• 한국연소학회지
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• 제12권2호
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• pp.26-33
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• 2007

To reveal the newly found liftoff height behavior of hydrogen jet, we have experimentally studied the stabilization mechanism of turbulent, lifted jet flames in a non-premixed condition. The objectives of the present research are to report the phenomenon of a liftoff height decreasing as increasing fuel velocity, to analyse the flame structure and behavior of the lifted jet, and to explain the mechanisms of flame stability in hydrogen turbulent non-premixed jet flames. The velocity of hydrogen was varied from 100 to 300m/s and a coaxial air velocity was fixed at 16m/s with a coflow air less than 0.1m/s. For the simultaneous measurement of velocity field and reaction zone, PIV and OH PLIF technique was used with two Nd:Yag lasers and CCD cameras. As results, it has been found that the stabilization of lifted hydrogen diffusion flames is related with a turbulent intensity, which means that combustion occurs at the point where the local flow velocity is balanced with the turbulent flame propagation velocity.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2006년도 제33회 KOSCO SYMPOSIUM 논문집
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• pp.73-78
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• 2006

The lifted oscillating flame has been studied using experiments of inverse diffusion flames that the air jet injected into a methane background. To find out the characteristics of inverse diffusion flames, fundamentally flame stabilized diagram is investigated with various air and fuel jet velocities. It has five regions - flame extinction, stable attached flame, anchored flame, liftoff flame and blow off region. In inverse diffusion flame, lifted flames were observed near the blow off region. As long as flames lift off, flames oscillate by periods. In this oscillating lifted flame region, the frequency of 1 and under were observed in various air and methane jet velocities. Characteristics of lifted flames are also examined by using the ICCD direct image. And intensity of flame chemiluminescence is very different in rising and falling period from photographs. For the present, it is predicted that the changes of flame structure are related with flame oscillation, but more experiments will be needed to make clear the phenomenon.

• 한국가스학회지
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• 제12권4호
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• pp.21-28
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• 2008

삼지화염과 포와젤(Poiseuille) 및 균일분포 연료출구 속도에 따른 부상화염의 부상거동 그리고 화염면 부근에서의 연소 유동 특성에 대하여 수치적 해석을 수행하였다. 부상화염에 대한 수치해석으로 기존 연구 결과를 검정하고 화염대 부근에서 구조적 특성을 살펴보며 포와젤 및 균일분포 연료출구속도 조건에서 운동량 유속으로 부상높이를 일반화하여 비교하였다. 또한 화염면 부근에서 속도, 압력, 온도, 화학반응속도 등으로 연소 유동 특성을 분석하고 규명하였다. 특히 중심선을 따른 속도 변화의 경우 노즐에서 화염대 부근까지는 전형적인 비반응제트 유동에 따른 속도 분포를 형성하지만 화염대 직전에서 속도가 급격히 감소하다가 화염대를 지나면서 급격히 증가한 후에 다시 감소하는 현상을 규명하였다. 화염대 전의 유동영역에서는 화염대가 장애물역할을 하다가 화염대를 지나고 나면 유동을 가속시켜주는 역할을 하고 있기 때문이다. 이러한 현상은 기존의 비반응 제트 유동으로 규명하지 못하였던 것이다.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2004년도 제28회 KOSCO SYMPOSIUM 논문집
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• pp.15-20
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• 2004

Characteristics of turbulent lifted flames in coflow jet have been investigated by varying initial temperature through the heating of coflow air. In the turbulent regime, liftoff height increases linearly with fuel jet velocity and decreases nonlinearly as the coflow temperature increases. This can be attributed to the increase of turbulent propagation speed, which is strongly related to laminar burning velocity. Dimensionless liftoff heights are correlated well with dimensionless jet velocity, which are scaled with parameters determining local flow velocity and turbulent propagation speed. This implies that the turbulent lifted flames are stabilized by balance mechanism between local turbulent burning velocity and flow velocity. Blowout velocity can be obtained from the ratio of mixing time to chemical time. Comparing to previous researches, thermal diffusivity should be evaluated from the initial temperature instead of adiabatic flame temperature.

• 한국연소학회지
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• 제9권1호
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• pp.32-38
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• 2004

Characteristics of turbulent lifted flames in coflow jet have been investigated by varying initial temperature through the heating coflow air. In the turbulent regime, liftoff height increases linearly with fuel jet velocity and decreases nonlinearly as the coflow temperature increases. This can be attributed to the increase of turbulent propagation speed, which is strongly related to laminar burning velocity. Dimensionless liftoff heights are correlated well with dimensionless jet velocity, which are scaled with parameters determining local flow velocity and turbulent propagation speed. This implies that the turbulent lifted flames are stabilized by balance mechanism between local turbulent burning velocity and flow velocity. Blowout velocity can be obtained from the ratio of mixing time to chemical time. Comparing to previous researches, thermal diffusivity should be evaluated from the initial temperature instead of adiabatic flame temperature.

• 대한기계학회논문집B
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• 제32권12호
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• pp.970-976
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• 2008

Characteristics of methane turbulent non-premixed flame have been studied experimentally in coflow jets with initial temperature variation. The results showed that the premixed flame model and the large-scale mixing model for turbulent flame stabilization were effective for methane fuel considered initial temperature variation. Especially, the premixed flame model has been improved by considering nitrogen dilution for the liftoff height of turbulent lifted flame. In estimating blowout velocity and the liftoff height at blowout with the premixed flame model and the large-scale mixing model, the two turbulent models were excellently correlated by considering the effect of physical properties and buoyancy for the initial temperature variation.

• 대한기계학회논문집B
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• 제32권3호
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• pp.190-197
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• 2008

It was experimentally studied that the stabilization mechanism of turbulent, lifted jet flames in a non-premixed condition to reveal the newly found liftoff height behavior of hydrogen jet. The objectives are to report the phenomenon of a liftoff height decreasing as increasing fuel velocity, to analyse the flame structure and behavior of the lifted jet, and to explain the mechanisms of flame stability in hydrogen turbulent non-premixed jet flames. The hydrogen jet velocity was changed from 100 to 300m/s and a coaxial air velocity was fixed at 16m/s with a coflow air less than 0.1m/s. For the simultaneous measurement of velocity field and reaction zone, PIV and OH PLIF technique was used with two Nd:Yag lasers and CCD cameras. As a result, it was found that the stabilization of lifted hydrogen diffusion flames is correlated with a turbulent intensity and Karlovitz number.

• 한국연소학회지
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• 제7권2호
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• pp.1-6
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• 2002

Characteristics of laminar lifted flames of propane highly-diluted with nitrogen have been investigated at various temperatures of coflow air. At various fuel mole fractions, the base of laminar lifted flames has the structure of tribrachial (or triple) flame. The liftoff heights are correlated well with the stoichiometric laminar burning velocity considering initial temperature at a given coflow velocity. It shows that lifted flames are stabilized on the basis of the balance mechanism between local flow velocity and the propagation speed of tribrachial flame, regardless of the temperature of coflow and fuel mole fraction. Lifted flames exist for a jet velocity even smaller than the stoichiometric laminar burning velocity, and liftoff velocity increases more rapidly than stoichiometric laminar burning velocity as coflow temperature increases. These can be attributed to the buoyancy effect due to the density difference.

• 대한기계학회논문집B
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• 제24권12호
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• pp.1662-1669
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• 2000

Various flame types are observed in a double concentric burner by varying equivalence ratio and flow rates in each tube. Observed flame types include bunsen-type flame, ring-shaped flame, outer lifted flame, inner lifted flame, and oscillatory lifted flame, The doman of existence of various flames is mapped with equivalence ratio and annular jet velocity. Each flame is investigated through direct photography and OH PLIF. As central air velocity increase, the blowout region is diminished and lifted oscillating flames are observed. Inner lifted flames are observed from bunsen flames or rich shaped flames by increasing central air velocity. For inner lifted flames, annular jet velocity, at flame liftoff decreases with increasing central air jet velocity. Axial velocity profile and temperature fie이 using LDV and CRS, respectively, for a typical inner lifted flame are also measured through which the role of tribrachial flame for stabilization in emphasized.

• 한국연소학회지
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• 제7권3호
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• pp.37-46
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• 2002

Characteristics of lifted flames in axisymmetric laminar coflow jets have been investigated experimentally. Approximate solutions for velocity and concentration accounting virtual origins have been proposed for coflow jets to analyze the behavior of liftoff height. From the measurement of Rayleigh intensity for probing the concentration field of propane, the validity of the approximate solutions was substantiated. From the images of OH PLIF and CH chemiluminescence and the Rayleigh concentration measurement, it has been shown that the positions of maximum luminosity in direct photography coincide with the tribrachial points, which were located along the stoichiometric contour. The liftoff height in coflow jets was found to increase highly nonlinearly with jet velocity and was sensitive to coflow velocity.