• Journal of Advanced Marine Engineering and Technology
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• v.34 no.2
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• pp.235-242
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• 2010

An experimental investigation has been conducted to clarify roughness effects of geothermal water scale deposited onto a heating surface upon its forced convection heat transfer characteristics. Examined was a circular cylinder, on which particles of silica scale having five different sizes are uniformly distributed. The Reynolds number was varied from 13000 through 50000. Local and mean heat transfer characteristics were measured as functions of particle size and Reynolds number. Subsequently the mean fouling resistance was estimated from those results, and its characteristics are clarified. It was found that the heat transfer of cylinders greatly varies with the fouling of geothermal water scale, especially its scale height. Further, the local and average Nusselt numbers strongly depend upon the cylinder spacing and the Reynolds number.

• Journal of the Korean Society for Railway
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• v.9 no.6 s.37
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• pp.772-777
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• 2006

Three-dimensional numerical study is performed for the flow analysis around the rolling stock with square cross section (Mugungwha train model). The height (H) of rolling stock is considered as the characteristic length and the total length of rolling stock is 40 which correspond to 1/2 unit of rolling stock. The gap between the surface and rolling stock is 0.17H which is average value. The relative velocity between the surface and rolling stock is assumed to be zero and Re=10,000 based on the characteristic length. Low Re ${\kappa}-{\epsilon}$[15] is employed for the calculation of turbulence which resolve all the way to the solid surface (laminar sub-layer). Large flow separation occurred at the front head of train and a pair of vortex is generated on both top and side of rolling stock. The behavior of vortices on the top of the rolling stock is believed to affect the performance of the pantograph which should be intensively investigated. The difference between the high pressure in the front stagnation region of train and the low pressure in the rear separated region causes a large pressure drag. A large pair or vortex are generated in the rear of train and the size of vortex is increased more than the size of cross section of train.

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.4
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• pp.86-100
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• 1998

The impingement of the fuel spray on the wall within the combustion chamber in compact high-pressure injection engines and on the intake port wall in port-fuel-inje- ction type engines is unavoidable. It is important to understand the characteristics of impinging spray because it influences on the rate of fuel evaporation and droplet distrib- ution etc. In this study, the numerical study for the characteristics of spray/wall interaction is performed to test the applicability and reliability of spray/wall impingement models. The impingement models used are stick model, reflect model, jet model and Watkins and Park's model. The head of wall-jet eminating radilly outward from the spray impingement site contains a vortex. Small droplets are deflected away from the wall by the stagnation flow field and the gas wall-jet flow. While the larger droplets with correspondingly higher momentum are impinged on the wall surface and them are moved along the wall and are rolled up by wall-jet vortex. Using the Watkins and Park's model the predicted results show the most reasonable trend. The rate of increase of spread and the height of the developing wall-spray is predicted to decrease with increased ambient pressure(gas density).

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.6
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• pp.2132-2146
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• 1991

Extensive experiments were carried out to investigate the mass transfer and flow structures around a circular cylinder with annular fins in crossflow. The naphthalene sublimation method was employed to measure the circumferential and longitudinal variations of mass transfer from the circular cylinder between annular fins and H is the height of the fin from the cylinder surface. A remarkable enhancement of mass transfer due to the horseshoe vortices was observed near the corner junction between the annular fin and circular cylinder. The present results indicate that the local circumferential Sherwood number shows the higher values on the front stagnation point. The maximum augmentation of mass transfer rate at the center of cylinder is found near L/H-0.15 due to the separation bubble along the annular fins. The secondary flows, which are the corner vortices V2 and V3 near the end wall of the annular fin, are fairly predicted from the distributions of local Sherwood number in the spanwise direction. The average Sherwood number of overall surface at L/H=0.15 is larger 2.0 times than that of without annualr fins. The correlations of total average mass transfer rate with L/H and Re$_{L}$ can also be obtained.d.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.4 s.235
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• pp.485-494
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• 2005

For the extensive investigation of local heat/mass transfer on the near-tip surface of turbine blade, experiments were conducted in a low speed stationary annular cascade. The turbine test section has a single stage composed of sixteen guide vanes and blades. The chord length and the height of the tested blade are 150 mm and about 125 mm, respectively. The blade has flat tip geometry and the mean tip clearance is about $2.5{\%}$ of the blade chord. Detailed mass transfer coefficient on the blade near-tip surface was obtained using a naphthalene sublimation technique. The inlet flow Reynolds number based on chord length and incoming flow velocity is changed from $1.0{\times}10^{5}\;to\;2.3{\times}10^{5}.$ Extremely complex heat transfer characteristics are observed on the blade surface due, to complicated flow patterns, such as flow acceleration, laminarization, transition, separation bubble and tip leakage flow. Especially, the suction side surface of the blade has higher heat/mass transfer coefficients and more complex distribution than the pressure side surface, which is related to the leakage flow. For all the tested Reynolds numbers, the heat/mass transfer characteristics on the turbine blade are the similar. The overall averaged $Sh_{c}$ values are proportional to $Re_{c}^{0.5}$ on the stagnation region and the laminar flow region such as the pressure side surface. However, since the flow is fully turbulent in the near-tip region, the heat/mass transfer coefficients are proportional to $Re_{c}^{0.8}.$

• Asian Journal of Atmospheric Environment
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• v.9 no.2
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• pp.114-127
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• 2015

In the Mexico City Metropolitan Area (MCMA), ozone ($O_3$) concentration is still higher than in other urban areas in developed countries. In order to reveal the current state of photochemical air pollution and to provide data for validation of chemical transport models, vertical profiles of meteorological parameters and ozone concentrations were measured by ozonesonde in two field campaigns: the first one, during the change of season from wet to dry-cold (November 2011) and the second during the dry-warm season (March 2012). Unlike previous similar field campaigns, ozonesonde was launched twice daily. The observation data were used to analyze the production and distribution of ozone in the convective boundary layer. The observation days covered a wide range of meteorological conditions, and various profiles were obtained. The evolution of the mixing layer (ML) height was analyzed, revealing that ML evolution was faster during daytime in March 2012 than in November 2011. On a day in November 2011, the early-morning strong wind and the resulting vertical mixing was observed to have brought the high-ozone-concentration air-mass to the ground and caused relatively high surface ozone concentration in the morning. The amount of produced ozone in the MCMA was estimated by taking the difference between the two profiles on each day. In addition to the well-known positive correlation between daily maximum temperature and ozone production, effect of the ML height and wind stagnation was identified for a day in March 2012 when the maximum ground-level ozone concentration was observed during the two field campaigns. The relatively low ventilation coefficient in the morning and the relatively high value in the afternoon on this day implied efficient accumulation of the $O_3$ precursors and rapid production of $O_3$ in the ML.

• Journal of the Korean earth science society
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• v.41 no.2
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• pp.111-128
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• 2020

This study presents a possibility of intensification of fine dust mass concentration due to the complex urban structure using data mining technique and clustering analysis. The data mining technique showed no significant correlation between fine dust concentration and regional-use public urban data over Seoul. However, clustering analysis based on nationwide-use public data showed that building heights (floors) have a strong correlation particularly with PM₁₀. The modeling analyses using the single canopy model and the micro-atmospheric modeling program (ENVI-Met. 4) conducted that the controlled atmospheric convection in urban area leaded to the congested flow pattern depending on the building along the distribution and height. The complex structure of urban building controls convective activity resulted in stagnation condition and fine dust increase near the surface. Consequently, the residual effect through the changes in the thermal environment caused by the shape and structure of the urban buildings must be considered in the fine dust distribution. It is notable that the atmospheric congestion may be misidentified as an important implications for providing information about the residual probability of fine dust mass concentration in the complex urban area.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.46 no.8
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• pp.611-620
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• 2018

For the past decades, the analytic model for distributed surface roughness has been developed to improve the accuracy of the icing simulation code. However, it remains limitations to validate the developed model and determine the empirical parameters due to the absence of the quantitative experimental data which were focused on the surface state. To this end, the experimental study conducted to analyze the ice covered surface state from a micro-perspective. Above all, the tendency of the smooth zone width which occurs near the stagnation point has been quantitatively analyzed. It is observed that the smooth zone width is increased as growing the ambient temperature and freestream velocity. Next, the characteristics of the ice covered surface under rime and glaze ice have been analyzed. For rime ice conditions, ice elements are developed as the opaque circular corn in the opposite direction of freestream. The height and interval of each circular corn are increased as rising the ambient temperature. For glaze ice conditions, numerous lumps of translucent ice can be observed. This is because the beads formed by gravity concentrate and froze on the lower surface.