Experimental study of the ignition of single droplets under forced convection

The convective ignition of suspended fuel droplets was experimentally investigated in the postflame gases (gas temperature T-infinity of 1000-2400 K; flow velocity 2 m/s) of a flat-flame burner, n-Heptane and hexadecane droplets with initial diameters (d(0)) ranging from 1000 to 1650 mu m and droplet Reynolds numbers of 10-30 were tested. For T-infinity = 1050-1240 K, ignition initiated far downstream (at a distance of 3-40 times d(0) and at a larger distance for a lower temperature) as a dim blue premixed flame, followed by rapid upstream propagation to form an envelope flame around the droplet. When T-infinity was increased to about 1270 K, ignition initiated near the rear stagnation point of the droplet and an envelope flame formed soon afterward. As T-infinity was increased to about 1350 K, the position of ignition moved to the front stagnation point. The position of the initial premixed flame appeared not to differ significantly for the two different fuels. Ignition positions and ignition delay times were measured for different T-infinity and d(0). The distance of the ignition position from the droplet was Found to increase with decreasing T-infinity in an exponential manner. The measured upstream flame propagation speed increased with T-infinity, especially for heptane droplets. Forced convection was found to impede droplet ignition in that the minimum ignitable temperatures were higher than those for nonconvective conditions. (C) 1997 by The Combustion Institute.