(690d) Internal Natural Convection Effects On the Self-Heating of Solids

Frank-Kamenetskii thermal ignition theory has been used to model the self-heating behavior of solids. The theory does not account for the effects of internal convection in the case of a porous body. A lignocellulosic solid (wood flour) experimental program was conducted to investigate the self-heating properties. The apparent kinetic parameters and physical parameters for the wood flour thermal ignition were developed from that study using the approach of Frank-Kamenetskii. The self-heating solid was modeled using FLUENT as a porous solid body with uniform thermal boundary conditions. Based upon comparing the density of the wood flour to solid wood, the porosity was calculated as 0.63 for the porous body with the balance consisting of air. The model was given a sensitivity analysis on permeability ranging from 10-6 m2 to 10-20 m2 (effectively impermeable) to investigate the effects on the self-heating behavior of the porous body. Changing the permeability was found to affect the location of the locus of ignition within the body and time to ignition, but was found to not affect the critical ambient temperature for ignition. Given the permeability for the experimental wood flour cubes, the model predicted a locus of ignition at the center of the cube identically to the experiments. By increasing the permeability of the porous model through a sensitivity analysis, the location of the hot spot for ignition could be translated upward along the z-axis from the geometric center of the cube. Additional cases were run with internally distributed gas generation to investigate the effects on the hot spot, time to ignition, and critical temperature. Gas generation within the cube was shown to prolong the induction time to ignition, but not to change the critical temperature or the location of the hot spot for ignition. These variations were found to lower the critical temperature at most by 2K from the conduction-only limit of 199.5K.