• Journal of the Korean Society of Safety
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• v.19 no.1
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• pp.124-130
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• 2004

The structure of the nonpremixed methane-air counterflow flames in microgravity was investigated by axisymmetric simulation with Fire Dynamics Simulator (FDS) to evaluate the numerical method and to see the effects of strain rate and fuel concentration on the diffusion flame structure in microgravity. Results of FDS for the methane mole fractions, $X_m$=20, 50, and 80% in the fuel stream, and the global strain rates $a_g$=20, 50, and $90s^{-1}$ for each methane mole fraction were compared with those of OPPDIF, an one-dimensional flamelet code. There was good agreement in the temperature and axial velocity profiles between the axisymmetric and one-dimensional computations. It was shown that FDS is applicable to the counterflow flames in a wide range of strain rate and fuel concentration by predicting accurately the flame thickness, flame positions and stagnation points.

• 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.

• 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 the Korean Society of Marine Environment & Safety
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• v.18 no.1
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• pp.55-60
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• 2012

In order to investigate the effect of n-heptane mixing on PAH and soot formation, small amount of n-heptane has been mixed in counterflow ethylene diffusion flame. Laser-induced incandescene and laser-induced fluorescene techniques were employed to measure soot volume fraction and polycyclic aromatic hydrocarbon(PAH) concentration, respectively. Results showed that the mixing of n-heptane in ethylene diffusion flame produces more PAHs and soot than those of pure ethylene flame. However, signals of LIF for 20% n-heptane mixture flame were lower than that of pure ethylene flame. It can be considered that the enhancement of PAH and soot formation by the n-heptane mixing of ethylene can be explained by methyl($CH_3$) radical in the low temperature region. And it can be found that reaction rate of H radical for 10% n-heptane plays a crucial role for benzene formation.

• 한국연소학회:학술대회논문집
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• 2012.11a
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• pp.167-170
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• 2012

Self-excitations of edge flame were studied in laminar lifted free- and coflow-jet as well as counterflow flames diluted with nitrogen and helium. The self-excitations, originated from variation of edge flame speed and found in the above-mentioned configurations, are discussed. A newly found self-excitation and flame blowout, caused by the conductive heat loss from premixed wings to trailing diffusion flame are described and characterized in laminar lifted jet flames. Some trials to distinguish Lewis-number-induced self-excitation from buoyancy-driven one with O(1.0 Hz) are introduced, and then the differences are discussed. In counterflow configuration, important role of the outermost edge flame in flame extinction is also suggested and discussed.

• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.1
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• pp.29-35
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• 2001

Radiation-induced oscillatory instability in CH$_4$/Air diffusion flames is numerically investigated by adopting detailed chemistry. Counterflow diffusion flame is employed as a model flamelet and optically thin gas-phase radiation is assumed. Attention is focused on the extinction regime induced by radiative heat loss, which occurs at low strain rate. Once a steady flame structure is obtained for a prescribed value of initial strain rate, transient solution of the flame is calculated after a finite amount of strain-rate perturbation is imposed on the steady flame. Depending on the initial strain rate and the amount of perturbed strain rate, transient evolution of the flame exhibits various types of flame-evolution behaviors. Basically, the dynamic behaviors can be classified into two types, namely oscillatory decaying solution and diverging solution leading to extinction.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.1
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• pp.72-80
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• 1988

Partially premixed diffusion system is analyzed using the matched asymptotic expansion technique adopting counterflow with supplying fuel and oxidizer from one side and fuel only from the other as a model problem. Results show that single-stage extinction always occurs as stretch increases, and the partially premixed diffusion flame can hardly exist. Depending on the initial mixture concentrations, either premixed or diffusion flame extinction leads to complete extinction of the system, and the diffusion flame can change its character to premixed flame such that two premixed flames can exist in the partially premixed-diffusion system.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.31 no.12
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• pp.1009-1016
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• 2007

Numerical study on preferential diffusion effects in flame structure in $CH_4-H_2$ diffusion flames is conducted with detailed chemistry. Comparison of flame structures with mixture-averaged species diffusion and suppression of the diffusivities of $H_2$ and H was made. Discernible differences in flame structures are displayed with three species diffusion models. The behaviors of maximum flame temperatures with those species diffusion models are not explained by scalar dissipation rate but by the nature of chemical kinetics. It is seen that the modifcation of flame structure is mainly due to the preferential diffusion of H2 and thereby the nature of chemical kinetics. It is also found that the behaviors of major species with the three species diffusion models are addressed to the nature of chemical kinetics, and this is evident by examining importantly contributing reaction steps to the production and destruction of those chemical species.