• 한국연소학회지
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• 제10권1호
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• pp.13-19
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• 2005

Measurements of flame length and NOx emissions have been conducted to investigate the effects of acoustic excitation on flame structure in turbulent hydrogen diffusion flames with coaxial air. When the acoustic excitation of a specific frequency is applied to coaxial air stream, flame length is dramatically reduced, resulting in reduction of flame residence time. Consequently, EINOx could decrease up to 35 % and this shows that acoustic excitation is effective in reducing NOx emissions. Mie scattering technique has been used to visualize the vortex structure induced by acoustic excitation and vortex formation, development and destruction were observed quantitatively. As a result, vortex entrains coflow air into fuel stream and mixing rate between fuel and air is significantly enhanced, which may contribute to reduction of NOx emissions.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2002년도 제25회 KOSCI SYMPOSIUM 논문집
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• pp.1-9
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• 2002

Simultaneous PIV and OH PLIF measurements are used for shear strain rates and flame locations, respectively. It is believed that the shear strain rates represent flow characteristics such as turbulence intensity and the OH intensity indicates the flame characteristics such as burning velocities. However, these are still lack of geometric information, which may be very important to flame quenching Hence, fractal dimensions 'Df) of the OH images are adopted as an additional information. Finally, the flame structure diagram proposed in this research has three parameters, which consist of strain rates, OH intensities and fractal dimensions. The results show that this diagram classifies turbulent premixed flames more effectively based on flame structures. The regime of weak turbulence is limited to narrow strain ranges and has the fractal dimension of about 2 In the regime of moderate turbulence, OH intensities increase as strain rates increase and the values of fractal dimensions are 1.8 Df 1.95. The regimes of thickened reaction and flame extinction (quenching) show bell-shaped and their values of fractal dimensions are 1.5 Df 1.7 and 0.9 Df 0.6, respectively.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2007년도 춘계학술대회
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• pp.775-778
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• 2007

The present study numerically investigate the detailed structure of the syngas diffusion flames. In order to realistically represent the turbulence-chemistry interaction, the transient flamelet model has been applied to simulate the combustion processes and $NO_X$ formation in the syngas turbulent nonpremixed flames. The single mixture fraction formulation is extended to account for the effects of the secondary inlet mixture. Computations are the wide range of syngas compositions and oxidizer dilutions. Based on numerical results, the detailed discussion has been made for the effects of syngas composition and oxidizer dilution on the structure of the syngas-air and syngas-oxygen turbulent nonpremixed flames.

• 한국화재소방학회논문지
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• 제28권4호
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• pp.35-43
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• 2014

3개의 난류모델과 3개의 연소모델로 구성된 9개의 모델조합을 이용하여 난류 부분예혼합 제트화염 구조에 대한 수치적 예측성능을 검토하였다. 이용된 난류모델은 표준 ${\kappa}-{\varepsilon}$ 모델(SKE), Realizable ${\kappa}-{\varepsilon}$ 모델(RKE) 및 Reynolds 응력모델(RSM)이며 연소모델들은 Eddy Dissipation Concept 모델(EDC), Steady Laminar Flamelet 모델(SLF)와 Unsteady Laminar Flamelet 모델(ULF)이다. 9개 모델조합의 예측성능을 평가하기 위하여 실험결과가 알려진 Sandia D 화염인 난류 부분예혼합 제트화염을 대상으로 수치계산을 수행하였다. 얻어진 결과로서, 화염길이의 예측은 RSM > SKE > RKE순으로 길게 예측하였으며, RKE 난류모델은 화염길이를 너무 과소 예측하는 것을 확인하였다. RSM + SLF과 RSM + ULF의 조합은 화염길이는 비교적 잘 예측하였지만 하류에서의 화염온도를 과대 예측하였다. 반면에 SKE와 연소모델의 조합에서 SLF 또는 ULF 조합은 화염길이 뿐만 아니라 하류에서의 화염온도도 비교적 잘 예측하였는 것을 확인하였다. 반경방향 화염온도 및 화학종 농도분포를 비교해 본 결과 SKE와 연소모델의 조합이 가장 예측성능이 뛰어났으며 SKE + ULF의 조합이 가장 우수한 예측성능을 갖는 것을 확인하였다.

• 동력기계공학회지
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• 제2권2호
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• pp.13-19
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• 1998

The purpose of study is to investigate the flame stability and Structure of turbulent diffusion flame behind the hollowed flame holder, which is located on the waste gas coming out from the test furnace. Fluctuating temperature and ion current measurement and their statistical treatment were used for the purpose. Three types of flame were stabilized and each of which were changed by adequate equivalence ration. And we found that have no periodicity near reacting zone.

• 한국연소학회지
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• 제11권4호
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• pp.27-35
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• 2006

The impacts of equivalence ratio on flow structure and flame dynamic in a model gas turbine combustor are investigated using large eddy simulation(LES). Dynamic k-equation model and G-equation flamelet model are employed as LES subgrid model for flow and combustion, respectively. As a result of mean flow field for each equivalence ratio, the increase of equivalence ratio brings about the decrease of swirl intensity through the modification of thermal effect and viscosity, although the same swirl intensity is imposed at inlet. The changes of vortical structure and turbulent intensity etc. near flame surface are occurred consequently. That is, the decrease of equivalence ratio can leads to the increase of heat release fluctuation by the more increased turbulent intensity and fluctuation of recirculation flow. In addition, the effect of inner vortex generated from vortex breakdown on the heat release fluctuation is increased gradually with the decrease of equivalence ratio. Finally, it can be identified that the variations of vortical structure play an important role in combustion instability, even though the small change of equivalence ratio is occurred.

• 대한기계학회논문집B
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• 제26권1호
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• pp.18-26
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• 2002

With the introduction of Fractal notation, various fields of engineering adopted fractal notation to express characteristics of geometry involved and one of the most frequently applied areas was turbulence. With research on turbulence regarding the surface as fractal geometry, attempts to analyze turbulent premised flame as fractal geometry also attracted attention as a tool for modeling, for the flame surface can be viewed as fractal geometry. Experiments focused on disclosure of flame characteristics by measuring fractal parameters were done by researchers. But robust principle or theory can't be extracted. Only reported modeling efforts using fractal dimension is flame speed model by Gouldin. This model gives good predictions of flame speed in unstrained case but not in highly strained flame condition. In this research, approaches regarding fractal dimension of flame as one representative value is pointed out as a reason for the absence of robust model. And as an extort to establish robust modeling, Presents methods treating fractal dimension as statistical variable. From this approach flame characteristics reported by experiments such as Da effect on flame structure can be seen quantitatively and shows possibility of flame modeling using fractal parameters with statistical method. From this result more quantitative model can be derived.

• 한국연소학회지
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• 제12권3호
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• pp.8-15
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• 2007

This study has numerically modelled the combustion processes of the turbulent swirling premixed lifted flames in the low-swirl burner (LSB). In these turbulent swirling premixed flames, the four tangentially-injected air jets induce the turbulent swirling flow which plays the crucial role to stabilize the turbulent lifted flame. In the present approach, the turbulence-chemistry interaction is represented by the level-set based flamelet model.. Two-dimensional and three-dimensional computations are made for the various swirl numbers and nozzle length. In terms of the centerline velocity profiles and flame liftoff heights, numerical results are compared with experimental data The three-dimensional approach yields the much better conformity with agreements with measurements without any analytic assumptions on the inlet swirl profiles, compared to the two-dimensional approach. Numerical clearly results indicate that the present level-set based flamelet approach has realistically simulated the. structure and stabilization mechanism of the turbulent swirling stoichiometric and lean-premixed lifted flames in the low-swirl burner.

• 대한기계학회논문집B
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• 제20권3호
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• pp.1050-1060
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• 1996

Recently, attention has been paid to the flame diagnostic by noncontact methods which dose not deform the flame shape. One of them is a method which is using the radical luminous intensity. Generally, this diagnostic method using radical luminous has been investigated its reliability by applying to laminar flame. This study, however, investigated each radical luminous signals through stocastical analysis like auto-correlation, cross-correlation, phase and coherence which were acquired from measuring radical luminous intensity of OH, CH, $O_{2}$, radicals in turbulent diffusion flame. To compare radical luminous intensity in flame with temperature, ion current and concentration , radious distribution of each properties was investigated and considered. In radical luminous intensity, correlation in the reaction zone of flame was higher than in correlation in combusted gas zone. And radious distribution of radical luminous intensity was corresponded with radious distribution of temperature, ion current and concentration. The result of the study confirms that a radical luminous flame diagnosis is possible in the turbulent diffusion flame.

• Journal of Mechanical Science and Technology
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• 제20권9호
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• pp.1459-1474
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• 2006

The present study is focused on the development of the RIF (Representative Interactive Flamelet) model which can overcome the shortcomings of conventional approach based on the steady flamelet library. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the RIF model can effectively account for the detailed mechanisms of $NO_x$ formation including thermal NO path, prompt and nitrous $NO_x$ formation, and reburning process by hydrocarbon radical without any ad-hoc procedure. The flamelet time of RIFs within a stationary turbulent flame may be thought to be Lagrangian flight time. In context with the RIF approach, this study adopts the Eulerian Particle Flamelet Model (EPFM) with mutiple flamelets which can realistically account for the spatial inhomogeneity of scalar dissipation rate. In order to systematically evaluate the capability of Eulerian particle flamelet model to predict the precise flame structure and NO formation in the multi-dimensional elliptic flames, two methanol bluffbody flames with two different injection velocities are chosen as the validation cases. Numerical results suggest that the present EPFM model has the predicative capability to realistically capture the essential features of flame structure and $NO_x$ formation in the bluff-body stabilized flames.