The Influence of Free Convection on the Ignition of Vertical Cellulosic Panels by Thermal Radiation

The dependence of time to ignition t i, on height x of a specimen is examined for radiant ignition of vertical cellulosic solids. There is a transitional irradiance value of about 4 cal/cm2 sec, below whicht i, decreases as X increases and above which t i, is essentially independent of X. In the present paper this phenomenon is explained on the basis of the time ts, required for establishment of a steady-state free-convection boundary layer. At low irradiance, t i, ≥ t ss, and x-dependent heat losses through the boundary layer cause the top of the panel to be hotter than the bottom and cause the pyrolysis rate and ignition time to depend on X. At high irradiance, t i, ≤ t ss, and ignition occurs before boundary-layer development, thereby causing t i, to be independent of X.