Countercurrent flame propagation in a packed bed of spherical PMMA particles: Effect of void structure on flamelets quenching

Countercurrent flame propagation against an upward flow of pure oxygen in a vertical cylindrical column packed with spherical polymethyl methacrylate (PMMA) particles was experimentally and numerically studied, with a particular attention given to the effect of void structure on flamelets quenching. In a typical experimental run, upon ignition from the top, a flame front composed, of many discrete blue flamelets, formed and propagated downwards, exhibiting characteristic flame front propagation velocity, mass loss rate and flamelets quenching behaviour that are dependent on the oxygen flow velocity. A decrease in oxygen flowrate leads to reduced flame propagation velocity with increasingly more small flamelets becoming quenched. A transient three-dimensional numerical model considering one-step finite-rate PMMA pyrolysis and gaseous phase oxidation reactions was developed and applied to simulate the flame structure and propagation process. By comparing the measured and predicted results, the validity of the numerical model was confirmed. The three-dimensional void structure of the particle packing is shown to significantly impact on the flamelets quenching characteristics. As the oxygen flow velocity decreases, the pyrolysis rate in the voids with larger characteristic length is drastically reduced, while the flamelets in the voids with smaller characteristic length may directly quench. Quenching of the flamelets leads to local oxygen bypassing and thus a significant decrease in pyrolysis rate.