• Atmosphere
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• v.30 no.4
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• pp.377-390
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• 2020

Not only emissions, but also atmospheric circulation is a key factor that affects local particulate matters (PM) concentrations in Korea through ventilation effects and transboundary transports. As part of the atmospheric circulation, air stagnation especially adversely affects local air quality due to weak ventilation. This study investigates the large-scale circulation related to air stagnation over Korea during winter and projects the climate change impacts on atmospheric patterns, using observed PM data, reanalysis and regional climate projections from HadGEM3-RA with Modified Korea Particulate matter Index. Results show that the stagnation affects the PM concentration, accompanied by pressure ridge at upper troposphere and weaken zonal pressure gradient at lower troposphere. Downscaling using HadGEM3-RA is found to yield Added-Value in the simulated low tropospheric winds. For projection of future stagnation, SSP5-8.5 and SSP1-2.6 (high and low emission) scenarios are used here. It has been found that the stagnation condition occurs more frequently by 11% under SSP5-8.5 and by 5% under SSP1-2.6 than in present-day climate and is most affected by changes in surface wind speed. The increase in the stagnation conditions is related to anticyclonic circulation anomaly at upper troposphere and weaken meridional pressure gradient at lower troposphere. Considering that the present East Asian winter monsoon is mainly affected by change in zonal pressure gradient, it is worth paying attention to this change in the meridional gradient. Our results suggest that future warming condition increase the frequency of air stagnation over Korea during winter with response of atmospheric circulation and its nonlinearity.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.3 no.1
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• pp.61-68
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• 1991

The purpose of this study is to investigate the heat transfer characteristics and the flow structure in the case of rectangular air jet impinging vertically on the flat heating surface. The maximum value of Nusselt number at stagnation point is observed at H/B=10. It is found that this trend has been caused by the effect of stretching of large scale vortex in the stagnation region. For potential core region the Nusselt number distribution in the downstream of the stagnation point decreases gradually and begins to increase at about X/B=3. From the flow visualization it could be seen that small eddy produced from the nozzle edge grows in large scale and that large scale eddy disturbed the thermal boundary layer on the heating plate. The local average Nusselt number becomes maximum at X/B=0.5 regardless of H/B variation.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.6 no.3
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• pp.291-301
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• 1994

This paper presents the promotion of heat transfer through the use of wire-mesh screens. To improve heat transfer in an impingement water system, the wire-mesh screens are installed between the nozzle-to-heater surfaces. When the wire-mesh screens are not employed, this report exhibits the maximum heat transfer and the secondary maximum value at the stagnation point. But in case of using the wire-mesh screens, the transfer coefficient value of maximum heat exists at the stagnation point, and the second maximum value doesn't occur. Therefore, the heat transfer is more improved than 4~6 times that of the mean Nusselt numbers of simple water jet system, Also, within the region presented in this study, the heat transfer was promoted by using the wire-mesh screens at the stagnation point ; thus, the heat transfer was more increased than 6-7. 5 times that of simple water jet system.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.768-773
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• 2004

When a gas expands through a convergent nozzle in which the ratio of the ambient to the stagnation pressures is higher than that of the critical one, the issuing jet from the nozzle is underexpanded. If a flat plate is placed normal to the jet at a certain distance from the nozzle, a detached shock wave is formed at a region between the nozzle exit and the plate. In general, supersonic moist air jet technologies with nonequilibrium condensation are very often applied to industrial manufacturing processes. In spite of the importance in major characteristics of the supersonic moist air jets impinging to a solid body, its qualitative characteristics can not even know. In the present study, the effect of the nonequilibrium condensation on the underexpanded moist air jet impinging on a vertical flat plate is investigated experimentally. Flow visualization and impact pressure measurement are performed for various relative humidities and flat plate positions. The obtained results show the plate shock and Mach disk are dependent on the nozzle pressure ratio and the relative humidity, but for a given nozzle pressure ratio, the diameters of the plate shock and Mach disk depend on the stagnation relative humidity. The impact pressure deviation from the flow of without condensation is large, as the relative stagnation humidity increases.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2154-2159
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• 2007

The potential of using a double-faced wall stagnation flow burner in mass production of carbon nanotubes was evaluated experimentally and computationally. With nitrogen-diluted premixed ethylene-air flames established on the Nickel-coated stainless steel double-faced wall, the propensities of carbon nanotube formation were experimentally determined using SEM and FE-TEM images and Raman spectroscopy, while the flame structure was computationally predicted using a 3-dimensional CFD code with a reduced reaction mechanism. The uniformity and yields of synthesized carbon nanotubes were evaluated in terms of the flame stretch rates. Results show substantial increase of area on the wall surface where uniform carbon nanotubes are synthesized with using the double-faced wall stagnation flow burner due to enhanced uniformity of temperature distribution along the wall surface and support the potential of using a double-faced wall stagnation flow burner in mass production of carbon nanotubes.

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.529-532
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• 2002

In coflow jets with relatively large size nozzle and low fuel jet velocity, the buoyancy effect arises from the density difference between fuel and air streams. The present study investigated the behavior of such a buoyant cold Jet both numerically and experimentally, especially when the fuel stream has higher density than air. It has been demonstrated that the cold jet has a circular cone shape since upwardly injected fuel jet decelerates and forms a stagnation region, when the fuel jet was composed of propane highly diluted with nitrogen. When the fuel was moderately diluted, numerical results showed the Kelvin-Helmholtz type instability along the mixing layer of the jet. The stagnation height increases nonlihearly with fuel jet velocity with the power of approximately 1.64.

• International Journal of Aeronautical and Space Sciences
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• v.9 no.1
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• pp.1-30
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• 2008

When the stagnation temperature of a perfect gas increases, the specific heats and their ratio do not remain constant any more and start to vary with this temperature. The gas remains perfect; its state equation remains always valid, except, it is named in more by calorically imperfect gas. The aim of this work is to trace the profiles of the supersonic axisymmetric Minimum Length Nozzle to have a uniform and parallel flow at the exit section, when the stagnation temperature is taken into account, lower than the dissociation threshold of the molecules, and to have for each exit Mach number and stagnation temperature shape of nozzle. The method of characteristics is used with the algorithm of the second order finite differences method. The form of the nozzle has a point of deflection and an initial angle of expansion. The comparison is made with the calorically perfect gas. The application is for air.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.3
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• pp.260-270
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• 1998

The viscous plane stagnation-flow solidification problem is theoretically investigated. An analytic solution at the beginning of solidification is obtained by expanding the temperature and thickness of solidified layer in powers of time. An exact expression for the steady-state thickness of solidified layer is also obtained. The .fluid flow toward the cold substrate inhibits the solidification process. As Stefan number becomes larger, or Prandtl number becomes smaller, the solidification is more strongly inhibited by the fluid flow. The transient heat flux at the liquid side of solid-liquid interface is increased, as Stefan number or Prandtl number is increased.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.1
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• pp.1-11
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• 2000

This study investigates the problem of phase change from liquid to solid in the inviscid stagnation flow. The solution of dimensionless governing equations is determined by the three dimensionless parameters of (temperature ratio/conductivity ratio), Stefan number, and diffusi-vity ratio. The solution at the initial stage of freezing is obtained by expanding it in powers of time, and the final equilibrium state is determined from the steady-state governing equations. The equilibrium state is dependent on (temperature ratio/conductivity ratio), but is independent of Stefan number and diffusivity ratio. The effect of fluid flow on the pure conduction problem can be clearly seen from the solution of the initial stage and the final equilibrium state, and the characteristics of the solidification process for all the dimensionless parameters are elucidated.

• Journal of the Semiconductor & Display Technology
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• v.20 no.3
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• pp.77-82
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• 2021

This numerical study focuses on the analysis of fume particle dispersion characteristics over the surface of target workpiece in laser micro-hole machining process. The effects of oblique stagnation flow over fume generating machining point are examined by carrying out a series of three-dimensional random particle simulations along with probabilistic particle generation model and particle drag correlation of low Reynolds number. Present computational model of fume particle dispersion is found to be capable of assessing and quantifying the fume particle contamination in precision hole machining which may influenced by different types of air flow patterns and their flow intensity. The particle size dependence on dispersion distance of fume particles from laser machining point is significant and the effects of increasing flow oblique angle are shown quite differently when slot blowing or slot suction flows are applied in micro-hole machining.