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Now showing 1 - 10 of 158

Experimental investigation on defrosting of a cold flat plate via ultrasonic vibration under natural convection
(2020-10-01)
Amer, Mohammed
;
Wang, Chi-Chuan
;
機械工程學系
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Department of Mechanical Engineering
The present experimental study examines the effect of ultrasonic vibration on the frost formation under a free convective environment. A stainless steel SS ANSI 316 flat surface is tested under 12-24 degrees C dry bulb temperature and 58-84% relative humidity. The experiments are conducted with direct contact and non-contact high-frequency ultrasonic source having continuous or intermittent vibrations. The ultrasonic transducer contains 28 +/- 0.5 kHz resonance frequency. It is found that increasing the relative humidity will increase the frost thickness and influences the frost property. Imposing ultrasonic vibration under high relative humidity and low ambient temperature increases the droplets' circularity with comparatively sparse distribution. Test results indicate that non-contact vibration has no effect on the frost formation. Intensive intermittent contact vibration is the most effective way to suppress frost growth. Compared to continuous contact vibration, either it is applied after each an hour or after every 30 min as an intensive continuous, intensive intermittent contact vibration shows a reduction in frost thickness as much as 24%. In addition, the ultrasonic vibration should be applied after the supersaturation stage since ultrasonic vibration enhances the supercooled process.
A mechanistic model for nucleate boiling heat transfer performance with lubricant-refrigerant mixture
(2020-10-01)
Hung, Jung-Tsung
;
Chen, Yu-Kai
;
Shih, Hou-Kuang
;
Wang, Chi-Chuan
;
機械工程學系
The present study proposes a rationally based model to investigate the influence of lubricant on the nucleate boiling characteristics of refrigerant-lubricant mixtures. This model is developed based on site-activation boiling mechanism along with detailed physical parameters in association with thermal and hydrodynamic processes, including waiting period for bubble incipience, bubble growth and departure period. Yet, the model also encompasses the polymer adsorption theory and energy gap concept to address the effect of lubricant with different chemical structure and physical property on the heat transfer phenomenon during each period of nucleate boiling individually. In addition, by using the partition function with Boltzmann energy distribution in different energy state, bubble density can be expressed in an analytic form to facilitate the calculation of heat transfer coefficient. Based on the proposed model, the presence of lubricant appreciably changes interfacial energy upon metal-liquid and liquid-bubble interfaces. The lubricant prefers lying on metal surface, thereby influencing surface coverage concentration when bubble is initiating. Such lubricant-rich layer near metal surface significantly alters the waiting period of nucleate boiling process, bubble size, growth and departure time, bubble density and superheat on heating surface. In essence, the presence of lubricant dramatically influences heat transfer performance. The proposed model is validated against some recent test data for R-134a/POE, R-1234ze/POE and R-134a/PVE refrigerant/lubricants mixtures. (C) 2020 Elsevier Ltd. All rights reserved.
Investigation of performance augmentation for natural convective heatsink with the help of chimney
(2020-09-01)
Abbas, Ali
;
Qasrawi, Fatima
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
The effect of adding a plastic chimney on the top of upward horizontal rectangular heatsink under natural convection was investigated experimentally and numerically. The effect of the chimney height, length, depth, inclination, view factor, and conductivity for different fins array dimensions were investigated as well. The presence of low thermal conductivity chimney induces airflow by creating effective pressure difference between the edge and the center of the chimney. Yet higher chimney height results in larger pressure difference. Yet the pressure difference is accentuated with a sharp edge chimney. A smooth edge chimney reduces the pressure gradient and entrained airflow. The chimney with low thermal conductivity is capable to entrain more airflow into the heatsink when comparing with high thermal conductivity chimney (aluminum). The entrained airflow can be as much as 25% higher than the metal chimney. A smaller fin spacing may block the airflow and reversed the heat transfer. For the present configuration, it is found that the chimney is only effective when the heatsink is placed horizontally upward. By employing a transparent chimney, the view factor of the heatsink can be appreciably improved when comparing with opaque chimney.
Thermal performance and entropy generation of novel X-structured double layered microchannel heat sinks
(2020-06-01)
Shen, Han
;
Xie, Gongnan
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
A novel X-structured Double layered Microchannel Heat Sink (X-DMHS) is proposed for performance augmentation on the traditional rectangular Double layered Microchannel Heat Sink (DMHS). The overall flow and thermal performances of parallel-flow and counter-flow arrangements on X-DMHSs are studied, and the entropy generation from the second law perspective is also quantitatively analyzed. From the analysis of the pressure drop penalty, thermal performance and entropy generation rate, it is found that the proposed X-DMHSs show much superior performance either from the heat transfer performance or from the second law perspective. The design offers higher entropy generation number and better thermal uniformity on substrate, meaning that proposed X-DMHSs can control the thermal gradients more significantly and lower the highest temperature on the heated substrate more effectively with affordable rise of pumping power as compared with the corresponding traditional DMHS. (C) 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Investigations regarding the influence of soft metal and low melting temperature alloy on thermal contact resistance
(2020-07-01)
Chu, Wen-Xiao
;
Khatiwada, Mahesh
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Wang, Chi-Chuan
;
機械工程學系
;
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%.
Energy-saving potential of separated two-phase thermosiphon loops for data center cooling
(2020-07-01)
Sulaiman, Mohammed W.
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Daraghmeh, Hafiz M.
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Wang, Chi-Chuan
;
機械工程學系
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Department of Mechanical Engineering
This study aims to experimentally examine the energy-saving potential by using R-134a filled separated two-phase thermosiphon loop (STPTL) for data center applications. A parametric study had been made to compare the energy consumption of two data center racks. Two fin-and-tube heat exchangers were attached to one of the racks to form two individual thermosiphon loops. The experiments were carried out subject to different operating conditions, including three ambient temperatures (20 degrees C, 23 degrees C, and 27 degrees C) and filling ratios ranging from 30 to 90% in association with heating loads ranging between 1.5 kW and 6 kW. Parametric influences regarding concentrated heat loading or uniform heat loading are studied. It was found that an appreciable energy-savings can be obtained at high filling ratios and a maximum of 49% energy-saving with the assistance of thermosiphon is observed. Accordingly, the rising of system pressure will result in noticeable savings. Relative to the uniform heat loading of the data rack, the thermosiphon shows even more energy-saving potential in concentrated heat loading. This phenomenon is more pronounced at a lower ambient temperature like 20 degrees C. On the other hand, there is no appreciable energy-saving for the thermosiphon between concentrated and uniform heating loads when the ambient temperature is high (27 degrees C). Furthermore, the influence of airflow rate was also investigated under various ambient temperatures with a 90% filling ratio and a heating load of 6 kW. The results revealed that the lower airflow rate in the thermosiphon yields comparatively better energy-saving than the higher flow rate. The study on the influence of using two STPTLs indicated that 15-23% energy-saving can be achieved at a 90% filling ratio and 6 kW heating load for all the studied ambient conditions if compared with testing each loop separately. Lower thermal resistance is seen at the higher filling ratios, ambient temperatures, and heating loads.
Performance improvement of photovoltaic modules via temperature homogeneity improvement
(2020-07-15)
Muneeshwaran, M.
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Sajjad, Uzair
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Ahmed, Tanveer
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Amer, Mohammed
;
Ali, Hafiz Muhammad
Though the PV module performance can be enhanced by cooling, the associated temperature nonuniformity across the PV panel can deteriorate its efficiency. In this study, a significant effort is conducted for an air-cooling system that can control the temperature uniformity across the PV panel. The cooling of a rooftop PV module is enabled by the air-conditioning unit installed on the building's roof. PV panel performance, with and without the cooling, is experimentally investigated for the uniform cold air duct. Experimental results showed that the cooled panel temperature is 6-12 degrees C lower than that of the uncooled one. However, even with the cooling, the temperature non-uniformity of 4-7 degrees C was noticed across the PV panel. To improve the temperature uniformity across the module, converging cold air ducts with different area ratios (AR) of 0.667 and 0.333 were designed and analyzed numerically. The effect of inlet air temperature (Tin-air = 18-23 degrees C) and inlet airflow velocity (U = 2-3 m/s) on panel temperature were also studied. The results suggested that the PV panel with CCD having an AR of 0.333 exhibited a temperature non-uniformity of 1.5-2.5 degrees C and demonstrated a 17-22% improvement in module efficiency under Tin-air = 18 degrees C and U = 3 m/s. (C) 2020 Elsevier Ltd. All rights reserved.
Non-Uniform Three-Dimensional Pulsating Heat Pipe for Anti-Gravity High-Flux Applications
(2020-06-01)
Tseng, Chih-Yung
;
Yang, Kai-Shing
;
Wang, Chi-Chuan
;
交大名義發表
;
National Chiao Tung University
This study proposes a novel high-flux pulsating heat pipe that can lift the major constraint of the conventional pulsating heat pipe (PHP) which is unable to function properly upon anti-gravity operations. The proposed PHP introduces additional unbalance force via uneven tube diameter/geometry in the adiabatic sections to tailor the problem in anti-gravity operation. The design contains a three-dimensional configuration circuitry with compact arrangement tubes on the evaporator and condenser. Through this design, the non-uniform three-dimensional pulsating heat pipe (3D-PHP) manipulates the uneven inner diameters of the adiabatic sections to form uneven vapor/liquid distributions in the adiabatic sections to yield a unitary flow pattern that is able to withstand a much higher input power. The present PHP uses methanol as working fluid, with 38% volumetric filling ratio, and has a high-flux of 22.9 W/cm(2)and a low the thermal resistance ratio (Ranti-gravity/Rgravity-assisted) of 1.05 when the input power is 800 W. Both the heat flux and thermal resistance ratio for the proposed design are far better than the existing literature.
OPTIMIZATION OF THE LOUVER FIN-AND-TUBE HEAT EXCHANGERS-A PARAMETRIC APPROACH
(2020-01-01)
Sadeghianjahromi, Ali
;
Kheradmand, Saeid
;
Nemati, Hossain
;
Wang, Chi-Chuan
;
機械工程學系
3D turbulent flow numerical simulations are performed to study heat transfer and flow friction characteristics of louver fin-and-tube heat exchangers. The effects of louver angle, fin pitch, transversal tube pitch, and longitudinal tube pitch on Colburn and friction factors are investigated in detail. Three stages of the effect of louver angle on the Colburn j factor are identified, including rising, level-off, and increasing again. A wide range for louver angle ranging from 0 degrees (plain fin) to 80 degrees is considered. Results show that the Colburn factor increases from a louver angle of 0 degrees (plain fin) to 20 degrees, followed by a level-off or slight decline until the louver angle reaches 45 degrees, and finally, the Colburn factor is increased again with a further rise of louver angle. However, the friction factor increases with an increase in louver angle from 0 degrees (plain fin) to 80 degrees. Colburn and friction factors are almost independent of fin pitch while they decrease with an increase in transversal and longitudinal tube pitches. The optimum louver angle is around 20 degrees upon carrying out the full factorial method in maximizing Colburn factor and minimizing friction factor.
A Novel Means Combining Corona Discharge and Electrostatic Force-Induced Vibration for Convective Heat Transfer
(2020-08-01)
Tsui, Yeng-Yung
;
Wei, Ting-Kai
;
Wang, Chi-Chuan
;
機械工程學系
;
Department of Mechanical Engineering
A new design, not reported in the existing literature, combining features of ionic wind and mechanical vibration to induce appreciable airflow is developed. Its feasibility is demonstrated in a cooling system to enhance heat transfer. Ionic wind is generated using a thin, flexible plate as the emitting electrode and a heated, vertical plate as the collecting electrode. By placing a metal inductor close to the discharge electrode, an electrostatic field is formed. The electrode is attracted and thus moves toward the inductor owing to the electrostatic force created. To sustain periodic oscillation and produce large vibrational amplitudes, the inductor is grounded using current-limiting resistors. Vibrational characteristics are highly dependent on the corona voltage, resistance of the resistor, and position of the induction plate, which are examined in the experiments. It was found that the heat transfer enhancement is not improved at high corona voltages because the ionic wind overwhelms the mechanical effect of vibration. The vibrational effect becomes more prominent at low corona voltages with which the electrical field created by the corona discharge is not so intense. The maximum increase of heat transfer coefficient over that without vibration can be as large as 13.4% at the lowest corona voltage considered in the tests.

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