The paper discusses the results of an experimental study on combustion development in a large volume filled with ultra-lean hydrogen-air mixture under terrestrial gravity conditions. For the first time, characteristicflame ball evolutionIn particular, ittheBesides, the lateral expansion of the flame ball plays an important role, defining the changes in curvature radius and stretching of the flame. Due to this, flame breaks up and secondary flame kernels arise. So, the developed structure of the ultra-lean flame occurs to be much more complex than earlier registered cap-shaped flames. Large-scale dynamics of the flame ball is of paramount interest for understanding the stability of near-limit flames and for estimation of hazards related to the development of flame balls under real conditions. Studied processes represent a possible way of energy transfer from the spatial areas filled with less reactive mixtures toward areas with high mixture reactivity. Such a phenomenon stages of the during its convective upward motion on large spatial scales are described. The physical mechanisms defining the particularities of ultra-lean flame evolution subjected to the natural convection are proposed. is shown that the key role in the process of flame propagation in the ultra-lean mixture belongs to the convective rise of the hot combustion products in the gravity field. Herewith, velocity of the flame upward motion occurs to be much higher than the laminar burning velocity in the considered ultra-lean hydrogen-air mixture. may become a reason for the development of emergencies and should be thoroughly investigated and taken into account while designing reliable fire and explosion safety systems. Experimental results are compared with previously obtained numerical data that allowed to demonstrate the correctness of the mathematical modeling and physical mechanisms identified on its basis.

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.

The paper is devoted to the numerical analysis of wave patterns behind a shock wave propagating in a tube filled with a gaseous mixture. It is shown that the flow inside the boundary layer behind the shock wave is unstable, and the way the instability develops fully corresponds to the solution obtained for the boundary layer over a flat plate. Vortical perturbations inside the boundary layer determine the nonuniformity of the temperature field. In turn, exactly these nonuniformities define the way the ignition kernels arise in the combustible mixture after the reflected shock interaction with the boundary layer. In particular, the temperature nonuniformity determines the spatial limitations of probable ignition kernel position relative to the end wall and side walls of the tube. In the case of low-intensity incident shocks the ignition could start not farther than the point of first interaction between the reflected shock wave and roller vortices formed in the process of boundary layer development. Proposed physical mechanisms are formulated in general terms and can be used for interpretation of the experimental data in any systems with a delayed exothermal reaction start. It is also shown that contact surface thickening occurs due to its interaction with Tollmien-Schlichting waves. This conclusion is of importance for understanding the features of ignition in shock tubes operating in the over-tailored regime.