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Large Eddy Simulation of a dilute particleladen turbulent flow over a backwardfacing step

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Science China(Technological Sciences) 2008年第11期51卷 1957-1970页

作者： ZHANG HuiQiang wang Bing CHAN CheongKi wang xilin Department of Engineering Mechanics Tsinghua UniversityBeijing 100084China Department of Applied Mathematics Hong Kong Polytechnic UniversityHong KongChina

Dilute gas-particle turbulent flows over a backward-facing step are numerically simulated by Large Eddy Simulation (LES) for the continuous phase and Lagran- gian particle trajectory method for the particle phase. Predicted results of mean velocities and fluctuating velocities of both phases agree well with the experimental data, and demonstrate that the main characteristics of the flow are accurately captured by the simulations. Characteristics of separation and reattachments as well as essential features of the coherent structure are obtained, in which the processes of vortex roll up, growth, pairing and breaking up are shown in details. Particle dispersions are then investigated through particles’ instantaneous distri- butions in coherent structure as well as the mean and fluctuating properties of particle number density (PND). The predicted mean PND agree well with experiment results. For small particles, the instantaneous distributions show much preferential concentration, while their mean PND shows more uniform distribution in down- stream region. On the contrary, for large particles, their instantaneous distributions are much uniform (without clear preferential concentration) due to less effect of large eddy coherent, while their mean PND across the section is not uniform for more particles are distributed in the main flow region. The preferential concentra- tion of particles by the large-scale eddies can lead to a high fluctuating PND.

关键词： two-phase flow particle dispersion Large Eddy Simulation backward-facing step flow spatially developing flow particle number density

Transports of air particulate matters in the atmospheric boundary layer-numerical studies using Eulerian and Lagrangian methods

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Science China(Physics,Mechanics & Astronomy) 2013年第3期56卷 645-650页

作者： LU Hao wang Bing ZHANG HuiQiang wang xilin School of Aerospace Tsinghua University The State of Key Laboratory of Nonlinear Mechanics

Transports of air particulate matters(PM) from face sources in the atmospheric boundary layer(ABL) are investigated by the Eulerian single fluid model and the Lagrangian trajectory method,*** eddy simulation is used to simulate the fluid phase for high accuracy in both two *** mean and fluctuating PM concentrations,as well as instantaneous PM distributions at different downstream and height positions,are *** mean and fluctuating particle concentrations are predicted by the Eulerian approach than the Lagrangian *** the Lagrangian method,PM distributions cluster near the ground-wall because of the preferential dispersion of inertial particles by turbulence structures in the ABL,while it cannot be obtained by the Eulerian single fluid method,because the two-phase velocity differences are neglected in the Eulerian method.

关键词： air pollution atmosphere boundary layer large eddy simulation Eulerian method Lagrangian method

Analysis of combustion instability via constant volume combustion in a LOX/RP-1 bipropellant liquid rocket engine

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Science China(Technological Sciences) 2012年第4期55卷 1066-1077页

作者： ZHANG HuiQiang GA YongJing wang Bing wang xilin Department of Engineering Mechanics Tsinghua UniversityBeijing 100084China

Turbulent two-phase reacting flow in the chamber of LOX/RP-1 bipropellant liquid rocket engine is numerically investigated in this paper. The predicted pressure and mean axial velocity are qualitatively consistent with the experimental measurements. The self-excited pressure oscillations are obtained without any disturbance introduced through the initial and boundary conditions. It is found that amount of abrupt pressure peaks appear frequently and stochastically in the head regions of the chamber, which are the important sources to drive and strengthen combustion instability. Such abrupt pressures are induced by local constant volume combustion, because local combustible gas mixtures with high temperature are formed and burnt out suddenly due to some fuel droplets reaching their critical state in a rich oxygen surrounding. A third Damkhler number is defined as the ratio of the characteristic time of a chemical reaction to the characteristic time of a pressure wave expansion to measure the relative intensity of acoustic propagation and combustion process in thrusters. The analysis of the third Damkhler number distributions in the whole thrust chamber shows that local constant volume combustion happens in the head regions, while constant pressure combustion presents in the downstream regions. It is found that the combustion instability occurs in the head regions within about 30 mm from the thruster head.

关键词： combustion instability constant volume combustion spray combustion LOX/RP-1 bipropellant liquid rocket engine third Damkohler number

Dense gas-particle flow in vertical channel by multi-lattice trajectory model

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Science China(Technological Sciences) 2012年第2期55卷 542-554页

作者： ZHANG HuiQiang LIU Min wang Bing wang xilin School of Aerospace Tsinghua UniversityBeijing 100084China

A multi-lattice deterministic trajectory(MLDT) model is developed to simulate dense gas-particle flow in a vertical *** actual inter-particle collision and particle motion are treated by a Lagrangian model with three sets of lattices to reduce computational *** formation and motion near the wall are successfully predicted with mean particle volume fraction and velocity,showing quantitatively agreement with experimental *** mechanism of particles concentrated near the wall is investigated by considering effects of gravity,particle-wall collisions,inter-particle collisions and velocity profiles of the gas *** is shown that the inter-particle collision and gas-phase velocity distribution are the essential factors for cluster formation near the wall,while gravity and particle-wall collision only have minor effects on particle concentration near the *** are unable to remain in the high velocity region due to the strong inter-particle collisions,while they tend to stay in the low velocity region for weak inter-particle *** addition,the effects of channel width and particle sizes on cluster formation are also investigated and it is found that particle concentration near the wall reduces with the decrease of channel width and increase of particle size.

关键词： multi-lattice deterministic trajectory (MLDT) model dense gas-particle channel flow core-annular regime clusterformation

A fabrication method for adaptive dielectric gradient insulating components

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High Voltage 2023年第1期8卷 59-69页

作者： Zikui Shen Zhidong Jia Yanpeng Hao Zhenyu Xin xilin wang School of Electric Power Engineering South China University of TechnologyGuangzhouChina Shenzhen International Graduate School(SIGS) Tsinghua UniversityShenzhenChina

Dielectric gradient components have advantages in electric field mitigation and insulation *** this paper,we propose a fabrication method for adaptive dielectric gradient components using in situ AC electric field,including the mechanism and the corresponding operational procedures for industrial *** on the electric polarisation and self-assembly effect of the filler particles in the liquid matrix,the chainlike structure in the high field strength region is constructed to enhance the local permittivity and mitigate the maximum of the spatial electric *** dielectric gradient basin insulator is prepared by this method,and its flashover voltage is increased by 12.8%compared with that of a homogeneous dielectric basin insulator,and the improvement is 20.8%when metal particles are present on the *** more non-uniform the initial electric field is,the greater the improvement in flashover *** method is expected to promote the industrial application of dielectric gradient insulating components.

关键词： dielectric gradient method

Analysis of solid particle clusters in coherent structures of turbulent channel flow

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Science China(Technological Sciences) 2013年第10期56卷 2525-2530页

作者： LU Hao wang Bing ZHANG HuiQiang QIN JianXiu wang xilin School of Aerospace Tsinghua University

A particle-laden turbulent channel flow is investigated to study particle clusters in large-scale turbulent coherent structures. The fluid phase is calculated by large eddy simulation and particles are tracked using Lagrangian trajectory method. The flow Reynolds number is 180 based on the friction velocity and half-width of the channel. The particle is lycopodium with St=0.93 which may well follow the fluid phase. The mean and fluctuating velocities of both two phases are obtained, which are in good agreement with previous data. The strongest accumulations of particles in low-speed streak structures are observed at y~=l 1.3. Moreover, once particles are captured in low-speed streaks, most of them will reside there for a long period. Particles clustered in low-speed streaks obtain smaller instantaneous wall-normal and spanwise velocities than those out of there, which induce a larger particle flux into low-speed streaks than that out of there. The study is important for understanding particle dispersion mechanisms in gas-particle turbulent channel flows.

关键词： particle cluster preferential concentration low-speed streak coherent structure large eddy simulation Lagrangiontrajectory method

Analytical model for the transient permittivity of uncured TiO_(2) whisker/liquid silicone rubber composites under an AC electric field

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High Voltage 2021年第3期6卷 470-479页

作者： Zikui Shen Zhenyu Xin xilin wang Xinyu Wei Zhidong Jia Engineering Laboratory of Power Equipment Reliability in Complicated Coastal Environment Shenzhen International Graduate SchoolTsinghua UniversityShenzhenChina

The electric field grading of dielectric permittivity gradient devices is an effective way of enhancing their insulation *** in situ electric field‐driven as-sembly is an advanced method for the fabrication of insulation devices with adaptive permittivity gradients;however,there is no theoretical guidance for *** this study,an analytical model with a time constant is developed to determine the transient permittivity of uncured composites under an applied AC electric *** model is based on optical image and dielectric permittivity monitoring,which avoids the direct processing of complex *** a composite with given components,the increased filler content and electric field strength can accelerate the transient *** with the finite element method based on differential equations,this statistical model is simple but efficient,and can be applied to any low‐viscosity uncured composites,which may contain multiple *** importantly,when a voltage is applied to an uncured composite insulating device,the proposed model can be used to analyse the spatiotemporal permittivity characteristics of this device and optimise its permittivity gradient for electric field grading.

关键词： composites permittivity transient

Analysis of conductance and interfacial polarisation behaviours of silicone rubber based on frequency domain spectroscopy

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High Voltage 2024年第1期9卷 56-65页

作者： Hao Hu Zhidong Jia xilin wang Ruiyang Guan Weinan Fan Simin Luo Tsinghua Shenzhen International Graduate School Tsinghua UniversityShenzhenChina Guangzhou Power Supply China Southern Power Grid Company Ltd.GuangzhouChina

Silicone rubber(SR)is widely used for cable insulation and operates in multi-stress coupling *** dielectric properties and ageing performance are the essential factors affecting cable *** this study,the frequency domain spectroscopy of unaged SR and SR aged under various conditions is *** dielectric responses before and after ageing are analysed in depth by the conjoint analysis via multiple complex planes(permittivity,conductivity,and modulus).Typical conductivity and interfacial polarisation properties,as well as property changes resulting from thermal and tensile ageing,are *** results show that tensile ageing promotes conductance and interfacial polarisation,while thermal ageing shows the opposite *** on the multi-core model and charge carrier hopping transport model,thermal and tensile ageing mechanisms on the conductance and interfacial polarisation behaviours of SR are *** behaviours exhibit positively correlated changing characteristics,suggesting that the two behaviours may originate from a common *** carrier transport process,promoted by the tensile effect and slightly suppressed by the thermal effect,is the primary factor and considered an important indicator to evaluate the dielectric properties of SR insulation.

关键词： interfacial rubber insulation

Gas-discharge induced flash sintering of YSZ ceramics at room temperature

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Journal of Advanced Ceramics 2022年第4期11卷 603-614页

作者： Yuchen ZHU Hongyang ZHOU Rongxia HUANG Nianping YAN xilin wang Guanghua LIU Zhidong JIA Engineering Laboratory of Power Equipment Reliability in Complicated Coastal Environments Tsinghua Shenzhen International Graduate SchoolTsinghua UniversityShenzhen 518055China School of Electromechanical Engineering Guangdong University of TechnologyGuangzhou 510006China State Grade Jiangxi Electric Power Research Institute Nanchang 330000China State Key Laboratory of New Ceramics and Fine Processing Tsinghua University School of Materials Science and TechnologyTsinghua UniversityBeijing 100084China

This is the first study to conduct the flash sintering of 3 mol%yttria-stabilized zirconia(3YSZ)ceramics at room temperature(25℃)under a strong electric field,larger than 1 kV/*** the standard atmospheric pressure(101 kPa),the probability of successful sintering is approximately half of that at low atmospheric pressure,lower than 80 *** success of the proposed flash sintering process was determined based on the high electric arc performance at different atmospheric pressures ranging from 20 to 100 *** 3YSZ samples achieved a maximum relative density of 99.5%with a grain size of~200 *** results showed that as the atmospheric pressure decreases,the onset electric field of flash sintering decreases,corresponding to the empirical formula of the flashover ***,flash sintering was found to be triggered by the surface flashover of ceramic samples,and the electric arc on the sample surfaces floated upward before complete flash sintering at overly high pressures,resulting in the failure of flash *** study reveals a new method for the facile preparation of flash-sintered ceramics at room temperature,which will promote the application of flash sintering in the ceramic industry.

关键词： flash sintering yttria-stabilized zirconia(YSZ) room temperature flashover AC field low atmospheric pressure