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Show TABLE I. Test Conditions for Selected Numerical Simulations of CJ4-Air Counterflow Diffusion Flames with Reactants at 300 K Case Reactant Methane N2 Diluent Fraction, Z No. Velocity (cm/s) Mass Flux (kg/s-m2) Fuel Side Air Side 1 50 0.326 0 0 2 50 0.326 0 0.15 3 50 0.106 0.15 0 4 127.8 0.326 0.15 0 Also shown in Fig. 3 are the reactant concentration profiles through the diluted-fuel flame having the same fuel mass flux as the undiluted flame (Case 4 in Table I). The profiles are nominally the same as the lower velocity flame with the same level of fuel dilution (Case 3, Z = 0.15); however, some oxygen penetration through the flame into the fuel is apparent, as would be expected as characteristic flow times get shorter and approach characteristic chemical reaction times. Temperature and velocity profiles for these flames are shown in Figs. 4 and 5, respectively. A key to understanding NO formation is the time-temperature relationship for gases flowing through the flame zone. To quantify this relationship in a meaningful way, we define a residence time for NOx formation as the time that a fluid particle exists at a temperature above 1500 K as follows: x2 (T= lS00 K) Residence time, 't - J u -1 (x) dx (5) xl (T = lS00 K) The choice of 1500 K is somewhat arbitrary, but this value is a reasonable lower-limit temperature for thermal NO production. The 1500 K level is shown as a horizontal line passing through the temperature profiles of the simulated flames (Fig. 4). Table II shows residence times for the four cases considered. The interesting observation here is that air-side dilution decreases residence times over the no-dilution case, while fuel-side dilution increases residence times for the same initial velocity (50 cm/s). On the other hand, fuel dilution with a fixed fuel flow rate shows a reduced residence time compared to no dilution. In 6 |