• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.2
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• pp.677-684
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• 1991

A numerical analysis is performed about the effects of aspect ratio on natural convective heat transfer from a vertical isothermal cylinder of 0.deg. C immersed in cold pure water. The results of analysis include velocity profiles, temperature profiles and mean Nusselt number of the steady flow region. As aspect ratio of vertical cylinder increases, the flow and heat transfer characteristics of vertical isothermal cylinder approach to those of vertical isothermal flat plate. Numerical solutions obtained for Rayleigh number and aspect ratio indicate the cylinders can be classified as short cylinder and long cylinder. In the cases of short cylinder and long cylinder, new heat transfer correlations are presented. Here, the coefficient values C of new heat transfer correlations are presented as the function of density extremum parameter $R^*/. Numerical results show that theoretical results are in close agreement with experimental results.ts.

• Journal of Korean Society of Occupational and Environmental Hygiene
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• v.11 no.1
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• pp.78-84
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• 2001

A CFD simulation of airflow and temperature field in a heated room has been described in this paper. The thermal wall jet created by a radiator greatly influences the airflow pattern, temperature distribution. The area close La a heat source has a higher risk of air-borne contamination and imposes a harmful effect on occupants in that area. The predicted flow field, temperature results show good agreement with the measured data. As the results were compared with experimental data, the applicability of CFD was satisfactorily verified. Also, the CFD simulation can capture the natural convective flow features. If a CFD simulation is applied ventilation design with a heat source, An effective design will be attained. Further study is required to improve the accuracy of CFD simulation.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.11
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• pp.1422-1436
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• 1997

Numerical investigations were conducted to study the convective phenomena of an initially stably stratified salt water solution with lateral heating in a uniformly rotating annulus. The method of investigation is the finite difference analysis of the basic conservation equation for an axisymmetric, unsteady, double-diffusive convection and calculation is made for R $a_{\eta}$=2*10$^{5}$ and Ta=10$^{7}$ ~ 2.5*10$^{8}$ . Formation of layered flow structure, merging process of layers, the corresponding temperature and concentration distributions, Nu variation with time are examined. Numerical results show that in each layer, the temperature profile looks 'S'-shaped and the concentration profile is uniform due to the convective mixing. At the interface between adjacent layers, the temperature changes smoothly but the concentration changes rapidly. As the effect of the rotation increases, the generation of rolls at hot wall, the formation and merging of layers are delayed. The average Nu shows the trend of conduction heat transferees the effect of the rotation increases.n increases.

• Journal of the Korean Society of Propulsion Engineers
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• v.6 no.3
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• pp.37-43
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• 2002

There have been numerous experimental studies for heat transfer measurement technique. This study investigates optical methods for measuring local heat transfer coefficient using thermochromic liquid crystal. Transient and steady methods have been utilized to measure local heat transfer coefficient on a cylinder with a cross flow. The steady method is based on the heat-coating technique and two transient methods adopt by-pass technique and insertion technique, respectively. Both techniques of transient method employ heating technique in which the flow is heated by using the electric heater and cooling technique which cools the preheated cylinder. Experimental results indicate that each methods have nearly similar results. Detailed discussions have been made for its own advantages and disadvantages.

• Journal of ILASS-Korea
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• v.5 no.1
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• pp.13-22
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• 2000

The present study has numerically analyzed the vaporization characteristics of fuel droplets in the high temperature convective flow field. The axisymmetric governing equations for mass, momentum, energy, and species are solved by an iterative and implicite unstructured finite-volume method. The moving boundary due to vaporization is handled by the deformable unstructured grid technique. The pressure-velocity coupling in the density-variable flows is treated by the SIMPLEC algorithm. In terms of the matrix solver, Bi-CGSTAB is employed for the numerically efficient and stable convergence. The n-decane is used as a liquid fuel and the initial droplet temperature is 300K. Computations are performed for the nonevaporating and evaporating droplets with the relative interphase velocity(25m/s). The unsteady vaporization process has been simulated up to the nondimensional time, 25. Numerical results indicate that the mathematical model developed in this study succesfully simulates the main features of the droplet vaporization process in the convective environment.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.2
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• pp.164-170
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• 2001

The influence of unsteady wake on the flow and heat transfer characteristics in a four-vane linear cascade was experimentally investigated. The unsteady wake was generated with four rotating rectangular plates located upstream of the cascade. Tested inlet Reynolds number based on chord length was set to 66,000 by controlling free-stream velocity. A hot-wire anemometer system was employed to measure turbulent velocity components. For the convective heat transfer coefficients measurement on turbine blade surface, thermochromic liquid crystal and gold film Intrex were used. It was found that the unsteady wake enhances the turbulent motion in the cascade passage and accordingly promotes the development and transition of boundary layer. It was found that the heat transfer coefficients on the blade surface increase as the plate rotating speed increases. However, the increasing of heat transfer coefficients is not significant in the case that Strouhal number is higher than 0.503.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.6 no.2
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• pp.155-165
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• 1994

In this study, the following factors are investigated from experiments for a vertical parallel plate heat exchanger under the frosting condition ; the growth of frost layer, the characteristics of heat and mass transfer, the change of mass flow rate of the air passing through the heat exchanger, and the pressure drop of the air in the heat exchanger. The amount of heat and mass flux of water vapor transferred from the air stream to the heat exchanger surface is large at the early stage of frosting and then decreases dramatically, and the extent of decreasing rate becomes moderate with time. The frost layer formed near the inlet of the heat exchanger is thicker and denser than that formed near the outlet. It is found that the gradient of the amount of frost along the flow direction increases with time. In the early period of frost formation, the thermal resistance between the air and the cooling plate increases dramatically and then the extent of change decreases with time. Initially the convective thermal resistance is dominant. Then, while the convective thermal resistance decreases with time, the conductive thermal resistance continues to increase with time and finally the conductive thermal resistance becomes dominant.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.7 no.4
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• pp.147-157
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• 1987

Five finite difference schemes (explicit, Bresler, implicit, upstream, and Chaudhari scheme) for the convective dispersion model are analyzed numerically to investigate their characteristics and applicabilities. Camparative study results show that the conditionally stable Chaudhari scheme has the smallest numerical dispersion and that the unconditionally stable Bresler scheme has the overshooting in regions of oscillation. Explicit scheme is the most accurate for a dispersion-dominated flow whereas Chaudhari scheme is for a convection-dominated flow. The computation time (CPU) is the shortest for the explicit or Chaudhari scheme with the same order of magnitude and is always the longest for the Bresler scheme.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.2
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• pp.169-176
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• 2000

Natural convection in a wide-gap horizontal annulus is considered, and the transition of flows and the bifurcation phenomenon are investigated for the fluids with Pr=0.2 and 0.3. At Pr=0.2, a bicellular flow pattern is observed at high Rayleigh number, and the solution is unique. At Pr=0.3, both the steady unicellular and bicellular flows exist above a certain critical Rayleigh number. For the fluids of Pr=0.2, the bicellular flow can be obtained by the impulsive heating of the inner cylinder, but it is not obtained from the zero initial condition for Pr=0.3. Hysteresis phenomena have not been observed. A transition from a bicellular flow to a unicellular flow occurs for Pr=0.3.

• Journal of KSNVE
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• v.7 no.4
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• pp.613-620
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• 1997

Steady and unsteady flows between rotating cylinders are of interest on lubrication, convective heat transfer and flow-induced vibration in large rotating machinery. Steady rotating flow is generated by rotating cylinder with constant velocity while the unsteady rotating flow by oscillating cylinder with homogeneoysly oscillating velocity. An analytical method is developed based on the simple radial coordinate transformation for the steady and unsteady rotating flows in concentric annulus. The governing equations are simplified from Navier-Stokes equatins. Considering the skin friction based on the radial variation of circumferential flow velocity, the torques acting on the fixed and the rotating cylinder are evaluated in terms of added-inertia and added-damping torque coefficients. The coefficients are found to be influenced by the oscillatory Reynolds number and the radius ratio of two cylinders; however, the effect of the oscillatory Reynolds number on the coefficients is minor in case of relatively low radius ratio.