• 한국추진공학회지
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• 제20권6호
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• pp.1-10
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• 2016

연소불안정 능동제어를 위해서는 음향 발생기나 2차 연료 분사를 통해 압력 섭동이나 열방출 섭동에 변화를 주어야 한다. 2차 연료 분사의 위치 및 시점을 결정하기 위해서는 연소불안정 시 발생하는 열방출 섭동의 분포를 알아야 한다. 본 연구에서는 탄화수소 연료, 유입 속도, 당량비, 음향가진 조건을 변화시키며 위상에 따른 열방출 섭동의 분포를 실험적으로 측정하였다. 와류 발생에 따른 열방출 섭동은 $Damk{\ddot{o}}hler$ 수에 의해 크게 영향을 받는 것을 알 수 있었다. $Damk{\ddot{o}}hler$ 수가 대략 4 - 5 보다 큰 경우는 와류의 leading edge에서 hot spot이 trailing edge에서 cold spot이 발생하였다. 이와는 반대로 $Damk{\ddot{o}}hler$ 수가 3 보다 작은 경우는 반대의 경향이 나타남을 확인할 수 있었다.

• 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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• 제23권9호
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• pp.1185-1191
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• 1999

Radiation-induced oscillatory instability in diffusion flames is numerically investigated with nonlinear dynamics considered. As the simplest flame model, a diffusion flame established in the stagnant mixing layer is employed with optically thin gas-phase radiation and unity Lewis numbers for all species. Attention is focused on the radiation-induced extinction regime, which occurs at large $Damk\ddot{o}hler$ number. Once the steady flame structure is obtained for a prescribed value of the initial $Damk\ddot{o}hler$ number, transient solution of the flame is calculated after a finite amount of the $Damk\ddot{o}hler$-number perturbation is imposed on the steady flame. Transient evolution of the flame exhibits three types of flame-evolution behaviors, namely decaying oscillatory solution, diverging solution to extinction and stable limit-cycle solution. A dynamic extinction boundary is identified for laminar flamelet library.

• 한국연소학회지
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• 제19권2호
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• pp.21-27
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• 2014

Experimental study in laminar propane coflow jet flames has been conducted to investigate self-excitations. For various propane mole fractions and jet velocities, two types of self-excitation were observed: (1) buoyancydriven self-excitation (hereafter called BDSE) and (2) Lewis-number-induced self-excitation coupled with (1) (hereafter called LCB). The mechanism of Lewis-number-induced self-excitation (hereafter called LISE) is proposed. When the system $Damk\ddot{o}hler$ number was lowered, LISE was shown to be launched. The LISE is closely related to heat loss, such that it can be launched in even helium-diluted methane coflow-jet flame (Lewis number less than unity). Particularly, The LISE becomes significant as the $Damk\ddot{o}hler$ number decreases and heat-loss is excessively large.

• 한국연소학회지
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• 제17권4호
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• pp.51-59
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• 2012

Experimental study in coflow jet flames has been conducted to investigate the helium-dilution effect of coflow air on self-excitation. For various helium mole fractions and jet velocities, two types of self-excitation were observed: buoyancy-driven self-excitation and Lewis-number-induced self-excitation(here after called Le-ISE) coupled with buoyancy-driven one. The difference between buoyancy-driven and Le-ISE is clarified by using the Mie-scattering visualization as well as exploring the different features. The mechanism of Le-ISE is proposed. When the system Damk$\ddot{o}$hler number was lowered, Le-ISE is shown to be launched. Le-ISE is closely related to heat loss, in that it can be launched in even methane jet flame (Lewis number less than unity) with helium-diluted coflow air. Particularly, Le-ISE becomes significant as the Damk$\ddot{o}$hler number decreases and heat-loss becomes significant.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2014년도 제49회 KOSCO SYMPOSIUM 초록집
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• pp.175-178
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• 2014

The flame cell dynamics in 2-D opposed nonpremixed tubular configuration was investigated using high-fidelity numerical simulations. The diffusive-thermal instability occurs as the $Damk{\ddot{o}}hler$ number, Da, approaches the 1-D extinction limit of the tubular flames and several flame cells are generated depending on Da, and flame radius. In general, the number of flame cells are found close to the largest wave number from the linear stability analysis. It was also found from the displacement speed analysis that during the local flame extinction and cell formation, negative edge flame speed is observed due to small gain from reaction compared to large loss from diffusion.

• 한국연소학회지
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• 제1권2호
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• pp.41-49
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• 1996

A numerical study is conducted to investigate the periodically unstable shock induced combustion around blunt bodies in stoichiometric hydrogen-air mixtures. Euler equations are spatially discretized by upwind-biased third order scheme and temporally integrated by Runge-Kutta method. Chemistry model used in this study involves 8 elementary kinetics steps and 7 species. At a constant Mach number, the effects of projectile size, inflow pressure and inflow temperature are examined with Lehr#s experimental condition as a reference. In addition to oscillation frequency, characteristic distances and time averaged values are found from the result to find an relation with dimensionless parameters. As a result, it is found that the effects of inflow pressure and body size are very similar and $Damk{\ddot{o}}hler$ number plays an important role in determining the instability characteristics.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2002년도 제24회 KOSCO SYMPOSIUM 논문집
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• pp.147-153
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• 2002

Experimental investigations are performed on the stability and the structure of bluff-body stabilized hydrogen flames. The velocities of coflow air are varied from subsonic to supersonic velocity of Mach 1.8 and OH PLIF images and Schilieren images are used for analysis. Three characteristic flame modes are classified into three regimes with the variation of fuel-air velocity ratio; a jet like flame, a central-jet dominated flame and a recirculation zone flame. Stability curves are drawn to find the blowout regimes and to show that flame stability is improved by increasing the lip thickness of fuel nozzle that works as bluff-body. $Damk{\ddot{o}hler$ number is adopted in order to scale the blowout curves of each flame obtained at different sizes of the bluff-body and all blowout curves are scaled successfully regardless of its bluff-body size.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2002년도 제25회 KOSCI SYMPOSIUM 논문집
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• pp.25-31
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• 2002

For the better understanding of the stability of turbulent combustion, more researches on extinction and re-ignition are needed. Flame interactions in non-premixed flame have also not been greatly researched. We made a hybrid twin jet flame, the combinations of diffusion flame and partially-premixed diffusion flame, in a twin jet counterflow configuration. The extinction limits of a crossed twin jet counterflow have been extended in comparison with those of a one-dimensional counterflow because of flame interactions through heat transfer and joint ownership of various radicals. Besides, we have obtain ignition $Damk\"{o}hler$ number by experimental method without external ignition source using the extinction characteristic in a crossed twin jet counterflow flame. From results, we can identify the hysteresis between extinction and ignition $Damk\"{o}hler$ number in S-curve.

• 한국연소학회지
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• 제17권3호
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• pp.9-16
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• 2012

Nonlinear dynamics of pulsating instability in radiating counterflow diffusion flames is numerically investigated by imposing Damk$\ddot{o}$hler number perturbation. Stable limit-cycle solutions occur in small ranges of Damk$\ddot{o}$hler numbers past bifurcation point of instability. Period doubling cascade and chaotic behaviors appear just before dynamic extinction occurs. Nonlinear dynamics is also studied when large disturbances are imposed to flames. For weak steady flames, the dynamic extinction range shrinks as the magnitudes of disturbances are increased. However, strong steady flames can overcome relatively large disturbances, thereby the dynamic extinction range extending. Stable limit-cycle behaviors reappears prior to dynamic extinction when the steady flames are strong enough.