• Journal of the Korean Society of Combustion
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• v.9 no.1
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• pp.32-38
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• 2004

Characteristics of turbulent lifted flames in coflow jet have been investigated by varying initial temperature through the heating coflow air. In the turbulent regime, liftoff height increases linearly with fuel jet velocity and decreases nonlinearly as the coflow temperature increases. This can be attributed to the increase of turbulent propagation speed, which is strongly related to laminar burning velocity. Dimensionless liftoff heights are correlated well with dimensionless jet velocity, which are scaled with parameters determining local flow velocity and turbulent propagation speed. This implies that the turbulent lifted flames are stabilized by balance mechanism between local turbulent burning velocity and flow velocity. Blowout velocity can be obtained from the ratio of mixing time to chemical time. Comparing to previous researches, thermal diffusivity should be evaluated from the initial temperature instead of adiabatic flame temperature.

• 한국연소학회:학술대회논문집
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• 2006.10a
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• pp.220-226
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• 2006

The effect of electric fields on the propagation speed of tribrachial flames has been investigated in a coflow jet by observing the transient flame propagation behavior after ignition. Without having electric fields, the propagation speed of tribrachial flame edges showed a typical behavior by having an inverse proportionality to the mixture fraction gradient at the flame edge. The behavior of flame propagation with the electric fields was investigated by applying high voltage to the central fuel nozzle and the enhancement of propagation speed has been observed by varying the applied voltage and frequency for AC electric fields. The propagation speed of tribrachial flame was also investigated by applying negative and positive DC voltages to the nozzle and similar improvements of the propagation speed were also observed. The propagation speeds of tribrachial flames in both the AC and DC electric fields were correlated well with the electric field intensity, defined by the electric voltage divided by the distance between the nozzle electrode and the edge of tribrachial flames.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.23 no.9
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• pp.1139-1150
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• 1999

The evolution of silica aggregate particles in coflow diffusion flames has been studied experimentally using light scattering and thermophoretic sampling techniques. The measurements of scattering cross section from $90^{\circ}$ light scattering have been utilized to calculate the aggregate number density and volume fraction using with combination of measuring the particle size and morphology through the localized sampling and a TEM image analysis. Aggregate or particle number densities and volume fractions were calculated using Rayleigh-Debye-Gans and Mie theory for fractal aggregates and spherical particles, respectively. Of particular interests are the effects of flame temperature on the evolution of silica aggregate particles. As the flow rate of $H_2$ increases, the primary particle diameters of silica aggregates have been first decreased, but, further increase of $H_2$ flow rate causes the diameter of primary particles to increase and for sufficiently larger flow rates, the fractal aggregates finally become spherical particles. The variation of primary particle size along the upward jet centerline and the effect of burner configuration have also been studied.

• 한국연소학회:학술대회논문집
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• 2000.05a
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• pp.9-15
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• 2000

Characteristics of lifted flames for highly diluted propane and methane with nitrogen in coflowing air is experimentally investigated. In case of propane, for various fuel mole fractions and jet velocities, three distinctive types of flames are observed; nozzle attached flames, stationary lifted flames, and oscillating lifted flames. When fuel jet velocity is much smaller than coflow velocity, the base of nozzle attached flame has a tribrachial structure unlike usual coflow difusion flames. Based on the balance mechanism of the propagation speed of tribrachial flame with flow velocity, jet velocity is scaled with stoichiometric laminar burning velocity. Results show that there exists two distinctive lifted flame stabilization; stabilization in the developing region and in the developed region of jets depending on initial fuel mole fraction. It has been found that lifted flame can be stabilized for fuel velocity even smaller than stoichiometric laminar burning velocity. This can be attributed to the buoyancy effect and flow visualization supports it. Lifted flames are also observed for methane diluted with nitrogen. The lifted flames only exist in the developing region of jet.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.2
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• pp.160-166
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• 2004

Combustion using oxygen enriched air is known as a technology which can increase flame stability as well as thermal efficiency due to improving the burning rate. Lift-off, blowout limit and flame length were examined as a function of jet velocity, coflow velocity and OEC(Oxygen Enriched Concentration). Blowout limit of the flame below OEC 25% decreased with increase of coflow velocity, but the limit above OEC 25% increased inversely. Lift-off height decreased with increase of OEC. In particular, lift-off hardly occurred in the condition above OEC 40%. Flame length of the flames above OEC 40% was increased until the blowout occurred. Great flame stability was obtained since lift-off and blowout limit significantly increased with increase of OEC.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.326-331
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• 2003

Combustion using oxygen enriched air is known as a technology which can increase flame stability as well as thermal efficiency due to improvement of the burning rate. Lift-off, blowout limit and flame length were examined as a function of jet velocity, coflow velocity and OEC(Oxygen Enriched Concentration). Blowout limit of the flame below OEC 25% decreased with coflow velocity, but the limit above OEC 25% increased inversely. Lift-off height decreased with increase of OEC. Especially lift-off hardly occurred in the condition above OEC 40%. Flame length of the flames above OEC 40% was increased until the blowout occurred. Flame stability became improved since lift-off and blowout limit increased much with increase of OEC.

• 한국연소학회:학술대회논문집
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• 2004.06a
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• pp.21-26
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• 2004

Characteristics of laminar lifted names in coflow jet with various coflow velocities have been studied experimently. USlI1g the fuel nozzle with d=0.254 for the pure propane, liftoff heights are fitted by using power equation with jet velocity. As coflow velocity increases up to 60 cm/s powers of fitting equation steeply decrease. From the result of numerical analysis using the FLUENT, the stoichiometry contour and the axial velocity nondimensionalized by initial jet velocity along the stoichiometry contour are changed with variations of coflow velocities, The change of axial velocity along stoichiometric contour is more sensitive than that of stoichiometric contour, For this reason, powers of fitting equation for liftoff height with jet velocity decreases with the increase of coflow velocity. Using the fuel nozzle with d=4,35 mm for the highly diluted propane by nitrogen, the liftoff height increases with the increase of coflow velocity when coflow velocity is less than the maximum value of initial jet velocity. But when coflow velocity is faster than that, the liftoff height decreases with the increase of coflow velocity.

• Journal of the Korean Society of Combustion
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• v.17 no.4
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• pp.51-59
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• 2012

Experimental study in coflow jet flames has been conducted to investigate the helium-dilution effect of coflow air on self-excitation. For various helium mole fractions and jet velocities, two types of self-excitation were observed: buoyancy-driven self-excitation and Lewis-number-induced self-excitation(here after called Le-ISE) coupled with buoyancy-driven one. The difference between buoyancy-driven and Le-ISE is clarified by using the Mie-scattering visualization as well as exploring the different features. The mechanism of Le-ISE is proposed. When the system Damk$\ddot{o}$hler number was lowered, Le-ISE is shown to be launched. Le-ISE is closely related to heat loss, in that it can be launched in even methane jet flame (Lewis number less than unity) with helium-diluted coflow air. Particularly, Le-ISE becomes significant as the Damk$\ddot{o}$hler number decreases and heat-loss becomes significant.

• 한국연소학회:학술대회논문집
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• 2003.05a
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• pp.255-260
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• 2003

Characteristics of methane jet diffusion flames using pure oxygen with recirculating $CO_{2}$ as an oxidizer were investigated experimentally. A coflow burner was considered, and the diameter of confinement was larger than that of the coflow. No stabilized flame could be observed over 75% of $CO_{2}$ volume percent. A comparison between air and $O_{2}/CO_{2}$ mixture was made in terms of liftoff velocity, flame liftoff height, and blowout conditions. As results, more stable flame could be observed with $O_{2}/CO_{2}$ mixture for the case of having similar flame temperature.

• 한국연소학회:학술대회논문집
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• 2000.12a
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• pp.160-167
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• 2000

We investigated a uniform temperature zone, produced by double flame structure of a coflow CH4/air partially premixed flame, to be used as a temperature calibration source for laser diagnostics. A broadband N2 CARS(coherent anti-Stokes Raman spectroscopy) system with a modeless laser was used for temperature measurement. When the stoichiometric ratio was 1.5, we found the uniform temperature zone in radial direction of the flame of which the averaged temperature was 2110 K with standard deviation 24 K. In the stoichiometric ratio range between 2.0 and 2.5, we found very stable temperature-varying zones in vertical direction at the center of the flame. The size of the zone was approximately 15 mm and it covered a temperature range from 300 K to 1900 K. We also suggest that this zone can be used as a calibration source for 2-D PLIF(planar laser induced flurescence) temperature measurement.