1,721,123 research outputs found
Self-balanced navigation-grade capacitive microaccelerometers using branched finger electrodes and their performance for varying sense voltage and pressure
No full textThis paper presents a navigation-grade capacitive microaccelerometer, whose low-noise high-resolution detection capability is achieved by a new electrode design based on a high-amplitude anti-phase sense voltage. We reduce the mechanical noise of the microaccelerometer to the level of 5.5 mug/rootHz by increasing the proof-mass based on deep RIE process of an SOI wafer. We reduce the electrical noise as low as 0.6 mug/rootHz by using an anti-phase high-amplitude square-wave sense voltage of 19 V. The nonlinearity problem caused by the high-amplitude sense voltage is solved by a new electrode design of branched finger type. Combined use of the branched finger electrode and high-amplitude sense voltage generates self force-balancing effects, resulting in an 140% increase of the bandwidth from 726 Hz to 1734 Hz. For a fixed sense voltage of 10 V, the total noise is measured as 2.6 mug/rootHz at the air pressure of 3.9 torr, which is the 51% of the total noise of 5.1 mug/rootHz at the atmospheric pressure. From the excitation test using I g, 10 Hz sinusoidal acceleration, the signal-to-noise ratio of the fabricated microaccelerometer is measured as 105 dB, which is equivalent to the noise level of 5.7 mug/rootHz. The sensitivity and linearity of the branched finger capacitive microaccelerometer are measured as 0.638 V/g and 0.044%, respectively
Development of a thermal-hydraulic analysis code for annular fuel assemblies
No full textA thermal-hydraulic analysis code which is capable of modeling both internally and externally cooled annular fuel pins was developed. The coolant flow distribution in the annular fuel-based assemblies is adjusted by a pressure drop model allowing for conditions such as non-equal velocity and non-saturated phases. The heat transfer fraction is determined by the ratio of cross-sectional areas distinguished by the radius at which the first derivative of the temperature within the annular fuel equals zero. The code predictions have been compared with calculations from Korea Atomic Energy Research Institute (KAERI) and MIT. The heat transfer fraction difference between the code and RELAP was about 3.9%, and the Departure from Nucleate Boiling Ratio (DNBR) prediction of the code agreed well with the MIT's result in the region below 3 m. For the application of the code, thermal-hydraulics of thorium-based fuel assemblies loaded with annular seed pins were compared with those of the existing thorium-based assemblies. The pressure drop in the assembly generally increased in the case of annular fuel due to the larger wetted perimeter. In the inner subchannels of the seed pins, mass fluxes were high due to the grid form losses in the outer subchannels. About 43% of the heat generated from the seed pin flowed into the inner subchannel and the rest into the outer subchannel. The minimum DNBRs (MDNBRs) of the annular fuel-based assemblies were higher than those of the existing ones. Because interchannel mixing cannot occur in the inner subchannels, temperatures and enthalpies were higher in the inner subchannels. (C) 2003 Elsevier B.V. All rights reserved
Numerical simulation of three-dimensional fiber orientation in injection molding including fountain flow effect
No full textIt is essential to predict the nature of flow field inside mold and flow-induced variation of fiber orientation for effective design of short fiber reinforced plastic parts. In this investigation, numerical simulations of flow field and three-dimensional fiber orientation were carried out in special consideration of fountain flow effect. Fiber orientation distribution was described using the second-order orientation tensor. Fiber interaction was modeled using the interaction coefficient C-I. Three closure approximations, hybrid, modified hybrid, and closure equation for C-I=0, were selected for determination of the fiber orientation. The fiber orientation routine was incorporated into a previously developed program of injection mold filling (CAMPmold), which was based on the fixed-grid finite element/finite difference method assuming the Hele-Shaw flow. For consideration of the fountain flow effect, simplified deformation behavior of fountain flow was employed to obtain the initial condition for fiber orientation in the flow front region. Comparisons with experimental results available in the literature were made for film-gated strip and center-gated disk cavities. It was found that the orientation components near the wall were accurately predicted by considering the fountain flow effect. Test simulations were also carried out for the filling analysis of a practical part, and it was shown that the currently developed numerical algorithm can be effectively used for the prediction of fiber orientation distribution in complex parts
Numerical simulation of three-dimensional fiber orientation in short-fiber-reinforced injection-molded parts
No full textIn this paper, a second-order orientation tensor, orientation average of dyadic product of orientation vector, was adopted to describe three-dimensional orientation distribution of short fibers in injection-molded parts. For calculation of the fiber orientation tensor, a closure approximation is needed to reduce the higher fourth-order orientation tensor to the lower second-order. A modified hybrid closure approximation, which can accurately describe random-in-space, random-in-plane, and uniaxial distribution of fiber orientations, is introduced to yield better computational results than existing solutions available in references. Comparisons between numerical calculations of the second-order orientation tensor and the orientation distribution function (ODF) in simple flow field were made in order to demonstrate the accuracy of the closure approximation proposed. Orientation tensor equation currently introduced was incorporated into a finite-element/finite-difference program for injection molding analysis. In addition, new numerical technique was developed to reasonably calculate velocity gradients using constant velocity elements. The developed program was applied to simulation of injection molding for the thin cavity of a sector of spherical shell. The analysis showed that the currently proposed numerical approach enhances the solution accuracy of fiber orientation prediction in injection-molded parts made of short-fiber-reinforced thermoplastics. (C) 2002 Elsevier Science B.V. All rights reserved
Compressible flow analysis of filling and post-filling in injection molding with phase-change effect
No full textIn order to predict the shrinkage, warpage and mechanical properties of the injection molded parts, it is necessary to know the history of the flow field during injection-molding processes. In the present investigation a numerical simulation program was developed to predict the flow field in filling and post-filling stages of injection molding. To simulate the real molding conditions more accurately, a generalized Hele-Shaw model for a non-Newtonian fluid was assumed considering the effects of phase change and compressibility of the resin. A finite-element-finite-difference (FEM-FDM) hybrid scheme with control volume approach was employed as the solving technique. For modeling the viscosity of the resin, a modified Cross model was used with a double-domain Tait equation of state being employed in describing the compressibility of the resin during molding. The energy balance equation, including latent-heat dissipation for semicrystalline materials, was solved in order to predict the solidified layer and temperature profile in detail. For verification of the numerical results obtained from the developed program, the simulation results were compared with the experimental results obtained from the test mold set designed in the current study using commercial-grade PP and the data available in the literature. Based on a comparison between experiments and simulations, it was found that the currently developed program was useful in unified simulations of filling and post-filling in injection-molding processes when considering the phase-change effect. (C) 1997 Elsevier Science Ltd.The authors are grateful for the grant from
Korea Science and Engineering Foundation
(KOSEF) through the Engineering Research
Center for Net Shape and Die Manufacturing at
Pusan National University under which this
work was possible, and Samsung Chemicals Co.
for providing PP materials
Modified hybrid closure approximation for prediction of flow-induced fiber orientation
No full textDistribution of fiber orientation in flow molding processes with short-fiber reinforcements is of great importance because it affects the mechanical properties of molded parts. Due to trade-off between the computational efficiency and accuracy, a second-order orientation tensor has been widely used to describe the fiber orientation distribution. For calculation of this fiber orientation tensor, a closure approximation has been introduced to reduce a higher fourth-order orientation tensor to a lower second order. Ln the present investigation, a hybrid closure approximation has been modified. Two parametric forms of the distribution function, which accurately describe the random-in-space, random-in-plane, and uniaxial distributions of the fiber orientation were linearly interpolated. The interpolating factor was obtained as a function of the fiber interaction coefficient by fitting the distribution function calculations. Test simulation in homogeneous flow and nonhomogeneous flow fields, respectively, showed that the proposed closure approximation gives good performance for a wide range of C-I values without showing nonphysical behavior. (C) 1999 The Society of Rheology. [S0148-6055(99)01603-X].The authors wish to thank the Korea Science and Engineering Foundation through the
Engineering Research Center for Net Shape and Die Manufacturing at Pusan National
University for a grant, under which this work was possible
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