• Journal of the Korean Society of Combustion
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• v.12 no.2
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• pp.34-41
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• 2007

The dynamic behaviors of counterflow non-premixed flame have been investigated experimentally to study effects of heat losses and Lewis number on edge flame oscillation, which result from the advancing and retreating edge flame motion of outer flame edge at low strain rate flame. For low strain rate flame, lateral conduction heat loss in addition to radiation heat loss could be more remarkable than the others. Oscillatory instabilities appear at fuel Lewis number greater than unity. But excessive lateral conduction heat loss causes edge flame instability even at fuel Lewis number less than unity. The excessive heat loss caused by the smaller burner diameter in which the flame length is an indicator of lateral conduction heat loss extends the region of flame oscillation and accelerates oscillatory instability in comparison to the previous study with the burner diameter of 26mm. Extinction behaviors quite different from the previous study are also addressed.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.379-381
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• 2014

Influence of fuel concentration gradient was investigated near flame extinction limit in buoyancy-suppressed non-premixed counterflow flame with triple co-flow burner. The use of He curtain flow produced a microgravity level of $10^{-2}-10^{-3}g$ in He-diluted non-premixed counter triple co-flow flame experiments. Flame stability map was presented based on flame extinction and oscillation near extinction limit. The stability map via critical diluent mole fraction with global strain rate was represented by varying outer and inner He-diluted mole fractions. The flame extinction modes could be classified into five: an extinction through the shrinkage of the outmost edge flame forward the flame center with and without self-excitation, respectively ((I) and (II)), an extinction via the rapid expansion of a flame hole while the outmost edge flame is stationary (III), both the outermost and the center edge flames oscillate, and then a donut shaped flame is formed or the flame is entirely extinguished (IV), a shrinkage of the outermost edge flame without self-excitation followed by shrinking or sustain the inner flame (V).

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.4 s.247
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• pp.343-349
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• 2006

Systematic experiments in $CH_4/Air$ counterflow diffusion flames diluted with He have been undertaken to study the oscillatory instability in which lateral flame size was less than burner nozzle diameter and thus lateral heat loss could be remarkable at low global strain rate. The oscillatory instability arises for Lewis numbers greater than unity and occurs near extinction condition. The oscillation is the direct outcome from the advancing and retreating edge flame. The dynamic behaviors of extinction in this configuration can be classified into three modes; growing, harmonic and decaying oscillation mode near extinction. As the global strain rate decreases, the amplitude of the oscillation becomes larger. This is caused by the increase of lateral heat loss which can be confirmed by the reduction of lateral flame size. Oscillatory edge flame instabilities at low global strain rate are shown to be closely associated with not only Lewis number but also heat loss (radiation and lateral heat loss).

• 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.

• 한국연소학회:학술대회논문집
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• 2012.11a
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• pp.23-26
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• 2012

Experimental study was conducted to elucidate flame extinction phenomena in counterflow flame. Using a curtain helium flow significantly reduced buoyancy such that the flame can be positioned at the center between the upper and lower nozzles even at the velocity ratio of 1.0. The curves of critical diluent mole fraction versus global strain rate have C-shapes. The flame oscillation was observed prior to low strain rate flame extinction at both flame conditions with and without minimizing buoyancy force. The results show that, at low strain rate flame, the self-excitation frequency with the order of 1.0 Hz in the case of utilizing pure helium gradually decreases in increase of $N_2$ mole fraction in the curtain flow, meaning that buoyancy suppresses the self-excitation of the outer edge flame.

• Journal of the Korean Society of Combustion
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• v.17 no.4
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• pp.38-43
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• 2012

Experimental study was conducted to elucidate flame extinction phenomena in counterflow flame. Using a curtain helium flow significantly reduced buoyancy such that the flame can be positioned at the center between the upper and lower nozzles even at the velocity ratio of 1.0. The curves of critical diluent mole fraction versus global strain rate have C-shapes. The flame oscillation was observed prior to low strain rate flame extinction at both flame conditions with and without minimizing buoyancy force. The results show that, at low strain rate flame, the self-excitation frequency with the order of 1.0 Hz in the case of utilizing pure helium gradually decreases in increase of $N_2$ mole fraction in the curtain flow, meaning that buoyancy suppresses the self-excitation of the outer edge flame.

• Journal of the Korean Society of Combustion
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• v.13 no.4
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• pp.16-25
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• 2008

Experiments have been conducted 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 examined in terms of velocity ratio, global strain rate, and nitrogen curtain flow rate. It is shown that multi-dimensional effects at low strain rate flames through global strain rate, velocity ratio, and curtain flowrate dominantly contribute to flame extinction and transition of shrinking flame disk to flame hole. Our concerns are particularly focused on the dynamic behavior of an edge flame in shrinking flame disk.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2006.05a
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• pp.275-278
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• 2006

Systematic experiments in $CH_4/Air$ counterflow diffusion flames diluted with He have been undertaken to study the oscillatory instability in which lateral heat loss could be remarkable at low global strain rate. The oscillatory instability arises for Lewis numbers greater than unity and occurs near extinction condition. The dynamic behaviors of extinction in this configuration can be classified into three modes; growing, harmonic and decaying oscillation mode near extinction. As the global strain rate decreases, the amplitude of the oscillation becomes larger. This is caused by the increase of lateral heat loss which ran be confirmed by the reduction of lateral flame size. Oscillatory edge flame instabilities at low global strain rate are shown to be closely associated with not only Lewis number but also heat loss (radiation and lateral heat loss).

• 한국가시화정보학회:학술대회논문집
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• 2004.12a
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• pp.147-155
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• 2004

Self-excited noise generation from laminar flames in thin annular jets of methane/air premixture has been investigated experimentally. Various flames were observed in this flow configuration, including conical shape flames, ring shape flames, steady crown shape flames, and oscillating crown shape flames. Self-excited noise with the total sound pressure level of about 70dB was generated from the oscillating crown shape flames for the equivalence ratio larger than 0.95. Sound pressure and $CH^*$ chemiluminescence were measured by using a microphone and a photomultiplier tube. The frequency of generated noise was measured as functions of equivalence ratio and premixture velocity. A frequency doubling phenomena have also been observed. The flame shape during flame oscillation was reconfirmed by a synchronized PIV experiment. The velocity and pressure field were obtained from PIV. The minimum pressure was formed near the edge of flame representing circulation. By comparing the results of sound pressure, flame luminosity and PIV, the noise source can be attributed to the flame front fluctuation near the edge of the oscillating crown-shape flames.

• International Journal of Aeronautical and Space Sciences
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• v.16 no.2
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• pp.165-176
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• 2015

A numerical analysis was conducted to investigate the inlet buzz and combustion oscillation in an axisymmetric ramjet engine with wedge-type flame holders. The physical model of concern includes the entire engine flow path, extending from the leading edge of the inlet center-body through the exhaust nozzle. The theoretical formulation is based on the Farve-averaged conservation equations of mass, momentum, energy, and species concentration, and accommodates finite-rate chemical kinetics and variable thermo-physical properties. Turbulence closure is achieved using a combined scheme comprising of a low-Reynolds number k-${\varepsilon}$ two-equation model and Sarkar's compressible turbulence model. Detailed flow phenomena such as inlet flow aerodynamics, flame evolution, and acoustic excitation as well as their interactions, are investigated. Mechanisms responsible for driving the inlet buzz are identified and quantified for the engine operating at subcritical conditions.