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Now showing 1 - 10 of 59

Response of a 50 kW Organic Rankine Cycle System Subject to Influence of Evaporators
(2014-01-01)
Lee, Yuh-Ren
;
Kuo, Chi-Ron
;
Liu, Chih-Hsi
;
Fu, Ben-Ran
;
Wang, Chi-Chuan
The variations of the evaporator on the system responses of a 50 kWe ORC system using R-245fa are investigated. The effect of exit superheat on the ORC system depends of the types of the evaporator. For the plate evaporator, an exit superheat less than 10 degrees C may cause ORC system unstable due to liquid entrainment. For maintaining stable operation, the corresponding Jakob number of the plate heat evaporator must be above 0.07. On the other hand, no unstable oscillation of the ORC system is observed for an exit superheat of 0 similar to 47 degrees C when the shell-and-tube heat exchanger is used as the evaporator. (C) 2014 The Authors. Published by Elsevier Ltd.
Improvements of Airflow Distribution in a Container Data Center
(2015-01-01)
Wang, I-Nuo
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Tsui, Yeng-Yung
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
The present study aims at improvements of the airflow and temperature distribution in a container data center. Simulation is carried out based on a raised floor design. It is found that the height of raised floor casts significant impact on the flow distribution. Generally, the flow rate of the perforated tile near the entrance region of CRAC unit is lower than that of the rear part of the perforated tile. The mal-distribution becomes even more severe when the height of the raised floor is reduced. The mal-distribution can be significantly improved using a smaller opening perforated tile but it also gives rise to higher pumping power and impairs the effective flow rate from CRAC. Hence, a variable opening design with a larger opening near the CRAC outlet, followed subsequently with smaller opening perforated tiles, the mal-distribution can be eliminated without additional pumping power. For improvement of the airflow/temperature distribution into the rack, we propose a drawer-type rack which can increase the effective hot aisle space and reducing the cold aisle space. Through this design, hot air recirculation and cold air bypass will be significantly reduced. As a result, the rack maximum inlet temperature of drawer-type rack can be reduced as much as 13.3 degrees C when compared with the traditional one. (C) 2015 The Authors. Published by Elsevier Ltd.
A Novel Thermal Module with 3-D Configuration Pulsating Heat Pipe for High-Flux Applications
(2018-12-01)
Tseng, Chih-Yung
;
Wu, Ho-Meng
;
Wong, Shwin-Chung
;
Yang, Kai-Shing
;
Wang, Chi-Chuan
A pulsating heat pipe (PHP) contains a wickless design with aligned serpentine tube configuration whose simple structure offers a comparatively easy manufacturing capability. The bends with large curvature are often used for serpentine PHPs. This eventually results in a decline in effective contact surface area between evaporator/condenser and PHP circuitry, thereby impairing the benefit of the wickless design of a PHP. A novel thermal module featuring a 3-D configuration pulsating heat pipe, an evaporator, and a fin-and-tube condenser is proposed to tackle the high-flux application. Methanol is used as the working fluid with a filling ratio of around 60%. Test results indicate the thermal resistance of the proposed module varies from 0.148 K/W to 0.0595 K/W when the supplied power changes from 100 to 1000 W. The proposed thermal module can handle a supplied power up to 1 kW and the corresponding power or heat flux is much higher than any existing literatures.
Compound Heat Transfer Enhancement of Wavy Fin-and-Tube Heat Exchangers through Boundary Layer Restarting and Swirled Flow
(2018-08-01)
Sadeghianjahromi, Ali
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Kheradmand, Saeid
;
Nemati, Hossain
;
Liaw, Jane-Sunn
;
Wang, Chi-Chuan
This study performs a 3D turbulent flow numerical simulation to improve heat transfer characteristics of wavy fin-and-tube heat exchangers. A compound design encompassing louver, flat, and vortex generator onto wavy fins can significantly enhance the heat transfer performance of wavy fin-and-tube heat exchangers. Replacement of wavy fins around tubes with flat fins is not effective as far as the reduction of thermal resistance is concerned, although an appreciable pressure drop reduction can be achieved. Adding two louvers with a width of 8 mm to the flat portion can reduce thermal resistance up to 6% in comparison with the reference wavy fin. Increasing the louver number and width can further decrease the thermal resistance. Also, it is found that the optimum louver angle is equal to the wavy angle for offering the lowest thermal resistance. Therefore, compound geometry with three louvers, a width of 12 mm, and the louver angle being equal to wavy angle with waffle height to be the same as fin pitch of the reference wavy fin has the most reduction in thermal resistance of 16% for a pumping power of 0.001 W. Adding punching longitudinal vortex generators on this compound geometry can further decrease thermal resistance up to 18%.
Airside Performance of H-Type Finned Tube Banks with Surface Modifications
(2019-02-02)
Bhale, Pradhyumn
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Kaushik, Mrinal
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Liaw, Jane-Sunn
;
Wang, Chi-Chuan
;
機械工程學系
The present study numerically investigates some novel modifications to augment the performance of the H-type finned tube banks, which are widely used in waste heat recovery in industries. The imposed modifications upon the original H-type finned tube banks include the following: (1) Design 1 contains some triangular cuts at the edge of the original rectangular fin; (2) Design 2 modifies the original rectangular geometry into a trapezoid shape; (3) Design 3 renders the original rectangular cross-section fin thickness into trapezoid cross-section; and (4) Design 4 changes the original rectangular shape into a circular shape. Based on the simulations, it is found that Design 1 shows barely any improvements in the heat transfer performance and surface area reduction. Design 2 can provide some weight saving and surface area reduction at a slightly inferior heat transfer performance. Design 3 can offer up to 14% improvements in the overall heat transfer performance without any pumping power penalty. Yet, Design 4 provides the maximum weight saving as compared to the original reference case. With 3-9% lesser surface area than the reference case, Design 4 still offers marginally higher heat transfer performance.
Investigation of Separated Two-Phase Thermosiphon Loop for Relieving the Air-Conditioning Loading in Datacenter
(2019-01-01)
Daraghmeh, Hafiz M.
;
Sulaiman, Mohammed W.
;
Yang, Kai-Shing
;
Wang, Chi-Chuan
;
機械工程學系
This study investigates the feasibility of using R-134a filled separated two-phase thermosiphon loop (STPTL) as a free cooling technique in datacenters. Two data center racks one of them is attached with fin and tube thermosiphon were cooled by CRAC unit (computer room air conditioning unit) individually. Thermosiphon can help to partially eliminate the compressor loading of the CRAC; thus, energy saving potential of thermosiphon loop was investigated. The condenser is a water-cooled design and perfluoroalkoxy pipes were used as adiabatic riser/downcomer for easier installation and mobile capability. Tests were conducted with filling ratio ranging from 0 to 90%. The test results indicate that the energy saving increases with the rise of filling ratio and an optimum energy savings of 38.7% can be achieved at filling ratios of 70%, a further increase of filling ratio leads to a reduction in energy saving. At a low filling ratio like 10%, the evaporator starves for refrigerant and a very uneven air temperature distribution occurring at the exit of data rack. The uneven temperature distribution is relieved considerably when the evaporator is fully flooded. It is also found that the energy saving is in line with the rise of system pressure. Overfilling of the evaporator may lead to a decline of system pressure. A lower thermal resistance occurs at high filling ratios and higher ambient temperature.
Effect of partial bypass on the heat transfer performance of dehumidifying coils
(2014-11-01)
Wang, Chi-Chuan
;
Cheng, Yu-Cheng
;
Liaw, Jane-Sunn
;
Tseng, Chih-Yung
;
機械工程學系
This study extends a so called "partial bypass" concept to dehumidifying fin-and-tube heat exchangers. Tests are performed for a 2-row and 4-row fin-and-tube heat exchangers having plain fin configuration. The inlet dry bulb temperature is fixed at 25 degrees C while the inlet relative humidities are 50% and 80%, respectively. Test results showed that for a 2-row coil under RH = 50%, the corresponding heat transfer augmentation ratio (Q(R)) and pressure penalty ratio (P-R) decrease with the rise of bypass ratio (BR). At a smaller frontal velocity, the regime of appropriate bypass ratio where Q(R) > 1 and P-R < 1 is more apparent. The effect of partial bypass decreases as the velocity increases. As the velocity gets close to 2 m s(-1), it shows that the performance at high bypass ratio becomes more efficient. At the same time, when bypass ratio becomes lager, the tendency of the decreasing of Delta P is more drastic than the decreasing of Q. Nevertheless, not for all the cases that the performances get better as the bypass ratio increases. For V-fr = 4 m s(-1), the experiment results show that the overall effect is less efficient\' than at V-fr = 2 m s(-1). The major reason is caused by the flow pattern. The rise of bypass airflow is quite large which may act as an air curtain to distort the main airflow, and result in higher pressure drop. This implies that the design of the bypass design is quite imperative in the practice of real applications. For a 4-row heat exchanger, the experiment data of QR and PR for the 4-row coil performs better than a 2-row coil. (C) 2014 Elsevier Ltd. All rights reserved.
A novel heat dissipation fin design applicable for natural convection augmentation
(2014-12-01)
Charles, Roody
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
In this study, a comparative study of heat sink having various fin assembly under natural convection is investigated. The fin pattern includes a rectangular, a trapezoidal and an inverted trapezoidal configuration. Tests were performed in a well controlled environmental chamber having a heat load ranging from 3 to 20 W. From the test results, the heat transfer coefficient of the conventional rectangular fins is higher than that of the trapezoidal fins while the heat transfer coefficient of the inverted trapezoidal fins is higher than the trapezoidal one by approximately 25%, and it exceeds that of convectional rectangular fin by about 10%. The heat transfer improvements of the inverted trapezoidal fin are mainly associated with a larger temperature difference and inducing more air flow into the heat sink. (C) 2014 Elsevier Ltd. All rights reserved.
System performance of R-1234yf refrigerant in air-conditioning and heat pump system - An overview of current status
(2014-12-22)
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
In this study, an overview of the system performance of R-1234yf in association with R-134a is carried out. Based on the existing researches, it is found that the COP and heat capacity of R-134a system may suffer from direct drop-in replacement of R-1234yf. The deterioration is around 0-27% depending on the operational conditions. With the introduction of internal heat exchanger, ejector, expander, or adjustment of the thermal expansion valve, the deterioration can be relieved, and a comparable performance becomes likely. For the heat transfer performance in the evaporator, R-1234yf is almost comparable with that of R-134a. However, the performance in the condenser is inferior to R-134a. The phenomenon may be quite severe for a water cooled condenser since the dominant thermal resistance may fall in the refrigerant side. The volumetric efficiency of R-1234yf system is slightly lower than that of R-134a due to higher frictional drop of R-1234yf. For the same thermal expansion valve for controlling the suction superheat, it appears that higher suction superheat may occur for R-1234yf refrigerant. Hence further adjustment of spring in the valve is required for soft optimization. (C) 2014 Elsevier Ltd. All rights reserved.
Investigation of bubble effect in microfluidic fuel cells by a simplified microfluidic reactor
(2010-09-01)
Shyu, Jin-Cherng
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Wei, Chung-Sheng
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Lee, Ching-Jiun
;
Wang, Chi-Chuan
;
機械工程學系
This study experimentally examines the influence of two-phase flow on the fluid flow in membraneless microfluidic fuel cells. The gas production rate from such fuel cell is firstly estimated via corresponding electrochemical equations and stoichiometry from the published measured current-voltage curves in the literature to identify the existence of gas bubble. It is observed that O(2) bubble is likely to be generated in Hasegawa's experiment when the current density exceeds 30 mA cm(-2) and 3 mA cm(-2) for volumetric flow rates of 100 mu L min(-1) and 10 mu L min(-1), respectively. Besides, CO(2) bubble is also likely to be presented in the Jayashree's experiment at a current density above 110 mA cm(-2) at their operating volumetric liquid flow rate, 0.3 mL min(-1). Secondly, a 1000-mu m-width and 50-mu m-depth platinum-deposited microfluidic reactor is fabricated and tested to estimate the gas bubble effect on the mixing in the similar microchannel at different volumetric flow rates. Analysis of the mixing along with the flow visualization confirm that the membraneless fuel cell should be free from any bubble, since the mixing index of the two inlet streams with bubble generation is almost five times higher than that without any bubble at the downstream. (C) 2010 Elsevier Ltd. All rights reserved.

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