• 전기화학회지
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• 제8권1호
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• pp.47-54
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• 2005

This article covers the fundamentals of stress-induced diffusion, focusing on the theoretical model for hydrogen transport through self-stressed electrode. First, the relationship between hydrogen diffusion and macroscopic deformation of the electrode specimen was briefly introduced, and then it was classified into the diffusion-elastic and elasto-diffusive phenomena. Next, the transport equation for the flux of hydrogen caused simultaneously by both the concentration gradient and the stress gradient was theoretically derived. Finally, stress-induced diffusion was discussed on the basis of the numerical solutions to the derived transport equation under the permeable and impermeable boundary conditions.

• 전기전자학회논문지
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• 제23권3호
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• pp.793-799
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• 2019

본 연구에서는 웨어러블 Fabric(Nylon) + MWCNT 가스센서 내부에서 열확산을 해석하기 위해서 요소해석 프로그램(comsol)을 이용하여 센서 내부에서의 열용량의 확산 과정과 열용량 분포 경도를 확인하였다. 열용량의 확산과정을 해석하기 위해서 가스센서의 구조체에 대하여 이차원으로 모델링을 진행하였으며, 제시된 모델링에 대해서 센서 구성 부분에 대한 특성값을 제시하여 메시 요소법(FEM)을 이용하여 설계된 웨어러블 가스센서에 대해서 열용량이 확산되는 정도와 열용량 경도를 구하기 위해서 지배방정식으로 1계 편미분방정식을 제안하여 해석하였으며, 열속도 전달식을 제안하여 전극층과 가스 검출부의 10[K/mm] 온도 경도를 구하였다.

• 멤브레인
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• 제12권2호
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• pp.90-96
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• 2002

멤브레인 여과 실험에서 얻어진 데이터 처리에 간단한 수치해석을 적용하여 삼투압(osmotic pressure) 과 구배확산계수(gradient diffusion coefficient)를 도출하는 새로운 방법론을 제시하였다. 삼투압과 구배확산계수는 이론 및 실험적으로 쉽게 구할 수 없는 물리적 특성치로서 멤브레인 여과의 특성 규명에 중요하다. 모델 라텍스 콜로이드의 여과시간에 따른 투과플럭스(permeate flux) 값과 이에 대한 수치적분과 수치미분 데이터로부터 분산된 입자농도의 함수인 삼투압 관계식을 구했다. 이로부터 계산된 열역학적 계수(thermodynamic coefficient)는 입자농도가 증가할수록 감소하는 거동을 보였고, 여기에 기존에 제시되어 있는 수력학적 계수(hydrodynamic coefficient)를 도입하여 구배확산계수를 산출하였다. 아울러, 본 연구에서 계산된 입자농도에 따른 구배확산계수의 결과와 동일한 멤브레인과 라텍스 콜로이드의 여과에 대해서 기존에 통계역학적 시뮬레이션으로 예측한 결과를 비교하였다.

• 대한기계학회논문집
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• 제11권3호
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• pp.528-536
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• 1987

본 연구에서는 간헐적인 유동에 의거한 체적 대류 모델을 설정하고, 구배 확 산 모델에 유동장의 중심부와 외부에서 서로 다른 무게값을 부여하는 혼성 확산 모델 (hybrid diffusion model)을 제안하며, 이 모델을 검증하기 위한 첫 단계로서 평면 제 트 유동에 대하여 수치 계산을 수행하다. 여기에는 간헐도에 대한 난류 전달 방정식 이 필요한데 이 방정식의 생성항(production term)은 곧 외부의 비회전 유동이 난류성 유체로 유입되는 정도를 의미하게 된다.

• Nuclear Engineering and Technology
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• 제51권5호
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• pp.1181-1194
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• 2019

A new moving-mesh Finite Volume Method (FVM) for the efficient solution of the two-dimensional neutron diffusion equation is introduced. Many other moving-mesh methods developed to solve the neutron diffusion problems use a relatively large number of sophisticated mathematical equations, and so suffer from a significant complexity of mathematical calculations. In this study, the proposed method is formulated based on simple mathematical algebraic equations that enable an efficient mesh movement and CV deformation for using in practical nuclear reactor applications. Accordingly, a computational framework relying on a new moving-mesh FVM is introduced to efficiently distribute the meshes and deform the CVs in regions with high gradient variations of reactor power. These regions of interest are very important in the neutronic assessment of the nuclear reactors and accordingly, a higher accuracy of the power densities is required to be obtained. The accuracy, execution time and finally visual comparison of the proposed method comprehensively investigated and discussed for three different benchmark problems. The results all indicated a higher accuracy of the proposed method in comparison with the conventional fixed-mesh FVM.

• Nuclear Engineering and Technology
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• 제52권3호
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• pp.485-498
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• 2020

A diffusion synthetic acceleration (DSA) technique for the S_N transport equation discretized with the linear discontinuous expansion method with subcell balance (LDEM-SCB) on unstructured tetrahedral meshes is presented. The LDEM-SCB scheme solves the transport equation with the discrete ordinates method by using the subcell balances and linear discontinuous expansion of the flux. Discretized DSA equations are derived by consistently discretizing the continuous diffusion equation with the LDEM-SCB method, however, the discretized diffusion equations are not fully consistent with the discretized transport equations. In addition, a fine mesh rebalance (FMR) method is devised to accelerate the discretized diffusion equation coupled with the preconditioned conjugate gradient (CG) method. The DSA method is applied to various test problems to show its effectiveness in speeding up the iterative convergence of the transport equation. The results show that the DSA method gives small spectral radii for the tetrahedral meshes having various minimum aspect ratios even in highly scattering dominant mediums for the homogeneous test problems. The numerical tests for the homogeneous and heterogeneous problems show that DSA with FMR (with preconditioned CG) gives significantly higher speedups and robustness than the one with the Gauss-Seidel-like iteration.

• Journal of Mechanical Science and Technology
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• 제17권7호
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• pp.1054-1062
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• 2003

The numerical simulations of flow and pollutant particle dispersion are described for two-dimensional bell shaped hills with various aspect ratios. The Reynolds-averaged incompressible Navier-Stokes equations with low Reynolds number $\kappa$-$\varepsilon$ turbulent model are used to simulate the flowfield. The gradient diffusion equation is used to solve the pollutant dispersion field. The code was validated by comparison of velocity, turbulent kinetic energy, Reynolds shear stress, speed-up ratio, and ground level concentration with experimental and numerical data. Good agreement has been achieved and it has been found that the pollutant dispersion pattern and ground level concentration have been strongly influenced by the hill shape and aspect ratio, as well as the location and height of the source.

• 한국세라믹학회지
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• 제40권9호
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• pp.827-831
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• 2003

In proton-conducting perovskites, oxygen ions and protons make a diffusion pair for a chemical diffusion and thus lead to the transport of $H_2O$ under its chemical potential gradient. The present manuscript develops relationships between the chemical diffusion coefficient of $H_2O$ and the diffusion coefficients of protons and oxygen vacancies with an emphasis on the thermodynamic behavior of the oxygen vacancies. Depending on the degree of hydration X, two different expressions of the chemical diffusion coefficient were obtained : equation omitted and equation omitted.

• 한국응용과학기술학회지
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• 제19권4호
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• pp.339-348
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• 2002

Self-diffusion coefficients of colloidal ass9Ciation structures in the aqueous solutions of anionic ammonium dodecyl sulfate (ADS) and cationic octadecyltrimethylammonium chloride (OTAC) surfactants were measured by pulsed-gradient spin echo NMR. The results were interpreted on the basis of the ADS/OTAC/water phase diagram. Crossing the phase boundaries, significant changes in self diffusion coefficients were observed and well correlated to the phase diagram. For the micelles their apparent radii were obtained from Stokes-Einstein equation. Their values were 15 for the ADS micelles and 54 ${{\AA}}$ for the OTAC micelles, respectively. For vesicles which were formed spontaneously at different relative amounts of the surfactants and total surfactant concentrations, the radius was measured as 50 to 200 nm. This result is in fair agreement with those by TEM and light scattering.

• Design & Manufacturing
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• 제12권1호
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• pp.1-6
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• 2018

In this study, we calculated the redistribution of vacancy concentration in metal specimens induced by stress-induced diffusion at a high temperature. To deduce the governing equation, we associated the unit volume change equation of strains with a differential equation of vacancy concentration as a function of stress using the stress-strain relationship. In this governing equation, we considered stress as the only chemical potential parameter to stay in the scope of this study, which provided the vacancy concentration equation as of stress gradient in metals. The equation was then mathematically delineated to derive a analytical solution for a transient, one-dimensional diffusion case. With the help of Korhonen's approximation and the boundary conditions, we successfully deduced a general solution from the governing equation. To visualize the feasibility of our solutions, we applied the solution to two different stress-induced cases - a rod with fixed concentrated stresses at both ends and a rod with varying concentrated stresses at both ends. Although it is necessary to legitimatized the model in the future for improvement, our results showed that the model can be used to interpret the location of structural defects, the formation of vacancy, and furthermore the high temperature behavior of metals.