Terms Of Velocity Ratio Research Articles

A Study on Flame Extinction Characteristics along a C-Curve

A study has been conducted both experimentally and numerically to clarify impacts of curtain flow and velocity ratio on low strain rate flame extinction and to further display transition of shrinking flame disk to flame-hole. Critical mole fractions at flame extinction are provided in terms of velocity ratio, nitrogen curtain flow rate, and global strain rate. Flame extinction modes are grouped into four on a C-curve, which is characterized by a flame-hole, the shrinking of edge flame, and edge flame oscillation. It is seen that varying curtain flow rate does not impact on edge flame oscillation, flame extinction, and even flame extinction modes. Variation of velocity ratio extends to the low strain rate flame extinction modes beyond the turning point. It is found that the expanding and shrinking flames always have negative flame propagation speed; it is also recognized that the decrease of flame radius is prone to extinguish due to the dominant role of radial conduction heat loss. The examination of ener...

이산화탄소로 희석된 메탄-공기 확산화염의 에지화염 불안정성

Experiments in low strain rate methane-air counterflow diffusion flames diluted with CO₂ have been conducted to investigate the flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss in addition to radiative loss could be remarkable at low global strain rates. The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. It is seen that flame length is closely relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations are categorized into three: a growing-, a harmonic- and a decaying-oscillation mode. Onset conditions of the edge flame oscillation and the relevant modes are examined with global strain rate and CO₂ mole fraction in fuel stream. A flame stability map based on the flame oscillation modes is also provided at low strain rate flames.

Edge flame instability in low-strain-rate counterflow diffusion flames

Experiments in low-strain-rate methane–air counterflow diffusion flames diluted with nitrogen have been conducted to study flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss, in addition to radiative loss, could be high at low global strain rates. 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 and nitrogen mole fraction in the fuel stream or in terms of fuel Lewis number. It is observed that flame length is intimately relevant to lateral heat loss, and this affects flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations, which result from the advancing and retreating edge flame motion of the outer flame edge of low-strain-rate flames, are categorized into three modes: a growing, a decaying, and a harmonic-oscillation mode. A flame stability map based on the flame oscillation modes is also provided for low-strain-rate flames. The important contribution of lateral heat loss even to edge flame oscillation is clarified finally.