Browsing by Author "Cheng, TC"

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Characteristics of vortex flow in a low speed air jet impinging onto a heated disk in a vertical cylindrical chamber
(2003-11-01)
Hsieh, JC
;
Cheng, TC
;
Lin, TF
;
機械工程學系
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Department of Mechanical Engineering
An experiment combining flow visualization and transient temperature measurement is carried out to investigate the characteristics of the mixed convective vortex flow resulting from a low speed air jet impinging onto a heated horizontal circular disk confined in a vertical adiabatic cylindrical chamber. Attention is focused on the conditions leading to the onset of the inertia and buoyancy driven vortex rolls and the effects of governing nondimensional groups on the steady and time dependent vortex flow. More specifically, experiments are conducted for the jet Reynolds number varied from 0 to 1082 and Rayleigh number from 0 to 18,790 for two different injection pipes. The results show that typically the steady vortex flow in the processing chamber consists of two inertia-driven and one buoyancy-driven circular vortex rolls. The secondary inertia-driven roll only appears at high jet Reynolds numbers. At low buoyancy-to-inertia ratio Gr/Re-j(2) the vortex rolls are steady and axisymmetric. But at certain high Gr/Re-j(2) the vortex flow becomes unstable and the vortex rolls are somewhat deformed. Besides, new vortex rolls can be induced by the additional thermal rising from the heated disk and the splitting of the primary inertia-driven roll. The temporal characteristics of the time periodic vortex flows are examined in detail. In the region dominated by the new rolls the flow oscillates significantly. Finally, empirical equations are proposed to correlate the oscillation frequency of the time periodic flow, and the size and location of the vortex rolls. Furthermore, the conditions for the onset of the buoyancy driven rolls are given. A flow regime map is provided to delineate the temporal state of the vortex flow. (C) 2003 Elsevier Ltd. All rights reserved.
Effects of thermal boundary condition on buoyancy driven transitional air flow in a vertical cylinder heated from below
(2000-06-01)
Cheng, TC
;
Li, YH
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Lin, TF
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機械工程學系
;
Department of Mechanical Engineering
A three-dimensional unsteady numerical computation was carried out here to investigate the effects of the thermal boundary condition on the convection flow in a vertical, bottom heated cylinder containing air. The thermal condition at the sidewall of the cylinder is assumed to be thermally well insulated or perfectly conducting. Results were obtained for air in a cylinder of finite aspect ratio (Gamma = 2) for various thermal Rayleigh numbers. The predicted results indicate that the flow in the sidewall insulated cylinder is highly asymmetric even at steady state and contains multicellular vortices. The flow formation processes leading to the above structures are relatively complicated. In the early transient two axisymmetric circular vortex rolls, one on top of another, appear. Then the rods merge asymmetrically. In the late stage the flow deflection by the cylinder top and bottom results in a very complex flow. In the cylinder with a perfectly conducting sidewall the transition from a steady to a time dependent flow is subcritical. However, in the cylinder with an insulated sidewall the flow transition is supercritical.
Stationary transverse rolls and U-rolls in limiting low Reynolds number mixed convective air flow near the convective threshold in a horizontal flat duct
(2002-03-01)
Cheng, TC
;
Lir, JT
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Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Combined experimental flow visualization and temperature measurement are carried out in the present study to explore the buoyancy driven vortex flow patterns in a limiting low Reynolds number mixed convective air flow through a bottom heated horizontal flat duct. In Particular, attention is paid to the flow approaching the natural convection limit (Re = 0) for Re = 1.0 and 2.0 with the Rayleigh number near the critical level for the onset of convection for 1200 less than or equal to Ra less than or equal to 4000. Results front the flow visualization have revealed two unfamiliar vortex flow patterns which were not seen in our earlier study [Int. J. Heat Mass Transfer 44 (4) (2001) 705]. One is characterized by the stable stationary transverse rolls in the duct entry and stable longitudinal rolls in the downstream. Another is in the form of U-rolls. The relations of these two patterns with those reported in the literature from analytical, numerical and experimental studies are discussed. Moreover, stable longitudinal rolls along with nonperiodic traversing waves, and mixed longitudinal and transverse rolls as well as irregular cells which appear in the higher Reynolds number for 3.0 less than or equal to Re less than or equal to 5.0 are also noted here. The temporal and spatial characteristics of the unfamiliar vortex flows are inspected in detail. In addition, the flow formation processes leading to tire two unfamiliar vortex flow structures are also examined carefully. During the flow formation we noted merging of longitudinal and transverse rolls to form U-rolls, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the generation of the longitudinal and transverse rolls. Finally, a flow regime map is provided to delineate various vortex flow structures observed in this study and in the previous study (cf. the above-mentioned reference) driven by the slightly supercritical and subcritical buoyancies for 1.0 less than or equal to Re less than or equal to 5.0. (C) 2002 Elsevier Science Ltd. All rights reserved.
Visualization of mixed convective vortex rolls in an impinging jet flow of air through a cylindrical chamber
(2002-07-01)
Cheng, TC
;
Chiou, PH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
In this study a combined buoyancy and inertia driven vortex flow in an air jet impinging onto a heated circular plate confined in a cylindrical chamber simulating that in a vertical single-wafer rapid thermal processor for semiconductor manufacturing is investigated experimentally by flow visualization. A copper plate is used here to simulate the wafer for its better uniformity of the surface temperature and air is used to replace the inert gases. We concentrate on how the inlet gas flow rate, temperature difference between the wafer and air jet, and chamber pressure affect the, ortex flow. The results show that typically the flow in the chamber is in the form of two-roll structure characterized by a circular vortex roll around the air jet along with another circular roll near the side wall of the chamber. Both rolls are somewhat deformed. The rolls are generated by the reflection of the jet from the wafer and by the deflection of the wall boundary layer flow along the wafer surface by the upward buoyancy due to the heated wafer. At low buoyancy and inertia the vortex rolls are steady and axisymmetric. At increasing buoyancy associated with the higher temperature difference and chamber pressure, the inner roll becomes slightly smaller and the outer roll becomes correspondingly bigger. Moreover, at a higher gas flow rate the inner roll is substantially bigger. Based on the present data, a correlation equation is provided to predict the location where the two rolls contact each other, providing the approximate size of the rolls, Moreover, at high buoyancy and inertia the flow becomes time dependent and does not evolve to a steady state. (C) 2002 Elsevier Science Ltd. All rights reserved.