AUTO-IGNITION OF A FLAT SOLID-FUEL IN A HIGH-TEMPERATURE OXIDIZER BOUNDARY-LAYER FLOW

A theoretical model is developed for the thermal ignition of a flat co mbustible solid in a hot, oxidizing boundary layer Bow. It is consider ed that the solid ignition is controlled by two primary processes, one being the solid heating and gasification, and the other the onset of the gas phase reaction after Fuel vaporization. With this approach, th e solid ignition delay is obtained by combining the time required for the fuel to start to pyrolyze, and the gas phase induction time. The s olid fuel pyrolysis time is calculated by using a simplified analysis of solid heating, with the assumption that because of the high activat ion energy characteristic of the solid pyrolysis process, this takes p lace exclusively at the surface when it reaches the solid pyrolysis te mperature. A one step, global reaction of the Arrhenius type is used t o describe the combustion reaction; thus, a large activation energy as ymptotic analysis is used to calculate the induction time. It is shown that for given gas flow conditions, the primary parameter determining both the pyrolysis and the induction times is the inverse how (invers e residence) time, given by the ratio between the flow velocity and th e distance from the solid leading edge to the location of ignition. As the inverse flow time is increased, the heat transfer to the solid su rface also increases, and consequently, the solid gasification time de creases. However, as the inverse flow time is increased, so is the ind uction time due to the convective cooling of the incipient combustion reaction; this increase in the induction time takes place slowly at fi rst, until a critical value of the flow time is reached, at which the induction time increases rapidly and becomes infinite. This critical v alue is given by the critical Damkohler number for ignition. Since the solid ignition time is given by the sum of the pyrolysis and inductio n times, the result is an ignition time that decreases, reaches a mini mum, and then increases with the inverse flow time. At a certain value of the inverse Bow time, the ignition time becomes infinite, and igni tion can no longer occur. The predictions of the model agree with a ph enomenological view of the solid fuel ignition process, and are in qua litative and order of magnitude agreement with the available experimen tal observations.