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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/1818


    Title: 附著性顆粒體在振動床內之動態行為探討;Dynamic Behavior of Cohesive Granular Materials in a Vibrated Bed
    Authors: 楊士震;Shie-Chen Yang
    Contributors: 機械工程研究所
    Keywords: 顆粒體;振動床;分離元素法;自我擴散;附著性;granular;vibrated bed;DEM;self-diffusion;cohesive
    Date: 2000-12-28
    Issue Date: 2009-09-21 11:32:26 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 本論文運用二維分離元素數值模擬法(DEM)探討顆粒體物質受到垂直振動時的動態行為,探討物質包括無附著性與附著性顆粒體。研究主題包括振動顆粒床體的迴流運動、顆粒床體自我擴散運動及混合問題的探討,數值模擬結果並與相關實驗結果比較。 本文首先以數值模擬的方法,定性探討無附著性顆粒振動床的迴流運動現象,模擬結果與相關文獻的實驗結果比較,頗為符合;進一步以定量的觀點分析振動顆粒床體粒子密度與粒子溫度與床體高度分佈的關係,同時計算迴流中心質量流率的大小,發現迴流中心質量流率與振動速度存在線性冪定律的關係。接著,以實驗及數值模擬的方法討論無附著性顆粒體振動床的自我擴散運動,粒子自我擴散運動乃是經由粒子間持續碰撞運動的變動速度所產生。在實驗方面,應用影像處理技術及粒子追蹤的方法,可準確量測並計算出追蹤粒子的位置與速度大小,藉由追蹤粒子的擴散位移與時間的關係可計算出粒子的自我擴散係數,由實驗及數值模擬結果顯示,自我擴散係數與振動參數如振動加速度、振動振幅與振動頻率的改變有重要的關係。此外,振動床體平均變動速度、粒子溫度的大小與自我擴散係數,由於受到垂直振動振動外力的影響,呈現非等向性分佈,在垂直方向分量大於水平方向分量,相關的研究在本文中均有深入的探討。 在附著性顆粒體方面,本文以顆粒體表面含有微量液體的黏性液橋力來分析探討粒子間黏滯力對振動顆粒床體運動的影響,文中以三種不同性質的黏性液體做為分析的對象,對於黏性液橋力的分析模式,以動態液體潤滑理論配合靜態毛細張力理論,結合DEM數值碰撞模式,模擬計算附著性顆粒振動床的各項動態運動行為。當顆粒床體受垂直振動力的作用時,床體的能量會因為某些因素的影響而消散,包括粒子間的摩擦阻力與非完全彈性碰撞、黏性液橋力的黏滯阻力與毛細結合力等,由數值模擬結果顯示,對於附著性顆粒體而言,能量的消散主要來自液體液橋力的黏滯阻力與粒子間的摩擦阻力與非完全彈性碰撞,毛細結合阻力的影響較不明顯,能量消散的大小並隨著粒子間液體多寡的增加而增加。至於在迴流中心質量流率方面,主要則是受到黏性液橋力的黏滯力、表面張力及粒子間的摩擦力交互作用影響。此外,附著性顆粒體振動床在自我擴散運動及粒子混合方面較非附著性顆粒體更為激烈,振動床體粒子間的混合過程極為複雜,基本上乃與粒子間的自我擴散運動有明顯地關係,並且受到粒子間黏性流體多寡的影響。 This thesis examines the dynamic behaviors of granular materials subjected to external vertical vibration. The cohesionless and cohesive materials are considered in this thesis. Two-dimensional discrete element computer simulation is used to study the flow behaviors, self-diffusion and mixing of vibration granular flows. Experiment was also performed to supplement the simulation results of the current study. The flow behaviors of convection cells of cohesionless materials under vertical vibration are first investigated by simulation. The flow pattern and velocity vectors are consistent with the former experimental results. The profiles of solid fractions and the granular temperatures with the altitude of granular bed are studied with different vibration conditions. A power law relation exists between the convection flow rate and the dimensionless vibration velocity. The influences of flow parameters on self-diffusion in the vibrated granular bed are studied by simulation and experiment. Employing the image processing technology and particle tracking method, the local displacements and velocities of particles are measured. The self-diffusion coefficients are determined from the history of particles’ diffusive displacements. The DEM simulation is performed to calculate the particles’ self-diffusive displacements with the same flow parameters and material properties as the experiment. The simulation results are compared to the experimental tests. The flow behaviors of convection cell are strongly related to the self-diffusion of particles induced by the energy input from the vertical external vibration. The velocity fluctuations, granular temperature and self-diffusions are anisotropic with greatest components in the vertical direction. The dependence of the diffusion coefficients on the dimensionless acceleration, vibration amplitude, vibration frequency, solid fraction, velocity fluctuations, restitution coefficient and granular temperature are discussed carefully. The wet particles with the effect of liquid bridge are used as the cohesive granular materials. Three types of viscous liquids with different tension and viscosity are used in this thesis. A simplified model of dynamic bridge strength based on the superposition of lubrication and circular capillary force approximation is incorporated in the DEM model. The energy dissipations during vertical vibration are generated from the friction and inelasticity between particles, viscous resistance and liquid bridge bond rupture due to the liquid bridge. For cohesive granular materials, the energy dissipation is mainly associated with the viscous force, the interparticle friction and the inelasticity of collision, rather than with the capillary force of liquid bridge. The liquid bridge force due to surface tension and viscosity interacts with frictional force mutually to determine the strength of the convection flow rate. The energy dissipation increases monotonously with the increase of the dimensionless interstitial liquid volume, and the distributions of the energy dissipation are strongly influenced by the properties of viscous liquids. The self-diffusion motions for cohesive materials are faster than those of cohesionless materials. The mixing of vibrated granular flow is strongly dependent on the self-diffusivity of particles and is related to the magnitude of interstitial liquid volume between particles.
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