On the Nearly Spherical Stratified Flame Propagation

Stable and reliable power is critical for not only modern conveniences, but also for basic goods and services needed to ensure protection of both life and property. To ensure a sustainable source of reliability in the global energy sector through current and future environmental and political changes and in concert with alternative and renewable production sources, current and future combustible fuels are needed to be accurately modeled. A key proponent of both natural gas and biogas is methane which has both current natural sources and future supply prospects. However there are still many fundamental questions regarding accurate modeling of the combustion of methane, and in particular within inhomogeneous mixtures. These stratification layers are less well understood in combustion environments than flames propagating through homogeneous mixtures, despite many of the current uses of this gaseous fuel in a variety of engineering systems.A set of spherical methane-air experiments within a constant volume chamber using Schlieren imaging and pressure traces as well as supporting one dimensional and three dimensional numerical modeling was undertaken to explore stratified flames propagating through methane-air mixtures. With comparisons to past work, an investigation of the effects of the stratification layer’s impact on the observed flame speed, product gas emissions, and to evaluate the possibility of extending the lean limit. To process the Schlieren images, a robust in-house edge tracking code was developed to track the progress of the flame observed in Schlieren images and closely evaluate the transient dynamics of the flame that occur within a flame burning through a stratification layer gradient set up between two mixture concentrations using a soap bubble. A speed up on the order of 20% higher than homogeneous equivalence ratio of 1.1 was observed in the rich to lean stratified cases. The experiments and numerical work agreed reasonably well with past experimental and numerical work. Higher CO was noted while burning in a stratified environment, while lower unburnt hydrocarbons and moderately lower NOx was also noted from stratification layers compared with an equivalent homogeneous mixture. The lean limit appeared to be extended, and a discussion is given in light of the prior work and capabilities within this work.While relative agreement was achieved experimentally with recent work, unexpected in- stabilities were noted in the flame that are difficult to be accounted for with the setup alone. This work adds to the possibility first mentioned and observed by Markstein, Behrens, and Einbinder of an inherent instability within stratified methane-air flames