• Proceedings of the KSME Conference
• /
• 2007.05b
• /
• pp.2983-2988
• /
• 2007

In the present study, the numerical investigation on the effects of water-mist characteristics has been carried out for the fire suppression mechanism. The FDS are used to simulate the interaction of fire plume and water mists, and program describes the fire-driven flows using LES turbulence model, the mixture fraction combustion model, the finite volume method of radiation transport for a non-scattering gray gas, and conjugate heat transfer between wall and gas flow. The numerical model is consisted of a rectangular enclosure of $L{\times}W{\times}H=1.5{\times}1.5{\times}2.0m$ and a water mist nozzle that be installed 1.8m from fire pool. In the study, the parameters of nozzle for simulation are the droplet size and the spray velocity. Finally, the droplet size influences to fire flume on fire suppression than spray velocity because of the effect of terminal velocity, and the optimal condition for fire suppression is that the droplet size and the spray velocity are $100{\mu}m$ and 20m/s, respectively.

• 한국연소학회:학술대회논문집
• /
• 2006.10a
• /
• pp.86-94
• /
• 2006

This paper investigates the effects of acoustic forcing on NOx emissions and mixing process in the near field region of turbulent hydrogen nonpremixed flames. The resonance frequency was selected to force the coaxial air jet acoustically, because the resonance frequency is effective to amplify the forcing amplitude and reduce NOx emissions. When the resonance frequency is acoustically excited, a streamwise vortex is formed in the mixing layer between the coaxial air jet and coflowing air. As the vortex develops downstream, it entrains both ambient air and combustion products into the coaxial air jet to mix well. In addition, the strong vortex pulls the flame surface toward the coaxial air jet, causing intense chemical reaction. Acoustic excitation also causes velocity fluctuations of coaxial air jet as well as fuel jet but, the maximum value of centerline fuel velocity fluctuation occurs at the different phases of $\Phi$=$180^{\circ}$ for nonreacting case and $\Phi$=$0^{\circ}$ for reacting case. Since acoustic excitation enhances the mixing rate of fuel and air, the line of the stoichiometric mixture fraction becomes narrow. Finally, acoustic forcing at the resonance frequency reduces the normalized flame length by 15 % and EINOx by 25 %, compared to the flame without acoustic excitation.

• Journal of the Korean Society of Combustion
• /
• v.21 no.4
• /
• pp.48-60
• /
• 2016

Experimental and numerical studies were conducted to investigate flame behaviors near flammable limits for downstream-interacting SNG-air premixed flames in a counter-flow configuration. The SNG fuel consisted of a methane, a propane, and a hydrogen with volumetric ratios of 91, 6, and 3%, respectively. The most appropriate priority for some reliable reaction mechanisms examined was given to the mechanism of UC San diego via comparison of lean extinction limits attained numerically with experimental ones. Flame stability map was presented with a functional dependencies of lower and upper methane concentrations in terms of global strain rate. The results show that, at the global strain rate of $30s^{-1}$, lean extinction boundary is slanted while rich extinction one is relatively less inclined because of the dependency of such extinction boundary shapes on deficient reactant Lewis number governed by methane mainly. Further increase of global strain rate forces both extinction boundaries to be more slanted and to be shrunk, resulting in an island of extinction boundary and subsequently one flame extinction limit. Extinction mechanisms for lean and rich, symmetric and asymmetric extinction boundary were identified and discussed via heat losses and chemical interaction.

• Journal of the Korean Society of Combustion
• /
• v.10 no.2
• /
• pp.1-8
• /
• 2005

Incomplete combustion due to quenching in a narrow confinement has been a major problem for realization of a reliable micro combustion device. In most micro combustors, effects of flows are absent in the quenching because the flow is laminar and no severe stretch is present. In such circumstance, quenching is caused either by heat loss or by removal of active radicals to the wall surface of the confinement. An experimental investigation was carried out to investigate the relative significance of these two causes of quenching of a premixed flame. A premixed jet burner with a rectangular cross section at the exit was built. At the burner exit, the flame stands between two walls with adjustable distance. The gap between the two walls at which quenching occurs was measured at different wall surface conditions. The results were analyzed to estimate the relative significance of heat loss to the wall and the removal of radicals at the surface. The measurements indicated that the quenching distance was independent of the wall surface characteristics such as oxygen vacancy, grain boundary, or impurities at low temperature. At high temperature, however, the surface characteristics strongly affect the quenching distance, implying that radical removal at the wall plays a significant role in the quenching process.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.19 no.3
• /
• pp.858-867
• /
• 1995

Characteristics of premixed flames in counter-flow system are numerically studied using a detailed chemical reaction mechanism including gas phase radiation. Without radiation effect accounted, low CO and high NO$_{x}$ emission indices are observed, when strain rate decreases, due to increased residence time and higher flame temperature. Higher NO$_{2}$ production has been also observed when two premixed flames are interacting or cold air stream is mixed with burned gas. The rate of NO$_{x}$ production and destruction is dependent upon the diffusional strength of H and OH radicals, the existence of NO and the concentration of HO$_{2}$. For radiating flames, the peak temperature and NO$_{x}$ production rate decreases as the strain rate decreases. At high strain rate, it is found that the effect of radiation on flame is little due to its negligible radiating volume. It is also found that NO$_{x}$ production from the interacting premixed flame is reduced due to reduced temperature resulting from radiation heat loss. It is concluded that the radiation from gas has significant effect of flame structure and on emission characteristics.ristics.

• 한국신재생에너지학회:학술대회논문집
• /
• 2007.11a
• /
• pp.449-452
• /
• 2007

The present study numerically investigate the effects of the Syngas chemical kinetics on the basic flame properties and the structure of the Syngas diffusion flames. In order to realistically represent the turbulence-chemistry interact ion and the spatial inhomogeneity of scalar dissipation rate. the Eulerian Particle Flamelet Model(EPFM) with multiple flamelets has been applied to simulate the combustion processes and NOx formation in the syngas turbulent nonpremixed flames. Due to the ability for interactively describing the transient behaviors of local flame structures with CFD solver, the EPFM model can effectively account for the detailed mechanisms of NOx format ion including thermal NO path, prompt and nitrous NOx format ion, and reburning process by hydrocarbon radical without any ad-hoc procedure. validation cases include the Syngas turbulent nonpremixed jet and swirling flames. Based on numerical results, the detailed discussion has been made for the sensitivity of the Syngas chemical kinetics as well as the precise structure and NOx formation characteristics of the turbulent Syngas nonpremixed flames.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.25 no.8
• /
• pp.1087-1096
• /
• 2001

The dynamic structures of unsteady CH$_4$/Air jet diffusion flame with a flame-vortex interaction were numerically investigated. A timed-dependent, axisymmetric computational model and a low mach number approximation were employed in the present calculation. A two-step global reaction mechanism which considers 6 species, was used to calculate the reaction rates. The predicted results including the gravitational effect show that the large outer vortices and the small inner vortices can be well simulated without any additional disturbances near nozzle tip. It was found that the temperature and species concentrations have deviated values even for the same mixture fraction in the flame-vortex interaction region. It was also shown that the flame surface is not deformed by the inner vortex in upstream region, while in downstream region, the flame surface is compressed or stretched by the outer vortex roll-up. The present unsteady jet flame configuration accompanying a flame-vortex interaction is expected to give good implications for the unsteady structures of turbulent flames.

• Journal of the Korean Society of Combustion
• /
• v.14 no.2
• /
• pp.26-31
• /
• 2009

Adding small amounts of air to the fuel is used in many commercial combustors to avoid sooty flame. But partially premixed jet flame has lower blowout velocity, $u_{b.o}$, than nonpremixed one. Increasing blowout limit would be one of the key factors to develope highly intense compact combustion devices. Swirling flow enhances fuel and air mixing and induces a highly turbulent recirculation zone, which helps flame stabilization. It was known that NOx emission decreases with swirl on the proper range of swirl number. And it was shown that the flame interaction in multiple jets also increases $u_{b.o}$ owing to the internal recirculation and reduces NO emission. If the effects of swirl and flame interaction are combined together in partially premixed flame, both $u_{b.o}$ increasement and NOx emission reduction could be achieved. Blowout limits of partially premixed interacting propane flame in the swirling air coflow are investigated experimentally. The results show that the flame is not extinguished up to the experimental limits, 210 m/s, at the swirl number of 0.32 and $X_{F,o}$ = 0.46.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.12 no.6
• /
• pp.1381-1389
• /
• 1988

The heat transfer by simultaneous conduction, convection and radiation between flame and fuel surface in a thermally radiating medium is investigated theoretically. The flame and fuel surface are assumed to be diffuse, gray, infinite, isothermal, parallel surfaces separated by a finite distance. The space between the plates is supposed is formulated exactly in terms of simultaneous interior-differential equations. The numerical results reveal the effect of the system parameters on the heat transfer characteristics and the temperature distributions. The numerical results reveal that the optically thick radiating medium has a blocking effect on the total beat transfer. The temperature distributions are observed to be convex upward for an optically thick radiating medium as the alberto decreases.

• Journal of the Korean Society of Safety
• /
• v.25 no.2
• /
• pp.12-17
• /
• 2010

Numerical simulations were performed for the prediction of the flame structure during the interaction between hydrogen and hydrocarbon flames. A counterflow flow geometry was introduced to establish the interacting two flames. Methane was used as a representative hydrocarbon fuel in this study. A well-known numerical code for the counterflow flame, OPPDIF, was used for the simulations. The detailed chemistry was adopted to predict the flame structure reasonably. The interaction of two one-dimensional premixed flames established in counterflow burner was investigated with the global strain rate and velocity ratio. It was found that the maximum temperature located near the methane flame surface while the heat release rate of methane was lower than hydrogen flame. The flame thickness become narrow with increasing the velocity ratio while the global strain rate was fixed. The local strain rate and heat release rate at the methane flame surface were correlated with the global strain rate, while those at the hydrogen flame were not correlated with the global strain rate. However, the maximum temperature of the interacting flames was correlated with the global strain rate.