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

Increasing Flashback Resistance in Lean Premixed Swirl-Stabilized Hydrogen Combustion by Axial Air Injection

[+] Author and Article Information
Thoralf G. Reichel

Chair of Fluid Dynamics,
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany
e-mail: thoralf.reichel@tu-berlin.de

Steffen Terhaar, Oliver Paschereit

Chair of Fluid Dynamics,
Hermann-Föttinger-Institut,
Technische Universität Berlin,
Müller-Breslau-Str. 8,
Berlin 10623, Germany

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received September 1, 2014; final manuscript received October 30, 2014; published online December 23, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(7), 071503 (Jul 01, 2015) (9 pages) Paper No: GTP-14-1522; doi: 10.1115/1.4029119 History: Received September 01, 2014; Revised October 30, 2014; Online December 23, 2014

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a nonswirling jet on the central axis of the radial swirl generator which influences the vortex breakdown (VB) position. In the present work, changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by means of the qualitative light sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

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Figures

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Fig. 1

Increase of both, (a) bulk outlet velocity u0(φ) and (b) momentum ratio J, with equivalence ratio strongly depends on ratio of inlet air to fuel temperature (Tin/Tfuel)

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Fig. 2

Schematic of burner model, indicating different volume flow pathways through swirl generator or axial injection

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Fig. 3

Experimental setup for simultaneous PIV and OH* measurements in atmospheric combustion test rig

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Fig. 4

Velocity vectors superimposed on normalized mean axial velocity of the isothermal flow field in the (a) absence (Dor = 0 mm) and (b) presence of a medium (Dor = 8.0 mm), and (c) high (Dor = 8.8 mm) amount of axial air injection (long mixing tube; S = 0.9), solid lines indicating zero axial velocity

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Fig. 5

Histogram of axial velocity at (x/D = 0.1, r/D = 0) for isothermal conditions in the absence (Dor = 0 mm) and presence of a medium (Dor = 8.0 mm) and high (Dor = 8.8 mm) amount of axial air injection (Re = 68,000)

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Fig. 6

Stability limits for varied air mass flows at two inlet temperatures; configurations 1–4 operated at stoichiometric conditions without flashback (symbol ×, not tested above stoichiometric). Hence, only the lower stability limits are displayed for each configuration.

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Fig. 7

Impact of increased equivalence ratio for the reacting flow (m· = 180 kg/h, tin = 450 k) for a high ((a)–(c), φ=0, 0.4, and 0.8) and medium ((d)–(f), φ=0, 0.4, and 0.6) amount of axial air injection. Solid lines indicate zero axial velocity.

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Fig. 8

Histogram of axial velocity at (x/D = 0.1, r/D = 0) revealing impact of fuel momentum in the presence of high (Dor = 8.8 mm) amount of axial air injection (Re = 75,000)

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Fig. 9

Observed VB types during PIV measurements of configurations 3 and 4 transferred to schematic in Ref. [18], in order to explain difference in character of axial air injection (χ ↑) and fuel momentum (φ↑)

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Fig. 10

Flame probability indicating the likelihood of the flame to appear in a certain region, hence, allowing to determine the upstream flame shape (Re = 75,000)

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Fig. 11

ABEL-deconvoluted OH* images normalized to the maximum intensity of the image at φ=0.8 (Re = 75,000)

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Fig. 12

NOx emissions (dry) over calculated adiabatic flame temperature for high axial air injection

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