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Research Papers: Fuel Combustion

Effects of Damköhler Number on Methane/Oxygen Tubular Combustion Diluted by N2 and CO2

[+] Author and Article Information
Baolu Shi

School of Aerospace,
Beijing Institute of Technology,
No. 5 ZhongGuanCun South Street,
Haidian, Beijing 100081, China
e-mail: shibaolu@bit.edu.cn

Qingzhao Chu

School of Aerospace,
Beijing Institute of Technology,
No. 5 ZhongGuanCun South Street,
Haidian, Beijing 100081, China
e-mail: 466363575@bit.edu.cn

Run Chen

Division of Artificial Systems Science,
Graduate School of Engineering,
Chiba University,
1-33 Yayoi-cho Inage-ku,
Chiba-shi, Chiba 263-8522, Japan
e-mail: chenrun@chiba-u.jp

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received June 27, 2016; final manuscript received November 19, 2016; published online December 21, 2016. Assoc. Editor: Reza Sheikhi.

J. Energy Resour. Technol 139(1), 012206 (Dec 21, 2016) (11 pages) Paper No: JERT-16-1265; doi: 10.1115/1.4035362 History: Received June 27, 2016; Revised November 19, 2016

To fundamentally elucidate the mixing and its effects on the characteristics of methane/oxygen flame in a rapidly mixed tubular flame burner, experiments were conducted under various oxygen mole fractions and flow rates. Two inert gases of nitrogen and carbon dioxide were used, respectively. The inert gas was added to both the oxidizer and fuel slits to maintain the oxidizer/fuel injection velocity ratio near unity. Based on flow visualization, the mixing process around injection slits and that in the axial downstream were discussed. The Damköhler number (Da1), defined as the ratio of molecular mixing time to reaction time, was selected as a parameter to quantitatively examine the criterion for the establishment of tubular flame from low to ultrahigh oxygen mole fractions (0.21–0.86). The mixing around slit exit determined the tubular flame establishment. Due to a flow time between two neighboring injection slits of fuel and oxidizer, part of the fuel was mixed in the downstream swirling flow, resulting in luminous helical structures. Hence, the Damköhler number (Da2), defined as the flow to the reaction time ratio, was examined. Detailed observations indicated that when Da2 was smaller than unity, the flame was uniform in luminosity, whereas the flame was nonuniform when Da2 ≥ 1. The value of Da2 was about 1.5 times as Da1; however, they correspond to different mixing zones and Da2 can be more easily calculated. The differences in flame stability between N2 and CO2 diluted combustion were also studied.

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Figures

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Fig. 1

Schematics of the tubular flame burner: (a) tubular flame burner and (b) fuel and oxidizer injection

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Fig. 2

Flow visualizations and mixing in the tubular flame burner (Sw = 5.89, W = 1 mm, L = 8 mm, and QO-total = QF-total = 0.045 m3/h): (a) in the cross section perpendicular to the tube axis and (b) in the plane containing the tube axis

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Fig. 13

Detailed observation of flame structure around the injection slits from an oblique angle illustrating the establishment of a tubular flame (lower right inset shows the position of the camera; case B: QO-total = 6.0 m3/h, β = 0.67, α = 1.0, and Φ = 1.0)

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Fig. 14

Detailed observation of flame structure around the injection slits from an oblique angle (case B: QO-total = 6.0 m3/h, β = 0.80, α = 1.3, and Φ = 1.0)

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Fig. 3

Flame appearances of rapidly mixed combustion diluted at the condition of Da1 = 0.5 and β = 0.21 (case A). The left pictures show the front view, and the right images show the side view.

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Fig. 4

Uniform methane/oxygen flames diluted by N2 atβ = 0.21 (case A: QO-total = QO-N2 + QO-N2 = 2.2 m3/h and QF-total = QF-CH4 + QF-N2 = 2.2m3/h)

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Fig. 5

Stoichiometric combustion of methane/oxygen mixture diluted by N2 at β = 0.30 (case A: Da1 = 1.5, 1.0, 0.75, and 0.56)

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Fig. 6

Flame appearances of CH4–N2/O2–N2 lean mixtures (case A: β = 0.30, Φ = 0.8, Da1 = 1.0 and 0.5)

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Fig. 7

Flame appearances of N2 diluted methane/oxygen combustion (case A: β = 0.40 and Da1 = 1.0)

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Fig. 8

Instantaneous flame images of N2 diluted methane/oxygen combustion illustrating unsteady combustion (case A: β = 0.5, Da1 = 1.0, QO-total = 6.0 m3/h, and Φ = 1.0), in which the red color zone represents reaction area with higher chemiluminescence (relative to the blue zone). Figure is available in color online.

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Fig. 9

Flame appearances of CO2 diluted methane/oxygen combustion (case B: Φ = 1.0, β = 0.4, and QO-total was raised to give various values of Da1)

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Fig. 10

Flame appearances of CO2 diluted methane/oxygen combustion (case B: β = 0.5, Φ = 1.0, and Da1 = 1.0, 0.67, and 0.5)

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Fig. 11

Stoichiometric methane/oxygen combustion under various oxygen mole fractions (case B: Φ = 1.0 and β = 0.60, 0.75, and 0.80)

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Fig. 12

Da1 for the establishment of steady methane/oxygen tubular combustion under various β (0.4–0.8) in case B

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Fig. 15

Adiabatic flame temperatures (a), laminar burning velocities (b), and reaction times (c) of the stoichiometric methane/oxygen mixtures diluted by N2 and CO2

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