Experimental investigations of single particle and particle group combustion in a laminar flow reactor using simultaneous volumetric OH-LIF imaging and diffuse backlight-illumination

The volatile combustion of Colombian high-volatile bituminous coal was experimentally studied in a laboratory laminar flow reactor. The volatile flames corresponding to the single particle and particle group combustion were visualized using non-intrusive multi-parameter optical diagnostics. In the present study, high-speed laser-induced fluorescence of OH radicals (OH-LIF) was applied to study igniting particles by temporally tracking OH-LIF signals in the gas-phase flame. A novel acousto-optic deflector combined with a 10 kHz dye laser was employed for laser scanning through a probe volume with a thickness of a few millimeters. The three-dimensional OH-LIF signals were used to reconstruct the volatile flame structures of burning particles. Simultaneously, diffuse backlight-illumination (DBI) is implemented to measure the size and the spatial distribution of particles to distinguish between single and group particle combustion. For single particles, starting from the onset of ignition, the OH-LIF intensity reaches its maximum within several milliseconds, which is temporally resolved by employing a laser scanning system. The gas-phase ignition starts downstream of the particles. As the particle size increases, the flame stand-off distance increases, whereas the ratio of the flame stand-off distance and the particle diameter decreases, which ranges from 2 to 4 for the coal particles investigated. For particle groups, the flame topology is evaluated for individual reconstructions with different particle number densities (PND). As the PND increases, the volatile flames are pushed outwards to the boundary of particle clouds and a non-flammable region emerges in the center of volatile flames. Soot formation is observed and becomes increasingly intensive as the PND increases.