doi:10.3850/978-981-08-7724-8_06-04

ABSTRACT

The combustion of clouds of fuel droplets is an important issue in the understanding and mitigation of explosion hazards. Such clouds might be generated during processing, handling, or by accidental spillage. Any subsequent unintentional ignition may result in an aerosol explosion with potentially devastating consequences. This paper reports a series of experiments on the combustion of initially quiescent aerosols in a confined vessel. Homogeneously distributed and near monodispersed iso-octane-air aerosol clouds were generated using a thermodynamic condensation method. Spherically expanding flames following central ignition were employed to quantify the flame structure and propagation rate. The effects of fine droplets on flame propagation at a wide range of equivalence ratios were investigated. Comparisons between gaseous and aerosol flame structure have shown quantitatively that the presence of liquid droplets causes earlier onset of instabilities and cellularity than for gaseous flames, particularly at rich conditions. This leads to an enhanced burning rate and is probably due to local disturbances due to droplet evaporation and subsequent diffusion processes. For rich flames, this is augmented by a reduction of gaseous phase equivalence ratio (towards stoichiometric) by collection of fuel into the liquid phase. In some situations with very rich mixtures, as might be expected following an accidental spillage, droplets can result in burning velocities which approach those of laminar stoichiometric gaseous flames. Conversely, for lean flames the reduction in gaseous phase equivalence ratio (away from stoichiometric) has a negative effect on burning rate which is only partly compensated by any droplet induced flame instabilities.

Keywords: Aerosol explosion, Flame speed, Droplets, Mist, Hazards.

© 2011 CPD Unit, University of Leeds