Transient dynamics of radiative extinction in low-momentum microgravity diffusion flames

Transient dynamics of growing diffusion flames produced by a low-momentum fuel flow emanating at the flat surface has not yet been properly addressed. The flames of interest resemble properties of laminar jet flames and the candle or droplet flames produced by a point fuel source. However, due to the considerable size of the fuel source, these flames appear to be dissimilar to either of the two cases. The practical importance of studying the stability and dynamics of these flames stems from the need to scrutinize the flammability of condensed fuels in normal and zero gravity. This paper presents comprehensive 3D simulations of the low-momentum zero- and normal gravity diffusion flames generated by the BRE (Burning Rate Emulator) burner. The scenarios of interest represent the subset of the ongoing orbital experiments in which flame development in the normal atmosphere is considered. The experimental observations have shown that these flames are prone to the local extinction followed by fluctuating instability and complete quenching. Such behavior was numerically predicted in the earlier ?blind? simulations and is now replicated for the orbital test conditions. Time to flame extinction predicted in this work agrees with the experimental observations. Extinction occurs due to the excessive radiative losses when the instantaneous radiative fraction grows up to 60-70% of the current heat release rate. The excessively high level of the radiative losses is facilitated by the long residence time in the low-momentum microgravity flame. The transient dynamics of flame disintegration after the local extinction onset is analyzed. The hook-like (bibrachial) structure of flame edge is predicted after the extinction onset. When the re-ignition occurs, a classical triple (tribrachial) flame develops and propagates along the stoichiometric surface. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.