• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.8 s.227
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• pp.1190-1195
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• 2004

A novel manipulation of microfluid width in a microchannel was presented by controlling inflation of a gas boundary. The gas boundary was formed by heating water with a microheater in a semicircular shape from a chamber which was connected symmetrically to the microchannel. The formed gas boundary inflated perpendicularly to the flow direction and, consequently, the microfluid width was narrowed. The inflation and contraction were flexibly like a virtual wall and dependent on two factors: one is the flow velocity of the microfluid and the other is the pressure inside the gas boundary. Dimensions of the chamber and the microchannel width were determined empirically as same of $300\;{\mu}m$ for stable operation. The width of microfluid was manipulated manually with the microheater and could be maintained as up to $22\;{\mu}m$. The stable focusing began to be distorted when the flow velocity exceeded 17.8 mm/s.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05a
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• pp.707-712
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• 1995

Grain boundary diffusion plays significant role in the fission gas release, which is one of the crucial processes dominating nuclear fuel performance. Gaseous fission products such as Xe and Kr generated inside fuel pellet have to diffuse in the lattice and in the grain boundary before they reach open space in the fuel rod. In the mean time, the grains in the fuel pellet grow and shrink according to grain growth kinetics, especially at elevated temperature at which nuclear reactors are operating. Thus the boundary movement ascribed to the grain growth greatly influences the fission gas release rate by lengthening or shortening the lattice diffusion distance, which is the rate limiting step. Sweeping fission gases by the moving boundary contributes to the increment of the fission gas release as well. Lattice and grain boundary diffusion processes in the fission gas release can be studied by 'tracer diffusion' technique, by which grain boundary diffusion can be estimated and used directly for low burn-up fission gas release analysis. However, even for tracer diffusion analysis, taking both the intragranular grain growth and the diffusion processes simultaneously into consideration is not easy. Only a few models accounting for the both processes are available and mostly handle them numerically. Numerical solutions are limited in the practical use. Here in this paper, an approximate analytical solution of the lattice and stationary grain boundary diffusion in a polycrystalline solid is developed for the tracer diffusion techniques. This short closed-form solution is compared to available exact and numerical solutions and turns out to be acceptably accurate. It can be applied to the theoretical modeling and the experimental analysis, especially PIE (post irradiation examination), of low burn up fission. gas release.

• Nuclear Engineering and Technology
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• v.29 no.1
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• pp.7-14
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• 1997

Grain boundary diffusion plays a significant role in fission gas release, which is one of the crucial processes dominating nuclear fuel performance. Gaseous fission produce such as Xe and Kr generated during nuclear fission have to diffuse in the grain lattice and the boundary inside fuel pellets before they reach the open spaces in a fuel rod. These processes can be studied by 'tracer diffusion' techniques, by which grain boundary diffusivity can be estimated and directly used for low burn-up fission gas release analysis. However, only a few models accounting for the both processes are available and mostly handle them numerically due to mathematical complexity. Also the numerical solution has limitations in a practical use. In this paper, an approximate analytical solution in case of stationary grain boundary in a polycrystalline solid is developed for the tracer diffusion techniques. This closed-form solution is compared to available exact and numerical solutions and it turns out that it makes computation not only greatly easier but also more accurate than previous models. It can be applied to theoretical modelings for low bum-up fission gas release phenomena and experimental analyses as well, especially for PIE (post irradiation examination).

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.559-564
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• 2001

Advanced gas turbine engines employ turbine entry temperatures so high that cooling of the turbine blades is essential. The coolant flow introduces losses which need to be minimized, and therefore it is important that the minimum amount of coolant is used. This work presents the result of the one-dimensional analysis and the effect of the boundary conditions on coolant flow rate in gas turbine blades.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.9
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• pp.888-894
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• 2001

Advanced gas turbine engines employ turbine entry temperatures so high that cooling of the turbine blades is essential. The coolant flow introduces losses which need to be minimized, and therefore it is important that the minimum amount of coolant should be used. This work presents the result of the one-dimensional analysis and the effect of the boundary conditions on coolant flow rate in gas turbine blades.

• Journal of computational fluids engineering
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• v.19 no.3
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• pp.77-84
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• 2014

The lattice Boltzmann (LB) method has been used to simulate rarefied gas flows in a micro-system as an alternative tool. However, previous results were mainly focused on a simple geometry with flat walls because the LB method is modeled on uniform Cartesian lattices. When previous boundary conditions for the microflows are applied to curved walls, the use of them requires approximation of the curved boundary by a series of stair steps, and introduces additional errors. For macroflows, no-slip curved wall boundary treatments have been developed remarkably in order to overcome these limits. However, the investigations for the slip curved wall boundary have rarely been performed for microflows. In this work, a curved boundary treatment of the LB method for a slip flow has been introduced. The results of the LB method for 2D microchannel and 3D microtube flows are in excellent agreement with the analytical solutions.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1996.11a
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• pp.27-41
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• 1996

In this study the heat transfer and fluid flow of the molten pool in stationary gas tungsten arc welding using argon shielding gas were investigated. Transporting phenomena from the welding arc to the base material surface, such as current density, heat flux, arc pressure and shear stress acting on the weld pool surface, were taken from the simulation results of the corresponding welding arc. Various driving forces for the weld pool convection were considered, self-induced electromagnetic, surface tension, buoyancy, and impinging plasma arc forces. Furthermore, the effect of surface depression due to the arc pressure acting on the molten pool surface was considered. Because fusion boundary has a curved and unknown shape during welding, a boundary-fitted coordinate system was adopted to precisely describe the boundary for the momentum equation. The numerical model was applied to AISI 304 stainless steel and compared with the experimental results.

• Journal of Electrical Engineering and Technology
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• v.5 no.2
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• pp.264-269
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• 2010

This paper presents the analysis of the dielectric characteristics of a hot $SF_6$ gas in a gas circuit breaker. Hot gas flow is analyzed using the FVFLIC method considering the moving boundary, material properties of real $SF_6$ gas, and arc plasma. In the arc model, the re-absorption of the emitted radiation is approximated with the boundary source layer where the re-absorbed radiation energy is input as an energy source term in the energy conservation equation. The breakdown criterion of a hot gas is predicted using the critical electric field as a function of temperature and pressure. To validate the simulation method, breakdown voltage for a 145kV 40kA circuit breaker was measured for various conditions. Consistent results between the simulation and experiment were confirmed.

• Journal of computational fluids engineering
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• v.19 no.2
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• pp.40-48
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• 2014

For rarefied gas flow regimes, physical phenomena such as velocity slip and temperature jump occur on the solid body surface. To predict these phenomena accurately, either the Navier-Stokes solver with a slip boundary condition or the direct simulation Monte Carlo method should be used. In the present study, flow simulations of a wedge were conducted in Mach-10 flow of argon gas for several different flow regimes using a two-dimensional Navier-Stokes solver with the Maxwell slip boundary condition. The results of the simulations were compared with those of the direct simulation Monte Carlo method to assess the present method. It was found that the values of the velocity slip and the temperature jump predicted increase as the Knudsen number increases. Also, the results are comparatively reasonable up to the Knudsen number of 0.05.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.4
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• pp.509-517
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• 1997

Results of flat plate compressible boundary layer calculation, based on discrete formulation of DSMC method, are presented in low Mach number and low Knudsen number range. The free stream is a uniform flow of pure nitrogen at various Mach numbers in low pressures (i.e. rarefied gas). Complete thermal accommodation and diffuse molecular reflections are used as the wall boundary condition, replacing unreal no-slip condition used in continuum calculations. In the discrete formulation of DSMC method, there is no need to use ad hoc assumptions on transport properties like viscosity and thermal conductivity, instead viscosity is calculated from values of other field variables (velocity and shear stress). Also the results are compared with existing self-similar continuum solutions. In all Mach number cases computed, velocity slip is most pronounced in regions near the leading edge where continuum formulation renders the solution singular. As the boundary layer develops further downstream, velocity slips asymptote to values that are between 10 to 20% of the magnitude of free stream velocity. When the free stream number density is reduced, so the gas more rarefied, the velocity slip increases as expected.