Bifurcating combustion behaviour (original) (raw)

Detailed numerical simulation of flame ball structure and dynamics

Combustion and Flame - COMBUST FLAME, 1999

A numerical study was conducted to examine the structure and dynamics of steady, source-free spherical premixed flames (“flame balls”), which have been observed in microgravity experiments. A time-dependent spherically symmetric code was used with detailed chemical, transport, and radiation submodels. Steady properties, stability limits, and dynamics of flame balls are computed for H2-air mixtures. The chemical and radiation models used were found to affect flame ball properties substantially. The special and unusual role of thermal radiation from the combustion products is described. In particular, the far-field radiative loss is found to affect the behavior of flame balls in a manner very different from propagating planar flames. One new feature was identified: mixtures capable of exhibiting both stable flame balls and steadily propagating flames depending on the initial condition. Numerical results are compared to theoretical predictions, prior steady-state numerical calculations, and prior experimental results on flame ball size. Additional experiments were performed to measure flame ball radiant emission. Qualitative agreement with theory and experiment is found; however, quantitative agreement with experiment is only fair, indicating the need for improved microgravity conditions, e.g., in orbiting spacecraft.

Localized Ignition And Subsequent Flame Spread Over Solid Fuels In Microgravity

Localized ignition is initiated by an external radiant source at the middle of a thin solid sheet under external slow flow, simulating fire initiation in a spacecraft with a slow ventilation flow. Ignition behavior, subsequent transition simultaneously to upstream and downstream flame spread, and flame growth behavior are studied theoretically and experimentally. There are two transition stages in this study; one is the first transition from the onset of the ignition to form an initial anchored flame close to the sample surface, near the ignited area. The second transition is the flame growth stage from the anchored flame to a steady fire spread state (i.e. no change in flame size or in heat release rate) or a quasi-steady state, if either exists. Observations of experimental spot ignition characteristics and of the second transition over a thermally thin paper were made to determine the effects of external flow velocity. Both transitions have been studied theoretically to determine...