Biomedical subjects

T S Zhao

Publications and source records attributed to T S Zhao.

16 recordsLinked to original sources

PubMed2006

Multiwalled carbon nanotube supported PtRu for the anode of direct methanol fuel cells.

The activity of the methanol oxidation reaction of a multiwalled carbon nanotube (MWCNT)-supported PtRu catalyst was investigated and compared with the Vulcan XC-72 carbon-supported catalyst. The PtRu nanoparticles with 1:1 and 7:3 atomic ratios (with similar PtRu loadings and morphological structures) were deposited both on the MWCNTs and on the carbon. Cyclicvoltammetry results demonstrated that the MWCNT-supported PtRu catalyst exhibited a higher mass activity (mA mg(-1) of PtRu) for the methanol oxidation reaction than the carbon-supported PtRu under the condition that both catalysts possess more or less the same PtRu loadings, particle sizes, dispersions, and electrochemical surface area. The direct methanol fuel cell performance test data showed that MWCNT-supported PtRu catalysts yielded about 35-39% higher power densities than the carbon-supported PtRu.

Journal Article↗

PubMed2006

Lattice Boltzmann method for incompressible flows with large pressure gradients.

Conventional lattice Boltzmann Bhatnagar-Gross-Krook (LBGK) models can simulate incompressible flows correctly only if the Mach number M and the density variation deltarho are negligibly small. However, the equation of state p=RTrho resulting from the conventional models limits their application to incompressible flows with a rather small pressure gradient. In this paper, based on the Enskog equation, we propose a finite difference lattice BGK model for isothermal incompressible flows with the resulting equation of state and transport properties suitable for nonideal fluids. We validated this model by simulating the plane Poiseuille flow, the two dimensional Womersley flow, and the backward-facing step flow and found that the numerical results obtained by the proposed model are more accurate than those by the conventional LBGK models when the pressure gradient imposed on the flows increases.

Journal Article↗

PubMed2005

Temperature dependence of the velocity boundary condition for nanoscale fluid flows.

Velocity slips may occur as a fluid flows over a solid surface in the nanometer scale. The slip length L(s) , characterizing the degree of slip, is usually used to describe the velocity boundary condition at the fluid/solid interface. In this work, we show that for a given wall-fluid system, the slip length L(s) generally varies with the system temperature T . In particular, we show that it is possible to create a pair of solid wall and fluid systems, in which the velocity slip becomes rather small and independent of temperature.

Journal Article↗

PubMed2005

A lattice Boltzmann algorithm for electro-osmotic flows in microfluidic devices.

In this paper, a finite-difference-based lattice Boltzmann (LB) algorithm is proposed to simulate electro-osmotic flows (EOF) with the effect of Joule heating. This new algorithm enables a nonuniform mesh to be adapted, which is desirable for handling the extremely thin electrical double layer in EOF. The LB algorithm has been validated by simulating a problem with an available analytical solution and it is found that the numerical results predicted by the algorithm are in good agreement with the analytical solution. The LB algorithm is also applied to modeling a mixed electro-osmotic/pressure driven flow in a channel. The numerical results show that Joule heating plays an important role in EOF.

Journal Article↗

PubMed2005

Simple kinetic model for fluid flows in the nanometer scale.

Fluid flows in the nanometer scale can be studied by molecular dynamics or Monte Carlo methods, but the time and length scales are usually limited to rather short ranges due to the computational expense. Kinetic theory is an alternative tool for studying nanoscale flows, but the existing models are rather complicated and difficult to implement. In this paper, we propose a simple Enskog-like kinetic model for nanoscale flows. The results predicted by this model compare well with molecular dynamics or Monte Carlo simulation results in the literature.

Journal Article↗

PubMed2005

Finite-difference-based lattice Boltzmann model for dense binary mixtures.

We propose a finite-difference-based lattice Boltzmann model for dense binary mixtures based on the Enskog theory. The model is applicable to a mixture composed of two dense fluids with different shear viscosities. The macroscopic hydrodynamic and diffusion equations are derived from the model through the Chapmann-Enskog procedure. The model is also validated numerically.

Journal Article↗

PubMed2004

Thermal lattice Bhatnagar-Gross-Krook model for flows with viscous heat dissipation in the incompressible limit.

In this paper, by introducing a different distribution function and starting from the Boltzmann equation as well as the Maxwell-Boltzmann distribution, we obtain a Boltzmann Bhatnagar-Gross-Krook (BGK) equation for thermal flows with viscous heat dissipation in the incompressible limit. The continuous thermal BGK model is then discretized over both time and phase space to form a lattice BGK model, which is shown to be consistent with some existing double distribution function lattice BGK models based on macroscopic governing equations. We have also demonstrated that the lattice BGK model derived theoretically in this work can be used to simulate laminar incompressible convention heat transfer with/without viscous heat dissipation.

Journal Article↗

PubMed2004

Preconditioned lattice-Boltzmann method for steady flows.

In this paper we propose a preconditioned lattice Boltzmann (LB) method for steady incompressible flows. For steady flows, the macroscopic equations derived from this LB model are equivalent to those from the standard LB model, but with an improved eigenvalue system. The proposed model can be viewed as an explicit solver for preconditioned compressible Navier-Stokes equations. Linear stability analysis is performed and the results show that the stability of the model is the same as that of the standard LB model for low Mach numbers. The proposed model retains the structure of the standard LB model and, hence, possesses all the advantages. Numerical tests show that the convergence rate can be enhanced as much as an order of magnitude compared to the standard lattice Boltzmann method. The accuracy of the solutions is improved as well.

Journal Article↗

PubMed2003

Discrete velocity and lattice Boltzmann models for binary mixtures of nonideal fluids.

In this paper, a discrete velocity model and a lattice Boltzmann model are proposed for binary mixtures of nonideal fluids based on the Enskog theory. The velocity space of the Enskog equation for each component is first discretized by applying a Gaussian quadrature, resulting in a discrete velocity model that can be solved by suitable numerical schemes. A lattice Boltzmann model is then derived from the discrete velocity model with a slightly modified equilibrium. The hydrodynamics of each model are also derived through the Chapmann-Enskog procedure.

Journal Article↗

PubMed2003

Explicit finite-difference lattice Boltzmann method for curvilinear coordinates.

In this paper a finite-difference-based lattice Boltzmann method for curvilinear coordinates is proposed in order to improve the computational efficiency and numerical stability of a recent method [R. Mei and W. Shyy, J. Comput. Phys. 143, 426 (1998)] in which the collision term of the Boltzmann Bhatnagar-Gross-Krook equation for discrete velocities is treated implicitly. In the present method, the implicitness of the numerical scheme is removed by introducing a distribution function different from that being used currently. As a result, an explicit finite-difference lattice Boltzmann method for curvilinear coordinates is obtained. The scheme is applied to a two-dimensional Poiseuille flow, an unsteady Couette flow, a lid-driven cavity flow, and a steady flow around a circular cylinder. The numerical results are in good agreement with the results of previous studies. Extensions to other lattice Boltzmann models based on nonuniform meshes are also discussed.

Journal Article↗

PubMed2002

Lattice Boltzmann model for incompressible flows through porous media.

In this paper a lattice Boltzmann model is proposed for isothermal incompressible flow in porous media. The key point is to include the porosity into the equilibrium distribution, and add a force term to the evolution equation to account for the linear and nonlinear drag forces of the medium (the Darcy's term and the Forcheimer's term). Through the Chapman-Enskog procedure, the generalized Navier-Stokes equations for incompressible flow in porous media are derived from the present lattice Boltzmann model. The generalized two-dimensional Poiseuille flow, Couette flow, and lid-driven cavity flow are simulated using the present model. It is found the numerical results agree well with the analytical and/or the finite-difference solutions.

Journal Article↗

PubMed1994

Rapid method for the determination of multiple pyrethroid residues in fruits and vegetables by capillary column gas chromatography.

A rapid and economical simplified multi-residue method is described for the determination of multiple pyrethroid insecticides in fruits and vegetables. The residues are extracted from crops with methanol and the crop co-extractives are removed by toluene partitioning and Florisil-charcoal minicolumn chromatography. The final extract is analysed by capillary column gas chromatography with electron-capture detection. The recoveries were determined by fortifying six different crops (apples, oranges, cabbages, pears, peppers and tomatoes) with eleven pyrethroids (Py-115, allethrin, biphenthrin, fenpropathrin, cyhalothrin, permethrin, cyfluthrin, flucythrinate, fluvalinate, fenvalerate and deltamethrin) at three levels, 0.01-0.07, 0.10-0.70 and 1.0-7.0 mg/kg. Three determinations were made at each level for each crop. Recoveries of the eleven pyrethroids ranged from 70.4 to 110.0% at the three different levels. The practical determination limit of the method was in the range 3.0-30.0 micrograms/kg for all the pyrethroid insecticides. The proposed method had major advantages that simplified steps were achieved for the extraction and the clean-up, the solvent consumption was reduced and the analysis time was shortened.

Chromatography, Gas↗

PubMedDate not supplied

Cleanup with two Florisil columns for gas chromatographic determination of multiple pyrethroid insecticides in products of animal origin.

A gas chromatographic method was developed for the simultaneous determination of 9 pyrethroid insecticides in products of animal origin. The multiresidues of the pyrethroids in different samples were extracted with acetone-petroleum ether (1 + 1), and extracts were cleaned up on a Florisil partition column and a Florisil adsorption column. Four animal tissue samples were examined at 0.05-0.25 ppm fortification levels. The average recoveries of all insecticides were 76.9-88.0%, and the coefficients of variation were < 4.6% for all insecticides except permethrin. The detection limits of the method were ca 5 ppb for all insecticides but permethrin, which had a detection limit of ca 10 ppb.

Animals↗

PubMedDate not supplied

Packed-column gas chromatographic method for the simultaneous determination of 10 pyrethroid insecticide residues in fruits, vegetables, and grains.

A simple, rapid, packed-column gas chromatographic method was developed for simultaneous determination of 10 pyrethroid insecticide residues (allethrin, biphenthrin, cyhalothrin, permethrin, cyfluthrin, flucythrinate, fenvalerate, fluvalinate, deltamethrin, and py-115) in fruits, vegetables, and grains. These multiresidues are extracted from various crops with acetone-petroleum ether and cleaned up on a Florisil column (for the fruits and vegetables) or on a Florisil-charcoal-alumina column (for the grains) prior to their determination by gas chromatography with an electron capture detector. Recoveries of 10 pyrethroid compounds from 12 different crops (maize, soybeans, wheat, sorghums, paddy, potatoes, cucumbers, cauliflowers, spinaches, apples, bananas, and oranges) fortified at levels of 0.02-5.00 ppm ranged from 58 to 130%. In a separate precision study, coefficients of variation were 5.5-14.6% at 0.1-0.5 ppm (n = 10, maize), and 4.1-12.1% at 0.010-0.050 ppm (n = 10, apples). The detection limits of the method ranged from 2.0 to 10.0 ppb on a crop basis.

Chromatography, Gas↗

PubMedDate not supplied

Modification of AOAC multiresidue method for determination of synthetic pyrethroid residues in fruits, vegetables, and grains. Part I: Acetonitrile extraction system and optimization of florisil cleanup and gas chromatography.

We present a multiresidue method for determination of synthetic pyrethroids in fruits, vegetables, and grains. The method is a modification of AOAC Method 970.52. Residues are extracted with acetonitrile (for fruits and vegetables) or acetonitrile-water (2 + 1) (for grains) and then transferred to hexane. Coextractives are removed by acetonitrile partitioning and open-column chromatography with deactivated Florisil. The final extract is analyzed by gas chromatography with electron capture detection (GC-ECD). An HP-17 wide-bore column is used to determine the individual isomeric contents of each insecticide. The method was used to recover 8 pyrethroids (biphenthrin, fenpropathrin, cyhalothrin, permethrin, cypermethrin, fluvalinate, fenvalerate, and deltamethrin) spiked at 0.01-4.0 mg/kg in 20 crops (apple, pear, peach, banana, grape, strawberry, potato, tomato, cucumber, pepper, cabbage, carrot, celery, polished rice, wheat, green gram, buckwheat, sorghum, maize, and barley). Recoveries of the 8 pyrethroid insecticides in 6 crops ranged from 83.8 to 112.8%, with a coefficient of variation (CV) of 2.00 to 12.09% for the narrow-bore capillary GC (n = 6) and from 82.8 to 106.4%, CV = 2.93-12.19%, for the wide-bore capillary GC (n = 6). The minimum detectable levels of 0.004-0.028 mg/kg (for fruits and vegetables) or 0.01-0.08 mg/kg (for grains) for the 8 pyrethroids are easy to detect.

Acetonitriles↗

PubMedDate not supplied

Modification of AOAC multiresidue method for determination of synthetic pyrethroid residues in fruits, vegetables, and grains. Part II: Acetone extraction system.

To optimize conditions and to evaluate further a multiresidue method for pyrethroids, various extraction solvents and partitioning conditions were examined. Acetone and acetonitrile (for fruits and vegetables) and acetone-water and acetonitrile-water (for grains) were used as solvents with or without acetonitrile partitioning. Twenty crops fortified with 8 pyrethroid insecticides at low, medium, and high levels were analyzed. For limit of detection (LOD) levels, repeatability tests were completed with acetone-water as extraction solvent. Recoveries of 8 pyrethroid insecticides at LOD levels were 76.2-99.6%, with coefficients of variation (CVs) of 2.08-10.90% for narrow-bore capillary gas chromatography (GC) (n = 6) and 80.1-107.3% with CVs of 3.76-15.38% for wide-bore capillary GC (n = 6). Both acetone or acetone-water extraction with acetonitrile partitioning and acetonitrile or acetonitrile-water extraction with acetonitrile partitioning are suitable for multiresidue analysis of pyrethroid insecticides. However, acetonitrile and acetonitrile-water as extraction solvents were better than acetone and acetone-water at LOD-fortified levels. This finding was confirmed chromatographically with pear, cucumber, and barley control samples.

Acetone↗