The design of a pressurized capillary rheometer operating at prescribed temperature is described to measure the viscosity of magnetic fluids (MFs) containing Fe3O4 magnetic nanoparticles (MNPs). The equipment cons...
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The design of a pressurized capillary rheometer operating at prescribed temperature is described to measure the viscosity of magnetic fluids (MFs) containing Fe3O4 magnetic nanoparticles (MNPs). The equipment constant of the rheometer was obtained using liquids with predetermined viscosities. Experimentally measured viscosities were used to evaluate different equations for suspension viscosities. Deviation of measured suspension viscosities from the Einstein equation was found to be basically due to the influence of spatial distribution and aggregation of Fe3O4 MNPs. By taking account of the coating layer on MNPs and the aggregation of MNPs in MFs, a modified Einstein equation was proposed to fit the experimental data. Moreover, the influence of external magnetic field on viscosity was also taken into account. Viscosities thus predicted are in good agreement with experimental data. Temperature effect on suspension viscosity was shown experimentally to be due to the shear-thinning behavior of the MFs.
In this article, comprehensive comparisons are made between the SIMPLER and IDEAL algorithms for four application examples. It is found that the IDEAL algorithm is efficient and stable not only for the simple, low-Re/...
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In this article, comprehensive comparisons are made between the SIMPLER and IDEAL algorithms for four application examples. It is found that the IDEAL algorithm is efficient and stable not only for the simple, low-Re/Ra or coarse-mesh flow cases, but also for the complex, high-Re/Ra or fine-mesh flow cases. For the low-Re/Ra, coarse-mesh flow cases, the ratio of CPU time of IDEAL to that of SIMPLER ranges from 0.029 to 0.7. For the high-Re/Ra, fine-mesh flow cases, the IDEAL algorithm can obtain convergent results but the SIMPLER algorithm cannot, even though the underrelaxation factors are adjusted. [ABSTRACT FROM AUTHOR]
Slug flow is a highly intermittent flow regime in which large bubble slugs flow alternately with liquid slugs at randomly fluctuating frequency. A one-dimensional model for the interfacial structure in bubble slug was...
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Slug flow is a highly intermittent flow regime in which large bubble slugs flow alternately with liquid slugs at randomly fluctuating frequency. A one-dimensional model for the interfacial structure in bubble slug was developed based on mass and momentum conservation for bubble nose, body and tail as well as for a hydraulic jump at the rear of the bubble. The model can reproduce experimental results accurately. The results indicate that the interfacial structure is strongly dependent on the Froude number defined by gas/liquid mixture velocity, pipe inclination, and bubble length.
Considering the fact that proton exchange membrane (PEM) fuel cell has different impedance performances under different discharge currents, a PEM fuel cell with single serpentine flow field was investigated with elect...
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Considering the fact that proton exchange membrane (PEM) fuel cell has different impedance performances under different discharge currents, a PEM fuel cell with single serpentine flow field was investigated with electrochemical impedance spectroscopy, and the R (QR) equivalent circuit was used to calculate the circuit element parameters. The experimental results show that water in the fuel cell increases from insufficient to sufficient and then excessive with the increasing discharge current. Accordingly, the electrochemical reaction rate on the electrodes increases first and then decreases, thus the charge transfer resistance decreases and then increases. The impedance of PEM fuel cell increases obviously for high discharge currents due to the accessional impedance produced by double-layer effect, while the impedance reaches minimum at medium discharge current (5 A).
The behavior of a single polyethylene polymer in aqueous solution confined between two hydrophilic walls is studied with molecular dynamics (MD) simulations. The thickness of the nano-slit ranges from 1.26 to 3.15 nm,...
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The behavior of a single polyethylene polymer in aqueous solution confined between two hydrophilic walls is studied with molecular dynamics (MD) simulations. The thickness of the nano-slit ranges from 1.26 to 3.15 nm, which is comparative to the polymer dimension. A monotonic transition from 3D- to 2D-like configurations is observed as the distance between the two walls narrows. Monomers are compressed into several layers and the preferred bond orientations alternate between parallel and normal to the walls accordingly. The diffusivity in the direction parallel to the wall is always larger than the one perpendicular to it. Calculation of the entropy and enthalpy changes during the folding of the polymer chain alone cannot explain the spontaneous process. The corresponding increase in water entropy due to volume expansion may be large enough to result in the overall free energy decrease.
Biodiesel is generally produced by the transesterification of vegetable oils or animal fats com- posed of C-C fatty acid triglycerides with methanol under the presence of a catalyst. The conventional biodiesel product...
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Biodiesel is generally produced by the transesterification of vegetable oils or animal fats com- posed of C-C fatty acid triglycerides with methanol under the presence of a catalyst. The conventional biodiesel production technology use base or acid *** are several
Gasification is a clean technique that converts coal into syngas, which can be cleaned much more easily than the coal itself. The raw sysgas exiting the gasifier is usually at a very high temperature. To protect the r...
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ISBN:
(纸本)189097725X
Gasification is a clean technique that converts coal into syngas, which can be cleaned much more easily than the coal itself. The raw sysgas exiting the gasifier is usually at a very high temperature. To protect the rearward equipment, the raw syngas needs to be cooled to allow appropriate cleaning of particulates, ashes, and sulfur before it exits the gasifier. Cooling of the syngas can be performed by employing heat exchangers or direct spray cooling. This paper focuses on conducting a 3-D CFD parametric study to aid in the design of an effective water spray cooling deployment. The studied parameters include water droplet sizes (100400m) and water injection angles. The goal is to cool the syngas from 1550C to 800C using the shortest spray section. The gasifier is designed to produce methanol at a full load condition, so the amount of sprayed water is fixed to ensure the syngas composition meets the specifications for methanol production. The gasifier is steam-oxygen blown with dry coal feed using approximately 960 tons per day. The results reveal that smaller droplets are effective in reducing the cooling section length. Moreover, using three water injection angles show that inclining the spraying angle upward 45 degrees could shorten the effective cooling length. This study provides a guide for cutting the cost of manufacturing a gasifier cooling section.
The Energy-Minimization Multi-Scale (EMMS) model (Li et al., 1988;Li & Kwauk, 1994) is employed in Eulerian-Eulerian simulations of the hydrodynamics of a pilot scale CFB riser, with an implicit cluster diameter c...
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ISBN:
(纸本)9783930400577
The Energy-Minimization Multi-Scale (EMMS) model (Li et al., 1988;Li & Kwauk, 1994) is employed in Eulerian-Eulerian simulations of the hydrodynamics of a pilot scale CFB riser, with an implicit cluster diameter correlation accounting for the effect of sub-grid scale heterogeneity on inter-phase drag force. The simulated solid circulation flux, radial and axial solid concentration distributions and the radial particle velocity profile agree well with corresponding measurements. It presents a further step in establishing sub-grid scale models, which has been a focus in the study of gas-solid flows, on the basis of variational principles.
Multi-scale heterogeneous structures are characteristic of the concurrent-up gas-solid flow in circulating fluidized bed (CFB) risers, which stand for a grand challenge to accurate simulation of industrial-scale CFB r...
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ISBN:
(纸本)9783930400577
Multi-scale heterogeneous structures are characteristic of the concurrent-up gas-solid flow in circulating fluidized bed (CFB) risers, which stand for a grand challenge to accurate simulation of industrial-scale CFB risers with reasonable computational cost. Coarse-grid simulations using traditional two-fluid models (TFM), although efficient, are not accurate enough for the purpose of design and control of industrial units, while direct numerical simulations (DNS) starting from below the Kolmogorov scale and particle scale are currently unrealistic for this purpose. Coarse-grid simulations with proper sub-grid scale (SGS) models are expected to provide both reasonable accuracy and efficiency, but the closure of SGS models has been a long-lasting controversial issue. In this presentation, we review our attempts in providing such a closure with specific stability condition for flows in CFB risers on the grid scale, that is, the so-called analytical multi-scale approach and its later improvements and extensions. The resultant SGS model has been incorporated into commercial software and applied in the simulation of industrial CFB risers with various sizes and bed materials. Further extension of the model to mass transfer processes shows that superficial Reynolds number is insufficient for correlating mass transfer rate as multi-scale heterogeneity is not reflected in this criterion. The presentation will be concluded by prospects in multi-scale computational fluid dynamics of multiphase systems.
Various process residues represent a kind of biomass resource already concentrated but containing water as much as 60 wt.%. These materials are generally treated as waste or simply combusted directly to generate heat....
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Various process residues represent a kind of biomass resource already concentrated but containing water as much as 60 wt.%. These materials are generally treated as waste or simply combusted directly to generate heat. Recently, we attempted to convert them into middle caloric gas to substitute for natural gas, as a chemical or a high-rank gaseous fuel for advanced combustion utilities. Such conversion is implemented through dual fluidized bed gasification (DFBG). Concerning the high water content of the fuels, DFBG was suggested to accomplish either with high-efficiency fuel drying in advance or direct decoupling of fuel drying/pyrolysis from char gasification and tar/hydrocarbon reforming. Along with fuel drying, calcium-based catalyst can be impregnated into the fuel, without much additional cost, to increase the fuel's gasification reactivity and to reduce tar formation. This article reports the Ca impregnation method and its resulting effects on gasification reactivity and tar suppression ability. Meanwhile, the principle of directly gasifying wet fuel with decoupled dual fluidized bed gasification (D-DFBG) is also highlighted.
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