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Show history of the jet fluid, and hence be appropriate for the chemical kinetic computation. Based upon these observations, the model considers reactions in two separate zones: diffusive layers and homogeneous regions. It follows the evolution of the two zones as they move downstream in the jet, suggesting the name Two-Stage Lagrangian model. The model represents the average time history that the fluid experiences as it travels through a steady jet. Empirical correlations provide the entrainment rate of surrounding fluid into the reactors. A limitation of the model is the need for these mixing correlations, which must come from experimental observations. MQx Emissions from H2! CO Flames The Two-Stage Lagrangian model has been applied to prediction of the NOx emissions from a variety of laboratory jet flames. As an example of the model performance, we present predictions for a H2/CO flame and compare emissions to the experimental measurements of M yhr and Turns 15. The fuel mixture is CO with 5 percent H2 by mass. While the weight of this fuel is comparable to methane, the stoichiometry requires much less oxidizer than methane does, which means these flames are significantly shorter than methane flames. As a result, non-equilibrium oxidation chemistry is important in correctly predicting the emissions. Figure 4 shows the predicted emissions index for NOx and CO for a series of H2/CO flames where the fuel stream is diluted with N2. The emission index is defined by the mass of NO and N02 (using the molecular weight of N02 for both species, by convention) to the mass of fuel. This definition produces an index that is independent of dilution with excess air. The predictions of NOx emissions are extremely accurate for this series of flames; the model captures the effect of dilution quite well. Even more impressive is the fact that the model predicts roughly the correct fraction of N02 in the NOx. The prediction of trace species like these in the correct proportion is a compliment to the detailed chemical kinetics research that has been performed over the past several years. The decrease in NOx emissions with dilution is caused by the reduced flame temperature. Dilution reduces the temperature by two means: (1) the larger average heat capacity, and (2) increased departure from equilibrium in the flame chemistry due to reduction in the residence time. As shown by the middle panel in Fig. 4, the CO emission index (mass CO per mass of fuel) is not predicted as well by the model. While the prediction is the right order of magnitude, the trend of CO emissions with dilution is opposite the observed behavior. The discrepancy may be due to uncertainty in the application of the experimental entrainment correlations, or the simplification of using a stirred reactor to approximate reaction in the diffusion layers within the jet. -7- |