Despite massive efforts in the field of nanofluids over the last two decades, nanofluids are primarily still used in a lab scale due to numerous controllable and uncontrollable barriers that impede their effective lar...
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Despite massive efforts in the field of nanofluids over the last two decades, nanofluids are primarily still used in a lab scale due to numerous controllable and uncontrollable barriers that impede their effective large-scale implementation. Nanofluids market uptake can be realized only when those barriers have been overcome. These barriers must be examined for their impacts on all aspects of nanofluids market adoption. In this study, barriers to the commercial applications of nanofluids in thermal energy technologies are identified in the literature and are assessed in consultation with experts in the field using a total interpretive structural modeling approach and cross-impact matrix multiplication applied to a classification analysis. It is discovered that most of the barriers are interrelated and can influence one another. Long-term stability issue is identified as the main driver in the effective implementation of nanofluids at commercial scale. Research in this direction might be able to help R&D institutions and researchers in this field to sort out the most influential barriers to nanofluids market uptake.
作者:
Jia, FeiGuo, LiejinLiu, HongtanXidian Univ
Sch Mechanoelect Engn Xian 710071 Shaanxi Peoples R China Xi An Jiao Tong Univ
State Key Lab Multiphase Flow Power Engn Int Res Ctr Renewable Energy Xian Shaanxi Peoples R China Univ Miami
Dept Mech & Aerosp Engn Clean Energy Res Inst Coral Gables FL 33124 USA
Studying dynamic characteristics of proton exchange membrane fuel cells (PEMFCs) during startups/shutdowns is of great importance to proposing strategies to improve fuel cell performance and durability. In this study,...
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Studying dynamic characteristics of proton exchange membrane fuel cells (PEMFCs) during startups/shutdowns is of great importance to proposing strategies to improve fuel cell performance and durability. In this study, internal current during startup and shutdown processes in PEMFC is investigated, and effects of gas supply/shutoff sequences and backpressure are analyzed by measuring local current densities and the cell voltage in situ. The experimental results show that when reactants were fed/shut off, internal current occurs and variation patterns of local current densities along the flow channel are different. During startups, local current densities in the downstream drop to negative values and internal current can be eliminated when air is first supplied into the cell. While during shutdowns, the results show that negative currents occur in the upstream, and if hydrogen is shut off first, all local current densities remain constant at zero, indicating the effectiveness of gas shutoff sequence in eliminating/mitigating internal current in PEM fuel cells. Further experimental results show that the magnitude of internal current increases with the pressure difference between the anode and the cathode.
Recent rapid development of inorganic thermoelectric (TE) materials has aroused enthusiasm for exploring low-cost, flexible, lightweight, and non-toxic organic TE materials. Great progress has been achieved in develop...
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Recent rapid development of inorganic thermoelectric (TE) materials has aroused enthusiasm for exploring low-cost, flexible, lightweight, and non-toxic organic TE materials. Great progress has been achieved in developing organic materials with high TE performance (figure of merit, ZT) over the past decade. However, it is still extremely challenging to obtain organic materials with high TE performance and a ZT over 0.5 because of the strong interrelationship between the three TE parameters: electrical conductivity, Seebeck coefficient, and thermal conductivity. In this review, we discuss current trends in developing strategies to decouple the electrical conductivity, Seebeck coefficient, and thermal conductivity, which to the best of our knowledge have not been discussed in previously published reviews. Methods such as solvent treatment, electrochemical doping, and nanostructure formation are analyzed. In addition, incorrect thermal conductivity values for highly electrically conducting organic materials are still frequently reported, even in papers published in high-impact journals. A description of this puzzling phenomenon is provided in this review. Finally, a discussion of the advantages of state-of-the-art fabrication techniques of organic TE modules is presented, which highlights the unique advantages of organic TE materials in supporting wearable/portable devices.
The single-phase thermal hydraulic characteristics of liquid metal sodium are very essential for the design and safety analysis of sodium-cooled fast reactor (SFR). In this paper, the pressure drop and heat transfer f...
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The single-phase thermal hydraulic characteristics of liquid metal sodium are very essential for the design and safety analysis of sodium-cooled fast reactor (SFR). In this paper, the pressure drop and heat transfer features of single-phase liquid sodium were experimentally investigated in a 7 rod bundle with the velocity range of 0-4 m/s, heat flux up to 120 kW/m(2) and the absolute pressure range of 0-0.2 MPa. The corresponding Reynolds number ranges from 4000 to 40,000, and the Pe number varies from 0 to 340. It was found that the critical Re number for transition-turbulent flow of single-phase liquid sodium is 13,500 in the hexagonal 7-rod bundle. Then the effects of relative axial position, wall heat flux and Re number on the heat transfer were discussed, respectively. Some existing correlations in the literatures were assessed and compared with the experimental data. Results indicated that these correlations could not predict the current experiments well because of the different geometries and working fluids. The new correlations for the friction factor and Nu number calculations were proposed based on the current experimental data. For 98.5% of heat transfer data produced by the other researchers, the prediction error of the new correlation is less than 30%. For most of the experimental data, it is less than 20%, which sufficiently proves that the correlation developed in this paper could give a good prediction of the experimental data obtained by other researchers.
An experimental study was performed to investigate the effect of non-condensable gas dissolved in sub cooled water on steam-water direct contact condensation (DCC). Non-condensable gas dissolved in the sub-cooled wate...
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An experimental study was performed to investigate the effect of non-condensable gas dissolved in sub cooled water on steam-water direct contact condensation (DCC). Non-condensable gas dissolved in the sub-cooled water was removed mostly by a degassing membrane device, and the mass fraction of dissolved oxygen was reduced from 7.6 mg/L to 4.6 mg/L and 1.1 mg/L respectively by degassing. Before and after degassing, the flow patterns, thickness of the mixture layer, temperature and pressure distributions, as well as average volumetric heat transfer coefficient were discussed respectively. The visualization results proved that the mixture layer between the water region and steam region mainly consisted of steam, hot water and a small amount of non-condensable gas dissolved out from water at saturated state. Furthermore, the formation mechanism of the mixture layer was qualitatively discussed. Besides, the temperature and pressure distributions on upper and bottom wall were hardly affected by the non-condensable gas dissolved in sub-cooled water. Moreover, the average volumetric heat transfer coefficient was investigated before and after degassing, and it was concluded that the dissolved non-condensable gas had a slight effect on the heat and mass transfer characteristic of DCC process. (C) 2019 Elsevier Ltd. All rights reserved.
ZSM-5 is active in n-hexane cracking to light alkenes, but with the disadvantage of fast deactivation. A long lifetime ZSM-5 with high activity and anti-carbon deposition performance was of great concern for industria...
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ZSM-5 is active in n-hexane cracking to light alkenes, but with the disadvantage of fast deactivation. A long lifetime ZSM-5 with high activity and anti-carbon deposition performance was of great concern for industrial application. The hierarchical ZSM-5 zeolites (xHF-ZSM5) were obtained by chemical etching with fluoride ions and characterized by XRD, FTIR, NH3-TPD, SEM, TEM, N-2 adsorption-desorption. They showed higher BET surface area, larger mesoporous volume, less defect zones, commensurable microporous structure and acid property compared with that of the parent ZSM-5. The defects were dissolved resulting in higher crystallinity zeolite. They showed better catalytic performance with less carbon deposition in catalytic cracking of n-hexane due to superior channel structure, large mesoporous and macroporous structure. In order to reduce the amount of carbon deposition further, 0.5HF-ZSM5 was selected and made phosphorus modification. After phosphorus modification, the catalytic activity and stability was maintained at a high level, and the carbon deposition of n hexane cracking was restrained due to the gradual decreasing of acid amount with the increase of phosphorus content. The amount of carbon deposition decreased and reached the minimum of 23 mg g(-1) for 0.5HF-ZSM5-1P, which was four times less than that of the parent ZSM-5.
The two-fluid model coupled with the improved local-structure-dependent drag model and the modified chemical kinetics is employed to account for the multi-scale phenomenon in a bubbling fluidized bed methanation proce...
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The two-fluid model coupled with the improved local-structure-dependent drag model and the modified chemical kinetics is employed to account for the multi-scale phenomenon in a bubbling fluidized bed methanation process for the production of synthetic natural gas. The accuracy of this multi-scale CFD model is verified by a series of experimental data under different operating temperatures, inlet compositions, inlet velocities and initial bed heights. The cold flow CFD simulations are further carried out to evaluate the effects of the chemical reactions on the meso-scale structure. The results demonstrate that the bubble volume fraction and bubble velocity decreases due to the gas volume contraction caused by the methanation reactions. Three-region distribution of the gas temperature is obtained by simultaneously solving the multi-scale CFD model and the energy equations and the simulation results are compared with those based on the isothermal flow assumption. The results indicate that the distributions of solid volume fraction and mole fractions obtained based on the isothermal flow assumption are almost coincident with those obtained by solving the energy equations, which confirms the rationality of the isothermal flow assumption in a fluidized bed reactor.
Passive residual heat removal (PRHR) system is a very important component of the passive safety systems in advanced passive safety pressurizer water reactors (PWRs) such as AP1000. The passive residual heat removal he...
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Passive residual heat removal (PRHR) system is a very important component of the passive safety systems in advanced passive safety pressurizer water reactors (PWRs) such as AP1000. The passive residual heat removal heat exchanger (PRHR HX) is a C-shape tube bundle heat exchanger immerged in the In-containment refueling water storage tank (IRWST) which removes the core decay heat during the accident transients. The performance of the PRHR HX is significant for the safety of the nuclear power plant (NPP). In this paper, the thermal hydraulics characteristics of the PRHR HX in the IRWST is analyzed using computational fluid dynamics (CFD). The tube region is modeled by the porous media approach along with the distributed resistance method. Heat transfer from the primary side fluid inside the tube to the secondary side fluid in the IRWST is considered. The simulation is carried out by the commercial CFD package FLUENT. The calculation of the flow resistance and heat transfer in the tube region is implemented using the User Defined Functions (UDF) in FLUENT based on the local flow conditions. Three dimensional distributions of the fluid velocity and temperature in the IRWST are obtained and thermal stratification is observed. The PRHR HX heat transfer capacity and the primary side fluid temperature distribution inside tubes are analyzed.
Methane conversion into ethanol and associated derivates under mild reaction conditions is of great significance, yet remains a challenge to date. Herein, we report efficient photocatalytic methane conversion into C2-...
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Methane conversion into ethanol and associated derivates under mild reaction conditions is of great significance, yet remains a challenge to date. Herein, we report efficient photocatalytic methane conversion into C2-hydrocarbons by using CO2 as a soft oxidant over a series of Zn-doped g-C3N4 composites. The results indicate that Zn atoms enter the lattice of g-C3N4 and are chemically coordinated therein in the form of ZneN bonds. The integration of Zn, on one hand, could increase the specific surface area thus surface alkaline sites, and broaden the light response ability as well, promoting the dehydrogenation of methane into methyl. One the other hand, it is found that the ZneN bonds could serve as electron channels, accelerating the charge separation inside g-C3N4 and enabling rapid electron transfer from g-C3N4 to surface photodeposited Ru cocatalyst. This unique behavior changes the photocatalytic pathway and promotes the formation of CH3CHO and CH3CH2OH, as demonstrated by our in-situ infrared spectroscopy. Our photocatalytic tests indicate that the 0.5% Ru/Zn-g-C3N4-1/20 composite possesses the best activity, with the yields of CO, CH3CHO, and CH3CH2OH up to 865.25, 330.38, and 1442.88 mu mol/g in 3 h, respectively. A proposed reaction mechanism based on the experimental results and above characterizations was finally deliberated.
Decarbonizing N 2 conversion is particularly challenging, but essential for sustainable development of industry and agriculture. Herein, we achieve electrocatalytic activation/reduction of N 2 on X/Fe−N−C (X=Pd, Ir an...
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Decarbonizing N 2 conversion is particularly challenging, but essential for sustainable development of industry and agriculture. Herein, we achieve electrocatalytic activation/reduction of N 2 on X/Fe−N−C (X=Pd, Ir and Pt) dual-atom catalysts under ambient condition. We provide solid experimental evidence that local hydrogen radical (H*) generated on the X site of the X/Fe−N−C catalysts can participate in the activation/reduction of N 2 adsorbed on the Fe site. More importantly, we reveal that the reactivity of X/Fe−N−C catalysts for N 2 activation/reduction can be well adjusted by the activity of H* generated on the X site, i.e., the interaction between the X−H bond. Specifically, X/Fe−N−C catalyst with the weakest X−H bonding exhibits the highest H* activity, which is beneficial to the subsequent cleavage of X−H bond for N 2 hydrogenation. With the most active H*, the Pd/Fe dual-atom site promotes the turnover frequency of N 2 reduction by up to 10 times compared with the pristine Fe site.
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