Impact of Cooling Air Injection on the Primary Combustion Zone of a Swirl Burner

Abstract

In most dry low NOx combustor designs the front panel impingement cooling air is directly injected into the combustor primary zone. As this air partially mixes with the swirling flow of premixed reactants from the burner prior to completion of heat release it reduces the effective equivalence ratio in the flame and has a beneficial effect on NOx emissions. However, the fluctuations of the equivalence ratio in the flame potentially increase heat release fluctuations and influence flame stability. Since both effects are not yet fully understood isothermal experiments are made in a water channel where high speed planar laser induced fluorescence (HSPLIF) is applied to study the cooling air distribution and its fluctuations in the primary zone. In addition the flow field is measured with high speed particle image velocimetry (HSPIV). Both, mixing and flow field are also analyzed in numerical studies using isothermal large eddy simulation (LES) and the simulation results are compared with the experimental data. Of particular interest is the influence of the injection configuration and cooling air momentum variation on the cooling air penetration and dispersion. The spatial and temporal quality of mixing is quantified with probability density functions (PDF). Based on the results regarding the equivalence ratio fluctuations regions with potential negative effects on combustion stability are identified. The strongest fluctuations are observed in the outer shear layer of the swirling flow, which exerts a strong suction effect on the cooling air. Interestingly, the cooling air dilutes the recirculation zone of the swirling flow. In the reacting case this effect is expected to lead to a decrease of the temperature in the flame anchoring zone below the adiabatic flame temperature of the premixed reactant, which may have an adverse effect on flame stability.