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

Dynamics of Premixed H2/CH4 Flames Under Near Blowoff Conditions

[+] Author and Article Information
Qingguo Zhang, Santosh J. Shanbhogue, Tim Lieuwen

School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0150

Much of the discussion in this section closely follows arguments from Ref. 28, made in the context of bluff body flames.

This quasilocal treatment of the flame holes neglects the fact that once a flame hole is initiated, it can advance into a region with a strain rate that is lower than κext, as discussed earlier.

This reasoning can also be generalized to the non quasisteady flamelet case. A more general parameterization of flame extinction and strain rate is with functions of the form κext=f(κ,κ̇,κ̈,) and P(κ,κ̇,κ̈,). We will not carry through the details of the calculation, but a similar integration can be performed.

J. Eng. Gas Turbines Power 132(11), 111502 (Aug 05, 2010) (8 pages) doi:10.1115/1.4000601 History: Received May 08, 2009; Revised September 03, 2009; Published August 05, 2010; Online August 05, 2010

Swirling flows are widely used in industrial burners and gas turbine combustors for flame stabilization. Several prior studies have shown that these flames exhibit complex dynamics under near blowoff conditions, associated with local flamelet extinction and alteration in the vortex breakdown flow structure. These extinction events are apparently due to the local strain rate irregularly oscillating above and below the extinction strain rate values near the attachment point. In this work, global temporally resolved and detailed spatial measurements were obtained of hydrogen/methane flames. Supporting calculations of extinction strain rates were also performed using detailed kinetics. It is shown that flames become unsteady (or local extinctions happen) at a nearly constant extinction strain rate for different hydrogen/methane mixtures. Based upon analysis of these results, it is suggested that classic Damköhler number correlations of blowoff are, in fact, correlations for the onset of local extinction events, not blowoff itself. Corresponding Mie scattering imaging of near blowoff flames also was used to characterize the spatio-temporal dynamics of holes along the flame that are associated with local extinction.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 4

Time series data of OH signal for methane flame at equivalence ratio of 0.54

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Figure 1

Two instantaneous OH PLIF images of acoustically forced swirling flame (left and middle) and associated line of sight image (right). Flow bottom to top. Reproduced from Ref. 22.

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Figure 2

Schematic of the combustor facility with Mie measurement window and optical probe

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Figure 3

Time series data of OH∗ signal of CH4 flame at equivalence ratios of 0.8 (top), 0.62 (middle), and 0.51 (bottom), where OHo∗ denotes time average of OH∗(t)

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Figure 5

Dependence of local extinction event frequency on equivalence ratio of CH4/H2 flames

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Figure 6

Dependence of κext, blowoff limits, and event rate on percentage of hydrogen and equivalence ratio. Contour lines of extinction strain rate are indicated at 1400 1/s, 1200 1/s, 800 1/s, 400 1/s, and 200 1/s. These contours were estimated by calculating κext at 0/100%, 20/80%, 40/60%, 50/50%, and 75/25% H2/CH4 mixtures with equivalence ratio steps of 0.02. Blowoff limits and event information (dash lines) were collected for 0/100, 20/80, 50/50, and 75/25 H2/CH4 mixtures.

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Figure 7

Dependence of extinction event rate upon Damköhler number of CH4/H2 flames

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Figure 8

Comparison of three chemical time scales at fuel/air ratios associated with event rate of 1 event/s

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Figure 9

Notional description of time variation of local Damköhler number at given point on flame sheet, illustrating local extinction events at two average Damköhler number values, Da¯, one farther (top) and closer (bottom) to blowoff

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Figure 10

Notional PDF of flame strain rate and extinction strain rate at a particular point along the flame, s

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Figure 11

Flame front in raw Mie scattering images (top left); Instantaneous isovorticity field and flame front (remaining images) of methane flame at equivalence ratio of 0.61

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Figure 12

Distribution of holes along the flame front for CH4 (◼) and 50/50 H2/CH4(O) flame at equivalence ratio of 0.62 and 0.43, respectively. Line denotes calculated result assuming holes are generated uniformly and randomly along the flame and propagate downstream with the speed of 30 m/s.



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