Browsing by Author "Lie, YM"

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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.
Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes
(2006-01-01)
Lie, YM
;
Su, FQ
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Lai, RL
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Lin, TF
;
機械工程學系
An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m(2) s, imposed heat flux q from 5 to 15 kW/m(2), inlet vapor quality x(in) from 0.2 to 0.8 and refrigerant saturation temperature T-sat from 5 to 15 degrees C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m(2) S with the other parameters varied in the same ranges as those for D-i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h, are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D-i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x(in), T-sat, G, q and D-i, refrigerant R-407C has a higher h(r) when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed. (c) 2005 Elsevier Ltd. All rights reserved.
Saturated flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct
(2005-12-01)
Lie, YM
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Experiments are conducted here to investigate how the channel size affects the saturated flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. The measured heat transfer data indicate that the saturated flow boiling heat transfer coefficient increases with a decrease in the gap of the duct. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing R-134a mass flux. Moreover, the bubble departure frequency increases at reducing duct size mainly due to the rising shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Correlation for the present saturated flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated. (c) 2005 Elsevier Ltd. All rights reserved.
Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct
(2006-07-01)
Lie, YM
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Experiments are conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and associated bubble characteristics of refrigerant R-134a in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm in this study. From the measured boiling curves, the temperature undershoot at ONB is found to be relatively significant for the subcooled flow boiling of R-134a in the duct. The R-134a subcooled flow boiling heat transfer coefficient increases with a reduction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a substantial increase in the subcooled boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are small in the narrow duct. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux significantly increases the bubble population, coalescence and departure frequency. The increase in the bubble departure frequency by reducing the duct size is due to the rising wall shear stress of the liquid flow, and at a high imposed heat flux many bubbles generated from the cavities on the heating surface tend to merge together to form big bubbles. Correlation for the present subcooled flow boiling heat transfer data of R-134a in the narrow annular duct is proposed. Additionally, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density are also correlated. (c) 2006 Elsevier Ltd. All rights reserved.