• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1166-1171
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• 2004

The combustion instability acts as a serious obstacle for the lean premixed combustion of gas turbine and even causes the fatal damage to the combustor and whole system. In this experiment, the pressure fluctuation is highly related to the stabilizing position of flame and fuel injection location. The fuel injection location is connected with the convection time of the fresh mixture, which is important time scale to refresh the mixtures near the flame stabilization location. The flame is extremely unstable when the alternative stabilization occurs and bulk mode frequency (${\sim}10Hz$) of pressure fluctuation is observed in this condition. It was found that the convection time scale of the fresh reactant coincided with the time scale of the bulk mode fluctuation. Hence this phenomenon results from the local equivalence ratio change caused by the pressure fluctuation induced by thermo-acoustic effects.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.11
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• pp.1582-1590
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• 1998

Combustion-driven oscillations in a surface burner have been investigated to clarify their characteristics. A model combustor is made and the oscillation frequencies are measured for various dimensions of the combustor. It is found that there are two modes of oscillations; one is the 'acoustic mode' at high frequencies, associated with the acoustic mode of the combustion system and the other is the 'combustion mode' at low frequencies around 100 Hz, associated with the instability of the flame. Acoustic mode is excited when the surface burner is placed where the phase of particle velocity leads that of acoustic pressure by $90^{\circ}$, for all the combustion conditions. Combustion mode is driven at high combustion rate by the lift of unstable flame near the lower limit of the combustible equivalence ratio. Combustion mode is greatly influenced by the inlet temperature of the premixed gas. When the inlet temperature is very high, the combustion mode does not occur.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.2
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• pp.616-623
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• 1996

This paper investigates the linear stability of reacting mixing layers with special emphasis on the existence of multiple unstable modes. The governing equations for laminar flows are from two-dimensional compressible boundary-layer equations. The chemistry is a finite rate single step irreversible reaction with Arrhenius kinetics. For the incompressible reacintg mixing layer with variable density. A necessary condition for instability has been derived. The condition requires that the angular momentum, not the vorticity, to have a maximum in the flow domain. New inflectional modes of instability are found to exist in the outer part of the mixing layer. For the compressible reacting mixing layer, supersonic unstable modes may exist in the abscence of a generalized inflection point. The outer modes at high Mach numbers in the reacting mixing layer are continuations of the inflectional modes of low Mach number flows. However, the generalized inflection point is less important at supersonic flows.

• Journal of the Korean Society of Combustion
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• v.7 no.4
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• pp.29-35
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• 2002

[$CO_2$] is a well-known greenhouse gas, which is the major source of global warming. Many researchers have studied to reduce $CO_2$ emission in combustion processes. The central method of low $CO_2$ emission is Oxygen/CxHy combustion. Theoretically Oxygen/CxHy combustion only produces $CO_2\;and\;H_2O$ and allows convenient recovery of $CO_2$. The combustion characteristics, flame stability, composition in the flame zone and temperature profile were studied experimentally for various compositions of oxidant by substituting $CO_2\;for\;N_2$ with the constant $O_2$ concentration. Results showed that flame became unstable due to the high heat capacity, low transport rate and strong radiation effect of $CO_2$ in comparison with those of $N_2$. The reaction zone was quenched and broadened, as the ratio of $CO_2\;to\;N_2$ was increased. The emission of NOx in flue gas decreased due to the decreased temperature of the reaction zone. As the conversion ratio of $CO_2\;to\;N_2$ was increased, the emission of CO and the higher temperature zone increased due to decrease of reaction rate by the a quenching effect.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.5
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• pp.750-756
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• 2002

The effect of equivalence ratio and velocity on the stability of flame in dump combustor was studied in an atmospheric pressure, laboratory scathe dump combustor operating natural gas. Traditionally, peak-to-peak pressure, fluctuation of the heat release rate and Rayleigh index were used to find and control the combustion instability. Cross correlation coefficients, Ci,j which is defined as the normalized value of the integration of the product of two of the mixer pressure, dump plane pressure and heat release rate, are introduced to see whether the flame is stable or not. Ci,j shows more sensitive to combustion status than Rayleigh index in steadily burning flame. Also, the result indicates that the amplitude of Ci,j between heat release and mixer pressure goes up before the flame at the rich de-stabilizing equivalence ratio near $\psi$=0.85. t means Ci:j at this case has a potential to detect the de-stablizing moment in prior to becoming unstable in dump combustor.

• Journal of the Korean Society of Combustion
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• v.19 no.1
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• pp.29-38
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• 2014

This paper described an experimental investigations of combustion instability mode in a lean premixed dual swirl combustor for micro-gasturbine system. When such the instability occurs, a strong coupling between pressure oscillations and unsteady heat release excites a self-sustained acoustic wave which results in a loud, annoyed sound and may also lead a structural damage to the combustion chamber. The detailed period of flame behavior and heat release in combustion instability mode have been examined with high speed OH and CH-PLIF system and $CH^*$ chemiluminescence measurement, flame tomography with operated at 10 kHz and 6 kHz each. Experiment results suggest that unstable flame behavior has a specific frequency with 200 Hz and this frequency is accords with about 1/2 sub-harmonic of combustor resonance frequency, not fundamental frequency. This is very interesting phenomenon that have not reported yet from other previous works. Therefore, when a thermo-acoustic instability with Rayleigh criterion occurs, the fact that the period of heat release and flame behavior are different each other was proposed for the first time through this work.

• Journal of the Korean Society of Combustion
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• v.19 no.3
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• pp.44-52
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• 2014

Solid oxide fuel cell(SOFC) makes electric power using hydrogen or reformed from methane and emits high temperature products that contain flammable species like hydrogen, carbon monoxide and methane which varies with operation condition. SOFC/gas turbine integrated system which uses thermal and chemical energy of the discharges is more efficient than SOFC itself. Burning character of the SOFC products will affect the efficiency and stability of the system. Experiments were conducted to know the characteristics of the flame for two typical composition of SOFC products, i.e. start-up and steady state composition. When coflowing air temperature was higher than $600^{\circ}C$, auto-ignitin occurred for both fuels. Though start-up fuel has higher contents of hydrogen, it makes longer flame than steady state composition. It was inferred that the amount of oxidizer necessary to burn makes this phenomenon. Steady state composition fuel was unstable since it contains lots of water. Nozzle that had 6 holes, distance between each hole was 16.7 times of hole diameter, improved the stability of the flame.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2005.11a
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• pp.437-444
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• 2005

Experiments were carried out in an atmospheric pressure, lab-scale gas turbine combustor to see the effect of partial premixing on unstable flame structure and Nox emission characteristics. The swirl angle is 45 deg., fuel-air mixing degrees were varied 0, 50 and 100% respectively at equivalence ratio ranging from 0.53 to 0.79. The evolution of phased-locked OH chemiluminescence images were acquired with an ICCD. NOx emission characteristics were also investigated at each experimental condition. The effect of the fuel-air mixing degree on the flame structure was obtained from phase-locked $OH^*$ images. And it was obtained from local heat release characteristics that the information about the region which the combustion instability was amplified or damped. It also could be confirmed that $\sigma$ has greatly influence on NOx emission characteristics at lean regimes. It would be expected that it could provide invaluable data for understanding the mechanism of combustion instability.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.8
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• pp.857-864
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• 2012

A linear stability analysis of a diffusion flame with radiation heat loss is performed to identify linearly unstable conditions for the Damk$\ddot{o}$hler number and radiation intensity. We adopt a counterflow diffusion flame with unity Lewis number as a model. Near the kinetic limit extinction regime, the growth rates of disturbances always have real eigenvalues, and a neutral stability condition perfectly falls into the quasi-steady extinction. However, near the radiative limit extinction regime, the eigenvalues are complex, which implies pulsating instability. A stable limit cycle occurs when the temperatures of the pulsating flame exceed the maximum temperature of the steady-state flame with real positive eigenvalues. If the instantaneous temperature of the pulsating flame is below the maximum temperature, the flame cannot recover and goes to extinction. The neutral stability curve of the radiation-induced instability is plotted over a broad range of radiation intensities.

• Applied Chemistry for Engineering
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• v.24 no.1
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• pp.77-81
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• 2013

In this study, the effect of halogen-phosphorus flame retardant on the flame retardancy and the mechanical properties of the rigid polyurethane foam (PUF) were studied. The reduced compressive strength and glass transition temperature of PUF decreased as contents of the flame retardant increased. After aging, the reduced compressive strength and glass transition temperature of PUF increased due to the reaction of unreacted isocyanate. The cell morphology effect of these flame retardants was also investigated using scanning electron microscope. The results of TCEP added to PUF showed an unstable and uneven cell morphology, leading to the increase of in thermal conductivity. The flame retardancy of vacuum aged PUF decreased compared to that of fresh PUF.