• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.1
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• pp.97-107
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• 2015

An analytical model was developed to estimate the viscous and squeeze-film damping ratios of heat exchanger tubes subjected to a two-phase cross-flow. Damping information is required to analyze the flow-induced vibration problem for heat exchange tubes. In heat exchange tubes, the most important energy dissipation mechanisms are related to the dynamic interaction between structures such as the tube and support and the liquid. The present model was formulated considering the added mass coefficient, based on an approximate model by Sim (1997). An approximate analytical method was developed to estimate the hydrodynamic forces acting on an oscillating inner cylinder with a concentric annulus. The forces, including the damping force, were calculated using two models developed for relatively high and low oscillatory Reynolds numbers, respectively. The equivalent diameters for the tube bundles and tube support, and the penetration depth, are important parameters to calculate the viscous damping force acting on tube bundles and the squeeze-film damping forces on the tube support, respectively. To calculate the void fraction of a two-phase flow, a homogeneous model was used. To verify the present model, the analytical results were compared to the results given by existing theories. It was found that the present model was applicable to estimate the viscous damping ratio and squeeze-film damping ratio.

• Journal of Advanced Marine Engineering and Technology
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• v.40 no.4
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• pp.270-274
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• 2016

The effect of the rib angle-of-attack on heat transfer in the convergent channel with ${\vee}/{\wedge}$-shaped ribs was examined experimentally. Four differently angled ribs (a = $30^{\circ}$, $45^{\circ}$, $60^{\circ}$, and $90^{\circ}$) were placed to only the one sided wall. The ribbed wall was manufactured with a fixed rib height (e) of 10 mm and rib spacing (p)-to-height (e) ratio of 10. The convergent channel had a length of 1,000 mm and a cross-sectional areas of $100mm{\times}100mm$ at inlet and $50mm{\times}100mm$ at exit. The measurement was conducted for the Reynolds numbers ranging from 22,000 to 75,000. The results show that the Nusselt number is generally higher at higher Reynolds number and that an angle-of-attack of $45^{\circ}$ at the ${\wedge}$-shaped rib produces the greatest Nusselt number.

• Journal of the Korean Society of Safety
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• v.33 no.1
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• pp.7-15
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• 2018

The friction factors according to the flow regimes in helical coil tubes depend on the coil diameter, the tube diameter, and the coil pitch. In previous studies, correlations for the laminar flow regime in helical coil tubes have been proposed. However, studies on the transition flow regime and the turbulent flow regime are insufficient and further researches are necessary. In this study, characteristics of the friction factors for the laminar, transition and turbulent flow regimes in helical coil tubes were experimentally investigated. The helical coil tubes used in the experiments were made of copper. The curvature ratios of the helical coil tubes, which means the ratio of helical coil diameter to inner diameter of the helical coil tube are 24.5 and 90.9. Experiments were carried out in the range of $529{\leq}Re{\leq}39,406$ to observe the flows from the laminar to the turbulent regime. The friction factors were obtained by measuring the differential pressures according to the flow rates in the helical coil tubes while varying the curvature ratios of the helical coil tubes. Experimental data show that the friction factors for the helical coil tube with 24.5 in the curvature ratio of the helical coil tube were larger than those in the straight tube in all flow regimes. As the curvature ratio of the helical coil tube increases, the friction factor in turbulent flow regime tends to be equal to that of the straight tube. In addition, it was confirmed that the transition flow regimes in the helical coil tubes were much wider than those in the straight tube, also the critical Reynolds numbers were larger than those in the straight tube. The results obtained in this experimental study can be used as basic data for studies on the water hammer phenomenon in helical coil tubes.

• Journal of the Korean Geotechnical Society
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• v.21 no.8
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• pp.63-72
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• 2005

Rocks are dumped to soft marine ground in order to improve trafficability and construction conditions in the tideland reclamation construction sites. Though this rock layer under earth fill has caused in a serious seepage problems after construction, seepage behaviors of this embankment structure is not correctly investigated. Water flow through rock layers is, in general, known as Non-Darcy's flow. However, the embedded rock layer under earth fill is not known whether its flow is governed by Darcy's or Non-Darcy's law. Therefore, a numerical analysis, laboratory model test and filed investigations were performed for analyzing the those seepage characteristics in this research. Results show that there is significance of $95\%$ of confidence between observed heads and seepage rates, and the calculated ones by SAMTLE which is developed under the assumption that the water flows through the two-layer system obey the Darcy's flow. And after operating the hydraulic gradient(i) of $0.10\~0.55$ upon laboratory model, these seepage characteristics of the embedded rock layer show that Reynolds Numbers are less than 10 and the relationship between these velocities of rock layer(v) and hydraulic gradients(i) is linearly proportional with more than 0.79 of the coefficient of correlation $(R^2)$. And the Reynolds Number of the velocity calculated by the relation of v=ki in the embedded rock layer of OO sea dike is $1\~6$. It shows also laminar flow. Based on these results, it is concluded that the seepage characteristics of embedded rock layer under earth fill can be laminar and Darcy's flow.

• Journal of the Korean Vacuum Society
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• v.19 no.4
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• pp.243-248
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• 2010

Sonic nozzles have been a standard device for measurement of steady state gas flow, as recommended in ISO 9300. This paper introduces two sonic nozzles of diameter ${\Phi}$ 0.03 mm and ${\Phi}$ 0.2 mm precisely machined according to ISO 9300. The constant volume flow meter(CVFM), readily set up in the Vacuum center of KRISS. was used to calibrate the discharge coefficients of both nozzles. The calibration results were shown to determine them within the 3% expanded measurement uncertainty. Calibrated sonic nozzles were found to be applicable for precision measurement of steady state gas flow in the vacuum process in the ranges of 0.6~1,800 cc/min. Those flow conditions are equivalent to the fine gas flow with Reynolds numbers of 26~12,100. Those encouraging results confirm that calibrated sonic nozzles enable precision measurement of extremely low gas flow encountered very often in th vacuum processes. Both calibrated sonic nozzles are proven to provide the precision measurement of the volume flow rate of the dry vacuum pump within one percent difference in reference to CVFM. Calibrated sonic nozzles are applied to a new 'in-situ and in-field' equipment designed to measure the volume flow rate of vacuum pumps in the semiconductor and flat display processes. Furthermore, they can provide other applications to flow control devices in vacuum, such as MFC, etc.

• Journal of Korean Society of Disaster and Security
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• v.14 no.1
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• pp.1-8
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• 2021

A corrugated pipe is widely used in firefighting equipment and sprinkler pipes because of its elasticity, which is less damaged by deformation and convenient facilities. However, the corrugated shape of the wall results in complex internal turbulent flow, and it is difficult to predict the pressure drop, which is an important design factor for pipe flow. The pressure drop in the corrugated tube is a function of the shape factors of the pipe wall, such as groove height, length, and pitch. Existing studies have only shown a study of pressure drop due to length changes in the case of D-shaped tubes with less than 5 pitch (P) and height (K) of the rectangular grooves in the tube. In this work, we conduct a numerical study of pressure drop for P/Ks with length and height changes of 2.8, 3.5 and 4.67 with Re Numbers of 55,000, 70,000 and 85,000. The pressure drop in the corrugated tube was interpreted to decrease with smaller P/K. We show that the pressure drop is affected by the change in the groove aspect ratio, and the increase in the height of the groove increases the recirculation area, and the larger the Reynolds number, the greater the pressure drop.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2402-2407
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• 2007

Heat transfer distributions and friction factors in square channels (3.5 ${\times}$ 3.5 cm) with twisted tape inserts and with twisted tape inserts plus interrupted ribs are respectively investigated. Tests are performed for Reynolds numbers ranging from 8,900 to 29,000. The rib height-to-channel hydraulic diameter, e/Dh, is kept at 0.057 and test section length-to-hydraulic diameter, L/Dh is 30. The twisted tape is 0.1 mm thick carbon steel sheet with diameter of 3.3cm, length of 90cm, and 2.5 turns. The square ribs are arranged to follow the trace of the twisted tape and along the flow direction defined as axial interrupted ribs. Each wall of the square channel is composed of isolated aluminum sections. The following conclusions from the experimental study were drawn as: 1) In the 4 heating wall channel with twisted tape inserts, Nusselt number based on bottom wall temperature is enhanced by 1.2 - 1.6 times if adding the axial interrupted ribs on the bottom wall only. 2) The twisted tape with interrupted ribs under the two-sided heating condition produces the highest heat transfer performance. 3) Friction factor data obtained for the square channel with twisted tape inserts plus axial interrupted ribs are less than those in the past publications for circular tubes with axial interrupted ribs and twisted tape inserts.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.6
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• pp.601-607
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• 2012

In this study, we performed computational fluid dynamic simulations to explore how the detailed design of drug-eluting stents affects both the flow field in the vicinity of the stent as well as the concentration of the eluted drug at the endothelial cell surface. Simulations were performed on three idealized stent geometries we developed and on geometries approximating three commercial stents,: Medtronic's Aurora stent, Cordis's BX Velocity stent, and Boston Scientific's Wallstent. An important contribution of the present study is the introduction of the stent effectiveness index (EI), which provides a quantitative assessment of stent performance and an objective basis for comparing the performance of different stents. Among the three commercial stents studied, our simulations have revealed that the BX Velocity stent is associated with the lowest in-stent EI values for the range of flow Reynolds numbers studied ($200{\leq}Re{\leq}800$). In addition to commercial stent designs, we investigated the EI in three idealized stents and determined that a spiral stent provides excellent performance (low EI) under all flow conditions investigated.

• Journal of the Society of Naval Architects of Korea
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• v.49 no.3
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• pp.213-221
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• 2012

The present study numerically investigates the interaction between a free-surface and flow around a circular cylinder over a moving wall. In order to simulate the flow past the circular cylinder over a moving wall near a free-surface, this study has adopted the direct-forcing/fictitious domain (DF/FD) method with the level set method in the Cartesian coordinates. Numerical simulation is performed for a Reynolds numbers of 100 in the range of $0.25{\leq}g/D{\leq}2.00$ and $0.5{\leq}h/D{\leq}2.00$, where g/D and h/D are the gaps between the cylinder and a moving wall and the cylinder and a free-surface normalized by cylinder diameter D, respectively. According to g/D and h/D, the vortex structures have been classified into three patterns of the two-row, one-row, steady elongation. In general, both of g/D and h/D have the large values which mean the cylinder is far away from the wall and the free-surface, two-row vortex structure forms in the wake. When g/D decreases, the two-row vortex structure gradually transfers into the one-row vortex structure. When the g/D reveals the critical value below which the flow becomes steady state, resulting in the steady elongation vortex.

• Clean Technology
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• v.7 no.4
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• pp.273-280
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• 2001

An absorber is a major component in the absorption refrigeration systems and its performance greatly affects the overall system performance. In this study, experimental analyses on heat transfer characteristics for removal of absorption heat in ammonia-water bubble mode absorber were performed. Heat transfer coefficients were estimated as the variations of input gas flow rate, solution flow rate, temperature, concentration, absorber diameter and height, and input flow direction. The increase of gas and solution flow rate affects positively in heat transfer. However, the increase of solution temperature and concentration affects negatively. Moreover, under the same Reynolds Numbers, countercurrent flow is superior to cocurrent flow in heat transfer performance. In addition, from these experimental data, empirical correlations which can explain easily the characteristics of heat transfer are derived.