Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2275±2282 HOMOGENEOUS IGNITION OF METHANE-AIR MIXTURES OVER PLATINUM: COMPARISON OF MEASUREMENTS AND DETAILED NUMERICAL PREDICTIONS

The homogeneous ignition of lean methane-air mixtures was investigated numerically and experimentally in a laminar plane channel ¯ow con®guration established by two externally heated catalytically active (Pt-coated) ceramic plates, 250 mm long by 100 mm wide, placed 7 mm apart. Preheated fuel-air mixtures with equivalence ratios of 0.31 and 0.37 and uniform velocities of 1 and 2 m/s were examined, resulting in incoming Reynolds numbers ranging from 190 to 380. Planar laser-induced ¯uorescence (PLIF) was used to map the OH concentration ®eld along the streamwise direction and thermocouples to monitor both catalyst plate temperatures. The numerical predictions included a two-dimensional elliptic model with detailed heterogeneous and homogeneous chemical reactions. The homogeneous ignition location strongly depends on the incoming velocity and mildly on the equivalence ratio. Following homogeneous ignition, a very stable V-shaped ¯ame is formed in all cases. Measured and predicted ¯ame sweep angles, OH levels, and the post-¯ame OH relaxation are in good agreement with each other, while the homoge-neous ignition distance is predicted within 9 % in all cases. The homogeneous ignition location is shown to be better identi®ed with changes of averaged (over the channel cross section) quantities rather than with changes in local wall gradients. The overall model performance suggests that the employed surface scheme is capable of capturing the coupling between surface and gaseous chemistries leading to homo-geneous ignition. Experiments and predictions were also carried out with noncatalytic plates. The resulting ¯ame is unstable and asymmetric, clearly showing the stability advantages of catalytically assisted com-bustion