The existence of both spatial variations of internal temperature and a temperature step at the surface of reacting, particulate cellulosic materials is established experimentally in small cube-shaped masses. When the temperature excess at the center of the mass is about 25K, the surface may be as high as 5K above that of the surrounding air. The effect is greatest in the smallest sizes.
The measured temperature gradient normal to the reactant surface and the magnitude of the surface step are used to evaluate the Blot number. For 50-100 mm cubes of the cellulosic reactant the Blot number is 5.75-14.00. This means that the critical values for Frank-Kamenetskii's dimensionless parameter 6 vary between 2.60 and 3.14 for the equivalent sphere in this range of cube sizes. ~Scr = 3.663 at the limit of pure conduction (Bi = co).
Such variations have to be taken into account if scaling rules are to be applied to predict the safe storage temperature for large stockpiles from small-scale laboratory tests. The present data are used to show the extent to which there is an overestimate of the safe conditions when allowance is not made for the changing Blot number.