| OCR Text |
Show 2.1.10 The accuracy of the approximation (19) was tested for a pure transient conduction case. The comparison between the calculated numerical results and exact analytical solution proved that a good accuracy is achieved, when the difference between the chosen successive isotherms T, and T. - is not too large. k k+1 The idea of these approximations (equations (18) and (19)) is that we can treat drying, pyrolysis and heat storage all as phase change processes. Instead of calculating the temperature distribution as a function of time, we calculate the locations of successice isotherms as functions of time. These isotherms are chosen in advance with a suitable interval. The isotherm 373 K is chosen as one of the isotherms,the transient location of which is followed. The pyrolysis is strong between the temperatures 473 K and 673 K, and a narrower difference T.-T, 1 K K- I can be chosen for this temperature area. The isotherms or the moving boundaries can be classified into different groups: - a moving core where no vaporization or pyrolysis take place having "the heat of phase change" p, .. c, . (T, -T, .) D 1 =n K~ I K- I K K.- 1 ' ok k - a moving core where the vaporization of the liquid water takes place having the heat of phase change p T-II+P^ ICV 1 ^TV~T V I^ where the latter term accounts for heating from temperature T, _1 to T, =373 K and the first term is due to the heat of vaporization. Steam is generated on this core. - a moving core where the generation of the pyrolysis products takes place having the heat of phase change p , 1, +P, -C. - (T, - T. -) where the first term accounts fot the heat of pyrolysis reactions - if the initial temperature of the particle is below 273 K, there is also a moving core where the meltinq of ice takes plac The heat of phase change is taken into account. By applying the approximations the equation (10) becomes |
