科创中国

Understanding the electroosmotic flow in microchannels is of both fundamental and practical significance for the design and optimization of various microfluidic devices to control fluid motion. In this paper, a lattice Boltzmann equation, which recovers the nonlinear Poisson-Boltzmann equation, is used to solve the electric potential distribution in the electrolytes, and another lattice Boltzmann equation, which recovers the Navier-Stokes equation including the external force term, is used to solve the velocity fields. The method is validated by the electric potential distribution in the electrolytes and the pressure driven pulsating flow. Steady-state and pulsating electroosmotic flows in two-dimensional parallel uniform and nonuniform charged microchannels are studied with this lattice Boltzmann method. The simulation results show that the heterogeneous surface potential distribution and the electroosmotic pulsating flow can induce chaotic advection and thus enhance the mixing in microfluidic systems efficiently.

G. H. Tang;Zhuo Li;J. K. Wang;雅玲 何;文铨 陶

Journal of Applied Physics

2006

In the current work, we present an experimental study to investigate the reaction mechanism of silica aerogel material at high temperature. The experimental study is based on simultaneous thermal analyzer 449F3 as well as Fourier Transform Infrared Spectroscopy (FTIR). The results show that when the material is exposed at high temperature, the mass of material will decrease about 1.98% accompanying with endothermic effect. Via FTIR analysis, it is found that the mass loss of material may be caused by the removal of adsorbed water. Besides the experimental study, a numerical heat transfer model is constructed to study the transient heat transfer characteristic of aerogel material by taking the endothermic reaction into account. The numerical heat transfer model is validated by a classical Stefan problem and a corresponding experimental measurement. Afterwards, a parametric study is conducted to investigate the effect of two parameters, reaction temperature and reaction heat, on transient heat transfer characteristics of aerogel insulating material. The results show that: (1) Before reaching the reaction temperature T_reaction, the material that with lower T_reaction possesses lower temperature. However, when it exceeds the reaction temperature, the material shows the highest temperature among other materials. (2) With the increase of reaction heat, the increasing trend of temperature is weakened and the emergent of the turning point that separates the temperature platform and temperature-rise period appears later.

Tao Xie;雅玲 何;Zi Xiang Tong;Wei Xu Yan;Xiang Qian Xie

International Journal of Heat and Mass Transfer

2014

Nano to micro tungsten films are widely used as low resistivity electrical connections in MEMS industry. Pyrolytic laser chemical vapor deposition (LCVD) can directly fabricate thin films or microstructures with the minimum thermal damage. The optimal conditions of the tungsten film growth in LCVD are studied by combining experiments and numerical simulations. A proper region of laser powers is determined by the systematical experiments, which is in the range 40–66 W. Based on a transient numerical model, computational chemistry method within the transition state theory is employed to investigate the deposition rate in the multi-line scanning process. It is found that the energy consumption and vacuum requirement of LCVD are the lowest of all tungsten-film fabrication technologies. From the numerical results the average growth rate is controlled by the laser power and the film uniformity mainly depends on the hatch space. The results provide the guidelines of choosing optimal growing conditions and are helpful in producing high quality tungsten films using laser technique.

Bo Ning;Tian Xia;Zi Xiang Tong;雅玲 何

International Journal of Heat and Mass Transfer

2019-9

In this work, a feedforward-feedback hybrid control strategy was proposed for concentrating solar thermal technology. Inside, the feedforward unit was designed to counteract the measured disturbance (mainly originated from the changeable weather), and the feedback unit employing proportional-integral controller to counteract some unmeasured and non-linear disturbances, e.g. local cloud cover, local defocus and mirror damage. For disturbance-rejecting and setpoint-tracking regulating issues, the hybrid strategy is superior to the single control mode in improving responsiveness, reducing steady-state error, as well as weakening overshoot. A feedforward-relaxed hybrid scheme allows enough tolerance for feedforward implementation and simultaneously helps determine the optimal regulation. For a closed power plant, the regulated operation contributes to additional 0.5%–1% daily efficiency in four typical meteorological days. Above all, the high-performance control strategy in energy-supply side lays the foundation for scheduling and dispatching the available energy and leaves enough room for the global energy management and market balance. It is suggested to incorporate feedforward information to enhance regulation performance, meet multi-grade energy demands, as well as turn the feedback-dominated regulation tide.

Lu Li;印实 李;Huajie Yu;雅玲 何

Renewable Energy

2020-7

The Direct Simulation Monte Carlo (DSMC) method models fluid flows using simulation particles which represent a large number of real molecules in a probabilistic simulation to solve the Boltzmann equation. Different opinions still exist on some basic understandings of the controlling factors of DSMC method, such as the proper grid dimension and the proper number of particles in a cell. In this contribution, DSMC simulation of Poiseuille flow was carried out to evaluate the dependence of simulation results on cell dimension and number of particles per cell. In the simulation process a self-adaptive block-structured grid system was employed to guarantee that the number of particles per cell is constant. The simulation covers both 2D and 3D, slip flow regime and transition flow regime, and for each regime, covers both high pressure and low pressure cases. Our simulation results indicate that the number of particles per cell and scaling factor have little influence on simulation result for both slip flow and transition flow when the number of particles per cell is greater than 5, but the dimension of cell influences the accuracy of results significantly. The error caused by cell dimension decreases with the decrease of cell dimension. It is concluded that in the DSMC method it is necessary to ensure that the cell is less than 1/2 of molecular mean free path.

Zhi Xin Sun;Zhen Tang;雅玲 何;文铨 陶

Computers and Fluids

2011-11

Microscale fluid dynamics has received intensive interest due to the rapid advances in microelectromechanical systems. In this work, the lattice Boltzmann method is applied to simulate isothermal gaseous slip flow in three-dimensional (3D) rectangular microducts and microscale porous structures. The flow characteristics in 3D microducts-including velocity profile, nonlinear pressure distribution, friction factor, and mass flow rate-are compared with analytical solutions, and the agreement is good. The flow-rate results show that due to the slip-velocity emergence at the walls, the lateral wall influence on the flow rate in 3D rectangular microducts is decreased. The predicted transport characteristics in 3D microscale porous media show that the rarefaction influence increases the gas permeability. The Klinkenberg effect is confirmed and the predicted gas permeability is qualitatively consistent with the experimental results. Furthermore, the nonlinear behavior of the porous flow at relatively higher Reynolds number is also observed. This study demonstrates that the lattice Boltzmann method can be employed to efficiently predict transport characteristics in microducts and microscale porous media.

G. H. Tang;文铨 陶;雅玲 何

Journal of Applied Physics

2005-5-15

A numerical method was developed to simulate the fouling processes on tubes. The detailed particle deposition and removal mechanisms were included in the model and the evolution of the shape of fouling layers was obtained. Multiple-relaxation-time lattice Boltzmann method (MRT-LBM) and finite volume method (FVM) were coupled to simulate the air flow. The particle motion was simulated by the probabilistic cellular automata model. The restitution coefficient was calculated by energy conservation and used as the deposition criterion. The particle removal was determined by the force and moment analysis. Since the simulation time was much shorter than the real time of the fouling process, a ratio was proposed for the time conversion between the simulation time and real time. The fouling processes for different particle diameters and inlet velocities were simulated by the proposed method. When the mass concentration was specified, small particles had large fouling rates. The fouling area grew linearly with time without removal mechanism, but grew exponentially to an asymptotic balance value when the removal mechanism was considered. The mass flux, particle deposition and removal were simultaneously influenced by the inlet velocity, so the relation between the inlet velocity and fouling rate was not monotonic. A velocity range existed in which the fouling rate was high. The removal was severe on the windward side and the area right behind the tubes, but relatively moderate on the laterals of the leeward side. The fouling layers grew on the entire leeward side of the tubes. On the windward side, the cone-shaped fouling layers were formed, which changed the flow and stopped the further particle deposition.

Zi Xiang Tong;Ming Jia Li;雅玲 何;Hou Zhang Tan

Applied Energy

2017-1-1

First the flow friction characteristics of nitrogen and helium in stainless steel microtubes, glass microtubes, square glass microchannels, and rectangular silicon microchannels are tested. The data in glass microtubes with diameters from 50 to 201 microns and in square glass channels with characteristic diameters from 52 to 100 microns show that the friction factors are in good agreement with the conventional predictions. The friction factors in stainless-steel tubes with diameters from 119 to 300 microns are much higher than the conventional ones. The results for two of the four silicon microchannels with characteristic diameters from 26-60 microns are in good agreement while those of the other two channels are larger. This discrepancy is resulted from the large relative surface roughness. Smaller friction factors in glass microtubes with diameters from 10 to 20 microns are obtained due to the rarefaction effect. Second the flow friction experimental data for deionized water flow in glass microtubes with diameters from 50 to 530 microns show that friction factors and transition Reynolds numbers are in good agreement with the conventional predictions. However, the friction factors in stainless steel microtubes with diameters from 50-1570 microns are much higher than the conventional predictions. This discrepancy is attributed to the large surface relative roughness or denser roughness distribution. Numerical simulations considering electroviscous effect are carried out. The simulation results show that the electroviscous effect does not play a significant role in the friction factor for channel dimensions of the order of microns though it does affect the velocity profile and hence it could be neglected in engineering applications for channel dimensions of the order of microns. Third the measured local Nusselt number distribution of deionized water along the axial direction of the stainless steel tubes of 373-1570 microns with uniform heat flux do not accord with the conventional results when Reynolds number is low and the relative thickness of the tube wall is high. Numerical study reveals that the large ratio of wall thickness to tube diameter at low Reynolds number causes significant axial heat conduction in the tube wall, leading to a non-linear streamwise distribution of the fluid temperature. The axial wall heat conduction effect is gradually weakened with the increase of Reynolds number and the decrease of the relative tube wall thickness. In conclusion, the conventional fluid flow and heat transfer theories should still be applied for single-phase flow in smooth microchannels. Nevertheless, micro-channels do raise some issues to be paid special attention to when being applied.

文铨 陶;Tang Gui-Hua;雅玲 何;Li Zhuo

2008

Obstructed flow around single and two paratactic circular cylinders were investigated with two-dimensional molecular dynamics simulation (MDS) methods in the view of discrete particles. The transient and time-averaged profiles of streamline and density were obtained in order to analyze the characteristics of the wake flow. For single cylinder case, different flow patterns, i.e. Stokes flow, steady vortices flow, periodic vortices-shedding flow with the Kármán vortex street and supersonic flow, were divided based on Reynolds number (Re), 4<Re<12, 12<Re<20, 20<Re<62 and Re>62 respectively. For two paratactic cylinders case, different flow patterns, namely periodic vortices-shedding flow, periodic vortices-shedding flow with gap-flow, bistable flow and synchronized vortex-shedding flow, were observed with different center-to-center pitch ratios (D*/d*), D*/d*=1.0, 1.0< D*/d*<1.1, 1.1<D*/ d*<1.8 and D*/d*>1.8 respectively. The results show qualitative or quantitative agreement with those obtained from experiments and other MDS and indicate that most macroscopic obstructed flow patterns still exist even in nanoscale.

Jie Sun;雅玲 何;印实 李;文铨 陶

2008

In order to improve the performance of pulse tube refrigerator(PTR), a new kind of PTR which is characterized by non-uniform cross-sectional area was proposed. Experiments show that lower cooling temperature can be reached by the new type PTR compared with that of the PTR of constant cross-sectional of area.

雅玲 何;Mingyao Xu;Chengming Gao;Zhongqi Chen;文铨 陶

Hsi-An Chiao Tung Ta Hsueh/Journal of Xi'an Jiaotong University

2001-9