Browsing by Author "Yan, YY"

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Condensation heat transfer and pressure drop of refrigerant R-134a in a plate heat exchanger
(1999-03-01)
Yan, YY
;
Lio, HC
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experimental refrigerant loop has been established in the present study to measure the condensation heat transfer coefficient h(r) and frictional pressure drop Delta P-f of R-134a in a vertical plate heat exchanger. Two vertical counter flow channels were formed in the exchanger by three plates of commercialized geometry with a corrugated sinusoidal shape of a chevron angle of 60 degrees. Downflow of the condensing R-134a in one channel releases heat to the cold upflow of water in the other channel. The effects of the refrigerant mass flux, average imposed heat flux, system pressure (saturated temperature) and vapor quality of R-134a on the measured data were explored in detail. The results indicate that at a higher vapor quality the condensation heat transfer coefficient and pressure drop are significantly higher. A rise in the refrigerant mass flux only causes a mild increase in the it, values h(r) most cases. The corresponding rise in the Delta P-f value is slightly larger. Furthermore, it is noted that the condensation heat transfer is only slightly better for a higher average imposed heat flux. But the associated rise in Delta P-f is larger. Finally? at a higher system pressure the h(r) value is found to be slightly lower. But the effect of the system pressure on Delta P-f is small. Correlations are also provided for the measured heat transfer coefficients and pressure drops in terms of the Nusselt number and friction factor, (C) 1998 Elsevier Science Ltd. All rights reserved.
Condensation heat transfer and pressure drop of refrigerant R-134a in a small pipe
(1999-02-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
The characteristics of condensation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe that has an inside diameter of 2.0 mm were investigated experimentally in this study. The effects of the heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured condensation heat transfer and pressure drop were examined in detail. When compared with the data for a large pipe (D-i = 8.00 mm) reported in the literature, the condensation heat transfer coefficient averaging over the entire quality range tested here for the small pipe is about 10% higher. Moreover, we noted that in the small pipe the condensation heat transfer coefficient is higher at a lower heal flux, at a lower saturation temperature and at a higher mass flux. In addition, the measured pressure drop is higher for raising the mass flux but lower for raising the heat flux. Based on the present data, empirical correlation equations were proposed for the condensation heat transfer coefficient and friction factors. The results are useful in designing more compact and effective condensers for various refrigeration and air conditioning systems using refrigerant R-134a. (C) 1998 Elsevier Science Ltd. All rights reserved.
Enhancement of pool boiling heat transfer in a horizontal water layer through surface roughness and screen coverage
(1996-11-01)
Tsay, JY
;
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experimental investigation was carried out to study the augmentation of heat transfer in saturated pool boiling of a liquid water layer on a heated horizontal stainless steel plate by roughing the surface and/or covering it with a single layer of stainless steel screen. The results were presented in terms of the boiling curves. Effects of various parameters - the surface roughness, liquid level and size of the stainless steel mesh on the boiling heat transfer were examined in detail. The measured data clearly indicated that a lowering of the liquid level from 60 to 5 mm in water depth causes heat transfer reduction. Roughing the surface was found to significantly enhance the heat transfer. Use a layer of metal screen to cover the heated surface was shown to substantially augment the heat transfer especially for a shallow water layer if the mesh size is comparable with the bubble departure diameter. Covering the rough surface with the metal mesh, however, reduced the heat transfer.
Enhancement of pool boiling heat transfer in a horizontal water layer through surface roughness and screen coverage
(1996-11-01)
Tsay, JY
;
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experimental investigation was carried out to study the augmentation of heat transfer in saturated pool boiling of a liquid water layer on a heated horizontal stainless steel plate by roughing the surface and/or covering it with a single layer of stainless steel screen. The results were presented in terms of the boiling curves. Effects of various parameters - the surface roughness, liquid level and size of the stainless steel mesh on the boiling heat transfer were examined in detail. The measured data clearly indicated that a lowering of the liquid level from 60 to 5 mm in water depth causes heat transfer reduction. Roughing the surface was found to significantly enhance the heat transfer. Use a layer of metal screen to cover the heated surface was shown to substantially augment the heat transfer especially for a shallow water layer if the mesh size is comparable with the bubble departure diameter. Covering the rough surface with the metal mesh, however, reduced the heat transfer.
Evaporation heat transfer and pressure drop of refrigerant 134A in a small pipe - Reply
(2003-03-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Evaporation heat transfer and pressure drop of refrigerant R-134a in a plate heat exchanger
(1999-02-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
The evaporation heat transfer coefficient and pressure drop for refrigerant R-134a flowing in a plate heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were formed in the exchanger by three plates of commercial geometry with a corrugated sine shape of a chevron angle of 60 deg. Upflow boiling of refrigerant R-134a in one channel receives heat from the hot down flow of water in the other channel. The effects of the mean vapor quality mass flux, heat flux, and pressure of R-134a on the el,evaporation heat transfer and pressure drop were explored. The quality change of R-134a between the inlet and outlet of the refrigerant channel ranges from 0.09 to 0.18. Even at a very low Reynolds number, the present flow visualization of evaporation in a plate heat exchanger with the transparent outer plate showed that the flow in the plate heat exchanger remains turbulent It is found that the evaporation heat transfer coefficient of R-134a in the plates is much higher than that in circular pipes and shows a very different variation with the vapor quality from that bl circular pipes, particularly in the convective evaporation dominated regime at high vapor quality. Relatively intense evaporation on the corrugated surface was seen from the flow visualization. Moreover, the present data showed that both the evaporation hear transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux the pressure drop is higher for the entire range of the vapor quality but the evaporation heat transfer is clearly better only at the high quality. Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower. Based an the present data, empirical correlations for the evaporation heat transfer coefficient and friction factor were proposed.
Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe
(1998-12-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (D-i greater than or equal to 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30-80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the-high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors. (C) 1998 Elsevier Science Ltd. All rights reserved.
Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe
(1998-12-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (D-i greater than or equal to 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30-80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the-high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors. (C) 1998 Elsevier Science Ltd. All rights reserved.
Subcooled flow boiling heat transfer of R-134a and bubble characteristics in a horizontal annular duct
(2000-06-01)
Yin, CP
;
Yan, YY
;
Lin, TF
;
Yang, BC
;
機械工程學系
Experiments were carried out to investigate the subcooled flow boiling heat transfer and visualize the associated bubble characteristics for refrigerant R-134a flowing in a horizontal annular duct having inside diameter of 6.35 mm and outside diameter of 16.66 mm. The effects of the imposed wall heat flux, mass flux, liquid subcooling and saturation temperature of R-134a on the resulting nucleate boiling heat transfer and bubble characteristics were examined in detail. In the experiment significant hysteresis was noted in the boiling curves during the onset of nucleate boiling (ONB) especially at low saturation temperature and high subcooling. The temperature undershoot at ONE is rather large for most cases. The boiling heat transfer was slightly higher for a lower saturation temperature and was little affected by the mass flux. However, for a higher subcooling of the refrigerant better heat transfer results. Furthermore, the flow visualization revealed that at higher imposed wall heat flux the heated surface was covered with more bubbles and the bubble generation frequency is higher. But the size of the bubbles departing from the heated surface was only slightly affected by the imposed heat flux. At high mass flux and subcooling the bubble generation was suppressed to a noticeable degree. Besides, the bubbles are much smaller at a higher subcooling. Finally, empirical correlations for the heat transfer coefficient and bubble departure diameter in the subcooled flow boiling of R-134a were proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.