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

Prediction of Confined Flame Flashback Limits Using Boundary Layer Separation Theory

[+] Author and Article Information
Vera Hoferichter

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, Germany
e-mail: hoferichter@td.mw.tum.de

Christoph Hirsch, Thomas Sattelmayer

Lehrstuhl für Thermodynamik,
Technische Universität München,
Garching 85748, 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 June 27, 2016; final manuscript received June 29, 2016; published online September 13, 2016. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(2), 021505 (Sep 13, 2016) (10 pages) Paper No: GTP-16-1278; doi: 10.1115/1.4034237 History: Received June 27, 2016; Revised June 29, 2016

Premixed combustion is a common technology applied in modern gas turbine combustors to minimize nitrogen oxide emissions. However, early mixing of fuel and oxidizer opens up the possibility of flame flashback into the premixing section upstream of the combustion chamber. Especially, for highly reactive fuels, boundary layer flashback (BLF) is a serious challenge. For high preheating and burner surface temperatures, boundary layer flashback limits for burner stabilized flames converge to those of so-called confined flames, where the flame is stabilized inside the burner duct. Hence, the prediction of confined flashback limits is a highly technically relevant task. In this study, a predictive model for flashback limits of confined flames is developed for premixed hydrogen–air mixtures. As shown in earlier studies, confined flashback is initiated by boundary layer separation upstream of the flame tip. Hence, the flashback limit can be predicted identifying the minimum pressure rise upstream of a confined flame causing boundary layer separation. For this purpose, the criterion of Stratford is chosen which was originally developed for boundary layer separation in mere aerodynamic phenomena. It is shown in this paper that it can also be applied to near-wall combustion processes if the pressure rise upstream of the flame tip is modeled correctly. In order to determine the pressure rise, an expression for the turbulent burning velocity is derived including the effects of flame stretch and turbulence. A comparison of the predicted flashback limits and experimental data shows high prediction accuracy and wide applicability of the developed model.

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References

Figures

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Fig. 1

Confined flame situation

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Fig. 2

Wrinkled flame front of stationary turbulent flame

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Fig. 3

Adiabatic flame temperature of hydrogen–air mixtures at different preheating temperatures

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Fig. 4

Unstretched laminar burning velocity at ambient temperature from one-dimensional free flame (FF) simulation in comparison to Eq. (10)

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Fig. 5

Unstretched laminar burning velocity Sl,0 (filled symbols) and stretched laminar burning velocity Sl,s (empty symbols) for different preheating temperatures at flashback conditions

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Fig. 6

Markstein length of hydrogen–air mixtures at ambient temperature

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Fig. 7

Distribution of normalized velocity fluctuation in the near-wall region of turbulent channel flow. Numbers in legend indicate flow Reynolds number.

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Fig. 8

Markstein length of hydrogen–air mixtures for different preheating temperatures

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Fig. 9

Effective Lewis number of hydrogen–air mixtures for different preheating temperatures

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Fig. 10

Flame stretch rate for different preheating temperatures at flashback conditions

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Fig. 11

Ratio of turbulence induced flame stretch rate to total flame stretch rate for different preheating temperatures at flashback conditions

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Fig. 12

Normalized averaged channel velocity at flashback at ambient temperature for a variation of stretch rate and different equivalence ratios

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Fig. 13

Maximum turbulent burning velocity for different preheating temperatures at flashback conditions

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Fig. 14

Wall distance of confined flame flashback for different preheating temperatures

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Fig. 15

Velocity fluctuations at location of flame flashback for different preheating temperatures

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Fig. 16

Reynolds number of turbulent channel flow at flashback limit for different preheating temperatures

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Fig. 17

Normalized averaged channel velocity at flashback at ambient temperature for a variation of normalized turbulent velocity fluctuations and different equivalence ratios

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Fig. 18

Predicted average channel velocity at flashback (empty symbols) compared to experimental data taken from Ref. [17] (filled symbols) at ambient and preheated temperatures

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Fig. 19

Predicted average tube velocity at flashback (empty symbols) compared to experimental data taken from Ref. [8] (filled symbols) at ambient temperature

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