THE EFFECT OF FLOW VELOCITY ON IGNITION AND EXTINCTION IN HOMOGENEOUS-HETEROGENEOUS COMBUSTION

Ignition and extinction points are calculated for homogeneous-heteroge neous combustion of a premixed propane-air stream in stagnation point flow over a platinum surface as functions of inlet gas velocity and co mposition and power input to the catalyst surface using global rate ex pressions for the homogeneous and surface reactions. Curves of surface temperature and surface concentration of fuel and oxygen at the ignit ion and extinction points are computed at several compositions in the fuel-lean regime. Ignition and extinction of the surface reaction and autothermal behavior are found over a very broad range of velocities. Homogeneous ignition occurs only at relatively low velocities. As the velocity increases from 0.5 cm/s to 50 cm/s, autothermal behavior is s upported by leaner mixtures, and the autothermal surface temperature i ncreases substantially Inlet compositions leaner than 1.4% propane can not support autothermal operation at any velocity. Homogeneous ignitio n takes place at higher surface temperatures as the velocity is increa sed, while the range of compositions that exhibit homogeneous ignition shrinks markedly as velocity increases.