OCR Text |
Show flame temperature. Several parameters were varied more widely to cover a likely range of operation. In most cases the new temperatures are higher than the baseline temperatures along the entire furnace length. For two cases, however, decreasing the excess air level and decreasing the flame diameter, the exit temperature decreased as the flame temperature increased. Decreasing the excess air level at a fixed firing rate has two consequences. The flame temperature is higher without the additional air because energy is exhausted in heating up excess nitrogen and oxygen. The exit temperatures are lower because of the loss in sensible energy associated with the decreased total mass flowrate. The slope of the temperature curve changes with flame diameter because of the redistribution of radiative transfer from the zones occupied by the flame. Recall the flame shape is assumed to be a cone frustum. At a fixed firing rate, decreasing the large diameter is equivalent to stretching the cone, because of the assumption of uniform volumetric chemical heat release. The first zone is cooler, because a smaller fraction of the heat is released there (less flame volume). The second zone is hotter, because more chemical heat is released there. The total radiative loss to the walls will be greater from the stretched flame cone, because its effective surface area is greater. Thus, the sensible energy entering the postflame region will be lower. Table 3 shows a gross rating of parameter effects on the temperature-position profile of the baseline furnace configuration. The inputs are divided into three categories according to their potential for influencing the postflame thermal environment. One caveat is appropriate: the model inputs can be varied independently of one another. In actual operation, this may not be possible. For example, the carbon:hydrogen ratio of the fuel, inasmuch as it effects the equilibrium C02.*H20 ratio and consequently the gas emissivity and enthalpy, has little effect on the heat loss rate. In practice, however, two fuels with different hydrocarbon compositions may have remarkably different heating values. The difference in furnace temperature history will be dramatic. The baseline bulk temperature model conditions (Table 1) were applied to the boundary layer model. Several runs were made with carbon tetrachloride as the injected waste. The additional inputs required by the second module are summarized in Table 4. The first five entries specify the calculation grid. "First zone" and "last zone" refer to the starting and ending zones for the integration; C/CQ 5.5.16 |