Dynamics of flame-ball formation from localized ignition: Effects of elevated pressure and temperature

A computational study was conducted on expanding spherical premixed flames to investigate the dynamics of flame-ball formation at elevated temperatures and pressures. Lean H₂/air mixtures were investigated using a time-dependent, spherically symmetric code with detailed chemistry, transport, and radiation submodels. Results show that, with increasing pressure, both the steady-state flame-ball radius and the H₂ consumption rate for a given mixture composition decrease monotonically up to 50 atm, varying approximately as p^-0.57. Furthermore, a window of pulsating flame behavior, near the upper dynamic flame-ball limit, was discovered and investigated. Within this window, an outwardly propagating flame begins to self-extinguish due to radiative losses but revives suddenly due to low-Lewis-number effects and evolves into a flame ball. More than one such cycle of behavior can occur for a given mixture concentration. Results further show that as the ambient mixture temperature is increased, the initial trend is a downward shift of the upper dynamic flame-ball limit. With reduced radiative loss, spherical flames continue to propagate outwardly for leaner mixture compositions without degenerating into flame balls, but at the same time, expand themselves into radiative extinction. Again, the role of radiative loss as both the requisite mechanism for and the limiting mechanism against the dynamic transformation of spherically propagating flames into flame balls is emphasized. Nonetheless, as the ambient temperature is increased to near 700 K (in an attempt to investigate the boundary defining the flameless combustion regime), steady flame balls are no longer attainable, with chemical reactions occurring at the boundary.