Flow structures in a lean-premixed swirl-stabilized combustor with microjet air injection

In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback, is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on flashback tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ = 0.2–1.0), bulk air velocity u0 = 30–90m/s and burner preheat temperature ranging from 300K to 700K. The burner is mounted in the atmospheric combustion test rig at the HFI, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against flashback occurrence by applying the axial air injection strategy. Axial air injection constitutes a non-swirling air jet on the central axis of the radial swirl generator, thus, influencing the vortex breakdown position. High axial air injection yields excellent flashback resistance and is used to investigate the whole inlet parameter space. In order to trigger flashback, the amount of axially injected air is reduced, which allowed to investigate the near flashback flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection alter the isothermal flow field and cause a downstream shift of the axial flame front location. Such a shift is proven beneficial for flashback resistance. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front likelihood xF. The temperature and equivalence ratio dependence of the parameters xF is identified to be governed by the momentum ratio between fuel and air flow J. These results contribute to the understanding of the superior flashback limits of configurations applying high amounts of axial air injection over medium or none air injection.

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

Volume 4B: Combustion, Fuels and Emissions ◽

10.1115/gt2014-27002 ◽

2014 ◽

Cited By ~ 1

Author(s):

Thoralf G. Reichel ◽

Steffen Terhaar ◽

Oliver Paschereit

Keyword(s):

Flow Field ◽

Gas Turbines ◽

High Speed ◽

Flame Temperature ◽

Air Injection ◽

Reacting Flow ◽

Nox Emissions ◽

Test Rig ◽

Swirl Number ◽

Lean Premixed

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 non-swirling jet on the central axis of the radial swirl generator which influences the vortex breakdown 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 the 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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Visualization and Analysis of Venting From a Single Microchannel Two-Phase Copper Heat Exchanger

ASME 2009 InterPACK Conference, Volume 2 ◽

10.1115/interpack2009-89192 ◽

2009 ◽

Author(s):

Milnes P. David ◽

Julie Steinbrenner ◽

Josef Miler ◽

Kenneth E. Goodson

Keyword(s):

Vapor Phase ◽

Liquid Flow ◽

Air Injection ◽

Flow Structures ◽

Static System ◽

Two Phase ◽

Flow Rates ◽

System Pressure ◽

The Impact ◽

Injection Rates

Two-phase microfluidic cooling solutions have the potential to meet the thermal and geometric requirements of high performance microprocessors. However, rapid nucleation and growth of the vapor phase in the micro-scale flow structures produce detrimental rise in the system pressure and create flow instabilities. In our previous work we developed a novel solution to these problems: to locally vent the vapor formed in the microstructures by capping the flow structures with porous, hydrophobic membranes that allow only the trapped vapor phase to escape the system. In this paper we present the results from a visualization study of this venting process in a copper microchannel with a porous hydrophobic Teflon membrane wall and determine the impact of varying flow conditions on the venting process. We tested liquid flow rates of 0.1, 0.25 and 0.5 ml/min with air injection rates varying from 0.2 to 6 ml/min, corresponding to mass qualities of 0.1% to 7%. Bubbly/slug and wavy flows are dominant at the lower liquid and air flow rates, with wavy-stratified and stratified flows becoming dominant at higher air injection rates. At the highest liquid flow rate, plug and annular flows were common. Analysis finds that venting effectiveness is insensitive to Reliq until the point where non-contact flow structures such as annular become dominant and result in a loss of effective venting area. We also find that venting area changes linearly with mass quality and that the maximum venting effectiveness can be improved by increasing the venting area or raising the total static system pressure. However, venting effectiveness is fundamentally limited by the membrane conductance.

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1002 Investigation of the hub secondary flow structures in a low pressure turbine stage with purge air injection

The Proceedings of the Fluids engineering conference ◽

10.1299/jsmefed.2014._1002-1_ ◽

2014 ◽

Vol 2014 (0) ◽

pp. _1002-1_-_1002-2_

Author(s):

Koki KUDO

Keyword(s):

Secondary Flow ◽

Low Pressure ◽

Air Injection ◽

Flow Structures ◽

Turbine Stage ◽

Low Pressure Turbine

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

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4029119 ◽

2015 ◽

Vol 137 (7) ◽

Cited By ~ 20

Author(s):

Thoralf G. Reichel ◽

Steffen Terhaar ◽

Oliver Paschereit

Keyword(s):

Flow Field ◽

Gas Turbines ◽

High Speed ◽

Flame Temperature ◽

Air Injection ◽

Reacting Flow ◽

Nox Emissions ◽

Test Rig ◽

Swirl Number ◽

Lean Premixed

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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Investigation of Lean Premixed Swirl-Stabilized Hydrogen Burner With Axial Air Injection Using OH-PLIF Imaging

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4031181 ◽

2015 ◽

Vol 137 (11) ◽

Cited By ~ 7

Author(s):

Thoralf G. Reichel ◽

Katharina Goeckeler ◽

Oliver Paschereit

Keyword(s):

Flow Field ◽

Flame Front ◽

Gas Turbines ◽

Equivalence Ratio ◽

Air Injection ◽

Turbulent Flame Speed ◽

Air Jet ◽

Wide Range ◽

Lean Premixed ◽

Inlet Conditions

In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback (FB), is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on FB tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ=0.2−1.0), bulk air velocity u0 = 30–90 m/s, and burner preheat temperature ranging from 300 K to 700 K. The burner is mounted in an atmospheric combustion test rig, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against FB occurrence by applying the axial air injection strategy. Axial air injection constitutes a nonswirling air jet on the central axis of the radial swirl generator. While a high rate of axial air injection yields excellent FB resistance, reduced rates of air injection are utilized to trigger FB, which allowed to investigate the near FB flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection, alter the isothermal flow field as they cause a downstream shift of vortex breakdown and, thus, the axial flame front location. Such a shift is proven beneficial for FB resistance from the recorded FB limits. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front probability xF. By these means, it was revealed that for hydrogen xF is shifted downstream with increasing equivalence ratio due to the added momentum of the fuel flow, superseding any parallel augmentation in the turbulent flame speed. The parameter xF is identified to be governed by J, the momentum ratio between fuel and air flow, over a wide range of inlet conditions. These results contribute to the understanding of the sensitivity of FB to changes in the flow field, stemming from geometry changes or specific fuel properties.

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