• 한국연소학회지
• /
• 제6권2호
• /
• pp.36-42
• /
• 2001

Usually, we use the flame thickness and turbulence scale to classify the flame structure on a phase diagram of turbulent combustion. The flame structure in turbulence is still in debate, and many studies have been done. Since the flame motion is rapid and its reaction zone thickness is very thin, it is difficult to estimate the flame thickness. Here, we propose a new approach to determine the reaction zone thickness based on ion current signals obtained by an electrostatic probe, which has enough time and space resolution to detect flame fluctuation. Since the signal depends on the flow condition and flame curvature, it may be difficult to analyze directly these signals and examine the flame characteristics. However, ion concentration is high only in the region where hydrocarbon-oxygen reactions occur, and we can specify the reaction zone. Based on the reaction zone existing, we estimate the reaction zone thickness. We obtain the thickness of flames both in the cyclone-jet combustor and on a Bunsen burner, compared with theoretically predicted value, the Zeldovich thickness. Results show that the experimentally obtained thickness is almost the same as the Zeldovich thickness. It is concluded that this approach can be used to obtain the local flame structure for modeling turbulent combustion.

• 대한기계학회논문집B
• /
• 제28권11호
• /
• pp.1417-1423
• /
• 2004

The thickness of laminar flame and preheat zone was computed from equation with burning velocity and the temperature profile, which is obtained by using premix code of Chemkin program for ethanol-air mixture. The computations were carried out under the unburned gas pressure 0.5bar-30bar and temperature of 300k-700K at 1.0. A difference flame thickness showed between temperature profile and equation with burning velocity. The ratio of flame thickness derived from the equation was about 45∼65% of the temperature profile, and the thickness of preheat zone was about 67.1% of the flame thickness. The flame thickness was decreased by increasing the pressure and temperature, but the effect of pressure is more significant than the effect of temperature on the flame thickness. The flame thickness was predicted by using the following equation. X(mm) = $X_{st}$ (T/300)$^{-0}$.65/(P)$^{-0}$.68/ (0.5bar$\leq$P$\leq$30bar, 300K$\leq$T$\leq$700K)K)

• 대한기계학회논문집B
• /
• 제27권9호
• /
• pp.1201-1208
• /
• 2003

The thickness of flame and preheat zone from burning velocity which was computed by using Premix code of Chemkin program for methane-air mixture. Also the thickness was evaluated from temperature profile which is also obtained from Premix code for the equivalence ratio of 0.5 to 1.6. The computations were carried out for the laminar flame thickness and burning velocity under the unburned gas temperature 0.5bat-30bar and temperature of 300K-700K at ${\Phi}=l.0$. Comparison of the results showed no difference between these two methods. The flame thickness was decreased by increasing the pressure and temperature, but, the affect of pressure is more significant than the effect of temperature on the flame thickness. The thickness of preheat zone was about 66.5% of the flame thickness, and flame thickness and burning velocity were also predicted by using empirical equation.

• 대한기계학회논문집
• /
• 제19권2호
• /
• pp.509-518
• /
• 1995

A study for the flame stability and the combustion characteristics of coaxial diffusion flame was conducted. The fuel employed was natural gas. The experimental variables were rim thickness of fuel tube, blockage ratio of the outer diameter of fuel tube to the inner diameter of air tube, and momentum ratio of fuel to air. It was consequently found that the stability in the neighborhood of the fuel rim depended on the rim thickness, especially in the case of above 3 mm, and that the stable region of the flame extended remarkably due to the formation of recirculation zone above rim. The effect of the blockage ratio on the flame stability was found to be minor in the case of above 3 mm of rim thickness. Between the momentum ratio 2 and 3, the stable flame zone was widely established as well good combustion. With increasing the fuel-air momentum ratio, axial velocity, turbulence intensity, and Reynolds stress increased.

• 한국안전학회지
• /
• 제19권3호
• /
• pp.124-129
• /
• 2004

초 록 : 확산화염 시뮬레이션에 대해 수치법을 검증하고 변형률과 연료농도가 화염반경과 두께의 변화에 미치는 영향을 조사하기 위해, Fire Dynamics Simulator (FDS)를 사용하여 무중력의 비예혼합 메탄-공기 대향류 화염을 축대칭으로 모사하였다. 연료 중 메탄의 몰분율 $X_m=20,\;50,\;80\%$와 각각의 몰분율에서 세 가지 변형률 $a_g=20,\;60,\;90s^{-1}$의 $1000^{\circ}C$ 기준 화염반경과 화염두께를 조사하였다. 변형률이 클수록 화염반경은 증가하였으나 화염두께는 거의 선형적으로 감소하였다. 또 화염반경은 메탄농도가 높을수록 감소하였으나, 변형률의 영향만큼 메탄농도에 민감하지 않았다. FDS와 OPPDIF로 각각 구한 무차원 화염두께가 잘 일치하므로, 넓은 범위의 연료농도와 변형률에서 FDS가 대향류 확산화염의 화염구조를 잘 예측할 수 있음을 확인하였다.

• International Journal of Safety
• /
• 제2권1호
• /
• pp.12-16
• /
• 2003

In Part 1, the flame structure of the counterflow nonpremixed flames computed by using Fire Dynamics Simulator was compared with that of OPPDIF for different concentrations of methane in the fuel stream. In this study, comparisons were made for the global strain rate that is an important parameter for diffusion flames for further evaluation of FDS. At each of the three fuel concentrations, $20% CH_4＋ 80% N_2, 50% CH_4 ＋ 50% N_2, 90% CH_4 ＋ 10% N_2$ in the fuel stream, the temperature and axial velocity profiles were investigated for the global strain rate in the range from 20 to $100s^{-1}$. Changes in flame thickness and radius were also compared with OPPDIF. There was good agreement in the temperature and axial velocity profiles between the axisymmetric simulations and the one-dimensional computations except for the regions where the flame temperature reach its peak and the axial velocity rapidly changes. The simulations of the axisymmetric flames with FDS showed that the flame thickness decreases and the flame radius increases with increasing global strain rate.

• 한국안전학회지
• /
• 제17권4호
• /
• pp.61-65
• /
• 2002

Nonpremixed counterflow flames at low strain rates, $ag=12s^{-1}$ and $12s^{-1}$, were numerically simulated to investigate the effects of the duct thickness on the flame structure in normal gravity. For small values of the duct thickness, the positions of the flame and stagnation point were highly sensitive to the duct thickness. When the duct thickness was greater than 6mm, however, the effects of the duct thickness on the flame structure were negligible. The computed temperature along the duct centerline agreed well with measurements.

• 대한기계학회논문집B
• /
• 제20권7호
• /
• pp.2337-2346
• /
• 1996

One of the most useful method for increasing combustion loading of premixed flame is to strengthen the turbulent intensity of unburned mixture. It produces an important information to a design of efficient combustion equipment that analysing microstructure of strong turbulence premixed flame. The flame structure and characteristics are depend on the turbulence of unburned mixture. Therefore, to strengthen the turbulent intensity of unburned mixture make flame scale small and accomplish efficient combustion. We measured the velocity of local flame front movements, local eddy radius and local reaction zone thickness quantitatively with increasing turbulent intensity of unburned mixture. We researched the microstructure of flame using ion currents that react sensitively in the reaction zone. Consequently, the velocity of local flame front movements is depend on the velocity of unburned mixture and local eddy scale is to be small with increasing turbulent intensity. But there is no change in local reaction zone thickness with turbulence.

• Journal of Advanced Marine Engineering and Technology
• /
• 제19권4호
• /
• pp.34-41
• /
• 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.

• 한국연소학회지
• /
• 제16권4호
• /
• pp.31-37
• /
• 2011

This paper presents both experimental and numerical investigation of the combustion characteristics of stretched premixed lift-off flames using synthetic gas($H_2$/CO) in an impinging burner. We used "Spin code" for numerical analysis. An ICCD camera was employed to measure flame location and flame thickness. The impinging surface temperature was affected by local strain rate K, equivalence ratio, and composition ratio of fuel. In spite of the difference of boundary conditions in experimental and numerical results, the tendencies of surface temperatures were agreed. From result of this work, we also found that flame location and flame thickness directly related to surface temperature are greatly affected by local strain rate K.