• Journal of Advanced Marine Engineering and Technology
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• v.23 no.4
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• pp.593-600
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• 1999

The structure of premixed turbulent flames in constant volume vessel was investigated by using a laser tomography. The flame structure was visualized by passing a laser sheet with 0.2mm thick and 2 cm wide through the turbulent flames to obtain their 2-D images. From the obtained images islands of reactants as well as of products were found at least in the 2-D images when the turbu-lence intensity was above 0.4m/s. Moreover in order to obtain the characteristic flamelet thickness the light intensities of them were digitized and processed into three colors incorporating two appro-priate threshold values in the image analysis. As the result the averaged value of charactertistic flamelet thickness was found to be about two times compared to laminar one.

• Journal of the Korean Society of Combustion
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• v.17 no.3
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• pp.46-52
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• 2012

3D DNS is performed for n-heptane spray turbulent jet combustion. Diesel-like conditions are considered including single and multiple injections. Conditional statistics are obtained for multiple Lagrangian flame groups of sequentially evaporating fuel. Each fuel group represents the conditional statistics of an independent Lagrangian flame group. Sequentially evaporating fuel goes through different histories and residence times over the ignition delay period. Multiple flame groups are required for accurate description of combustion of a spray jet that goes through a long injection duration or multiple injections.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.11a
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• pp.377-380
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• 2007

Large Eddy Simulation has been conducted to investigate both stable and unstable flame structures in a swirl turbulent combustor. While a flame is stabilized with periodic dynamic structure at 600K, a slight increase in the flame temperature of inlet mixture, 660K, lead to bifurcation of flame at swirl angle 45 degrees. It was observed that both swirl number and mixture temperature affect a flame bifurcation and the former is a major parameter. One major mechanism contributing to the unstable flame is that the local flame speed overshadows the local flow velocity near the wall of the combustor.

• Transactions of the Korean Society of Automotive Engineers
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• v.10 no.6
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• pp.51-58
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• 2002

The RPV(Reaction Progress Variable) combustion model has been applied to numerically investigate the effects of Damkohler number on the superequilibrium concentration and flame structure in the nonpremixed turbulent flames. Computations are performed for the two turbulent jet flames of CO/H$_2$/N$_2$(40/30/30 volume percent) having the same jet Reynolds number of 16,700 but different nozzle diameters(4.58mm and 7.72mm). The detailed discussions have been made for the interaction between fluid dynamics and chemistry in the flame field.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.5
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• pp.1269-1279
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• 1995

An experimental study is carried out on turbulent diffusion flames stabilized by a circular cylinder in a divergent duct flow. A commercial grade gaseous propane is injected from two slits on the rod as fuel. Flame stability limits, as well as size and temperrature of recirculation zone, are measured by direct and schlieren photographs to clarify the characteristics and structure of diffusion flames and to assess the effect of various divergent angle of duct. The results of the present study are as follows. Temperature in the recirculation zone decreases with increasing divergent angle. The blow-off velocity in parallel duct is higher than that in divergent duct. Critical blow-off velocity is expected to be about 8-12 degree through blow-off velocity pattern. Regardless of divergent angles, the length of recirculation zone is nearly constant, and this length becomes longer with rod diameter. Pressure gradient has an effect on the eddy structure in shear layer behind the rod. With the increase of divergent angle, large scale eddies by dissipated energy in shear layer are split into small scale eddies, and the flame becomes a typical distributedreacting flame.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.1
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• pp.125-132
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• 2002

An experimental study on the characteristics of stability of propane turbulent nonpremixed jet flames discharged normal to air free-streams with uniform velocity profile is conducted. Experimental observations are focused on the flame shape, the stability considering two kinds of flame, lift-off distance, and the flame length according to velocity ratio. In order to investigate the mixing structure of the flame base at the lower limit, we employ the RMS technique and measure the species concentration by a gas chromatography. In the results of the stability curve and lifted flame, it is fecund that the dependency of nozzle diameter is closely related to the large-scale vortical structure representing counter-rotating vortices pair. Also, the detailed discussion on the phenomenon of blowout due to this large vortical motion, is provided.

• 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.

• Journal of Advanced Marine Engineering and Technology
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• v.13 no.4
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• pp.33-39
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• 1989

A turbulent premixed flames of layer formed between burned hot gas and unburned mixture were investigated by means of schlieren photograph with fluctuations of temperature and ion current. The combustion intensity between burned hot gas and shear layer was higher than the intensity between unburned mixture and shear layer. A wrinkled laminar flame and flamelet were appeared at downstream to exist and distributed reaction zone was at upstream as a result of analyzed probability density functions of temperature fluctuation. The initial combustion intensity of reaction zone of eddy between burned hot gas and shear layer was higher than that of final, flowing downstream, and vice versa between unburned mixture and shear layer.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.11a
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• pp.393-396
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• 2008

The study of nitrogen dilution effect on flame stability was experimentally investigated in non-premixed turbulent lifted hydrogen jet with coaxial air. hydrogen gas was used as a fuel and coaxial air was injected to make flame liftoff. And both of the fuel jet and coaxial air velocity were fixed as $u_F$=200 m/s and $u_A$=16 m/s, while nitrogen diluents mole fraction was varied from 0 to 0.2. For the analysis of flame structure and flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF had been performed. It was found that the turbulent flame propagation velocity increased as decreasing of nitrogen mole fraction. We concluded that the turbulent flame propagation velocity was expressed as a function of turbulent intensity, even though the mole fraction of nitrogen diluents gas was changed.

• 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.