Browsing by Author "Hu, Yu-Chang"

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Analysis of the flow agitated by disc impellers with pitched blades
(2008)
Tsui, Yeng-Yung
;
Lin, Shih-Chao
;
Shen, Shih-Jhen
;
Hu, Yu-Chang
;
機械工程學系
In this study we are concerned with the flow in a tank stirred by disc impellers with pitched blades. A mathematical method, based on multiframe of reference and unstructured grid technology, was employed in the flow simulation. It was learned from the study by Tsui et al. [12] that for a pitched-blade impeller, which does not include the disc, the flow pattern in the agitated tank is transformed from the axial type to the radial type at a particular angle when the blade angle is increased. A similar phenomenon was observed in the present study. However, in contrast to the results of Tsui et al., the change of flow pattern brings about a sharp decrease in power requirement and a jump in pumping capacity. The cause of the difference is related to the disc which obstructs the fluid when it flows in the axial direction at low blade angles and makes it flow smoothly in the radial direction along the disc at high blade angles. It was shown in the results that the transition angle is increased when the impeller size, the disc size as well as the off-bottom clearance are reduced.
Blade angle effects on the flow in a tank agitated by the pitched-blade turbine
(2006-07-01)
Tsui, Yeng-Yung
;
Chou, Jian-Ren
;
Hu, Yu-Chang
;
機械工程學系
;
Department of Mechanical Engineering
This paper presents a study of the influence of the blade angle on the flow in a tank stirred by the pitched-blade turbine. The flow induced by the pitched-blade turbine is usually described as an axial type with a principal ring vortex dominating the flow structure. However it is known that as the blade becomes vertical, i.e., as the pitch angle of the blade becomes 90 deg, the flow is of the radial type, with two main ring vortices occupying the tank. Thus, a transition of flow type must take place when the blade angle is varied. This motivates the current study. A computational method was developed, which incorporates the unstructured grid technique to deal with the complex geometry in the tank. Multiframe of reference was employed to handle the rotation of the impeller. The results show that the transition from the axial type to the redial type is not progressive, but occurs all of a sudden at a particular angle, depending on the configuration. This critical angle decreases as the off-bottom clearance and the impeller size are increased. Its influences on the flow angle of the discharge stream, the power requirement, the induced flow rate through the impeller, and the pumping efficiency are discussed. The mechanism to cause the sudden change of flow type is addressed through observing the flow on the surface of the turbine blade.
FLOW CHARACTERISTICS IN MIXERS AGITATED BY HELICAL RIBBON BLADE IMPELLER
(2011-09-01)
Tsui, Yeng-Yung
;
Hu, Yu-Chang
;
機械工程學系
;
Department of Mechanical Engineering
The main concern of this study is to investigate the flow mixing generated by helical ribbon blade impellers and to show that with the help of CFD the performance of the mixing system can be significantly improved by optimizing the geometric configuration of the impeller. To fulfill this objective, a numerical model is developed to solve the Navier-Stokes equations for the flow field. However, difficulties arise due to the rotation of the impeller in the vessel. In order to ease the problem, the velocity field is assumed to be in a quasi-steady state and the multiframe of reference is adopted to tackle the rotation of the impeller. For discretization the fully conservative finite volume method, together with unstructured grid technology, is incorporated. It is shown that the flow in the mixer can be regarded as a flow in an open channel with a wall moving at an angle with respect to the channel. The influences of the blade pitch, the blade width, and the clearance gap between the blade and the surrounding wall are examined. The mechanism to cause these effects is delineated in detail. It is demonstrated that after optimization of the blade geometry, the circulating flow rate induced by the impeller is largely increased, leading to significant reduction in mixing time. In addition, the power demand is reduced. It is also evidenced that by enlarging the clearance, it is difficult for the fluid in this region to be mixed.
Mixing flow characteristics in a vessel agitated by the screw impeller with a draught tube
(2008-04-01)
Tsui, Yeng-Yung
;
Hu, Yu-Chang
;
機械工程學系
;
Department of Mechanical Engineering
The circulating flow in a vessel induced by rotating impellers has drawn a lot of interests in industries for mixing different fluids. It used to rely on experiments to correlate the performance with system parameters because of the theoretical difficulty to analyze such a complex flow. The recent development of computational methods makes it possible to obtain the entire flow field via solving the Navier-Stokes equations. In this study, a computational procedure, based on multiple frames of reference and unstructured grid methodology, was used to investigate the flow in a vessel stirred by a screw impeller rotating in a draught tube. The performance of the mixer was characterized by circulation number, power number, and nondimensionalized mixing energy. The effects on these dimensionless parameters were examined by varying the settings of tank diameter, shaft diameter, screw pitch, and the clearance between the impeller and the draught tube. Also investigated was the flow system without the draught tube. The flow mechanisms to cause these effects were delineated in detail.