• 한국연소학회지
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• 제18권2호
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• pp.42-50
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• 2013

Linear stability analysis of radiating counterflow diffusion flames is numerically conducted to examine the instability characteristics of cellular patterns. Lewis number is assumed to be 0.5 to consider diffusional-thermal instability. Near kinetic limit extinction regime, growth rates of disturbances always have real eigen-values and neutral stability condition of planar disturbances perfectly falls into quasi-steady extinction. Cellular instability of disturbance with transverse direction occurs just before steady extinction. However, near radiative limit extinction regime, the eigenvalues are complex and pulsating instability of planar disturbances appears prior to steady extinction. Cellular instability occurs before the onset of planar pulsating instability, which means the extension of flammability.

• 한국연소학회지
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• 제16권2호
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• pp.23-32
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• 2011

To investigate cell formation in hydrocarbon/hydrogen/carbon monoxide-air premixed flames, the outward propagation and cellular instabilities were experimentally studied in a constant pressure combustion chamber at room temperature and elevated pressures. Unstretched laminar burning velocities and Markstein lengths of the mixtures were obtained by analyzing high-speed schlieren images. In this study, hydrodynamic and diffusional- thermal instabilities were evaluated to examine their effects on flame instabilities. The experimentally-measured unstretched laminar burning velocities were compared to numerical predictions using the PREMIX code. Effective Lewis numbers of premixed flames with methane addition decreased for all of the cases; meanwhile, effective Lewis numbers with propane addition increased for lean and stoichiometric conditions and increased for rich and stoichiometric cases for hydrogen-enriched flames. With the addition of propane, the propensity for cell formation significantly was diminished, whereas cellular instabilities for hydrogen-enriched flames were promoted. However, similar behavior of cellularity was obtained with the addition of methane to the reactant mixtures.

• 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.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2012년도 제45회 KOSCO SYMPOSIUM 초록집
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• pp.207-209
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• 2012

The present experiment was conducted to measure the unstretched laminar burning velocity and cellular instability of DME-air and syngas (in steps of 25 %) added DME-air premixed flames using propagating spherical flame. The experimental results were discussed in two focuses which are effects of syngas fraction and initial pressure on Markstein length, unstretched laminar burning velocities, and cellular instability. The flame instability was evaluated by the Markstein length and cellularity which is caused by diffusional-thermal instability and hydrodynamic instability.

• 대한기계학회논문집B
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• 제21권9호
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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.

• 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.

• 한국수소및신에너지학회논문집
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• 제15권1호
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• pp.12-22
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• 2004

In order to evaluate the potential of partial hydrocarbon substitution to improve the safety of hydrogen use in general and the performance of internal combustion engines in particular, the outward propagation and development of surface cellular instability of spark-ignited spherical premixed flames of mixtures of hydrogen, hydrocarbon, and air were experimentally studied at NTP (normal temperature and pressure) condition in a constant-pressure combustion chamber. With propane being the substituent, the laminar burning velocities, the Markstein lengths, and the propensity of cell formation were experimentally determined, while the laminar burning velocities and the associated flame thicknesses were computed using a recent kinetic mechanism. Results show substantial reduction of laminar burning velocities with propane substitution, and support the potential of propane as a suppressant of both diffusional-thermal and hydrodynamic cellular instabilities in hydrogen-air flames.

• 대한기계학회논문집B
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• 제38권6호
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• pp.493-500
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• 2014

Damk$\ddot{o}$hler수가 클 때 복사열손실에 의한 소염근처에서 셀모양의 대향류확산화염의 특성에 대하여 수치해석적으로 연구하였다. Lewis 수를 0.5로 두고 일차원 정상상태의 화염의 해에 매우 작은 교란을 가하여 시간에 따른 화염전개를 계산하였다. 천이과정 초기에는 선형안정성 해석에서 예측된 결과와 매우 비슷하게 진행된다. 시간이 증가함에 따라 증가율이 가장 강한 파동수를 갖는 교란파가 성장하고, 완전히 발달되면 소염영역과 화염영역이 번갈아 나타나는 셀모양의 화염구조를 갖는다. 화염온도는 총엔탈피의 국소 이득 때문에 일차원 정상상태의 화염온도보다 높다. 셀모양의 확산화염은 Damk$\ddot{o}$hler 수가 증가함에 따라 셀의 모양이 원형으로 되며 일차원 정상상태 소염조건보다 큰 Damk$\ddot{o}$hler 수에서도 셀모양의 화염은 꺼지지 않고 살아남는다.

• 대한기계학회논문집B
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• 제35권2호
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• pp.179-188
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• 2011

본 연구에서는 합성가스-공기 화염의 셀 불안정성에 있어서 탄화수소 연료의 첨가효과를 알아보기 위하여 상온, 고압, 정적상태의 연소실에서 실험을 수행하였다. 층류화염전파속도는 상세반응기구와 전달물성치를 사용하여 계산하였고 이를 실험으로 측정된 값과 비교하였다. 탄화수소 연료가 첨가된 합성가스-공기 화염의 셀 불안정성은 수력학적 불안정성과 확산-열 불안정성의 관점에서 평가되며 희박예혼합 화염에 대해 실험으로부터 측정된 셀불안정성을 유발하는 임계 Peclet 수는 이론적으로 얻어진 값과 비교하였다. 실험결과는 반응혼합물에 탄화수소 계 연료의 첨가량이 증가할수록 화염전파속도는 감소함을 보였다. 합성가스-공기화염에 프로판과 부탄을 첨가하였을 경우 수력학적 불안정성과 확산-열 불안정성이 감소하여 셀 형성은 현저하게 감소하였다. 반면 메탄을 첨가하였을 경우 셀 불안정성이 완화되는 효과는 없었다.