OCR Text |
Show 2.1 .21 In the following examples the standard conditions from which perturbations are made are: Am=0.6 W/mK, T =873 K, Tr=79 3 K, u=0.07, p0=439 kg/m3, a=0.9, hcQ=26 W/m2K (0.9 m/s), plate, R=3 mm, V/S=R/(1+T)=3 mm, r=0, T0=293 K, nitrogen atmosphere, wood. MASS FLOW RATE/INITIAL MASS 1 _ 0 - MASS/IMITIAL \v X PLATE MASS CYLINDER y = TB_ = s I+r 3 TG=873 K,TR-793 K lk • 610 WJ/Jcq ^N. SPHERE MM ' 1 0.01 (1/s) 0.005- U=0.07 .PLATE 0 150 TIME (SECONDS) 300 0 150 TIME (SECONDS) Fig.6. The effect of shape on the transient mass and mass flow rate (calculated). The effect of the shape is shown in Fig.6. Plates, cylinders and spheres having the same volume/surface area ratio are compared. The rate of mass loss is fastest for plates and slowest for spheres. The difference between the curves would be smaller for smaller values of 1, and for dried particles. There are peaks in the mass flow rate. For example, for cylinders, the first peak i due to vaporization of water on the surface layer, when the surface reaches 373 K. The second peak is due to the start of the pyrolysis, the third peak is due to the intensification of the pyrolysis. The reason for the last peak is that the cylinder has completely been dried and the cooling effect of the steam is finished. The effect of the heat of pyrolysis is shown in Fig.7. The reactions are assumed to be endothermic. The effect of the heat conductivity of the pore walls (which is not the true local heat conductivity, eq. (40)) can be seen in Fig.8. |