Browsing by Author "Leu, SW"

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Heat transfer enhancement by a multilobe vortex generator in internally finned tubes
(1999-04-01)
Tsui, YY
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Leu, SW
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機械工程學系
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Department of Mechanical Engineering
A three-dimensional computational method is employed to study the flow and heat transfer in internally finned tubes with a multilobe vortex generator inserted. Governing equations are discretized using the finite volume method The irregular lobe geometry is treated using curvilinear nonstaggered grids. The linear interpolation method is adopted to calculate face velocities. The results show that secondary flows induced by the lobes are transformed to become axial vortices downstream of the vortex generator. As a consequence of the transport by the vortex flow, the core flow is moved to the fins and the tube wall, while the wall flow moves to the core. In this way, both heat transfer and flow mixing are enhanced. When the fin height is increased the axial vortex is more restricted in the centerline region, and the strength of the vortex flow, represented by circulation, is decreased. In turn, the total pressure loss is also decreased However, the heat transfer increases with fin height. Consequently, efficiency is greatly promoted.
Heat transfer enhancement by multilobe vortex generators: Effects of lore parameters
(2000-05-12)
Tsui, YY
;
Leu, SW
;
Lin, CC
;
Wu, PW
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機械工程學系
A three-dimensional computational method is developed to study the flow and heat transfer in a circular tube with a multilobe vortex generator inserted. Governing equations are discretized by the finite volume technique. The irregular lobe geometry is treated using curvilinear nonstaggered grids. Examination of the flow field reveals that the radial and counterradial flows induced inside and outside the lobe are forced to form axial vortices downstream of the vortex generator Because of the transport of the high-speed, low-temperature core flow toward the tube wad by the vortex, heat transfer is enhanced. Also because of the vortex flow, the high-temperature wall flow is carried away from the wall to mix with the low-temperature core flow. Consequently, very effective augmentation in the heat transfer and mixing of flow temperature results. It is shown also that since the slope of the lobe is increased by enlarging the lobe penetration, reducing the lobe length, and making a concave contour geometry, the circulation, representing the strength of the axial vortex induced by the lobe, is promoted, leading to higher heat transfer and flow mixing By increasing the lobe number both the circulation and Nusselt number (Nu) decrease because of the narrowed lobe passage.