• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.12
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• pp.1061-1069
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• 2010

In this study, a simplified axisymmetric model is proposed for the problem of compressible internal flow past a microgap. Using numerical and experimental methods, the phenomena of choked flows are observed; these flows are induced by the acceleration of subsonic flows past the narrow cross-section of an annular shape made by a microgap. The relation between mass flow rate and differential pressure is obtained, and by comparing the result with experimental results, the reliability of the numerical results is discussed. The generation of a supersonic jet flow and its diffraction are visualized by performing the numerical analysis of axisymmetric compressible Navier-Stokes equations. This investigation greatly extends the physical understanding of the axisymmetric compressible flow, which has a wide range of engineering applications, e.g., in the case of valves in automotive power systems.

• Journal of Biosystems Engineering
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• v.29 no.3
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• pp.217-224
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• 2004

This research was conducted to test the feasibility of utilizing high-pressure water jets of over 1.0㎫ as a means of breaking and peeling garlic bulbs. High-pressure plunger pumps and flat-spray nozzles of varying orifice diameters and spray angles were utilized to supply water jets into a prototype peeling chamber made of transparent acrylic plates. Water jets were discharged from a total of six nozzles installed in such a way that three parallel nozzles face the other three. The cross-sectional area of the peeling chamber and the installation angle of the nozzles had critical effects on peeling performance. Small cross-sectional area was required so that total impact force of water jets on garlic could be increased. The optimum installation angles were around 4, 8, and 16$^{\circ}$ for the nozzles having 15, 40, and 65$^{\circ}$ spray angles, respectively. Best performance with 61.4% of completely-peeled garlics was obtained at a pressure of 1.94㎫ and a flow rate of 9.07 $\ell$/min for each nozzle. The peeling efficiency of the system was generally unsatisfactory due to the limited flow rate of the plunger pumps utilized. For better performance, it is recommended to increase flow rate while reducing operating pressure by utilizing other type of pumps.

• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.1
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• pp.105-115
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• 2014

A numerical analysis on the smoke behavior and evacuee safety has been performed with computational fluid dynamics. The purpose of this study is to build computational processes for an evacuation and prevention of a fire disaster of a 3 km-length tunnel in Korea. To save computational cost, 1.5 km of the tunnel that can include a few cross-passing tunnels is considered. We are going to assess the fire safety in a road tunnel according to the smoke level, which consists of the smoke density and the height from the floor. The smoke density is obtained in detail from three-dimensional unsteady CFD analysis. To obtain proper temperature distributions on the tunnel wall, one-dimensional conduction equation is considered instead of an adiabatic wall boundary or a constant heat flux. The tunnel considered in this study equips the cross passing tunnels for evacuees every 250 m. The distance is critical in both safety and economy. The more cross passing tunnels, the more safe but the more expensive. Three different jet fan operations can be considered in this study; under- and over-critical velocities for normal traffic condition and 0-velocoty operation for the traffic congestion. The SE (smoke environment) level maps show a smoke environment and an evacuating behavior every moment.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.4
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• pp.619-625
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• 2019

It is difficult to design because of the plug-holing phenomenon in which the amount of smoke discharged from the vertical vent is smaller than the designed amount of smoke. In this study, the effect of cross-sectional area ratio of tunnel and natural ventilation and heat release rate of fire source on plug-holing phenomenon occurring in natural ventilation system was experimentally analyzed. In the experiment model reduced to 1/20 size, the aspect ratio of the tunnel and the vertical vent was fixed, and the influence on the plug-holing phenomenon was confirmed by varying the sectional area ratio of the tunnel and the vertical vent. Experimental results show that the plug-holing phenomenon is caused by the comparison of the smoke boundary layer temperature with the temperature in the vertical vents, and the flow and temperature distribution characteristics under the vertical vents are changed as the cross-sectional area ratio of the tunnel and vertical vents increases. The plug-holing phenomenon is affected by the cross-sectional area ratio between the tunnel and the vertical ventilation. The greater the cross-sectional area ratio, the greater the probability of plug-holing.

• Journal of Ocean Engineering and Technology
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• v.30 no.1
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• pp.44-50
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• 2016

An experimental study was performed to investigate the behavior of green water on a structure with a rectangular cross section under wave conditions, along with the flow characteristics in bubbly water flow. An experiment was conducted in a two-dimensional wave flume using an acrylic model (1/125) of FPSO BW Pioneer operating in the Gulf of Mexico under its design wave condition. The occurrence of green water, including its development, in front of the model was captured using a high-speed Charge Coupled Device (CCD) camera with the shadowgraph technique. Using consecutive images, the generation procedure for green water on the model was divided into five phases: flip through, air entrapment, wave run-up, wave overturning, and water shipping. In addition, the distinct water elevations of the green water were defined as the height of flip through, height of splashing jet, and height of freeboard exceedance, and showed a linear relationship with the incoming wave height.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.21 no.1
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• pp.54-62
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• 2009

The present study investigates the behaviour of overtopping flows falling on the leeside of a caisson breakwater with dissipating blocks through laboratory measurements. The falling overtopping flows in the leeside are expected to directly affect the leeside stability of the breakwater. This study focuses on not the resultant stability but the characteristic pattern of the overtopping flows depending on wave conditions through examining front velocity and plunging distance in the leeside. Regular waves were used to investigate the dependence of the overtopping flow pattern on wave conditions and a modified image velocimetry combining the shadowgraphy and cross-correlation method was employed for measurements of image and velocity. From the measurements, it is shown that the plunging distance and front velocity of the overtopping flow in the breakwater leeside increase as the wave period or height increases. From non-dimensional relationships between the variables, empirical formula for the velocity and overtopping distance are suggested.

• 한국전산유체공학회:학술대회논문집
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• 2005.10a
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• pp.93-97
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• 2005

Air interlacing serves to protect the yarn against damage, strengthens inter-filament compactness or cohesion, and ensures fabric consistency. The air interlacing nozzle is used to introduce intermittent nips to a filament yarn so as to improve its performance in textile processing. This study investigates the effect of interlacing nozzle geometry on the interlacing process. The geometries of interlacing nozzles with multiple air inlets located across the width of a yarn channels are investigated. The basic interlacing nozzle is the yarn channel, with a perpendicular single air inlet in the middle. The yarn channel shapes are cross sections with semicircular or rectangular shapes. This paper presents three doubled sub air inlets with main air inlet and one of them is slightly inclined doubled sub air inlets with main air inlet. The compressed air coming out from the inlet hits the opposing wall of the yarn channel, divides into two branches, flows trough the top side of yarn channel, joins with the compressed air coming out from the sub air inlet and then creates two free jets at both ends of the yarn channel. The compressed air moves in the shape of two opposing directional vortices. The CFD-FASTRAN was used to perform steady simulations of impinging jet flow inside of the interlace nozzles. The vortical structure and the flow pattern such as pressure contour, particle traces, velocity vector plots inside of interlace nozzle geometry are discussed in this paper.

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

• Journal of the Korean Society of Propulsion Engineers
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• v.8 no.1
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• pp.61-69
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• 2004

This paper describes numerical efforts to characterize the flame-holding and air-fuel mixing process of model SCRamjet engine combustor, where a hydrogen jet injected into a supersonic cross flow and in a cavity Combustion phenomena in a model SCRamjet engine, which has been experimentally studied at University of Queensland and Australian National University using a free-piston shock tunnel, was observed around separation region of upstream of the normal injector and inside of cavity. The results show that the separation region and cavity generates several recirculation zones, which increase the fuel-air mixing. Self ignition occurs in the separation-freestream and cavity-freestream interface.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.8
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• pp.503-510
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• 2016

Incinerators generally emit pollutants such as NOx and CO during the combustion process. In this paper, pollutant emissions and temperature distributions were studied in a simulated incinerator with a reversed (relative to the flue gas flow) secondary air injection system. The experiments were performed by using a lab-scale furnace in order to evaluate the effects of the injection location, direction and flow rate of secondary air jets. The emission of NOx was lower in the case of reversed secondary air injection than in the case of cross injection, due to the recirculation and mixing of the exhaust gas. In the reversed air injection cases, thermal NOx emissions decreased as secondary air ratio increased from 30 to 60 and slightly increased at secondary air ratios higher than 60. In most cases, CO emissions were not detected except for a few reversed secondary air injection cases, in which cases CO concentrations below 2ppm were observed.