• 대한기계학회논문집B
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• 제32권1호
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• pp.7-14
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• 2008

It has been reported that partially premixed interacting flame could be sustained till sonic exit velocities if eight small nozzles are arranged optimally and one nozzle on the center is fed small amount of fuel. But the equivalence ratios in this experiments were 20-60. In this research, experiments were conducted to know the effects of lean-rich staging in multiple jet flames on the blowout velocity. The fuel mole tractions in the fuel-air mixture, the nozzle exit velocity and the diameter between adjacent nozzles were alternatively changed. When the lower mole fraction fuel was fed to the nozzles located near the center and small amount of fuel to the center nozzle, flame was not extinguished even at the nozzle exit velocity of 200m/s. Also the interacting flame could be sustained till that velocity when four small size nozzles for lean mixture were located within the arrangement of four nozzles for rich mixture and configured optimally.

• Journal of Mechanical Science and Technology
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• 제16권12호
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• pp.1710-1718
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• 2002

The effect of fuel concentration gradient on the propagation characteristics of tribrachial (or triple) flames has been investigated experimentally in both two-dimensional and axisymmetric counterflows. The gradient at the stoichiometric location was controlled by the equivalence ratios at the two nozzles; one of which is maintained rich, while the other lean. Results show that the displacement speed of tribrachial flames in the two-dimensional counterflow decreases with fuel concentration gradient and has much larger speed than the maximum speed predicted previously in two-dimensional mixing layers. From an analogy with premixed flame propagation, this excessively large displacement speed can be attributed to the flame propagation with respect to burnt gas. Corresponding maximum speed in the limit of small mixture fraction gradient was estimated and the curvefit of the experimental data substantiates this limiting speed. As mixture fraction gradient approaches zero, a transition occurs, such that the propagation speed of tribrachial flame approaches stoichiometric laminar burning velocity with respect to burnt gas. Similar results have been obtained for tribrachial flames propagating in axisymmetric counterflow.

• 한국연소학회지
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• 제13권1호
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• pp.10-16
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• 2008

Numerical simulations of freely propagating premixed flames burning mixtures of methane and chlorinated hydrocarbons in fuel are performed at atmospheric pressure in order to understand the effect of chlorinated hydrocarbons on the formation of nitrogen oxide. A detailed chemical reaction mechanism is used, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. Chlorine atoms available from chlorinated hydrocarbons inhibit the formation of nitrogen oxides by lowering the concentration of radical species. The reduction of NO emission index calculated with thermal or prompt NO mechanism is not linear and is probably related to the saturation effect as $CH_3Cl$ addition is increased, In the formation or consumption of nitrogen oxide, the $NO_2$ and NOCl reactions play an important role in lean flames while the HNO reactions do in rich flames. The molar ratio of Cl to H in fuel has an effect on the magnitude of NO emission index.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2015년도 제51회 KOSCO SYMPOSIUM 초록집
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• pp.53-56
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• 2015

A combined experimental and numerical study has been conducted to investigate the downstream interaction between simulated SNG-air premixed flames in fuel composition of 91% $CH_4$ + 6% $C_3H_8$ + 3% $H_2$. In this study, the effects of fuel molar concentration(lean-rich) and strain rate($a_g$) were major parameters. A main focus is to investigate flames behavior and chemical interaction at flames downstream. The numerical results were calculated by OPPDIF application. The reaction mechanism adopted was USC-II model including C3 reaction.

• 한국연소학회지
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• 제20권2호
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• pp.28-35
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• 2015

In this study, the CH4/air lean-rich combustion system with exhaust gas recirculation (EGR) was investigated to explore the potential for lowering pollutant emissions. To achieve this purpose, experiments of lean-rich combustion system with EGR were conducted to measure the changes in the characteristics of the pollutant emission and flame shape with various equivalence ratios and EGR rates. Here, this study was applied to the fuel distribution ratio of 3:1 for the formation of the lean and rich flames. Additionally, the results were compared with $CH_4$/air lean premixed combustion system. The results show that flame shape of lean-rich combustion system was determined by lean and rich equivalence ratios (${\Phi}_L$ and ${\Phi}_R$) and stratified flame was formed with increasing ${\Phi}_R$. According to the pollutant emission characteristics based on experimental results, the NOx and CO emission index (EINOx and EICO) decreased with increasing EGR rate. Especially, in the range needed to form a stable flame, the reduction rates of EINOx and EICO were approximately 47% and 48% for an EGR rate of 25%, global equivalence ratio of 0.85 and ${\Phi}_L$ of 0.80 compared with lean premixed combustion system (${\Phi}$ = 0.78).

• 한국자동차공학회논문집
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• 제14권6호
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• pp.171-178
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• 2006

The performance in the practical combustion system including reciprocating engines and gas turbine combustors is being much governed by turbulent reacting flow that is often analyzed by both a laminar flamelets concept and flame interaction. The characteristics of laminar flame interaction have been investigated numerically to provide basic understanding of wrinkled turbulent flames under concentration interaction resulting from inhomogeneity in fuel-air mixing, especially focused on the transition of flame characteristics such as diffusion flame, partially premixed diffusion flame, and triple-layer flame by the variation in the degree of premixedness. The extinction stretch rates to the premixedness have also been obtained in this paper. The boundary defining the regime of the existence of triple-layer flames as functions of both stretch rate and premixedness has been determined which agrees well with previously reported experiment measuring OH radical concentration peaks based on PLIF.

• 한국연소학회지
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• 제18권4호
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• pp.29-36
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• 2013

Numerical study was conducted to clarify effects of added $CO_2$ for the downstream interaction between $H_2$-air and CO-air premixed flames in counterflow configuration. The reaction mechanism adopted was Davis model which had been known to be well in agreement with reliable experimental data. The results showed that both lean and rich flammable limits were reduced. The most discernible difference between the two with and without having $CO_2$ addition into $H_2$-air and CO-air premixtures was two flammable islands for the former and one island for the latter at high strain flame conditions. Even a small amount of $H_2$, in which $H_2$-air premixed flame cannot be sustained by itself, participates in CO oxidation, thereby altering the CO-oxidation reaction path from the main reaction route $CO+O_2{\rightarrow}CO_2+O$ with a very long chemical time in CO-air flame to the (H, O, OH)-related reaction routes including $CO+OH{\rightarrow}CO_2+H$ with relatively short chemical times. This intrinsic nature alters flame stability maps appreciably. The results also showed that chemical effects of added $CO_2$ suppressed flame stabilization. Particularly this phenomenon was appreciable at flame conditions which lean and rich extinction boundary was merged. The detailed discussion of chemical effects of added $CO_2$ was addressed to the present downstream interaction.

• 한국연소학회지
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• 제13권4호
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• pp.8-15
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• 2008

When a circular cylinder is placed at the center of a slot burner nozzle, once stable Woflhard-Parker type laminar lean premixed flame is changed to an oscillating flame with self-induced noise. The wrinkled flame surface showed the same pattern and frequency of the Karman vortex street at the downstream of a circular cylinder. The interaction of flame with Karman vortex street is observed to be responsible for flame oscillation. The measured flame oscillation frequency is very similar to the estimated Karman vortex shedding frequency based on the St-Re relationship of the flow past circular cylinder, which could be considered as a strong evidence for the interaction between laminar pre-mixed flame and a Karman vortex street. As Reynolds number increases oscillation frequency decreases and the self-induced noise level increases as well as the flame front is more severly wrinkled. This result suggests that the flame/vortex interaction becomes more active at higher Re.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2013년도 제46회 KOSCO SYMPOSIUM 초록집
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• pp.111-112
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• 2013

An experimental study of a dual swirl burner was conducted to analyze NOx emission in the lean conditions. The dual swirl burner is composed of a combination of swirling jet premixed(main section) and diffusion flames(pilot section). It was operated with a co-swirling configuration and overall equivalence ratios between 0.6 and 0.8. The purpose of this study is to analyze experimentally the characterization of flame temperature and NOx concentration in reacting zone and to supply the useful experimental data for numerical simulations. The measurements of temperature and NOx concentration were captured using a thin digitally-compensated thermocouple and a sampling quartz probe with quenching effect of sudden expansion, and were measured by the NOx analyzer of chemiluminescence method. We could analyse the NOx emission characteristics comparing the temperature distributions in the lean equivalence ratios.

• 한국연소학회:학술대회논문집
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• 한국연소학회 2006년도 제32회 KOSCO SYMPOSIUM 논문집
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• pp.149-156
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• 2006

Structure of edge flame established in a mixing layer, formed between two uniformly flowing pure $CH_4$ and pure $O_2$ streams, is numerically investigated by employing a detailed methane-oxidation mechanism. The numerical results exhibited the most outstanding distinction of using pure oxygen in the fuel-rich premixed-flame front, through which the carbon-containing compound is found to leak mainly in the form of CO instead of HC compounds, contrary to the rich $CH_4-air$ premixed flames in which $CH_4$ as well as $C_2H_m$ leakage can occur. Moreover, while passing through the rich premixed flame, a major route for CO production, in addition to the direct $CH_4$ decomposition, is found to be $C_2H_m$ compound formation followed by their decomposition into CO. Beyond the rich premixed flame front, CO is further oxidized into $CO_2$ in a broad diffusion-flame-like reaction zone located around moderately fuel-rich side of the stoichiometric mixture by the OH radical from the fuel-lean premixed-flame front. Since the secondary CO production through $C_2H_m$ decomposition has a relatively strong reaction intensity, an additional heat-release branch appears and the resulting heat-release profile can no longer be seen as a tribrachial structure.