OCR Text |
Show Temperature distribution. Figure 3 shows the calculated temperature distributions on the central vertical cross section and three horizontal cross sections under the three simulation cases. W h e n the fresh air is preheated from 323 K to 1223 K, the combustion intensity is seen to have increased a lot. Not only the maximum flame temperature increases from 2186 K to 2627 K, but also the averaged furnace temperature reaches a high level. W h e n the oxygen concentration in the highly preheated air is lowered to 4 vol% by air dilution with nitrogen, the maximum flame temperature greatly decreases from 2627 K to 1571 K which is Distance from AH wet [mm] even lower than the normal combustion in the first Figure 4 Profiles of maximum and case However, it can been seen from the figures that averaged temperature the averaged furnace temperature in this case still keeps a high level. In order to have a quantitative impression on the temperature changes in these cases, the profiles of the calculated maximum flame temperature and the averaged temperature along the height of the combustion chamber are plotted in Fig 4. It is clearly seen that the highly preheated fresh air combustion provides higher levels of both the maximum flame temperature and the averaged furnace temperature in comparison with the normal air combustion. O n the other hand, the highly preheated and diluted air combustion has lower maximum flame temperature but higher averaged furnace temperature in comparison with the normal combustion. And the difference between the maximum flame temperature and the averaged furnace temperature is the lowest among the three simulation cases, which implies that the temperature distribution in the furnace tends to become more uniform in the highly preheated and diluted air combustion. Here a ratio of the maximum flame temperature to the averaged furnace temperature is introduced to quantitatively evaluate the uniformity of temperature distribution in the furnace. The value of the ratio closing to unity represents a more uniform temperature distribution. The profiles of Tmax/Tmcan along the height of the furnace under the three cases are shown in Fig 5. The highly preheated and diluted air combustion provides the best uniformity of temperature distribution. Such an improvement on the uniformity of temperature distribution is one of the important benefits provided by the highly preheated and diluted air combustion. 2600 2400 2200 2000 _ 1800 £ 1600 i, MOO H 1200 1000 - / T. ' " ' -1 • 1 J323K. 21%0,) \T~ T_ T_ T_ T_J323K. 21%0,) I • 1 ' (1223K.21%0,) \ (1223K.4%0]) J1223K. 21*0,) • J1223K.4SO,) ^"\ - |