OCR Text |
Show 6 dimensional boundary layer. The pre-mixed gas of C O G and air was ejected from a nozzle of inside diameter 4.2mm at an average 1.22m/s as shown in Figure 11. The fuel for numerical simulation is C O G shown in Table 1.14 species and 30 reaction equations are considered in the calculation and are shown in Table 2. 4.2. Influence of Air preheating Figure 12 shows the distribution of flame temperature and chemical species at the nozzle's central axis at an air ratio of 0.85, at a preheating air temperature of 20°C, in the exhaust gas atmosphere. The combustion reacts from the nozzle rim toward the central axis, and reaches the central axis at a distance of 3 7 m m from the nozzle tip. This position corresponds to the boundary of the oxidation area and the reduction area at the top of the Bunsen inside the flame, as shown in Figure 13. While CH4 concentration decreases rapidly as a result of the thermal decomposition reaction and is consumed completely at this stage, H2 concentration decreases slowly by diffusion and remains until the latter stage of combustion. Reducible C H 2 0 radical is generated in an excess fuel and in the low temperature region at the early stage of combustion. But the thermal decomposition reaction of C H 2 0 proceeds in the high temperature region at the latter stage of combustion, and C H 2 0 is consumed almost completely. Reducible H radical and oxidizable O and OH radical are continuously generated in the high temperature part of the latter stage of combustion. It is suspected that C H 20 governs the reduction reaction in the low temperature region of the reaction, and the competition of H and 0,OH decide the oxidation and the reduction reaction in the high temperature part. The radial concentration distribution of radical species at the position where the combustion reached the central axis is shown Rgure 14 in the case of a preheated air temperature of between 20°C and 500°C. The temperature of the flame is raised by the air preheating, and the radical concentration of each species increases. 4.3. Influence of Oxygen Enrichment Next, the effect of the oxygen enrichment was examined. The radial concentration distribution of radical species at an air ratio of 0.5 and an oxygen concentration of 5 0 % is shown in Figure 15(a)-(d). In Figure 15(a) and (b) which corresponds to the early stage of combustion, C H 2 0 is generated inside the flame when there is excess fuel, and H is generated at high temperature and the reactive part of the outside of the C H 2 0 layer. In Rgure 15(c) and (d) which correspond to the latter stage of combustion, C H 2 0 is completely consumed, and a lot of H is generated. H diffuses easily compared with CH20, and has |