• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.8
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• pp.1087-1096
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• 2001

The dynamic structures of unsteady CH$_4$/Air jet diffusion flame with a flame-vortex interaction were numerically investigated. A timed-dependent, axisymmetric computational model and a low mach number approximation were employed in the present calculation. A two-step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including the gravitational effect show that the large outer vortices and the small inner vortices can be well simulated without any additional disturbances near nozzle tip. It was found that the temperature and species concentrations have deviated values even for the same mixture fraction in the flame-vortex interaction region. It was also shown that the flame surface is not deformed by the inner vortex in upstream region, while in downstream region, the flame surface is compressed or stretched by the outer vortex roll-up. The present unsteady jet flame configuration accompanying a flame-vortex interaction is expected to give good implications for the unsteady structures of turbulent flames.

• Journal of the Korean Society of Combustion
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• v.9 no.2
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• pp.10-17
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• 2004

A two-dimensional direct numerical simulation is performed to investigate the flame structure of $CH_4/N_2-Air$ counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman#s detailed reaction mechanism are adopted in this calculation. To quantify the strain on flame induced by a vortex, a scalar dissipation rate (SDR) is introduced. The results show that fuel-side and air-side vortex cause an unsteady extinction. In this case, the flame interacting with a vortex is extinguished at much larger SDR than steady flame. It is also found that air-side vortex extinguishes a flame more rapidly than fuel-side vortex. The unsteady effect induced by flame-vortex interaction does not lead to a transient OH overshoot of the maximum steady concentration observed in experiment, while $HO_2$ radical increases more than the maximum steady concentration with increasing SDR. In addition, it is seen that NO and $NO_2$ are not sensitive to the unsteady variation of SDR.

• Journal of Mechanical Science and Technology
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• v.19 no.9
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• pp.1811-1820
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• 2005

Thermo-acoustic instability of methane/ air flames in an industrial gas-turbine combustor is numerically investigated adopting CFD analysis. The combustor has 37 EV burners through which methane and air are mixed and then injected into the chamber. First, steady fuel! air mixing and flow characteristics established by the burner are investigated by numerical analysis with single burner. And then, based on information on the flow data, the burners are modeled numerically via equivalent swirlers, which facilitates the numerical analysis with the whole combustion system including the chamber and numerous burners. Finally, reactive flow fields within the chamber are investigated numerically by unsteady analysis and thereby, spontaneous instability is simulated. Based on the numerical results, scaling analysis is conducted to find out the instability mechanism in the combustor and the passive control method to suppress the instability is proposed and verified numerically.

• Proceedings of the Korea Institute of Fire Science and Engineering Conference
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• 2008.04a
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• pp.234-237
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• 2008

A direct numerical simulation (DNS) code suitable for the prediction of buoyant jet diffusion flames was developed in this study. The thermodynamic and transport properties were evaluated using CHEMKIN package to enhance the prediction performance of the developed DNS code. A two dimensional simulations were performed for the jet diffusion flames in normal and zero-gravity conditions where the Froude numbers are 5 and infinity, respectively. The simulated buoyant jet diffusion flame in normal gravity showed that the unsteady and dynamic motion although the reynolds number is low (400). It was identified that the flame in normal gravity flickered periodically. The periodic motion of the flame disappeared in zero-gravity condition. The dynamic motion of the buoyant jet diffusion flame could be well understood by comparing the flame structures obtained by the simulations of normal and zero-gravity conditions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.12
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• pp.1557-1566
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• 2004

A flame extinction phenomenon is a typical unsteady process in combustion. Flame extinction is characterized by various physical phenomena, such as convection, diffusion, and the production of heat and mass. Flame extinction can be achieved by either increasing the strain rate or curvature, by diluting an inert gas or inhibitor, or by increasing the thermal or radiant energy loss. Though the extinction is an inherently transient process, steady and quasi-steady approaches have been used as useful tools for understanding the flame extinction phenomenon. Recently, unsteady characteristics of flames have been studied by many researchers, and various attempts have been made to understand unsteady flame behavior, by using various extinction processes. Representative parameters for describing flame, such as flame temperature, important species related to reactions, and chemi-luminescence of the flame have been used as criterions of flame extinction. In these works, verification of each parameter and establishing the proper criterions of the extinction has been very important. In this study, a time-dependent flame temperature and an OH radical concentration were measured using optical methods, and the instantaneous change of the flame luminosity was also measured using a high-speed ICCD (HICCD) camera. We compare the unsteady extinction points obtained by three different methods, and we discuss transient characteristics of maximum flame temperature and OH radical distribution near the extinction limit.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.494-502
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• 1997

Experiments on corresponding jet flames with stagnant point diffusion flames have been carried out in initial injection periods. A compensated measurement of maximum flame temperature, which is based on the ion signal, has been employed to inspect flame responses to time-varying strain rates. The flame responses are obtained at two conditions for the slowly time-varying strain rate and the case of flame extinction, and analyzed to confirm similarity between a stagnant point diffusion flame and an evolving jet diffusion flame. Nonsteady effects are addressed via the comparison between several time scales. The time variation with low strain rates, in which illustrates the flame behavior of the upper branch far from extinction in the well-known S-curve, is confirmed to produce a quasi-steady flame response through the nonsteady experiments. The time variation with strain rates in the case of flame extinction indicates an unsteady effect of flame response. It is therefore found that the flame responses near jet tip depend on time histories of characterized strain rates in the developing process.

• 한국연소학회:학술대회논문집
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• 2006.04a
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• pp.26-31
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• 2006

Flickering behaviors of lean premixed flame of propane/air and methane/air flame anchored by a pilot flame in a tube were investigated. Unsteady behaviors of the flame were monitored by a high speed ICCD camera and the flickering frequency was defined as the number of flame curvatures passing a fixed spatial point in a second. Unlike previous studies in which flames are in open condition so that the flickering mechanism is an unstable interaction of hot buoyant products with the ambient air, flames in this study are surrounded by a tube which means they are not open to ambient air, so that there is no interaction between hot buoyant products and ambient air. Despite the fact, there exists flickering phenomena and the flickering frequency ranges from 10 Hz to 50 Hz which is wider compared to previous studies. We relate the flickering mechanism to flame-generated vorticity and analytic solution for locally approximated flow is used. As a result, the relationship between flickering wavelength and dimensionless vorticity is acquired and the cause of higher range of flickering frequency is explained.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.1
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• pp.1-8
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• 2012

Acoustic-pressure response of diluted hydrogen-air diffusion flames is investigated numerically by adopting a fully unsteady analysis of flame structures in low and high pressure regimes. As acoustic frequency increases, finite-rate chemistry is enhanced through a nonlinear accumulation of heat release rate for any pressure regime, leading to a high amplification index. Same numerical results are analyzed with application of a pressure-sensitive time lag model, and thereby, interaction index and time lag are calculated for each pressure regime. The interaction index has the largest value in each pressure regime at an acoustic frequency near 1000 Hz. In a high-pressure regime, flames are more unstable than in a low-pressure regime. The interaction index shows a good agreement with the amplification index.

• Journal of Mechanical Science and Technology
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• v.18 no.10
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• pp.1859-1868
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• 2004

Monitoring of OH chemiluminescence through an optical fiber was demonstrated to be a useful method in detecting self-excited combustion oscillations. OH chemiluminescence intensity detected by the optical fiber showed mostly excellent agreement with those obtained by high speed CCD camera measurements when combustion oscillations were strong. 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, we have found and proposed unique measures to tell 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.

• Journal of ILASS-Korea
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• v.22 no.2
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• pp.55-61
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• 2017

This study has investigated the relationship between heat release fluctuations and the flow perturbations in a partially premixed gas turbine combustor using a commercial CFD code. Special focus of the current work is placed on the effect of equivalence ratio on the flame dynamics in a partially-premixed system. As the first step for this combustion dynamics study in the non-perfectly premixed combustor, flame behaviors are modeled and then compared with measured results under both steady and unsteady conditions. The calculated results of the flame transfer function with equivalence ratio fluctuation are found to well capture the main qualitative characteristics of the combustion dynamics for the partially-premixed flames.