Browsing by Author "Chu, Wen-Xiao"

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A novel micro-channel heat sink with trapezoid drainage for enhancing condensation heat transfer of dielectric fluid
(2019-09-01)
Chu, Wen-Xiao
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Chen, Chuan-Yu
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Liao, Ying-Hao
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Wang, Chi-Chuan
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機械工程學系
This study experimentally investigates the condensation of dielectric fluid HFE-7100 in a micro-channel heat sink with the hydraulic diameter of 1.21 mm. Tests are conducted at a fixed pressure of 110 kPa, with vapor mass quality (x(in)) ranging from 0.1 to 0.9, mass fluxes (G) from 150 to 250 kg.m(-2).s(-1), inclined angle ( theta) from -90 degrees to +90 degrees. A novel trapezoid drainage design is proposed to enhance the condensation heat transfer by effectively entraining condensation film. When G is lower than 200 kg.m(-2).s(-1), the two phase heat transfer coefficient (h(tp)) for the trapezoid drainage design exceeds the conventional rectangular micro-channel heat sink by 10-26% while the corresponding pressure drop is about 22-45% lower. When G is increased to 250 kg.m(-2).s(-1), the drainage design can still dramatically decrease the pressure drop, however, it shows a negative effect on h(tp) when the x(in) is greater than 0.2. With inclined arrangements, the pressure drop would dramatically increase due to gravity effect. Besides, at the conditions of theta = -45 degrees and -90 degrees, the trapezoid drainage channel can improve the h(tp) by 43-45% when G is lower than 200 kg.m(-2).s(-1). On the contrary, with the arrangement of theta = +45 degrees and +90 degrees, the trapezoid drainage channels may decrease the h(tp) by 8-15%. This phenomenon is especially pronounced at a high z i p. In addition, the heat transfer enhancement and deterioration are both analyzed based on stress distribution at vapor phase and condensation film.
A review on airflow management in data centers
(2019-04-15)
Chu, Wen-Xiao
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
This study provides a review upon airflow management in data centers. Based on the available airflow path, cooling systems in data centers are categorized as long-distance cooling or short-distance cooling systems. Investigations on airflow management include tests in real data center or in simulated data center. Besides, the computational fluid dynamics (CFD) had been widely employed upon the thermal and airflow management of the data centers. For the long-distance cooling system, the airflow management normally adopts raised-floor configuration and hot/cold aisle arrangement. The major problems in airflow management include hot-air recirculation, cold-air bypass, leakages, over-provisioned and under-provisioned air supply, and airflow/temperature non-uniformity. The aforementioned effects often interact with the geometry layout of the data center. Related literatures regarding the effect of plenum depth, perforated tiles, enhanced facility such as induced bypass fans, infrastructure layout, aisle containment and leakage are discussed and compared. In addition, studies on the overhead air supply method are also examined and compared with the raised-floor ones. For the short-distance cooling system, the effect of server layout and heat exchanger layout concerning the airflow uniformity are investigated. It is found that the appropriate management of the original design into centralized server layout can ease the mal-distribution of airflow into the severs by 30%. This review aims to emphasize the criteria of implementing airflow management to data centers that serve as a reference guide for energy saving in data center as far as airflow arrangement is concerned. Moreover, some recommended future research efforts are also addressed.
Airside performance of sinusoidal wavy fin-and-tube heat exchangers subject to large-diameter tubes with round or oval configuration
(2020-01-05)
Chu, Wen-Xiao
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Sheu, Wen-Jenn
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Hsu, Chen-Chieh
;
Wang, Chi-Chuan
;
機械工程學系
A comparative study regarding airside performance of sinusoidal wavy fin-and-tube heat exchangers (FTHXs) having round and oval tube configurations is experimentally investigated and numerically verified. The computational fluid dynamics (CFD) method is also adopted to analyze detailed local heat transfer performance. The diameter of round tube is 16.3 mm while the oval tube contains diameters of 21.0 mm (major) and 10.6 mm (minor) subject to tube rows of 2-6. Results show that the FTHXs having round configuration performs higher pressure drop when the fin pitch (F-p) is 3.0 mm, however, the oval configuration conversely shows higher pressure drop with the F-p of 1.8 mm. Meanwhile, the effect of F-p on the heat transfer performance is rather small for round configuration with the tube row of 2 and 4. When the tube row increasing to 6, the heat transfer coefficient can be improved by about 10-20% and 9.3-25.2% for FTHXs with round and oval configurations, respectively, by increasing the fin pitch from 1.8 mm to 3.0 mm. The existing experimental correlations for wavy FTHX having round configuration fail to predict the present experimental data due to different ranges of fin pitch, tube pitch and tube diameter. The overall performance of twelve tested samples are also compared using JF ratio factor.
Assessment on rack intake flowrate uniformity of data center with cold aisle containment configuration
(2020-07-01)
Chu, Wen-Xiao
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Wang, Rui
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Hsu, Po-Hao
;
Wang, Chi-Chuan
;
機械工程學系
The present paper investigates the rack intake flowrate uniformity in a simulated container data center subject to cold aisle containment (CAC) configuration. Based on a novel flowrate uniformity index (FUI) in prior study, a strong flowrate mal-distribution at the rack inlets is observed by examining the standard deviation of FUI (sigma(FUI)). When raising the supplied volumetric flowrate from 2.52 m(3) s(-1) to 3.40 m(3) s(-1), the outlet temperature of each cabinet is remarkably reduced. Meanwhile, applying a gradient fan speed for cabinets can reduce the sFUI by 19.5-34.0%. However, the mal-distribution for the top and bottom cabinets is still prominent. In essence, adding air supply flowrate or changing rack intake layout show limited improvements and may even have potentially deleterious effects. To tailor the mal-distribution problem, a novel configuration featuring the concept of Coanda effect is proposed. A triangular deflector is implemented to guide the cold air sweeping through fan trays in front of the data rack. The sFUI can be effectively reduced by about 51.3-62.0% compared to the original design, showing a significant improvement on the intake flowrate distribution for all racks.
CFD analysis and experimental verification on a new type of air-cooled heat sink for reducing maximum junction temperature
(2020-02-01)
Chu, Wen-Xiao
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Tsai, Ming-Kun
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Jan, Shun-Yuan
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Huang, Hsiang-Ho
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Wang, Chi-Chuan
This paper investigates a new type of plate-fin heat sink having a rectangular fin-cut at the entrance while containing a fin-protrusion part at the rear of the heat sink. The design not only offers appreciable heat transfer enhancement without pressure drop penalty, but also pronouncedly reduces the maximum junction temperature (T-max). It is experimentally validated that the effective thermal resistance can be reduced by 5.5-7.2% under the heating load of 250 W, and the temperature at the rear heat source can be reduced by about 2.5 K under same pumping power compared to the original plate-fin heat sink. Triangular fin-protrusion applying at the rear part shows more pronounced improvement on thermal performance compared to a rectangular one because of better heat spreading. Meanwhile, by placing a partition baffle below the rectangular fin-cut can further reduce the T-max to reach as low as 354.5 K under the pumping power of 0.3 W. The effect of partition baffle and fin angle (theta) is quantitatively studied in more details. When theta is increased over 45 degrees, the partition baffle shows further improvement on T-max at fixed pumping power. The lowest T-max is obtained when the fin angle (theta) of the fin-protrusion area reaches 65 degrees It is also observed that the rectangular fin-cut is superior to a triangular one, suggesting a "Coanda-like" effect. In addition, a correlation is developed to predict the T-max of the new air-cooled heat sink. The predictive deviations of the correlation against numerical results are within +/- 12.0 K, showing a reasonably predictive ability. (C) 2019 Elsevier Ltd. All rights reserved.
Experimental and Numerical Investigations on Jet Impingement Cooling for Electronic Modules
(2019-10-01)
Chu, Wen-Xiao
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Huang, Kuan-Chang
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Amer, Mohammed
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Wang, Chi-Chuan
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機械工程學系
In this paper, the influence of outlet arrangement and plenum structure on impingement cooling is experimentally and numerically investigated in a typical 1-U confined server space. Three outlets include Z-type, bilateral, and U-type arrangements, and the plenum configurations contain partially inclined, fully inclined, and staged layouts. As a result, using the U-type outlet or staged plenum may prominently compromise the impingement cooling performance on the target plates with lower pumping power. With numerical investigation, it is found that, for the case with Z-type outlet, the flowrate of jet impingement increases alongside the streamwise direction. Besides, the impingement stagnation region on target plates with the minimum thermal resistance may shift toward the outlet. Meanwhile, the uniformity of jet impingement can be improved by 10.7% and 50.3% when the bilateral and U-type outlets are applied, respectively, and the jet impingement is changed to perpendicular direction due to the opposite cross flow from the coming flow direction. On the other hand, by applying the inclined plenum and staged plenum, the uniformity of jet impingement can be dramatically improved by 113.9% and 215.1%, respectively. However, the local jet impingement velocity distribution is still nonuniform. Hence, a novel design of impingement plate based on the concept of Coanda effect is proposed. The peak value of the thermal resistance on target plate can be reduced by 21.8% and 16.0% at the center region and the fore part of the jet array.
Experimental and numerical study on the performance of passive heat sink having alternating layout
(2019-06-01)
Chu, Wen-Xiao
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Lin, Yu-Chieh
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Chen, Ching-Yao
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Wang, Chi-Chuan
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機械工程學系
In this study, a novel passive heat sink with triangular fins having alternating layout to promote heat transfer performance of a passive heat sink is proposed and verified by experimental and numerical examinations. The heat sinks with rectangular fins (case A), triangular fins with right angle, 90 degrees (case B), triangular fins with right angle having alternating layout (case C), triangular fins with obtuse angle of 120 degrees (case D) and triangular fins with obtuse angle of 120 degrees having alternating layout (case E) are tested with vacuum pressure ranging from 400 Pa to atmosphere pressure. It is indicated that the triangular fin having alternating layout offer some benefits like entraining more airflow into the heat sink and a higher effective view factor, thereby showing an appreciable increase of effective heat transfer coefficient (HTC). Results obtained in low vacuum pressure show that the effective HTC of cases B, C, D and E can be improved by 1.5%, 5.5%, 3.0% and 12.1%, respectively as compared to the reference case A. Yet the effective HTC can be improved by 3.2%, 6.1%, 4.6% and 13.0%, respectively in atmosphere pressure. The numerical study is in line with the experimental results and the simulations suggest that the optimum angle of the proposed triangular fins having alternating layout can improve the effective HTC by up to 14.6% and 16.0% with obtuse angles of 150 degrees and 140 degrees when fin area is 2000 mm(2) and 3000 mm(2) , respectively. The benefits of triangular fins having alternating layout show even more superior performance when the heat sink is placed vertically and the effective heat transfer coefficient for triangular fins having alternating layout with 150 degrees is improved by up to 20.3%. (C) 2019 Elsevier Ltd. All rights reserved.
Experimental and Numerical Study Upon Uniformity of Impingement Cooling With Pin-Fin Heat Sink
(2020-01-01)
Chu, Wen-Xiao
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Chiou, Po-Hsiang
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
In this paper, the impingement cooling performance on pin-fin heat sinks is experimentally and numerically investigated with different outlet arrangements and pin-fin layouts. Compared with one-way opening arrangement, results show that the two-way opening arrangement can offer a much lower pressure loss (Delta p) due to shorter flow distance of airflow from outer jets. Meanwhile, it is surprising that the two-way opening arrangement still offers appreciable improvements on effective heat transfer coefficient (h(eff)). This is because the fountain may shift toward the outer jets more rapidly with one-way opening arrangement. Yet the implementation of staggered pin-fin layout has minor improvement on h(eff) . However, Delta p can be obviously reduced by 11.1% due to the more effective spreading of impinging airflow in radial direction. The h(eff) value can be further increased by 8.8% when a smaller jet-to-target distance is applied. With priority consideration of hot spots in electronic cooling, the nonuniformity of local Nusselt number (S) is further studied through the computational fluid dynamics (CFD) method. S approaches 18.5% when the airflow directly impinges on a flat surface with optimized jet array. Implementing pin-fin heat sinks can reduce the nonuniformity of Nusselt number distribution to fall below 8.0%. The nonuniformity of local Nusselt number can be further reduced to 1.8% by applying four-way opening arrangement and larger central jet at the same time.
Experimental investigation on heat transfer enhancement with twisted tape having various V-cut configurations
(2020-05-25)
Chu, Wen-Xiao
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Tsai, Ching-An
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Lee, Bing-Hung
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Cheng, Kai-Yueh
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Wang, Chi-Chuan
This paper experimentally investigates the thermo-hydraulic performance of tubes with various twisted tapes (TTs) with the Reynolds number ranging from 4000 to 10,000 (at the turbulent transition region). The Nusselt number (Nu) and friction factor (f) of the tested plain tube shows good agreement with the Gnielienski and Filonenko equations. The average deviations are within 9.5% and 2.8%, respectively. For the successive TT tube, the existing correlation which includes the effect of gap between the TT insert and tube can well predict the present measurement, and the deviation upon Nu and f is within +/- 10%. Meanwhile, the successive TT with the twisted ratio of 4 can achieve the highest comprehensive thermal performance (CTP) by 1.08-1.10. Yet the Nu and f will simultaneously drop by 6.7% and 11.6% when raising the gap ratio from 0.043 to 0.135. The CTP is also pronouncedly reduced and can be negatively reversed than the plain tube at the turbulent transition region. On the other hand, it is validated that the U-cut and isosceles V-cut configurations might provide similar improvement on Nu and f compared to the successive TT. The obtuse V-cut configuration shows more pronounced improvement on Nu and f, and yields the highest CTP of 1.18-1.23 within the test range. The existing correlations applicable for the V-cut TTs are unable to predict the present measurements accurately.
Experimental investigation on thermal management for small container data center
(2019-01-01)
Chu, Wen-Xiao
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Hsu, Chang-Sheng
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Tsui, Yeng-Yung
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Wang, Chi-Chuan
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機械工程學系
The energy consumptions of data centers are increasing dramatically and cooling systems using for controlling the operating temperature usually consume more than half of these energy. Thus, effective strategies on thermal management in data centers may have benefit on energy savings. In order to improve the power usage effectiveness (PUE) of data centers, the effects of air supply flowrate by computer room air handler (CRAH), intake flowrate of rack cooling fans, air supply grilles and heat load distribution on the cooling performance of a typical small container data center having overhead air supply system are investigated in this paper. The rack cooling index (RCI) and supply heat index (SHI) are incorporated to evaluate the local and average cooling performance of data racks. It is found that the intake flowrate of rack cooling fans plays dominant role, which is suggested to be greater than the air supply flowrate in normal operation. Meanwhile, the volumetric flowrate distribution of grilles is studied. The additional front grille before entering the cold aisle may improve the RCI and SHI of front rack (rack A), in which severe hot-air recirculation may occur in typical operation. Conversely, additional rear grille at the exit of cold aisle has negative effect on cooling performance of the last rack (rack E). Based on the measurement of local temperature distribution, the non-uniform heat load distribution in racks is proposed. Result shows that the RCI can be improved to 64% if more power is placed at lower half part, which is eligible for energy saving. However, the non-uniform heat load distribution shows limited improvement on the rear rack (rack E).
Experimental Study on Thermal Contact Resistance Improvement for Optical Fiber by Using Low-Melting Temperature Alloy
(2020-03-01)
Chu, Wen-Xiao
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Lin, Hao-Yu
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
This paper presents an experimental analysis for minimizing the thermal contact resistance (R) between an optical fiber and copper heat sink by using the low-melting temperature alloy (LMTA) as the thermal interface material (TIM) subject to high-flux operation (up to 250 W .m(-1)). For the cases without LMTA, the temperature rise (Delta T) can easily surpass 195 degrees C at a heating load of 25 W .m(-1). By contrast, Delta T is dramatically reduced to be less than 1 degrees C with LMTA as TIM with a much higher heating power of 150 W .m(-1). The corresponding thermal resistance (R) can be reduced from 6.5-8.2 K .m .W-1 to 0.004-0.013 K .m .W-1. The improvement is far superior to existing studies. Besides, decreasing the surface roughness and increasing contact pressure also help to reduce R, especially for the cases when the LMTA is not melted. As the LMTA melts, a significant reduction of R by 56% is achieved as compared to the case without melting. The effect of surface roughness and contact pressure on the thermal contact resistance is also examined, and it is found that the influences are small once LMTA melts.
Impact of Overhead Air Supply Layout on the Thermal Performance of a Container Data Center
(2020-03-01)
Chu, Wen-Xiao
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Wu, Jui-Lin
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Tsui, Yeng-Yung
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Wang, Chi-Chuan
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機械工程學系
This study focused on the improved designs of airflow management in container data centers having overhead air supply. The computational fluid dynamics (CFD) model is first validated with experimental results. Then, the impact of grille diameter, deflector angle, and air supply layout on the data center thermal performance is investigated. The results show that the larger grille diameter may reduce the volumetric flowrate through the upstream grille, causing insufficient air supply and strong hot-air recirculation at the first rack A1. By decreasing the grille diameter from 335 mm to 235 mm, the average rack cooling index (RCI) and supply heat index (SHI) can be improved from 25.4% and 0.292 to 65% and 0.258, respectively. However, implementing small diameter grilles is not an economic way for data center performance improvement as far as the energy consumption is concerned due to the high pumping power. Meanwhile, raising the deflector angle below 30 deg in grille S1 can provide moderate improvement on temperature of the A1 rack. A further rise in the deflector to 40 deg may impose severe deterioration with a pronounced hot-spot area. The data center performance can be improved by changing from center-cold-aisle arrangement to center-hot-aisle layout. The layout provides much higher return air temperature and the RCI and SHI can be improved by 32.7% and 34.5%, respectively.
Improvement on dehumidifier performance using a plastic assisted condenser
(2020-02-25)
Chu, Wen-Xiao
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Chiu, Chuang-Hui
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
This paper proposed a new design concept for improving daily moisture removal rate and energy-efficiency factor of a dehumidifier. The plastic straws with diameters of 12 mm, 8.0 mm and 6.0 mm are inserted between the evaporator and condenser as an assisted condenser. A total of four cases are experimentally investigated, and the numerical analysis is conducted to further explain the performance improvement mechanism. Compared to the original dehumidifier, the assisted condenser with twelve straws having a diameter of 12 mm can pronouncedly improve the moisture removal rate of the dehumidifier by 6.9-9.2% and 1.2-6.4% at the standard condition and over-loading condition, respectively. By reducing the straw diameter to 8.0 mm, the improvement is reduced and the assisted condenser even reveal deteriorated effect at a high fan speed condition. Yet more pronounced degradation is seen when introducing 8.0 mm straws having two rows with staggered layout. It can be explained from numerical analysis that the pressure drop across the assisted condenser plays an important role. The optimum design depends on the balance between the main airflow, entrained airflow, and the flow resistance of the assisted condenser.
Investigations regarding the influence of soft metal and low melting temperature alloy on thermal contact resistance
(2020-07-01)
Chu, Wen-Xiao
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Khatiwada, Mahesh
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
In this paper, the thermal contact resistance (TCR) of thermal interface materials (TIMs) is experimentally investigated. The effects of contact pressure, heating load and surface roughness are examined in details. Results shows that the low melting temperature alloy (LMTA) can pronouncedly improve the TCR at high heating load condition when compared to indium TIM. Meanwhile, a larger contact pressure or a smoother interface also reduce the TCR effectively. For the solid-state indium TIM, an apparent hysteresis effect is seen for the TCR against contact pressure while there is no hysteresis effect regrading TCR against heating load. The indium TIM is reduced from 0.141 K.W-1 to 0.119 K.W-1 after loading and unloading of contact pressure, showing an apparent hysteresis effect. On the other hand, the TCR of LMTA can be improved by about 13.9-16.0% after increase/decrease heating process. Yet, LMTA contains some overflow issue upon melting. Hence a novel design of LMTA with indium confinement seal is proposed to eliminate overflow problem. Through this design, the TCR remains barely unchanged even after hundred cyclic operations. Additionally, a correlation for predicting the TCM of solid-state TIMs is proposed, and the correlation can predict 98% of experimental data within the error of +/- 30%.
Numerical analysis of thermohydraulic behavior in a directional solidification furnace
(1970-01-01)
Chu, Wen-Xiao
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Yang, Ching-Hsiang
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
In this study, the crystallization process in a directional solidification furnace is studied by using the transient numerical simulation. The flow field in melt silicon in conjunction with different solidification stages is investigated. A separated side-heater design is proposed, showing a pronounced improvement on temperature gradient in axial direction normal to the solidification interface at late period. Result shows that the Voronkov ratio which can reflect the crystal quality is reduced by 16.5-24.8% and the energy consumption can be saved by 6% simultaneously. On the other hand, the airflow management for argon gas in the crucible chamber, which contains advantage to reduce chemical impurity in crystal during solidification process, is also numerically studied. By installing a guiding plate beneath the graphite cover can appreciably lower the oxygen mass fraction in contact with the cover plate and reduce the potential chemical reaction accordingly. The plate diameter and the spacing between the plate and graphite cover are optimized. Compared to the original design, the oxygen mass fraction at the graphite cover can be pronouncedly reduced by 10.6-33.3%, which can be further improved by combining a corner outlet configuration. Meanwhile, under the same volumetric flow rate of argon gas, yet the argon gas could be saved by almost half (e.g., from 40 SLM to 20 SLM) for the same chemical concentration with the optimized configuration.
Utilization of low-melting temperature alloy with confined seal for reducing thermal contact resistance
(2019-12-25)
Chu, Wen-Xiao
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Tseng, Po-Hsiang
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Wang, Chi-Chuan
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機械工程學系
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Department of Mechanical Engineering
This paper investigates a novel design of thermal interface material (TIM) using low-melting temperature alloy (LMTA) with confined seal. For solid TIM like indium foil, the thermal contact resistance is reduced with the rise with contact pressure, and an apparent hysteresis phenomenon may occur during loading/unloading of contact pressure. For the solid-state TIM, the indium TIM outperforms those of copper, lead and tin TIMs. The molten LMTA can further reduce the thermal contact resistance, however, the overflow and dislocation problems are observed. Hence, a novel confined design by using LMTA with an annular metal seal is proposed that can completely prevent aforementioned problems. Yet the reliability of the novel TIM is also investigated after hundred thermal cycles.