Browsing by Author "Lir, JT"

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Stabilization and elimination of transient unstable mixed convective vortex flow of air in a bottom heated horizontal flat duct by top plate heating
(2004-09-01)
Chen, SW
;
Chang, CY
;
Lir, JT
;
Lin, TF
;
機械工程學系
An experiment combining flow visualization and temperature measurement is carried out here to study the possible stabilization and elimination of the buoyancy driven unstable longitudinal, transverse and mixed vortex flow in mixed convection of air in a bottom heated horizontal flat duct by the top plate heating. The top plate temperature is varied systematically to examine its effects on the spatial and temporal flow structures in the duct. How the top plate temperature and the Reynolds and Rayleigh numbers of the flow affect the vortex flow characteristics is investigated in detail. Specifically the experiment is conducted for the Reynolds number varying from 1 to 50, Rayleigh number from 4000 to 8000 and the non-dimensional top plate temperature from 0 to 1 at an interval of 1/8, covering a wide range of the buoyancy-to-inertia ratio. The results indicate that the top plate heating substantially stabilizes and for some cases even eliminates the longitudinal, transverse, mixed longitudinal and transverse, and irregular vortex flows induced by the buoyancy associated with the heated bottom plate of the duct. At the high top plate temperature even the entire irregular vortex flow can be eliminated and the flow becomes unidirectional in the duct. Obviously the transient velocity and temperature oscillations in the flow are completely suppressed. (C) 2004 Published by Elsevier Ltd.
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
;
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 roll patterns in Rayleigh-Benard convection of air in a rectangular shallow cavity
(2001-08-01)
Lir, JT
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Flow visualization is conducted here to study the vortex flow patterns associated with the Rayleigh-Benard convection in a horizontal shallow cavity of air. The cavity is a rectangular enclosure characterized by the aspect ratios A(x) = 16 and A(z) = 20. Attention is focused on the convection rolls driven at slightly supercritical and subcritical buoyancies. The results show that at slightly subcritical Ra the induced vortex flow is in the form of rectangular rolls along the cavity sides and short straight parallel rolls in the cavity core. At slightly higher Ra near Ra-c proportional to (= 1708) more rectangular rolls appear and the short straight rolls in the cavity core merge together to form a serpentine roll. At slightly supercritical buoyancy with 2000 less than or equal to Ra less than or equal to 3000 the entire cavity is filled with the straight rolls all parallel to the short sides of the cavity. At an even higher Ra of 4000 the vortex rolls become irregular and time dependent. Moreover, the processes through which various vortex flow structures evolve during the transient stage are shown to be rather complicate and the vortex flow patterns during the how formation are significantly affected by the heating rate in raising the buoyancy force. Furthermore, the wavenumber reduction at higher Ra for the parallel vortex roll pattern was noted to mainly result from the splitting of some rectangular rolls into cells and the subsequent merging of the cells into bigger rolls at the intermediate stage of the flow formation. (C) 2001 Elsevier Science Ltd. All rights reserved.
Vortex flow patterns near critical state for onset of convection in air flow through a bottom heated horizontal flat duct
(2001-02-01)
Lir, JT
;
Chang, MY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
In this paper, an experiment is carried out to investigate the buoyancy driven vortex how at slightly subcritical and supercritical Rayleigh numbers in a mixed convective air flow through a bottom heated horizontal flat duct. Particular attention is paid to the flow at a very low Reynolds number for 3.0 less than or equal to Re less than or equal to 5.0. Results from the flow visualization have revealed two new vortex flow patterns in addition to those often seen in literature such as longitudinal rolls (L rolls), moving transverse rolls (T rolls), and mixed longitudinal:transverse rolls (M rolls). The newly observed vortex flow patterns include the stable longitudinal rolls near the duct sides along with nonperiodic traversing transverse waves in the duct core, and the mixed longitudinal and transverse rolls as well as irregular cells. Moreover, steady longitudinal rolls and nonperiodic traversing transverse waves are noted even at subcritical Rayleigh numbers. A flow regime map delineating various vortex flow patterns is given. Temporal characteristics of the flow are also inspected. Furthermore, a correlation is given to estimate the local onset locations of the longitudinal rolls. Meanwhile, the oscillation frequency and convection speeds of the transverse rolls are correlated from the present data. Finally, many complicate processes during the vortex flow formation are noted, such as the generation of the L and T rolls and transverse waves, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the moving and bending of T rolls. (C) 2001 Elsevier Science Ltd. Ail rights reserved.