Young-Don Choi

Analysis of periodically fully developed turbulent flow and heat transfer by k-ε equation model in artificially roughened annulus

Abstract Periodic fully developed turbulent flows with heat transfer from an inner pipe in the artificially roughened annulus are analysed by the k - e equation model. A new streamline curvature correction model of eddy viscosity and cyclic TDMA are applied in order to increase the accuracy and converging rate of numerical solutions. This paper illustrates the new method of obtaining the roughness functions from numerically predicted velocity and temperature profiles. A predicted roughness function related to the velocity profile agrees well with previous semi-empirical correlations, but that for the temperature profile shows a different shape from the previous ones.

Compressibility Factor Effect on the Turbulence Heat Transfer of Super-critical Carbon Dioxide by an Elliptic-blending Second Moment Closure

The present contribution describes the application of elliptic-blending second moment closure to predict the gas cooling process of turbulent super-critical carbon dioxide flow in a square cross-sectioned duct. The gas cooling process under super-critical state experiences a drastic change in thermodynamic and transport properties. Redistributive terms in the Reynolds stress and turbulent heat flux equations are modeled by an elliptic-blending second moment closure in order to represent strongly non-homogeneous effects produced by the presence of walls. The main feature of Durbin`s elliptic relaxation second moment closure that accounts for the nonlocal character of pressure-velocity gradient correlation and the near-wall inhomogeneity guaranteed by the elliptic blending second moment closure.

Computational Study of Energy Loss in a Pipe of Refuse Collecting System

This paper describes energy loss in a pipe line of refuse collecting system. Analysis of energy loss in a pipe line is the decisive factor in a design of refuse collecting system. Using the results of energy loss analysis, we can determine the power of turbo-blower. The flow characteristics of the pipe line with refuse bags were analyzed by three-dimensional CFD. The refuse bag is modeled by using the shape obtained from profile measurement. Friction factors were calculated with changing the refuse bag size, mixing ratio and Reynolds number. And drag coefficients were calculated using the CFD results. From the results we can calculate energy loss in a pipe line of refuse collecting system and predict the capacity of turbo-blower.

A Study on the Development of a Multi-Heat Supply Control Algorithm in a District Heating Apartment Building in Accordance with the Variation of Outdoor Air Temperature

In this study, we developed a heat supply control algorithm that minimizes the heat loss in the heat distribution pipelines used for supplying heat energy to shared group housing. Controlling the temperature and flow rate of the hot water supplied to the heat exchanger for shared group housing enables us to develop a heat supply control technique that meets the heating load required by each household in a shared apartment building in accordance with changes in the outdoor air temperature, and that minimizes the heat loss occurring in the heat distribution pipeline. A one-year study in 2008 on a 1,473-household D-apartment building in Hwaseong, Gyeonggi-do, South Korea, compared the heat capacity used by each household, as well as the heat capacity supplied to the heat exchanger room of the apartment housing building, to calculate the amount of heat loss in the heat distribution pipeline. The results confirmed that 24.1% of the heat supplied was lost in the piping.

Large Eddy Simulation of Heat Transfer Performance Enhancement due to Unsteady Flow in Compound Channels

In the present article, we investigate numerically turbulent flow of air through compound rectangular channels. Large eddy simulation(LES) is employed for unsteady turbulence modeling. LES gives better predictions for the axial mean velocity distribution than those of other turbulent models. Strong large-scale quasi-periodic flow oscillations are observed in most of the geometries investigated. Such large-scale flow oscillations in compound rectangular channels are similar to the quasi-periodic flow pulsation through the gaps between fuel rod bundle in nuclear reactor. It exists in any longitudinal connecting gap between two flow channels. The frequency of this flow oscillation is determined by the geometry of the gap. The large scale cross motions through the rectangular compound channels induce significant heat transfer enhancement of the compound channel flow.

Experimental study on the secondary flow characteristics of a supercritical carbon dioxide flow in a gas cooling process within a square duct

Abstract The carbon dioxide properties change sharply near the critical or pseudo-critical point in the heat transfer processes. The reduction in turbulent, convective heat transfer parameters observed in some supercritical data and in experiments with common gases can be due to property variation, acceleration, buoyancy or combinations of these phenomena, depending on the conditions of the applications. In this study, the measurement for the secondary flow driven by buoyancy was carried out on the supercritical carbon dioxide turbulent flows in the different boundary condition with the constant mass flow rate. The available measuring techniques were used to clarify the behaviour of any supercritical fluid. Laser Doppler Velocimeter (LDV) and a special device was used to measure the secondary velocity and turbulent characteristics of the supercritical flows. † 책임저자, 회원, 고려대학교 기계공학과 E-mail : ydchoi@korea.ac.kr TEL : (02)3290-3355 FAX : (02)928-1067 * 고려대학교 기계공학과 ** 서울대학교 기계공학과

Near-Wall Modelling of Turbulent Heat Fluxes by Elliptic Equation

A new second-moment closure model for turbulent heat fluxes is proposed on the basis of the elliptic equation. The new model satisfies the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also has the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. The predictions of turbulent heat transfer in a channel flow have been carried out with constant wall heat flux and constant wall temperature difference boundary conditions respectively. The velocity field variables are supplied from the DNS data and the differential equations only fur the mean temperature and the scalar flux are solved by the present calculations. The present model is tested by direct comparisons with the DNS to validate the performance of the model predictions. The prediction results show that the behavior of the turbulent heat fluxes in the whole region is well captured by the present model.

The Radiation Heat Treansfer Analysis of the Cryochamber

The cooling load, which should be removed by the cooling system to maintain the nominal operating temperature of a cryochamber, depends on the thermal insulation efficiency of the cryochamber. Therefore, to decrease and minimize the heat loss of the cryochamber has become a big problem in the design stage, in considering the very low thermal efficiency and small cooling capacity of the cryocooler. Radiation and conduction heat transfer occur simultaneously in a cryochamber. The radiation heat transfer from the hot surface to the cold surface depends on the emissivity, temperature, area and radiation exchange factor.In the present work, the energy equation, which includes the rarefied gas conduction, radiation between the boundary surfaces and solid conduction, is solved to evaluate the steady cooling loads at the cold finger of a small cryocooler. The surface to surface radiation model is used for the calculation of the radiation exchange factor. This paper presents the effects of the emissivity of the ma...

A Study of Beat Transfer Characteristics of Large Scale Vortex Flow Mixing Vane of Nuclear Fuel Rod Bundle

Mixing vanes have been installed in the space grid of nuclear fuel rod bundle to improve turbulent heat transfer. Split mixing vanes induce the vortex flow in the cooling water to swirl in sub-channel of fuel assembly. But, The swirling flow decays rapidly so that the heat transfer enhancing effect limited to short length after the mixing vane. In thi present study, the large scale vortex flow(LSVF) is generated by rearranging the mixing vanes to the coordinated directions. This LSVF mixing vanes generate the most strong secondary flow vortices which maintain about 35 after the spacer grid. The streamwise vorticity generated by LSVF sustain two times more than that split mixing vane. Heat transfer in the rod bundle occurs greatly at the same direction to cross flow, and maximum temperature at the surface of bundle drops about 1.5K

Measurement of Developing Turbulent Flows in a 90-Degree Square Bend with Spanwise Rotation

Mean flow and turbulence properties of developing turbulent flows in a 90 degree square bend with spanwise rotation were measured by a hot-wire anemometer. A slanted wire is rotated into 6 orientations and the voltage outputs from them are combined to obtain the mean velocity and Reynolds stress components. The combinative effects of the centrifugal and Coriolis forces due to the curvature and the rotation of bend on the mean motion and turbulence structures are investigated experimentally. Results show that the two body forces can either enhance or counteract each other depending on the flow direction in the bend.