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    ABSTRACT: New energy and energy saving are the two basic energy issues. This study proposes an innovative optimal method to design LEDs with high thermal spreading holder. The optimization uses the FEM combined with the simplified conjugated gradient method (SCGM). The minimal temperature of the MCPCB backside of the LEDs is obtained throughout this design. The simulations have been proofed by the experiment of IR and thermal couple measurement. The optimal temperature decreases at about 4 °C compared with the original temperature without any active cooling device. This design can prolong the LEDs׳ life to benefit the energy saving obviously. In addition, this design combined with the active cooling device will achieve better heat dissipation on cooling of the electronic component.
    Microelectronics Journal 04/2014; 45:904-909. · 0.91 Impact Factor
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    ABSTRACT: This study is focused on computation optimization of the geometry for the flow channels in a micro-reformer used for methanol steam reforming. Three-dimensional mass and momentum transport phenomena with a pure fluid simulation in a micro-reformer are predicted using a commercial computational fluid dynamics code. Meanwhile, a simplified conjugate-gradient method is adopted to seek the optimal manifold shape and channel width of the micro-reformer iteratively using a Python interface. In the present study, the geometrical optimization tasks involve the designs of the inlet manifold and outlet manifold shapes as well as channel width distribution, and the design purpose is to obtain a uniform flow distribution throughout the entire micro-reformer so as to increase the hydrogen gas production rate. Cubic-spline interpolation is used in shape design to fit the points on the manifold shape more smoothly. The results show that the velocity standard deviation decreased from 0.14 to 0.048 and 0.051 after searching the optimal manifold shapes and channel widths, respectively. The manifold shapes of the inlet and outlet as well as the channel widths can efficiently lead to significant uniformity in the flow fields using a simplified conjugate-gradient method.
    Energy 01/2014; · 4.16 Impact Factor
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    Chin-Hsiang Cheng, Yen-Fei Chen
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    ABSTRACT: A novel computational approach based on a non-constrained formulation with a volume-of-solid (VOS) function equation is firstly presented for topology design of heat conductive solid paths between constant-temperature objects. In the first step of the approach, the distributions of the VOS function and the temperature in the original design domain are carried out by simultaneously solving the VOS function equation and the heat conduction equation. Secondly, the shape outline of the heat conduction path leading to a maximum heat transfer rate per unit solid mass is determined by selecting a cut-off value of the VOS function. Performance of this approach is tested for three two-dimensional test cases. Various thermal boundary configurations are taken into consideration to demonstrate the validity of the present method. Results show that the present computational method is capable of predicting the optimal shapes of the heat conduction paths for the test cases efficiently.
    International Journal of Thermal Sciences 01/2014; 78:16–25. · 2.47 Impact Factor
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    ABSTRACT: This paper examines the field emission synthesis of multi-walled carbon nanotubes (MWCNTs) grown on various interlayers on carbon cloth (CC) substrates, such as Ni/CC, Ni/Ti/CC, and Ni/Al/CC, by thermal chemical vapor deposition using ethylene (C2H4) as the carbon source and nickel (Ni) as the catalyst. The Al interlayer activates the substrate for MWCNTs growth on carbon cloth with Ni as the active catalyst. Field emission SEM was utilized to study the morphology of the catalyst and MWCNTs, and a Raman spectrometer was used to characterize the quality of CNTs. As a result, catalyst particles initiate the nanotube growth and the tube diameters were found to be governed by the size of the associated particles. In our experiment, the smallest size and highest density of catalyst nanoparticles was formed on Al/CC substrate, so that the MWCNTs have a smaller diameter and higher density on this than on Ni/CC and Ni/Ti/CC substrate. Therefore, MWCNTs grown on flexible carbon cloth with an Al interlayer have the best quality and field emission characteristics of the materials examined in this paper.
    Current Nanoscience 01/2014; 10(4). · 1.36 Impact Factor
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    Yu-Xian Huang, Jiin-Yuh Jang, Chin-Hsiang Cheng
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    ABSTRACT: The aim of the present article is to study the fractal channel pattern design and the gradient catalyst layer in relation to their effects on the performance of a micro methanol steam reformer. A three-dimensional simulation model is established for the purpose of predicting the effects of bio-channel design on the performance of a micro-reformer. The CO concentration in the production gases, which is necessary to avoid the poisoned catalyst layers of low temperature fuel cells, is also investigated. In addition, the distributions of velocity and gas concentrations are predicted, and the methanol conversion ratios are also evaluated. Due to further decreases of the CO in product gases, a gradient catalyst layer arrangement is proposed to delay the timing of hydrogen generation and thus avoid the presence of hydrogen in the catalyst layer too long. This catalyst arrangement can effectively decrease the possibility of a reverse water gas shift reaction to reduce CO generation. Results showed that the fractal channel design increases the conversion ratio, decrease CO as well as decrease the pressure drop in the channels. Relative to a parallel channel design, the CO and methanol conversion ratio of this fractal channel design pattern with uniform catalyst layer can be decreased and increased by 17% and 8%, respectively, based on a 0.3 cc/min flow rate, respectively. Meanwhile, the pressure drops in the parallel channel design and in the fractal channel design were found to be 254 Pa and 51 Pa, respectively. From an energy consumption point of view, a low pressure drop also implies low input pumping power. Furthermore, compared to the fractal design with a uniform catalyst layer, the gradient catalyst layer was demonstrated to effectively increase the conversion ratio by 8.5% and decrease CO by 11% when the inlet liquid flow rate was fixed at 1.0 cc/min.
    International Journal of Hydrogen Energy 01/2014; 39(5):1998–2007. · 3.55 Impact Factor
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    ABSTRACT: The present study is concerned with dynamic simulation of thermal-lag Stirling engines. A dynamic model is built and incorporated with a thermodynamic model to study the engine start process. A prototype engine is designed and simulated by using the dynamic model. In the simulation, different operating modes, including rotating mode, swinging mode, swinging-to-rotate mode, and swinging-to-decay mode, have been observed. The rotating mode is desired and can be achieved if the operating parameters are properly designed. In a poor design, the engine may switch to the swinging or even the swinging-to-decay mode. In addition, it is found that geometric parameters, such as bore size, stroke, and volume of working spaces, also determine the operating mode of the engine. Brake thermal efficiency of the engine is monotonically reduced by increasing engine speed. However, study of the dependence of the shaft power of the engine speed shows that there exists a maximum value of the shaft power at an optimal operating engine speed. The optimal engine speed leading to maximum shaft power is significantly influenced by the geometrical parameters.
    Applied Energy. 10/2013; 108:466–476.
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    ABSTRACT: The aim of this work is to investigate argon scattering behaviors on the smooth and rough tungsten surfaces. Current work deals with numerical simulation of nanoscale heat transfer process accompanying with rarefied gas-solid substrate interactions using molecular dynamics (MD) method. Taking into account that this method is very time consuming, MD simulation using CUDA capable Graphic Cards is implemented. The results found that imperfection of the surface significantly influences on gas atom's momentum change upon collision. However, the energy exchange rate remains unchanged regardless to the surface roughness. This finding is in contrast with the results in extant literatures. We believed the results found in this paper are important for both numerical and theoretical analyses of rarefied gas flow in micro- and nano-systems where the choice of boundary conditions significantly influences flow.
    Journal of molecular graphics & modelling 08/2013; 45C:45-49. · 2.17 Impact Factor
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    Chi-Kang Chan, Chin-Hsiang Cheng
    08/2013;
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    ABSTRACT: This paper develops an inverse problem approach to optimize the geometric structure of TECs (thermoelectric coolers). The approach integrates a complete multi-physics TEC model and a simplified conjugate-gradient method. The present TEC model couples the heat and electrical conductions and accounts for all physical mechanisms occurred within TECs. Three geometric parameters, the semiconductor pair number, N, leg length of semiconductor column, Hpn, and base area ratio of semiconductor columns to TEC, g, are optimized simultaneously at fixed current and fixed temperature difference. The cooling rate on the cold end is the objective function to be maximized to obtain the optimal TEC geometry. The effects of applied current and temperature difference on the optimal geometry are discussed. The results show that at temperature difference of 20 K, the geometry optimization increases the TEC cooling rate by 1.99-10.21 times compared with the initial TEC geometry, and the optimal N decreases from 100 to 47 with invariable r=0.95 and Hpn=0.2 mm, as the applied current varies from 1.0 A to 3.0 A. With the increase in temperature difference, the optimal N increases at smaller currents of I �<1.0 A, however, it is almost invariable at larger currents of I > 1.5 A.
    Energy 08/2013; 59:689-697. · 4.16 Impact Factor
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    Chin-Hsiang Cheng, Hang-Suin Yang, Lam Keong
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    ABSTRACT: In this study, a beta-type 300-W Stirling engine is developed and tested, and a non-ideal adiabatic model is built and applied to predict performance of the engine. Engine torque, engine speed and shaft power output are measured under various operating conditions. The experiments are conducted for two different working gases (air and helium) and at various charged pressures and heating temperatures. Effects of regenerator wire mesh on the shaft power output are also examined. Results show that the shaft power output of the engine is much higher using helium as the working fluid than using air. Furthermore, as the charged pressure and the heating temperature are set at 8 bars and 850 �C and a No. 120 wire mesh is used in the regenerator, the shaft power of the engine can reach 390 W at 1400 rpm with 1.21-kW input heat transfer rate (32.2% thermal efficiency). The experimental data are compared with the numerical predictions to verify the theoretical model. It is found that the experimental data of the shaft power output closely agree with the numerical predictions. This implies that the theoretical model is valid and helpful in the engine design.
    Energy 08/2013; 59:590-599. · 4.16 Impact Factor
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    ABSTRACT: Residual stress in MEMS is of inherent importance in various respects. This study proposes a specific method using ANSYS including the birth and death method and combined with the optimal method (SCGM) to reduce the residual stresses during the CMOS fabrication process. The suitable cooling temperature for decreasing the residual stress is proposed and available. It demonstrates that the suitable parameter on the fabrication can reduce the residual stress in MEMS devices without any extra manufacturing process or external apparatus. The proposed method can expand to simulate the realistic MEMS model effectively.
    JOURNAL OF MECHANICS 08/2013; · 0.33 Impact Factor
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    M S Ozhgibesov, T S Leu, C H Cheng
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    ABSTRACT: The aim of this work is to investigate argon flow behaviors through the channels with three types of boundary conditions. Current work deals with numerical simulations of rarefied gas flow through nano-channels using the Molecular Dynamics method. Taking into account that this method is very time consuming, we implemented all the simulations using CUDA capable graphic cards. We found that the well-known and relatively simple Maxwell model of boundary conditions is able to reproduce gas flow through a tungsten channel with irregularities and roughness, while it results in a significant error in the case of a smooth metal surface. We further found that the flow rate through a relatively short channel correlates nonlinearly with the channel's length. This finding is in contrast with the results available in extant literature. Our results are important for both numerical and theoretical analyses of rarefied gas flow in micro- and nano-systems where the choice of boundary conditions significantly influences flow.
    Journal of molecular graphics & modelling 03/2013; 42C:32-38. · 2.17 Impact Factor
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    Chin-Lung Chen, Chin-Hsiang Cheng
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    ABSTRACT: Numerical study has been performed to investigate the effects of cavity shape on flow and heat transfer characteristics of the lid-driven cavity flows. Dependence of flow and thermal behaviour on the aspect ratio of the cavities is also evaluated. Three types of the cross-sectional shape, namely, circular, triangular, and rectangular, and four aspect ratios, 0.133, 0.207, 0.288, and 0.5, are taken into account to construct twelve possible combinations; however, attention is focused on the small-aspect-ratio situations. Value of the Reynolds number considered in this study is varied between 100 and 1800. For the cases considered in this study a major clockwise vortex driven by the moving lid prevailing in the cavity is always observed. When the Reynolds number is fixed, the rectangular cavity produces strongest lid-driven flow, and the triangular cavity weakest. For the cases at small aspect ratio and low Reynolds number, the streamlines appear symmetric fore-and-aft with respect to the central line at x/L = 0.5. Data for the local and average Nusselt numbers are also provided. For rectangular cavities, it is observed that case 1/5R produces the highest average Nusselt number at any Reynolds number. Among the twelve possible geometric cases considered herein, the highest and lowest average Nusselt numbers are found with cases 1/6T and 1/2C, respectively.
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    Chin-Lung Chen, Chin-Hsiang Cheng
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    ABSTRACT: Numerical study has been performed to investigate the effects of cavity shape on flow and heat transfer characteristics of the lid-driven cavity flows. Dependence of flow and thermal behaviour on the aspect ratio of the cavities is also evaluated. Three types of the cross-sectional shape, namely, circular, triangular, and rectangular, and four aspect ratios, 0.133, 0.207, 0.288, and 0.5, are taken into account to construct twelve possible combinations; however, attention is focused on the small-aspect-ratio situations. Value of the Reynolds number considered in this study is varied between 100 and 1800. For the cases considered in this study a major clockwise vortex driven by the moving lid prevailing in the cavity is always observed. When the Reynolds number is fixed, the rectangular cavity produces strongest lid-driven flow, and the triangular cavity weakest. For the cases at small aspect ratio and low Reynolds number, the streamlines appear symmetric fore-and-aft with respect to the central line at x/L = 0.5. Data for the local and average Nusselt numbers are also provided. For rectangular cavities, it is observed that case 1/5R produces the highest average Nusselt number at any Reynolds number. Among the twelve possible geometric cases considered herein, the highest and lowest average Nusselt numbers are found with cases 1/6T and 1/2C, respectively.
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    Chin-Hsiang Cheng, Hang-Suin Yang
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    ABSTRACT: In the present study, optimization of rhombic drive mechanism used in a beta-type Stirling engine is performed based on a dimensionless theoretical model toward maximization of shaft work output. Displacements of the piston and the displacer with the rhombic drive mechanism and variations of volumes and pressure in the chambers of the engine are firstly expressed in dimensionless form. Secondly, Schmidt analysis is incorporated with Senft's shaft work theory to build a dimensionless thermodynamic model, which is employed to yield the dimensionless shaft work. The dimensionless model is verified with experimental data. It is found that the relative error between the experimental and the theoretical data in dimensionless shaft work is lower than 5.2%. This model is also employed to investigate the effects of the influential geometric parameters on the shaft work, and the optimization of these parameters is attempted. Eventually, design charts that help design the optimal geometry of the rhombic drive mechanism are presented in this report.
    Energy 01/2013; · 4.16 Impact Factor
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    Chin-Hsiang Cheng, Hang-Suin Yang
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    ABSTRACT: A theoretical model for predicting thermodynamic behavior of thermal-lag Stirling engine is presented in this study. Without a displacer and its link, the thermal-lag engine contains only a moving part (piston) and a static part (regenerative heater) in engine’s cylinder and hence, is regarded as a unique type of Stirling engines that featuring rather simple mechanical structure. In this study, a numerical simulation of thermodynamic behavior of the thermal-lag Stirling engine is performed based on the theoretical model developed. Transient variations of temperatures, pressures, pressure difference, and working fluid masses in the individual working spaces of the engine are predicted. Dependence of indicated power and thermal efficiency on engine speed has been investigated. Then, optimal engine speeds at which the engine may reach its maximum power output and/or maximum thermal efficiency is determined. Furthermore, effects of geometrical and operating parameters, such as heating and cooling temperatures, volumes of the chambers, thermal resistances, stroke of piston, and bore size on indicated power output and thermal efficiency are also evaluated.
    Energy 12/2012; 49:218-228. · 4.16 Impact Factor
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    ABSTRACT: Thermoelectric devices (TEs) can achieve direct conversion of heat and electricity by semiconductor materials, coupling of heat transfer and electric conduction is important to accurately predict the performance of TEs. This paper develops a general, three-dimensional numerical model of TEs with consideration of coupling of temperature field and electric potential field. The model is used to figure out the performance of thermoelectric coolers (TECs) with the temperature-dependent thermal conductivity, electric conductivity, and Seebeck coefficient of semiconductor materials. A miniature TEC is considered and Bi2(Te0.94Se0.06)3 and (Bi0.25Sb0.75)Te3 are selected as the n-type and p-type thermoelectric materials, respectively. The effect of parameters such as the temperature difference and the current is investigated under conditions of variable material properties as well as radiation and convection heat transfer occurred between the TEC and the ambient gas. The results show that the variable properties and the heat losses to the ambient gas have significant effects on the cooling capacity and the coefficient of performance (COP) of the TEC. Three-dimensional temperature distributions within the semiconductors is observed under convective boundary condition and it becomes remarkable at large temperature differences and high currents.
    Energy 12/2012; 47:488-497. · 4.16 Impact Factor
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    ABSTRACT: This study describes the performance of micro methanol steam reformers with channel widths optimized using the simplified conjugate gradient method (SCGM), which uses a minimum objective function of the H2 mass fraction standard deviation in channels. A three-dimensional numerical model and optimal simplified conjugate gradient algorithm were built to predict and search for the effects of channel widths and flow rate on the performance of chemical reactions. Furthermore, this simulation model was compared to; and corresponded well with existing experimental data. Distributions of velocity, temperature, and gas concentrations (CH3OH, CO, H2, and CO2) were predicted, and the methanol conversion ratio was also evaluated. The mole fraction of CO contained in the reformed gas, which is essential to preventing poisoning of the catalyst layers of fuel cells, is also investigated. In the optimization search process, the governing equations use the continuity, momentum, heat transfer, and species equations to evaluate the performance of the steam reformer. The results show that channel width optimization can not only increase the methanol conversion ratio and hydrogen production rate but also decrease the concentration of carbon monoxide. The velocity and mixture gas density distributions in channels are discussed and plotted at various locations for an inlet liquid flow rate of 0.3 cc min−1. Full development is not obtained in the downstream channel flow, the velocity in channel is increased from 1.28 m s−1 to 2.36 m s−1 at location Y = 1 mm–32 mm, respectively. This can be attributed to a continuous increase in the lightweight H2 species as a result of chemical reactions in the channels.
    International Journal of Hydrogen Energy 11/2012; 37(22):16974–16985. · 3.55 Impact Factor
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    Jiin-Yuh Jang, Chin-Hsiang Cheng, Yu-Xian Huang
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    ABSTRACT: The present study is aimed at integrating a power system for reformer, PrOx and PEM fuel cells using a simulation model. A three-dimensional numerical model is established for predicting the effects of channel patterns and the inlet and outlet manifold configuration with a fixed inlet flow rate on a reformer. Distributions of velocity, gases concentrations and current density are predicted, and the methanol conversion ratios are evaluated as well. In addition, the mole fraction of CO is contained and removed in the reformer and PrOx reactions, respectively. The solution model is used to improve the design of both the micro reformer and the fuel cell. Results showed that a CO mole fraction can be decreased through the PrOx reaction effectively; furthermore, the methanol conversion ratio and the concentration of hydrogen can be improved from 83% to 99% and from 67.11% to 74.4%, respectively. Additionally, the relative standard deviations of velocity in channels are decreased from 58.68% to 0.048%, according to the manifold configuration design; from the point of view of a fuel cell, high fuel usage and current density are obtained using a z-serpentine channel pattern. The current density is increased by 184% on the basis of inlet flow rate for a steam reformer from Design 1 to Design 2.
    International Journal of Hydrogen Energy 09/2012; 37(18):13797–13805. · 3.55 Impact Factor
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    Chin-Hsiang Cheng, Shu-Yu Huang
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    ABSTRACT: In this study, numerical simulation of transient thermal behavior of thermoelectric coolers is attempted based on a non-uniform-current model. In the present model, the thermoelectric cooler is divided into seven blocks, including aluminum plates, ceramic plates, and P-type and N-type thermoelectric elements. Numerical solution for three-dimensional distributions of electric voltage and electric current vectors in the thermoelectric cooler are firstly obtained. The obtained distribution of the electric current vectors is introduced into energy equation for the solutions of the transient thermal behavior of thermoelectric coolers. A parametric study is performed to investigate the effects of the influential parameters including applied electric current, heat transfer coefficient on the ceramic plate of hot end, heat transfer coefficient on surfaces of the P-type and N-type elements, height of the P-type and N-type elements, and cooling load per unit area. To verify the numerical predictions, experimental data have been presented for variation of temperatures of both cold and hot ends. It is observed that the numerical predictions closely agree with the experimental temperature data.
    Applied Energy 08/2012; 100:326-335. · 5.26 Impact Factor

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