Browsing by Author "Chu, HS"

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Analysis of thermal and water management with temperature-dependent diffusion effects in membrane of proton exchange membrane fuel cells
(2004-04-22)
Yan, WM
;
Chen, FL
;
Wu, HY
;
Soong, CY
;
Chu, HS
In the present work, the detailed thermal and water management in the membrane of proton exchange membrane fuel cells (PEMFC) is investigated numerically. The coupling effects of mass diffusion and temperature gradient on the water distribution in the membrane are taken into account with consideration of the temperature-dependent diffusivity. Thermal and water transport equations with various boundary conditions are solved by the control volume finite difference method. Predictions show that under the conditions of fixed water concentration at the cathode side, the effect of cathode temperature, T-c, on the water concentration is significant. Increases in T-c may lead to an increase in membrane dehydration. At the water-flux condition on the cathode side, the influence of the operating temperature on the water distribution in the membrane shows a similar trend. The effects of the anode temperature, T-a, on the water management in the membrane are also examined. It is found that T-a has considerable impact on the water content in the membrane. In addition, high current density may cause non-uniformity of the temperature distribution in the membrane. (C) 2003 Elsevier B.V. All rights reserved.
Application of inverse problem algorithm for temperature uniformity in rapid thermal processing
(2002-01-01)
Lin, S
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Chu, HS
;
機械工程學系
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Department of Mechanical Engineering
This article presents a finite-difference-method formulation to the application of inverse problem algorithms for uniform temperature tracking of several different linear ramp-up rates in rapid thermal processing. A one-dimensional thermal model and temperature-dependent thermal properties of silicon wafers are used. The required incident-heat-flux profiles for temperature uniformity across 300-mm-diameter 0.775-mm-thick silicon wafer were intuitively evaluated. Our numerical results indicate theft temperature non-uniformity occurring during the ramp increase with the ramp-up rate. Although a linear ramp-up rate of 300 degreesC/s was used and random errors did reach 3.864 degreesC, the temperature over the wafer was maintained within 0.665 degreesC of the wafer center if the incident-heat-flux profiles were dynamically controlled according to the inverse results. These temperature non-uniformities could be acceptable in the advanced rapid thermal processing system. (C) 2002 Elsevier Science B.V. All rights reserved.
Combined conduction and radiation heat transfer in plane-parallel packed beds with variable porosity
(1999-03-01)
Wu, JW
;
Chu, HS
;
機械工程學系
;
Department of Mechanical Engineering
The primary concern of this study deals with the prediction of thermal performance of packed beds with variable porosity. Combined conduction and radiation heat transfer in plane-parallel packed beds of spherical particles was investigated numerically. The effects of variable porosity, effective thermal conductivity, and thermal radiation were taken into account. The method for radiative transfer equation was based on the P-3-spherical harmonic approximation, while the finite-difference scheme was employed to solve the energy equation. Four different models of porosity distributions were tested. It was found that variable porosity and effective thermal conductivity effects on the mean total effective thermal conductivity and radiative heat flux distributions are important and should not be ignored. (C) 1999 Elsevier Science Ltd. All rights reserved.
Combined natural convection and radiation in a vertical annulus
(1996-08-01)
Weng, LC
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Chu, HS
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機械工程學系
;
Department of Mechanical Engineering
This paper presents a numerical study of the effects of thermal radiation on the heat transfer of steady natural convection in a participating fluid contained inside a cylindrical annulus. The fluid absorbs and emits radiation energy. Uniform temperature difference is imposed across the inner and outer cylinder walls. The radiation part of the problem is treated by using the two-dimensional P-l approximation. The numerical calculations were executed using the PROJECTION algorithm with a power law scheme. The analysis are performed for Rayleigh number ranging from 10(4) to 10(6), aspect ratio varying between 2 and 10, and the radius ratio ranging from 1.25 to 10 for N=0.1, 1, and infinity where N is the conduction-to-radiation parameter. The thermal radiation greatly influences the flow field and temperature distribution inside the enclosure for different aspect ratio and radius ratios. Those results are presented vividly by the streamlines and isotherms. Based on the results, the average Nusselt number is correlated by a relation of the form <(Nu)over bar = a+blnA> and <(Nu)over bar = c+d> exp(-eta/e). The coefficients for various N are tabulated.
Combined natural convection and radiation in a vertical annulus
(1996-08-01)
Weng, LC
;
Chu, HS
;
機械工程學系
;
Department of Mechanical Engineering
This paper presents a numerical study of the effects of thermal radiation on the heat transfer of steady natural convection in a participating fluid contained inside a cylindrical annulus. The fluid absorbs and emits radiation energy. Uniform temperature difference is imposed across the inner and outer cylinder walls. The radiation part of the problem is treated by using the two-dimensional P-l approximation. The numerical calculations were executed using the PROJECTION algorithm with a power law scheme. The analysis are performed for Rayleigh number ranging from 10(4) to 10(6), aspect ratio varying between 2 and 10, and the radius ratio ranging from 1.25 to 10 for N=0.1, 1, and infinity where N is the conduction-to-radiation parameter. The thermal radiation greatly influences the flow field and temperature distribution inside the enclosure for different aspect ratio and radius ratios. Those results are presented vividly by the streamlines and isotherms. Based on the results, the average Nusselt number is correlated by a relation of the form <(Nu)over bar = a+blnA> and <(Nu)over bar = c+d> exp(-eta/e). The coefficients for various N are tabulated.
Effect of diatomic islands on step morphological stability of a terrace edge in molecular beam epitaxy
(1999-09-29)
Lin, SP
;
Hwang, CC
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Chu, HS
;
機械工程學系
;
Department of Mechanical Engineering
The effect of two-dimensional diatomic-island nucleation on the linear stability of the step morphology during molecular beam epitaxy is investigated numerically via shooting methods. It is found that the effect of diatomic islands on the step morphological stability is significant. The greater the effects of diatomic islands, the more stable the step morphology. increasing capture efficiency can decrease the critical surface capillary length and shift the critical wave number toward short-wavelength regimes. The unstable region is shrunk with increasing capture efficiency. Further, increasing flux coverage and/or surface coverage can decrease the critical surface capillary length and shift the critical wave number toward short-wavelength regimes. (C) 1999 Elsevier Science S.A. All rights reserved.
Effect of diatomic islands on step morphological stability of a terrace edge in molecular beam epitaxy (vol 353, pg 174, 1999)
(2000-01-31)
Lin, SP
;
Hwang, CC
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Chu, HS
;
機械工程學系
;
Department of Mechanical Engineering
Effect of interface thermal resistance on heat transfer in a composite medium using the thermal wave model
(2000-03-01)
Lor, WB
;
Chu, HS
;
機械工程學系
;
Department of Mechanical Engineering
This study analyzes how interface resistance affects heat transfer in a two-layered composite media under an incident pulse energy exerting on the exterior surface of one layer by using the hyperbolic heat conduction equation. The incident energy is absorbed and a pulse width temperature wave is generated within the skin depth of the first layer, which subsequently emanates to the second layer. Reflection and transmission occur when the initial pulse wave impacts the contact surface of the dissimilar material. In addition, the radiation-boundary-condition model based on Acoustic Mismatch Model or Diffuse Mismatch Model is used to predict the interface resistance. Analysis results indicate that the reflection-transmission combination phenomena strongly depend on the interface condition. Moreover, the thermal resistance restricts the energy transmission across the interface, creating a temperature difference at the interface and ultimately alerting the reflected wave feature. Our results further demonstrate that the critical thermal resistance magnitude to approach the perfect contact condition varies with the two-layered properties ratio and the absorption skin depth. (C) 1999 Elsevier Science Ltd. All rights reserved.
Effect of interface thermal resistance on thermal wave phenomena in a two-layer composite media
(1999)
Lor, WB
;
Chu, HS
;
機械工程學系
;
Department of Mechanical Engineering
This article numerically analyzes the heat conduction in a two layers composite media under an initial temperature pulse emanating from the exterior surface of one layer. The thermal wave model, which allows for both a transient heat flux and a finite speed of propagation, predicts significantly different results with those predicted by the Fourier's diffusion model at small time scale. The differences between the two heat transfer models diminish with increasing time. The thermal wave model further demonstrates that the internal reflection and transmission occur at the contact surface of the dissimilar material. To consider the thermal resistance at the contact interface, an interfacial-layer model (ILM) and a radiation heat flux model at the interface based on acoustic mismatch model (AMM) or diffuse mismatch model (DMM) are employed. Analysis results indicate that the thermal boundary resistance significantly influences the wave pattern and strength. The reflection-transmission-combination phenomena strongly varies with the two layers properties ratio, skin depth, and the interface condition.
Effects of flow distributor geometry and diffusion layer porosity on reactant gas transport and performance of proton exchange membrane fuel cells
(2004-01-02)
Yan, WM
;
Soong, CY
;
Chen, FL
;
Chu, HS
;
機械工程學系
A study has been conducted on the effects of flow distributor and diffusion layer geometries and morphologies on the transport phenomena of reactant gas from flow channels to gas diffuser, and on the performance of a proton exchange membrane fuel cell. The emphasis of the analysis is placed on the effects of the cross-section of the channel of the anode flow distributor and the porosity of the gas-diffusion layer. By applying a two-dimensional mass transport model, the effects of the channel width fraction, lambda= l(c)/l(b) the channel number, N, the porosity of the diffuser layer, E, and the surface overpotential of the catalyst layer, eta, on the resultant current density are investigated. The cell performance is evaluated by the predicted voltage-current density curves. It is disclosed that an increase in either;, Nor E may lead to a better cell performance. At relatively low overpotential, better uniformity in current density distribution along the width of the cell (x-direction) can be attained. Based on the present results, a correlation of overpotentials is proposed, namely, eta(cr) = 0.165lambda(0.0935) N-0.2033 epsilon(0.0856), which may serve as a guideline to justify the validity of a simplified one-dimensional model. (C) 2003 Elsevier B.V. All rights reserved.
Effects of porosity change of gas diffuser on performance of proton exchange membrane fuel cell
(2003-09-15)
Chu, HS
;
Yeh, C
;
Chen, F
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機械工程學系
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Department of Mechanical Engineering
An investigation is made of the effects of the change of the porosity of the gas diffuser layer (GDL) on the performance of a proton exchange membrane (PEM) fuel cell. The analysis of fuel cell performance with non-uniform porosity of GDL is a necessity because the presence of liquid water in the GDL leads to a non-uniformly distributed porosity in the GDL. To implement this performance analysis, a half-cell model which considers the oxygen mass fraction distribution in the gas channel, the GDL and the catalyst layer, and the current density and the membrane phase potential in the catalyst layer and the membrane is investigated. Four continuous functions of position are employed to describe the porosity, and differential equations are derived based on oxygen transportation and Ohm's law for proton migration and solved numerically. Results show that a fuel cell embedded with a GDL with a larger averaged porosity will consume a greater amount of oxygen, so that a higher current density is generated and a better fuel cell performance is obtained. This explains partly why fuel cell performance deteriorates significantly as the cathode is flooded with water (i.e. to give a lower effective porosity in the GDL). In terms of the system performance. a change in GDL porosity has virtually no influence on the level of polarization when the current density is medium or lower, but exerts a significant influence when the current density is high. This finding supports the scenario proposed by previous studies that the polarization at high current density corresponds mainly to mass transfer through (or the concentration activation of) the membrane assembly. (C) 2002 Elsevier Science B.V. All rights reserved.
Heat transfer in open cell polyurethane foam insulation
(1998-11-01)
Wu, JW
;
Chu, HS
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機械工程學系
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Department of Mechanical Engineering
This paper study systematic investigates the combined conductive and non-gray radiative heat transfer of open cell polyurethane (PU) foam in the pressure range between 760 and 0.02 Torr. Direct transmission measurements are also taken using Fourier transform infrared (FTIR) spectrometer. In doing so, experimental results are obtained for the spectral extinction coefficient from 2.5 to 25 mu m. In addition, the P-3 approximation method along with the box model is employed to calculate the non-gray radiative heat flux. The diffusion approximation method is also applied to calculated the radiative conductivity. Also tested herein are three samples with different cell sizes ranging from 330 to 147 mu m. According to those results, the spectral extinction coefficient increases with a decrease of cell size, leading to a decrease of thermal conductivity. Moreover, evacuating the gases in the foam cells can reduce the thermal conductivity of the PU foam by as much as 75%. Furthermore, radiative heat transfer accounts for about 4% of total heat transfer at 760 Torr and increases to 20% at 0.02 Torr.
Hyperbolic heat conduction in thin-film high T-c superconductors with interface thermal resistance
(1999-09-01)
Lor, WB
;
Chu, HS
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機械工程學系
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Department of Mechanical Engineering
This article numerically analyzes the hyperbolic heat conduction problem in the film and substrate composites under an imposed surface heat flux on the exterior film surface. The radiation heat flux model is employed to take account of the interface thermal resistance. The reflection and transmission occur at the contact surface of the dissimilar material which depends on the substrate properties and interface conditions. The interface resistance restricts the energy transmission across the interface and alerts the reflected and transmitted waves' strength. Neglecting the interface thermal resistance causes the temperature distribution in the film to be greatly underestimated. Moreover, the hyperbolic equation predicts significantly different results with those predicted by the parabolic equation at small time scales. The discrepancies between the solutions in an investigation of superconductor Y-Ba-Cu-O film depositions on several commonly used substrates are examined. (C) 1999 Elsevier Science Ltd. All rights reserved.
Influence of thermal boundary resistance on bolometric response of high-T-c superconducting films
(1998-09-01)
Wu, JP
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Shih, CU
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Chu, HS
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機械工程學系
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Department of Mechanical Engineering
This work presents a thermal analysis on predicting the temperature increase and the voltage response of high-T-c superconducting bolometers. To consider the thermal boundary resistance between film and substrate, an acoustic mismatch model (AMM), a diffusive mismatch model (DMM), and an interfacial layer model (ILM) are employed. The thermal boundary resistance significantly influences the voltage response. Additionally, several common substrates are examined. SrTiO3 (100) or LaAlO3 (100) is a better substrate for high-T-c superconducting bolometers. Furthermore, we demonstrated that there are several factors to affect the voltage response, such as the pulse duration for a constant total incident energy or a fixed highest incident heat flux, the pulse distribution function in time, optical penetration depth, the thermal conductivities of the film and substrate, initial operating temperature, thickness ratio, and the amount of incident heat flux. One interesting finding was that when compared with experimental data, all the theoretical values from the present study as well as the other previously theoretical treatment overestimate the voltage response near the transition temperature. (C) 1998 Elsevier Science Ltd. All rights reserved.
Intrinsic thermal stability and quenching recovery of thin-film superconductors with thermal boundary resistance
(1997-05-01)
Wu, JP
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Chu, HS
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機械工程學系
;
Department of Mechanical Engineering
The stability behaviour of a thin-film superconductor under a localized release of thermal disturbance is investigated. Two-dimensional conjugate film/substrate conduction equation with anisotropic thermal conductivity of the film, and Joule heat are employed to investigate effects of substrate and thermal properties on the intrinsic stability and quenching recovery. To consider the thermal boundary resistance between film and substrate, an interfacial-layer model (ILM) with very low diffusivity and an acoustic mismatch model (AMM) are employed. Results show that the thermal boundary resistance influences strongly the intrinsic stability. Thermal boundary resistance increases intrinsic stability if the thermal conductivity of the substrate or the disturbance energy is large. Higher Riot numbers and thermal conductivity ratios of film to substrate in longitudinal direction influence stability favorably. We demonstrate also that operation of a film/substrate system, such as YBCO/MgO, is either intrinsically stable or irrecoverably unstable.
Intrinsic thermal stability and quenching recovery of thin-film superconductors with thermal boundary resistance
(1997-05-01)
Wu, JP
;
Chu, HS
;
交大名義發表
;
機械工程學系
;
National Chiao Tung University
The stability behaviour of a thin-film superconductor under a localized release of thermal disturbance is investigated. Two-dimensional conjugate film/substrate conduction equation with anisotropic thermal conductivity of the film, and Joule heat are employed to investigate effects of substrate and thermal properties on the intrinsic stability and quenching recovery. To consider the thermal boundary resistance between film and substrate, an interfacial-layer model (ILM) with very low diffusivity and an acoustic mismatch model (AMM) are employed. Results show that the thermal boundary resistance influences strongly the intrinsic stability. Thermal boundary resistance increases intrinsic stability if the thermal conductivity of the substrate or the disturbance energy is large. Higher Riot numbers and thermal conductivity ratios of film to substrate in longitudinal direction influence stability favorably. We demonstrate also that operation of a film/substrate system, such as YBCO/MgO, is either intrinsically stable or irrecoverably unstable.
Inverse determination of surface temperature in thin-film/substrate systems with interface thermal resistance
(2004-07-01)
Wu, SK
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Chu, HS
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機械工程學系
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Department of Mechanical Engineering
This work examines the effect of interface thermal resistance on the estimation of surface temperatures in thin-film/ substrate systems. A radiation-boundary-condition model based on the acoustic mismatch model (AMM) is employed to consider the interface thermal resistance between thin-film and substrate. The inverse beat conduction problem is solved using the space-marching technique. The influences of interface thermal resistance, measurement locations, and measurement errors on this method are studied in detail. Numerical results show that the inverse method accurately estimates surface conditions and temperature distributions in a two-layer system even with an abrupt temperature drop at the interface. Sensor locations and interface thermal resistance only slightly affect the accuracy of the inverse estimation during the transient process when the exact input data (without measurement errors) are applied. However, the inaccuracy might be amplified by the interface thermal resistance and sensor locations if measurement errors exist. (C) 2004 Elsevier Ltd. All rights reserved.
Non-Fourier heat conduction with radiation in an absorbing, emitting, and isotropically scattering medium
(2002-06-15)
Chu, HS
;
Lin, SP
;
Lin, CH
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機械工程學系
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Department of Mechanical Engineering
This article numerically analyses the combined conductive and radiative heat transfer in an absorbing, emitting, and isotropically scattering medium. The non-Fourier heat conduction equation, which includes the time lag between heat flux and the temperature gradient, is used to model the conductive heat transfer in the medium. It predicts that a temperature disturbance will propagate as a wave at finite speed. The radiative heat transfer is solved using the P-3 approximation method. In addition, the MacCormack's explicit predictor-corrector scheme is used to solve the non-Fourier problem. The effects of radiation including single scattering albedo, conduction-to-radiation parameter, and optical thickness of the medium on the transient and steady state temperature distributions are investigated in detail. Analysis results indicate that the internal radiation in the medium significantly influences the wave nature. The thermal wave nature in the combined non-Fourier heat conduction with radiation is more obvious for large values of conduction-to-radiation parameter, small values of optical thickness and higher scattering medium. The results from non-Fourier-effect equation are also compared to those obtained from the Fourier equation. Non-Fourier effect becomes insignificant as either time increases or the effect of radiation increases. (C) 2002 Elsevier Science Ltd. All rights reserved.
Nonlinear diffusion-induced stresses in a long bar of square cross section
(1999-01-01)
Hwang, CC
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Lin, SP
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Chu, HS
;
Lee, WS
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機械工程學系
This paper is to investigate the nonlinear effect of the self-induced electric field on the diffusion-induced stresses in a long bar. We first approximate the nonlinear concentration-dependent diffusivity asa series of third-degree polynomials by the least-squares curve-fitting techniques, and then calculate the distributions of concentration by the Galerkin method. Afterwards, the diffusion-induced stresses inside the bar are determined analytically by introducing the Goodier displacement potential and Airy stress function. It is found that the nonlinear self-induced electric fields can depress both the concentration gradient and the maximum diffusion-induced stresses apparently, and these effects are more significant at short times than at long times. (C) 1998 Elsevier Science Ltd. All rights reserved.
Propagation and reflection of thermal waves in a rectangular plate
(1999-07-01)
Wu, JP
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Chu, HS
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機械工程學系
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Department of Mechanical Engineering
The wave nature of heat propagation in a two-dimensional rectangular plate with an instantaneous thermal disturbance released in an arbitrary position is investigated by solving the hyperbolic heat conduction equation. The exact analytical solutions are developed for the temperature field and heat flux using the Green's function technique to deal with two limiting boundary conditions, the constant wall temperature and the adiabatic condition, around the region. The disturbance gives rise to a severe thermal wave front, which differs completely front that obtained through one-dimensional analysis, traveling through the medium at a finite speed with a sharp peak at the leading edge. The significant findings in these results are that a negative trailer is generated and follows behind the wave front. In addition, the magnitude of the front is significantly attenuated from the side adjacent to the trailer because the increasing area available to it for diffusion, and decays exponentially along its path of travel, since the thermal energy is dissipated in the wake of the moving wave front. The results also reveal that different boundary conditions strongly influence the reflection of a thermal wave front from the exterior surfaces and the reflection and interaction among thermal waves are more complicated than those found through one-dimensional analysis.