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

Flame Propagation Following the Autoignition of Axisymmetric Hydrogen, Acetylene, and Normal-Heptane Plumes in Turbulent Coflows of Hot Air

[+] Author and Article Information
Christos N. Markides, Epaminondas Mastorakos

Hopkinson Laboratory, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, Cambridgeshire CB2 1PZ, UK

Quantities are reported to the least significant figure allowed by the uncertainty. Uncertainties are relative and are reported at one significant figure and 95% confidence level (or a confidence interval of two standard deviations (s.d.)).

Recall that the default injector had d=2.24mm.

J. Eng. Gas Turbines Power 130(1), 011502 (Dec 13, 2007) (9 pages) doi:10.1115/1.2771245 History: Received June 15, 2006; Revised March 05, 2007; Published December 13, 2007

Axisymmetric plumes of hydrogen, acetylene, or n-heptane were formed by the continuous injection of (pure or nitrogen-diluted) fuel into confined turbulent coflows of hot air. Autoignition and subsequent flame propagation was visualized with an intensified high-speed camera. The resulting phenomena that were observed include the statistically steady “random spots” regime and the “flashback” regime. It was found that with higher velocities and smaller injector diameters, the boundary between random spots and flashback shifted to higher air temperatures. In the random spots regime the autoignition regions moved closer to the injector with increasing air temperature and/or decreasing air velocity. After a localized explosive autoignition event, flames propagated into the unburnt mixture in all directions and eventually extinguished, giving rise to autoignition spots of mean radii of 25mm for hydrogen and 610mm for the hydrocarbons. The average flame propagation velocity in both the axial and radial directions varied between 0.5 and 1.2 times the laminar burning speed of the stoichiometric mixture, increasing as the autoigniting regions shifted upstream.

Copyright © 2008 by American Society of Mechanical Engineers
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Figure 1

Top left: Typical instantaneous PMT (with OH* filter) voltage profile of single acetylene autoignition spot. Top right: Average PMT OH* profile (over many as shown on the left). Bottom: Fast OH* image sequence (left to right and top to bottom) taken simultaneously with the PMT measurement shown on the top left at 10kHz. Flow direction upwards. Reproduced from Markides (2).

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Figure 2

Apparatus sketch

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Figure 3

Acetylene autoignition and flashback. Fast image sequence (left to right, top to bottom) at 13.5kHz. Conditions Tair=809K, Uair=10.4m∕s, υinj=0.93, YC2H2=0.75. Flow direction upwards.

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Figure 4

Temperature-velocity operation envelope limits for hydrogen. (a) YH2 fixed to 0.15. Data points signify transition limit to flashback. (b) Filled data points are for YH2=1.00, υinj=3.2–5.6. Empty data points for YH2=0.05–0.40, υinj=0.35–12.6. Data points signify stable random spots behavior. Lines signify a transition limit to Flashback.

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Figure 5

“Autoignition spot” definitions. Flow upwards. Vectors showing a positive displacement.

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Figure 6

Pdfs of upstream flame front thickness (solid) and autoignition spot half-size (dash-dot) for n-heptane with Tair=1100–1140K, Uair=14m∕s, 18m∕s, Yfuel=0.95, and υinj=1.0–1.2

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Figure 7

Mean upstream flame front thickness (filled) and autoignition spot longitudinal half-size (empty) as a function of Lmin for n-heptane with Tair=1100–1120K, Uair=14m∕s, Yfuel=0.95, and υinj=1.0 (circles) and Tair=1100–1140K, Uair=18m∕s, Yfuel=0.95, and υinj=1.0–1.2 (squares)

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Figure 8

Pdfs of upstream flame front (solid) and autoignition spot center advection (dash-dot) velocity for n-heptane. Conditions as in Fig. 6.

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Figure 9

Mean velocities as a function of Lmin for n-heptane with conditions as in Fig. 6. (a) Absolute longitudinal upstream flame front (solid) and autoignition spot advection (empty) velocities. (b) Upstream longitudinal flame front velocity.

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Figure 10

Mean transverse velocity of left and right flame fronts for n-heptane with conditions as in Fig. 6

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Figure 11

Mean longitudinal upstream flame front velocities as a function of Lmin. (a) Acetylene with Uair=11.1m∕s, Tair=806–824K, Yfuel=0.6, and υinj=1.0 (circles) and Uair=17.5m∕s, Tair=850–879K, Yfuel=0.6, and υinj=1.0 (squares). (b) Hydrogen with Tair=950K, Uair=25m∕s, Yfuel=0.2–1.0, and υinj=0.9–1.0.

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Figure 12

Evolution of spot flame fronts at 13.5kHz. Flow direction shown and scale given. Reproduced from Ref. 1.

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Figure 13

(a) Mean acetylene decay time from the instant of autoignition to 10% (solid) and 1% (empty) decay thresholds as a function of Lmin. Conditions as in Fig. 1. (b) Difference in mean decay time as a function of the time from autoignition from 10% to 5% (solid) and from 5% to 1% (empty) decay thresholds. Calculated based on data in (a).



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