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Flocculation of suspended cohesive particles in homogeneous isotropic turbulence

JOURNAL OF FLUID MECHANICS 2021年 921卷 A17-A17页

作者： Zhao, K. Pomes, F. Vowinckel, B. Hsu, T. -J. Bai, B. Meiburg, E. UC Santa Barbara Dept Mech Engn Santa Barbara CA 93106 USA Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Peoples R China Tech Univ Carolo Wilhelmina Braunschweig Leichtweiss Inst Hydraul Engn & Water Resources D-38106 Braunschweig Germany Univ Delaware Ctr Appl Coastal Res Dept Civil & Environm Engn Newark DE 19716 USA

We investigate the dynamics of cohesive particles in homogeneous isotropic turbulence, based on one-way coupled simulations that include Stokes drag, lubrication, cohesive and direct contact forces. We observe a transient flocculation phase, followed by a statistically steady equilibrium phase. We analyse the temporal evolution of floc size and shape due to aggregation, breakage and deformation. Larger turbulent shear and weaker cohesive forces yield smaller elongated flocs. Flocculation proceeds most rapidly when the fluid and particle time scales are balanced and a suitably defined Stokes number is . During the transient stage, cohesive forces of intermediate strength produce flocs of the largest size, as they are strong enough to cause aggregation, but not so strong as to pull the floc into a compact shape. Small Stokes numbers and weak turbulence delay the onset of the equilibrium stage. During equilibrium, stronger cohesive forces yield flocs of larger size. The equilibrium floc size distribution exhibits a preferred size that depends on the cohesive number. We observe that flocs are generally elongated by turbulent stresses before breakage. Flocs of size close to the Kolmogorov length scale preferentially align themselves with the intermediate strain direction and the vorticity vector. Flocs of smaller size tend to align themselves with the extensional strain direction. More generally, flocs are aligned with the strongest Lagrangian stretching direction. The Kolmogorov scale is seen to limit floc growth. We propose a new flocculation model with a variable fractal dimension that predicts the temporal evolution of the floc size and shape.

关键词： suspensions sediment transport cohesive sediments

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Gasification of unsymmetrical dimethylhydrazine in supercritical water: Reaction pathway and kinetics

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INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 2018年第18期43卷 8644-8654页

作者： Yi, Lei Guo, Liejin Jin, Hui Kou, Jiajing Zhang, Deming Wang, Runyu Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn SKLMF Xian 710049 Shaanxi Peoples R China

Unsymmetrical dimethylhydrazine (UDMH) is a high N-containing (as much as nearly 50%) substance. Traditional treatment methods such as incineration will inevitably cause the formation of nitric oxide and secondary pollution. Supercritical water is a preferred transformation medium due to its unique physicochemical properties. However, at present most of studies are limited to supercritical water oxidation (SCWO) which tends to produce hydrogen nitrate resulting in corrosion to the reactor. To conquer this problem, we propose supercritical water gasification (SCWG) technology which is in a reducing environment, realizing both harmless treatment and resource utilization. In order to promote its industrialization process, the reaction pathways and kinetic parameters should be studied. In this paper, the reaction pathways and kinetics of UDMH in supercritical water were conducted under the conditions of 400 C-degrees-550 C-degrees in quartz reactor, which avoids the catalytic effect on the reaction kinetics. From the resource utilization perspective, the most abundant quantitatively detectable gaseous product is methane, together with less hydrogen, carbon monoxide and ethane orderly. All these gaseous products are combustible. The maximum of carbon efficiency is 90.25% at 550 C-degrees, 10 min. In the point of view of harmless treatment, the organic compounds contained in the residual liquid are detected with H-1 NMR, FTIR and GC/MS. Results show that UDMH could be fully degraded within 3 min and the ultimate organic compounds in the residual liquid are mainly dimethylamino acetonitrile and trimethylamine. In addition, a reaction pathway for UDMH disposed in supercritical water is developed. Finally, the quantitative kinetic model for describing the gaseous products and ammonia-nitrogen in the residual liquid is brought forward. The pyrolysis activation energy for UDMH in supercritical water is 49.98 +/- 7.38 kJ/mol. (C) 2018 Hydrogen Energy Publications LLC. Published

关键词： UDMH SCWG Resource utilization Harmless treatment Reaction pathway Kinetics

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Analysis on the behavior of dispersed plate-type fuel based on fluid-solid coupling method

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PROGRESS IN NUCLEAR ENERGY 2020年 126卷 103398-103398页

作者： Li, Chenxi Wu, Yingwei Wang, Yangyang Wang, Kunpeng Wang, Mingjun Tian, Wenxi Su, G. H. Qiu, Suizheng Xi An Jiao Tong Univ Sch Nucl Sci & Technol Shanxi Key Lab Adv Nucl Energy & Technol State Key Lab Multiphase Flow Power Engn Xian 710049 Peoples R China Minist Ecol & Environm Peoples Republ China Nucl & Radiat Safety Ctr Beijing 100082 Peoples R China

In this study, a three-dimensional bi-directional fluid-solid coupling method FLUENT-ABAQUS-MpCCI was used to analyze the behavior of the fuel plate. ABAQUS was used to simulate the solid domain, including the heat conduction, mechanical behavior and irradiation effects. Several subroutines were developed and validated, including CREEP, UHARD, USDFLD and UEXPAN. FLUENT was used to simulate the fluid domain, including pressure and temperature. FLUENT and ABAQUS were coupled by MpCCI to consider the interaction between fluid domain and solid domain. Many parameters were obtained, including fluid temperature distribution, Mises stress distribution and strain distribution. The influences of the fluid domain, particle volume fraction, creep behavior and burnup were analyzed. The results showed that the zero-dimensional model or one-dimensional model on fluid could not accurately simulate the mechanical behavior of the fuel plate. The results also showed that the creep behavior could reduce the Mises stress on the cladding.

关键词： Fluid-solid coupling analysis Irradiation effects Fuel behavior analysis

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Reactivity of Ni, Cr and Zr doped ceria in CO₂ splitting for CO production via two-step thermochemical cycle

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INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 2018年第30期43卷 13754-13763页

作者： Zhu, Liya Lu, Youjun Li, Feng Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn SKLMF Xian 710049 Shaanxi Peoples R China

This work focuses on modification and screening of ceria-based oxides for solar H2O/CO2 splitting via two-step thermochemical cycle. Ce1-xMxO2-delta (M = Zr, Ni, Cr;x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized via sol-gel method and tested for CO2-splitting via two-step thermochemical cycles. Reduction was conducted at 1500 degrees C through a ramp rate of 10 degrees C/min and oxidation was performed at 1000 degrees C isothermally. Both Ni and Cr showed low solubility in ceria and no or very limited promoting effect on CO productivity. Cr could be reduced in the first reduction step but cannot be oxidized by CO2 in the following oxidation step. Zr doped sample showed advantages in both CO productivity and lattice stability. 15% Zr doped exhibited the best performance with the CO productivity of 315.40 mu mol/g. However, the oxidation rate of Zr doped samples was much lower than that of pure ceria. Compromise between fuel productivity and fast kinetics should be made in practical application. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

关键词： CO2 splitting Thermochemical cycle Solar energy Ceria-based oxides

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Characteristics of interfacial oscillation caused by condensation of steam jet in subcooled water flow

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EXPERIMENTAL THERMAL AND FLUID SCIENCE 2019年 109卷 109864-000页

作者： Xu, Qiang Chu, Xiaona Yao, Tian Liu, Weizhi Guo, Liejin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

An experimental study is conducted to investigate interfacial characteristics of direct contact condensation of steam jet in counter-current subcooled water flow in confined conditions. A customized visualization window and a high-speed camera are combined together to capture and record the dynamical interface behavior of the condensing jet. The transient interface behavior of the jet is analyzed and discussed quantitatively by utilizing digital image processing technology. With increase of steam mass flux, the bubbling and jetting regime appear successively at subsonic conditions and sonic or supersonic conditions, respectively. The transient radial interface position along all axial locations downstream of the jet is obtained quantitatively for these two typical condensation regimes. The probability distribution function and power spectrum density of the radial interface position along all axial locations of a certain jet follow almost the same principles in the time-frequency domain. Significant variations of the probability distribution function and power spectrum density between the bubbling and jetting regimes happen at transonic condition. With increase of axial distance away from the nozzle exit, the probability of the pinch-off events first rise and then drops in subsonic and sonic conditions. While the jet interface is very stable and no pinch-off events occur in supersonic condition.

关键词： Steam condensation Interface position Jet stability Condensation regime multiphase flow

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Central-pointy to central-concave icing transition of an impact droplet by increasing surface subcooling

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INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER 2019年 108卷 104326-000页

作者： Shang, Yuheng Liu, Xiufang Bai, Bofeng Zhong, Xin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

The effect of substrate temperature is investigated experimentally on the ice profile of an impact water droplet. A hydrophilic substrate, a relatively high impact velocity which can lead to a larger contact area, and a low surface temperature are employed aiming at enhancing the heat transfer rate through the droplet. The frozen droplet is uniquely central-concave at low surface temperatures, in contrast to the widely observed central-pointy ice formed at cold surfaces. The formation of the central-concave icing is attributed to the additional interior circular-like freezing front advancing both upwards and outwards during the solidification. Upon decreasing the surface temperature, the nucleation is triggered earlier, and the freezing rate is higher than that of a quasi-steady droplet freezing on a similar subcooled surface due to the impact-induced flattening of the droplet. A freezing regime map described by the dimensionless number representing the heat conduction rate through the substrate and Weber number consists of two regimes for the formation of the central-pointy and central-concave ice profile respectively.

关键词： Impact droplet Freezing Ice profile Spread Heat conduction rate

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Role of solid-liquid interaction energy on anomalous thermal conductivity enhancement in well-dispersed dilute nanofluids studied by equilibrium molecular dynamics

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CHEMICAL PHYSICS 2020年 539卷 110943-110943页

作者： Wang, Xin Jing, Dengwei Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Peoples R China Xi An Jiao Tong Univ Int Res Ctr Renewable Energy Xian 710049 Peoples R China

In this study, Green-Kubo formulism was employed to quantitatively assess thermal conductivity of nanofluids with different solid-liquid interaction energy. Anomalous thermal conductivity enhancement was observed in well-dispersed dilute nanofluids with strong solid-liquid interaction energy. The simulation results indicated that solid-liquid interaction is the key factor for the understanding of the heat conduction mechanism in nanofluids. Green-Kubo theory and Brownian motion were studied in detail to derive the physical origin of thermal conductivity enhancement. The results suggested that the phonons across the solid-liquid interface or micro-convection induced by Brownian motion of nanoparticles is responsible for thermal conductivity enhancement. In addition, a wide range of thermal conductivity enhancement and nonlinear increasing with volume fraction was observed, which is consistent with experiment investigations. The effect of solid-liquid interaction energy on the viscosity of nanofluids was also studied and was found to be an insignificant factor. Our results indicated that the nanofluids with strong solid-liquid interaction energy should have more significant application potential.

关键词： Molecular dynamics Nanofluids Thermal conductivity Viscosity Micro-convection Brownian motion

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Hybrid Photoelectrochemical Water Splitting Systems: From Interface Design to System Assembly

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ADVANCED ENERGY MATERIALS 2020年第11期10卷

作者： Niu, Fujun Wang, Degao Li, Fei Liu, Yanming Shen, Shaohua Meyer, Thomas J. Xi An Jiao Tong Univ Int Res Ctr Renewable Energy State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China Univ N Carolina Dept Chem CB 3290 Chapel Hill NC 27599 USA Dalian Univ Technol State Key Lab Fine Chem Dalian 116024 Peoples R China

Photoelectrochemical (PEC) water splitting has attracted increasing attention due to its potential to mitigate energy and environmental issues. Hybrid PEC systems containing semiconductor photosensitizers and molecular catalysts are reported to be highly active and stable for water splitting with great potential for facilitating clean fuels production. In this review, following a showcasing of the fundamental details of hybrid PEC systems for water splitting, semiconductor/molecular catalyst interface designs are highlighted, with a focus on interfacial physicochemical interactions and binding, and interfacial energetics and dynamics for efficient charge transfer. Recent advances in hybrid system assemblies for PEC water splitting are also briefly introduced. Finally, future challenges and directions in the field of hybrid PEC water splitting for solar energy conversion are reviewed. The current review provides state-of-the-art strategies for optimized interface design for creating highly active and stable PEC water splitting assemblies.

关键词： artificial photosynthesis interface design photoelectrode assembly water splitting

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Effect of alkali and alkaline metals on gas formation behavior and kinetics during pyrolysis of pine wood

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FUEL 2021年 290卷 120081-120081页

作者： Li, Jinhu Burra, Kiran G. Wang, Zhiwei Liu, Xuan Gupta, Ashwani K. Univ Maryland Dept Mech Engn Combust Lab College Pk MD 20742 USA China Univ Geosci Fac Engn Wuhan 430074 Hubei Peoples R China Henan Univ Technol Coll Environm Engn Zhengzhou 450001 Peoples R China Xi An Jiao Tong Univ Sch Energy & Power Engn State Key Lab Multiphase Flow Power Engn Xian 710049 Peoples R China

This study examines the acid and alkaline treatment of pine wood to help understand the effect of alkali and alkaline earth metals (AAEMs) content on the pyrolysis behavior at different temperatures. Acid and alkali pretreatment of pine wood were conducted to modify AAEMs content by ion-exchanging method. Thermal kinetic behavior of the pretreated samples was first conducted by a thermogravimetric analyzer (TGA) at different heating rates that provided activation energies of pyrolysis. Gas formation behavior of the samples for different extents of conversion was carried out in a fixed-bed reactor at two different temperatures of 823 K and 1073 K. Evolutionary behavior of the gas components during pyrolysis, including flow rate, and yield and their energy content were measured and compared. Results showed that the values of activation energies increased with the extent of conversion for all the pretreated samples examined. The effect of AAEMs on pyrolysis behavior of biomass varied with the extent of conversion. The presence of AAEMs in biomass decreased the decomposition energy at low conversion while it greatly improved under high conversion. Besides, alkali treated sample with higher AAEM content enhanced the gas and char yield while it reduced the bio-oil production at low temperature with low conversion. However, at high temperature the opposite trend was observed. Presence of AAEMs was favorable for the generation of H-2, CO, CO2 and CnHm at low temperature, while it showed an inhibition effect on CO and CnHm yield and syngas energy at high temperature. The catalytic mechanism of AAEMs on the pyrolysis behavior at different temperatures was discussed based on activation energy and gaseous formation. Results revealed decomposition of carboxylate at low temperature and formation of stable biomass-Na (BM-Na) structure at high temperature that led to the variation of activation energy and changed gaseous products yield and syngas energy. Acid washing pretreatment w

关键词： Kinetic behavior Mechanism Ion-exchanging Gas components Activation energy Syngas yield

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Characteristics of frictional pressure drop of two-phase nitrogen flow in horizontal smooth mini channels in diabatic/adiabatic conditions

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APPLIED THERMAL engineering 2019年 162卷 114312-114312页

作者： Chen, Xingya Hou, Yu Chen, Shuangtao Liu, Xiufang Zhong, Xin Xi An Jiao Tong Univ State Key Lab Multiphase Flow Power Engn Xian 710049 Shaanxi Peoples R China

The two-phase frictional pressure drop of nitrogen under diabatic and adiabatic conditions is investigated experimentally in two horizontal mini channels. The effects of factors in a variety, including the channel diameter, mass flux, heat flux, inlet pressure and vapor quality are examined on the frictional pressure drop and liquid-phase multiplier of the two-phase flow. The two-phase frictional pressure drop predicted by previous correlations from other studies are compared with the experimental results of this study, and the lowest mean absolute deviation is 18.3% for adiabatic conditions and 10.9% for adiabatic conditions. Based on the experimental results of nitrogen flow, two new correlations in the form of the Chisholm-C correlation are developed to predict the two-phase frictional pressure dropby involving the effect of surface tension, and the mean absolute deviation of which is 12.3% and 10.4% for the adiabatic and diabatic conditions respectively. The new correlations are also shown able to well predict the experimental frictional pressure drop of other studies under different experimental conditions.

关键词： Frictional pressure drop Liquid nitrogen Mini channel Two-phase flow Correlation

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