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標題: 水平管道中二相粒子懸浮流場之探討
Investigation of two phase particle-suspended flow in a horizontal channel
作者: 張景貿
Chang, Ching-Mao
關鍵字: two-phase flow;二相流;stokes number;bubble stokes number;vorticity shear lift force;懸浮層流流場;史托克數;氣泡史托克數;渦漩浮力
出版社: 機械工程學系
本研究主要目的,為探討各種粒子與載運流體間密度比對粒子運動特性之影響。為能更深入瞭解各種大小及密度之粒子存在於氣體或液體流場中,粒子所受力量之變化情形,本研究擬集中探討二相粒子懸浮層流流場,並針對兩個極端情形,即gas-solid particle(高密度比)及liquid-bubble flow(密度比接近零)粒子所受之力量,進行詳細之分析,並藉此瞭解粒子運動之機制。
在本研究中,在載運相流體之計算是利用流函數---渦度模式(Sreamfunction-vorticity Model)求得。在粒子運動部分,以Eulerian-Lagrangian法來作分析,採用一階Euler方法,對粒子之運動方程式進行積分。
由研究中,在gas-solid particle flow方面,可驗證得其高密度粒子在流場中的運動,可由史托克數來表示之,在同一史托克數之下其運動軌跡均相似,同一入口速度之下,不同粒徑、密度比及重力影響皆不明顯。當雷諾數不同亦即福得數不同,也會影響軌跡,雷諾數增大,粒子受入口速度增大,福得數增大,受重力影響變小,故再著點長度會增加。
在liquid-bubble flow方面,密度接近零之氣泡,考慮其與重力之影響為零,運動軌跡與雷諾數、氣泡史托克數相關。低雷諾數、低氣泡史托克數時,其運動軌跡會有震盪之情形,為尋找其震盪之原因,本研究擬針對速度、力方向研究,在比對有無渦漩浮力時,發掘震盪原因應為渦漩浮力。為驗證此一理論,將流場複雜化,亦可得相同之結論。

The major goal of the present study is to investigate the effect of density ratio on the motion of particles that suspended in a flow. Two extreme cases of particle suspended flows, the gas-solid particle flow and the liquid-bubble flow are investigated numerically in the present study.
The two-phase system is assumed to be sufficient dilute and therefore the carrier and particle phase can be solved independently. In the carrier phase, a numerical model based on streamfunction-vorticity formulation is employed for obtaining the velocity and vorticity field. The particle motion is obtained by integrating its equation of motion. The two-phase flow system is restricted to be laminar and inside a horizontal channel with a block mounted on the lower surface of the channel.
In the gas-solid particle flow, it is found that the traditionally defined Stokes number is the major control parameter of solid particle motion in the flow. Density ratio plays insignificant role in the particle motion. For reducing Reynolds number of the flow field, it is found that gravity effect becomes important resulting early particle deposition on the lower wall of the channel.
In the liquid-bubble channel flow, it is found that the major control parameters of bubble motion are the bubble Stokes number and Reynolds number Re of flow field. For low Re, it is found that bubble exhibits oscillatory trajectories for small values of . The bubble follows the streamlines of the flow field as increasing. For large Reynolds number, the oscillatory trajectories of bubble are still can be found, however, with longer period due to the effect of the flow velocity. It is found oscillating bubble trajectories are caused by the vorticity lift force that acting on bubble.
The bubble motion in a channel with surface-mounted block is also examined. The oscillatory trajectories can also be found for small values. While increases, the bubble trajectories become complicated due to the reasons of existence of recirculation zones and complicated vorticity variation in the flow field. The bubble may be entrapped into the recirculation zone for certain values of .
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