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RISS 인기검색어
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- 저자 : Yiguang Ju (검색결과 4 건)
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Sohn, Chae Hoon,Chen, Zheng,Ju, Yiguang Elsevier 2015 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.86 No.-
<P><B>Abstract</B></P> <P>This work investigates numerically the effects of spectral dependent radiation on laminar flame speed determination using spherically propagating CH<SUB>4</SUB>/air and H<SUB>2</SUB>/air flames with CO/CO<SUB>2</SUB>/H<SUB>2</SUB>O dilutions at elevated pressures. Three different models, adiabatic, optically thin radiation, and fitted statistically narrow band correlated <I>k</I> (FSNB-CK) models, are employed. The effects of radiation-induced negative burned gas velocity, increased density ratio, and chamber confinement induced flow compression are investigated. It is found that compared to the FSNB-CK model, the adiabatic flame model over-predicts the flame speed by 7% and the optically thin model makes more significant under-prediction. Moreover, this discrepancy increases with pressure. The results also show that a large negative velocity in the burned gas is induced by radiative heat loss and magnified further by the flow compression in a small combustion chamber. The radiation-induced negative burned gas velocity causes an under-estimation of flame speed. Moreover, radiation also increases the density ratio between the burned and the unburned gases. The use of the density ratio of adiabatic flame also causes under-prediction of flame speed. Two radiation corrections taking into account of the negative burned gas velocity and the increased density ratio are recommended for flame speed determination using propagating spherical flame for radiative mixtures. The corrections proposed in this study reduce the uncertainty of flame speed due to radiation.</P>
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손채훈,손진우,Sang Hee Won,Yiguang Ju 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.3
The diffusion cool flame structures of n-heptane are studied numerically with a comprehensively reduced chemistry. For this study, nheptane/oxygen diffusion flame is self-sustained in a counterflow flame configuration with ozone sensitization. N-heptane is diluted bynitrogen to lower flame temperature and a small amount of ozone is added to oxygen to enhance oxygen atom production. N-heptanediffusion flame shows both hot-flame and cool-flame behaviors in high and low temperature regimes, respectively. Its S-shaped curvehas two upper branches of hot and cool flame branches. The upper branch of cool flame is observed at low strain rates with the order ofmagnitude from O(1) to O(102) s-1 and its zone is much narrower than that of hot flame. N-heptane cool flame survives irrespective ofozone addition, but ozone extends the viable or stable flame zone to higher strain rate. Cool-flame temperatures are below 800 K andradical-induced ignition comes into play. N-heptane is highly diluted by nitrogen and extinction strain rate is more sensitive to mole fractionof n-heptane rather than mole fraction of ozone. Heat release rates of elementary reaction steps and concentrations of major radicalsare far lower in cool flames than in hot flames. Main characteristics of cool flame structures at low strain rates and low temperatures areprovided although the present low-temperature kinetics is not complete.
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Thermo-kinetic dynamics of near-limit cool diffusion flames
Sohn, Chae Hoon,Han, Hee Sun,Reuter, Christopher B.,Ju, Yiguang,Won, Sang Hee Elsevier 2017 Proceedings of the Combustion Institute Vol.36 No.1
<P><B>Abstract</B></P> <P>The dynamics of near-limit cool diffusion flames are investigated experimentally and numerically by studying transient flame evolution and instability. In order to observe the effects of chemistry-transport coupling on ignition and instability in a cool diffusion flame, diffusion flames of n-heptane and pure oxygen are sensitized by ozone in a counterflow burner. First, it is found from experimental observations that the immediate addition of ozone to the oxidizer side of a frozen flow either initiates a cool diffusion flame or triggers a two-stage ignition into a hot flame. Second, a cool flame near its extinction limit can be destabilized by a perturbation of the fuel mole fraction, eventually leading to flame extinction. The recorded flame chemiluminescence signals reveal repeated flame oscillations before extinction. Next, details of the transient dynamics of near-limit cool diffusion flames are explored through numerical calculations by adopting the same perturbation method. Experimental observations of unsteady flame initiation and instability are simulated, and it is found that the ignition process is highly sensitive to the perturbed ozone mole fraction and that the ignition delay times in a counterflow configuration are significantly affected by the diffusive transport of species with high concentration. The instability mechanism of a cool flame is found to be distinct from that of diffusive-thermal instability. The results show that the instability behavior of a cool flame is a thermo-kinetic instability, which is triggered and controlled by the chemical kinetics associated with the OH radical population in the negative temperature coefficient chemical kinetic regime coupled with heat production and loss.</P>
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