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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

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

[+] Author and Article Information
Denise Ahrens

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: denise.ahrens@rolls-royce.com

Michael Kolb, Christoph Hirsch, Thomas Sattelmayer

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received November 30, 2015; final manuscript received December 15, 2015; published online March 15, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(8), 081506 (Mar 15, 2016) (10 pages) Paper No: GTP-15-1555; doi: 10.1115/1.4032420 History: Received November 30, 2015; Revised December 15, 2015

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.

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References

Figures

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Fig. 1

Outline of the reacting jet in hot cross flow experiment

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Fig. 2

Definition of the coordinate systems

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Fig. 3

Sketch of the large scale test rig

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Fig. 4

Measured mixture fractions in the y–z plane 7.7 jet diameters downstream of the jet injection (OP212), and the measured points are marked with crosses

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Fig. 5

Measured NOx concentrations in the y–z plane at 7.7 jet diameters downstream of the jet injection (OP212), and the measured points are marked with crosses

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Fig. 6

Comparison of mixture and NOx emission profiles over the channel height at y/D = 0, normalized with their minimum and maximum values (OP212)

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Fig. 7

Comparison of jet trajectories and the measured velocity field in the x–z plane (OP212)

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Fig. 8

Comparison of the extracted trajectories and the position of the flame in the x–z-center plane with the mixture fraction at x/D = 7.7 (OP212)

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Fig. 9

Reactor network model

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Fig. 10

Influence of preflame mixing on in-flame NOx formation at 1 bar and 20 bar (OP212)

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Fig. 11

Scenario for the investigation of the influence of postflame mixing on NOx formation

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Fig. 12

Sketch of the mixture fraction f along the trajectory of a JIC

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Fig. 13

Influence of the preflame mixing on the overall NOx level after τpost = 5 ms for 1 and 20 bar (OP212)

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Fig. 14

Comparison of total NOx emissions with in-flame NOx for 1 bar (left) and 20 bar (right) (OP212)

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Fig. 15

Influence of residence time and preflame mixture fpf on overall NOx formation at 20 bar (OP212)

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Fig. 16

Scenario for the postflame zone model with constant reactor temperature

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Fig. 17

Temperature dependency of postflame NOx formation for 1 and 20 bar and a residence time of τpost = 5 ms (OP212)

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Fig. 18

Computed NOx emission field at 20 bar using the minimum and maximum values from Table 3 and Eq. (10) for pressure scaling (OP212, τpost = 3 ms)

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Fig. 19

NOx reduction potential at 20 bar for two different equivalence ratios in the second stage (OP212 and OP211) with unmixedness (F2 and F3)

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