• 대한기계학회논문집B
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• 제28권5호
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• pp.569-579
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• 2004

The lift-off characteristics of lifted laminar propane jet flames highly diluted with nitrogen are investigated introducing acoustic forcing with a fuel tube resonance frequency. A flame stability curve is obtained according to forcing strength and the nozzle exit velocity for N2 diluted flames. Flame lift-off behavior is globally classified into three regimes; 1) a weakly varying partially premixed behavior caused by a collapsible mixing for large forcing strength, 2) a coexistent behavior of the edge flame and a weakly varying partially premixed behavior for moderate forcing strength, and 3) edge flame or triple flame behavior for small forcing. It is shown that the laminar lifted flame with forcing affects flame lift-off behavior considerably, and is also clarified that the flame characteristic of flame base is well described with the penetration depth of the degree of mixing, ${\gamma}$$\_$$\delta$/. It is also confirmed that the weakly varying partially premixed flame caused by a collapsible mixing fur large forcing strength behaves as that just near flame blow-out in turbulent lift-off flame.

• 한국전산구조공학회논문집
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• 제30권1호
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• pp.47-53
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• 2017

1차원 등엔트로피 모델과 통합된 경계층 적분법은 초음속 노즐의 설계과정에서 내열재 표면의 열전달을 예측하는데 효과적으로 사용되고 있지만 노즐 목과 같이 2차원 효과와 경계층과 노즐 코어유동의 상호작용이 발생하는 지점에서는 경계층 외부유동 해석의 부정확성으로 해석의 정확도가 감소한다. 따라서 본 연구에서는 경계층 적분법을 이용한 열전달 예측의 정확도를 향상시키기 위해 CFD를 이용하여 2차원 효과와 노즐 코어유동의 상호작용이 고려된 경계층 외부유동 조건을 도출하고 이를 경계조건으로 하는 해석기법을 개발하였다. 오일러 모델과 SST $k-{\omega}$ 모델을 CFD로 해석하여 경계조건으로 적용했으며 계산방법을 검증하기 위해 선행문헌의 실험노즐에 대해 해석을 수행하였다. 계산 결과 CFD를 통해 경계층 외부유동 조건을 도출한 해석에서 노즐 열전달의 정확도가 향상되는 것을 확인하였으며 특히 노즐 목 후방과 팽창부에서의 차이가 크게 나타났다. SST $k-{\omega}$모델로 도출된 계산결과는 1차원 등엔트로피 모델과 비교 시 팽창부에서 실험결과와의 오차가 16% 감소하였다. 본 연구에서 개발된 해석기법은 향후 로켓노즐의 내열설계에 유용하게 사용될 것으로 평가된다.

• Journal of Advanced Marine Engineering and Technology
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• 제33권2호
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• pp.259-266
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• 2009

An experimental study was conducted to investigate the effects of forcing amplitude on the flow structure near the nozzle exit of forced jet diffusion flames. The jet was excited up to the blowout occurrence by a considerable large amplitude with a periodic velocity fluctuation at the tube resonating frequency. In the attached flame regime, we disclosed the very interesting result newly that adding of a moderate forcing amplitude caused the jet flame to become longer in spite of being forced. Particular attention is focused on the turnabout mechanism of vortex roll-up around the elongated flame, which has not been reported previously, and on the inner coherent structure of the forced jet in the attached flame regime. From the velocity and flow visualization results, it was ascertained that the surrounding air due to the occurrence of negative velocity parts was suck into the fuel nozzle. To aid in understanding the rotating phenomenon of coherent structure, we present a schematic diagram of the turnabout mechanism of vortex roll-up. The mechanism of vortex turnabout phenomenon can be easily understood by considering the positive and negative velocity amplitudes about the instantaneous velocity of the forcing flow, as shown in this diagram.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.365-368
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• 2002

The design of industrial arc plasma systems is still largely based on trial and error although the situation is rapidly improving because of the available computational power at a cost which is still fast coming down. The desire to predict the behavior of arc plasma system, thus reducing the development cost, has been the motivation of arc research. To interrupt fault current, the most enormous duty of a circuit breaker, is achieved by separating two contacts in a interruption medium, $SF_{6}$ gas or air etc., and arc plasma is inevitably established between the contacts. The arc must be controlled and interrupted at an appropriate current zero. In order to analyze arc behavior in $SF_{6}$ gas circuit breakers, a numerical calculation method combined with flow field and electromagnetic field has been developed. The method has been applied to model arc generated in the Aachen nozzle and compared the results with the experimental results. Next, we have simulated the unsteady flow characteristics to be induced by arcing of AC cycle, and conformed that the method can predict arc behavior in account of thermal transport to $SF_{6}$ gas around the arc, such as increase of arc voltage near current zero and dependency of arc radius on arc current to maintain constant arc current density.

• 대한기계학회논문집B
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• 제26권10호
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• pp.1348-1354
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• 2002

We investigate the flame behavior and spatial distribution of OH, PAHs and soot in a confined buoyant diffusion flame with decrease of the coflowing air flow rate. Direct photographs and Schlieren images represent that flame is Ally occupied by blue flame and becomes unstable, which is partially detached to the fuel nozzle tip in a near extinction flame under extremely reduced oxidizer condition. Laser induced fluorescence profiles clearly shows that OH is still generated in near-extinction flame, although intensity becomes weak with decreasing air flow rate. But soot scattering image cannot be seen any more in an oxidizer deficient ambience and simultaneously the PAHs are widely distributed downstream. These results are due to that a decrease of oxygen concentration in the combustion chamber leads to a temperature drop of flame, as a consequence, to a delay in soot growth and to a expanding of the PAHs, as soot precursors.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.871-877
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• 2001

Experimental study was conducted to investigate the effects of axial forcing on the flame structures near the nozzle exit of non-premixed flame. The most notable observation is that the direction of vortical motions is changed at some ranges, according to the increase of excitation amplitude. Especially, the elongation flame and the phenomenon of In-burning are always occurred when the vortical motion turnabout. In an analysis of the flame/flow visualization by means of direct photography and RMS technique, a plausible explanation can be made that above phenomena are related only to the amplitude of phase average velocity between the instantaneous velocity elements of excited flow.

• 한국연소학회지
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• 제14권1호
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• pp.1-8
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• 2009

Experiment is conducted to grasp effects of flame curvature on flame behavior in laminar lifted-jet flames. Nozzle diameters of 0.1 and 1.0mm are used to vary flame curvature of edge flame. There exist three types of edge flame oscillation. These edge flame oscillations may be caused by radial heat loss at all flame conditions, by fuel Lewis numbers near or larger than unity with the help of appreciable radial conduction heat loss, and by buoyancy effects. These are confirmed by the analysis of oscillation frequencies. It is however seen that the change of lift-off height through edge-flame oscillation is mainly due to radial heat loss irrespective of Lewis number and buoyancy.

• 한국연소학회지
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• 제22권3호
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• pp.17-22
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• 2017

An experimental study has been conducted to elucidate the effect of AC electric field on behaviors of laminar lifted flame in nitrogen-diluted methane coflow-jets. Our concerns are focued on the regime to show a decrease in liftoff height, $H_L$ with increasing nozzle exit velocity, $U_O$ (hereafter, $decreasing-H_L$). The $H_L$ with $U_O$ near flame extinction were measured by varying the applied AC voltage, $V_{AC}$ and frequency, $f_{AC}$ in a single electrode configuration. The behavior of $H_L$ with a functional dependency of $V_{AC}$ and $f_{AC}$ was categorized into two regime : (I) $H_L$ decreased for nozzle diameter, D = 1.0 mm, and (II) $H_L$ increased in the increase of $f_{AC}$ for a fixed $V_{AC}$ in a D = 4.0, 8.4 mm. The lifted flames in $decreasing-H_L$ region was unstable in high voltage regimes while the $H_L$ showed a decreasing tendency with $U_O$ except them. Such behaviors in $H_L$ were also characterized by functional dependencies of related physical parameters such as $V_{AC}$, $f_{AC}$, $U_O$, fuel mole fraction ($X_{F.O}$) and D.