• Coupled systems mechanics
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• 제7권4호
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• pp.373-393
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• 2018

This paper is concerned with the wave propagation behavior of rotating functionally graded temperature-dependent nanoscale beams subjected to thermal loading based on nonlocal strain gradient stress field. Uniform, linear and nonlinear temperature distributions across the thickness are investigated. Thermo-elastic properties of FG beam change gradually according to the Mori-Tanaka distribution model in the spatial coordinate. The nanobeam is modeled via a higher-order shear deformable refined beam theory which has a trigonometric shear stress function. The governing equations are derived by Hamilton's principle as a function of axial force due to centrifugal stiffening and displacement. By applying an analytical solution and solving an eigenvalue problem, the dispersion relations of rotating FG nanobeam are obtained. Numerical results illustrate that various parameters including temperature change, angular velocity, nonlocality parameter, wave number and gradient index have significant effect on the wave dispersion characteristics of the understudy nanobeam. The outcome of this study can provide beneficial information for the next generation researches and exact design of nano-machines including nanoscale molecular bearings and nanogears, etc.

• 산업기술연구
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• 제6권
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• pp.19-31
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• 1986

The residence time distribution of liquid flow in a 4.0cm diameter column packed with porous $Al_2O_3$ spheres of 0.37cm diameter were measured with pulse injections of a tracer under cocurrent trickling flow conditions. The mean residence time of liquid flow and liquid hold-up calculated by the transient curve of tracer were unaffected by gas flow rates under experimental ranges of liquid flow rates from 2.4 to $4.5(kg/m^2\;sec)$ and gas flow rates from 0 to $0.13(kg/m^2\;sec)$. The axial dispersion coefficient of liquid stream and apparent diffusivity of tracer in a micropore of solid particle were estimated from the response curve of tracer. The calculated Peclet No. were increased in ranges of 68-to 82 with a increasing of liquid mass velocity, and the external effective contacting efficiency between liquid and solid which can be expressed. by $(D_i)_{app}/D_i$ varied in ranges of 0.54 to 0.68 depending on the liquid flow rates. The gas to liquid(water) volumetric mass transfer coefficient were determined from desorption experiments with oxygen at $25^{\circ}C$ and 1 atm. The measured mass transfer coefficients were increased with liquid flow rates and the effect of gas flow rates on the mass transfer coefficient was insignificant.

• 한국자동차공학회논문집
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• 제14권1호
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• pp.17-24
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• 2006

This study was performed to figure out spray behavior and fuel atomization characteristics of a piezo-driven injector and a solenoid-driven injector in the common-rail injection system under the same design parameters and test conditions. The process of spray injection was visualized by using the spray visualization system composed of a Nd:YAG laser and an ICCD camera. The atomization characteristics were investigated in terms of axial mean velocity, Sauter mean diameter(SMD) and droplet distributions obtained from a phase Doppler particle analyzer system. Compared with solenoid-driven injector, the piezo-driven injector has short injection delay and reaches quickly to the maximum injection value. Spray tip penetration shows some difference, however, spray angle of piezo-driven injector is wider than that of solenoid-driven injector. Sauter mean diameter of piezo-driven type injector is smaller than that of solenoid-driven type.

• 대한기계학회논문집B
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• 제28권11호
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• pp.1407-1416
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• 2004

A two-dimensional direct numerical simulation was performed to investigate the flame structure of CH$_4$/$N_2$-Air counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman's detailed chemistry were adopted in this computation. The results showed that an initially flat stagnation plane, on which an axial velocity was zero, was deformed into a complex-shaped plane, and an initial stagnation point was moved far away from a vortex head when the counterflow field was perturbed by the vortex. It was noted that the movement of stagnation point could alter the species transport mechanism to the flame surface. It was also identified that the altered species transport mechanism affected the distributions of the mixture fraction and the scalar dissipation rate.

• 대한기계학회논문집B
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• 제26권5호
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• pp.629-639
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• 2002

In-vitro flow characteristics downstream of a polyurethane artificial heart valve and a Bjork-Shiley Monostrut mechanical valve have been comparatively investigated in pulsatile flow using particle image velocimetry (PIV). With a triggering system and a time-delayed circuit the velocity distributions on the two perpendicular measurement planes downstream of the valves are evaluated at any given instant in conjunction with the opening behaviors of valve leaflets during a cardiac cycle. The regions of stasis and high shear stress can be found simultaneously by examining the entire view of the instantaneous velocity and Reynolds shear stress fields. It is known that high shear stress regions exist at the interface between strong axial jet flows along the wall and vortical flows in the central area distal to the valves. In addition. there are large stagnation or recirculation regions in the vicinity of the valve leaflet, where thrombus formation can be induced by accumulation of blood elements damaged in the high shear stress zones. A correlation between the unsteady flow patterns downstream of the valve and the corresponding opening postures of the polyurethane valve membrane gives useful data necessary for improved design of the frame structure and leaflet geometry of the polyurethane valve.

• 한국추진공학회지
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• 제9권2호
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• pp.97-104
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• 2005

많은 장점에도 불구하고 유동함수를 이용한 수치해석용 격자생성 좌표변환기법의 단점은 저속영역에서의 격자간격이 고속영역에 비해 상대적으로 큼에 따라 수치적 처리에 많은 오차를 내포하고 있다는 점이다. 본 연구에서는 이러한 저속영역에서의 단점을 보완하기 위하여 격자간격을 속도크기 및 영역에 따라 적절히 조절할 수 있도록 수학적으로 변형된 압축성 유동함수를 이용한 좌표변환기법을 제안하고 가스터빈엔진에 주로 적용되는 유동모델로서 동심원상 두개 이상의 난류제트혼합유동에 대해 적용하였으며 해당 실험치, 즉 축 방향 평균속도분포, 난류운동에너지, 그리고 난류전단응력분포와 비교하여 난류운동에너지가 약간 과소평가 된 대칭축을 제외한 혼합경계층 내에서 $3.5\%$ 이내의 신뢰성을 확보하였다. 본 기법은 특히 터보팬엔진에 대한 내부흐름들의 혼합유동을 규명하거나 또는 난류전단응력에 의한 제트소음발생 및 저감방법을 도모하는데 유용하게 활용될 것으로 기대된다.

• 한국진공학회지
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• 제17권3호
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• pp.189-194
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• 2008

플라즈마 진단법 중 손쉽고 구동이 용이한 장점으로 인해 범용 플라즈마 진단 장치로 사용되는 랑뮤어 (Langmuir) 탐침법은 탐침이 직접 플라즈마 내에 삽입됨으로써 플라즈마를 간섭하기 때문에 플라즈마 고유의 특성을 변형시키는 약점이 있다. 본 연구에서는 대기압 아르곤 자유 연소 아크 (free-burning arc)에 삽입된 랑뮤어 탐침에 의한 교란 영향을 고찰하기 위하여 탐침 유무에 따른 열플라즈마 상태를 수치계산 하였고, 온도장과 속도장이 왜곡된 정도를 비교 분석하였다. 열플라즈마를 모델링하기 위하여 열유동장과 전자기장에 관한 두 종류의 지배방정식을 상용 CFD 프로그램과 자체 제작한 서브루틴을 이용하여 연계 계산하였다. 삽입된 탐침에 의해 열플라즈마 온도장은 탐침의 앞뒤에서 모두 플라즈마 축의 수직 방향으로 큰 변화를 나타내는 열적 교란이 발생되었다. 속도장에서는 탐침 선단의 정체 영역과 후단 후류 영역에서 유동 교란이 발생되었으며, 삽입 된 탐침의 영향이 국부적인 것이 아닌 플라즈마 유동장 대부분 영역에 영향을 미침을 확인할 수 있었다. 따라서 탐침법의 신뢰성을 높이기 위해서는 탐침 삽입에 의한 플라즈마의 열유체역학적 교란을 고려해야 한다.

• Nuclear Engineering and Technology
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• 제50권7호
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• pp.1138-1147
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• 2018

Delayed hydride cracking (DHC) is an important failure mechanism for Zircaloy tubes in the demanding environment of nuclear reactors. The threshold stress intensity factor, $K_{IH}$, and critical hydride length, $l_C$, are important parameters to evaluate DHC. Theoretical models of them are developed for Zircaloy tubes undergoing non-homogenous temperature loading, with new stress distributions ahead of the crack tip and thermal stresses involved. A new stress distribution in the plastic zone ahead of the crack tip is proposed according to the fracture mechanics theory of second-order estimate of plastic zone size. The developed models with fewer fitting parameters are validated with the experimental results for $K_{IH}$ and $l_C$. The research results for radial cracking cases indicate that a better agreement for $K_{IH}$ can be achieved; the negative axial thermal stresses can lessen $K_{IH}$ and enlarge the critical hydride length, so its effect should be considered in the safety evaluation and constraint design for fuel rods; the critical hydride length $l_C$ changes slightly in a certain range of stress intensity factors, which interprets the phenomenon that the DHC velocity varies slowly in the steady crack growth stage. Besides, the sensitivity analysis of model parameters demonstrates that an increase in yield strength of zircaloy will result in a decrease in the critical hydride length $l_C$, and $K_{IH}$ will firstly decrease and then have a trend to increase with the yield strength of Zircaloy; higher fracture strength of hydrided zircaloy will lead to very high values of threshold stress intensity factor and critical hydride length at higher temperatures, which might be the main mechanism of crack arrest for some Zircaloy materials.

• 대한환경공학회지
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• 제27권12호
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• pp.1305-1310
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• 2005

하상여과에서 집수관의 최적 설계를 위한 방편으로 여러 개의 모래통을 이용하여 모형 집수관 실험을 수행하였다. 집수정의 수위, 집수관의 직경, 집수관의 길이, 그리고 모래의 투수계수를 변화시키면서 모형 집수관을 운영하였으며, 집수관 표면에서의 잔류 수두와 집수관 각 부위에서의 여과수 유입율, 그리고 집수정에서의 산출유량을 측정하였다. 실험결과 모래층에서의 저항에 비해 집수관에서의 저항이 작아서 집수관 표면에서의 잔류수두가 수평에 가까운 경우 효율적인 집수관임을 알 수 있었다. 집수관의 여과수 유입율은 집수정에 근접할수록 지수함수적으로 증가하였고, 집수관의 직경이 작을수록 그리고, 대수층의 투수계수가 클수록 이 지수함수의 기울기는 증가하였다. 집수관의 길이와 직경이 증가하면 산출유량은 증가하지만 그 한계생산성은 감소하였다. 효율적인 하상여과를 위해서는 집수관에서의 축방향 유속을 줄이는 것이 중요함을 알 수 있었으며, 본 연구의 조건에서는 이 유속이 1 m/sec를 넘을 경우 우물의 효율이 크게 감소함을 알 수 있었다.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1996년도 춘계학술발표회논문집(2)
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• pp.233-240
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• 1996

Analyses of the viscous and end effects on electromagnetic (EM) pumps of annular linear induction type for the sodium coolant circulation in Liquid Metal Fast Breeder Reactors have been carried out based on the MHD laminar flow analysis and the electromagnetic field theory. A one-dimensional MHD analysis for the liquid metal flowing through an annular channel has been performed on the basis of a simplified model of equivalent current sheets instead of three-phase currents in the discrete primary windings. The calculations show that the developed pressure difference resulted from electromagnetic and viscous forces in the liquid metal is expressed in terms of the slip, and that the viscous loss effects are negligible compared with electromagnetic driving forces except in the low-slip region where the pumps operate with very high flow velocities comparable with the synchronous velocity of the electromagnetic fields, which is not applicable to the practical EM pumps. A two-dimensional electromagnetic field analysis based on an equivalent current sheet model has found the vector potentials in closed form by means of the Fourier transform method. The resultant magnetic fields and driving forces exerted on the liquid metal reveal that the end effects due to finiteness of the pump length are formidable. In addition, a two-dimensional numerical analysis for vector potentials has been performed by the SOR iterative method on a realistic EM pump model with discretely-distributed currents in the primary windings. The numerical computations for the distributions of magnetic fields and developed pressure differences along the pump axial length also show considerable end effects at both inlet and outlet ends, especially at high flow velocities. Calculations of each magnetic force contribution indicate that the end effects are originated from the magnetic force caused by the induced current ( u x B ) generated by the liquid metal movement across the magnetic field rather than the one (E) produced by externally applied magnetic fields by three-phase winding currents. It is concluded that since the influences of the end effects in addition to viscous losses are extensive particularly in high-velocity operations of the EM pumps, it is necessary to find ways to suppress them, such as proper selection of the pump parameters and compensation of the end effects.