Influence of Preflame and Postflame Mixing on NOx Formation in a Reacting Premixed Jet in Hot Cross Flow

Axial staging in premixed gas turbine combustors is a promising option for the increase in firing temperature without NOx penalty and for the improvement of turndown ratio, which is limited by the onset of CO-emissions. The configuration of greatest interest is the combination of state of the art premixed combustion in the primary stage with secondary injectors near the turbine inlet, which feed additional jets of premixed combustible mixture into the hot cross flow. Regarding NOx, this configuration is particularly beneficial (1) if the overall mixing quality in the first stage is limited, (2) if the difference between primary zone flame temperature and turbine inlet temperature due to air addition along the combustor is large, and (3) if a high degree of mixing in the second stage is achieved. The potential of this promising combustion concept was investigated in a large scale atmospheric test rig. For the study presented below, scaling of the second stage according to Karlovitz number similarity was chosen. This leads to smaller jet diameters and higher injection velocities compared to scaling based on Damköhler number applied in an earlier study. The impact of the higher velocities at the injector outlet on the flow field, on the liftoff height of the flame and on NOx formation is analyzed. A chemical network model is presented, which illustrates the effects of preflame and postflame mixing on NOx formation under atmospheric and high pressure conditions. In addition, this model is used to study the interactions of chemistry with mixing between the reacting jet and cross flow. On the basis of atmospheric testing and reactor modeling, predictions for engine pressure are made assuming similar liftoff as well as pre and postflame mixing. These results are further analyzed regarding the NOx reduction potential at different equivalence ratios and residence times. Finally, it is discussed under which conditions the investigated configuration can be beneficially applied to reduce NOxemissions of real engines.