OCR Text |
Show while a negative velocity corresponds to a flow from right to left. Since the density of N2 is greater than that of Cf4, fuel-side dilution increases the momentum flux of the fuel stream significantly. This, in turn, causes the stagnation plane to move toward the oxidizer inlet. Figure 2 takes this shift into account by defining the axial coordinate relative to the stagnation plane. From Fig. 2, we clearly see that the stoichiometric concentration for the N2-diluted fuel stream lies closer to the stagnation plane than the air-diluted case. This position also corresponds to a significantly lower velocity magnitude. The importance of this is that the same qualitative behavior occurs for the reacting flows, i.e., the fuel-diluted flames lie in lower velocity regions compared to their air-diluted counterparts. This, in tum, results in larger residence times at high temperatures for the fuel-diluted flames, which is an important factor in NO production. We discuss this in greater detail below. Reacting Flows Two types of flow conditions were considered for the reacting flows: first, variable dilution, but with fixed inlet velocities of 50 cm/s for all cases; and second, variable dilution, but with a fixed fuel mass flux. This second condition of fixed fuel mass flux was met by increasing the velocities at both the exit of the fuel and air nozzles as diluent was added to the fuel stream. For example, with Z = 0.15, the nozzle exit velocity is 128 cm/s compared to 50 cm/s with no dilution. The air stream velocity was increased to match the velocity of the fuel stream thus keeping the flame roughly centered between the two nozzles and, thereby, preventing any artifactual heat loss through the boundaries. Flow and Flame Structure Calculations were performed with the same nozzle velocities (uo = UL = 50 crn/s) as the nonreacting flows discussed above, and a parametric investigation conducted in which increasing amounts of N2 were added to either the fuel stream or oxidizer (air) stream. Calculations were performed with reactants entering at 300 K and at 500 K. Figure 3 shows CH4, 02, and N2 mole fraction profiles between the opposed nozzles for no dilution, air dilution with Z = 0.15, and fuel dilution with Z = 0.15, i.e., test cases 1,2, and 3 shown in Table I. 5 |