Countercurrent flame propagation inside vertical PMMA cylinders against pure oxygen flow

Abstract

The countercurrent flame propagation against pure oxygen flow inside a vertical polymethyl methacrylate (PMMA) cylinder under oxygen-limited conditions was systematically investigated experimentally and numerically, with a particular attention given to the effect of the inner diameter (di = 6, 5, and 4 mm) of the cylinder. Metered pure oxygen was flowed through the cylinder from the bottom and, when ignition was initiated at the top, a blue flame was formed and propagated downwards in a quasi-steady state. The flame propagation velocity (Sf) was determined by analysing the digital video record of the entire flame propagation process. The flame was apparently fuelled by the volatiles from the pyrolysis of the inner surface the PMMA cylinder. The rate of pyrolysis (mpy) was estimated by measuring the initial and remaining masses of the PMMA cylinder. A transient two-dimensional axisymmetric 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 Sf and mpy, the validity of the numerical model was confirmed. The flame structure was depicted and the role of the gas – solid heat transfer was quantified. A decrease in di leads to an increase in the heat flux that preheats the unburnt PMMA ahead of the flame front. This results in a greater pyrolysis rate of the unburnt PMMA and a faster flame propagation velocity. Effects of thickness of the PMMA cylinder and radiative heat transfer from the flame to the inner PMMA surface were also discussed.