• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.2
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• pp.181-188
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• 2012

The present study on nitrogen-diluted non-premixed counterflow flames with finite burner diameters experimentally investigates the important role of the outer edge flame in flame extinction. Flame stability diagrams mapping the flame extinction response of nitrogen-diluted non-premixed counterflow flames to varying global strain rates in terms of the burner diameter, burner gap, and velocity ratio are explored. There exists a critical nitrogen mole fraction beyond which the flame cannot be sustained, and also the curves of the critical nitrogen mole fraction versus the global strain rate have C-shapes in terms of burner diameter, burner gap, and velocity ratio. In flames with sufficiently high strain rates, the curves of the critical nitrogen mole fractions versus global strain rate collapse into one curve, and the flames can have the 1-D flame response of typical diffusion flames. Three flame extinction modes are identified: flame extinctions through the shrinkage of the outer edge flame with and without an oscillation of the outer edge flame prior to the extinction and flame extinction through a flame hole at the flame center. The measured flame surface temperature and a numerical evaluation of the fractional contribution of each term in the energy equation show that the radial conductive heat loss at the flame edge destabilizes the outer edge flame, and the conductive and convection heat addition to the outer edge from the trailing diffusion flame stabilizes the outer edge flame. The radial conductive heat loss at the flame edge is the dominant extinction mechanism acting through the shrinkage of the outer edge flame.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.279-282
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• 2006

Experiments in low strain rate methane-air counterflow diffusion flames diluted with $CO_2$ have been conducted to investigate the flame extinction behavior and edge flame oscillation The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. Onset conditions of the edge flame oscillation and the relevant modes are also provided with global strain rate. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Edge flame oscillations are categorized into three: a growing-, a decaying-, and a harmonic-oscillation mode.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.7
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• pp.535-540
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• 2009

Experiments are conducted to elucidate effects of counterflow burner diameter on flame extinction behaviors in C-curve. Present experimental results with burner diameters of 18, 26, and 50 mm in normal-gravity are compared with the numerical result of Oppdif code as well as the previous experimental results in micro-gravity. The turning point migrates to a higher global strain rate as burner diameter decreases. It is shown that the C-curve with the burner diameter of 50mm is best-fitted to the numerical result of Oppdif code and the previous micro-gravity results also excurse to the numerical result. This suggests that the precise C-curve can be obtained only with an appreciably large burner. The main reason why these differences appear is shown to be attributed to the transition of shrinking flame disk to flame hole due to strong effects of radial conduction heat loss, which is the typical extinction characteristics of low strain rate flames with a finite burner diameter in a counterflow diffusion flame.