• Journal of the Korean Society of Propulsion Engineers
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• v.17 no.2
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• pp.16-23
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• 2013

An experimental study has been conducted to investigate the effects of equivalence ratio on the propagation characteristics of $CH_4$-air premixed flame intervened by an ultrasonic standing wave. A Schlieren photography was used for the flame structure visualization, and the flame propagation behavior was investigated in detail throughout the post-processing analysis. It is found that the structural variation of methane/air premixed flame caused by the intervention of ultrasonic standing wave give rise to the enhancement of combustion reaction and flame propagation velocity. Effectiveness of the standing wave on the flame velocity decreases as the equivalence ratio increases. Larger flame velocity with the standing wave becomes undistinguishable in a specific range of equivalence ratios.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.3 s.234
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• pp.376-383
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• 2005

In the paper we investigate characteristics of a transitional behavior from a premixed flame to a triple flame in a lifted flame according to the change of equivalence ratio. In previous study, we showed that the stabilized laminar lifted flame regime is categorized by regimes of premixed flame, triple flame and critical flame. A gas-chromatograph is used to measure concentration field, a smoke-wire system is used to measure streak line, and a PIV system is used to measure velocity field in lifted flame. In the visualization experiment of smoke wire, the flow divergence and redirection reappeared in premixed flame as well as triple flame. Thus we cannot express the flame front of lifted flame has a behavior of triple flame with only flow divergence and redirection. In PIV measurement, flow velocity for those three flames has minimum value at the tip of flame front. To differentiate triple flame and premixed flame, $\Phi$ value of partially premixed fraction is employed. The partially premixed fraction $\Phi$ was constant in premixed flame. In critical flame small gradient appears over the whole regime. In triple flame, typical diffusion flame shape is obtained as parabolic distribution type due to diffusion flame trailing.

• Journal of the Korean Society of Industry Convergence
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• v.11 no.1
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• pp.5-10
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• 2008

A flame speed of methane mixture of water vapor and air have been measured to study the process of flame propagation using schlieren photographs. The quantity of water vapor contained were changed 5% and 10% of total mixture, and equivalence ratio of mixture between 0.8 and 1.2 were tested under the ambient temperature 323K and 373K. The results showed that the burning velocity was decreased by increasing the water vapor contents due to the interruption of flame development. And, the reduction rate of burning velocity was smaller by increasing the water contents under the same ambient temperature. The effects of ambient temperature on burning velocity was decreased by increasing the water vapor contents.

• 한국연소학회:학술대회논문집
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• 2006.04a
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• pp.63-68
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• 2006

Effects of oxidizer inlet velocity on NO emission characteristics of 0.2MW oxy-fuel combustor have been experimentally investigated. The NO formation process in the oxy-fuel combustion is extremely sensitive even for the small fraction of nitrogen in oxidizer. By increasing the oxidizer velocity, flame length is reduced due to the enhanced turbulent mixing. The increased oxidizer velocity also results in the decreased flame temperature through the elevated entrainment rate of the recirculated product and the corresponding NO emission is drastically decreased. Experimental results clearly indicate that the entrained product gases play a crucial role to decrease the temperature at the flame zone and the post flame zone where the thermal NO is mainly formed.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.4
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• pp.847-864
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• 1988

The main purpose of this study is to investigate the local velocity acceleration in a boundary layer diffusion flame over a flat plate. In order to know the effect of separation on the local velocity acceleration, two typical cases, flows with and without separation, are considered. For these cases, flow visualization using paraffine smoke tracers has been made. Mean velocity and r.m.s. value of fluctuating velocity are measured by using a laser Doppler velocimeter. In addition, measurements of time-mean concentration and time-mean temperature have been made. Time-mean density profiles have been obtained from the data of concentration and temperature. The obtained results are summarized as follows : (1) In the case without separation, the local velocity acceleration is clearly observed near the visible flame zone for all flow conditions. On the while, in the case with serration, the local velocity acceleration is observed only at low free stream velocity and high fuel injection velocity. As increasing the free stream velocity or decreasing the fuel injection velocity, it is not distinctly observed in the mean velocity profile. (2) The r.m.s. value of fluctuating velocity is significantly decreased by combustion in the case with separation. But in the case without separation, the r.m.s. value is increased near the visible flame zone in comparison with cold flow. In both cases, the peak value of r.m.s. appeared just at the visible flame zone, where the mean velocity gradient is not too high.

• Transactions of the Korean Society of Automotive Engineers
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• v.1 no.1
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• pp.59-65
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• 1993

In a spark ignition engine, in order to make research on flame propagation, attentive concentration should be paid on initial combustion stage about the formation and development of flame. In addition, the initial stage of combustion governs overall combustion period in a spark ignition engine. With the increase of the size of flame kernel, it could reach initial flame stage easily, and the mixture could proceed to the combustion of stabilized state. Therefore, we must study the theoretical calculation of minimum flame kernel radius which effects on the formation and development of kernel. To calculate the minimum flame kernel radius, we must know the thermal conductivity, flame temperature, laminar burning velocity and etc. The thermal conductivity is derived from the molecular kinetic theory, the flame temperature from the chemical reaction equations and the laminar burning velocity from the D.K.Kuehl's formula. In order to estimate the correctness of the theoretically calculated minimum flame kernel radius, the researcheres compared it with the RMaly's experimental values.

• Journal of the Korean Society of Combustion
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• v.1 no.2
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• pp.21-29
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• 1996

Flame propagation characteristics of propane-air mixtures were experimentally investigated in constant-volume combustion chambers. Flame propagation process was observed as a function of mixture strength, initial mixture temperature and initial mixture pressure in quiescent mixtures. A cylindrical combustion chamber and a spherical combustion chamber contain a pair of parallel windows through which optical access into the chamber can be provided. Laser two beam deflection method was adopted to measure the local flame propagation, which gave information on the flame size and flame propagation speed. Pressure development was also measured by a piezoelectric pressure transducer to characterize combustion in quiescent mixtures. Burning velocity was calculated from flame propagation and pressure measurements. The effect of flow on flame propagation was also investigated under flowing mixture conditions. Laser two beam method was found to be feasible in measuring flame propagation of quiescent mixtures. Flame was observed to propagate faster with higher initial mixture temperature and lower initial pressure. Combustion duration was shortened in the highly turbulent flowing mixtures.

• Journal of the Korean Institute of Gas
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• v.14 no.3
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• pp.8-13
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• 2010

Hydrogen is expected to become a new, clean source of energy for the next generation. Therefore, many studies have investigated the characteristics of the hydrogen flame. However, because the hydrogen flame has high temperature, the flame does not emit visible light, and the flame propagates at a high velocity, investigating its characteristics is difficult. In the present study, in order to simultaneously examine the flame temperature and flame propagation velocity of hydrogen/air mixtures, ultra fine thermocouples with diameters of 12.7, 25.4, and 50.8 ${\mu}m$ are utilized. The results show that it is possible to detect the arrival time of the flame. Due to the temperature compensation with the time constants of thermocouples, it is also possible to estimate the flame temperature.

• International Journal of Aeronautical and Space Sciences
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• v.13 no.3
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• pp.386-397
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• 2012

The stability and structure of bluff-body stabilized hydrogen flames were investigated numerically and experimentally. The velocity of coflowing air was varied from subsonic velocity to a supersonic velocity of Mach 1.8. OH PLIF images and Schlieren images were used for analysis. Flame regimes were used to classify the characteristic flame modes according to the variation of the fuel-air velocity ratio, into jet-like flame, central-jet-dominated flame, and recirculation zone flame. Stability curves were drawn to find the blowout regimes and to show the improvement in flame stability with increasing lip thickness of the fuel tube, which acts as a bluff-body. These curves collapse to a single line when the blowout curves are normalized by the size of the bluff-body. The variation of flame length with the increase in air flow rate was also investigated. In the subsonic coflow condition, the flame length decreased significantly, but in the supersonic coflow condition, the flame length increased slowly and finally reached a near-constant value. This phenomenon is attributed to the air-entrainment of subsonic flow and the compressibility effect of supersonic flow. The closed-tip recirculation zone flames in supersonic coflow had a reacting core in the partially premixed zone, where the fuel jet lost its momentum due to the high-pressure zone and followed the recirculation zone; this behavior resulted in the long characteristic time for the fuel-air mixing.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.10
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• pp.1458-1464
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• 2002

$CO_2$ is well known greenhouse gas which is the major source of global warming. Reducing $CO_2$ emission in combustion process can be achieved by increasing combustion efficiency, oxygen enriched combustion and recirculation of the emitted $CO_2$ gas. Stability of non-premixed flame in oxygen enriched environment will be affected by the amount of oxygen, kind of diluents and fuel exit velocity. The effects of these parameters on flame liftoff and blowout are studied experimentally oxidizer coflowing burner. Experiments were divided into three cases according as where $CO_2$gas was supplied. - 1) to coflowing air, 2) to fuel with 0$_2$-$N_2$ coflow, 3) to coflowing oxygen. Flame in air coflowing case was lifted in turbulent region. Flame lift and blowout in laminar region with the increase in $CO_2$ volume fraction in $CO_2$-Air mixture makes flame lift and blowout in laminar region. Increase in oxygen volume fraction makes flame stable-i.e. flame liftoff and blowout occur at higher fuel flowrates. Liftoff height was non-linear function of nozzle exit velocity and affected by the $O_2$ volume fraction. It was found that the flame in $O_2$-$N_2$ coflow case was more stable than $O_2$-$CO_2$ case, Liftoff heights vs (nozzle exit velocity/laminar burning velocity)$^{3.8}$ has a good correlation in $O_2$-$CO_2$ oxidizer case.