Abstract
Development of the combustion process in the gaseous mixtures of near-limit composition is of great interest for fundamental aspects of combustion theory and fire-safety applications. The dynamics of ultra-lean gaseous flames in near-limit mixtures is governed by many effects, such as buoyancy, preferential diffusion, radiation, and instability development. Though ultra-lean combustion was extensively studied in microgravity conditions, the influence of gravity on the ultra-lean flame structure and stability is still poorly understood. The paper is devoted to deepening the knowledge of ultra-lean flame dynamics in hydrogen-air mixtures under terrestrial gravity conditions. The spatial structures of the flame developing under the effect of buoyancy forces are investigated employing detailed numerical analysis. Different modes of near-limit flame evolution are observed depending on the mixture concentration. In particular, we registered and described three distinct spatial structures: individual kernels tending to extinguish in leanest compounds, complex multi-kernel structures in marginal compositions, and stable cap-shaped flames in more chemically active mixtures. We apply the flame-bubble analogy to interpret flame dynamics. On this basis, the diagram in the Re-Fr plane is developed. That allows classifying the emerging flame structures and determine flame stability. Additionally, different ignition modes are studied, and the mechanisms determining the impact of ignition mode on the flammability limits are distinguished. Obtained results provide useful insights into the processes of flame quenching and development in near-limit hydrogen-air mixtures under real gravity conditions and can be applied in the design of contemporary fire-safety systems.
Highlights
Even today, fire and explosion safety remains a relevant problem for energy, technology, and industrial applications
We aim to study the ultra-lean flames by means of numerical modeling and classify the flame structure by employing flame-bubble analogy
We carried out numerical analysis of the ultra-lean hydrogen-air flame dynamics in terrestrial gravity conditions
Summary
Fire and explosion safety remains a relevant problem for energy, technology, and industrial applications. Among the most acute issues is the determination of conditions and mechanisms of non-stationary flame propagation, flame acceleration, and transition to fast subsonic and supersonic flame propagation regimes, including detonations. Another critical problem is related to the assessment of flammability limits and, in particular, to the investigation of combustion modes in lean and ultra-lean flammable gaseous mixtures. In large volumes characteristic for the real technical environments, very intense release and long periods are required for a flammable gas to mix with the oxidizer atmosphere and form a highly active gaseous mixture with nearly stoichiometric composition. The dynamics of near-limit flames and physical mechanisms determining the development of combustion in real large-scale environments under terrestrial gravity conditions are still poorly understood
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
