Browsing by Author "Tuh, JL"

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Delayed onset of return flow by substrate inclination in model horizontal longitudinal MOCVD processes
(2005-01-15)
Kuo, WS
;
Wang, CY
;
Tuh, JL
;
Lin, TF
;
機械工程學系
In this study an experimental flow visualization is carried out to investigate how the substrate inclination affects the buoyancy-induced return flow structure in mixed convection of gas in a horizontal rectangular duct. The return flow is driven by a heated circular disk embedded in the bottom plate of the duct. simulating that in a horizontal longitudinal MOCVD reactor. Specifically, the bottom plate of the duct is inclined so that the gas flow in the duct is accelerated, causing the buoyancy-to-inertia ratio to decrease in the main flow direction. In the experiment. the Reynolds and Rayleigh numbers of the flow at the duct inlet are respectively varied from 3.7 to 79.7 and from 90,40 to 24,000 for the inclined angle of the bottom plate fixed at 0degrees, 0.34degrees and 0.97degrees. Particular attention is paid to delineating the spatial changes of the return flow structure with the plate inclination angle and to how the bottom plate tilting possibly, suppresses and stabilizes the flow. The results show a substantial delay in the onset of the return flow and the effctive suppression of the buoyancy-driven unstable vortex flow by the bottom plate inclination. Besides, the bottom, plate inclination can effectively weaken the return flow at slightly higher Reynolds numbers. An empirical equation is provided to correlate the present data for the onset of the return flow in the duct with its bottom inclined at 0: and 0.97degrees. (C) 2004 Elsevier B.V. All rights reserved.
Structure of mixed convective longitudinal vortex air flow driven by a heated circular plate embedded in the bottom of a horizontal flat duct
(2003-04-01)
Tuh, JL
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
In this study experimental flow visualization combined with transient temperature measurement are conducted to investigate the structure of the buoyancy driven longitudinal vortex rolls in low Reynolds number mixed convective air flow through a horizontal flat duct with an isothermally heated circular disk embedded in the bottom plate of the duct for the Reynolds number ranging from 15.1 to 99.2 and Rayleigh number from 3506 to 29,493. How the circular geometry of the heated surface affects the longitudinal vortex flow characteristics is investigated in detail. The results indicate that the longitudinal vortex rolls (L-rolls) in the duct core are induced at more upstream locations than those near the duct sides, which is completely opposite to those induced in a duct with a uniformly heated bottom. Besides, the thermals driven by the circular heated surface are not evenly spaced in the spanwise direction and are slightly asymmetric. It is of interest to note that at a given Rayleigh number Ra the thermals are unstable at high Reynolds numbers, suggesting the existence of the inertia driven instability. Thus the L-rolls evolved from these thermals are also unstable with the presence of nonperiodic generation and disappearance of new L-rolls. But at slightly lower Re the thermals and L-rolls are steady and regular. The vortex flow becomes unstable and irregular for a further reduction in the Reynolds number, which obviously results from the buoyancy driven instability. The simultaneous presence of these two instability mechanisms explains the appearance of the reverse steady-unsteady transition in the vortex flow. Based on the present data, a flow regime map is given to delineate various L-roll patterns driven by the circular heated plate. In addition, the boundaries separating these patterns are empirically correlated. Empirical correlations for the onset points of the L-rolls are also provided. (C) 2002 Elsevier Science Ltd. All rights reserved.
Visualization of return flow structure in mixed convection of gas over a heated circular plate in a horizontal flat duct
(2003-09-01)
Tuh, JL
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Experimental flow visualization is conducted here to investigate the buoyancy-induced return flow structure in mixed convection of gas over a heated circular plate mounted on the bottom of a horizontal flat duct, simulating that in a horizontal longitudinal flow reactor. In particular, how the return flow induced in the upstream portion of the duct is related to the vortex flow in the downstream is delineated. The results show that at a low buoyancy-to-inertia ratio only steady longitudinal vortex rolls (L-rolls) are induced in the exit half of the duct and no return flow appears in the duct. At an intermediate buoyancy-to-inertia ratio a small flow recirculation exists around the duct inlet near the top wall and the L-rolls become unsteady. When the buoyancy-to-inertia ratio is high and the Reynolds number is small with Re<10, the upstream return flow is strong and occupies a large region. There is another return flow zone around the exit end of the duct. Over the heated circular plate the flow is dominated by the moving transverse rolls (T-rolls). Moreover, the return flow around the duct inlet at steady or statistically stable state is in the form of a semicircular roll around the upstream edge of the circular plate. Flow visualization also reveals that the splitting of the downstream tip of the return flow zone and the rising of the buoyancy-driven spanwisely extended thermal under the tip generate a pair of counter-rotating transverse rolls in the entry portion of the duct. In addition, the criterion for the onset of the return flow near the duct inlet, the size and the center position of the return flow, based on the present experimental data, are correlated empirically. (C) 2003 Elsevier B.V. All rights reserved.