• International Journal of Air-Conditioning and Refrigeration
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• v.10 no.3
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• pp.117-128
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• 2002

This paper describes the smoke filling process of a fire field model based on a self-deve-loped SMEP (Smoke Movement Estimating Program) code to the simulation of fire induced flows in the two types of atrium space containing a ceiling heat flux. The SMEP using PISO algorithm solves conservation equations for mass, momentum, energy and species, together with those for the modified k-$\varepsilon$ turbulence model with buoyancy production term. Also it solves the radiation equation using the discrete ordinates method. Compressibility is assumed and the perfect gas law is used. Comparison of the calculated upper-layer average tempera-ture and smoke layer clear height with the zone models has shown reasonable agreement. The zone models used are the CFAST and the NBTC one-room. For atrium fires with ceiling glass the ceiling heat flux by solar heat causes a high smoke temperature near the ceiling. However, it has no effect on the smoke movement such as the smoke layer clear heights that are important in fire safety. In conclusion, the smoke layer clear heights that are important in evacuation activity except the early of a fire were not as sensitive as the smoke layer tem-perature to the nature of ceiling heat flux condition. Thus, a fire sensor in atrium with ceiling glass has to consider these phenomena.

• 한국해양학회지
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• v.30 no.5
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• pp.512-521
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• 1995

To investigate a short-term (from 2 hours to 24 hours) variability of a mixed layer, oceanographical data (water temperature, salinity, current) and meteorological data (wind, air temperature, solar radiation) were collected at a site in the Korea Strait at the interval of one hour for 48 hours from October 12 to 14, 1993. The average rates of temporal variations of the mixed layer depth (MLD) and temperature of the mixed layer (MLT), which are very weakly correlated with the wind stress and buoyancy flux at the sea surface, are about 5.2 m/hour and 0.2$^{\circ}C$/hour, respectively. The mixed layer is relatively shallow when both MLT and MLS (salinity of the mixed layer) are low, while MLD is relatively deep when they are high. MLT shows a sudden decrease or increase. Analysis of satellite infrared images and XBT data shows that sudden increase of MLT is caused by advection of warm water. These results suggest that the short-term variation of the mixed layer in the Korea Strait in autumn, in which surface current is relatively strong and different water masses exist, is mainly determined by advection rather than air0sea interaction such as wind stress or buoyancy flux.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.11
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• pp.1509-1520
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• 1998

An experiment was carried out for the fully developed turbulent flow of water in tube coils on the condition of uniform heat flux. The present work was conducted for various ranges for Dean number(1794~1321), Prandtl number (2.5~4.5), curvature ratio parameters (22~60). Heat transfer to steady viscous flow in coiled tubes of circular cross section was studied for fully developed velocity and temperature fields under the thermal boundary condition of uniform heat flux. The peripherally local Nusselt number correlated as a function of Dean and Prandtl numbers. We studied the flow in heat coiled tubes under the influence of both centrifugal and buoyancy forces in order to gain insight into the flow pattern. In the present study, we obtained three emperical formulas, $Nu_v=0.0231Re^{0.84}Pr^{0.4}(a/R)^{0.13}$ (vertical) $Nu_c=0.0241Re^{0.86}Pr^{0.4}(a/R)^{0.08}$ (corrugated) $Nu_h=0.0227Re^{0.84}Pr^{0.4}(a/R)^{0.09}$ (horizontal).

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.30 no.3
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• pp.117-122
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• 2020

In last few decades, although thermal and/or solutal buoyancy-driven recirculating flows in a closed ampoule have been intensively studies as a model problem, there exist interesting total molar flux of Hg₂Br₂ that have been unreported in the literature. It is concluded that the total molar flux of Hg₂Br₂(A) increases linearly and directly as the temperature difference regions in the range of 10℃ ≤ ΔT ≤ 50°, 3.5 × 10³ ≤ Gr_t ≤ 4.08 × 10³, 4.94 × 10⁴ ≤ Gr_s ≤ 6.87 × 10⁴. For the range of 10 Torr ≤ P_B ≤ 150 Torr, the total molar flux of Hg₂Br₂(A) decays second order exponentially as the partial pressure of component B (argon as an impurity), P_B increases. From the view point of energy transport, the fewer the partial pressure of component B (argon), P_B is, the more the energy transport is achieved.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.7 no.3
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• pp.248-256
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• 1995

The numerical study on the surface buoyant jets has remained of requiring more intensive investigation for problems due to the treatments of free surface, Reynolds stress/flux terms in turbulent flow and especially buoyancy effects on the turbulent fluctuation. etc. The verification of predicted results from the numerical study continues in the qualitative study. because of the lack of experimental data, which seems to be due to the difficulties in measuring the turbulent fluctuations in concentration or temperature fields. In this study, the computer program of Algebraic Stress Model has been developed to investigate the behaviours of two-dimensional surface buoyant jets with free surface boundary condition. The computational results are compared with published experimental data. By comparing these results with experimental data. it is found that this model can predict fairly well the flow characteristics of two-dimensional surface buoyant jets in the momentum-dominant region and buovancy-dominant region. Especially, it is proved that this model can predict the flow characteristics reasonably in buoyancy-dominant region stably stratified due to buoyancy effect.

• Nuclear Engineering and Technology
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• v.51 no.6
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• pp.1487-1503
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• 2019

The methods and performance of a pin-level nuclear reactor core thermal-hydraulics (T/H) code ESCOT employing the drift-flux model are presented. This code aims at providing an accurate yet fast core thermal-hydraulics solution capability to high-fidelity multiphysics core analysis systems targeting massively parallel computing platforms. The four equation drift-flux model is adopted for two-phase calculations, and numerical solutions are obtained by applying the Finite Volume Method (FVM) and the Semi-Implicit Method for Pressure-Linked Equation (SIMPLE)-like algorithm in a staggered grid system. Constitutive models involving turbulent mixing, pressure drop, and vapor generation are employed to simulate key phenomena in subchannel-scale analyses. ESCOT is parallelized by a domain decomposition scheme that involves both radial and axial decomposition to enable highly parallelized execution. The ESCOT solutions are validated through the applications to various experiments which include CNEN $4{\times}4$, Weiss et al. two assemblies, PNNL $2{\times}6$, RPI $2{\times}2$ air-water, and PSBT covering single/two-phase and unheated/heated conditions. The parameters of interest for validation include various flow characteristics such as turbulent mixing, spacer grid pressure drop, cross-flow, reverse flow, buoyancy effect, void drift, and bubble generation. For all the validation tests, ESCOT shows good agreements with measured data in the extent comparable to those of other subchannel-scale codes: COBRA-TF, MATRA and/or CUPID. The execution performance is examined with a mini-sized whole core consisting of 89 fuel assemblies and for an OPR1000 core. It turns out that it is about 1.5 times faster than a subchannel code based on the two-fluid three field model and the axial domain decomposition scheme works as well as the radial one yielding a steady-state solution for the OPR1000 core within 30 s with 104 processors.

• Journal of Mechanical Science and Technology
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• v.16 no.10
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• pp.1346-1354
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• 2002

The purpose of this work is to study the effects of specularly reflecting wall under the combined radiative and laminar free convective heat transfer in an infinite square duct. An absorbing and emitting gray medium is enclosed by the opaque and diffusely emitting walls. The walls may reflect diffusely or specularly. Boussinesq approximation is used for the buoyancy term. The radiative heat transfer is evaluated using the direct discrete ordinates method. The parameters under considerations are Rayleigh number, conduction to radiation parameter, optical thickness, wall emissivity and reflection mode. The differences caused by the reflection mode on the stream line, and temperature distribution and wall heat fluxes are studied. Some differences are observed for the categories mentioned above if the order of the conduction to radiation parameter is less than order of 10$\^$-3/ fer the range of Rayleigh number studied. The differences at the side wall heat flux distributions are observed as long as the medium is optically thin. As the top wall emissivity decreases, the differences between these two modes are increased. As the optical thickness decreases at the fixed wall emissivity, the differences also increase. The difference of the streamlines or the temperature contours is not as distinct as the side wall heat flux distributions. The specular reflection may alter the fluid motion.

• Environmental Sciences Bulletin of The Korean Environmental Sciences Society
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• v.1 no.2
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• pp.119-124
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• 1997

Cloud streets were successfully simulated by numerical model (RAMS) including an isolated mountain near the coast, large sensible heat flux from the sea surface, uniform stratification and wind velocity with low Froude number (0.25) in the inflow boundary. The well developed cloud streets between a pair of convective rolls are simulated at a level of 1 km over the sea. The following five results were obtained: 1) For the formation of the pair of convective rolls, both strong static instability and a topographically induced mechanical disturbance are strongly required at the same time. 2) Strong sensible heat flux from the sea surface is the main energy source of the pair of convective rolls, and the buoyancy caused by condensation in the cloud is negligibly small. 3) The pair of convective rolls is a complex of two sub-rolls. One is the outer roll, which has a large radius, but weak circulation, and the other is the inner roll, which has a small radius, but strong circulation. The outer roll gathers a large amount of moisture by convergence in the lower marine boundary, and the inner roll transfers the convergent moisture to the upper boundary layer by strong upward motion between them. 4) The pair of inner rolls form the line-shaped cloud streets, and keep them narrow along the center-line of the domain. 5) Both by non-hydrostatic and by hydrostatic assumptions, cloud streets can be simulated. In our case, non-hydrostatic processes enhanced somewhat the formation of cloud streets. The horizontal size of the topography does not seem to be restricted to within the small scale where non-hydrostatic effects are important.

• Journal of Ocean Engineering and Technology
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• v.16 no.3
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• pp.8-18
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• 2002

Formulation of the far-field method for the prediction of time-mean hydrodynamic force and moment acting on a 3-D surface-piercing body in waves is reviewed. It is found that the inequality between the weight of the floating body and its buoyancy force permits the replacement of the fluid particles inside the control surface by the fluid particles outside the control surface. Under such circumstances, momentum exchanges across the control surface make the time-mean value of the time rate of the momentum of the fluid inside the control surface non-vanishing. It is a second-order quantity which is hard to calculate by the far-field method. The drift forces and moments on half-immersed ellipsoids are calculated by both the far-field method and the near-field method. The discrepancy between two numerical results is presented and discussed.

• Water Engineering Research
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• v.3 no.1
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• pp.9-22
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• 2002

In general, two dimensionless numbers are used in predicting the front propagation rate of density currents: the densimetric Froude number and the dimensionless front velocity. The former expresses the front speed in terms of the characteristic length and reduced gravitational acceleration. Previous papers report that the range of this dimensionless number is wide. The other is the dimensionless front velocity, which is a function of the buoyancy flux per unit width. This paper presents the state of the art review of the dimensionless numbers for the front propagation rate of density currents. Values of the densimetric Froude number are found to be consistent when the proper characteristic length is used for normalization. Then, the densimetric Froude number and the dimensionless front velocity are compared by using the experimental data of density currents over a horizontal surface.