• 한국연소학회:학술대회논문집
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• 2004.06a
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• pp.49-56
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• 2004

A two-dimensional twin-jet counterflow system has been designed, in which two streams from two double-slit nozzles form a counterflow. This flow system enables one to systematically investigate various effects on non-premixed flames, including the non-premixed flame interaction, the edge flame behavior and the effect of curvature. Non-premixed flame interaction in the twin-jet counterflow system has been investigated numerically for methane fuel diluted with nitrogen. Three types of non-premixed flame(conventional counterflow flame, crossed twin-jet flame and petal shaped flame) were simulated depending on the combination of fuel/oxidizer supply to each nozzle. The extinction characteristics of non premixed methane flame in the twin-jet counterflow have been investigated numerically. The boundary of the existence of petal-shaped flames was identified for the twin-jet counterflow flames. Due to the existence of the unique petal-shaped flames, the extinction boundary for the twin-jet counterflow can be extended significantly compared to that for the conventional counterflow non-premixed flames, through the interaction of two flames. Through the comparison of the crossed twin-jet flame and the conventional counterflow flame, structure of the crossed twin-jet counterflow flame is analysed. Through the comparison of the petal shaped flame and the conventional counterflow flame, the extension of the extinction boundary for the twin-jet counterflow is investigated.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.494-502
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• 1997

Experiments on corresponding jet flames with stagnant point diffusion flames have been carried out in initial injection periods. A compensated measurement of maximum flame temperature, which is based on the ion signal, has been employed to inspect flame responses to time-varying strain rates. The flame responses are obtained at two conditions for the slowly time-varying strain rate and the case of flame extinction, and analyzed to confirm similarity between a stagnant point diffusion flame and an evolving jet diffusion flame. Nonsteady effects are addressed via the comparison between several time scales. The time variation with low strain rates, in which illustrates the flame behavior of the upper branch far from extinction in the well-known S-curve, is confirmed to produce a quasi-steady flame response through the nonsteady experiments. The time variation with strain rates in the case of flame extinction indicates an unsteady effect of flame response. It is therefore found that the flame responses near jet tip depend on time histories of characterized strain rates in the developing process.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.11
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• pp.1481-1487
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• 2002

The thin SiC filament technique has been employed to identify the possibility of measuring flame temperature, and especially unstable near-extinction flame temperature in an oxidizer deficient ambience, by comparing the relative visible (non-IR) luminosities of SiC filaments with thermocouple measured temperature in co-flowing, laminar propane/air diffusion flames. The results show good agreement between the digitized relative visible luminosity profiles of the SiC filaments and temperature profiles measured using a thermocouple at temperatures above $700^{\circ}C$, although, a non-linear calibration is probably required far the whole temperature range. The highest radial peak temperature exists near to the nozzle exit. and the centerline temperatures were virtually unchanged with increasing flame height in an oxidizer deficient near-extinction flame.

• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.2
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• pp.232-240
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• 1986

Extinction characteristics of the two interacting premixed flames are analyzed for the effects of flame stretch and preferential diffusion using large activation energy asymptotic analysis by adopting counterflow system as a model problem. Results show that the flammable limit of the thermally interacting premixed flames is extended compared to the single flame, and the extinction mechanism is classified into weak and strong interactions. As the lewis number of the deficient species increases, the region of strong interaction diminishes which can explain the different characteristics of the extinction boundaries of the lean (rich) methane/air and butane/air flames. The influence of the flame stretch to the interaction boundaries is also studied.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.9 s.240
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• pp.988-996
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• 2005

Flame structure and extinction mechanism of counterflow methane/air non-premixed flame diluted with nitrogen are studied by NASA 2.2 s drop tower experiments and two-dimensional numerical simulations with finite rate chemistry and transport properties. Extinction mechanism at low strain rate is examined through the comparison among results of microgravity experiment, 1D and 2D simulations with a finite burner diameter. A two-dimensional simulation in counterflow flame especially with a finite burner diameter is shown to be very important in explaining the importance of multidimensional effects and lateral heat loss in flame extinction, effects that cannot be understood using a one-dimensional flamelet model. Extinction mechanism at low strain rate is quite different from that at high strain rate. Low strain rate flame is extinguished initially at the outer flame edge, the flame shrinks inward, and finally is extinguished at the center. It is clarified from the overall fractional contribution by each term in energy equation to heat release rate that the contribution of radiation fraction with 1D and 2D simulations does not change so much and the overall fractional contribution is decisively attributed to radial conduction ('lateral heat loss'). The experiments by Maruta et at. can be only completely understood if multi-dimensional heat loss effects are considered. It is, as a result, verified that the turning point, which is caused only by pure radiation heat loss, has to be shifted towards much lower global strain rate in microgravity flame.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.1
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• pp.125-131
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• 1997

The extinction of premixed flames under the influence of stretch is studied numerically. A wide range of fuel (hydrogen, ethylene, acetylene, methane, propane and methanol) and air mixtures are established in an opposed jet and their flame properties such as flame speed, flame thickness, thermal diffusivity, and stretch rate at extinction are computed. Computations are made using several chemical kinetic mechanism (Smooke, Kee et al. and Peters). The major result is that, in contrast to the various previous claims of extinction Karlovitz number varying over three orders of magnitude, it is found to be constant around two for all of the mixtures tested. That is, premixed flames are extinguished when the physical flow time decreases (due to increased stretch rate) to the point where it approximately equals the chemical reaction time. Here the relevant chemical reaction time is not the one computed using the one-dimensional flame properties as originally suggested in the formulation of Karlovitz number, but rather it is the one obtained using the stretched flame properties which fully reflect the effect of straining on the flame structure.

• Journal of the Korean Society of Safety
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• v.26 no.1
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• pp.21-26
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• 2011

Extinction and ignition characteristics of $CH_4$-air counterflow diffusion flame were numerically investigated using a Flame-Controlling Method(FCM). A skeletal reaction mechanism, which adopts 17 species and 58 reactions, was used in the simulation. The extinction and ignition conditions of the $CH_4$-air diffusion flames were investigated with varying the global strain rate. Upper and middle branches of S-curve for the peak temperature in the inverse of the global strain rate space were obtained with the FCM. The structures of diffusion flames in the upper and middle branches of S-curve were compared. It was found that the global strain rate was not correlated with the local strain rate well in the low global strain rate region. It is expected that the FCM is very useful to obtaining the extinction and ignition condition of diffusion flame, such as fires.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.10
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• pp.1401-1411
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• 2003

A two-dimensional direct numerical simulation was performed to investigate the flame structure of CH$_4$$N_2$-air counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman's detailed chemistry were adopted in this simulation. The characteristic vortex and chemical time scales were introduced to quantify and investigate the extinction phenomenon during a flame-vortex interaction. The results showed that fuel- and air-side vortex cause an unsteady extinction. In this case, the flame interacting with a vortex was extinguished at much larger scalar dissipation rate than steady flame. It was also found that the air-side vortex extinguished a flame more rapidly than the fuel-side vortex. Furthermore, it was noted that the degree of unsteady effect experienced by a flame can be investigated by comparing the above two characteristic time scales, and this analysis could give an appropriate reason for the results of the previously reported experiment.

• 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.