OCR Text |
Show - 11 - because of the low Mach numbers of both deOagration waves PI = P 2' Moreover, if the overall orders of the reactions in both zones, as well as the avemge molecular weights, before the first and second flame wnes are similar, f will be much smaller than (T tiT cJ2. Under these conditions, and considering the presence of a exothermic first and second reaction zone (TilT 0>1 and T iTI>I), it is found that the ratios of the activation energies are bounded by [11 ] Equation (11) suggests that the activation energy of the second stage is always larger than the activa-tion energy of the first stage. It is funher interesting to note that the same analysis may be applied through the reaction wne of a single flame. 'The implications are then, that the "overall" activation energy increases within the flame proceeding from the unburned mixture towards the burned products. Since the second-stage flame is expected to have an overall activation energy higher than that of the first-stage flame, the second reaction zone would be more sensitive to tempemture. This appears to be consistent with the observations presented in Fig. 5. Here, the oxygen concentration was increased only by 2% which produced a small temperature rise resulting in ignition of the second-stage flame. Further oxygen addition increased the flame speed of the second reaction wne. The relatively insensi-tive nature of the first flame to temperature is apparent from the invariance of the shape of the inner flame. The higher activation energy of the second stage may contribute to a higher sensitivity to flame stretch than the first stage [31,32,33]. It is speculated from the above considerations that in practical incinerators, the different levels of turbulence will affect the total emissions profoundly through extinc-tion phenomena of the second stage; in these incinerators, significant fractions of the reaction wne are known to operate in the flamelet regime [31]. This suggests that the design rule used for incinerators to maximize turbulence may well have far-reaching deleterious effects on emissions. Although the results presented here are not quantitatively applicable to turbulent flames, it is highly likely that the required critical H/Cl ratio to maintain a second-stage flame is much larger. From the present results , |