Mechanisms of flame propagation in jet fuel sprays as revealed by OH/fuel planar laser-induced fluorescence and OH* chemiluminescence

Abstract

Previous work on spray flames has shown that different propagation mechanisms may occur depending on the size and number density of droplets. In this work, the structure and propagation of flames in uniformly dispersed sprays of low-volatility fuels is experimentally examined. The effect of the Sauter mean diameter (SMD) of the spray (16–33 µm) on the propagation modes, flame speed, and flame curvature is assessed in weakly turbulent sprays, with the ratio of axial velocity rms to the gaseous laminar burning velocity uz′/SL,g ranging from 0.5–2.5, and overall equivalence ratio ϕ of 0.8, 1, and 1.4. The growth of the flame is evaluated from OH*-chemiluminescence and schlieren visualisation, which combined with OH/fuel planar laser-induced fluorescence visualisation reveal details of the propagation mechanisms. The aviation fuels investigated – Jet A and a renewable alternative, ATJ-8 – exhibited similar flame speed behaviour due to changes in SMD in each of the propagation modes identified: the droplet, inter-droplet, and gaseous-like modes. Concentrated reactions around large droplets found in lean conditions (ϕ = 0.8) allowed for a slowly propagating flame front which, in turn, ignited new droplets. Stoichiometric to rich conditions (ϕ = 1, 1.4) were marked by stronger evaporation ahead of the flame and, therefore, higher and more uniform heat release across the flame. Still, droplets penetrated the flame, locally inducing regions of negative flame curvature and continuing to evaporate in the burnt products. The droplet-induced effects disappeared at low SMD (16 µm, ϕ = 1.4), giving rise to a fully gaseous layer ahead of the flame and the highest flame speeds. At rich conditions and high SMD, Jet A had a lower flame speed than ATJ-8.