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Influence of electrode configuration on the heat transfer performance of a LED heat source
(2014-10-01)
Chen, Ing Youn
;
Chen, Chien-Jen
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
In this study, the effects of electrode configuration on the cooling of LED are presented. A total of six needle type electrodes, including sharp, 0.08, 0.12, 0.37, 0.49 R and flat tip, are used to generate ionic wind for cooling of the LED. The effects of electrode configuration, vertical separation height, and tilt angle on the cooling performance of a LED are reported in this study. It is found that the thermal resistance is reduced with the supplied voltage and a maximum 50% reduction is achieved before the spark-over voltage. For the same supplied voltage, the thermal resistance with a larger vertical separation is also higher. However, the operational range is also longer. The effect of tilt angle on the cooling performance of LED depends on the supplied voltage. With a supplied voltage being less than 6.5 kV, it is found that the thermal resistance is increased when the tilt angle is reduced with a tip radius of 0.12 R. However, the trend is reversed when the supplied voltage is higher than 6.5 kV where the thermal resistance of a smaller tilt angle is lower than that of a larger tilt angle. For the same mesh ground, the effect of electrode configurations cast little influence on the final thermal resistance. All the electrodes show an approximately 50% reduction of thermal resistance in association with its original thermal resistance. However, the corresponding threshold voltage and operational voltage differs significantly. The threshold voltage is lowest for the sharp needle and is increased with the rise of tip radius. On the other hand, the corresponding operational range becomes narrower when the tip radius is increased. Yet the spark-over voltage is also insensitive to change of electrode configuration. (C) 2014 Elsevier Ltd. All rights reserved.
Two-phase pressure drops and flow pattern observations in 90 degrees bends subject to upward, downward and horizontal arrangements
(2015-11-01)
Hsu, Li-Chieh
;
Chen, Ing Youn
;
Chyu, Chien-Min
;
Wang, Chi-Chuan
;
機械工程學系
In this study, single-phase/two-phase frictional characteristics and flow patterns of 900 bends are experimentally investigated. The two bends have inner diameters (D) of 5.5 and 9.5 mm with tube curvature ratio (2R/D) of 5.4 and 4.2, respectively. The bends are installed either an upward, horizontal or downward arrangement. For results concerned with single-phase flow, the bend friction factor (f(B)) is always greater than that of the straight tubes at the same liquid Reynolds number. On the other hand, the f(B) values for D = 9.5 mm with smaller curvature ratio are greater than those of D = 5.5 mm. The resultant two-phase frictional pressure drops of 90 bend are well predicted by the Sanchez Silva et al. [14]\'s correlation. For the two-phase frictional pressure gradient, the upward arrangement is greater than that of the horizontal arrangement due to swirled motion and liquid flow reversal. The flow reversals are observed for upward flow arrangement at specific flow pattern and pressure gradient. However, no swirled motion and liquid flow reversal is seen in the downward arrangement. As mass flux and quality increase, the differences of pressure drop among the three arrangements are getting smaller due to the appreciable offset by the rising flow inertia. Meanwhile, the liquid flow reversal is more evidently observed in the large diameter bend. Flow pattern map showing the region of liquid flow reversal is shown on the diagram of the superficial gas velocity (U-GS) versus superficial liquid velocity (U-LS). The region for liquid flow reversal for D = 9.5 mm is much greater than that of D = 5.5 mm. For D = 5 mm, there is no liquid flow reversal when U-GS is above 2 m/s. On the other hand, the threshold value is raised to 4 m/s for the 9.5 mm internal diameter tube. (C) 2015 Elsevier Inc. All rights reserved.
A new correlation and the review of two-phase flow pressure change across sudden expansion in small channels
(2010-09-01)
Wang, Chi-Chuan
;
Tseng, Chih-Yung
;
Chen, Ing Youn
;
機械工程學系
;
Department of Mechanical Engineering
In this study, the authors first present a short overview on the single-phase flow across sudden expansion, followed by a thorough review of the relevant literature for two-phase flow across sudden expansion, and examination of the applicability of the existing correlations. Also, 282 data available from five publications are collected. It is found that none of the existing correlations can accurately predict the entire database. Most of the correlations highly over predict the data with a mini test section which has a Bond number being less than 0.1 in which the effect of surface tension dominates. Also, some of the correlations significantly under predict the data for very large test sections. Among the models/correlations being examined, the homogeneous model shows a poor predictive ability than the others, but it is handy for the engineering aspect. Hence by taking account the influences of Bond number, Weber number, Froude number, liquid Reynolds number, gas quality and area ratio into the original homogeneous model, a modified homogeneous model is proposed that considerably improves the predictive ability over existing correlations with a mean deviation of 23% and a standard deviation of 29% to all the data with wider ranges of operating parameters for application. (C) 2010 Elsevier Ltd. All rights reserved.
An experimental investigation of air cooling thermal module using various enhancements at low Reynolds number region
(2010-12-01)
Yang, Kai-Shing
;
Li, Shu-Lin
;
Chen, Ing Youn
;
Chien, Kuo-Hsiang
;
Hu, Robert
This study examines the airside performance of heat sinks having fin patterns of plate fin (Type I), interrupted fill geometry (Type II), dense vortex generator (Type III), and loose vortex generator (Type IV). Test results indicate that the heat transfer performance is strongly related to the arrangement of enhancements. The interrupted and dense vortex generator configurations normally contribute more pressure drop penalty than improvements of heat transfer. This is especially pronounced when operated at a lower frontal velocity. Actually the plain fin geometry outperforms most of the enhanced fin patterns such as of Type II and Type Ill at the fully developed region. This is because a close spacing prevents the formation of vortex, and the presence of interrupted surface may also suffer from the degradation by constriction of conduction path. The results suggest that the vortex generators operated at a higher frontal velocity is more beneficial than that of plain fin geometry. In association with the VG-1 criteria (same pumping power and same heat transfer capacity), the results show that effective reduction of surface area can be achieved when the frontal velocities are at 3-5 m s(-1) and the fin patterns are triangular, triangular attack, or two-groups dimple. The result from the present experiment suggests that the asymmetric combination such as using loose vortex generator (Type IV) can be quite effective. The triangular attack vortex generator is regarded as the optimum enhancement design for it could reduce 12-15% surface area at a frontal velocity around 3-5 m s(-1). The asymmetric design is still applicable even when the fin pitch is reduced to 1 mm. (C) 2010 Elsevier Ltd. All rights reserved.
Influence of electrode configuration on the heat transfer performance of a LED heat source
(2014-10-01)
Chen, Ing Youn
;
Chen, Chien-Jen
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
In this study, the effects of electrode configuration on the cooling of LED are presented. A total of six needle type electrodes, including sharp, 0.08, 0.12, 0.37, 0.49 R and flat tip, are used to generate ionic wind for cooling of the LED. The effects of electrode configuration, vertical separation height, and tilt angle on the cooling performance of a LED are reported in this study. It is found that the thermal resistance is reduced with the supplied voltage and a maximum 50% reduction is achieved before the spark-over voltage. For the same supplied voltage, the thermal resistance with a larger vertical separation is also higher. However, the operational range is also longer. The effect of tilt angle on the cooling performance of LED depends on the supplied voltage. With a supplied voltage being less than 6.5 kV, it is found that the thermal resistance is increased when the tilt angle is reduced with a tip radius of 0.12 R. However, the trend is reversed when the supplied voltage is higher than 6.5 kV where the thermal resistance of a smaller tilt angle is lower than that of a larger tilt angle. For the same mesh ground, the effect of electrode configurations cast little influence on the final thermal resistance. All the electrodes show an approximately 50% reduction of thermal resistance in association with its original thermal resistance. However, the corresponding threshold voltage and operational voltage differs significantly. The threshold voltage is lowest for the sharp needle and is increased with the rise of tip radius. On the other hand, the corresponding operational range becomes narrower when the tip radius is increased. Yet the spark-over voltage is also insensitive to change of electrode configuration. (C) 2014 Elsevier Ltd. All rights reserved.
Effect of cannelure fin configuration on compact aircooling heat sink
(2011-07-01)
Wang, Chi-Chuan
;
Yang, Kai-Shing
;
Liu, Yang-Ping
;
Chen, Ing Youn
;
機械工程學系
The present study proposed a novel cannelure fin structure applicable to compact air-cooled heat sink under cross flow condition. A total of eight heat sinks were made and tested, including plain fin, oblique dimple type (I), oblique dimple type (II), straight cannelure, full oblique cannelure, half oblique cannelure with dimple/cavity type, half oblique cannelure with dimple/cavity type (II), and full oblique cannelure with dimple/cavity. Test results indicate that the proposed full cannelure fin along with the dimple/cavity structure can attain approximately 25% increase of heat transfer performance, and accompanies a friction reduction of about 20%. Moreover, converse to those interrupted fin or triangular vortex generator, it is found that the proposed fin structure still offers significant augmentation in fully developed region. The observed IR image of the temperature measurement of the test fin configurations also confirms the measurements. (C) 2011 Elsevier Ltd. All rights reserved.
Enhanced cooling for LED lighting using ionic wind
(2013-01-15)
Chen, Ing Youn
;
Guo, Mei-Zuo
;
Yang, Kai-Shing
;
Wang, Chi-Chuan
;
機械工程學系
This study employs ionic wind to augment heat transfer of a LED mounted on a substrate. The size of the LED chip is 0.9 mm x 0.9 mm with a nominal power of 1 W. A needle type electrode is used to generate ionic wind with the applied voltage ranging from 4 to 11 kV. The effects of aligned angle, electrode polarity, separation distance, and ground configuration on the thermal resistance of the LED substrate are examined in this study. For the same applied voltage, it appears that the thermal resistance for the negative polarity is lower than that for the positive one and the negative electrode also has a wilder operation range. The thermal resistance can be reduced as much as 50% in the test range. The thermal resistance is slightly reduced when the aligned angle is increased from 0 degrees to 20 degrees, but a further increase of aligned angle casts no further reduction on the thermal resistance. It is found that the influence of vertical separation distance between the needle and ground electrode is moderately higher than that of horizontal separation distance. Test results also indicate that the mesh ground electrode shows moderately lower thermal resistance than those of point or line electrode. (C) 2012 Elsevier Ltd. All rights reserved.
Characteristics of flow distribution in compact parallel flow heat exchangers, part I: Typical inlet header
(2011-11-01)
Wang, Chi-Chuan
;
Yang, Kai-Shing
;
Tsai, Jhong-Syuan
;
Chen, Ing Youn
;
機械工程學系
This study experimentally and numerically investigates the single-phase flow into parallel flow heat exchangers with inlet and outlet rectangular headers having square cross section and 9 circular tubes. The effects of inlet flow condition, tube diameter, header size, area ratio, flow directions (Z and U-type), as well as the gravity are investigated. The experimental results indicate that flow distribution for U-type flow is more uniform than Z-type flow. Depending on the inlet volumetric flow rate, the flow ratio at the first several tubes can be lower than 50% of the last tube for Z-type arrangement, and this phenomenon becomes more and more pronounced with the rising velocity at the intake conduit. The mal-distribution can be eased via reducing the branching tube size or increasing the entrance settling distance at the intake conduit. It is found that the influence of gravity on mal-distribution is negligible and the mal-distribution is associated with the jet flow pattern. (C) 2011 Elsevier Ltd. All rights reserved.
Characteristics of flow distribution in compact parallel flow heat exchangers, part II: Modified inlet header
(2011-11-01)
Wang, Chi-Chuan
;
Yang, Kai-Shing
;
Tsai, Jhong-Syuan
;
Chen, Ing Youn
;
機械工程學系
This study presents the experimental results of liquid flow distribution in compact parallel flow heat exchanger through a rectangular and 5 modified inlet headers (i.e., 1 trapezoidal, one multi-step, 2 baffle plates and 1 baffle tubes header). The basic header has a rectangular shape with 9 x 9 mm cross section and 90 mm long header length having a 4 mm inlet tube for flow into the header and distributed to nine 3 mm parallel tubes with 400 mm length. A jet stream induced at the header inlet associated with vortexes affecting the flow distribution to the front tubes. The flow distribution in the header highly depends on the header shape and the total flow rate. Normally the first several tubes have the lowest flow ratios for the conventional headers and the flow distribution is significantly improved by lifting the jet stream using the modified header with baffle tube, followed by the baffle plate and multi-step header. The baffle tube yields the best flow distribution for it removed the vortex flow, and it is applicable for all the flow rates. (C) 2011 Elsevier Ltd. All rights reserved.
A Comparative Study of Nozzle/Diffuser Micropumps with Novel Valves
(2012-02-01)
Yang, Kai-Shing
;
Chao, Tzu-Feng
;
Chen, Ing Youn
;
Wang, Chi-Chuan
;
Shyu, Jin-Cherng
This study conducts an experimental study concerning the improvement of nozzle/diffuser micropump design using some novel no-moving-part valves. A total of three micropumps, including two enhancement structures having two-fin or obstacle structure and one conventional micro nozzle/diffuser design, are made and tested in this study. It is found that dramatic increase of the pressure drops across the designed micro nozzles/diffusers are seen when the obstacle or fin structure is added. The resultant maximum flow rates are 47.07 mm(3)/s and 53.39 mm(3)/s, respectively, for the conventional micro nozzle/diffuser and the added two-fin structure in micro nozzle/diffuser operated at a frequency of 400 Hz. Yet the mass flow rate for two-fin design surpasses that of conventional one when the frequency is below 425 Hz but the trend is reversed with a further increase of frequency. This is because the maximum efficiency ratio improvement for added two-fin is appreciably higher than the other design at a lower operating frequency. In the meantime, despite the efficiency ratio of the obstacle structure also reveals a similar trend as that of two-fin design, its significant pressure drop (flow resistance) had offset its superiority at low operating frequency, thereby leading to a lesser flow rate throughout the test range.

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