Investigation of the transition from single to group coal particle combustion using high-speed scanning OH-LIF and diffuse backlight-illumination.

The transition from single to group high-volatile bituminous (hvb) coal particle combustion is experimentally investigated in a laminar flow reactor using state-of-the-art laser diagnostics. Simultaneous volumetric OH-LIF imaging using a novel laser scanning technique is combined with time-resolved diffuse backlight-illumination (DBI) providing fundamental insights into flame topologies. Three-dimensional visualizations of volatile flames are first demonstrated on single particle combustion and then applied to characterize the flame topology associated with group particle combustion. The particle number density (PND) determined by instantaneous DBI images covers a wide range from the individual particle combustion to group combustion enabling the determination of transitional effects. While an enclosed volatile flame is observed with dominant spherical structures at low PND, enveloping flames surrounding non-flammable regions reveal distinct features as PND increases. The corresponding physical process is described by exploring the effect of local gas temperatures. The particle velocity affected by the inter-particle and particle-gas interaction is analyzed along the axial and radial direction. The ignition delay time increases gradually as PND increases. The non-flammable region is quantified by evaluating the non-flammable volume ratio R nf. The non-flammability becomes pronounced if PND exceeds a limit of approximately 0.37 mm − 3 , which corresponds to an inter-particle distance of 4d P in this study. [ABSTRACT FROM AUTHOR]