Kun Hyuk Sung

A numerical study on the effect of hematocrit on hemodynamic characteristics in arteriovenous graft

Stenosis at an arteriovenous graft is related with the critical ranges of hemodynamic characteristics. Hematocrit has a significant effect on the blood viscosity. During hemodialysis, hematocrit is changed by the dialysis machine. The effect of hematocrit on hemodynamic characteristics is investigated by numerical study. A multiphase non-Newtonian blood model was used to analyze the changes of hematocrit. The hematocrit of blood flows at injection needle changed 40%, 50%, and 60%. As a result, the blood viscosity increased by about 6% point. Also, the high wall shear stress region (over 3 Pa) increased about 6% point when the hematocrit at the vein anastomosis increased by about 2% point. When the hematocrit increased by 4% at the vein anastomosis, an extremely high wall shear stress region (over 7.5 Pa) increased by 3 times. Thus, the variation of hematocrit should be predicted using a multiphase blood model to avoid the critical range of wall shear stress when hematocrit changes regionally.

Numerical Investigation of the Flow Characteristics Considering the Human Workloads in a Human Airway

The flow characteristicscan be sensitively changed by the respiration flow rate due to the complex geometries, e.g., bends, constrictions in a human airway. Also, caused by the switching the flow direction, the disturbance destabilizes the flow when a human breathes. Thus, the real breathing conditions, e.g., peak flow rate and the breath cycle time related to workload, should be considered to investigate on the flow characteristics in a human airway. In this study, realated to the human workloads, the effect of the change of the respiratory flow rate on the flow characteristics is investigated with the numerical method at the 4-specific normalized times in a breath cycle. In almost time of a breath cycle, as the workload increases, the flow recirculation zone decreases slightly because the energy transport toward the wall increases due to the turbulence fluctuation and the secondary motion.However, when the flow acclecrates in the expiratory phase, the flow recirculation zone increases because of the unsteady effect and the glottal jet. Thus, respiratory flow rate considering the human workloads and the unsteady effect caused by the real respiration should be considered to investigate the flow characteristics and the particle movement in a human airway.

Experimental study on the effect of initial liquid droplet size on the evaporation in a heterogeneous droplet

In the present work, we experimentally investigated the effect of initial liquid droplet size on the evaporation in the heterogeneous droplet. Spherical carbon and water were used for particle and liquid droplet comprising the heterogeneous droplet. four initial droplet volumes of 1, 2, 3 and 4 μl were considered when the diameter of the particle was 5 mm. The heterogeneous droplet was suspended with a rod at 20 cm away from the radiator which surface temperature was fixed to 473 K. Ambient temperature and relative humidity remained 296 K and 40 %, respectively, during the experiment. As the results, the evaporation rate of 4 μl case increased about 1.8 times compared with that of 1 μl case. The evaporation rate increased almost linearly with the volume ratio, and that is related closely with the contact surface between particle and water droplet. Contact surface area remained almost constantly with time, whereas it increased with the initial volume of water droplet. The energy from radiator can be accumulated at the contact surface at the side of particle, thereby intensifying the evaporation of water droplet because more heat transfers from particle to droplet through the contact surface. Consequently, the initial volume of liquid droplet is one of the influence factors on the evaporation rate in the heterogenous droplet. DOI: http://dx.doi.org/10.4995/ILASS2017.2017.4744

Numerical Study on Characteristics of Gas Leakage in an Underground Combined Cycle Power Plant

Abstract The present study numerically investigated the gas leakage characteristics in a simplified underground combined cycle power plant. The effect of obstacles near a crack location on the gas concentration in the confined space was analyzed by using the lower flammable limit (LFL) of methane gas. When the jet flow was close to the vertical walls, the longitudinal leakage distance increased by about 60% (when an obstacles was present) compared to the case without any obstacle, because these obstacles prevented transverse flows. In addition, when an air filterwas installed near to the trajectory of the gas flow, the longitudinal leakage distance was similar to the distance between the crack and obstacle, whereas the transverse leakage distance increased up to 8 times compared to the casewithout any obstacle. As the jet flow impacts on the obstacle and changes its direction, the gas flows recirculate.Therefore, it is necessary to consider the effect of the structure and facility locations on the trajectory of the jet flowto propose an accident prevention system in confined spaces.

Development of Water Film Nozzle for Glass Curtain Wall System Using Numerical Analysis

초고층 건축물이란, 건축물 시행령 제2조(용어의 정의)에서 ‘건축물의 높이가 200 m 이상 또는 건축물의 층수가 50층 이 상의 건축물’로 정의하고 있다. 초고층 건축물은 대형화, 고층 화, 지하시설 활용 들을 통해 공간 효율을 증가 하였으나 화 재시 수직적 구조로 인해 지상으로 피난거리 증가, 소방대원 진입의 어려움으로 인하여 대형사고 발생 가능성이 높다 (Kim et al., 2003; Son and Yi, 2004). 특히 초고층 건축물에는 외장재로 주로 커튼월 구조가 적용 되고 있으며 유리커튼월 시장은 매년 약 20% 이상의 증가율 을 보이고 있다(Jeon, 2012). 커튼월(Curtain wall)이란 하중 을 지지하지 않는 외장용 건축 벽체를 총칭하며(Cho et al., 2010) 유리를 외장재로 사용하는 유리커튼월이 현재 많이 사 용되고 있다. 하지만 화재 발생 시 하층부의 유리커튼월이 파손되어 화염 이 건물 외측으로 분출되며 이에 따라 상층부의 커튼월이 파 손되어 상층부로 화재가 전파되는 Leapfrog effect가 발생되 Abstract

Numerical Study on the Prediction Method of Flashover in a Compartment

In building fires, irradiation from walls and the fire plume increases burning rates, then fire spreads throughout a compartment within a few minutes due to flashover phenomena. Many researchers have experimentally suggested empirical correlations and conditions for the flashover which applications are limited in specific spaces or vents. Therefore, it is necessary to investigate prediction of the flashover using numerical results. The objective of this study is to numerically investigate the prediction method of flashover by estimating oxygen concentrations and mean flame surface at the compartment which size is 5 m×6 m×3 m. In the results, flashover occur at 70s and the minimum required heat release rate is about 4.14 MW.

Numerical Investigation of Micro-particle Transport and Deposition in an Upper Airway Considering Human Workloads

During a respiration, the process of transport /deposition of inhaled particles is influenced by the morphological characteristics of airway and respiratory conditions related to the human workload. Especially in a heavy workload or a panic state, the flow rate and the breath frequency remarkably increase with compared to a normal state, and then the particle transport and the flow characteristics change dramatically in the airway. In this study, the flow characteristics and microparticle deposition patterns were numerically investigated in a representative human upper airway model using the real respiratory waveform considering workloads. Also, the mean Stokes number for particles is in the equivalent range of 0.00015 < St mean < 0.03377 at each workload. In the results, the deposition fraction (DF) steeply declines with a decrease in the mean Stokes number at the upper region from the oral cavity to the pharynx, while the change in the DF is relatively small at the lower region from the larynx to the trachea in all workloads. This is because the mechanism of the transport/deposition process is diffusion dominant due to turbulent dispersions at the lower region, whereas at the upper region is impaction dominant. Also, compared with the light workload, the total DF increases in whole mean Stokes numbers at the heavy workload, while the dominant region where particles deposited differs with the workload. These implies that the human workload as well as the particle size are important to investigate the optimal drug aerosol targeting and the effect of the inhaled toxic materials in the human airway.

A Study on the Effects of Various Disk Shape of Hydrant on the Pressure Drop

In this study, the effects of various disk shapes of hydrant on the pressure drop are experimentally and numerically analyzed. The test methods for measuring pressure drop of hydrant are comply with standard of Underwriters Laboratory (UL). The hydrant as used in this study has one inlet, diameter 150 mm, and three outlet, 114.3 mm diameter for one outlet and 63.5 mm diameter for the others. The pressure of the hydrant are measured in the range 760 L/min~2,270 L/min for 63.5 mm outlet and 3,030 L/min~6,060 L/min for 114.3 mm outlet. Also, the numerical results of pressure drop are compared with the experiments to verify the accuracy and to analyze the of various valve shape of hydrant on the pressure drop. The engineering parameters, flow coefficients, are reduced from 181.57 to 136.35 (L/min/kPa 0.5 ) with inclined angle of disk from 0 o to 45 o . These results are able to practical use for design hydrant to minimize pressure drop.

Numerical Analysis on the Influence of the Unsteady Effect upon the Flow Characteristics in a Human Airway

The focus is on the unsteady effect on the flow characteristics considering the real respiration in a human airway. In order to capture the unsteady effect, the flow characteristics were investigated and compared with that of steady flow by using the computational fluid dynamics technique. The real respiratory flow rate in a light workload was appliedto the inlet of airway. The unsteady effects especially appear when the flow rate accelerates in the inspiratory and the expiratory phase than other times in a breath cycle. The size of the flow recirculation zone, the turbulence fluctuation and the secondary flow intensity decrease because the unsteady effect reduces the strength of jets passing through the constriction of airway by interfering with the flow. These results imply that the unsteady effect should be considered to investigate the flow characteristics and the particle movement in a human airway.

Numerical Analysis on the Flow Characteristics Considering the Inspiratory Flow Rate in a Human Airway

The inspiratory flow rate of a human is changed with the amount of the workload. The flow characteristic is affected by the inspiratory flow rate. In the flow field of airway, the both of turbulence intensity and secondary flow affect the deposition pattern of particles which is important for the drug-aerosol targeting. Thus the analysis of the flow characteristic in a human airway is important. The purpose of this study is to investigate the effects of the inspiratory flow rate on the flow characteristics in a human airway. The tubular airway is con- sistent with the oral cavity, pharynx, larynx and trachea. The relatively inspiratory flow rate is used at each case of human states regarding the workload. By the effect of geometric airway changes, transition to turbulent airflow after the larynx can occur with relaminarization further downstream. The low Rey- nolds number k-ω turbulence model is used for analysis with flow regime. As the inspiratory flow rate is larger, the turbulence kinetic energy and secondary flow intensity increase in airway. On the other hand, the area of recirculation zone is smaller.