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Research Papers

The Effect of Intrinsic Instabilities on Effective Flame Speeds in Under-Resolved Simulations of Lean Hydrogen–Air Flames

[+] Author and Article Information
Peter Katzy

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

Josef Hasslberger, Lorenz R. Boeck, Thomas Sattelmayer

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

1Corresponding author.

Manuscript received September 26, 2016; final manuscript received May 18, 2017; published online July 31, 2017. Assoc. Editor: Guoqiang Wang.

ASME J of Nuclear Rad Sci 3(4), 041015 (Jul 31, 2017) (11 pages) Paper No: NERS-16-1113; doi: 10.1115/1.4036984 History: Received September 26, 2016; Revised May 18, 2017

The presented work aims to improve computational fluid dynamics (CFD) explosion modeling for lean hydrogen–air mixtures on under-resolved grids. Validation data are obtained from an entirely closed laboratory-scale explosion channel (GraVent facility). Investigated hydrogen–air concentrations range from 6 to 19 vol %. Initial conditions are p = 0.1 MPa and T = 293 K. Two highly time-resolved optical measurement techniques are applied simultaneously: (1) 10 kHz shadowgraphy captures line-of-sight integrated macroscopic flame propagation and (2) 20 kHz planar laser-induced fluorescence of the OH radical (OH-PLIF) resolves microscopic flame topology without line-of-sight integration. This paper presents the experiment, measurement techniques, data evaluation methods, and simulation results. The evaluation methods encompass the determination of flame tip velocity over distance and a detailed time-resolved quantification of the flame topology based on OH-PLIF images. One parameter is the length of wrinkled flame fronts in the OH-PLIF plane obtained through automated postprocessing. It reveals the expected enlargement of flame surface area by instabilities on a microscopic level. A strong effect of mixture composition is observed. Simulations based on the new model formulation, incorporating the microscopic enlargement of the flame front, show a promising behavior, where the impact of the augmented flame front on the observed flame front velocities can be detected.

Copyright © 2017 by ASME
Topics: Flames , Hydrogen , Simulation
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References

Figures

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Fig. 1

Shadowgraphy images of the hydrogen–air flame propagating from left to right in the GraVent facility. Flame fronts with and without instabilities are visible.

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Fig. 2

Fresh mixture is flowing from right to left passing the wrinkled flame front. The wrinkled flame front is burning with the velocity sl perpendicular to itself, whereas st can be defined based on a theoretical smooth flame front. The connection between these values is shown in Eq. (7).

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Fig. 3

Channel cross section showing the height (z-direction) and the width (y-direction) of the GraVent facility

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Fig. 4

Sectional top view of the GraVent facility. The flame propagates mainly in x-direction.

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Fig. 5

Simulated volume-averaged values of specific turbulent kinetic energy over waiting time after injection

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Fig. 6

Depiction of the photodiode positions (diamonds) along the channel axis

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Fig. 7

Setup of simultaneous shadowgraphy and OH-PLIF measurements

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Fig. 8

Synchronization scheme of the measurement techniques. Square signals represent trigger signals, and shaded areas represent the exposure times of cameras and pulse length of the laser.

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Fig. 9

Overview of data evaluation process for a single OH-PLIF image

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Fig. 10

Depiction of the steps conducted within the data evaluation process

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Fig. 12

Velocity of the flame front over distance determined from OH-PLIF recordings

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Fig. 13

Development of flame front lengths over time corresponding to a hydrogen–air mixture of 9.2% H2. At 25 ms, the flame front has left the visibility range of the camera at approximately x = 0.11 m.

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Fig. 14

Flame propagation process divided into three stages. Solid line corresponds to an experimentally determined flame front Aeff of a 9.2% H2 mixture at various times.

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Fig. 15

Development of flame front lengths over time corresponding to a hydrogen–air mixture of 17.6% H2. At 6 ms, the flame front has left the visibility range of the camera at approximately x = 0.11 m.

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Fig. 16

Development of the wrinkling factor over time for three different hydrogen–air mixtures and corresponding calculated mean wrinkling factors in regime C

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Fig. 17

Left: Depiction of a smooth surface. Right: A depicts the area of the smooth surface.

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Fig. 18

Left: Depiction of a wrinkled surface in the x-direction. Right: Area A depicts the base area of the wrinkled surface, meaning an area without wrinkles. Area B shows the enlargement of area A due to wrinkling.

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Fig. 19

Left: Depiction of a wrinkled surface in the y-direction. Right: Area A depicts the base area of the wrinkled surface, meaning an area without wrinkles. Area C shows the enlargement of area A due to wrinkling.

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Fig. 20

Left: Depiction of a wrinkled surface in the x- and y-direction. Right: Area A depicts the base area of the wrinkled surface, meaning an area without wrinkles. Areas B, C, and D show the enlargement of area A due to wrinkling.

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Fig. 21

Mean three-dimensional wrinkling factors plotted over hydrogen–air concentration with a linear fitted curve showing the trend

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Fig. 22

Mean three-dimensional wrinkling factor plotted over effective Lewis number of the hydrogen–air mixture with a fitted solid curve showing the trend. The dashed lines indicate a variation of the exponent by ±0.1.

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Fig. 23

Unstretched laminar burning velocity over hydrogen mole fraction; measured values: circles: Dowdy et al. [17], crosses: Kwon and Faeth [18], stars: Tse et al. [19], diamonds: Vagelopoulos et al. [20], and squares: Wu and Law [21]; and calculated fit: Boeck [22]

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Fig. 24

Flame front velocity uF over distance for a hydrogen concentration of 19.1%. Comparison of experimental data and simulations. Shaded area depicts the influence of flame front enlargement on flame front propagation velocity. Error bars show a deviation of ±2σsd.

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Fig. 25

Flame front velocity uF over distance for a hydrogen concentration of 13.4%. Comparison of experimental data and simulations. Shaded area depicts the influence of flame front enlargement on flame front propagation velocity. Error bars show a deviation of ±2σsd.

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