• 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.

• 한국연소학회:학술대회논문집
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• 2012.11a
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• pp.55-58
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• 2012

EINOx scaling for $H_2/CO$ non-premixed turbulent jet flame was conducted. NOx concentration and flame length were measured simultaneously with varying flow conditions. Flame length increases with Reynolds number which means the flames in buoyancy-momentum transition region. We assessed the previous Chen & Driscoll's scaling with present results. However, the scaling cannot satisfy the present results. We proposed new scaling which is addressed the simplified flame residence time. The new scaling satisfies the results of $H_2/CO$ syngas flame as well as pure hydrogen flames.

• Journal of ILASS-Korea
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• v.8 no.4
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• pp.24-30
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• 2003

This paper presents the characteristics of lifted height and flame length from non-premixed jet flames in highly preheated air to investigate the detail combustion mechanism in the gas turbine or HCCI engine, etc. Special attention was paid to the effect of preheated air temperature, oxygen concentration and fuel injection flow rate on flame length and lifted hight. By using highly preheated air, stable flames were formed even in low oxygen concentration condition. The lifted height increased with decreasing preheated air temperature, where the flame length showed opposed phenomena. The flamelet model, which is thought to have very thin flamelet, is difficult to applicable to the present flame conditions which is formed In low oxygen concentration in highly preheated air.

• 한국연소학회:학술대회논문집
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• 1998.10a
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• pp.131-138
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• 1998

Flame characteristics in a double concentric burner has been studied experimentally. Air is supplied through a central nozzle, methane/air premixture is supplied in a inner annular part, and coflowing shield air is supplied to minimize outside disturbances. Depending on flow rate and concentration, various flame shapes can be observed. As the flow rate difference between central air jet and annular premixed jet is varied, several distinctive flames are observed. Conditions of partially premixed flames are further investigated; nozzle attached rich premixed flame, inner lifted flame, and outer lifted flame. Using the Abel transformation of digitized images of flames, cross- sectional images of flames can be obtained, from which overall structure of flames can be identified. PLIF measurement of OR radical was also conducted. OR radicals were mainly distributed in diffusion flame region. From the difference of OR distribution between nozzle attached and lifted flames, similarity of OR distribution between tribrachial flame and lifted flames in this study are observed.

• Journal of ILASS-Korea
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• v.10 no.2
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• pp.41-47
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• 2005

Exhaust gas emissions from internal combustion engines are one of the major sources of air pollution. And. it is extremely difficult to increase gasoline engine efficiency and to reduce NOx and PM(particulate matter) simultaneously in diesel combustion. This paper offers some basic concepts to overcome the above problems. To solve the problems, a recommended technique is CAI(controlled auto-ignition) combustion. In this paper. internal EGR(exhaust gas recirculation) effect is suggested to realize CAI combustion. An experimental study was carried out to achieve CAI combustion using homogeneous premixed gas mixture in the constant volume combustion chamber(CVCC). A flame trap was used to simulate internal EGR effect and to increase flame propagation speed in the CVCC. Flame propagation photos and pressure signals were acquired to verify internal EGR effect. Flame trap creates high speed burned gas jet. It achieves higher flame propagation speed due to the effect of geometry and burned gas jet.

• Transactions of the Korean Society of Automotive Engineers
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• v.11 no.2
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• pp.96-103
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• 2003

In this paper, the heat loss to the constant volume vessel wall was investigated using instantaneous heat flux sensor, schlieren visualization, pressure rise curve. And the heat loss characteristics of plasma jet ignition were compared with conventional spark ignition. In case of plasma jet ignition, the flame kernel moves toward the center of combustion vessel in the initial period of combustion, and the flame surface spread out to the vessel wall. However, in case of conventional spark ignition, the flame surface contact with combustion vessel wall in the initial period of combustion. As a result, heat loss in the combustion duration for conventional spark ignition increase faster than that of plasma jet ignition. And the combustion enhancement rate of plasma jet ignition is higher than that of conventional spark ignition, and it was found that the heat loss rate is inversely proportional to the combustion enhancement rate.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.4
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• pp.264-271
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• 2009

Characteristics of a counter flowing diffusion flame, which is formulated by an oppositely-injected methane-jet flow in a narrow channel of a uniform air flow. The location of the flame fronts and the flame lengths were compared by changing the flow rates of fuel. To distinguish the effects of the narrow channel on the diffusion flame, a numerical simulation for an ideal two-dimensional flame was conducted. Overall trends of the flame behavior were similar in both numerical and experimental results. With the increase of the ratio of jet velocity to air velocity flame front moved farther upstream. It is thought that the flow re-direction in the channel suppresses fuel momentum more significantly due to the higher temperature and increased viscosity of burned gas. Actual flames in a narrow channel suffer heat loss to the ambient and it has finite length of diffusion flame in contrast to the numerical results of infinite flame length. Thus a convective heat loss was additionally employed in numerical simulation and closer results were obtained. These results can be used as basic data in development of a small combustor of a nonpremixed flame.

• Fire Science and Engineering
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• v.24 no.6
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• pp.170-175
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• 2010

The effect of carbon dioxide addition on soot formation was investigated in jet diffusion flames in coflow. Flame temperature were measured with R-type thermocouple and the boundary temperature between blue and yellow flame was confirmed. Light-extinction method was introduced for the relative soot density (1-I/$I_0$) in the in-flame region. He-Ne laser with wave length at 632.8 nm was used for the light source, and the signal attenuated by absorption and scattering was detected directly. Oxidizer velocity effect on soot formation was studied to know that the thermal influence for soot formation. The results showed that the temperature of both blue and yellow flame were decreased according to the dilution of carbon dioxide but boundary temperature was nearly constant. The relative soot density was lower when carbon dioxide was added in oxidizer stream and oxidizer velocity increased. These were caused by the reduction of flame temperature and shorter residence time for soot growth. Also carbon dioxide addition enhanced the instability of jet flames like flickering, so the flame length was a little longer than pure ethylene/air flame.

• Journal of the Korean Society of Combustion
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• v.5 no.1
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• pp.19-30
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• 2000

The characteristics of NOx emissions in pure hydrogen nonpremixed flames with coaxial air are analyzed numerically for the three model cases of coaxial air flames classified by varying coaxial air velocity and/or fuel velocity. In coaxial air flames, the flame length is reduced by coaxial air and can be represented as a function of the ratio of coaxial air to fuel velocity. Coaxial air decreases flame reaction zone, resulting in reducing flame residence time significantly. Finally, the large reduction of EINOx is achieved by the decrease of the flame residence time. It is found that because coaxial air can break down the flame self-similarity law, appropriate scaling parameters, which are different from those in the simple jet flames, are recommended. In coaxial air flames, the flame residence time based on the flame volume produces better results than that based on a cube of the flame length. And some portion of deviations from the 1/2 scaling law by coaxial air may be due to the violation of the linear relationship between the flame volume and the flame reaction zone.

• 한국연소학회:학술대회논문집
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• 2012.04a
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• pp.83-85
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• 2012

Experimental study in coflow jet flames has been conducted to investigate the effects of adding $N_2$, $CO_2$ and He to coflowing air-side in self-excitations. Differences in the behaviors between buoyancy-driven and diffusive-thermal self-excitations with similar frequency range are explored and discussed in laminar coflow jet flames.