• 대한기계학회논문집B
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• 제26권5호
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• pp.695-702
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• 2002

Nonlinear dynamics of striped diffusion flames, by the diffusional-thermal instability with Lewis numbers sufficiently less than unity, is numerically investigated by examining various two-dimensional flame-structure solutions. The Lewis numbers for fuel and oxidizer are assumed to be identical and an overall single-step Arrhenius-type chemical reaction rate is employed in the model. Particular attention is focused on identifying the flame-stripe solution branches corresponding to each distinct stripe pattern and hysteresis encountered during the transition. At a Damkohler number slightly greater than the extinction Damkohler number, eight-stripe solution first emerges from one dimensional solution. The eight-stripe solution survives Damkohler numbers much smaller than the extinction Damkohler number until the transition to four-stripe solution occurs at the first forward transition Damkohler number. At the second forward transition Damkohler number, somewhat smaller than the first transition Damkohler number, the transition to two-stripe solution occurs. However, anu further transition from two-stripe solution to one-stripe solution is not always possible even if one-stripe solution can be independently accessed for particular initial conditions. The Damkohler number ranges for two-stripe and one-stripe solutions are found to be virtually identical because each stripe is an independent structure if distance between stripes is sufficiently large. By increasing the Damkohler number, the backward transition can be observed. In comparison with the forward transition Damkohler numbers, the corresponding backward transition Damkohler numbers are always much greater, thereby indicating significant hysteresis between the stripe patterns of strained diffusion flames.

• 한국자동차공학회논문집
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• 제10권6호
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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.

• Journal of Advanced Marine Engineering and Technology
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• 제19권4호
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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.

• 대한기계학회논문집B
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• 제34권9호
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• pp.859-866
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• 2010

대향류 확산화염에서 확산-전도 불안정성에 의한 맥동불안정성의 비선형 거동을 수치 해석적으로 연구하였다. Lewis 수를 1보다 충분히 크게 두고 일차원 준정상상태의 화염의 해로부터 Damkohler 수를 섭동시켜 시간에 따른 화염의 전개를 계산하였다. 맥동 불안정성에 의한 비선형 화염전개는 세 가지 다른 형태, 즉 교란이 점점 감소되는 경우, 교란이 증폭되어 안정된 주기적 진동이 일어나는 경우, 그리고 교란이 계속 증폭되어 화염이 소염되는 경우 등으로 나타났다. 스트레치를 받지 않는 화염의 결과와 달리 대향류 유동장의 화염에서는 안정된 한계순환 맥동 불안정이 존재하였다. 세 가지 다른 형태의 화염 전개를 보이는 임계 Damkohler 수를 계산하여 동적 소염이 일어나는 영역을 표시하였고, 이는 층류소화염의 국소소염 계산에 이용될 수 있다. 불안정성이 나타나는 갈래질의 구조는 초임계 및 임계이하 Hopf 갈래질로 나타났다. 특정한 Damkohler 수의 영역에서 안정된 한계 순환 갈래질이 나타났으며, 화염온도가 증가함에 따라 영역이 축소되어 안정된 한계순환이 일어나는 영역은 사라지고 불안정한 한계순환 갈래질이 나타났다. 안정된 한계순환 영역이 확장되는 영역이 존재하며, 이는 단순한 한계순환 불안정성이 주기배증에 의한 Rossler 갈래질이 나타나면서 한계 영역이 확장되었다.

• Journal of Mechanical Science and Technology
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• 제18권8호
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• pp.1470-1478
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• 2004

In this paper, a new 'presumed' Probability Density Function (PDF) approach coupled with a Lagrangian tracking method is proposed for turbulent combustion modeling. The test and the investigation of the model are conducted by comparing the model results with DNS data for a premixed flame subjected in a decaying turbulent field. The newly constructed PDF, which incorporates the instantaneous chemical reaction term, demonstrates consistent improvement over conventional assumed PDF models. It has been found that the time evolution of the mean scalar, the variance and the mean reaction rate are strongly influenced by a parameter deduced by a Lagrangian equation which takes into account explicitly the local reaction rate. Tests have been performed for a moderate Damkohler number, and it is expected the model may cover a broader range of Damkohler number. The comparison with the DNS data demonstrates that the proposed model may be promising and affordable for implementation in a moment-equation solver.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2004년도 제28회 KOSCO SYMPOSIUM 논문집
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• pp.124-132
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• 2004

Activation energy asymptotics (AEA) for Linan's diffusion-flame regime is revisited in this paper. The main purpose of the paper is to carefully re-examine each AEA analysis step in order to clarify the some concepts that are often misunderstood among the ordinary practitioners of the AEA. Particular attention is focused on the different AEA regimes arising from the double limit of large Zel'dovich and Damkohler numbers. In addition. the expansion procedures are shown in detail and the method that the turning point condition, commonly known as the Linan's extinction condition, is found is explained.

• 한국연소학회지
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• 제9권2호
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• pp.1-9
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• 2004

Activation energy asymptotics (AEA) for Linan#s diffusion-flame regime is revisited in this paper. The main purpose of the paper is to carefully re-examine each AEA analysis step in order to clarify the some concepts that are often misunderstood among the ordinary practitioners of the AEA. Particular attention is focused on the different AEA regimes arising from the double limit of large Zel#dovich and Damkohler numbers. In addition, the expansion procedures are shown in detail and the method that the turning point condition, commonly known as the Linan#s extinction condition, is found is explained.

• Journal of Mechanical Science and Technology
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• 제18권3호
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• pp.499-512
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• 2004

The effects of swirl intensity on non-reacting and reacting flow characteristics in a flat flame burner (FFB) with four types of swirlers were investigated. Experiments using the PIV method were conducted for several flow conditions with four swirl numbers of 0, 0.26, 0.6 and 1.24 in non-reacting flow. The results show that the strong swirling flow causes a recirculation, which has the toroidal structures, and spreads above the burner exit plane. Reacting flow characteristics such as temperature and the NO concentrations were also investigated in comparison with non-reacting flow characteristics. The mean flame temperature was measured as the function of radial distance, and the results show that the strong swirl intensity causes the mean temperature distributions to be uniform. However the mean temperature distributions at the swirl number of 0 show the typical distribution of long flames. NO concentration measurements show that the central toroidal recirculation zone caused by the strong swirl intensity results in much greater reduction in NO emissions, compared to the non-swirl condition. For classification into the flame structure interiorly, the turbulence Reynolds number and the Damkohler number have been examined at each condition. The interrelation between reacting and non-reacting flows shows that flame structures with swirl intensity belong to a wrinkled laminar-flame regime.

• 대한기계학회논문집B
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• 제28권12호
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• pp.1625-1631
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• 2004

Investigation on Flue Gas Recirculation(FGR) flame and Fuel Injection Recirculation(FIR) flame was performed with numerical method. Quantitative Reaction Path Diagram(QRPD) is utilized to compare the different chemistry effects between FGR flame and FIR flame. In order to compare flamelets in various oxygen-enrichment conditions reasonably, the adiabatic flame temperature and Damkohler number were held fixed by modulating the amount of diluents to fuel and oxidizer stream and by varying global strain rate of flame respectively. Basic flame structures were compared and characteristics of CH$_4$ decomposition and NO formation were analyzed based on QRPD analysis between FGR flame and FIR flame.

• 대한기계학회논문집B
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• 제26권2호
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• pp.336-344
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• 2002

In this study, the flow and combustion characteristics of flat flame burner with twirler were investigated. There are several factors that define the characteristics of burner. Among them, the experiments was focused on swirl effect by four types of twirler in terms of flow structure, distribution of temperature and emission characteristics. In PIV(Particle Image Velocimetry) experiment, the less of swirl number, axial flow is dominant at the center. As swirl number increases, the flow develops along the burner tile and backward flow becomes stronger at center. From the combustion characteristics, as long as combustion load increases, blow-off limit was improved. But at the higher swirl number, the limit is decreased. At swirl number 0, the temperature is shown typical distribution of long flame burner. but swirl number increases, the temperature distribution is uniform in front of round tile. Therefore, the temperature distribution is coincided with flow structure. As excess air ratio increases, NO concentrations are high. But high swirl number gives rise to become low NO concentrations. The flame characteristics are comprised in wrinkled laminar-flame regime according to turbulence Reynolds number(Rel) and Damkohler number(Da).