• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.3 s.234
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• pp.390-401
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• 2005

In this study, extinction limit extension of unsteady $(CH_{4}+N_{2})$/air diffusion flames was investigated experimentally. A spatially locked flame in an opposing jet burner was perturbed by linear velocity variation, and time-dependent flame luminosity, transient maximum flame temperature and OH radical were measured over time with the high speed camera, Rayleigh scattering method and OH laser-induced fluorescence, respectively. Unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames, and unsteady extinction limits extend as the slope of the strain rate increases or the initial strain rate decreases. We verified the validity of the equivalent strain rate concept by comparing the course of unsteady extinction process and steady extinction process, and it was found that the equivalent strain rate concept represents well the unsteady effect of a convective-diffusive zone. To investigate the reason of the unsteady extinction limit extension, we subtracted the time lag of the convective-diffusive zone by using the equivalent strain concept. Then the modified unsteady extinction limits become smaller than the original unsteady extinction limits, however, the modified unsteady extinction limits are still larger than the steady extinction limits. These results suggest that there exist the unsteady behavior of a diffusive-reactive zone near the extinction limit due to the chemical non-equilibrium states associated with unsteady flames.

• Journal of Mechanical Science and Technology
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• v.14 no.1
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• pp.103-112
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• 2000

Numerical simulations of unsteady opposed-flow flames are performed using an adaptive time integration method designed for differential-algebraic systems. The compressibility effect is considered in deriving the system of equations, such that the numerical difficulties associated with a high-index system are alleviated. The numerical method is implemented for systems with detailed chemical mechanisms and transport properties by utilizing the Chemkin software. Two test simulations are performeds hydrogen/air diffusion flames with an oscillatory strain rate and transient ignition of methane against heated air. Both results show that the rapid transient behavior is successfully captured by the numerical method.

• Journal of the Korean Society of Safety
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• v.24 no.6
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• pp.27-31
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• 2009

An experimental study on the unsteady effect of the extinction limit was performed in ethene jet diffusion flames. To impose the unsteadiness on jet flames, the amplitude and frequency of a co-flow velocity was varied, and the two inert gases, $N_2$ and $CO_2$, were used to dilute the oxidizer for extinguishing concentration. The experimental results shows that large amplitude of velocity induces a low extinguishing concentration, which implies that flow variation affects the blow out mechanism. Also, the flow oscillation effects under high frequency attenuates the flame extinction. These results means that flow unsteadiness extends the extinction limit and finally minimum extinction concentration by inert gases. When the Stoke's 2nd Problem is introduced to explain the flow unsteadiness on extinction concentration, the solution predicts the effect of amplitude and frequency of velocity well, and hence it is concluded the effect of low frequency velocity excitation was attributed only to flow effect.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.2
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• pp.243-250
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• 2003

The extinction behavior and the unsteady response of augmented reduced mechanism(ARM) have been investigated by adopting an OPPDIF code and a numerical solver for the flamelet equations. By comparing the performance of the ARM based on Miller and Bowman's mechanism(MB-ARM) with that of the ARM based on GRI-Mech 3.0(GRI-3.0-ARM), it is identified that the MB-ARM is more suitable for the unsteady calculation because it is relatively less stiff than GRI-3.0-ARM during an ignition process. The steady results using the MB-ARM, which is modified to predict reasonably the extinction point of experiment, are in excellent agreement with those from full mechanism. Under the sinusoidal transient disturbances of scalar dissipation rate, the unsteady responses of the flame temperature and species concentrations using a modified MB-ARM show in very close agreement with those from full mechanism. It is presumed that above modified MB-ARM is very suitable for the unsteady simulation of turbulent flames because it gives not only a low computational cost but also a good prediction performance for flame structure, extinction point and unsteady response.

• 한국연소학회:학술대회논문집
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• 2000.12a
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• pp.113-122
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• 2000

Dynamic structures of unsteady $CH_4$/Air jet diffusion flames with flame-vortex interaction were numerically investigated. A time-dependent, axisymmetric computational model was adopted for this calculation. Two step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including gravitational effect show that the large outer vortices and the small inner vortex street can be well simulated without any additional disturbances in the downstream of nozzle tip. It was found that the temperature and species concentrations had various values for the same mixture fraction in flame-vortex interaction region. This unsteady jet flame configuration accompanying flame-vortex interaction is expected to give good implications for the structure of turbulent flames.

• Journal of Advanced Marine Engineering and Technology
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• v.28 no.7
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• pp.1111-1122
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• 2004

An idea to suppress the self-excited combustion oscillation was applied to the flames. The characteristics of unsteady combustion were examined and the unsteady combustion was driven by forced pulsating mixture supply that can modulate its amplitude and frequency. The self-excited combustion oscillation having weaker flow velocity fluctuation intensity than that of the forced pulsating supply can be suppressed by this method. The effects of the forced pulsation amplitude and frequency on controlling self-excited combustion oscillations were also investigated comparing with the steady mixture supply. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillation. Symptoms of self-excited combustion oscillation were also studied in order to predict the onset of combustion oscillation before it proceeded to a catastrophic failure For the purpose, the unique measures to observe the onset of self-excited combustion oscillations based on the careful statistics of fluctuating properties in flames, such as pressure or emission of OH radicals, have been proposed.

• 한국연소학회:학술대회논문집
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• 2000.12a
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• pp.9-16
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• 2000

The flamelet concept has been widely applied to numerically simulate complex phenomena occurred in nonpremixed turbulent flames last two decades, and recently broadened successfully the applicable capabilities to various combustion problems from simple laboratory flames to gas turbine engine, diesel spray combustion and partially premixed flames. The paper is focused on brief review of recently noticeable work related to flamelet modeling, which includes Lagrangian flamelet approach, RIF concept as well as steady flamelet approach. The limitation of steady flamelet assumption, the effect of transient behavior of flamelets, and the effect of spray vaporization on PDF model have been discussed.

• Journal of Mechanical Science and Technology
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• v.17 no.11
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• pp.1820-1831
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• 2003

Characteristics of self-excited combustion oscillation are experimentally studied using confined premixed flames stabilized by a rearward-facing step. A new idea to suppress combustion oscillation was applied to the flames. The characteristics of unsteady combustion were examined, which is driven by forced pulsating mixture supply that can modulate its amplitude and frequency. The self-excited combustion oscillation having weaker flow velocity fluctuation intensity than that of the forced pulsating supply can be suppressed by the method. The effects of the forced pulsation amplitude and frequency on controlling self-excited combustion oscillations were also investigated comparing with the steady mixture supply. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillations, and it also exhibits desirable performances, from a practical point of view, such as high combustion load and reduced pollutant emissions of nitric oxide.

• 한국연소학회:학술대회논문집
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• 2005.10a
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• pp.189-197
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• 2005

The usefulness of unsteady combustion was experimentally investigated using confined premixed flames stabilized by a rearward-facing step. For this purpose, apparatus of forced pulsating mixture supply, which could be modulated its amplitude and frequency, was designed. The unsteady combustion used in this experiment plays an important role in controlling self-excited combustion oscillations and furthermore it exhibits desirable performance, from a practical point of view, such as high combustion and reduction of pollutant emission like nitric oxide.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.320-325
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• 2003

Acoustic-Pressure Response of diluted hydrogen-air diffusion flames is investigated numerically by adopting a fully unsteady analysis of flame structures. In the low-pressure regime, the amplification index remains low and constant at low frequencies. As acoustic frequency increases, finite-rate chemistry is enhanced through a nonlinear accumulation of heat release rate, leading to a high amplification index. Finally, the flame responses decrease at high frequency due to the response lag of the transport zone. For a medium-pressure operation and low-frequency excitation, the amplification index is low and constant. It then decreases at moderate frequencies. As frequency increases further, the amplification index increases appreciably due to an intense accumulation effect.