• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.4
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• pp.352-356
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• 2008

In this study of lifted hydrogen jet with coaxial air, we have experimentally studied the characteristics of stabilization point in turbulent diffusion flames. The objectives are to present the phenomenon of a liftoff height decreasing as increasing fuel velocity and to analyse the flame structure and behavior including liftoff mechanisms. The fuel jet exit velocity was changed from 100 up to 300 m/s and a coaxial air velocity was fixed at 16 m/s with a coflow air less than 0.1 m/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. It has been suggested that the stabilization of lifted hydrogen diffusion flames was correlated with a turbulent intensity, $S_t{\sim}u^{\prime}$, and jet Reynolds number, $S_t{\sim}Re^{0.017}_{jet}$.

• 한국연소학회:학술대회논문집
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• 2001.11a
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• pp.119-124
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• 2001

Propane coflow diffusion flames have been experimentally studied to investigate the liftoff and reattachment characteristics. Flame properties such as velocity and density distribution were measured by LDV and shadowgraphy, respectively. It is shown that as the velocity of coflowing air increases, liftoff velocity decreases nonlinearly in turbulent jets and linearly in laminar jets, while reattachment velocity decreases nonlinearly. Meanwhile, as inner nozzle tip thickness increases, liftoff velocity increases with the reattachment velocity nearly unchanged. Liftoff phenomena in these flames can be categorized into three classes as a function of coflow velocity, such as laminar liftoff, turbulent liftoff, and transient liftoff.

• Fire Science and Engineering
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• v.18 no.2
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• pp.34-40
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• 2004

The effects of external flow excitation with various frequencies and amplitudes on the flame behavior and pollution emission characteristics from a laminar jet flame are experimentally investigated. Measurements of $NO_x$ emission indices ($EINO_x$), performed in vertical lifted flame like turbulent with various exciting amplitude at a constant resonance frequency, have been conducted. It was also conducted to investigate the effects of excited frequency at a constant exciting amplitude on $NO_x$ emissions with a various frequency ranged 0 Hz to 2 KHz. From the vertical lifted turbulent flame of the excited jet with resonance frequency by strong excitation was shown that the dependence of $NO_x$ emission could be categorized into three groups Group I of long flame length with high disturbances yielding high $NO_x$ emission, Group II of intermediate flame length and relative narrow flame volume with low disturbance yielding low $NO_x$ emission and Group III of long flame length and large flame volume with high time ＆ space disturbances behaviour yielding high $NO_x$ emission.

• Fire Science and Engineering
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• v.23 no.1
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• pp.55-62
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• 2009

A general combustion characteristics of forcing nonpremixed jet in laminar flow rates have been conducted experimentally to investigate the effect of forcing amplitude with the resonant frequency of fuel tube. There are two patterns of the flame lift-off feature according to the velocity increasing; one has the decreasing values of forcing amplitude on the lift-off occurrence when a fuel exit velocity is increasing, while the other has the increasing values. These mean that there are the different mechanisms in the lift-off stability of forced jet diffusion flame. Especially, the characteristics of attached jet flame regime are concentrically observed with flame lengths, shapes, flow response and velocity profiles at the nozzle exit as the central figure. The notable observations are that the flame enlogation, in-homing flame and the occurrence of a vortical motion turnabout have happened according to the increase of forcing amplitude. It is understood by the velocity measurements and visualization methods that these phenomena have been relevance to an entrainment of surrounding oxygen into the fuel nozzle as the negative part of the fluctuating velocity has begun at the inner part of the fuel nozzle.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.453-458
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• 2001

The stability of turbulent nonpremixed interacting flames is investigated in terms of nozzle configuration shapes which depend on the existence of the center nozzles. Six nozzle arrangements which are cross 4, 5, 8, 9, square 8 and circular 8 nozzles are used for the experiment. Those are arranged to see the effect of the center nozzle out of multi-nozzle. There are many parameters that affect flame stability in multi-nozzle flame such as nozzle separation distance, fuel flowrates and nozzle configuration, but the most important factor is the existence of nozzles in the center area from the nozzle arrangement. As the number of nozzle in the area is reduced, more air can be entrained into the center of flame base and then tag flame is formed. In the case of circular 8 nozzles, blowout flowrates are above 5.4 times compared with that of single equivalent area nozzle.

• 한국연소학회:학술대회논문집
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• 2004.06a
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• pp.7-14
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• 2004

The lift-off characteristics of the triple flame have been studied experimentally with various mean velocities and concentration gradients using a multi-slot burner, which can control the concentration gradient and the mean velocity independently, Lift-off height, axial maximum velocity, flame temperature, and some other characteristics were examined for methane and propane flame, It was found that minimum values of the lift-off heights exist at a certain concentration gradient for constant mean velocity, and this result implies that the propagation velocity has a maximum value at this condition, OH radical distribution was measured with LIF method and velocity variation along streamline was measured with PlV system. In addition maximum temperature along streamline was measured with CARS system. The intensity of the diffusion flame affects on the propagation velocity of triple flame in the region of very weak concentration gradient.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.04a
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• pp.371-374
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• 2011

High-resolution numerical study is carried out to investigate the flame stability of the turbulent supersonic combustion in a Dual-Combustion Ramjet (DCR). The auto-ignition in a shear layer between hydrogen/carbon-monoxide syngas and air was studied at elevated enthalpy condition. Comparison of a constant area combustor and a combustor with a small divergence angle shows that the supersonic combustion has a characteristics of the lifted flame and its stability is influenced significantly by the compressibility.

• Journal of the Korean Society of Combustion
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• v.7 no.3
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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.

• Journal of the Korean Society of Propulsion Engineers
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• v.4 no.1
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• pp.30-35
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• 2000

The experimental study on the lift-off height of diffusion flames was conducted to investigate the damage of swirl-coaxial injector used in $GO_2$/kerosene rocket engine during initial stage of ignition. To investigate the causes of damage and to prevent further damage of the injector, experimental injector was designed and hot fire tests were performed with varying propellant momentum ratio($\frac{Momentum of {GO_2}}{Momentum of Kerosene}$) from 1 to 12. In experimental coaxial injector, kerosene is sprayed from the central nozzle with swirl and $GO_2$ sprayed around the kerosene nozzle in the direction parallel to the axis of combustion chamber. Chamber pressure are close to the atmospheric condition. Lift-off height was measured by still images from camcoder and average values were used as data.

• Journal of the Korean Society of Propulsion Engineers
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• v.22 no.6
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• pp.56-63
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• 2018

In low swirl combustors the flame is lifted above the nozzle to achieve balance between the flame speed and velocity field at the exit of the nozzle. Characterization of the flame liftoff height is important because it affects the stability of the combustor and degradation of the nozzle material. In experiments, a counter-intuitive trend of flame liftoff heights with respect to inlet velocities was observed. To elucidate the complicated flow field in a low swirl combustor having swirl vanes and a turbulence generator, a series of numerical simulations of non-reacting flows was conducted by varying the inlet velocity. The flow structures at the exit of the nozzle with respect to the inlet velocities are investigated to support the observation in the experiments.