Gas-phase ignition of a solid fuel in a hot stagnation-point flow

Experimental ignition times of a solid fuel in a hot oxidant stagnation-point flow are obtained. It is found that by allowing flow velocity to change, at constant flow temperature, the ignition time takes on a minimum value at a certain velocity. The lower velocity range up to the minimum point corresponds to a pyrolysis-controled region. Ignition times, which are nearly equal to gasification times, decrease as flow velocity increases. At high velocities above the minimum point, the pyrolysis-control region changes to a reaction-control region and ignition can not occur rapidly even if gasification is completed. A counterflow field of vaporized fuel gas and hot oxidant gas is formed after rapid vaporization, and in this stretched-flow field the exothermic reaction time in the gas phase lengthens as the flow velocity increases. Experimental ignition times, measured by changing either the oxygen concentration or the gas temperature of a flow, demonstrage the transition from a pyrolysis-control to a reaction-control region. Measured surface temperature histories verify a slow gas-phase reaction period.