• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.9
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• pp.1218-1229
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• 1997

The diffusional-thermal instability of diffusion flames in the premixed-flame regime is studied in a constant-density two-dimensional counterflow diffusion-flame configuration, to investigate the instability mechanism by which periodic wrinkling, travelling or pulsating of the reaction sheet can occur. Attention is focused on flames with small departures of the Lewis number from unity and with small values of the stoichiometric mixture fraction, so that the premixed-flame regime can be employed for activation-energy asymptotics. Cellular patterns will occur near quasisteady extinction when the Lewis number of the more completely consumed reactant is less than a critical value( ~ =0.7). Parametric studies for the instability onset conditions show that flames with smaller values of the Lewis number and stoichiometric mixture fraction and with larger values of the Zel'dovich number tend to be more unstable. For Lewis number greater than unity, near-extinction flame are found to exhibit either travelling instability or pulsating instability.

• 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 Combustion
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• v.18 no.2
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• pp.1-7
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• 2013

Flame stabilization characteristics of premixed flames in narrow annular coaxial tubes (NACT) were investigated experimentally. The NACT burner was proposed as a model of a cylindrical refractory burner, and it was made of quartz tubes. Flame stabilization conditions affected by the characteristic length of the burner was investigated with the variation of the equivalence ratio and the flow rates. Flame behaviors in narrow spaces could be directly observed. Conclusively, more wide flame stabilization conditions could be obtained at the case of the decreased channel scale. A flame instability, such as combustion noise was detected concerned with the flame oscillation observed at the surface of multi channel stage. Some flame propagation characteristics had complicated tendencies that may exist in practical porous-media combustors. Therefore, this NACT burner can be a basic configuration for the development of flame stabilization model in porous media combustor, and it will enhance our understanding about the behavior of flames in meso-scale combustion spaces.

• Journal of the Korean Society of Combustion
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• v.18 no.3
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• pp.24-30
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• 2013

Flame stabilization characteristics of premixed flames in an annular coaxial 5-tubes burner (AC5TB) were investigated experimentally. The AC5TB was made of five quartz tubes, and the flame stabilization conditions in that burner were investigated with the variation of equivalence ratio and the flow velocities. Flame behaviors inside of narrow annular tubes could be observed directly. Overall flame stabilization conditions were similar to that of the previous study, while the flame behaviors and structures were different mainly due to the controlled uniform distribution of the velocities in channels. Flame flashback conditions were thought to be governed by the competition between heat release rate, heat loss and heat recirculation in each channel. Stationary flames at a fixed location were compared in its velocity distribution and burned gas temperature across the channel. This AC5TB can be a basic configuration for the development of flame stabilization model of porous media combustors, and it will help understand about the real behavior of flames in meso-scale combustion spaces.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.7
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• pp.818-825
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• 2004

In this paper, the study of effects of flow parameters on flame structure and NOx emission concentration was performed in co-axial. laminar partially premixed methane/air flames. Such (low parameters as equivalence ratio(${\Phi}$), fuel split percentage($\sigma$), and mixing distance(x/D$\_$i/) were defined as a premixing degree and varied within ${\Phi}$=1.36∼9.52, $\sigma$=50∼100, and x/D$\_$i/=5∼20. The image of OH$\^$*/ and NOx concentration were obtained with an ICCD camera and a NOx analyzer. The flame structure observations show a categorization of partially premixed flames into three distinct flame regimes corresponding to ${\Phi}$<1.7(premixed flame structure), 1.7<${\Phi}$<3.3(hybrid structure), and ${\Phi}$>3.3(diffusion flame structure existing a luminous sooting region) at $\sigma$=75%, and x/D$\_$i/=10. As o decreases from 100% to 50%, and x/D$\_$i/ decreases, nonpremixed flame structure appear at low equivalence ratio relatively. In addition, the measured emissions for NOx rise steeply from ${\Phi}$=1.7, to ${\Phi}$=3.3, then constants ${\Phi}$>4.76. NOx emissions decrease with increase the level of premixing level. In conclusion, the main effect on flame structure and NOx production was at first equivalence ratio(${\Phi}$), and next fuel split percentage($\sigma$), and finally mixing distance(x/D$\_$i/).

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2372-2377
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• 2007

Characteristics of turbulent lifted flames in coflow jets with the varying initial temperature have recently been investigated about only propane case diluted by nitrogen. The investigation has firstly improved a premixed flame model and a large scale mixing model among competing theories on the stabilization mechanism of turbulent flame to be suitable for a high temperature condition. In this research, about methane with good availability to apply for a practical combustor as clean fuel, its characteristics of turbulent nonpremixed flame have been studied experimentally. The results have shown an effectiveness of the premixed flame model and the large scale mixing model considered initial temperature variation. Additionally, considering the axial distance where the mean fuel concentration falls below the stoichiometric level along the center line of the jet according to diluting nitrogen, the premixed flame model have more accurately been improved.

• 한국연소학회:학술대회논문집
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• 2004.11a
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• pp.97-106
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• 2004

Stability of laminar premixed planar flames inclined in gravitational field which generate vorticity is asymptotically examined. The flame structure is resolved by a large activation energy asymptotics and a long wave approximation. The coupling between hydrodynamics and diffusion processes is included and near-unity Lewis number is assumed. The results show that as the flame is more inclined from the horizontal plane it becomes more unstable due to not only the decrease of stabilizing effect of gravity but also the increase of destabilizing effect of rotational flow. The obtained dispersion relation involves the Prandtl number and shows the destabilizing effect of viscosity. The analysis predicts that the phase velocity of unstable flame wave depends on not only the flame angle but also the Lewis number. For relatively short wave disturbances, still much larger than flame thickness, the most unstable wavelength is nearly independent on the flame angle and the flame can be stabilized by gravity and diffusion mechanism.

• Journal of the Korean Society of Combustion
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• v.9 no.4
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• pp.9-21
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• 2004

Stability of laminar premixed planar flames inclined in the gravitational field is asymptotically examined. The flame structure is resolved by a large activation energy asymptotics and a long wave approximation. The coupling between hydrodynamics and diffusion processes is included and near-unity Lewis number is assumed. The results show that as the flame is more inclined from the horizontal plane it becomes more unstable due to not only the decrease of stabilizing effect of gravity but also the increase of destabilizing effect of rotational flow. The obtained dispersion relation involves the Prandtl number and shows the destabilizing effect of viscosity. The analysis predicts that the phase velocity of unstable flame wave depends on not only the flame angle but also the Lewis number. For relatively short wave disturbances, still much larger than flame thickness, the most unstable wavelength is nearly independent on the flame angle and the flame can be stabilized by gravity and diffusion mechanism.

• 한국연소학회:학술대회논문집
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• 2012.11a
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• pp.325-328
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• 2012

Radiation effects on the partially premixed methane flames diluted with water vapor in the air stream were numerically investigated. OPPDIF code and GRI-v3.0 were used in the numerical simulation. Adiabatic condition was compared with two different radiation models, optically-thin and WSGGM models. It was found that the radiation effect on the flame structure for the equivalence ratio (${\Phi}$) of 2.5 was less than ${\Phi}=1.5$. Extinction limit was not affected significantly, however, local flame structure was markedly influenced by the radiation models as increasing the water vapor concentration.

• 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.