Browsing by Author "Wang, Chi-Chuan"

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A 3-D NUMERICAL ANALYSIS OF THE EFFECT OF FIN PATTERN ON THE THERMO-HYDRAULIC PERFORMANCE OF PLATE HEAT EXCHANGERS
(2014-01-01)
Li, Kun-Hao
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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 numerically examines some commercially available plate patterns of plate heat exchangers using a 3-D CFD simulation. Detailed flow distribution and heat transfer characteristics subject to three different plate patterns are examined in this study. The plate pattern include GC26 and NT10 (double chevron) and SW26. The effective surface area of the associated plate patterns are 0.8671, 0.6808, and 0.6721 m(2), respectively. The corresponding chevron angle are 33 degrees, 64 degrees (double chevron) and 61 degrees, repectively. The calculated results show that the heat transfer efficient for NT10 is higher than that of GC26 by approximately 6.35% and is higher than SW26 by 10.3%. The results indicate that the heat transfer characteristics for the double chevron plate outperform that of the single chevron plate. On the other hand, the pressure drop is also increased with the double chevron as well as chevron angle. However, it is found that the double chevron design provides a better heat transfer capability subject to identical pressure drop.
A Comparative Study of Nozzle/Diffuser Micropumps with Novel Valves
(2012-02-01)
Yang, Kai-Shing
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Chao, Tzu-Feng
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Chen, Ing Youn
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Wang, Chi-Chuan
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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.
A Comparative Study of the Oil-Free Centrifugal Water Chillers with the Flooded or Falling Film Evaporator-A Case Study
(2019-07-01)
Hung, Kuo-Shu
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Chung, Jenn-Chyi
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Liu, Chung-Che
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Lin, Jun-Jie
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Wang, Chi-Chuan
A comparative study regarding the performance of real-scale oil-free centrifugal chillers having the flooded evaporator or falling film evaporator was conducted in this study. The nominal capacity for the test chillers was around 200 similar to 230 USRT (US refrigeration ton) (703 similar to 809 kW). The compressors of the two chillers were identical and R-134a was used as the working fluid. Both evaporators employed the same enhanced tubes (GEWA-B) to fulfill phase change. Tests were conducted in full, 75%, 50%, and 25% loading. Test results indicate that both chillers contained a comparable system performance with an integrated part-load value of around 8.62 similar to 8.63. The overall heat transfer coefficient for the flooded evaporator was appreciably higher (20 similar to 40%) than the falling film evaporator. This is because the falling film flowrate was below the threshold value and the heat transfer was dominated by evaporation mode. Yet, the heat transfer performance for the falling film evaporator was further jeopardized due to starvation of the film flowrate (partial dry-out), especially in the middle or bottom of the tube bundle. This phenomenon became even more pronounced at partial loading (25%), whereas the flooded evaporator did not reveal such a performance dip at partial loading.
A Criterion of Heat Transfer Deterioration for Supercritical Organic Fluids Flowing Upward and Its Heat Transfer Correlation
(2020-02-01)
Li, Yung-Ming
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Liaw, Jane-Sunn
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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
The main objective of this study was to develop the supercritical heat transfer correlation applicable for organic fluids when flowing upward in smooth tubes based on the available experimental data. The organic fluids contain R-22, R-134a, R-245fa and Ethanol and the associated heat transfer characteristics were compared with non-organic fluids like water and carbon-dioxide (CO2). It was found that the limit heat flux may result in heat transfer deterioration (HTD) of organic fluid and the corresponding values are much smaller than water or CO2. A new criterion to predict the HTD was developed and this criterion yields the best predictive ability against database. It was found that HTD occurs can be well described by the acceleration parameter evaluated at the wall condition rather than at bulk condition. For estimation of the supercritical heat transfer coefficient (HTC) for organic fluid, the present study proposes a new correlation with a physically based correction factor, which gives satisfactory predictions against the HTC of supercritical organic fluid. The new correlation can offer the smallest average deviation of 0.007 and standard deviation of 0.181 among the existing correlations.
A generalized log-linear poisson-modeled correlation to predict the optimal heat rejection pressure of transcritical CO2 systems
(2018-01-01)
Sian, Rony A.
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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, a comprehensive literature survey of all the experimental data available in literature regarding optimal heat rejection pressure on transcritical carbon dioxide systems is carried out. Based on the phenomenological analysis and quantitative assessment of the data, a Poisson regression analysis is performed and a new dimensionless correlation to predict the optimal heat rejection pressure is developed. The influences of relevant parameters on heat rejection pressure, such as ambient temperature and temperature at the gas cooler are taken into account in the correlation development. The proposed correlation is tested against experimental measurements and is further validated by comparison to all existing experimental data collected from literature. Additionally, the predictive ability of the newly proposed correlation is discussed, tested against experimental data found in literature and compared with existing correlations. Upon comparison with existing correlations, the new generalized dimensionless correlation proposed in this study predicts the optimal heat rejection pressure with an average deviation of 1.31% and a standard deviation of 4.26 bar. Thus, becoming the most accurate correlation up to date, yet providing a much wider range of temperature applicability than the correlations currently available in literature.
A mechanistic model for nucleate boiling heat transfer performance with lubricant-refrigerant mixture
(2020-10-01)
Hung, Jung-Tsung
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Chen, Yu-Kai
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Shih, Hou-Kuang
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Wang, Chi-Chuan
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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.
A new correlation and the review of two-phase flow pressure change across sudden expansion in small channels
(2010-09-01)
Wang, Chi-Chuan
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Tseng, Chih-Yung
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Chen, Ing Youn
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機械工程學系
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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.
"A novel ""partial bypass"" concept to augment the performance of air-cooled heat exchangers"
(2012-09-01)
Wang, Chi-Chuan
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Chen, Kuan-Yu
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Liaw, Jane-Sunn
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Tseng, Chih-Yung
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機械工程學系
"This study proposes a novel ""partial bypass"" concept which facilitates the heat transfer augmentation without the pumping power penalty. The basic idea is to introduce bypass cold stream which bypasses the front part of the heat exchanger and mixes with the hot stream across the front part of the heat exchanger, and the mixed stream flows through the rear part of the heat exchanger. Through this concept, heat transfer performance at the front part is slightly reduced but the rear part is substantially increased. The concept can be implemented with either maintaining the same total flowrate, heat transfer rate but accompanies with a much lower pressure drop or it can be applied with the same pumping power but accompanies with a much higher heat transfer rate. It is found that the ""partial bypass"" concept is especially fruitful for a heat exchanger with a comparatively large number of transfer unit (NTU). The concept is especially helpful in the design of a deep tube bank operated at lower frontal velocity. For validating the proposed concept, the calculation is also compared with the experimental measurements, and good agreement amid the calculations and measurements are reported. (C) 2012 Elsevier Ltd. All rights reserved."
A novel "partial bypass" concept to augment the performance of air-cooled heat exchangers
(2012-09-01)
Wang, Chi-Chuan
;
Chen, Kuan-Yu
;
Liaw, Jane-Sunn
;
Tseng, Chih-Yung
;
機械工程學系
This study proposes a novel "partial bypass" concept which facilitates the heat transfer augmentation without the pumping power penalty. The basic idea is to introduce bypass cold stream which bypasses the front part of the heat exchanger and mixes with the hot stream across the front part of the heat exchanger, and the mixed stream flows through the rear part of the heat exchanger. Through this concept, heat transfer performance at the front part is slightly reduced but the rear part is substantially increased. The concept can be implemented with either maintaining the same total flowrate, heat transfer rate but accompanies with a much lower pressure drop or it can be applied with the same pumping power but accompanies with a much higher heat transfer rate. It is found that the "partial bypass" concept is especially fruitful for a heat exchanger with a comparatively large number of transfer unit (NTU). The concept is especially helpful in the design of a deep tube bank operated at lower frontal velocity. For validating the proposed concept, the calculation is also compared with the experimental measurements, and good agreement amid the calculations and measurements are reported. (C) 2012 Elsevier Ltd. All rights reserved.
A Novel Condensate-Free Refrigerated Cold Plate for Electronic Cooling
(2010-01-01)
Wu, Yu-Lieh
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Yang, Kai-Shing
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Chen, Min-Yi
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Wang, Chi-Chuan
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機械工程學系
A novel cooling design featuring a two-stage expansion process is proposed in this study. Without any help from insulation, the design can minimize or even entirely eliminate condensate formation outside the cold-plate surface even for a very low evaporation temperature. The design incorporates a double-pipe inlet/outlet and a two-container cold plate, and its performance is compared to a conventional cold plate. For the conventional cold plate, the outlet pressure/temperature hold quite steadily at a light heat load, and the outlet pressure/temperature shows a substantial rise when the refrigerant is completely evaporated. The outlet pressure/temperature of the condensate-free cold plate shows an opposite trend at a heavy heat loading. The benefit of this characteristic is that the wall surface temperature and the outlet temperature of the cold plate can be maintained comparatively more steady than the conventional cold plate.
A novel design of pulsating heat pipe with fewer turns applicable to all orientations
(2012-10-01)
Chien, Kuo-Hsiang
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Lin, Yur-Tsai
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Chen, Yi-Rong
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Yang, Kai-Shing
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Wang, Chi-Chuan
This study presents a novel pulsating heat pipe (PHP) concept that is functional even when PHP is with fewer turns and is operated horizontally. Two heat pipes were made of copper capillary tubes with an overall size of 122 mm x 57 mm x 5.5 mm is investigated, one had 16 parallel square channels having a uniform cross-section of 2 mm x 2 mm (uniform CLPHP), and the other had 16 alternative size of parallel square channels (non-uniform CLPHP; a cross-section 2 mm x 2 mm and a cross-section of 1 mm x 2 mm in alternating sequence). Test results showed that the performance of PHP rises with the inclination but the uniform channel CLPHP is not functional at horizontal configuration whereas the proposed non-uniform design is still functional even at horizontal arrangement. The thermal resistance for uniform PHP is relative insensitive to change of inclination when the inclination angle exceeds certain threshold value. (c) 2012 Elsevier Ltd. All rights reserved.
A novel double pipe pulsating heat pipe design to tackle inverted heat source arrangement
(2016-08-05)
Tseng, Chih-Yung
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Yang, Kai-Shing
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Chien, Kuo-Hsiang
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Wu, Shih-Kuo
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Wang, Chi-Chuan
Traditional pulsating heat pipe with fewer turns is functional only when the heat source is placed at the bottom while the heat sink is on top. This study proposes a novel design having alternative double-pipe tube diameter with some extra open connections of these double-pipes. By introducing extra un-balanced pressure force, capillary force and gravity force, the PHP can be easily starting up even for an opposite heat source/sink arrangement while the conventional single tube design fails to start up with this opposite arrangement. The thermal resistance of double pipe PHP can be lowered to 0.0729 W/K with an effective thermal conductivity of 12,603 W/m.K when the heat source is placed on top. Yet the thermal resistance for the proposed double pipe design is only half of the single tube design. (C) 2016 Elsevier Ltd. All rights reserved.
A novel heat dissipation fin design applicable for natural convection augmentation
(2014-12-01)
Charles, Roody
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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, 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.
A Novel Means Combining Corona Discharge and Electrostatic Force-Induced Vibration for Convective Heat Transfer
(2020-08-01)
Tsui, Yeng-Yung
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Wei, Ting-Kai
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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
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.
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 novel oxidized composite braided wires wick structure applicable for ultra-thin flattened heat pipes
(2017-11-01)
Yang, Kai-Shing
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Tu, Cheng-Wei
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Zhang, Wen-Hua
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Yeh, Chih-Ting
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Wang, Chi-Chuan
This present study proposed a novel low-cost and high performance braided wire wick structure having superhydrophilic treatment applicable for ultra-thin heat pipe. The test wick structures include mono and composite wick structure. The composite braided wire adopts two different diameter wire design, using a large diameter (0.1 mm) in the core while the exterior of the core is covered by smaller diameter copper wire (0.05 mm). The mono design contains only one uniform wire diameter (0.1 mm) in core region. Test results indicate that via oxidizing the surface of the braided wires can largely enhance the surface roughness and result in a much higher capillary force as comparing to the un-oxidized one. In addition, both mono (m-FHP) and composite braided wire (c-FHP) with oxidization could possess a maximum heat transfer capability over 15 W under horizontal operation, and test results showed that the heat transfer capability of composite design outperforms mono one by more than 32.5% in all orientations. It is also found that the filling loading ratio plays essential role on the overall performance of FHP. Either a low filling loading ratio or a high filling loading ratio may yield a lower maximum heat transfer capability and a higher thermal resistance. An optimum filling loading ratio of 125% yields the lowest thermal resistance around 0.12 K/W with the best heat transfer capability of 20W.
A Novel Thermal Module with 3-D Configuration Pulsating Heat Pipe for High-Flux Applications
(2018-12-01)
Tseng, Chih-Yung
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Wu, Ho-Meng
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Wong, Shwin-Chung
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Yang, Kai-Shing
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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.
A novel trapezoid fin pattern applicable for air-cooled heat sink
(2015-11-01)
Chen, Chien-Hung
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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
The present study proposed a novel step or trapezoid surface design applicable to air-cooled heat sink under cross flow condition. A total of five heat sinks were made and tested, and the corresponding fin patterns are (a) plate fin; (b) step fin (step 1/3, 3 steps); (c) 2-step fin (step 1/2, 2 steps); (d) trapezoid fin (trap 1/3, cutting 1/3 length from the rear end) and (e) trapezoid fin (trap 1/2, cutting 1/2 length from the rear end). The design is based on the heat transfer augmentation via (1) longer perimeter of entrance region and (2) larger effective temperature difference at the rear part of the heat sink. From the test results, it is found that either step or trapezoid design can provide a higher heat transfer conductance and a lower pressure drop at a specified frontal velocity. The effective conductance of trap 1/3 design exceeds that of plate surface by approximately 38 % at a frontal velocity of 5 m s(-1) while retains a lower pressure drop of 20 % with its surface area being reduced by 20.6 %. For comparisons exploiting the overall thermal resistance versus pumping power, the resultant thermal resistance of the proposed trapezoid design 1/3, still reveals a 10 % lower thermal resistance than the plate fin surface at a specified pumping power.
A numerical investigation of the geometric effects on the performance of plate finned-tube heat exchanger
(2011-03-01)
Lu, Chi-Wen
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Huang, Jeng-Min
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Nien, W. C.
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Wang, Chi-Chuan
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機械工程學系
This study numerically examines the geometric parameters on the performance of a two-row fin-and-tube heat exchanger. Effects of fin pitch, tube pitch, fin thickness, and tube diameter are termed with. The simulation indicates that the performance, in terms of Q/Delta P and COP, increases with longitudinal tube pitch or with transverse tube pitch, and it decreases with larger tube diameter or fin thickness. An optimum value for Q/Delta P occurs at a 6-8 fpi at a fixed flow rate condition. There is not much difference in choosing the index of Q/Delta P or COP under fixed flow rate condition. However, when the simulation are performed with the actual axial fan whose P-Q curve being implemented. It is found that Q/Delta P peaks at 12 fpi while COP peaks at 16 fpi. (C) 2010 Elsevier Ltd. All rights reserved.
A numerical study of the supercritical CO2 plate heat exchanger subject to U-type, Z-type, and multi-pass arrangements
(2018-01-01)
Zhu, Chen-Xi
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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 proposes a numerical model for plate heat exchanger that is capable of handling supercritical CO2 fluid. The plate heat exchangers under investigation include Z-type (1-pass), U-type (1-pass), and 1-2 pass configurations. The plate spacing is 2.9 mm with a plate thickness of 0.8 mm, and the size of the plate is 600 mm wide and 218 mm in height with 60 degrees chevron angle. The proposed model takes into account the influence of gigantic change of CO2 properties. The simulation is first compared with some existing data for water-to-water plate heat exchangers with good agreements. The flow distribution, pressure drop, and heat transfer performance subject to the supercritical CO2 in plate heat exchangers are then investigated. It is found that the flow velocity increases consecutively from the entrance plate toward the last plate for the Z-type arrangement, and this is applicable for either water side or CO2 side. However, the flow distribution of the U-type arrangement in the water side shows opposite trend. Conversely, the flow distribution for U-type arrangement of CO2 depends on the specific flow ratio (C*). A lower C* like 0.1 may reverse the distribution, i.e. the flow velocity increases moderately alongside the plate channel like Z-type while a large C* of 1 would resemble the typical distribution in water channel. The flow distribution of CO2 side at the first and last plate shows a pronounced drop/surge phenomenon while the channels in water side does not reveal this kind of behavior. The performance of 2-pass plate heat exchanger, in terms of heat transfer rate, is better than that of 1-pass design only when C* is comparatively small (C* < 0.5). Multi-pass design is more effective when the dominant thermal resistance falls in the CO2 side.