共查询到16条相似文献,搜索用时 15 毫秒
1.
Nada ZamelEfim Litovsky Xianguo Li Jacob Kleiman 《International Journal of Hydrogen Energy》2011,36(19):12618-12625
Carbon paper, a fibrous material, is often used as the gas diffusion layer in polymer electrolyte membrane (PEM) fuel cells, which are being vigorously developed as a zero-emission power source for transportation applications. The temperature field and heat transfer in this material is determined by its thermal conductivity and diffusivity, which are directly dependent on the operating temperature. In this work, we use a quasi-steady method known as the thermal capacitance (slug) method to experimentally measure the through-plane thermal conductivity of TORAY carbon paper for a temperature range from −50 to +120 °C. The effects of compression and PTFE loading on the overall thermal conductivity are also investigated. Compression leads to a decrease in thermal resistance between the carbon fibers; hence, an increase in the overall thermal conductivity. However, it is also found that this thermal resistance is highly dependent on the temperature and the PTFE loading. In contrast with our in-plane thermal conductivity measurements from a previous study, the through-plane thermal conductivity is found to increase with an increase in temperature in this study. This finding suggests that the thermal expansion of the carbon fibers is a direction dependent quantity. 相似文献
2.
Phosphoric acid-doped polybenzimidazole-membrane-based PEM fuel cells were tested in the temperature range of 120–200 °C, with ambient backpressure and 0% RH. AC impedance spectroscopy, surface cyclic voltammetry and fuel cell performance simulation were used to obtain the exchange current densities for the cathodic oxygen reduction reaction (ORR) and anodic hydrogen oxidation reaction (HOR) on platinum-based catalysts at such high temperatures. The activation energies for ORR, HOR and membrane conductivity were also obtained separately. The results showed that temperature significantly affects the charger transfer and gas (O2 and H2) diffusion resistances. The effect of O2 stoichiometry (STair) on fuel cell performance was also investigated. Increasing STair can effectively increase the O2 partial pressure in the feed air, leading to improvements in both the thermodynamics and the kinetics of the fuel cell reactions. In addition, it was observed that increasing STair could also improve the gas diffusion processes. 相似文献
3.
Sergei F. Burlatsky Vadim V. Atrazhev Mallika Gummalla Dave A. Condit Fuqiang Liu 《Journal of power sources》2009,190(2):485-492
Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively. 相似文献
4.
Nam-Jin KimSung-Seek Park Sang Hoon LimWongee Chun 《International Communications in Heat and Mass Transfer》2011,38(3):313-318
The proper mixture ratio of nanofluids was examined by measuring thermal conductivity via transient hot-wire method. Comparisons are made with the nanofluid prepared by dispersing oxidized Multi-Walled Carbon NanoTubes (MWCNTs) in distilled water (herein referred to as “oxidized nanofluid”). Viscosity measurements were also carried out for the PVP-added nanofluids and oxidized nanofluids by using a digital viscometer. The nanofluids with 300 wt.% PVP and oxidized MWCNTs exhibited better thermal conductivity than that reported in previous studies. The thermal conductivity of oxidized carbon nanofluids was the highest of those compared and the use of additives in the nanofluid preparation deems to increase viscosity. For industrial applications, the chemical dispersion method applied in the preparation of oxidized carbon nanofluids should be considered as it offers high thermal conductivity with a slight increase in viscosity. 相似文献
5.
碳纤维作为一种被广泛应用的微纳米材料,对其导热性能的测量研究一直被作为对碳纤维性能研究的重要内容。在利用氦气的气体液化基础上搭建的超低温实验环境中,基于瞬态电热法对处于290到10 K温度内的碳纤维样品的导热性能进行研究。实验发现,当实验温度低于某一特定温度后,材料的热扩散率表现出与声子散射分析相反的实验结果。通过引入热扩散系数倒数这一理论研究声子热阻在低温下的变化,分析得出,当实验环境温度低于某一特定温度后,低温会造成碳纤维材料内的石墨微晶体结构发生变化,从而造成材料热扩散率下降。 相似文献
6.
A two-dimensional two-phase model based on the classical two-fluid model is used to analyze electrochemical and thermal transport in a PEMFC. The model is extended to account for the dependence of interfacial area density on liquid volume fraction. At a given fixed voltage, the fuel cell generates maximum current density for low through-plane and high in-plane thermal conductivities at high humidity operating conditions. It is also predicted that for low humidity operating conditions, the fuel cell generates maximum current density if the GDL is tailored to have high through-plane thermal conductivity near the inlet and progressively decreasing through-plane thermal conductivity at distances away from the inlet. At fully humidified cathode inlet conditions, narrower current collector ribs generate higher current densities at all voltages by reducing the resistance to diffusion of reactants and products through the GDL. In order to maximize the current density at low humidities, ribs must be wider near the inlet and narrower away from the inlet. The proposed methodology for tailoring GDL through-plane thermal conductivities and rib widths reduces the risk of membrane dehydration near inlet and also reduces the possibility of excessive liquid accumulation in the region away form the inlet. 相似文献
7.
The purpose of this study is to investigate the effect of ploytetrafluoroethylene (PTFE)-treatment and microporous layer (MPL)-coating on the electrical conductivity of gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells (PEMFCs). The results show that, for PTFE-treated GDLs, the electrical conductivity in orthogonal in-plane directions is almost invariant with the PTFE loading. On the other hand, the in-plane conductivity of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be higher than that of the counterpart SGL 10BC (25% PTFE) and this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Further, the conductivity of each GDL sample was measured in two perpendicular in-plane directions in order to investigate the in-plane anisotropy. The results show that the electrical conductivity of the GDL sample in one direction is different to that in the other direction by a factor of about two. The contact resistance, the main factor affecting the through-plane conductivity, of PTFE-treated GDLs shows a different trend to the corresponding in-plane conductivity, namely it increases as the PTFE loading increases. On the other hand, the contact resistance of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be lower than that of the counterpart SGL 10BC (25% PTFE) and again this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Also, it was noted that the MPL coating appears to have a positive effect in reducing the contact resistance between the GDL and the bipolar plate. This is most likely due to the compressibility of the MPL layers that allows them to fill in the ‘gaps’ that exist in the surface of the bipolar plates and therefore establishes a good contact between the latter plates and the GDLs. Finally, good curve fitting of the contact resistance as a function of the clamping pressure has been achieved. 相似文献
8.
《International Journal of Hydrogen Energy》2014,39(35):20127-20138
Maintaining optimal temperature of the stack is critical for efficient operation of high temperature polymer electrolyte membrane fuel cells. While a number of possibilities of thermal management exist for small stacks, the problem becomes more complicated for larger stacks. In the present study, the thermal management of stacks in the power range of 1–10 kWe is considered through computational fluid dynamics simulations. It is shown that large stacks need to have dedicated cooling plates through which a coolant is circulated. Further, stacks of the size of 10 kWe can have reasonably low cell temperature variations (∼20 K) only by passing pre-heated liquid coolant through the coolant plates. Estimates show that the concomitant increase in the coolant flow rate induces large pressure drops, of the order of 30 bar, if a four-parallel serpentine is used on an active cell area of 30 cm × 30 cm. It is therefore necessary to use parallel channel flow fields with carefully designed feeder manifolds to maintain optimal cell temperatures and reasonably low coolant pressure drops in large stacks. 相似文献
9.
Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL.A novel test bed that allows separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described in this paper. Measurements are performed using Toray carbon paper TGP-H-120 samples with varying polytetrafluoroethylene (PTFE) content at a mean temperature of 65-70 °C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The in-plane effective thermal conductivity is found to remain approximately constant, k ≈ 17.5 W m−1 K−1, over a wide range of PTFE content, and its value is about 12 times higher than that for through-plane conductivity. 相似文献
10.
An attempt to obtain conductive carbon layer (CCL) on the LiMn2O4 and LiMn2O4−ySy spinels, using the radical precipitation polymerization of acrylonitrile (AN) and styrene–acrylonitrile (SAN) in water suspension of the spinels followed, by pyrolysis of the composed precursors was made. In the case of PAN–spinel precursors the CCL on spinel particles was effectively formed. Phase transitions, as well as thermal and electrical properties of the carbon-sulphided spinel composites were characterized. An attempt with SAN copolymer failed due to the reaction between SAN and the spinels during pyrolysis, even in the argon stream. 相似文献
11.
Ian Turner Romain Rémond Patrick Perré 《International Journal of Heat and Mass Transfer》2010,53(4):715-725
A comprehensive heat and mass transfer computational model is used to analyse the intricate two-way coupling arising from the activated chemical reactions involved in the heat treatment of wood. The 2D version of a drying code known as TransPore is used to simulate the coupled heat and mass transfer phenomena. This code accounts for the internal pressure in the porous medium. The pyrolysis model describing the chemical reactions occurring in the main constituents within the cell walls of wood (cellulose, hemicelluloses and lignins) is derived using data taken from the literature. Refined computational strategies were required to address the two-way coupling between the heat and mass transfer and chemical mechanisms, including the thermal activation of the chemical reactions, together with the treatment of heat sources (or sinks) and the production of volatiles. The experimental set-up allows the overall weight loss, and the internal temperature and pressure at specific locations within the board to be determined during processing. The reported simulations highlight that the model is able to capture two particular phenomena observed during the heat treatment of wood: the double pressure peak due to water evaporation and volatiles production; and the temperature overshoot during the heat treatment phase. 相似文献
12.
A new type of internal combustion engine with distinct advantages over conventional piston-engines and gas turbines in small power ranges is presented. The engine has analogies with piston engine operation, but like gas turbines it has dedicated spaces and devices for compression, burning and expansion. The engine operates on a modified limited-pressure thermodynamic cycle. The core of the engine is a nutating non-rotating disk, with the center of its hub mounted in the middle of a Z-shaped shaft. The two ends of the shaft rotate, while the disk nutates. The motion of the disk circumference prescribes a portion of a sphere. In the single-disk configuration a portion of the surface area of the disk is used for intake and compression, a portion is used to seal against a center casing, and the remaining portion is used for expansion and exhaust. The compressed air is admitted to an external accumulator, and then into an external combustion chamber before it is admitted to the power side of the disk. The external combustion chamber enables the engine to operate on a variable compression ratio cycle. Variations in cycle temperature ratio and compression ratio during normal operation enable the engine to effectively become a variable-cycle engine, allowing significant flexibility for optimizing efficiency or power output. The thermal efficiency is similar to that of medium sized diesel engines. For the same engine volume and weight this engine produces approximately twice the power of a two-stroke engine and four times the power of a four-stroke engine. The computed sea-level engine performance at design and off-design conditions in the 2 to 500 kW power range is presented. 相似文献
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14.
The colligative properties of acidic solution inside Nafion® 117 membranes have been investigated, in a large temperature range, by two different methods. 相似文献
15.
2-Amino-2-methyl-1,3-propanediol (AMP), 2-amino-2-hydroxymethyl-1,3-propanediol (TRIS) and the mixture (AMP+TRIS, mole ratio 50:50) are being considered as potential candidates for the thermal storage of energy. The thermal conductivities have been measured with an uncertainty of ±3% from 20°C to the supermelting points of these substances by means of a calorimeter equipped with a thermistor. The experimental thermal conductivities of the substances under investigation are smoothed by fitting for different values of temperature and reported at intervals of 10 K. In addition, the solid–solid transition and melting temperatures are also determined to be 80 and 108°C for AMP, 135 and 175°C for TRIS and 70 and 142°C for (AMP+TRIS), based on the thermal conductivity–temperature diagrams of these substances. 相似文献
16.
The effect of increasing the reaction temperature to 300 °C on the activity, stability and deactivation behavior of a 4.5 wt.% Au/CeO2 catalyst in the water gas shift (WGS) reaction in idealized reformate was studied by kinetic and spectroscopic measurements at 300 °C and comparison with previously reported data for reaction at 180 °C under similar reaction conditions [A. Karpenko, Y. Denkwitz, V. Plzak, J. Cai, R. Leppelt, B. Schumacher, R.J. Behm, Catal. Lett. 116 (2007) 105]. Different procedures for catalyst pretreatment were used, including annealing at 400 °C in oxidative, reductive or inert atmospheres as well as redox processing. The formation/removal of stable adsorbed reaction intermediates and side products (surface carbonates, formates, OHad, COad) was followed by in situ IR spectroscopy (DRIFTS), the presence of differently oxidized surface species (Au0, Au0′, Au3+, Ce3+) was evaluated by XPS. The reaction characteristics at 300 °C generally resemble those at 180 °C, including (i) significantly higher reaction rates, (ii) comparable apparent activation energies (44 ± 1/50 ± 1 kJ mol−1 vs. 40 ± 1 kJ mol−1 at 180 °C), (iii) a correlation between deactivation of the catalyst and the build-up of stable surface carbonates, and (iv) a decrease of the initially significant differences in activity after different pretreatment procedures with reaction time. Different than expected, the tendency for deactivation did not decrease with higher temperature, due to enhanced carbonate decomposition, but increases. 相似文献