OCR Text |
Show method [2J as adapted by Kee and co-workers at Sandia [7J, the adiabatic flame temperature is calculated to be 3528°F at atmospheric pressure. The chemical species concentrations at this final state are listed in order of concentration in the first column below ~S~pe~c~i~e~s __ ~F~u~l~l_l~i~s~t~S~h~o~r~t_l~i~s~t __ ~C~O ____ ~H2_ N2 H20 C02 CO 02 H2 OH NO H o Temp (0 K) 0.71212 0.18010 0.08629 0.00859 0.00439 0.00342 0.00265 0.00189 0.00036 0.00020 2220 0.71812 0.18633 0.09555 2318 0.71450 0.71378 0.18539 0.18177 0.08499 0.08631 0.01008 0.00863 0.00504 0.00605 0.00343 2252 2240 with all other species such as H02, N02, N20, and H202 less than 1 ppm (mole fraction of less than 1.0 x 10-6). If we repeat the same calculation but assume that the only possible products are N2, C02, H20, and 02, the adiabatic flame temperature is then found to be 2318K or 3713 of, with the composition shown in the second column of the above table. The third column shows the computed results when CO is added to the product list, and the fourth column shows the results when H2 is then included. Comparisons of these results with the complete list in the first column above indicate that the greatest improvement comes from inclusion of CO, which is the most important minor product, but that the changes resulting from consideration of the other species are still quite noticeable. Using the above reference case for comparison, we can now address the influences of variations in three common operating parameters on adiabatic flame temperature and equilibrium composition, with NO noted in particular. These basic parameters are the fuel-air equivalence ratio ~, the amount of excess oxygen, and the preheat temperature of the air. We will treat these quantities as independent parameters, although in practice they are often used together to fit a particular need for a given combustor. EQUIVALENCE RATIO - The most important quantity that changes with ~ is the adiabatic flame temperature, reaching a maximum value for equivalence ratios close to stoichiometric (~ = 1). Because most combustion reactions depend quite strongly on temperature [3J, the overall combustion rate and related quantities like the flame speed also are found to reach maximum values for ~ = 1. Similarly, the equilibrium concentrations of most of the radical species generally are greatest near stoichiometric. 146 Using the same computer model as used earlier, we can compute the adiabatic flam: temperature and product species concentratlons for methane-air mixtures initially at atmospheric temperature and pressure. The results for the temperature and some illustrative species concentrations are shown in Figures 1-3. 2500~----------------------------~ ~ :> 2000 ...... Q) s... ~ 1500 ~ 1000 s... Q) 0.. S Q) E-< 500 O~--~----~----~--~----r---~ 0.4 0.6 0.8 1 1.2 1.4 1.6 Eq ui valence ratio Fi gu re 1 Adiabatic flame temperature as a function of fuel-air equivalence ratio for mixtures initially at room temperature and pressure. 0.20 -r------------------------------, 0.15 ~ 0 ·M +oJ C,) ro s... 0.10 ~ Q) ...... 0 ::E 0.05 .......... ...••..•.....c ._•o • . 2. . ... /. /' ...... Ccy' ........... . , .-/ 0.00 +-----,.--~...a...::.--,------r-----~--~ 0.4 0.6 0.8 1 1.2 1.4 1.6 Eq ui valence ratio F i gu re 2 Selected species concentrations at equilibrium, plotted as functions of equivalence ratio. As already noted, the peak values of temperature and the important radical species such as 0, OH, H, and H02 lie close to stoichiometric. There are some shifts in peak values with equivalence ratio; for example, the H atom concentration peaks at ~ = 1.1, and |