Papers by Prof. Jeong Park
A two-dimensional direct numerical simulation is performed to investigate the flame structure of ... more A two-dimensional direct numerical simulation is performed to investigate the flame structure of counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman#s detailed reaction mechanism are adopted in this calculation. To quantify the strain on flame induced by a vortex, a scalar dissipation rate (SDR) is introduced. The results show that fuel-side and air-side vortex cause an unsteady extinction. In this case, the flame interacting with a vortex is extinguished at much larger SDR than steady flame. It is also found that air-side vortex extinguishes a flame more rapidly than fuel-side vortex. The unsteady effect induced by flame-vortex interaction does not lead to a transient OH overshoot of the maximum steady concentration observed in experiment, while radical increases more than the maximum steady concentration with increasing SDR. In addition, it is seen that NO an...
International Journal of Spray and Combustion Dynamics, 2009
A study was conducted to clarify flame characteristics through the evaluation of critical mole fr... more A study was conducted to clarify flame characteristics through the evaluation of critical mole fractions at flame extinction and edge-flame oscillation of low strain rate flames using the global strain rate, velocity ratio, and burner distance as experimental variables. The transition from a shrinking flame disk to a flame hole was verified through gradient measurements of the maximum flame temperature. Evidence of edge-flame oscillation in flame disks was also found using numerical simulations in zero and normal gravity. The main mechanisms of flame extinction and edge-flame oscillation were analyzed by comparing the energy fractions in the energy equation. For low strain rate flame disks, radial conduction heat loss rather than flame radiation was a significant contributor to flame extinction and even edge-flame oscillation. This was experimentally demonstrated by evaluating the critical mole fraction at flame extinction and edge-flame oscillation, as well as measurements of the f...
Transactions of the Korean Society of Mechanical Engineers B, 2009
Experiments are conducted to elucidate effects of counterflow burner diameter on flame extinction... more Experiments are conducted to elucidate effects of counterflow burner diameter on flame extinction behaviors in C-curve. Present experimental results with burner diameters of 18, 26, and 50 mm in normal-gravity are compared with the numerical result of Oppdif code as well as the previous experimental results in micro-gravity. The turning point migrates to a higher global strain rate as burner diameter decreases. It is shown that the C-curve with the burner diameter of 50mm is best-fitted to the numerical result of Oppdif code and the previous micro-gravity results also excurse to the numerical result. This suggests that the precise C-curve can be obtained only with an appreciably large burner. The main reason why these differences appear is shown to be attributed to the transition of shrinking flame disk to flame hole due to strong effects of radial conduction heat loss, which is the typical extinction characteristics of low strain rate flames with a finite burner diameter in a counterflow diffusion flame.
Transactions of the Korean Society of Mechanical Engineers B, 2012
Fuel, 2016
h i g h l i g h t s Chemical effects of additional CO 2 on flame extinction are exhibited and dis... more h i g h l i g h t s Chemical effects of additional CO 2 on flame extinction are exhibited and discussed. Effects of radiation heat loss on flame extinction are compared in oxymethane flames. A capability of predicting extinction strain rate via a radical index and oxidizer Lewis number is evaluated. Impact of radiation and chemical effects on reduction of NO x formation is discussed.
The present study was conducted to investigate the flame instability(evaluated by Markstein lengt... more The present study was conducted to investigate the flame instability(evaluated by Markstein length and cellular instability) and laminar burning velocity in a constant volume combustion chamber at room temperature and elevated pressure up to 0.3 MPa to suggest the possibility of utilizing mixtures of syngas added DME-air premixed flames in internal combustion engines. The experimentally measured laminar burning velocities were compared to predictions calculated the PREMIX code with Zhao reaction mechanism. Discussions were made on effects of syngas addition into DME-Air premixed flames through evaluating laminar burning velocity, Markstein length, and cellular instability. Particular concerns are focused on cellular instability caused by hydrodynamic instability and diffusive-thermal instability.
International Journal of Spray and Combustion Dynamics, 2010
Experiments and numerical simulations were conducted to investigate the effects of the burner dia... more Experiments and numerical simulations were conducted to investigate the effects of the burner diameter on the flame structure and extinction limit of counterflow non-premixed methane flames in normal gravity and microgravity. Experiments were performed for counterflow flames with a large inner diameter ( d) of 50 mm in normal gravity to compare the extinction limits with those obtained by previous studies where a small burner ( d < 25 mm) was used. Two-dimensional (2D) simulations were performed to clarify the flame structure and extinction limits of counterflow non-premixed flame with a three-step global reaction mechanism. One-dimensional (1D) simulations were also performed with the same three-step global reaction mechanism to provide reference data for the 2D simulation and experiment. For microgravity, the effect of the burner diameter on the flame location at the centerline was negligible at both high ( ag = 50 s−1) and low ( ag = 10 s−1) strain rates. However, a small burn...
Materials Science Forum, 2011
Experiments in a constant pressure combustion chamber at room temperature and elevated pressures ... more Experiments in a constant pressure combustion chamber at room temperature and elevated pressures using schlieren system were conducted to investigate the cellular instabilities in hydrogen/carbon monoxide/methane (or propane)–air premixed flames. In the present study, hydrodynamic and diffusional-thermal instabilities were evaluated to elucidate their effects to flame instabilities. Effective Lewis numbers of premixed flames with methane addition decrease for all of the cases. Meanwhile, the effective Lewis numbers with propane addition increase for lean and stoichiometric conditions, but they increase for rich and stoichiometric cases for hydrogen-enriched flames. With propane addition, the propensity for cells formation is significantly diminished whereas the cellular instabilities for hydrogen enriched flames are promoted. With methane addition, the similar behavior of cellularity is obtained, indicating that methane is not a candidate for suppressing cells formation in hydrogen/carbon monoxide/methane–air premixed flames.
Journal of Mechanical Science and Technology, 2008
The compressible flow field is numerically analyzed in a two-dimensional converging-diverging noz... more The compressible flow field is numerically analyzed in a two-dimensional converging-diverging nozzle of which the area ratio, exit to throat, is 1.8. The solver is FLUENT and the embedded RNG k ε − model is adopted to simulate turbulent flow. The plume characteristics such as shock-cell structure are discussed when nozzle pressure ratio and stagnation temperature at the nozzle entrance are varied. The downstream flow field can be classified into two types based on the shock shapes generated near the nozzle exit. First, a reiterative pattern in the plume is not formed between the slip streams in case that a strong lambda-type shock wave exists. Second, when oblique shock waves are crossing each other on the nozzle centerline, a shock cell structure appears in the plume field. Even when the flow field is changed due to stagnation temperature, the upstream of the shock wave is little affected. Especially, the pressure distributions on the nozzle centerline behind the shock wave are rarely influenced by the stagnation temperature, that is, the product of density and temperature is nearly constant provided that the working fluid is a perfect gas. Therefore, the pressure field shows quasi-isobaric behavior far downstream.
International Journal of Hydrogen Energy, 2009
International Journal of Hydrogen Energy, 2010
Experiments were conducted in a constant pressure combustion chamber using schlieren system to in... more Experiments were conducted in a constant pressure combustion chamber using schlieren system to investigate the effects of carbon dioxide/nitrogen/helium diluents on cellular instabilities of syngas-air premixed flames at room temperature and elevated pressures. Laminar burning velocities and Markstein lengths were calculated by analyzing high-speed schlieren images at various diluent concentrations and equivalence ratios. Experimental results showed substantial reduction of the laminar burning velocities and of the Markstein lengths with the diluent additions in the fuel blends. Effective Lewis numbers of helium-diluted syngas-air flames increased but those of carbon dioxide-and nitrogen-diluted syngas-air flames decreased in increase of diluents in the reactant mixtures. With helium diluent, the propensity for cells formation was significantly diminished, whereas the cellular instabilities for carbon dioxide-diluted and nitrogen-diluted syngas-air flames were not suppressed.
International Journal of Hydrogen Energy, 2010
Numerical and experimental studies are conducted to grasp downstream interactions between premixe... more Numerical and experimental studies are conducted to grasp downstream interactions between premixed flames stratified with two different kinds of fuel mixture. The selected fuel mixtures are methane and a nitrogen-diluted hydrogen with composition of 30% H 2 þ 70% N 2. Extinction limits are determined for methane/air and (30% H 2 þ 70% N 2)/air over the entire range of mixture concentrations. These extinction limits are shown to be significantly modified due to the interaction such that a mixture much beyond the flammability limit can burn with the help of a stronger flame. The lean extinction limit shows both the slanted segments of lower and upper branches due to the strong interaction with Lewis numbers of deficient reactant less than unity, while the rich extinction limit has a square shape due to the weak interaction with Lewis numbers of deficient reactant larger than unity. The regimes of negative flame speed show an asymmetric aspect with a single wing shape. The negative flame always appears only when methane is weak. The extent of interaction depends on the separation distance between the flames, which are the functions of the mixtures' concentrations, the strain rate, the Lewis numbers, and the preferential diffusions of the penetrated hydrogen from the nitrogen-diluted hydrogen flame. The important role of preferential diffusion effects of hydrogen in the flame interaction is also discussed.
International Journal of Hydrogen Energy, 2008
International Journal of Hydrogen Energy, 2008
Heat release rate H 2 /CO syngas flame diluted CO 2 dilution Oxidation reaction pathway Radiative... more Heat release rate H 2 /CO syngas flame diluted CO 2 dilution Oxidation reaction pathway Radiative heat loss a b s t r a c t Numerical study is conducted to understand the impact of fuel composition and flame radiation in flame structure and their oxidation process in H 2 /CO synthetic gas diffusion flame with and without CO 2 dilution. The models of Sun et al. and David et al., which have been well known to be best-fitted for H 2 /CO synthetic mixture flames, are evaluated for H 2 / CO synthetic mixture flames diluted with CO 2. Effects of radiative heat loss to flame characteristics are also examined in terms of syngas mixture composition. Importantly contributing reaction steps to heat release rate are compared for the synthetic gas mixture flames of high contents of H 2 and CO, individually, with and without CO 2 dilution. The modification of the oxidation pathways is also addressed.
International Journal of Hydrogen Energy, 2009
Numerical study is conducted to grasp flame characteristics in H 2 /CO syngas counterflow diffusi... more Numerical study is conducted to grasp flame characteristics in H 2 /CO syngas counterflow diffusion flames diluted with He and Ar. An effective fuel Lewis number, applicable to premixed burning regime and even to moderately stretched diffusion flames, is suggested through the comparison among fuel Lewis number, effective Lewis number, and effective fuel Lewis number. Flame characteristics with and without the suppression of the diffusivities of H, H 2 , and He are compared in order to clarify the important role of preferential diffusion effects through them. It is found that the scarcity of H and He in reaction zone increases flame temperature whereas that of H 2 deteriorates flame temperature. Impact of preferential diffusion of H, H 2 , and He in flame characteristics is also addressed to reaction pathways for the purpose of displaying chemical effects.
International Journal of Hydrogen Energy, 2011
To investigate cell formation in methane (or propane)/hydrogen/carbon monoxideeair premixed flame... more To investigate cell formation in methane (or propane)/hydrogen/carbon monoxideeair premixed flames, the outward propagation and development of surface cellular instabilities of centrally ignited spherical premixed flames were experimentally studied in a constant pressure combustion chamber at room temperature and elevated pressures. Additionally, unstretched laminar burning velocities and Markstein lengths of the mixtures were obtained by analyzing high-speed schlieren images. In this study, hydrodynamic and diffusional-thermal instabilities were evaluated to examine their effects on flame instabilities. The experimentally-measured unstretched laminar burning velocities were compared to numerical predictions using the PREMIX code with a H 2 /CO/C 1 eC 4 mechanism, USC Mech II, from Wang et al. [22]. The results indicate a significant increase in the unstretched laminar burning velocities with hydrogen enrichment and a decrease with the addition of hydrocarbons, whereas the opposite effects for Markstein lengths were observed. Furthermore, effective Lewis numbers of premixed flames with methane addition decreased for all of the cases; meanwhile, effective Lewis numbers with propane addition increase for lean and stoichiometric conditions and increase for rich and stoichiometric cases for hydrogen-enriched flames. With the addition of propane, the propensity for cell formation significantly diminishes, whereas cellular instabilities for hydrogen-enriched flames are promoted. However, similar behavior of cellularity was obtained with the addition of methane, which indicates that methane is not a candidate for suppressing cell formation in methane/hydrogen/carbon monoxideeair premixed flames.
International Journal of Hydrogen Energy, 2011
The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed... more The effect of strain rate in downstream interactions between lean (rich) and lean (rich) premixed syngas flames with the fuel composition of 50% H 2 and 50% CO is numerically investigated by varying the strain rate in the range of 5w500 s À1. The flame stability maps for several strain rates are presented and main concerns are focused on the downstream interactions on the lean and rich extinction boundaries. The fuel composition of 50% H 2 and 50% CO with effective Lewis numbers larger than unity for both lean and rich extinction boundaries is chosen for grasping the important role of hydrogen with the deficient reactant Lewis numbers much smaller than unity. The results show that the lean extinction boundaries have the slanted shape, thereby leading to strong interactions; meanwhile the rich extinction boundaries at appropriately low strain rates are of square, indicating weak interactions. However, at highly strained interacting rich flames, the rich extinction boundaries show a slanted shape, thereby leading to strong interactions even for Lewis numbers much larger than unity. In such situations, thermal and chemical interactions are explained in detail. It is found that, in interacting flames, the excessive heat loss of the stronger flame partly to the weaker flame and mostly to the ambience is the mechanism of flame extinction.
International Journal of Energy Research, 2002
ABSTRACT The dilution effect of air stream according to agent type on flame structure and NO emis... more ABSTRACT The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically analysed with detailed chemistry. The adopted fuel is hydrogen diluted with the argon of volume percentage 50 per cent and the volume percentage of diluents (H2O, CO2 and N2) in air stream is systematically changed from 10 to 50. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour, especially at low strain rates.The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO2&gt;H2O&gt;N2. The qualitative tendency of the numerically predicted mole fractions of H, O and OH is well described using a simplified formula, based on a partial equilibrium concept. It is seen that the H2O addition to air stream is the most effective for reducing NO emission. In the case of the addition of H2O and N2 the NO emission behaviour is governed by the thermal effect and in the case of CO2 addition it is governed by both the thermal effect and the chemical effect. But the chemical effect, which is mainly attributed by the Fenimore mechanism to the breakdown of CO2, is much more predominant in comparison with the thermal effect. Copyright © 2002 John Wiley &amp; Sons, Ltd.
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Papers by Prof. Jeong Park