Quenching distance of flames in hybrid methane–aluminum mixtures

Abstract

Quenching distances for laminar flames in hybrid methane–aluminum fuel mixtures (16.3%O2/8.1%CH4/75.6% N2/Al) were experimentally measured at different concentrations of aluminum suspensions with the aluminum particles having an average size of about d32=5.6μm. Experiments were performed with freely propagating flames in a 48-mm-inner-diameter Pyrex glass tube. A set of parallel equidistant metal plates was installed at about 50cm from the open tube end, forming parallel channels. The flame was initiated at the upper open tube end and propagated downwards. Benchmark experiments performed with an inert silicon carbide (SiC) powder suspension in the same gaseous mixture have demonstrated a steady increase of the quenching distance from about 3.5mm for an unseeded methane flame to about 9.5mm for a flame seeded with 270g/m3 of SiC. At higher SiC concentrations, the flame failed to propagate down the tube. Coupled aluminum–methane flame fronts only appeared above a threshold aluminum concentration around 300g/m3. Below this concentration, the appearance and quenching behaviors of the methane flame seeded with reactive aluminum and inert SiC dusts were similar. The coupled aluminum–methane flame demonstrated two quenching modes. At concentrations below 400g/m3, the aluminum flame decoupled from the methane flame and quenched while the methane flame still propagated. The quenching of a decoupled methane flame in a narrow channel resembles the quenching behavior of the inert powder-seeded flame. At concentrations above 400g/m3, only a coupled aluminum–methane flame could be observed to propagate, or be extinguished, as a whole and exhibited a weaker dependence of the quenching distance on aluminum concentration. These results were interpreted in terms of models for binary fuel mixtures wherein the second fuel to react is completely decoupled, separated by a fixed distance, or merged with the initial flame front.