OCR Text |
Show due to the fact that by shifting the peak downstream, the residence time is longer (local velocity decays approximately as J/x in the jet), so there is more time for chemical production. The fact that the air toxic species are consumed even before the mixture reaches stoichiometri~ conditions is emphasized by Figure 18, which shows the mole fractions of a group of species plotte against theoretical air, instead of distance. Although the species peak at different locations due to different production pathways, they are all consumed before the mixture entrains 80% of the theoretical air. Integration of the jet model for the individual jets proceeds to the point where the fuel mixture fraction is 0.07, consistent with the experimental species measurements at the quarl exit. To ap~ly the jet model a second time for the fuel rich mixture entering the larger furnace chamber, the solutIon for the homogeneous mixture becomes the initial condition for the merged jet. Application of the jet model to the merged flow from the individual jets requires some engineering judgment. Where possible, experimental measurements provide information to reduce the level of uncertainty in choosing conditions. The observed velocity is roughly 18 mls; conservation of the total mass flow rate suggests an equivalent diameter of 25 cm. The surrounding fluid now contains some recirculated flue gas, defined by taking the measured oxygen mole fraction of about 0.15 at the quarl exit plane and diameter. The measured temperature of approximately 1000 K provides the temperature of the jet surroundings. The merged jet entrains this flue gas/air mixture until its composition reaches 125% theoretical air. From this point on, the air/fuel ratio is constant, and continued reaction occurs in the plug-flow computation until the measured total residence time of 5.25 seconds has elapsed. The plug-flow model includes the effect of heat transfer using a linear cooling rate between the maximum temperature of the merged jet solution and the observed furnace exit temperature of 1100 K. Figure 19 shows the solution for NOx and CO for all three regions of the simulation: the individual jet, the merged jet, and the plug-flow. The individual jet rapidly produces CO and NOx as fuel mixes with air and bums. There is a brief period of NOx consumption at the end of the individual flame, when the mixture passes through the slightly-rich composition at which reburning occurs. NOx production continues in the merged jet until the flame tip. Beyond this, the temperature falls due to further entrainment, and the NO chemistry essentially freezes below 2000 K, when the NO level becomes constant (the mole fractions in these figures are corrected to a fixed dilution of 30/0 oxygen on the dry basis). Meanwhile, CO is consumed in the merged jet as it continues to receive oxygen from the surroundings. The plug-flow computation shows continued CO burnout, after mixing is completed. The final CO level is reasonable considering the fact that the measurement was below the detection limit of 5 ppm. The final NOx concentration is 106 ppm, compared to the measured value of 118 ppm. Since NO production is extremely sensitive to temperature, and the maximum temperature of the jet flames is only crudely adjusted using a simple radiation loss model, this agreement is as good as should be expected. 9 |