• The KSFM Journal of Fluid Machinery
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• v.19 no.3
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• pp.10-13
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• 2016

The wet-air oxidation in gravity pressure reactor is effective for organic waste treatment with energy saving under high pressure and high temperature. But its oxidation control is difficulty because its multi-phase flow characteristics is very complicated. The flow characteristics of an oxidizer feed section in the gravity pressure reactor were investigated using numerical method which are verified by comparison with experimental results. In this study, the results showed that the flow rate of oxidizer have an effect on the generation of bubble around feed section.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1190-1195
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• 2004

In this study, numerical investigation has been performed on the evolution of key-hole geometry during high-energy density laser welding process. Unsteady phase-change heat transfer and fluid flow with the surface tension and recoil pressure are simulated. To model the overheated surface temperature and recoil pressure considering subsonic/sonic vapor flow, the one-dimensional vaporization models proposed by Ganesh and Knight are coupled over liquid-vapor interface. It is shown that the present model predicts well both the vaporization physics and the fluid flow in the thin liquid layer over the other model.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1990.11a
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• pp.16-35
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• 1990

The mechanism of failure of lubricated surfaces at high sliding speeds was Investigated. Experiments were performed with the ball-on-flat and cylinder-on-flat geometries, using lubricants of four different chemical reactivities. Surface failure was found to not be predictable using the ratlo, $\lambda$, of fluid film thickness to composite surface roughness except when chemically inert lubricants are used. Even then the influence of temperature rise on fluid film thickness does not adequately explain the low load carrying capacity of lubricants at high sliding speeds. which causes surface failure. The protective layers on sliding surfaces that form by chemical reaction with the lubricant were found to reduce the surface roughentrig and Increase the load carrying capacity of surfaces to values of $\lambda$ as low as 0.03. Neither the surface toughening nor the formation of the protective layers have been incorporated Into failure models for lubricated systems.

• Tribology and Lubricants
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• v.7 no.1
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• pp.16-27
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• 1991

The mechanism of failure of lubricated surfaces at high sliding speeds was investigated. Experiments were performed with the ball-on-flat and cylinder-on-flat geometries, using lubricants of four different chemical reactivities. Surface failure was found to not be predictable using the ratio, $\lambda$, of fluid film thickness to composite surface roughness except when chemically inert lubricants are used. Even then the influence of temperature rise on fluid film thickness does not adequately explain the low load carrying capacity of lubricants at high sliding speeds, which causes surface failure. The protective layers on sliding surfaces that form by chemical reaction with the lubricant were found to reduce the surface roughening and increase the load carrying capacity of surfaces to values of $\lambda$ as low as 0.03. Neither the surface roughening nor the formation of the protective layers have been incorporated into failure models for lubricated systems.

• Transactions of the Korean Society of Mechanical Engineers
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• v.8 no.2
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• pp.178-183
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• 1984

The stresses in lubricants by external force lead to rise in temperature and drop in viscosty, and the performance of lubricants decrease by this phenomena. The processes of shear stress generation and relaxation are linear under light load condition but those are changed into nonlinearly over a certain limit of load and speed, and this phenomena influences to viscosty change. This study investigates dense fluid which carries property change for high shear rate by using molecular dynamics, and that result can be related to research a behavior of property change of lubricants under high speed and heavy load.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.561-564
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• 2006

Because of new requirements related to the employment of SMT(Surface Mounting Technology) manufacturing and the diversity of components on high density PCB(printed circuit boards), Thermal control of the reflow process is required in oder to achieve acceptable yields and reliability of SMT assemblies. Accurate control of the temperature distribution during the reflow process is one of the major requirements, especially in lead-free assembly. This study has been performed for reflow process using the commercial CFD tool(Fluent) for predicting flow and temperature distributions. There was flow recirculation region that had a weak point in the temperature uniformity. Porous plate was installed to prevent and minimize flow recirculation region for acquiring uniform temperature in oven. This paper provided design concept from CFD results of the steady state temperature distribution and flow field inside a reflow oven.

• Journal of Electrical Engineering and Technology
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• v.9 no.1
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• pp.114-120
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• 2014

In this paper, a method for multi-physics analysis of the temperature-dependent properties of an oil-immersed transformer is discussed. To couple thermal fields with electromagnetic and fluid fields, an algorithm employing a user defined function (UDF) is proposed. Using electromagnetic analysis, electric power loss dependent on temperature rise is calculated; these are used as input data for multi-physics analysis in order to predict the temperature rise. A heat transfer coefficient is applied only at the outermost boundary between transformer and the atmosphere in order to reduce the analysis region. To verify the validity of the proposed method, the predicted temperature rises in high-voltage (HV) and low-voltage (LV) windings and radiators were compared with the experimental values.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.2094-2099
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• 2004

An experimental study on heat transfer characteristics near the critical pressure has been performed with an internally-heated vertical annular channel cooled by R-134a fluid. Two series of tests have been completed: (a) steady-state critical heat flux (CHF) and (b) heat transfer tests for pressure reduction transients through the critical pressure. In the present experimental range, the steady-state CHF decreases with the increase of the system pressure For a fixed inlet mass flux and subcooling, the CHF falls sharply at about 3.8 MPa and shows a trend toward converging to zero as the pressure approaches the critical point of 4.059 MPa. The CHF phenomenon near the critical pressure does not lead to an abrupt temperature rise of the heated wall because the CHF occurred at remarkably low power levels. In the pressure reduction transient experiments, as soon as the pressure passed through the critical pressure, the wall temperatures rise rapidly up to a very high value due to the occurrence of the departure from nucleate boiling. The wall temperature reaches a maximum at the saturation point of the outlet temperature, then tends to decrease gradually.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.11
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• pp.1622-1629
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• 2002

To enhance isothermal characteristics of glass-farming surface, a rectangular parallelepiped heat pipes was fabricated, tested, and analyzed. The working fluid was sodium and the wall material was stainless steel 304. The dimension of the heat pipe was 210 (L) $\times$ 140(W) $\times$ 92(H)mm. A lattice structure covered with screen mesh was inserted to promote return of working fluid. The bottom side of heat pipe was heated electrically and the top side was cooled by liquid circulation. The temperature distribution at the bottom surface was of major concern and was monitored to determine isothermal characteristics. A frozen start-up of rectangular parallelepiped liquid metal heat pipe was tested. The operating mode of the sodium heat pipe was affected by the temperature of cooling zone, input heat flux, and the operating temperature of heat pipe. The heat pipe operated in a normal fashion as long as the heat flux was over 5.78W/cm$^2$, and the inside wall temperature of condenser part was above 95$^{\circ}C$ The maximum temperature difference at the bottom surface was observed to be 32$^{\circ}C$ when the operating temperature of the heat pipe was operating normally around 50$0^{\circ}C$. The result showed that a sodium heat pipe was very effective in reducing significantly the temperature difference in the glass-forming surface.

• Tribology and Lubricants
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• v.33 no.6
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• pp.288-295
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• 2017

In order to improve the efficiency and reliability of the machine, the friction should be minimized. The most widely used method to minimize friction is to maintain the fluid lubrication state. However, we can reduce friction only up to a certain limit because of viscosity. As a result of several recent studies, surface texturing has significantly reduced the friction in highly sliding machine elements, such as mechanical seals and thrust bearings. Thus far, theoretical studies have mainly focused on isothermal/iso-viscous conditions and have not taken into account the heat generation, caused by high viscous shear, and the temperature conditions on the bearing surface. In this study, we investigate the effect of dimple depth and film-temperature boundary conditions on the thermohydrodynamic (THD) lubrication of textured parallel slider bearings. We analyzed the continuity equation, the Navier-Stokes equation, the energy equation, and the temperature-viscosity and temperature-density relations using a computational fluid dynamics (CFD) code, FLUENT. We compare the temperature and pressure distributions at various dimple depths. The increase in oil temperature caused by viscous shear was higher in the dimple than in the bearing outlet because of the action of the strong vortex generated in the dimple. The lubrication characteristics significantly change with variations in the dimple depths and film-temperature boundary conditions. We can use the current results as basic data for optimum surface texturing; however, further studies are required for various temperature boundary conditions.