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Show V AFRC85-3 6 dimensionless temperature M viscosity v stoichiometric coefficient i P c(t) * • Subscripts c e F g i ig IH N 0 o P R s w Lee's transformati density penetration depth coupling equation, stream function conduction free stream fuel gas species i Ignition inert heating Nitrogen oxygen initial condition pyrolysis reaction solid at the wall 1 . INTRODUCTION Ignition studies of pyrolyzlng solid fuel under exposure to radiative and convective heating have been conducted for years and many aspects of the problem have been explained. The AFRC85-4 existence of a threshold pyrolyzate flow rate, surface temperature and incident flux for Ignition to occur were determined and reported by references (1-41. The variation of ignition time and surface ignition temperature with flow velocity, temperature and oxygen concentration have also been evaluated (5-10). Despite these many studies, the subject that has drawn a great deal of controversy is the principal site of the ignition reaction. Host conclusions have been made that ignition occurs in the gas phase [7,11-13]. Others (2,8-9,14] assumed it to occur in a thin layer of the solid at its surface. Niioka et al. [7] measured Ignition time for PMMA and other polymers and observed that with increasing flow velocity the Ignition time first decreases to its minimum value and then increases with further Increase of the flow velocity. Siailar behavior of Ignition time with respect to the flow velocity was also observed by Isakov and Grishln (8,9]. They also observed that at low flow velocity the unsteady state heating of the surface of the PMMA reaches up to approximately 600*K at the time of ignition and the molar concentration of the monomer does not exceed a value of 0.01*. This is considerably lower than the concentration limits for an ignition reaction to occur in the gas phase. Therefore, the ignition mechanism must be controlled by the chemical processes at the surface. Later they developed a heterogeneous model that predicts very well ignition time for the flow velocity less than 900 1/s. However, ignition time predicted by their heterogeneous model continues to decrease monotonically for flow velocity higher than this value while the measured value turns upward and Increases with the increase of the flow velocity. This is because the ignition has changed its mechanism and mass transfer, heat transfer and the chemical processes in the gas phase have become significant. The gas phase control of ignition has also been recognized by many other investigators [11-13,15-18]. It is believed from these results that, ignition of a pyrolyzlng solid fuel is of heterogeneous and homogeneous |