• 상하수도학회지
• /
• 제19권5호
• /
• pp.565-571
• /
• 2005

The method to decide media sphericity on the filter has been investigated. The sphericity, the ratio of the surface area of an equal volume sphere to the real surface area of the particles, is one of major physical characters of media affecting the bed expansion during backwash. The media in each treatment plant may have different sphericity, and the sphericity of the media in the filter may be changed as backwashing has been conducted regularly for a long time. Media from twelve water treatment plants under KOWACO have been collected and selected to insure various and practical sphericities. The sphericity of each media has been calculated by using well known equations. For example, Kozeny equation, Dahmarajah equation and so on. The experiment results have indicated that the sphericity of each water treatment plant is different. Although the sphericity values measured by different methods were turned out to be diverse values, the order in the magnitude seemed to be the same. The sphericity values of sand media were in the range of 0.71-0.82 and those of anthracite were placed between 0.49 and 0.56 by the Dharmarajah equation.

• 상하수도학회지
• /
• 제21권6호
• /
• pp.763-770
• /
• 2007

There are many design parameters affecting filtration efficiency such as filteration rate, media packing depth, size distribution, and so on. The sphericity, the ratio of the surface area of an equal volume sphere to the real surface area of the particles, is one of major physical characters of media. The effect of sphericity on the performance of anthracite filter has been investigated. Media from eight water treatment plants have been collected. The sphericity of each media has been calculated by using well known headloss equations such as Kozeny equation, Dahmarajah equation etc.. Columns packed with anthracite media having different sphericity have been used to compare headloss development, floc accumulation in the bed, particles in bed water, filtrate turbidities after backwash and so on. The repeated experiments have indicated that the sphericity of anthracite media may not have remarkable influence on the filter performance as it has been suspected. It also has been prospected in the experiment that the media of higher sphericity would store more particles in the bed and give better filtrate quality, if provided that the effective size and the size distribution of media would be the same.

• Communications for Statistical Applications and Methods
• /
• 제12권2호
• /
• pp.473-481
• /
• 2005

The testing problem for sphericity structure of the covariance matrix in a multivariate normal distribution is introduced when there is a sample with 2-step monotone missing data pattern. The maximum likelihood method is described to estimate the parameters on the basis of the sample. Using these estimates, the likelihood ratio criterion for testing sphericity is derived.

• 한국정밀공학회지
• /
• 제17권9호
• /
• pp.145-150
• /
• 2000

Out-of-sphericity is degree of deformation of an air bearing sphere deviated from a perfect sphere. This paper investigates numerically the effect of out-of-sphericity error on the radial stiffness of an air bearing Three types of out-of-sphericity modes are considered. in this study the stiffness is calculated from pressure distribution at the bearing surface which is obtained by solving th Reynolds equation. in some cases large out-of-sphericity errors are found to improve the stiffnesses of air bearings. This implies that an air bearing of perfect hemispheres is not necessarily of the best performance. Thus much labor and cost in manufacturing air bearings can be saved, In addition the radial stiffness of an air bearing depends greatly on the application direction.

• 반도체디스플레이기술학회지
• /
• 제21권2호
• /
• pp.1-6
• /
• 2022

This computational investigation of micro-sized particle dispersion concerns the fume particle contamination over target surface in high-precision laser line machining process of semiconductor and display device materials. Employing the random sampling based on probabilistic fume particle generation distributions, the effects of sphericity for nonspherical fume particles are analyzed for the fume particle dispersion and contamination near the laser machining line. The drag coefficient correlation for nonspherical particles in a low Reynolds number regime is selected and utilized for particle trajectory simulations after drag model validation. When compared to the corresponding results by the assumption of spherical fume particles, the sphericity of nonspherical fume particles show much less dispersion and contamination characteristics and it also significantly affects the particle removal rate in a suction air flow patterns.

• Food Science and Biotechnology
• /
• 제16권5호
• /
• pp.715-721
• /
• 2007

In this study, the sphericity of calcium alginate gel beads was optimized using response surface methodology. The optimum conditions for bead sphericity were a concentration of 2.24% sodium alginate, a flow rate of 0.059 mL/sec for the sodium alginate solution, and a 459 rpm rotation for the calcium chloride solution. The predicted and experimental bead sphericities under the optimum conditions were 94.5 and 96.7%, respectively, showing close agreement. We also investigated the processing condition effects for the physical properties of the optimized calcium alginate gel beads. Immersion in hot water slightly decreased bead size and rupture strength. NaCl treatment increased bead size and decreased rupture strength. While the pH of the calcium chloride solution had little effect on bead sphericity, the bead sizes and gel strengths decreased with longer times in each pH solution. The beads coated with pectin and glucomannan showed no significant changes in sphericity, but their sizes decreased with time. The coated beads showed higher rupture strengths than the uncoated beads.

• Journal of the Korean Statistical Society
• /
• 제13권2호
• /
• pp.73-80
• /
• 1984

In this paper, likelihood-ratio test has been derived for testing multisample sphericity in complex multivariate Gaussian populations. The $h^{th}$ moment of the test statistic is given and its exact distribution has been derived using inverse Mellin transform. Asymptotic distribution of the statistic is also given.

• Korean Chemical Engineering Research
• /
• 제54권6호
• /
• pp.786-791
• /
• 2016

친환경 환원제인 글루코스를 사용하여 액상 반응으로 그래핀 산화물을 환원시킨 후 에어로졸 분무건조 공정을 통하여 구형의 그래핀 볼(Graphene Ball, GB)을 제조하였다. 제조된 입자는 FE-SEM 분석을 통하여 구형임을 관찰하였고, XRD 분석으로 구형 입자들의 결정형이 그래핀임을 확인하였다. GB의 구형도는 온도, 글루코스의 양, 암모니아수($NH_4OH$)의 주입에 따라 조절할 수 있었다. 제조된 GB 중, 암모니아수 주입 하에 온도가 높은 조건에서 제조된 GB가 높은 구형도를 보였다. 암모니아수 주입하에 진행된 액상반응에 주입된 글루코스의 양이 증가할수록 구형도가 높은 GB가 생성되었다. 가장 높은 구형도를 가지는 GB의 구형도는 1.1이었다. 상대적으로 높은 구형도를 가지는 GB가 낮은 구형도를 가지는 GB보다 응집성이 감소됨을 확인할 수 있었다. 추가적으로 GB의 전기화학 분석 결과를 통해 GB가 커패시터의 전극 재료로서의 가능성을 보여주었다.

• 한국주조공학회지
• /
• 제16권6호
• /
• pp.565-575
• /
• 1996

In this study, to gain the semi-solid alloy we employed the electromagnetic rotation by a induction motor of 3-phases and 2-poles for Al-7wt%Si alloy and observed the size of primary solid particle, distribution state of primary solid particle, the degree of sphericity, and fraction of primary solid for the evaluation of its results. The size of primary solid particle increases from $98{\mu}m$ to $118{\mu}m$ as solid fraction increases from 0.2 to 0.5. The degree of sphericity increased as the solid fraction increased. Solid particles obtained from the microstructures of isothermally held sample were coarsened and the degree of sphericity was enhanced as isothermal holding time increased. However, when the sample was stirred for more than 40min, solid particles merged together and liquid phase was entrapped within the cluster of solid particles. The size of primary solid particle was not changed significantly with the variation of input voltages by 160V over which solid particles began to merge together to be a large cluster of about $170{\mu}m$ at 180V. The standard deviation and the degree of sphericity were not changed significantly with the variation of input voltage.

• 한국분말재료학회지
• /
• 제29권2호
• /
• pp.99-109
• /
• 2022

Powder flowability is critical in additive manufacturing processes, especially for laser powder bed fusion. Many powder features, such as powder size distribution, particle shape, surface roughness, and chemical composition, simultaneously affect the flow properties of a powder; however, the individual effect of each factor on powder flowability has not been comprehensively evaluated. In this study, the impact of particle shape (sphericity) on the rheological properties of Ti-6Al-4V powder is quantified using an FT4 powder rheometer. Dynamic image analysis is conducted on plasma-atomized (PA) and gas-atomized (GA) powders to evaluate their particle sphericity. PA and GA powders exhibit negligible differences in compressibility and permeability tests, but GA powder shows more cohesive behavior, especially in a dynamic state, because lower particle sphericity facilitates interaction between particles during the powder flow. These results provide guidelines for the manufacturing of advanced metal powders with excellent powder flowability for laser powder bed fusion.