• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1971-1978
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• 1992

A turbulent flow in a conical diffuser with total divergence angle of 8.deg. was numerically studied. The low Reynolds number k-.epsilon. model(Launder-Sharma model) was adopted to simulate the turbulence. The continuity and time averaged Navier-Stokes equations in a nonorthogonal coordinate system were solved by a finite volume method based on the fully elliptic formulation. The low Reynolds number k-.epsilon. model reasonably simulates the pressure recovery and the mean velocity components. However, there are also considerable discrepancies between predicted and measured shear stress distribution on the wall and turbulent kinetic energy distributions. It is necessary to investigate the flow structure at the entry of the diffuser, numerically as well as experimentally.

• Journal of Environmental Science International
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• v.27 no.6
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• pp.359-369
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• 2018

A model has been developed to predict natural ventilation in a single zone building with large openings. This study first presents pressure-based equations on natural ventilation, that include the combined effect of wind and thermal buoyancy. Moreover, the concept of neutral pressure level(NPL) is introduced to consider the two-way flow through a large opening. The total pressure differences across the opening and the NPL are calculated, and nonlinear equations are solved to find the zonal pressure to satisfy mass conservation. For this analysis, an iterative technique of successively approximating the zonal pressure is used. The results of applying this study model to several simple cases are as follows. When there is no wind and only the stack effect is caused, a one-way flow occurs in both the top and bottom openings in the case of two openings of equal-area, and a one-way flow occurs in the top opening; however, a two-way flow occurs in the bottom opening in the case of two openings of unequal-area. When there is a wind effect, regardless of whether the outside air temperature is lower or higher than the indoor air temperature, air flows into the room through the bottom opening and out of the room through the top opening. As the wind velocity increases, the wind effect appears to be more influential than the stack effect owing to the temperature difference.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.1 s.139
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• pp.1-9
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• 2005

Waterjet propulsion is widely used to thrust high speed marine vessels in excess of 30-35 knots by virtue of the high propulsive efficiency, good maneuverability, and less cavitation. From the aspect of power loss, approximately $7-9\%$ of the total power is lost in intake duct due to the flow separation, nonuniformity, etc. Thus, detail understanding of flow phenomena occurring within intake duct is essential to reduce the power loss, as well as noise and vibration. The present work solved 30 incompressible RANS equations to provide complicated viscous flow features of intake duct. The numerical results were compared with experiments and good agreements were obtained for three jet velocity ratios.

• Nuclear Engineering and Technology
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• v.31 no.6
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• pp.548-560
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• 1999

An experimental investigation has been peformed to examine the effects of various geometrical parameters and an initial operating condition on the air-water countercurrent How limitation (CCFL) in a simulated PWR hot leg. A total of 118 experimental data for the onset of CCFL and zero liquid penetration were obtained for various combinations of test parameters. It was observe that the CCFL can be classified into three different categories: (the onset of CCFL, (the partial liquid delivery, and (r) the zero liquid penetration. The observed mechanisms of the onset of CCFL were different depending on the inlet water flow rate. The parametric effects of pipe diameter, horizontal pipe length, horizontal pipe length-to-diameter (L/D) ratio, and initial water level in the horizontal pipe of the test section on the onset of air-water CCFL were also examined. An empirical correlation for the onset of CCFL in a horizontal pipe connected to an inclined riser was developed in terms of Wallis flooding parameters for the low inlet water flow rate region. Comparisons of the present empirical correlation with the air-water CCFL data of large pipe diameters show that the present correlation agrees more closely with the experimental data than the existing CCFL correlations.

• 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.

• 한국전산유체공학회:학술대회논문집
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• 1998.05a
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• pp.168-173
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• 1998

A numerical analysis of axisymetric backward facing step main nozzle flow in air jet loom has been accomplished. To obtain basic design data for an optimum main nozzle for an air-jet loom and to predict the transonic/supersonic flow, a characteristic based upwind flux difference splitting compressible Navier-Stokes method has been used. The wall static pressure of the main nozzle and the flow velocity changes in the nozzle tube were analyzed by changing air tank pressures and acceleration tube lengths. The flow inside the nozzle experiences double choking one at the needle tip and the other at the acceleration tube exit at tank pressures over $4kg_f/cm^2$. The tank pressure $P_t$ leading to the critical condition depends on the acceleration tube length; i.e, $P_t$ is higher for longer acceleration tubes. The $P_t$ value required to bring the acceleration tube exit to the critical condition is nearly constant regardless of acceleration tube length. The round needle tip shape might lead to less total pressure loss when compared with step shape.

• Journal of Mechanical Science and Technology
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• v.19 no.12
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• pp.2312-2320
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• 2005

The aerodynamic effects of leading-edge accretion can raise important safety concerns since the formulation of ice causes severe degradation in aerodynamic performance as compared with the clean airfoil. The objective of this study is to develop a numerical simulation strategy for predicting the particle trajectory around an MS-0317 airfoil in the test section of the NASA Glenn Icing Research Tunnel and to investigate the impingement characteristics of droplets on the airfoil surface. In particular, predictions of the mean velocity and turbulence diffusion using turbulent flow solver and Continuous Random Walk method were desired throughout this flow domain in order to investigate droplet dispersion. The collection efficiency distributions over the airfoil surface in simulations with different numbers of droplets, various integration time-steps and particle sizes were compared with experimental data. The large droplet impingement data indicated the trends in impingement characteristics with respect to particle size ; the maximum collection efficiency located at the upper surface near the leading edge, and the maximum value and total collection efficiency were increased as the particle size was increased. The extent of the area impinged on by particles also increased with the increment of the particle size, which is similar as compared with experimental data.

• Membrane and Water Treatment
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• v.12 no.2
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• pp.65-73
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• 2021

Sponge ball cleaning can generate an abrasion effect, which leads to an attractive increasing in both permeate flux and membrane rejection. The aim of this study was to investigate the influence of the daily sponge ball cleaning (SBC) on the performance of different UF cross-flow membrane modules integrated with a bioreactor. Two 1"-membrane modules and one 1/2"-membrane module were tested. The parameters measured and controlled are temperature, pH, viscosity, particle size, dissolved organic carbon (DOC), total suspended solids (TSS), and permeate flux. The permeate flux could be improved by 60%, for some modules, after 11 days of daily sponge ball cleaning at a transmembrane pressure of 350 kPa and a flow velocity of 4 m/s. Rejection values of all tested modules were improved by 10%. The highest permeate flux of 195 L/㎡.h was achieved using a 1"-membrane module with the aid of its negatively charged membrane material and the daily sponge ball cleaning. In addition, the enhancement in the permeate flux caused by daily sponge ball cleaning improved the energy specific demand for all tested modules. The negatively charged membrane showed the lowest energy specific demand of 1.31 kWh/㎥ in combination with the highest flux, which is a very competitive result.

• Nuclear Engineering and Technology
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• v.54 no.4
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• pp.1495-1507
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• 2022

The improvement of thermal-hydraulic analysis techniques is essential to ensure the safety and reliability of nuclear power plants. The one-dimensional two-fluid model has been adopted in state-of-the-art thermal-hydraulic system codes. Current constitutive equations used in the system codes reach a mature level. Some exceptions are the partition method of wall friction in the momentum equation of the two-fluid model and the interfacial drag force model for a horizontal two-phase flow. This study is focused on deriving the partition method of wall friction in the momentum equation of the two-fluid model and modeling the interfacial drag force model for a horizontal bubbly flow. The one-dimensional momentum equation in the two-fluid model is derived from the local momentum equation. The derived one-dimensional momentum equation demonstrates that total wall friction should be apportioned to gas and liquid phases based on the phasic volume fraction, which is the same as that used in the SPACE code. The constitutive equations for the interfacial drag force are also identified. Based on the assessments, the Rassame-Hibiki correlation, Hibiki-Ishii correlation, Ishii-Zuber correlation, and Rassame-Hibiki correlation are recommended for computing the distribution parameter, interfacial area concentration, drag coefficient, and relative velocity covariance of a horizontal bubbly flow, respectively.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.5
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• pp.488-494
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• 2005

The ejector is used to obtain a vacuum state, and it has been applied to a lot of industry field such as a heat engine, a fluid instrument power plant. a food industry, an environment industry etc., because there is no problem even it is mixed with any kind of liquid, gas. and solid. The flow characteristics in the ejector was investigated by a PIV and a CFD. The agreement between numerical analysis and experiment shows the validity of this study and the results of this study would be useful to the engineers who design for the flow systems for heating. ventilation. air conditioning and wastewater purification plants.