• 한국연소학회:학술대회논문집
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• 2006.04a
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• pp.105-110
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• 2006

Many previous works have been performed to provide correlations of flame length, theoretically and experimentally. Most of these results studied were conducted in vertical turbulent flame with no coaxial air condition. The present study analyzes the flame length scaling with coaxial air. In turbulent hydrogen non-premixed jet flames with coaxial air, flame length scaling theoretically proposed so far has been related with the concept of a far-field equivalent source. At high coaxial air to fuel velocity ratio, $U_A/U_F$, however, this scaling theory has some difference with experimental flame length data. This difference is understood to be due to the fact that the theory is based on far-field notion, while the effect of coaxial air on jet flame occurs in the region near the nozzle exit. Therefore, we define effective jet density $P_{eff}$ involving the concept of near-field so that effective jet diameter can be extended to the near-field region. In this condition, we modify the correlation and compare with experimental data.

• 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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• 2014.11a
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• pp.133-134
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• 2014

The full transport equation approach for laminar non-premixed flame with detailed chemistry, soot and radiation has an advantage in accuracy and describing for emission pathway, but this approach requires the excessive computational cost especially for a higher-order hydrocarbon fuel flames. On the other hand, the standard flamelet model has an efficiency and accuracy for non-premixed flame, though this model is not suitable for simulating slow processor like soot and radiation in laminar non-premixed flame situation. To overcome this limitation, modified transient flamelet model is developed which coupled with two-equation soot model involved in soot formation and evolution mechanism such as nucleation, surface growth, oxidation and agglomeration.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.485-490
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• 2004

Sawtooth mixing device used in a non-premixed burner is evaluated for flame stabilization and NO$_{x}$ reduction. Three mixers with different blade angles are tested. Methane is delivered through the fuel jet and air passes through the co-flow annulus. The flame mode changes (attached flame, lifted flame and extinction) against the fuel flow speed are measured, and the stability diagram is drawn. Moreover, by traversing thermocouple and sampling probe in the flame, the distribution of temperature and NO$_{x}$ mole fraction are measured. With the change in blade angle, flame shape, flame stabilization, the distribution of temperature and NO$_{x}$ mole fraction are changed considerably.rably.

• 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.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.7
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• pp.477-485
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• 2009

The study of nitrogen dilution effect on the flame stability was experimentally investigated in a non-premixed turbulent lifted hydrogen jet with coaxial air. Hydrogen gas was used as a fuel and coaxial air was used to make flame liftoff. Each of hydrogen and air were injected through axisymetric inner and outer nozzles ($d_F=3.65\;mm$ and $d_A=14.1\;mm$). And both fuel jet and coaxial air velocity were fixed as $u_F=200\;m/s$ and $u_A=16\;m/s$, while the mole fraction of nitrogen diluents gas was varied from 0.0 to 0.2 with 0.1 step. For the analysis of flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF laser diagnostics had been performed. The stabilization point was selected in the most upstream region of the flame base and defined as the point where the turbulent flame propagation velocity was equal to the axial component of local flow velocity. We found that the turbulent flame propagation velocity increased with the decrease of nitrogen mole fraction. We concluded that the turbulent flame propagation velocity was expressed as a function of turbulent intensity and axial strain rate, even though nitrogen diluents mole fraction was changed.

• Journal of the Korean Society of Combustion
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• v.17 no.1
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• pp.22-29
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• 2012

The detachment stability characteristics of syngas $H_2$/CO jet attached flames were studied. The flame stability was observed while varying the syngas fuel composition, coaxial nozzle diameter and fuel nozzle rim thickness. The detachment stability limit of the syngas single jet flame was found to decrease with increasing mole fraction of carbon monoxide in the fuel. In hydrogen jet flames with coaxial air, the flame detachment stability was found to be independent of the coaxial nozzle diameter. However, velocities of appearance of liftoff and blowout velocities of lifted flames have dependence. At lower fuel velocity range, the critical coaxial air velocity leading to flame detachment increases with increasing fuel jet velocity, whereas at higher fuel velocity range, it decreases. This increasing-decreasing non-monotonic trend appears for all $H_2$/CO syngas compositions (50/50~100/0% $H_2$/CO). To qualitatively understand the flame behavior near the nozzle rim, $OH^*$ chemiluminescence imaging was performed near the detachment limit conditions. For all fuel compositions, local extinction on the rim is observed at lower fuel velocities(increasing stability region), while local flame extinction downstream of the rim is observed at higher fuel velocities(decreasing stability region). Maximum values of the non-monotonic trends appear to be identical when the fuel jet velocity is normalized by the critical fuel velocity obtained in the single jet cases.

• 한국연소학회:학술대회논문집
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• 2014.11a
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• pp.211-213
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• 2014

Simultaneous measurements of planar laser-induced fluorescence (PLIF) of OH radicals and particle image velocimetry (PIV) were used to investigate the strain rates and OH structure characteristics of turbulent syngas non-premixed jet flames close to blowoff. Mean values of the maximum principal strain rate on OH layer decreases with the axial distance, and its standard deviation is significantly large upstream. Strain rate on stabilization region of the stable flame is only about a half of that of the flame near blowoff.

• Journal of the Korean Society of Combustion
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• v.10 no.4
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• pp.33-40
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• 2005

The propagation rates of advancing and retreating non-premixed edge flames in a slot-jet counterflow were measured as a function of strain rate for varying jet spacing, mixture strength, stoichiometric mixture fractions $(Z_{st})$ and Lewis numbers (Le). Methane and propane fuels were tested and nitrogen and carbon dioxide were used as inerts. As results, we could identify igniting fronts, retreating fronts, two total extinction limits, and short-length edge flames. A burner separation affected to a low extinction limit only. Regimes for advancing and retreating edges together with total extinction were mapped in terms of normalized flame thickness and heat loss factor for $CH_4/O_2/N_2$ mixtures. Edge flames for $Z_{st}$ > 0.5 behaved like a stronger mixture while for $Z_{st}$ < 0.5 showed deteriorated feature, because of relative locations of a non-premixed flame and intermediate species such as CO and $H_2$. Furthermore, due to the relative importance of heat loss, propagating speeds of edge flames were significantly enhanced in $CH_4/O_2/CO_2$ mixtures (Le < 1) demonstrating increasing stability limits. However $C_3H_8/O_2/N_2$ mixtures (Le > 1) showed opposite result.