• Journal of the Korean Society of Combustion
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• v.5 no.2
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• pp.19-27
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• 2000

A turbulent combustion model, based on edge flame dynamics, is discussed in order to predict global extinction of turbulent flames. The model is applicable to the broken flamelet regime of turbulent combustion, in which global extinction of turbulent flame is achieved by gradual expansion of flame holes. The edge flame dynamics is the key mechanism to describe the flame hole expansion or contraction. For flames with Lewis numbers near unity, there is a $Damk{\ddot{o}}hler$ number, namely the crossover $Damk{\ddot{o}}hler$ number, at which edge flame changes its direction of propagation. The parametric region between the quasi-steady extinction condition and the edge-flame crossover condition is a metastable region, in that flames without edge can stay in their burning states while flames with edge have to retract to expand quenching holes. Using the above properties of edge flame, Hartley and Dold proposed a Lagrangian hole dynamics, which allows us to simulate transient variation of quenching holes. In their model, each stoichiometric surface is subjected to a random sequence of scalar dissipation rate compatible to the equilibrium turbulence. Then, each stoichiometric surface will evolve, according to the combustion map, dependent on the scalar dissipation rate and existence of flame edge, If all the burning surfaces are annihilated, the event can be declared as a global extinction. The consequence obtained from the above model also can be used as a subgrid model to determine local extinction occurring in a calculation grid.

• 한국연소학회:학술대회논문집
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• 2004.11a
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• pp.85-90
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• 2004

We have presented characteristics of a transitional behavior from a premixed flame to a triple flame in a lifted flame according to the change of equivalence ratio. The experimental apparatus consisted of a slot burner and a contraction nozzle for a lifted flame. As concentration difference of the both side of slot burner increases, the shape of flame changed from a premixed flame to a triple flame, and the liftoff height is decreased to the minimum value and then increase again. Around this minimum point, it is confirmed a transition regime from premixed flame to triple flame. Consequently, the experimental results of the liftoff height, flame curvature and luminescence intensity showed that the stabilized laminar lifted flame regime is categorized by regimes of premixed flame, triple flame and critical flame. In the visualization experiment of smoke wire, the flow divergence and redirection reappeared in premixed flame as well as triple flame. Thus we cannot express the flame front of lifted flame has a behavior of triple flame with only flow divergence and redirection. To differentiate triple flame and premixed flame, ${\Phi}$ value of partially premixed fraction is employed. The partially premixed fraction ${\Phi}$ was constant in premixed flame. In critical flame small gradient appears over the whole regime. In triple flame, typical diffusion flame shape is obtained as parabolic distribution type due to diffusion flame trailing.

• Transactions of the Korean Society of Automotive Engineers
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• v.9 no.1
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• pp.45-56
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• 2001

A constant volume combustion chamber is employed to investigate the initial flame kernel development and flame propagation of gasoline-air mixtures with various ignition systems, ignition energy and spark plug electrodes. To do this research, four ignition systems are designed and manufactured, and the ignition energy is controlled by varying the dwell time. Several kinds of spark plugs are also made to analyze the effects of electrodes on flame kernel development. The velocity of flame propagation is measured by the laser deflection method. The output laser beam from He-Ne laser is divided into three parallel beams by a beam splitter. The splitted beams pass through the combustion chamber. They are deflected when contacted with flame front, and the voltage signals from photodiodes change due to deflection. The results show that higher ignition energy raises the flame propagation speed especially under the fuel lean operation. The wider electrode gap, smaller electrode diameter and sharper electrode tip make the speed of the initial flame propagation faster. The speed of the initial flame propagation is affected by electrode material as well. Electrode material with lower melting temperature help the initial flame propagation.

• Journal of the Korean Society of Propulsion Engineers
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• v.16 no.6
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• pp.1-8
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• 2012

An experimental study has been conducted to scrutinize into the influence of ultrasonic standing wave on the propagating behavior and structural stabilization of propane/air premixed flame at various equivalence ratios in half-open rectangular duct. Evolutionary features of the flame fronts are caught by high-speed images, and the variation of flame structure and local flame velocities along the propagation are analyzed. It is revealed that the propagation velocity agitated by the ultrasonic standing wave is greater than that without the agitation: the velocity enhancement diminishes as the equivalence ratio approaches the stoichiometric. Influence of standing wave on the flame overwhelms that of the buoyancy which slants the flame front towards top of the duct, and thus the standing wave contributes to the structural stabilization of propane/air premixed flame.

• Journal of the Korean Society of Safety
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• v.27 no.6
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• pp.70-77
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• 2012

Experimental investigations were performed to examine the characteristics of propagating flame fronts around multiple bars within a rectangular chamber. The explosion chamber is 400 mm in height, $700{\times}700mm^2$ in cross-section and has a large top-venting area, $A_v$, of $700{\times}210mm^2$. This results in a value of 0.44 for $A_v/V^{2/3}$ and a L/D value of 0.57. The multiple obstacles of length 700 mm with a blockage ratio of 30 % were placed within the chamber. Temporally resolved flame front images were recorded by a high speed video camera to investigate the interaction between the propagating flame and the obstacles. Results showed that the flame propagation speeds before the flame impinges onto the obstacle almost equal to the laminar burning velocity. As the propagating flame impinged on the obstacle, the central region of flame began to become concave, this resulted in the flame deceleration in the region. As the flame interacted with the modified flow filed generated behind the central obstacle, the probability density functions(PDFs) of the local flame displacement speed were extensively distributed toward higher speeds.

• 한국전산유체공학회:학술대회논문집
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• 1998.11a
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• pp.204-210
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• 1998

A method is developed to include the effect of volume expansion in the description of the flame dynamics using G-equation. Line volume-source is used to represent the effect of the exothermic process of combustion with source strength determined by the density difference between the burned and the unburned region. Volume expansion adjusts the flow field to accommodate the increased volume flow rate crossing the flame front. Test result predicted the measured velocity field qualitatively. The method was applied to study the interaction of vortex and premixed flame. Increased volume expansion did not change the initial growth rate of flame area. However, the residence time and flame surface area increased with higher expansion ratios.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.12
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• pp.1701-1709
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• 1997

The diagnostic used to observe the early flame development was a fiber optic spark plug, which enabled measurement of the flame front arrival times on a cycle-to-cycle basis. The data obtained with this fiber optic spark plug were analyzed to obtain two parameters to describe the behavior of the flame kernel : an expansion speed and a convection velocity. In addition, synchronized cylinder pressure data were taken to compare with the fiber optic spark plug data on a cyclic basis. Heat release analysis was performed on the cylinder pressure data to obtain the mass burning profile of the charge for each cycle. There was a significant correlation observed in the initial flame duration and the kernel expansion speed with dwell angle.

• Journal of Advanced Marine Engineering and Technology
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• v.19 no.4
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• pp.34-41
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• 1995

The structure of premixed tubulent flames in a constant-volume vessel was investigated using a schlieren method and microprobe method. The schlieren method was used to observe the flame structure qualitatively. The microprobe method, which detects a flamelet by detecting its flame potential signal, was used to investigate the deeper flame structure behind the flame front. The flame potential signal having one to six peaks was obtained in the case of turbulent flames, each of them being regarede as a flamelet existing in the flame zone. Based on this consideration, the flame propagation speed, the thickness of the flame zone, the number of flamelets and the separation distance between adjacent flamelets in the flame zone were measured. Moreover, the thickness of flamelet which could not be attempted in the conventional electrostatic probe method was also investigated. The experimental results of this work suggest the existence of "reactant islands" in the reaction zone, and show that the averaged number of flamelets increases with an increase in the turbulence intensity and/or a decrease in the Damkohler number. The mean thickness of flamelet in the case of turbulent flames was found to be about two times compared to laminar values.ar values.

• Journal of the Korean Society of Combustion
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• v.6 no.1
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• pp.1-6
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• 2001

To evaluate the effect of lobed structure on pollutant emission, an experimental study examines NOx and CO emissions associated with four burner geometries, such as a conventional circular burner and three lobed ones. Rapid mixing allowed by the lobed burner to produce lean premixed flames, with narrower flame stability diagram than for the conventional circular one. Conventional circular burner of wide and uniform burner rim has an advantage of flame stabilization. Correlation on fuel discharge velocity for flame blowout should be included a variable related to the wall effect of the burner. NOx emission reduces by about 5% at the burner with lobed structure in fuel discharge side compared to conventional circular one. This is due to lower flame temperatures through flame elongation and increased radiative heat losses, caused by partially luminous flame in flame front. Meanwhile, at the burner with lobed structure in air discharge side and both fuel and air discharge sides, NOx emission somewhat increases with reduced radiative heat losses in spite of flame elongation. Therefore, the rapid mixing by lobed structure does not always have an advantage on NOx reduction.

• Journal of Advanced Marine Engineering and Technology
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• v.35 no.2
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• pp.204-215
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• 2011

Flame stability is the one of the main mechanism of laminar lifted flame and flame propagation velocity becomes a yardstick to measure the flame stability. Bilge has presented the flame propagation velocity of the triple flame and the flame stability mechanism related the flame configuration and mixture fraction. However, there was not able to observe all process of flame ignition and extinction for small nozzle diameter. In this paper, we have subdivided the flame configuration and stability mechanism and classified the flame behavior with a nozzle diameter. Also we have subdivided the 'triple flame propagation opened' and the 'triple flame propagation closed' from the triple flame propagation of triple flame criterion.