OCR Text |
Show c AFRC85-15 found for t > 2-3 s at T& < 1140*K, with a flow velocity of 300 1/s and YQ p of 0.98. These results cannot compare to those in reference [8] due to different value of the flow velocity used by this work. However, the curve corresponding to 1400 1/s agrees with that shown in Fig. 6 of the reference [71. 4.2. Ignition Mode Transition It was shown previously that at low flow velocity ignition is controlled by the heterogeneous processes and switches to the gas phase mode when the flow velocity is high. To explain this behavior. Table 2 tabulates values of Y„ , Y and T at F , w 0 , w s , w Ignition, as function of the flow velocity and temperature. This analysis predicts that when the unsteady-state heating of the surface is attained up to approximately 680*K, at flow velocity less than 900 1/s at the moment of ignition, the fraction of the fuel at the surface is at most 1.7*. The fuel fraction introduced into the reaction zone is very much lower than the concentration limits for ignition reaction of a mixture of hydrocarbon compounds. In the present analysis, this is calculated to be 0.17 to 0.273 (see Table 3), for the specified oxygen concentration. Therefore, the gas phase is not involved in the Ignition process. At the same time, the rate of gasification is small and nearly all oxygen can diffuse to the surface. These results lead to a strong chemical process at the surface. At high velocity, information from Tables 2 and 3 reveals that more fuel is available in the gas phase and less oxygen at the surface. These conditions result in strong chemical processes in the gas phase and the ignition reaction must be initiated at some position within the boundary layer where fuel and oxygen mix together at a stoichiometric proportion. It is clear from these results that when the flow velocity increases the ignition mechanism exhibits its transition from the surface ignition mode to the gas phase ignition mode. For the conditions used by this analysis, such transition occurs only for ! AFRC85-16 Table 2. Effects of Flow Velocity and Temperature on Y„ , Yn and T at Ignition; F,w 0,w s,w Yn = 0.7177 0, e T (K) a(l/s) F,w 0,w 1050 1200 300 500 700 900 1100 1300 300 500 7C0 900 1100 1300 1450 0.003 0.005 0.520 0.589 0.648 0.718 0.002 0.002 0.005 0.017 0.589 0.734 0.691 0.715 0.715 0. 145 0.119 0.092 0.072 0.716 0.716 0.714 0.705 0.145 0. 105 0. 103 627 633 745 756 771 787 618 630 654 660 756 780 786 Table 3 Effects Of Flow Velocity And Temperature On Y . Y . o at Ignition In The Gas Phase; F* 0' 0,e 0.7177 T (K) a(l/s; 1050 1113 1200 700 900 1100 1300 630 890 1160 1410 1100 1300 1450 0. 170 0. 218 0. 207 0. 268 0. 154 0. 189 0. 219 0.281 0. 194 0. 277 0. 273 0. 308 0.259 0. 277 0.237 0. 345 0. 292 0.295 0. 285 0. 338 0.324 0. 281 2 . 5 2.6 2 .7 2.75 2.2 2 .4 2.5 2 .4 2. 18 2 .04 2 .23 flow temperature equal to or higher than 1050*K. At T lower e than this value, the ignition mechanism is completely homogeneous. This is consistent with the result reported by Isakov and Grlshin [8]. |