Browsing by Author "Yan, Wei-Mon"

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A critical review on heat transfer augmentation of phase change materials embedded with porous materials/foams
(2019-06-01)
Tauseef-ur-Rehman
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Ali, Hafiz Muhammad
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Janjua, Muhammad Mansoor
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Sajjad, Uzair
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Yan, Wei-Mon
Phase change material (PCM) is promising media for thermal energy storage owing to its extensive value of latent heat (140-970 KJ/Kg). However, thermal conductivity of PCMs is too low which obstructs energy storage and retrieval rate. In recent days, thermally enhanced PCMs are considered promising materials for efficient heat transfer in many applications. This article designates the review on improved thermal properties and heat transfer of PCMs by using porous materials. Enhanced heat transfer of PCMs can be achieved using extended surfaces (triangular, conical, square, and rectangular fins), heat pipes, and addition of highly conductive nanoparticles (e.g. Cu, Al2O3, Au, SiC, SiO2 and TiO2). Major focus of this article is to study the enhanced heat transfer of PCMs through metallic (copper, nickel, and aluminum) and carbon based (carbon, graphite and expanded graphite) porous materials/foams. Effects of porosity and pore density on heat transfer, thermal conductivity, specific heat, latent heat and charging/discharging time are critically reviewed. Porous materials/foams are reported to be efficient for heat transfer/thermal conductivity enhancement by 3-500 times. Furthermore, correlations to find the effective thermal conductivity of PCM/foam are reported. Important applications of PCM/foam reported by different researchers are also discussed in this paper. Finally, conclusions and recommendations are presented to highlight the research gap in this area. (C) 2019 Elsevier Ltd. All rights reserved.
Bubble characteristics in time periodic evaporation flow of R-134a in a narrow annular pipe due to heat flux oscillation
(2016-12)
Chen, C. A.
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Lin, T. F.
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Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
In this work, bubble characteristics of periodic evaporation flow with refrigerant R-134a in a horizontal narrow annular pipe were examined experimentally in details. Attention is focused on the time periodic evaporation flow characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The photos of the R-134a time periodic evaporating flow taken from the duct side are presented to show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. The results show that at the low vapor quality, the bubbles get smaller with time and become less crowded in the duct in the first half of the cycle in which the R-134a heat flux decreases. The changes of the bubble characteristics with the instantaneous heat flux become more pronounced for an increase in the amplitude of the heat flux oscillation. At the very high mean vapor quality the bubble nucleation can be barely seen in the entire periodic cycle since the liquid film covering the heating surface is very thin. In addition, the duct flow is dominated by the annular two-phase flow at all time. (C) 2016 Elsevier Ltd. All rights reserved.
Bubble characteristics in time periodic saturated flow boiling of R-134a in a narrow annular pipe due to heat flux oscillation
(2016-11)
Chen, C. A.
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Lin, T. F.
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Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
In this work, experiments have been conducted to investigate how the imposed time periodic heat flux oscillation affects the bubble characteristics of saturated flow boiling with refrigerant R-134a in a horizontal narrow annular pipe. The test section for the horizontal annular duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow annular duct. In particular, attention is focused on the time periodic saturated flow boiling characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The results show that the bubble departure diameter, bubble frequency and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the boiling the resulting oscillation amplitudes of the bubble parameters, such as the bubble departure diameter, bubble frequency and active nucleation site density, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. (C) 2016 Elsevier Ltd. All rights reserved.
Bubble dynamics in evaporation flow of R-134a in narrow annular ducts due to flow rate oscillation
(2019-01-01)
Chen, C. A.
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Lin, T. F.
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Ali, Hafiz Muhammad
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Amani, Pouria
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Yan, Wei-Mon
This article aims to investigate the influence of imposed flow rate oscillation on the bubble behaviors of evaporative flow of R-134a in horizontal narrow annular ducts. Particularly, the effects of amplitudes and periods of the oscillating flow rates along with the vapor quality of the R-134a on the bubble characteristics are examined in details. Moreover, the R-134a flow photos captured at the middle axial location are presented to illustrate the shift between the annular two-phase flow pattern and the bubble nucleation at different operating conditions and time instances. The observations reveal that the imposed mass flow rate oscillation results in the fluctuation of bubble departure diameter and frequency along with the density of active nucleation sites with the same frequencies. The larger amplitudes and longer periods of the mass flow rate oscillation lead to the stronger fluctuations in bubble dynamics. Furthermore, it is found that the bubble nucleation on the heating surface is the dominant mechanism at the low vapor quality of 0.05. However, the annular two-phase flow prevails over a relatively large portion of the periodic cycle at the dryness fraction of 0.5 and over the entire cycle at the dryness fraction of 0.95. Finally, the evaporative heat transfer of R-134a subject to the flow rate oscillation is correlated as a function of the mass flow rate and the vapor quality.
Degradation of proton exchange membrane fuel cells due to CO and CO(2) poisoning
(2009-03-01)
Yan, Wei-Mon
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Chu, Hsin-Sen
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Lu, Meng-Xi
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Weng, Fang-Bor
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Jung, Guo-Bin
The Co and CO(2) poisoning effects on the degradation of cell performance or proton exchange membrane fuel cell (PEMFC) tinder transient stage were investigated. The mechanism of CO poisoning lies in the preferential adsorbing or CO to the platinum surface and the blocking or active sites of hydrogen. These phenomena were described with adsorption, desorption, and electro-oxidation processes of CO and hydrogen in the present work. In addition, it is well known that the reverse water gas shift reaction (RWGS) is the main effect of the CO(2) poisoning, through which a large part of the catalytic surface area becomes inactive due to the hydrogen dissociation. The predicted results showed that, by contaminating the fuel with 10 ppm CO at the condition of P(H) = 0.8 atm and P(co2) = 0.2 atm, the current density of the PEM fuel cell was lowered 28% with rate constant of RWGS k(rs) from zero to 0.02. With 50 ppm CO, the performance drop was only 18%. For the reformed gas, CO(2) poisoning became much more significantly when the CO content in the reactant gas was small. (C) 2008 Elsevier B.V. All rights reserved.
Effects of chlorides on the performance of proton exchange membrane fuel cells
(2011-05-01)
Yan, Wei-Mon
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Chu, Hsin-Sen
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Liu, You-Lun
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Chen, Falin
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Jang, Jer-Huan
This paper investigates the effects of cathode gases containing chloride ions on the proton exchange membrane fuel cell (PEMFC) performance. Chloride solutions are vaporized using an ultrasonic oscillator and mixed with oxygen/air. The salt concentration of the mixed gas in the cathode is set by varying the concentration of the chloride solution. Five-hour tests show that an increase in the concentration of sodium chloride did not significantly affect the cell performance of the PEMFC. It is found that variations in the concentration of chloride do not show significant influence on the cell performance at low current density operating condition. However, for high current density operating conditions and high calcium chloride concentrations, the chloride ion appears to have a considerable effect on cell performance. Experimental results of 108-h tests indicate that the fuel cell operating with air containing calcium chloride has a performance decay rate of 3.446 mV h(-1) under the operating condition of current density at 1 A/cm(2). From the measurements of the I-V polarization curves, it appears that the presence of calcium chloride in the cathode fuel gas affects the cell performance more than sodium chloride does. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Experimental study on bubble characteristics of time periodic subcooled flow boiling in annular ducts due to wall heat flux oscillation
(2020-08-01)
Chen, A.
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Lin, T. F.
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Ali, Hafiz Muhammad
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Yan, Wei-Mon
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機械工程學系
The significant effects of imposed wall heat flux oscillation on bubble characteristics of subcooled flow boiling with R-134a in horizontal annular ducts are experimentally performed in details. Specific emphasis has been applied on the bubble dynamics of the time periodic subcooled flow boiling characteristics influenced by the mean levels, amplitudes, and periods of the wall heat flux oscillations. The obtained experimental results have disclosed that the bubble parameters like the departure diameter, frequency and active nucleation site density, do oscillate periodically in time and are at the same frequency as the imposed wall heat flux oscillation. Nevertheless, these parameters also increase at long periods, high amplitudes of imposed heat flux oscillation or high values of mean imposed heat flux. Furthermore, the bubble size reduced after the time lag when the imposed heat flux decreases with time. (C) 2020 Elsevier Ltd. All rights reserved.
Experimental study on time periodic evaporation heat transfer of R-134a in annular ducts due to wall heat flux oscillation
(2017-03)
Chen, C. A.
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Lin, T. F.
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Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
An experimental study was carried out to examine the time periodic evaporation heat transfer for refrigerant R-134a flowing in horizontal narrow annular ducts subjected to wall heat flux oscillation in the form of triangular waves. The experiment was performed for the gap of the annular duct delta = 1.0, 2.0 and 5.0 mm. Besides, the amplitude of the imposed heat flux oscillation Delta q was set at 0, 10, 30, 50, 80 and 100% of mean imposed heat flux (q) over bar. Moreover, the period of the heat flux oscillation t(p) was fixed at 2, 20, 30, 60,120 and 600 s. Attention was mainly paid to examining the effects of the amplitude and period of the wall heat flux oscillation on the time periodic evaporation heat transfer in the narrow annular pipe. The measured evaporation heat transfer data are expressed in terms of the evaporation heat transfer coefficient. The thermal characteristics of the oscillatory evaporation heat transfer were illustrated by showing the time variations of the instantaneous heated pipe wall temperature T-w, and evaporation heat transfer coefficient h(r). Measured results showed that the time-average heat transfer coefficients for the oscillatory evaporation heat transfer of R-134a were not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Besides, the stronger wall temperature oscillation was noticed for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. (C) 2016 Elsevier Ltd. All rights reserved.
Numerical study of heat and mass transfer in a plate methanol steam micro reformer with methanol catalytic combustor
(2010-06-01)
Hsueh, Ching-Yi
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Chu, Hsin-Sen
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Yan, Wei-Mon
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Chen, Chiun-Hsun
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機械工程學系
A numerical study is performed to examine the characteristics of heat and mass transfer and the performance of a plate methanol steam micro reformer with a methanol catalytic combustor. The effects of the flow configurations for co- and counter-current flows are explored in the present study. The influences of the Reynolds number (Re) and various geometric parameters on heat and mass transfer phenomena in the channels are also investigated numerically. It is expected that the Reynolds number (Re) and various geometric parameters can be improved by thermal management to enhance the chemical reaction and thus augment the micro reformer performance. Comparing the co- and counter-current flows via numerical simulation, the results show that the methanol conversion for counter-current flow could be improved by 10%. This is due to the fact that counter-current flow leads to a better thermal management, which in turn improves fuel conversion efficiency. With a higher Reynolds number on the combustor side, the wall temperature is increased and the methanol conversion can thus be enhanced. Meanwhile, a reduced Reynolds number on the micro reformer side would increase the methanol conversion. The results also reveal that appropriate geometric parameters exist for a micro reformer with a combustor to obtain better thermal management and methanol conversion. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Numerical study of heat and mass transfer in the plate methanol steam micro-reformer channels
(2010-08-01)
Hsueh, Ching-Yi
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Chu, Hsin-Sen
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Yan, Wei-Mon
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Chen, Chiun-Hsun
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Chang, Min-Hsing
Effects of geometric and thermo-fluid parameters on performance and heat and mass transfer phenomena in micro-reformer channels were investigated by mathematical modeling. The geometric parameters considered were the channel length, channel height, catalyst thickness and catalyst porosity, while the thermo-fluid parameters included wall temperature, inlet fuel temperature, fuel ratio and Reynolds number. The results of the modeling suggest that the methanol conversion could be improved by 49%-points by increasing the wall temperature from 200 degrees C to 260 degrees C. The results also show that the CO concentration would be reduced from 1.72% to 0.95% with the H(2)O/CH(3)OH molar ratio values ranging from 1.0 to 1.6. The values of parameters that enhance the performance of micro-reformer were identified, such as longer channel length, smaller channel height, thicker catalyst layer, larger catalyst porosity, lower Reynolds number and higher wall temperature. (C) 2010 Elsevier Ltd. All rights reserved.
Numerical study of phonon radiative transfer in porous nanostructures
(2008-07-15)
Li, Sheng-Yen
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Chu, Hsin-Sen
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Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
This study analyzes the phonon radiative transfer in two-dimensional porous silicon nanostructures with a phonon transport model based on the Boltzmann transportation equation. We focus on the inter-scattering between pores. The numerical results show that when the aspect ratio is less than 1.22, the scale factor dominates the thermal conductivity, and the thermal conductivity of nanostructures with in-line arrangement pores is determined by the dependent phonon scattering effect. In nanostructures with staggered arrangement pores, the phonons are prevented from transporting through the material. In general, the results show that the larger the pore size, the lower the thermal conductivity of the nanostructure. The results presented in this study provide a useful reference for the development of high-efficiency thermoelectric structures. (C) 2008 Elsevier Ltd. All rights reserved.
Numerical study on micro-reformer performance and local transport phenomena of the plate methanol steam micro-reformer
(2009-02-15)
Hsueh, Ching-Yi
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Chu, Hsin-Sen
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Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
The objective of this work is to investigate the transport phenomena and performance of a plate steam methanol micro-reformer. Micro channels of various height and width ratios are numerically analyzed to understand their effects on the reactant gas transport characteristics and micro-reformer performance. In addition, influences of Reynolds number and geometric size of micro channel on methanol conversion of micro-reformer and gas transport phenomena are also explored. The predicted results demonstrated that better performance is noted for a micro channel reformer with lower aspect-ratio micro channel. This is due to the larger the chemical reaction surface area for a lower aspect-ratio channel reformer. It is also found that the methanol conversion decreases with increasing Reynolds number Re. The results also indicate that the smaller micro channel size experiences a better methanol conversion. This is due to the fact that a smaller micro channel has a much more uniform temperature distribution, which in turn, fuel utilization efficiency is improved for a smaller micro channel reformer. (C) 2008 Elsevier B.V. All rights reserved.
Time periodic evaporation heat transfer of R-134a in a narrow annular duct due to mass flow rate oscillation
(2018-03-01)
Chen, C. A.
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Lin, T. F.
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Yan, Wei-Mon
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Amani, Mohammad
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機械工程學系
An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal annular duct was experimentally conducted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations. (C) 2017 Elsevier Ltd. All rights reserved.
Time periodic saturated flow boiling heat transfer of R-134a in a narrow annular duct due to heat flux oscillation
(2017-03)
Chen, C. A.
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Lin, T. F.
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Yan, Wei-Mon
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機械工程學系
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Department of Mechanical Engineering
The time periodic saturated boiling heat transfer in a horizontal annulus was investigated experimentally where the walls are under an oscillating heat flux. The fluid enters the duct with zero vapor quality (saturated liquid state). The amplitude of the imposed heat flux oscillation Delta q varies from 0% to 50% of mean imposed heat flux (q) over bar and four different periods of heat flux oscillation t(p) including 20, 30, 60 and 120 s are applied to the system. The measured data display that when the applied heat flux is close to that for the onset of stable flow boiling, intermittent flow boiling appears in which nucleate boiling on the heated surface only exists in a partial interval of each periodic cycle and the heat flux oscillation does not noticeably affect the time-average boiling curves and heat transfer coefficients. Besides, the heated wall temperature and evaporating flow pattern are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the persistent boiling the resulting oscillation amplitudes of the heated surface temperature, heat transfer coefficient gets larger for a longer period and larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux. The substantial time lag in the heated surface temperature oscillation is observed. In the first half of the periodic cycle in which the heat flux reduces with time, after the time lag the heated wall temperature decreases with time. The inverse processes occur in the second half of the cycle in which imposed heat flux increases with time. Finally, flow regime maps are provided to explain the boundaries separating different boiling regimes for the R-134a saturated boiling in the duct. (C) 2016 Elsevier Ltd. All rights reserved.
Transient analysis of water transport in PEM fuel cells
(2006-11-22)
Yan, Wei-Mon
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Chu, Hsin-Sen
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Chen, Jian-Yao
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Soong, Chyi-Yeou
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Chen, Falin
This paper theoretically studies the water transport phenomena in PEM fuel cells, mainly investigating the transient behavior in the gas diffusion layer (GDL), catalyst layer (CL) and proton exchange membrane (PEM). In the PEM, both diffusion and electro-osmosis processes are considered, while in the GDL and CL, only diffusion process is taken into account. The process of water uptake is employed to account for the water transport at the interface between the PEM and CL. The results indicate that the water content in the PEM and the time for reaching the steady state in the start-up process are influenced by the humidification constant, k, the humidification, and the thickness of PEM. The rise of the k increases the water content in the membrane and shortens the time for reaching the steady state. Insufficient humidification causes relatively small water content and long steady time. When the PEM is thinner, the water is more uniformly distributed, the water content gets higher, and the time for reaching the steady state is distinctly shorter. (c) 2006 Elsevier B.V. All rights reserved.
Transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field design
(2010-10-01)
Hsueh, Ching-Yi
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Chu, Hsin-Sen
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Yan, Wei-Mon
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Chen, Chiun-Hsun
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機械工程學系
A numerical investigation of the transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field as a function of wall temperature, fuel ratio and Reynolds number are presented. The fuel Reynolds number and H(2)O/CH(3)OH molar ratio (S/C) that influence the transport phenomena and methanol conversion are explored in detail. In addition, the effects of various wall temperatures on the plates that heat the channel are also investigated. The predictions show that conduction through the wall plays a significant effect on the temperature distribution and must be considered in the modeling. The predictions also indicate that a higher wall temperature enhances the chemical reaction rate which, in turn, significantly increases the methanol conversion. The methanol conversion is also improved by decreasing the Reynolds number or increasing the S/C molar ratio. When the serpentine flow field of the channel is heated either through top plate (Y = 1) or the bottom plate (Y = 0), we observe a higher degree of methanol conversion for the case with top plate heating. This is due to the stronger chemical reaction for the case with top plate heating. (C) 2010 Published by Elsevier Ltd.
Transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field design
(2010-10-01)
Hsueh, Ching-Yi
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Chu, Hsin-Sen
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Yan, Wei-Mon
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Chen, Chiun-Hsun
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機械工程學系
A numerical investigation of the transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field as a function of wall temperature, fuel ratio and Reynolds number are presented. The fuel Reynolds number and H(2)O/CH(3)OH molar ratio (S/C) that influence the transport phenomena and methanol conversion are explored in detail. In addition, the effects of various wall temperatures on the plates that heat the channel are also investigated. The predictions show that conduction through the wall plays a significant effect on the temperature distribution and must be considered in the modeling. The predictions also indicate that a higher wall temperature enhances the chemical reaction rate which, in turn, significantly increases the methanol conversion. The methanol conversion is also improved by decreasing the Reynolds number or increasing the S/C molar ratio. When the serpentine flow field of the channel is heated either through top plate (Y = 1) or the bottom plate (Y = 0), we observe a higher degree of methanol conversion for the case with top plate heating. This is due to the stronger chemical reaction for the case with top plate heating. (C) 2010 Published by Elsevier Ltd.