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NOx Emissions Characteristics in the Wake of an Air Jet Injected into a Fuel Rich Vitiated Crossflow

  • Author(s): Lee, Howard Hokyung;
  • Advisor(s): Samuelsen, G.Scott;
  • et al.

An air jet injected into a fuel rich vitiated crossflow was studied in order to gain detailed understanding of the NOx emissions characteristics in the wake of the reacting jet. The fuel rich vitiated crossflow was produced by premixed combustion of methane and air with an equivalence ratio of 1.5 at STP condition. Detailed in-situ measurements of NOx emissions and CFD simulations were conducted. The varied parameters include location of the sample points relative to the jet, the jet-to-crossflow momentum-flux-ratio, and the jet diameter size. The resulting matrix consist five cases in which one parameter was varied. All of the cases were studied at the same 5 axial distances normalized by the jet diameter. 17 sampling points were used at each axial distance separated by 1/16'' between each point in transverse direction and 3 distances (0.25'', 0.50'', 0.75'') away from the wall. Total of 5 different testing cases were developed based on varying the momentum-flux-ratios or the jet diameters. The horse-shoe shape of the NOx emissions levels were found in the majority of the data set especially within lower axial distances. However, the horse-shoe shapes were replaced by a shape of parabola at a higher axial distance. The locations of the highest NOx emissions levels moved to farther downstream from the jet injection plane with increasing distance away from the wall. The overall area of the region where significant levels of NOx were found grew in both axial and radial directions with increasing distance away from the wall as well. However, both were found to be moving higher and growing with decreasing momentum-flux-ratios. As the rate of air diffusion remains constant regardless of momentum-flux-ratio, the higher momentum-flux-ratio causes the jet deformations to occur at a farther distance from the wall and causes the outwardly air diffusion along the jet trajectory to occur in the later distance. Hence, the distance away from the wall can be normalized by momentum-flux-ratios and can be rearranged in the order of jet development along the jet trajectory. The CFD simulations results revealed that the reacting jet causes elevated temperature in the wake of the jet. The cross-section temperature profiles show the evidence that the both jet and the jet influenced high temperature zone develop at the same rate. Hence, the shape and the size of the high temperature zone can be correlated to the NOx concentrations levels found in the wake of the jet explaining the trends with the locations of the highest NOx emissions level and the regions of significant NOx levels. The simplified reaction simulation within the CFD simulations revealed evidences to support that the Fenimore NO mechanism is occurring in the shear layer of the reacting jet while the thermal NO mechanism is occurring in the wake of the jet, especially within the high temperature zone. The current research also found that the momentum-flux-ratio has the biggest impact on the NOx emissions levels and characteristics in the wake of the jet. The varying jet diameters had minimum impact on the NOx emissions behavior but the different jet penetration associated with the different jet diameters caused the NOx emissions characteristics to differ. Finally, it was concluded that higher jet penetration is correlated with lowering NOx emissions in a full scale RQL combustor based on current research. The higher momentum-flux-ratio case returned the lowest NOx levels while the jet diameter size does not impact the NOx levels in the wake of the jet near the wall. Also, the previous work done by Vardakas in 1999, where the larger jet diameter increased the NOx emissions level in the wake of the jet but showed the lowest overall NOx levels when averaged by weighted area. Combining these results, the higher jet penetration due to higher momentum-flux-ratio would minimize the NOx in the wake of the jet near the wall, and even though the momentum-flux-ratio is fixed, the larger jet diameter would cause the jet to penetrate farther into crossflow causing the area of the lean equivalence ratio in the center of the combustor to increase and minimizes the area of high NOx emissions in the wake of the jet.

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