Bifurcation behavior in homogeneous-heterogeneous combustion: I. Experimental results over platinum

Abstract

Ignition, extinction, and autothermal behavior of the homogeneous and heterogeneous oxidation reactions in air of NH 3, CH 4, C 3H 8, and an equimolar NH 3 CH 4 mixture over resistively heated polycrystalline platinum foils was studied in an atmospheric pressure flow reactor as a function of flow rate, fuel concentration, preheat temperature, and reactor geometry. Surface temperature versus power curves were experimentally determined for different compositions in these systems, which were in turn summarized in bifurcation diagrams of surface temperature or power as a function of fuel composition. Two ignitions (heterogeneous and homogeneous), one extinction and one self-sustaining autothermal steady state, were observed for all systmes except NH 3 CH 4 oxidation. The heterogeneous ignition occurred around 600°C for CH 4 and 200°C for all other systems and was weakly dependent on the fuel composition. Long transients due to carbon formation were observed in the autothermal behavior for fuel-rich compositions in the CH 4 system. A second heterogeneous ignition occurred in NH 3 CH 4 oxidation, which ranged from 400° to 600°C. Homogeneous reaction ignited at surfaca temperatures ranging from 1050°C for C 3H 8 oxidation to 1500°C for CH 4 and NH 3 oxidation. For C 3H 8 and CH 4 oxidation, the flame generally left the foil, but in NH 3 oxidation a stable boundary layer flame was also observed under some conditions.