• 한국연소학회:학술대회논문집
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• 1999.10a
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• pp.165-172
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• 1999

Landfill gas has merely half heating value compared with liquified natural gas but can be greatly utilized as a commercial fuel. The authors have examined emission characteristics as well as measured burning velocity of LFG mixed gas which contains plenty of $CO_{2}$. With the viewpoint of fuel utilization, flame stability could be one of important characteristics of LFG. In this study, the comparison experiments are conducted between $CH_{4}$ and LFG for searching the region of flame stabilization based upon the flame blowout at maximum fuel stream velocity. As a result, it is found that stabilization region of LFG is not improved with that of $CH_{4}$ in non-swirl/or weak swirl jet diffusion flame. However, it is also known that flame stability is hardly affected by inert gas in the strong swirl with considering widened flame stabilization region of LFG rather than LNG.

• 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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• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.2
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• pp.223-229
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• 2004

Combustion using oxygen enriched air is known as a technology which can increase thermal efficiency due to increase of the flame temperature. Flame shapes, schlieren photos, OH radical chemiluminescence and local flame temperature were examined as a function of OEC(Oxygen Enriched Concentration) in a coaxial non-premixed jet. With increase of OEC, flame length and width decreased, but its brightness increased significantly, and the size of vortices in the flame also increased. Especially, the reaction around the flame surface became active. The strong OH intensity appeared to be made and moved from middle stream to upper one with increase of OEC, which shows combustion reaction in the upper stream becomes more dominant In addition, the temperature distributions of the flames showed similar tendency with OH radical intensities. A flame with high temperature and strong stability was obtained with increasing OEC of the coflow.

• Journal of the Korean Society of Combustion
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• v.12 no.2
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• pp.26-33
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• 2007

To reveal the newly found liftoff height behavior of hydrogen jet, we have experimentally studied the stabilization mechanism of turbulent, lifted jet flames in a non-premixed condition. The objectives of the present research are to report the phenomenon of a liftoff height decreasing as increasing fuel velocity, to analyse the flame structure and behavior of the lifted jet, and to explain the mechanisms of flame stability in hydrogen turbulent non-premixed jet flames. The velocity of hydrogen was varied from 100 to 300m/s and a coaxial air velocity was fixed at 16m/s with a coflow air less than 0.1m/s. For the simultaneous measurement of velocity field and reaction zone, PIV and OH PLIF technique was used with two Nd:Yag lasers and CCD cameras. As results, it has been found that the stabilization of lifted hydrogen diffusion flames is related with a turbulent intensity, which means that combustion occurs at the point where the local flow velocity is balanced with the turbulent flame propagation velocity.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.3
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• pp.190-197
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• 2008

It was experimentally studied that the stabilization mechanism of turbulent, lifted jet flames in a non-premixed condition to reveal the newly found liftoff height behavior of hydrogen jet. The objectives are to report the phenomenon of a liftoff height decreasing as increasing fuel velocity, to analyse the flame structure and behavior of the lifted jet, and to explain the mechanisms of flame stability in hydrogen turbulent non-premixed jet flames. The hydrogen jet velocity was changed from 100 to 300m/s and a coaxial air velocity was fixed at 16m/s with a coflow air less than 0.1m/s. For the simultaneous measurement of velocity field and reaction zone, PIV and OH PLIF technique was used with two Nd:Yag lasers and CCD cameras. As a result, it was found that the stabilization of lifted hydrogen diffusion flames is correlated with a turbulent intensity and Karlovitz number.

• Journal of the Korean Society of Safety
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• v.29 no.4
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• pp.23-27
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• 2014

The use of electromagnetic energy and non-equilibrium plasma for enhancing ignition and combustion stability is receiving increased attention recently. The conventional technologies have adapted the electrical devices to make the electromagnetic field, which resulted in various safety issues such as high-maintenance, additional high-cost system, electric shock and explosion. Therefore, an electrodeless microwave technology has an advantage for economic and reliability compared with conventional one because of no oxidation. However, the application of microwave has been still limited because of lack of interaction mechanism between flame and microwave. In this study, an experiment was performed with jet diffusion flames induced by microwaves to clarify the effect of microwave on the combustion stability and pollutant emissions. The results show that microwave induced flames enhanced the flame stability and blowout limit because of abundance of radical pool. However, NOx emission was increased monotonically with microwave intensity except 0.2 kW, and soot emission was reduced at the post flame region.

• 한국연소학회:학술대회논문집
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• 2002.11a
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• pp.33-39
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• 2002

Unsteady behaviors of counterflow flame were studied experimentally in opposing jet counterflow burner using diluted methane. To generate the unsteadiness on the flame, the counterflow diffusion flame was perturbed by velocity changes made by the pistons installed on both sides of the air and fuel stream. The velocity changes were measured by Hot wire and Laser Doppler Velocimetry, and the flame behaviors were observed by High speed ICCD and ICCD. In this investigation, the spatial irregularity of the strain rate caused the flame to extinguish from the outside to the axis during the extinction, and we found the following unsteady phenomena. First, the extinction strain rates of unsteady cases are much larger than those of the steady ones. Second, the extinction strain rates become larger as the slope of the change of the strain rate increases. Third, the unsteady extinction strain rates become smaller with the increase of the initial strain rate.

• 한국연소학회:학술대회논문집
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• 2003.05a
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• pp.65-73
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• 2003

An experimental study on flame lift-off characteristics of propane jet flame highly diluted with nitrogen has been conducted introducing acoustic forcing with a tube resonant frequency. A flame stability curve is attained according to forcing strength and nozzle exit velocity for $N_2$ diluted flames. Flame lift-off behavior with forcing strength and nozzle exit velocity is globally categorized into three; a well premixed behavior caused by a collapsible mixing for large forcing strength, a coexistent behavior of well-premixed and edge flames interacting with well-organized inner fuel vortices for moderate forcing strengths, and edge flame behavior for small forcing strengths. Special focus is concentrated on the coexistent behavior of the flame base in lifted flame since this may give a hint to a possibility which the flame base behaves like a well-mixed premixed flame in highly turbulent lifted flames. It is also shown that the acoustic forcing to self-pulsating laminar lifted flame affects flame lift-off behavior considerably which is closely related to downstream flow velocity, mixture strength, effective fuel Lewis number, and flame stretch.

• 한국가시화정보학회:학술대회논문집
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• 2007.11a
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• pp.149-154
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• 2007

This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen nonpremixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NOx emissions. Acoustic excitation causes the flame length to decrease by 15 % and consequently, a 25 % reduction in EINOx is achieved, compared to a flame without acoustic excitation. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NOx emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface.