Pulsating flame propagation of PMMA particle cloud in microgravity

Although a large number of studies have been made on heterogeneous combustion of solid particle clouds, the combustion mechanisms of these systems are not well understood due to the complexity of the combustion processes and difficulties in conducting experiments. The purpose of this paper is to show a particular phenomenon found in flame propagation of PMMA particle clouds. The experiments were performed in microgravity to prevent sedimentation of the particles and the effect of buoyancy on the flame using a closed-vessel method and spherical polymethylmethacrylate (PMMA) particles as fuel. Observation of the flame using a CCD video camera and the time history of pressure and ionization current showed that the flame propagation with alternating fast and slow modes occurred and that the flame was characterized by oscillation. This phenomenon is called "pulsating flame" in this paper. The pulsating flame appears only near the lean flammability limit. As fuel concentration increases from the lean flammability limit, the frequency of the pulsating flame increases first and then decreases; beyond a certain equivalence ratio, the pulsating flame does not occur. The frequency also decreases with increasing mean diameter of the particles. Explanation of the mechanism of this pulsating flame phenomenon was attempted based on the concept of heat absorption of the particles in burned gas and radiative heat transfer to the particles in a fresh mixture.