Browsing by Author "Chang, W. R."

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Experimental and numerical studies of local current mapping on a PEM fuel cell
(2008-10-01)
Hwnag, J. J.
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Chang, W. R.
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Peng, R. G.
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Chen, P. Y.
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Su, A.
Local current distribution on a PEM fuel cell has been mapped experimentally by using a special-designed single cell fixture. it is composed of a composite cathodic flow-field plate, a membrane electrode assembly (MEA) and a stainless-steel anodic flow-field plate. An array of 16 individual conductive segments was distributed on the composite plate. A self-made MEA is in direct contact with the segmented current collectors. Regional-averaged current through each segment is determined by using the Hall-effect sensor. To ensure the data reliability, a comparison of polarization curves was made between the composite flow-field plate and the conventional flow-field plate. Then, the effects of flow-field patterns, dew points of the cathodic feedings and cathodic stoichiometrics on the local current distribution were examined. The transient variation of the local current distribution on the cathode under supersaturated conditions was further visualized to illustrate the flooding phenomena in different flow patterns. This technique developed by the present work has contributed to knowledge and understanding the local current distributions in a PEM fuel cell that is helpful in designing the fuel-cell components. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Saturated flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip
(2007-09-01)
Lie, Y. M.
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Ke, J. H.
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Chang, W. R.
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Cheng, T. C.
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Lin, T. F.
An experiment is carried out here to investigate flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted in the bottom of a horizontal rectangular channel. Besides, three different micro-structures of the chip surface are examined, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 mu m x 200 mu m x 70 mu m (width x length x height) and 100 mu m x 100 mu m x 70 mu m. The pitch of the fins is equal to the fin width for both surfaces. The effects of the FC-72 mass flux, imposed heat flux, and surface microstructures of the silicon chip on the FC-72 saturated flow boiling characteristics are examined in detail. The experimental data show that an increase in the FC-72 mass flux causes a delay in the boiling incipience. However, the flow boiling heat transfer coefficient is not affected by the coolant mass flux. But adding the micro-pin-fin structures to the chip surfaces can effectively enhance the single-phase convection and flow boiling heat transfer. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for a rise in the FC-72 mass flux. A higher coolant mass flux results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed at a higher imposed heat flux. We also note that adding the micro-pin-fins to the chips decrease the bubble departure diameter and increase the bubble departure frequency. However, the departing bubbles are larger for the pin-finned 100 surface than the pin-finned 200 surface but the bubble departure frequency exhibits an opposite trend. Finally, empirical equations to correlate the present data for the FC-72 single-phase liquid convection and saturated flow boiling heat transfer coefficients and for the bubble characteristics are provided. (c) 2007 Elsevier Ltd. All rights reserved.
Subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip
(2010-11-01)
Chang, W. R.
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Chen, C. A.
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Ke, J. H.
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Lin, T. F.
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機械工程學系
Experiments are conducted here to investigate subcooled flow boiling heat transfer and associated bubble characteristics of FC-72 on a heated micro-pin-finned silicon chip flush-mounted on the bottom of a horizontal rectangular channel. In the experiments the mass flux is varied from 287 to 431 kg/m(2) s, coolant inlet subcooling from 2.3 to 4.3 degrees C, and imposed heat flux from 1 to 10 W/cm(2). Besides, the silicon chips contain three different geometries of micro-structures, namely, the smooth, pin-finned 200 and pin-finned 100 surfaces. The pin-finned 200 and 100 surfaces, respectively, contain micro-pin-fins of size 200 mu m x 200 mu m x 70 mu m (width x length x height) and 100 mu m x 100 mu m x 70 mu m. The measured data show that the subcooled flow boiling heat transfer coefficient is reduced at increasing inlet liquid subcooling but is little affected by the coolant mass flux. Besides, adding the micro-pin-fin structures to the chip surface can effectively raise the single-phase convection and flow boiling heat transfer coefficients. Moreover, the mean bubble departure diameter and active nucleation site density are reduced for rises in the FC-72 mass flux and inlet liquid subcooling. Increasing coolant mass flux or reducing inlet liquid subcooling results in a higher mean bubble departure frequency. Furthermore, larger bubble departure diameter, higher bubble departure frequency, and higher active nucleation site density are observed as the imposed heat flux is increased. Finally, empirical correlations for the present data for the heat transfer and bubble characteristics in the FC-72 subcooled flow boiling are proposed. (C) 2010 Published by Elsevier Ltd.
Subcooled flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct
(2009-06-01)
Chen, C. A.
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Chang, W. R.
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Li, K. W.
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Lie, Y. M.
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Lin, T. F.
An experiment is conducted here to investigate how the channel size affects the subcooled flow boiling heat transfer and the associated bubble characteristics of refrigerant R-407C in a horizontal narrow annular duct with the gap of the duct fixed at 1.0 and 2.0 mm. The measured boiling curves indicate that the temperature overshoot at ONB is relatively significant for the subcooled flow boiling of R-407C in the duct. Besides, the subcooled flow boiling heat transfer coefficient increases with a reduction in the duct gap, but decreases with an increase in the inlet liquid subcooling. Moreover, raising the heat flux imposed on the duct 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 slighter. Visualization of the subcooled flow boiling processes in the duct reveals that the bubbles are suppressed to become smaller and less dense by raising the refrigerant mass flux and inlet subcooling. Raising the imposed heat flux, however, produces positive effects on the bubble population, coalescence and departure frequency. Meanwhile, the present heat transfer data for R-407C are compared with the R-134a data measured in the same duct and with some existing correlations. We also propose empirical correlations for the present data for the R-407C subcooled flow boiling heat transfer and some quantitative bubble characteristics such as the mean bubble departure diameter and frequency and the active nucleation site density. (C) 2009 Elsevier Ltd. All rights reserved.
Time periodic flow boiling heat transfer of R-134a and associated bubble characteristics in a narrow annular duct due to flow rate oscillation
(2010-09-01)
Chen, C. A.
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Chang, W. R.
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Lin, T. F.
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機械工程學系
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Department of Mechanical Engineering
An experiment is conducted here to investigate the effects of the imposed time periodic refrigerant flow rate oscillation in the form of nearly a triangular wave on refrigeriant R-134a flow boiling heat transfer and associated bubble characteristics in a horizontal narrow annular duct with the duct gap fixed at 2.0 mm. The results indicate that when the imposed heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface does not exist in an entire periodic cycle. At somewhat higher heat flux persistent boiling prevails. Besides, the refrigerant flow rate oscillation only slightly affects the time-average boiling curves and heat transfer coefficients. Moreover, the heated wall temperature, bubble departure diameter and frequency, and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed mass flux oscillation. Furthermore, in the persistent boiling the resulting heated wall temperature oscillation is stronger for a longer period and a larger amplitude of the mass flux oscillation. And for a larger amplitude of the mass flux oscillation, stronger temporal oscillations in the bubble characteristics are noted. The effects of the mass flux oscillation on the size of the departing bubble and active nucleation site density dominate over the bubble departure frequency, causing the heated wall temperature to decrease and heat transfer coefficient to increase at reducing mass flux in the flow boiling, opposing to that in the single-phase flow. But they are only mildly affected by the period of the mass flux oscillation. However, a short time lag in the wall temperature oscillation is also noted. Finally, a flow regime map is provided to delineate the boundaries separating different boiling regimes for the R-134a flow boiling in the annular duct. (C) 2010 Published by Elsevier Ltd.