DEPENDENCE OF FLAMMABILITY LIMITS OF A COMBUSTIBLE PARTICLE CLOUD ON PARTICLE DIAMETER DISTRIBUTION

Flammability limits and flame structure of a cloud of pure stearic aci d (CH3(CH2)(16)COOH) particles have been investigated under a wide ran ge of particle diameter distributions. The stearic acid in a reservoir was heated to become liquid of 74 degrees C just above its melting po int (68-71 degrees C), and sprayed by a gas-liquid two phase nozzle. T he liquid droplets turned into solid particles quickly during rising a long a duct to the test section. Particle diameter distributions were controlled by widely changing spraying conditions. The structures and propagating velocities of particle cloud flames under various conditio ns were also examined by recording the direct emission from the flames , the images caused by laser light scattering, and the schlieren image s of flames. Two flammability limits were found to exist. One is the l ower flammability limit which mainly depends on the mass density of sm aller particles (smaller than 60 mu m in diameter). The mass density o f smaller particles at the lower flammability limit is about 3.0 x 10( -5) g/cm(3). The other is the upper flammability limit which depends o n the amount of all particles in the cloud. The mass density of all pa rticles at upper flammability limit is about 34.0 x 10(-5) g/cm(3). (C ) 1998 Elsevier Science Ltd. All rights reserved.