Investigations On The Pollutant Emissions Of The Axial-Fuel-Staged Combustion And Characteristics Of The Jet-in-Crossflow Flames

Increasing the combustor outlet temperature is one of the effective methods to increase the gas turbine efficiency.However,increasing the combustor outlet temperature would significantly increase the NO_x emissions.To deal with the conflict between the temperature and NO_x emission,the design of axial-fuel-stage combustor has been paid more and more attention and has a potential to be applied to the advanced（1975 K）gas turbines.In the present work,both numerical and experimental investigations have been performed to evaluate the NO_x-reduction potential of the axial-fuel-staged combustion and figure out the key factors impacting on the NO_x and CO emissions,as well as to understand the flame characteristics in the secondary stage.The main contents of the current research are as follows.Firstly,based on the assumptions of the perfect and imperfect mixing between the secondary reactants and the vitiated gases,two simplified chemical reactors network models were constructed respectively.Impacts of the fuel distribution,residence time distribution,mixing in the secondary stage,wall heat loss,inlet temperature and pressure have been investigated parametrically.Calculation at gas turbine conditions shows that at the combustor outlet temperature of 1975 K,the NO_x emission will approach 60 ppm@15%O₂ at single-stage mode.Whereas the NO_x emission will be reduced to 16 ppm@15%O₂ at the axial-fuel-staged mode（with a corresponding secondary fuel fraction of 20%）.Hence,there is large NO_x-reduction potential when applying the axial-fuel-staged combustion to advanced gas turbines.Increasing the secondary fuel fraction and shortening the secondary stage residence time can reduce the NO_x emission and the former reduces the NO_x emission most effectively.The NO_x emission can be reduced by approximately 40% as the secondary fuel fraction approaches 10% at the perfect mixing conditions.However,the imperfect mixing between the secondary jet reactants and the vitiated gases will increase the pollutant emissions of the staged combustion,and even produce more pollutant emissions than the single-stage combustion in the worst case.After confirming the feasibility of the axial-fuel-staged combustion to reduce the NO_x emissions numerically,a model combustor was constructed and the impacts of the fuel distribution,first and secondary equivalence ratios and secondary jet velocity on the NO_x and CO emissions were investigated experimentally.Results show that the NO_x-reduction effectiveness of the axial-fuel-staged combustion is dependent upon the combustor outlet temperature due to the imperfect mixing in the secondary stage.A maximum NO_x reduction of 40% is achieved by the axial-fuel-stage combustion at the combustor outlet temperature of 1975 K,and the corresponding NO_x emission at single stage mode is approximately 8 ppm@15%O₂.But the NO_x-reduction benefits will be eliminated as the combustor outlet temperature is reduced.Moreover,the linear positive correlation rather than the exponential correlation between the NO_x increase and the temperature increase in the secondary stage is observed,indicating that the conditions of the high-temperature and low-oxygen concentration environment and the fuel-rich injection at the secondary stage depress the NO_x formation.Investigations on the secondary jet velocity show that increasing the jet velocity at a constant temperature rise of the secondary stage will decrease the NO_x increase in the secondary stage,and higher jet equivalence ratios correspond to a larger NO_x decrease at the higher jet velocity.Optical measurement was also conducted based on the model combustor.Results show that when the secondary jet equivalence ratio is greater than 1,further increasing the secondary jet equivalence ratio or increasing the first equivalence ratio will reduce the OH* intensity,which demonstrates a low-NO_x emission potential of the Lean-Rich combustor at high combustor outlet temperature conditions.Analysis on the flame liftoff behavior shows that the lift-off height is decreased with the equivalence ratios of the both stages.And the flame is attached to jet nozzle exit when the jet equivalence ratio is greater than 1,raising the risks of the nozzle and wall overheating.In addition,due to the accumulation of the OH radicals in the post-flame region,the signals of the OH-PLIF cannot mark the distribution of the heat release region correctly.Subsequent 1D flame model calculation supports the abovementioned experimental results fairly well,and observes a potential NO-reburing mechanism in the inner-flame region,which further demonstrates the low-NO_x emission scheme of the Lean-Rich combustor.Considering the limited flowfield information obtained from the experiments,a typical jet-in crossflow pattern normally employed to the axial-fuel staged combustion was studied numerically.The comparison results show that the RANS approaches fail to predict the magnitudes of Reynolds stress correctly,thus causing significant deviations on the prediction of the scalar distribution.The scale adaptive simulation（SAS）with higher resolutions was validated to confirm its applicability to the jet-incrossflow flames.The non-reacting validation results show that the SAS model predicts the distribution of both velocity and Reynolds stress correctly,thus improving its prediction on the scalar distribution.Results of the reacting simulation with SAS model coupled with the eddy dissipation model show that the velocity field can be predicted accurately,and the scalar decay tendency along the jet center line is also captured in a reasonable way.Also,the reacting results show that attached flame occurs as the fuelrich injection is perfectly premixed,which subsequently increases the wall temperature near the nozzle and causing the risks of the nozzle or wall overheating.To deal with this problem,a nozzle configuration with non-uniform fuel profiles was proposed.The simulating results on this configuration show that the flame lift-off height is increased and the formation of local high-temperature region at the leeward side of the jet flame is also depressed,thus the performance of the secondary jet nozzle is improved correspondingly.