• 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.32 no.6
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• pp.413-420
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• 2008

The unsteady heat release characteristics play a significant role in combustion instabilities observed in low emissions gas turbine combustors. Such combustion instabilities are often caused by coupling mechanisms between unsteady heat release rates and acoustic perturbations. A generalized model of the turbulent flame response to acoustic perturbations is analytically formulated by considering a distributed heat release along a curved mean flame front and using the flame's kinematic model that incorporates the turbulent flame development. The effects of the development of flame speed on the flame transfer functions are examined by calculating the transfer functions with a constant or developing flame speed. The flame transfer function due to velocity fluctuation shows that, when a developing flame speed is used, the transfer function magnitude decreases faster with Strouhal number than the results with a constant flame speed at low Strouhal numbers. The flame transfer function due to mixture ratio fluctuation, however, exhibits the opposite results: the transfer function magnitude with a developing flame speed increases faster than that with a constant flame speed at low Strouhal numbers. Oscillatory behaviors of both transfer function magnitudes are shown to be damped when a developing flame speed is used. Both transfer functions also show similar behaviors in the phase characteristics: The phases of both transfer functions with a developing flame speed increase more rapidly than those with a constant flame speed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.5
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• pp.569-579
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• 2004

The lift-off characteristics of lifted laminar propane jet flames highly diluted with nitrogen are investigated introducing acoustic forcing with a fuel tube resonance frequency. A flame stability curve is obtained according to forcing strength and the nozzle exit velocity for N2 diluted flames. Flame lift-off behavior is globally classified into three regimes; 1) a weakly varying partially premixed behavior caused by a collapsible mixing for large forcing strength, 2) a coexistent behavior of the edge flame and a weakly varying partially premixed behavior for moderate forcing strength, and 3) edge flame or triple flame behavior for small forcing. It is shown that the laminar lifted flame with forcing affects flame lift-off behavior considerably, and is also clarified that the flame characteristic of flame base is well described with the penetration depth of the degree of mixing, ${\gamma}$$\_$$\delta$/. It is also confirmed that the weakly varying partially premixed flame caused by a collapsible mixing fur large forcing strength behaves as that just near flame blow-out in turbulent lift-off flame.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.9 s.252
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• pp.905-912
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• 2006

Experiments in low strain rate methane-air counterflow diffusion flames diluted with $CO_2$ have been conducted to investigate the flame extinction behavior and edge flame oscillation in which flame length is less than the burner diameter and thus lateral conductive heat loss in addition to radiative loss could be remarkable at low global strain rates. The critical mole fraction at flame extinction is examined in terms of velocity ratio and global strain rate. It is seen that flame length is closely relevant to lateral heat loss, and this sheets flame extinction and edge flame oscillation considerably. Lateral heat loss causes flame oscillation even at fuel Lewis number less than unity. Edge flame oscillations are categorized into three: a growing-, a harmonic- and a decaying-oscillation mode. Onset conditions of the edge flame oscillation and the relevant modes are examined with global strain rate and $CO_2$ mole fraction in fuel stream. A flame stability map based on the flame oscillation modes is also provided at low strain rate flames.

• Journal of the Korean Society of Industry Convergence
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• v.2 no.1
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• pp.121-130
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• 1999

It is necessary to measure pressure, temperature, chemical equilibrium and the shape of flame in order to understand the combustion process in a combustion chamber. In particular, the flame formulation and combustion process of divided combustion chamber are different from those of a single chamber, And the variable diameter of a jet hole can effect not only physical properties like ejection velocity, temperature and time of combustion but also a chemical property like the reaction mechanism. Accordingly temperature is one of the most important factors which influence the combustion mechanism. This paper observed shape of flame by using the schlieren photographs and measured the pressure in a combustion chamber and the reaching time of the flame by ion probe By doing these, we investigation the formulation of the flame and the process of propagation. These measurement methods can be advanced in understanding the combustion process and process and propagation of flame.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.2
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• pp.509-518
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• 1995

A study for the flame stability and the combustion characteristics of coaxial diffusion flame was conducted. The fuel employed was natural gas. The experimental variables were rim thickness of fuel tube, blockage ratio of the outer diameter of fuel tube to the inner diameter of air tube, and momentum ratio of fuel to air. It was consequently found that the stability in the neighborhood of the fuel rim depended on the rim thickness, especially in the case of above 3 mm, and that the stable region of the flame extended remarkably due to the formation of recirculation zone above rim. The effect of the blockage ratio on the flame stability was found to be minor in the case of above 3 mm of rim thickness. Between the momentum ratio 2 and 3, the stable flame zone was widely established as well good combustion. With increasing the fuel-air momentum ratio, axial velocity, turbulence intensity, and Reynolds stress increased.

• 한국연소학회:학술대회논문집
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• 2006.04a
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• pp.32-38
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• 2006

A premixed flame propagating in a tube suffers strong variation in its shape and structure depending on boundary conditions. The effects of thermal boundary conditions and flow fields on flame propagation are numerically investigated. Navier-Stokes equations and species equations are solved with a one-step irreversible global reaction model of methane-air mixture. Finite volume method using an adaptive grid method is applied to investigate the flame structure. In the case of an adiabatic wall, friction force on the wall significantly affected the flame structure while in the case of an isothermal wall, local quenching near the wall dominated flame shapes and propagation. In both cases, variations of flow fields occurred not only in the near field of the flame but also within the flame itself, which affected propagation velocities. This study provides an overview of the characteristics of flames in small tubes at a steady state.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.1
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• pp.321-327
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• 1996

Experiments were performed on the laminar premixed flame stabilized in a porous medium to know whether the flame downstream of the combustor exists or not. In previous theoretical studies, a stable flame has been predicted in the downstream region of the combustor, but it has never been observed in experiments. In this study, a stable downstream flame could be obtained for the lower burning velocity through circumferential heating by a blue flame positioned outside the periphery of the specially devised combustor. The existence of the stable downstream flame was confirmed by a direct photography of soot line, and temperature measurements. The effect of combustor diameter to flame stability was also considered. As the diameter of the combustor increases, the lean flammability limit was extended.

• 한국연소학회:학술대회논문집
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• 2013.06a
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• pp.41-43
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• 2013

In order to simulate and visually observe combustion phenomena in cylindrical radial-flow porous inert media, a radial multi-channel burner, made of transparent quartz plates, was fabricated. Flame stabilization characteristics and its pulsating instability in the burner were experimentally investigated with respect to various mixture flow rates and equivalence ratio. As a result, five different flame behaviors, such as stable flame, pulsating instability, sudden extinction, blowout and unstable extinction, were observed. Mean radial position of circularly arranged multi-flame and its averaged burning velocity were measured and then compared to the freely propagating flame. The multi-flame pulsation frequency is about several tens of Hz and it is supposed to be generated by the heat diffusion enhancement to cold pre-mixture by the intensive gas-solid interaction.

• Journal of the Korean Institute of Gas
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• v.7 no.4 s.21
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• pp.24-29
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• 2003

Numerical analysis was conducted to predict the damage of indoor gas explosion for the propagation of explosion flame. Indoor gas diffusion distribution due to gas leakage was obtained by diffusion equation that adopted initial conditions from reference. Enthalpy of each gas-mixture ratio and reduced mechanism was applied to calculate flame temperature, and laminar combustion velocities with the variant of each gas concentration from reference were applied to the gas mixture. Turbulent combustion velocity was modeled by coupling of turbulent energy and laminar combustion velocity in k-$\epsilon$ model. For the analysis of flame propagation cartesian and cylindrical coordinate were used to indoor position and flame propagation respectively. The study analyzes the cause of pressure rising with the variation of flame propagation by glass damage, and the result shows that indoor pressure rising with ignition position varies window dimension.