Experimental and mechanistic study of dispersed micrometer-sized particle resuspension in a square straight duct with rough walls

The resuspension of graphite dust is an important phenomenon in the release of radioactivity and the safety of nuclear reactors during severe accidents.In this study,a visualization experimental platform is constructed to study effects of particle size,flow velocity,and wall roughness on the resuspension characteristics of graphite particles.A statistical model of particle resuspension applicable to monolayer dispersed particles is developed based on the moment equilibrium of the particles and the flow field characteristics,as calculated by the large-eddy simulation framework.The results show that particle resuspension can be divided into short-and long-term resuspension stages.Most particle resuspension occurs during the short-term stage.With increases in flow velocity and particle diameter,the aerodynamic or adhesion force acting on the particles increases,and corresponding particle resuspension fraction increases.The influence of rough walls on particle resuspension is related to both the force on the particles and the arm ratio between the wall morphology and the particle diameter.A comparison with the experimental results demonstrates that the particle resuspension model developed in this study accurately predicts the impact of flow velocity,particle size,and wall roughness on particle resuspension.

Modeling micro-particle deposition in human upper respiratory tract under steady inhalation

A representative human upper respiratory tract （URT） with idealized oral region and asymmetric tracheo- bronchial （TB） airway has been modeled, and laminar-to-turbulent airflow for typical inhalation modes as well as micro-particle transport and deposition has been simulated using CFX10.0 software from Ansys Inc. on a personal computer. The asymmetric TB airway could not be replaced by an extended straight tube as outlet of the oral region while investigating the tracheal airflow field and particle deposition. Compared to an idealized oral airway with an extended straight tube, several differences could be noted： （i） The laryngeal jet extends further down the trachea and inclines towards the anterior wall; （ii） the turbulence level in trachea is less and decays more quickly; （iii） three recirculation zones are visible with intense adverse current after the glottis; （iv） deposition of small particles in trachea is reduced due to lower turbulence. Refined unstructured mesh with densified boundary layer mesh could be a proper substitute for the structured mesh in the human URT model with asymmetric TB airway. Based on the refined unstructured mesh, the physiological structure of uvula in the soft palate is properly simulated in the present human URT model.

New Fabrication and Mechanical Properties of Styrene-Butadiene Rubber/Carbon Nanotubes Nanocomposite

A novel technology to prepare styrene-butadiene rubber （SBR）/carbon nanotubes （CNTs） composites was developed by combining a spray drying method and a subsequent mechanical mixing process. The cross-linking degrees of the vulcanized composites increased gradually with the additive CNTs contents. By comparing with those of the pure SBR composites, the mechanical properties such as tensile strength, tear strength and hardness of the composites filled with CNTs at certain contents were dramatically improved almost by 600%, 250% and 70%, respectively. The fabrication of the CNTs filled with SBR composites by combination of the spray drying method and subsequent mechanical mixing process was effective for enhancing the reinforcement effects of CNTs in rubbers. The novel technology can also open a new route for the modification and reinforcement on the nanocomposites with large amount of CNTs.

Study on the kinetic model of Cr/H/Air/Cl mixture

Constructing a reasonable reaction kinetic model for Cr-containing waste incineration is of high significance to study the effect of various factors,such as temperature,Cl,on its hexavalent chromium compounds.In this paper,a primary Cr/H/Air/Cl reaction kinetic model is built and studied by Gaussian 03 code with quantum chemistry theory.Numerical results show that there are two overall reactions in the model,each of the two overall reactions consists of their respective two elementary reactions.So the two overall reactions should be omitted from the model.Instead,the four elementary reactions should be added in the Cr/H/Air/Cl reaction kinetic model.The activation energies of the four elementary reactions are also calculated.

Deposition of non-spherical microparticles in the human upper respiratory tract

We investigated the deposition pattern of microparticles with different particle diameters, shape factors, and initial flow conditions in a realistic human upper respiratory tract model. We identified a close relationship between the deposition fraction and the particle shape factor. The deposition fraction of the particles decreased sharply with increasing particle shape factor because of the decreasing drag force. We also found that the deposition varied at different positions in the upper respiratory tract. At low shape factors, the highest fraction of particles deposited at the mouth and pharynx. However, with increasing shape factor, the deposition fraction in the trachea and lungs increased. Moreover, for a given shape factor, larger particles deposited at the mouth and pharynx, which indicates that the deposition fraction of microparticles in the human upper respiratory tract is affected first and foremost by particle inertia as well as by the drag force.

Graphite dust deposition in the HTR-10 steam generator

Graphite dust has an important effect on the safe operation of a high-temperature gas-cooled reactor （HTR）. The present study analyzes temperature and flow field distributions in the HTR-10 steam genera- tor. The temperature and flow field distributions are then used to study thermophoretic deposition and turbulent deposition. The results show that as the dust diameter increases, the thermophoretic deposition decreases, while the turbulent deposition first decreases and then increases. The thermophoretic deposi- tion is higher at higher reactor powers, with turbulent deposition growing more rapidly at higher reactor power. For small particles, the thermophoretic deposition effect is greater than the turbulent deposition effect, while for large particles, the turbulent deposition effect is dominant.

Evaluation of thermophoretic effects on graphite dust coagulation in high-temperature gas-cooled reactors

In high-temperature gas-cooled reactors,graphite dust particles within the reactor core and the heat transfer equipment experience large temperature gradients.Under such conditions,thermophoresis may play an important role in determining aerosol evolution.This study presents a theoretical and numerical analysis of the thermophoretic effects on aerosol coagulation within these reactors.The coagulation rates for Brownian versus thermophoretic coagulation are calculated and compared for various temperature gradients.Our results show that thermophoretic coagulation dominates over Brownian coagulation for large temperature gradients.We defined an enhancement factor to evaluate the role of thermophoretic coagulation under various reactor conditions.The enhancement factor increased dramatically with increasing temperature gradient,decreasing pressure and increasing particle diameter,but was not very sensitive to temperature change.The time evolution of the particle size distribution related to combined Brownian and thermophoretic coagulation was simulated using a log-skew-normal method of moments.The simulation results indicate that aerosol evolution can be strongly accelerated by thermophoretic coagulation under large temperature gradients.

Graphite dust resuspension in an HTR-10 steam generator

Graphite dust has an important effect on the safety of high-temperature gas-cooled reactors（HTR）.The flow field in the steam generator was studied by the computational fluid dynamics（CFD） method,with the results indicating that the friction velocity in the windward and the leeward of the heat transfer tubes is relatively low and is higher at the sides.Further analysis of the resuspension of graphite dust indicates that the resuspension fraction reaches nearly zero for particles with a diameter less than 1 μm,whereas it will increases as the helium velocity in the steam generator increases for particle size larger than 1 μm.Moreover,the resuspension fraction increases as the particle size increases.The results also indicate that resuspension of the particles with sizes larger than 1 μm exhibited obvious differences in different parts of the steam generator.

SOOT PARTICLES ANALYSIS IN LAMINAR PREMIXED PROPANE/OXYGEN(C_(3)H_(8)/O_(2))FLAMES USING PUBLISHED MEASUREMENT DATA

A laminar premixed Propane/Air flame with a fuel equivalence ratio of 2.1 was employed for analysis of soot particles. Zeroth-order lognormal distributions (ZOLD) were used in the analysis of experimental distribution phe-nomena at different residence times during soot formation in the flame. Rayleigh抯 theory and Mie抯 scattering theory were combined with agglomerate analysis using scattering and extinction data to determine the following soot charac-teristics: agglomerate parameters, volumetric fractions, mass flow rates and surface growth rate. Soot density meas-urements were carried out to determine density variations at different stages of growth. The measured results show that for long residence times the soot clearly crystallizes with higher density (up to 1.8 g.cm-3). The increases of soot volu-metric fraction and mass flow rate indicate that the surface growth rate of soot particles exceeds the oxidation rates in the flame studied. The data obtained in this work would be used to study soot oxidation rate under flaming condition.

Effects of thermophoresis on Brownian coagulation of spherical particles over the entire particle size regime

In many energy and combustion applications.particles experience large temperature gradients,which can affect the coagulation process due to thermophoresis.This study presents a rigorous theory of thermophoretically modified Brownian coagulation in the entire particle size regime.The theoretical derivations are based on the kinetictheory forthe free-molecular regime and the harmonic mean method for the transition regime.The coagulation kernels in different size regimes can be expressed as the basic Brownian coagulation kernel times an enhancement factor,The enhancement factor represents the coagulationrate enhancement induced by thermophoresis and is a function of specific dimensionless numbers.Based on the enhancement factor,the thermophoretic enhancement effects on particle coagulation are further analyzed under a wide range of gas and particle conditions.The results show that thermophoretic enhancement effects are ignorable in the free-molecular regime,but need to be considered in the continuum regime and the transition regime.In addition,the enhancement effects increase significantly with increase of gas temperature and temperature gradient while decrease with increase of gas pressure.The present study can improve understanding ofthermophoretic effects on Brownian coagulation in the entire size regime and provide a useful tool to calculate the coagulation rates in presence of thermophoresis.