalence ratio was matched at the location of peak heat release. Back supported flames (where the products were closer to stoichiometric than the reactants) propagated significantly faster and with reduced thickness compared to perfectly premixed flames which, in turn, were faster and thinner than the corresponding front supported flames. This observation, which corroborates the findings of previous studies, holds in both the steady and unsteady configurations. In order to understand the underlying processes, the composition and species flux were analyzed through the reaction zone in the steady cases. At the location of peak heat release, back supported flames exhibit temperatures and levels of major species which are similar to premixed flames, while radical species concentrations, including OH, as well as the heat release rate are enhanced. The increased (reduced) levels and fluxes of OH in back (front) supported flames were attributed to the shifting equilibrium of the CO-H2 recombination reactions due to the stratification induced temperature gradient. The stratified flames were found to respond to equivalence ratio variations following a delay on the order of the characteristic flame time-scale. The unsteadiness adds to the effects of the instantaneous flame normal equivalence ratio gradient observed in the steady flames. When the equivalence ratio oscillates with a period similar to, or less than, the flame time scale the flame response is attenuated. The presence of equivalence ratio variation through the flame implies that flame speed, flame thickness or reaction rate cannot be modeled accurately as functions of only the progress variable and equivalence ratio. Models accounting for temporal and spatial equivalence ratio variation may be advantageous

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ORCID for E.S. Richardson: orcid.org/0000-0002-7631-0377

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