Browsing by Author "Kuo, WS"

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Condensation heat transfer and pressure drop of refrigerant R-410A flow in a vertical plate heat exchanger
(2005-12-01)
Kuo, WS
;
Lie, YM
;
Hsieh, YY
;
Lin, TF
;
機械工程學系
Heat transfer and associated frictional pressure drop in the condensing flow of the ozone friendly refrigerant R-410A in a vertical plate heat exchanger (PHE) are investigated experimentally in the present study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60 degrees. Downflow of the condensing refrigerant R-410A in one channel releases heat to the upflow of cold water in the other channel. The effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data are explored in detail. The results indicate that the R-410A condensation heat transfer coefficient and associated frictional pressure drop in the PHE increase almost linearly with the mean vapor quality, but the system pressure only exhibits rather slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger frictional pressure drop. Besides, the imposed heat flux exhibits stronger effects on the heat transfer coefficient and pressure drop than the refrigerant mass flux especially at low refrigerant vapor quality. The friction factor is found to be strongly influenced by the refrigerant mass flux and vapor quality, but is almost independent of the imposed heat flux and saturated pressure. Finally, an empirical correlation for the R-410A condensation heat transfer coefficient in the PHE is proposed. In addition, results for the friction factor are correlated against the Boiling number and equivalent Reynolds number of the two-phase condensing flow. (c) 2005 Elsevier Ltd. All rights reserved.
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.