• 한국연소학회:학술대회논문집
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• 2006.04a
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• pp.7-12
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• 2006

The propagation rates ($U_{edge}$) of various premixed edge-flames were measured as a function of global strain rate (${ \sigma}$), mixture strength, and Lewis number (Le). Using a counterflow slot-jet burner with electrical heaters at each end, both advancing (positive $U_{edge}$) and retreating (negative $U_{edge}$) edges can be studied as they propagate along the long dimension of the burner. Preliminary results are presented for single and twin premixed hydrocarbon edge-flames in terms of the effects on $U_{edge}$. A low-${\sigma}$ extinction limit has been discovered for all mixtures tested but further analysis is necessary for full characterization since sufficiently $high-{\sigma}$ leads to an apparent stability limit. Propagation rates clearly show a strong dependence on Le. Future work will focus on completing the premixed hydrocarbon edge-flame analysis and include investigations into non-premixed edge-flames and edge-flames composed of fuels such as hydrogen ($H_2$) with significantly lower Le.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1185-1191
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• 1999

Radiation-induced oscillatory instability in diffusion flames is numerically investigated with nonlinear dynamics considered. As the simplest flame model, a diffusion flame established in the stagnant mixing layer is employed with optically thin gas-phase radiation and unity Lewis numbers for all species. Attention is focused on the radiation-induced extinction regime, which occurs at large $Damk\ddot{o}hler$ number. Once the steady flame structure is obtained for a prescribed value of the initial $Damk\ddot{o}hler$ number, transient solution of the flame is calculated after a finite amount of the $Damk\ddot{o}hler$-number perturbation is imposed on the steady flame. Transient evolution of the flame exhibits three types of flame-evolution behaviors, namely decaying oscillatory solution, diverging solution to extinction and stable limit-cycle solution. A dynamic extinction boundary is identified for laminar flamelet library.

• 한국연소학회:학술대회논문집
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• 2004.06a
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• pp.124-132
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• 2004

Activation energy asymptotics (AEA) for Linan's diffusion-flame regime is revisited in this paper. The main purpose of the paper is to carefully re-examine each AEA analysis step in order to clarify the some concepts that are often misunderstood among the ordinary practitioners of the AEA. Particular attention is focused on the different AEA regimes arising from the double limit of large Zel'dovich and Damkohler numbers. In addition. the expansion procedures are shown in detail and the method that the turning point condition, commonly known as the Linan's extinction condition, is found is explained.

• Journal of the Korean Society of Combustion
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• v.9 no.2
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• pp.1-9
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• 2004

Activation energy asymptotics (AEA) for Linan#s diffusion-flame regime is revisited in this paper. The main purpose of the paper is to carefully re-examine each AEA analysis step in order to clarify the some concepts that are often misunderstood among the ordinary practitioners of the AEA. Particular attention is focused on the different AEA regimes arising from the double limit of large Zel#dovich and Damkohler numbers. In addition, the expansion procedures are shown in detail and the method that the turning point condition, commonly known as the Linan#s extinction condition, is found is explained.

• Journal of the Korean Mathematical Society
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• v.36 no.5
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• pp.923-943
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• 1999

In this paper we consider functionals of the empirical age distribution of supercritical Bellman-Harris processes. Let f : R+ longrightarrow R be a measurable function that integrates to zero with respect to the stable age distribution in a supercritical Bellman-Harris process with no extinction. We present sufficient conditions for the asymptotic normality of the mean of f with respect to the empirical age distribution at time t.

• Journal of the Korean Society of Combustion
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• v.18 no.4
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• pp.44-52
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• 2013

Important role of chemical interaction in flame extinction was numerically investigated in downstream interaction among lean(rich) and lean(rich) premixed as well as partially premixed $H_2$-air and CO-air flames. The strain rate varied from 30 to $5917s^{-1}$ until interacting flame could not be sustained anymore. Flame stability diagrams mapping lower and upper limit fuel concentrations for flame extinction as a function of strain rate are presented. Highly stretched interacting flames were survived only within two islands in the flame stability map where partially premixed mixture consisted of rich $H_2$-air flame, extremely lean CO-air flame, and a diffusion flame. Further increase in strain rate finally converges to two points. Appreciable amount of hydrogen in the side of lean $H_2$-air flame also oxidized the CO penetrated from CO-air flame, and this reduced flame speed of the $H_2$-air flame, leading to flame extinction. At extremely high strain rates, interacting flames were survived only by a partially premixed flame such that it consisted of a very rich $H_2$-air flame, an extremely lean CO-air flame, and a diffusion flame. In such a situation, both the weaker $H_2$-air and CO-air flames were parasite on the stronger diffusion flame such that it could lead to flame extinction in the situation of weakening the stronger diffusion flame. Particular concerns are focused on important role of chemical interaction in flame extinction was also discussed in detail.

• Journal of the Korean Society of Combustion
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• v.17 no.1
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• pp.22-29
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• 2012

The detachment stability characteristics of syngas $H_2$/CO jet attached flames were studied. The flame stability was observed while varying the syngas fuel composition, coaxial nozzle diameter and fuel nozzle rim thickness. The detachment stability limit of the syngas single jet flame was found to decrease with increasing mole fraction of carbon monoxide in the fuel. In hydrogen jet flames with coaxial air, the flame detachment stability was found to be independent of the coaxial nozzle diameter. However, velocities of appearance of liftoff and blowout velocities of lifted flames have dependence. At lower fuel velocity range, the critical coaxial air velocity leading to flame detachment increases with increasing fuel jet velocity, whereas at higher fuel velocity range, it decreases. This increasing-decreasing non-monotonic trend appears for all $H_2$/CO syngas compositions (50/50~100/0% $H_2$/CO). To qualitatively understand the flame behavior near the nozzle rim, $OH^*$ chemiluminescence imaging was performed near the detachment limit conditions. For all fuel compositions, local extinction on the rim is observed at lower fuel velocities(increasing stability region), while local flame extinction downstream of the rim is observed at higher fuel velocities(decreasing stability region). Maximum values of the non-monotonic trends appear to be identical when the fuel jet velocity is normalized by the critical fuel velocity obtained in the single jet cases.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.38 no.6
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• pp.493-500
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• 2014

Nonlinear characteristics of cellular counterflow diffusion flame near the radiative extinction limit at large Damk$\ddot{o}$hler number are numerically investigated. Lewis number is assumed to be 0.5 and flame evolution is calculated by imposing an infinitesimal disturbance to a one-dimensional(1-D) steady state flame. The early stage of nonlinear development is very similar to that predicted in a linear stability analysis. The disturbance with the wavenumber of the fastest growing mode emerges and grows gradually. Eventual, an alternating pattern of reacting and quenching stripes is developed. The cellular flame temperature is higher than that of 1-D flame because of the gain of the total enthalpy. As the Damk$\ddot{o}$hler number is further increased, the shape of the cell becomes circular to increase the surface area per unit reacting volume. The cellular flames do not extinguish but survive even above the 1-D steady state extinction condition.

• Journal of Advanced Marine Engineering and Technology
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• v.30 no.8
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• pp.853-862
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• 2006

The present study is numerically investigated the flame structure of non-premixed counterflow jet flames using the laminar flamelet model Detailed flame structures with the fuel composition of 40% CO, 30% $H_2$. 30% $N_2$ and an oxidizer composition of 79% $N_2$ and 21% $O_2$ in a non-premixed counterflow flame are studied numerically. This study is aimed to investigate the effects of axial velocity gradient and combustion atmosphere pressure on flame structure. The results show that the role of axial velocity gradient on combustion processes is globally opposite to that of combustion atmosphere pressure. That is, chemical nonequilibrium effects become dominant with increasing axial velocity gradient, but are suppressed with increasing ambient pressure. Also, the flame strength is globally weakened by the increase of axial velocity gradient but is augmented by the increase of ambient pressure. However, flame extinction is described better on the basis of only chemical reaction and in this study axial velocity gradient and ambient pressure play a similar role conceptually such that the increase of axial velocity gradient and ambient pressure cause flame not to be extinguished and extend the extinction limit, respectively. Consequently it is suggested that a combustion process like flame extinction is mainly influenced by the competition between the radical formation reaction and the third-body recombination reaction.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.243-244
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• 2014

The opposed flow flame in a mesoscale channel was constructed to observe the flame stabilization behaviors at low strain rate conditions (<$10s^{-1}$). The purpose of this study is to get the overall flame behaviors of partially premixed flames with oxygen enhanced conditions at low strain rates. The oxygen ratio in oxidizer was changed from 18 to 30 %. Conclusively, the flame extinction limit approached to about $1s^{-1}$, and divided into three representative regimes corresponding to self propagating flame, transitional flame, quenching flame regimes.