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Research Papers: Fuel Combustion

Flame Hysteresis Effects in Methane Jet Flames in Air-Coflow

[+] Author and Article Information
N. J. Moore, S. D. Terry, K. M. Lyons

Department of Mechanical and Aerospace Engineering,  North Carolina State University, Raleigh, NC 27695-7910

J. Energy Resour. Technol 133(2), 022202 (May 26, 2011) (5 pages) doi:10.1115/1.4003806 History: Received September 25, 2009; Revised February 25, 2011; Published May 26, 2011; Online May 26, 2011

Presented are the results of experiments designed to investigate flame lift-off behavior in the hysteresis regime for low Reynolds number turbulent flows. The hysteresis regime refers to the situation where the jet flame has dual positions favorable to flame stabilization: attached and lifted. Typically, a jet flame is lifted off of a burner and stabilized at some downstream location at a pair of fuel and coflow velocities that is unique to a flame at that position. Since the direction from which that condition is arrived at is important, there is an inherent hysteretic behavior. To supplement previous research on hysteretic behavior in the presence of no coflow and low coflow velocities, the current research focuses on flames that are lifted and reattached at higher coflow velocities, where the flame behavior includes an unexpected downstream recession at low fuel velocities. Observations on the flame behavior related to nozzle exit velocity and coflow velocity are made using video imaging of flame sequences. The results show that a flame can stabilize at a location downstream despite a decrease in the local excess jet velocity and assist in determining the effect of coflow velocity magnitude on hysteretic behavior. These observations are of utility in designing maximum turndown burners in air coflow, especially for determining stability criteria in low fuel-flow applications.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Methane is delivered from the nozzle that is surrounded by coflowing air. h is the distance from the fuel nozzle to the lifted flame front.

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Figure 2

Flame position for various coflow velocities. As indicated by the arrows, the fuel velocity was increased until the flame lifted and then incrementally decreased until reattachment. In each case, a change in the direction of movement of the flame is observed even though the fuel velocity is being decreased.

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Figure 3

Sequence of images of a methane flame with coflow velocity constant at 0.5 m/s. The corresponding fuel velocity, U0 , for each image is given as well as the height of the flame front. The first image was taken just as the flame lifted off of the nozzle. In the last image, the flame is moving upstream to reattach.

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Figure 4

Sequence of images of a methane flame with coflow velocity constant at 0.7 m/s

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Figure 5

The normalized radius, r/d, for each case

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Figure 6

The ratio of coflow velocity to fuel velocity for each case, d =  3.5 mm, at three different instances: lift-off, the local minimum, and reattachment. Projections of the data show that below a normalized coflow velocity of approximately 50 s−1 the existence of a local minimum is unlikely.

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Figure 7

The ratio of coflow velocity to fuel velocity for d = 2.5 and 4.0 mm from Terry [16]. Trends support 50 s−1 as the minimum normalized coflow velocity at which a local minimum will be observed.

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