• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.9
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• pp.859-866
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• 2010

Nonlinear dynamics of pulsating instability-diffusional-thermal instability with Lewis numbers sufficiently higher than unity-in counterflow diffusion flames, is numerically investigated by imposing a Damkohler number perturbation. The flame evolution exhibits three types of nonlinear behaviors, namely, decaying pulsating behavior, diverging behavior (which leads to extinction), and stable limit-cycle behavior. The stable limit-cycle behavior is observed in counterflow diffusion flames, but not in diffusion flames with a stagnant mixing layer. The critical value of the perturbed Damkohler number, which indicates the region where the three different flame behaviors can be observed, is obtained. A stable simple limit cycle, in which two supercritical Hopf bifurcations exist, is found in a narrow range of Damkohler numbers. As the flame temperature is increased, the stable simple limit cycle disappears and an unstable limit cycle corresponding to subcritical Hopf bifurcation appears. The period-doubling bifurcation is found to occur in a certain range of Damkohler numbers and temperatures, which leads to extend the lower boundary of supercritical Hopf bifurcation.

• Bulletin of the Korean Chemical Society
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• v.4 no.5
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• pp.228-230
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• 1983

Molecular reorientation of oblate symmetric top molecules with internal rotation is investigated theoretically and an analytic expression for the overall reorientational correlation time in terms of the internal angular momentum correlation time is derived. This expression is quite different from the expression for prolate symmetric top molecules but reduces to the same expression in the spherical top limit. Fast internal rotation is treated by a modified version of the extended rotational diffusion while the bulky symmetric top mainbody is treated by the rotational diffusion model.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.8
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• pp.857-864
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• 2012

A linear stability analysis of a diffusion flame with radiation heat loss is performed to identify linearly unstable conditions for the Damk$\ddot{o}$hler number and radiation intensity. We adopt a counterflow diffusion flame with unity Lewis number as a model. Near the kinetic limit extinction regime, the growth rates of disturbances always have real eigenvalues, and a neutral stability condition perfectly falls into the quasi-steady extinction. However, near the radiative limit extinction regime, the eigenvalues are complex, which implies pulsating instability. A stable limit cycle occurs when the temperatures of the pulsating flame exceed the maximum temperature of the steady-state flame with real positive eigenvalues. If the instantaneous temperature of the pulsating flame is below the maximum temperature, the flame cannot recover and goes to extinction. The neutral stability curve of the radiation-induced instability is plotted over a broad range of radiation intensities.

• Korean Journal of Environmental Biology
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• v.23 no.4
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• pp.374-378
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• 2005

A diffusion - precipitation method was developed to determine acid volatile sulfide (AVS) concentrations in freshwater sediments. This method uses silver nitrate as a sulfide trap solution and the concentration of trapped sulfide is determined gravimetrically. The proposed diffusion - precipitation method is more rapid and less expensive than previously developed purge- and - trap methods. Spiked sodium sulfide recoveries using this method $(97\~120\%)$ were similar with a previously developed diffusion - absorption method $(93.8\~115\%)$ and about $20\%$ greater than a previously developed purge-and-trap method $(74.6\~105\%)$. Detection limit of this method $(0.1\;{\mu}mole\;S\;g^{-l})$ was comparable with that of diffusion-absorption method $(0.06\;{\mu}mole\;S\;g^{-l})$ and purge-and-trap method $(0.05\~0.5\;{\mu}mole\;S\;g^{-l})$.

• Communications for Statistical Applications and Methods
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• v.21 no.4
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• pp.327-334
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• 2014

Various modified GARCH(1, 1) models have been found adequate in many applications. We are interested in their continuous time versions and limiting properties. We first define a stochastic integral that includes useful continuous time versions of modified GARCH(1, 1) processes and give sufficient conditions under which the process is exponentially ergodic and ${\beta}$-mixing. The central limit theorem for the process is also obtained.

• Bulletin of the Korean Chemical Society
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• v.17 no.2
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• pp.192-198
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• 1996

We present a theoretical formulation of diffusion process on solid surface based on multidimensional transition state theory (TST). Surface diffusion of single adatom results from hopping processes on corrugated potential surface and is affected by surface vibrations of surface atoms. The rate of rare events such as hopping between lattice sites can be calculated by transition state theory. In order to include the interactions of the adatom with surface vibrations, it is assumed that the coordinates of adatom are coupled to the bath of harmonic oscillators whose frequencies are those of surface phonon modes. When nearest neighbor surface atoms are considered, we can construct Hamiltonians which contain terms for interactions of adatom with surface vibrations for the well minimum and the saddle point configurations, respectively. The escape rate constants, thus the surface diffusion parameters, are obtained by normal mode analysis of the force constant matrix based on the Hamiltonian. The analysis is applied to the diffusion coefficients of W, Ir, Pt and Ta atoms on the bcc(110) plane of W in the zero-coverage limit. The results of the calculations are encouraging considering the limitations of the model considered in the study.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.3 s.234
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• pp.390-401
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• 2005

In this study, extinction limit extension of unsteady $(CH_{4}+N_{2})$/air diffusion flames was investigated experimentally. A spatially locked flame in an opposing jet burner was perturbed by linear velocity variation, and time-dependent flame luminosity, transient maximum flame temperature and OH radical were measured over time with the high speed camera, Rayleigh scattering method and OH laser-induced fluorescence, respectively. Unsteady flames survive at strain rates that are much higher than the extinction limit of steady flames, and unsteady extinction limits extend as the slope of the strain rate increases or the initial strain rate decreases. We verified the validity of the equivalent strain rate concept by comparing the course of unsteady extinction process and steady extinction process, and it was found that the equivalent strain rate concept represents well the unsteady effect of a convective-diffusive zone. To investigate the reason of the unsteady extinction limit extension, we subtracted the time lag of the convective-diffusive zone by using the equivalent strain concept. Then the modified unsteady extinction limits become smaller than the original unsteady extinction limits, however, the modified unsteady extinction limits are still larger than the steady extinction limits. These results suggest that there exist the unsteady behavior of a diffusive-reactive zone near the extinction limit due to the chemical non-equilibrium states associated with unsteady flames.

• Bulletin of the Korean Chemical Society
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• v.32 no.6
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• pp.1975-1979
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• 2011

The modification of a gas diffusion layer (GDL), a vital component in polymer electrolyte fuel cells, is described here for use in the electrochemical detection of antibody-antigen biosensors. Compared to other substrates (gold foil and graphite), mouse anti-rHBsAg monoclonal antibody immobilized on gold-coated GDL (G-GDL) detected analytes of goat anti-mouse IgG antibody-ALP using a relatively low potential (-0.0021 V vs. Ag/AgCl 3 M NaCl), indicating that undesired by-reactions during electrochemical sensing should be avoided with G-GDL. The dependency of the signal against the concentration of analytes was observed, demonstrating the possibility of quantitative electrochemical biosensors based on G-GDL substrates. When a sandwich method was employed, target antigens of rHBsAg with a concentration as low as 500 ng/mL were clearly measured. The detection limit of rHBsAg was significantly improved to 10 ng/mL when higher concentrations of the 4-aminophenylphosphate monosodium salt (APP) acting on substrates were used for generating a redox-active product. Additionally, it was shown that a BSA blocking layer was essential in improving the detection limit in the G-GDL biosensor.

• Journal of the Korean Society of Combustion
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• v.20 no.4
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• pp.19-25
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• 2015

An experimental study was conducted to find the effect of a pilot flame on the flammability of inert-gas-diluted methane and propane. The diffusion pilot flame was formed with propane at the innermost nozzle of a concentric triple co-flow burner. The main diffusion flame was formed with nitrogen-diluted methane or propane at the outermost nozzle of the burner. An air flow was located in-between. The results showed that the existence of the pilot flame helped stabilizing the main flame even at the flammability limit concentration of nitrogen-diluted fuel. The co-flow burner generated re-circulation zones and local variation of equivalence ratio depending on the flow rates of the reactants, which are known to help flame stabilization. Hot-wire experiments confirmed that both heating of the reactants and supplying of active chemical species by the pilot flame contributed to stabilization of the main flame. The results of this study would suggest a design concept for an efficient SVRU system that minimizes the emission of unburned hydrocarbon fuel from ship fuel tanks.

• Journal of the Korean Society of Safety
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• v.24 no.6
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• pp.27-31
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• 2009

An experimental study on the unsteady effect of the extinction limit was performed in ethene jet diffusion flames. To impose the unsteadiness on jet flames, the amplitude and frequency of a co-flow velocity was varied, and the two inert gases, $N_2$ and $CO_2$, were used to dilute the oxidizer for extinguishing concentration. The experimental results shows that large amplitude of velocity induces a low extinguishing concentration, which implies that flow variation affects the blow out mechanism. Also, the flow oscillation effects under high frequency attenuates the flame extinction. These results means that flow unsteadiness extends the extinction limit and finally minimum extinction concentration by inert gases. When the Stoke's 2nd Problem is introduced to explain the flow unsteadiness on extinction concentration, the solution predicts the effect of amplitude and frequency of velocity well, and hence it is concluded the effect of low frequency velocity excitation was attributed only to flow effect.