Yeon-Won Lee

Thermal performance of smart heat sinks for cooling high power LED modules

This paper reports the thermal performance of smart heat sinks (SHSs) consisting of hybrid pin fins (HPFs) containing internal channels and integrated with plate fins. The SHSs are mainly aimed at cooling high power LED modules under natural convection condition or forced convection condition with a moderate air velocity. The computational fluid dynamics (CFD) models of SHSs are generated utilizing a commercial CFD software package. The CFD analysis evaluates the cooling performance as well as the basic thermal behavior of the SHSs under various parametric conditions such as heat dissipations ranging from 5 to 20W, air velocities ranging from 0 to 3m/s, and fin spaces of 12, 15, and 20mm. The cooling performance of the SHSs is compared with those of conventional pin fin heat sinks (PHSs). Parametric study has found that thermal resistances of the SHSs are typically smaller than those of the PHSs; for example, they are 15% smaller than the PHS under natural convection. Parametric results show that 15mm is the best fin space for SHSs. This study also explores the effect of the declination angles between the symmetry axis of the fin and the axis of the gravity on the performance of HPFs under natural convection. The results show the thermal resistance value of the HPF decreases with the increase of the declination angle.

Study on Wave Energy Generation of Multi-Floating Bodies for Energy Absorption by CFD

In order to design a wave energy generating system, a 6-DOF analysis technique is applied to CFD analysis on of a floating body and the behavior is interpreted according to the nature of the incoming waves. A spring constant is adopted to control the motion of multi floating bodies and to calculate the total average power absorption. Three cases of different wavelengths namely 20D, 30D and 40D have been modeled to analyze the total average power absorption. The average power absorption not only varies with the position of the floating body but also varies with wavelength. From the results obtained, it is concluded that the maximum total average power absorption is 9W approximately in wavelength 30D and the minimum total average power absorption is 4.3W approximately in wavelength 40D.

A Study on the Change of Free Surface Vortex according to Intake Conditions in the Pump Sump

In this study the change of free surface vortex is represented at different times according to height of water and with or without curtain wall installation. The air volume fraction is investigated at each condition of water level and the influence about creation of vortex is analyzed. The height of sump intake is taken as 100mm and the water level is divided into 5 steps. The sump model is the TSJ model and the curtain wall is applied by HI standard of America. The results shows that the free surface vortex can be identified on the isotropic surface at air volume fraction 1%~5% and the vortices make an air column from the free surface to the sump intake and are created and destroyed repeatedly. In the higher water level, less air is absorbed into the intake pipe. After curtain wall installation, the suction rate of the air volume fraction is decreased by 6.7%. The result of the vortex motion according to time, works on a cycle.

Large eddy simulation of flow around a bluff body of vehicle shape

The turbulent flow with wake, reattachment and recirculation is a very important problem that is related to vehicle dynamics and aerodynamics. The Smagorinsky Model (SM), the Dynamics Subgrid Scale Model (DSM), and the Lagrangian Dynamic Subgrid Scale Model (LDSM) are used to predict the three-dimensional flow field around a bluff body model. The Reynolds number used is 45,000 based on the bulk velocity and the height of the bluff body. The fully developed turbulent flow, which is generated by the driver part, is used for the inlet boundary condition. The convective boundary condition is imposed on the outlet boundary condition, and the Spalding wall function is used for the wall boundary condition. We compare the results of each model with the results of the PIV measurement. First of all, the LES predicts flow behavior better than the k-ε turbulence model. When we compare various LES models, the DSM and the LDSM agree with the PIV experimental data better than the SM in the complex flow, with the separation and the reattachment at the upper front part of the bluff body. But in the rear part of the bluff body, the SM agrees with the PIV experimental results better than them. In this case, the SM predicts overall flow behavior better than the DSM and the LDSM.

Study on copper end-tab shape for maximum heat discharging performance

When implementing butt joint welding of two plates, it is useful to attach end-tabs made of a metal with high heat conductivity (e.g., copper) at the front and back sides of the welded plates to prevent the bead from rolling down and prevent defects that may occur at the tips of the weld zone. In this study, the fin shape, which is known to have good heat discharging characteristics by natural convection, has been applied to enhance the cooling performance of the end-tab. From both experiment and numerical analysis, it was confirmed that end-tabs with fin-shaped holes have better heat discharging performance than end-tabs without holes. Through thermal and fluid flow analysis, the cooling rates of end-tabs with different hole shapes were estimated in order to figure out characteristics of shape factor that are important for the heat discharging performance. As a result, we found that the structure including vertical fins with optimal fin gap was the best-performing shape.

LBM simulation on natural convection flow in a triangular enclosure of green house under winter day conditions

Natural convection flow of roof top cross section in a green house has been numerically investigated using a thermal lattice Boltzmann method (TLBM). Two types of thermal boundary condition are considered; uniform and nonuniform bottom heating with symmetrically cold inclined walls. The results are presented as velocity and temperature profiles as well as stream function and temperature contours for different Rayleigh number, Ra, ranging from 103 to 105 with other controlled parameters. The intensity of circulation is found to be higher and symmetric for lower values of Ra and the asymmetric behavior of the flow about the geometric centre line is seen for higher values of Ra in the case of uniform bottom heating. However, for nonuniform case, multiple circulation cells are observed for different Ra maintaining the symmetrical fluid properties. In addition, the average rate of heat transfer in terms of Nusselt number indicated the lower heat transfer rates for the nonuniform case compared to the uniform heating case. Finally, the results have been compared with the previous published works and found a good agreement.

Flow Visualization of Plastic type PCV Valve with Horizontal Force

PCV(Positive Crankcase Ventilation) system is designed to remove blowby gas. In this system, a PCV valve is attached in a manifold suction tube to control the flow rate of blowby gas which generates various operating conditions of an automotive engine. As this valve plays a crucial role, the demand in its design is high owing to the small size and high velocity. For this reason, a numerical investigation was carried out to understand both the spool dynamic motion and internal fluid flow characteristics. As a result, the spool dynamic characteristics(i.e. displacement, velocity, acting force), increase in direct proportion to the magnitude of the pressure difference and indicate periodic oscillating motions. Moreover, the velocity at the orifice region decreases according to the increase in differential pressure due to energy loss caused by the sudden decrease of flow area at the orifice region and the increase of flow volume in front of the spool head. Finally, the mass flow rate at the outlet decreases with the increase of spool displacement.

A Study on Optimum Shape of Shield Gas Nozzle for Bead Shape Control in TIG Welding using Gas Force (I) - Design and Performance Analysis of Venturi Nozzle -

Bead shape control with gas force process has been developed to overcome the concave back bead in pipe orbital welding. However, It is impossible to make a convex back bead using the existing gas nozzle, because it has high gas-consuming and low gas force. The purpose of this paper, to develop optimum shape of nozzle which to reduce the consumption of gas, maximizing the shield gas force with low cost and high productivity coincide the Green welding. In this paper venturi-type nozzle was designed by using the Venturi meter and compared velocity, pressure, arc shape in the flat position with existing CP-nozzle. As a result, Venturi-type nozzle`s maximum velocity and pressure was improved at the same flow rate. Also heat input was increased by the arc contraction in the flat position.

A Study on Optimum Shape of Shield Gas Nozzle for Bead Shape Control in TIG Welding using Gas Force (II) - Effect of Molten Metal Control by Venturi Nozzle in Overhead Position -

Bead shape control with gas force process has been developed to overcome the concave back bead in pipe orbital welding. However, It is impossible to make a convex back bead using the existing gas nozzle, because it has high gas-consuming and low gas force. The purpose of this paper, to develop optimum shape of nozzle which to reduce the consumption of gas, maximizing the shield gas force with low cost and high productivity coincide the Green welding. In this paper venturi-type nozzle was compared with existing CP-type nozzle by TIG pulse welding in overhead position. As a result, CP-type occurs the wormholes in the overhead position, but the Venturi-type without the pore and formed a good bead appearance.

Visualization of Air Absorption Induced by Free Surface Vortex in the Pump Sump Using Multi-phase Flow Simulation

Abstract. In this study the change of free surface vortex is expressed through the time volume fractionusing multiphase unsteady condition in sump, because in previous studies of the pump sump did not rep-resent the behavior of the free surface vortex exactly due to the reason it was calculated using single phaseand steady condition. The reliability of the computational analysis is verified through comparing exper-imental results with that of present numerical analysis. Homogeneous free surface model is used to applyinteractions of air and water. The results show that the free surface vortex can be identified on the iso-tropic surface at air volume fraction 1%~5%. The vortices make an air column from the free surface tothe sump intake and are created and destroyed repeatedly. The behavior of free surface vortex at numericalanalysis is quite similar to experimental test. The result of vortex motion according to time, works ona cycle.Key Words:Free Surface Vortex (자유표면 보텍스), Homogeneous Model (등균질 모델), Multi-phaseFlow (다상유동), Volume Fraction (체적비), Unsteady Condition (비정상 조건)