OCR Text |
Show 9 To estimate the effect of air ratio fluctuations on NO formation , the model proposed by Heywood et al. (1972) was used. In this model, the burned gas is assumed to be composed of a large number of fluid eddies having a Gaussian distribution of air ratio about the mean value. The NO formed in the burned gas in the presence of air ratio fluctuations is calculated from equations (1) and (2). The standard deviation () in equation (2), which represents the degree of air ratio fluctuation, is expressed as a function of the air ratio. The function approximates the data in Figure 8. [NO]= f[NO](AC,t). f(AC). dAC (1) f (AC)= 1 /cr{i;. EXP[ - (AC -AC )2/ 2cr2] (2) where, [NOH,.\ c,t) : NO formed in the burned gas at the adiabatic flame temperature for ,.\ c with residence time of t sec. () : standard deviation Figures 12-a and 12-b show the predicted NO levels using the measured standard deviations in upstream and downstream respectively. In Figure 12-a, clear differences in the NO formation can be observed for perfect mixing and partial mixing. Near the stoichiometric air ratio, the NO levels in the flames of perfect mixing are higherthan those for partial mixing. However, in the leaner air ratios, more NO is formed in flames of partial mixing than perfect mixing. In Figure 12-b, the difference between perfect mixing and partial mixing is smaller than that in Figure 11-a The reason, as described above, is that fluctuations in the air ratio are attenuated by passing through the multi-port burner. From the results of the simulation, it can be concluded that if the air ratio fluctuates, NOx formation decreases near the stoichiometric air ratio, while NOx formation increases for the lean air ratio. Comparing the predicted NOx with the experimental results, the predicted effect of air-fuel fluctuations on NOx formation using the measured standard deviation in the downstream side agrees well with the experimental results. Figure 13 shows the relationship between the standard deviations and the ratio of the predicted NO in partial mixing to those in perfect mixing, for each air ratio. When the air ratio is 1.3 and 1.4, as the standard deviation increases, the ratio of the NO increases |