A lean flammability limit of polymethylmethacrylate particle-cloud in microgravity

Lean flammability limits of heterogeneous mixtures of combustible solid par ticles and air were measured in a microgravity field. Polymethylmethacrylat e (PMMA) spherical particles with mass median diameters of 5.0, 8.4, 13.5, 30.4, and 48.5 mu m were instantaneously and uniformly dispersed into a cyl indrical closed vessel using an air jet dispersion device. The mixture beca me quiescent without sedimentation of the PMMA particles at 6 seconds after the dispersion in microgravity. The mixture was ignited at the center of t he vessel by a hot wire, and then a spherically propagating flame was forme d. Flame behavior in the vessel near the lean flammability limit was observ ed with pressure histories, CCD camera images, and flame speeds measured by ionization probes. The experiments were performed for the initial conditio ns at room temperature and atmospheric pressure in microgravity. Results sh owed that the equivalence ratios at the lean flammability limits for the pa rticles whose mean diameters were 8.4, 13.5, 30.4, and 48.5 mu m were 0.68, 0.76, 0.86, and 1.05, respectively, indicating that the equivalence ratio at the limit increased linearly as the particle diameters were increased. ( C) 1999 by The Combustion Institute.