OCR Text |
Show 3 3.Resurts and Discussion 3.1. CO Concentration Profile The local CO concentration in the test rig, Cco depends on the initial value of CO concentration, Cco'nrt,al . It is appropriate to use 1 - 77, instead of Cco, to compare combustion characteristics on the different initial values of concentration in fuel gas, where 77 is the combustion ratio of C O in flue gas, defined as: V = (cco in*al - Cco) / (Cco Mai - C c o find) x 100 (1) where Cco final is the final concentration of C O in flue gas. Figure 2a, 2b, 2c, 2d, 2e, 2f and 2g present a set of 1- 77 axial profile data obtained from the experiments shown Table 1, compared with the predictions, respectively. The following equations are used in these calculations: (a)Mass conservation equation, (b)Momentum equation, (c)Conservation equations of k- e turbulence model, (d)Species transport equation and (e)Energy transport equations. The Eddy-Dissipation model of Magnussen and Hjertager[1] is incorporated into the species transport equation to take account into the mass rate of generation or dissipation by combustion of CO. In this model, the rate of reaction Rti is given by the smaller of the two expressions below F\j = v ^ A p ( £ / k ) ( m R / ^ M R ) (2) Rij = i/i JMiABp(e/k)(Zpmp/Zpi/pJMp ) (3) where R is the reactant species, A is an empirical constant equal to 4.0, and B is an empirical constant equal to 5.0. [2] These equations were solved in a coupled manner using the FLUENT, that is the computational fluid dynamics software package(FLUENT Inc.). As shown in these figures, each 1 - 77 axial profile decays toward the downstream as the consequence of combustion, and the experimental data and the calculation curve mostly has a good agreement. Figure 2a and 2b present the effect of the 02/N2 burner nozzle inner diameter concerning I-77 axial profile. Judging from the fact the decreasing rate of I-77 along the flow axis in Figure 2a is faster than in Figure 2b it has been found that the condition in the case of 7.6 millimeter nozzle in diameter, had larger combustion rate than that in the case of 12.4 millimeter nozzle in diameter,. Figure 2b, 2c, 2d and 2e present the effect of the initial C O concentration in the fuel gas on the I-77 axial profile. It can be seen the larger the initial C O concentration was, the larger the decreasing rate of 1- 77 became. Figure 2b, 2f and 2g present the effect of the 0 2 concentration in oxidized gas on the 1- 77 axial profile. It can be seen the larger the 0 2 concentration in oxidized gas is, the smaller the decreasing rate of I-77 became. The results show the opposite tendency to the effect of the initial C O concentration above mentioned; it suggests that there is another factor besides the chemical kinetics to increase the reaction rate of combustion, as below mentioned. |