AN IMPROVEMENT TO THE SSF MODEL FOR PREDICTION OF LIGHT PARTICLE DISPERSION

This study investigates the predicted motion of small, light particles using the stochastic separated flow models proposed by Shuen et al. (1983 Shuen , J. S. , Chen , L. D. , and Faeth , G. M. ( 1983 ). Evaluation of a stochastic model of particle dispersion in a turbulent round jet , AIChE J. , 29 , 167 – 170 . [CSA] [CROSSREF] [Crossref], [Web of Science ®] , [Google Scholar]) and Lightstone and Raithby (1998 Lightstone , M. F. and Raithby , G. D. ( 1998 ). A stochastic model of particle dispersion in a turbulent gaseous environment , Combust. Flame , 113 , 424 – 441 . [CSA] [CROSSREF] [Crossref], [Web of Science ®] , [Google Scholar]). Model predictions are compared to the benchmark experiment of Snyder and Lumley (1971 Snyder , W. H. and Lumley , J. L. ( 1971 ). Some measurements of particle velocity autocorrelation functions in a turbulent flow , J. Fluid Mech. , 48 , 41 – 71 . [CSA] [CROSSREF] [Crossref], [Web of Science ®] , [Google Scholar]) for particles released from a point source in grid-generated turbulence. It is found that the predictions of particle velocity variance and dispersion exhibit nonphysical waves. This behavior is explored in order to understand its mathematical basis arising from the model formulation. A modification to the model is presented. The modified model is successful in removing the characteristic waves.     

Mechanism of Particle Separation in Small Hydrocyclone

The mechanism of particle separation in a small hydrocyclone is analyzed in this study. The separation efficiency of particles is affected by two major effects, the centrifugal and underflow effects. The fundamentals of these effects on the selectivity curve are derived from the theories of hydrodynamics. Three JIS particulate samples are separated in a 10-mm hydrocyclone. The proposed mechanism can be demonstrated by the available data. An increase in split ratio leads to an increase in the separation efficiency only for fine particles; consequently, a decrease in the separation sharpness results in a remarkable fishhook phenomenon. When the feed rate is increased by increasing the pressure drop through the hydrocyclone, the entire partial separation efficiency will be improved, but the separation sharpness and fishhook depth have no substantial varieties. The proposed mechanism can be used for explaining these phenomena, for the selection of optimum operating conditions, and for the hydrocyclone design.     

A Parametric Study of Spray Drying of a Solution in a Pulsating High-Temperature Turbulent Flow

Spray drying of a concentrated common salt (NaCl) solution carried out in the intense oscillating high-temperature turbulent flow field generated in the tailpipe of a pulse combustor was simulated. Simulation of such transport process problems is especially crucial since the environmental conditions are too hostile for detailed and reliable measurements. The momentum, heat, and mass transfer processes between the gas and droplet phases during drying were simulated using a computational fluid dynamic solver. The simulated profiles of flow field, temperature, and humidity of gaseous phase, and particle trajectories in a drying chamber are presented and discussed. The effects of gas temperature, pulse frequency and amplitude, and gas mass flow rate on the transient flow patterns, droplet trajectories, and overall dryer performance were investigated. Different turbulence models were also tested. Simulation results show that the flow field and droplet drying conditions vary widely during a single pulsating period. Very short drying times and very high drying rate characterize pulse combustion spray drying. Thus, pulse combustion drying can be applied to drying of fine droplets of highly heat-sensitive materials although the jet temperature initially is extremely high.     

Validation of the Lagrangian Approach for Predicting Turbulent Dispersion and Evaporation of Droplets within a Spray

The accuracy of the Lagrangian approach for predicting droplet trajectories and evaporation rates within a simple spray has been addressed. The turbulent dispersion and overall evaporation rates of droplets are modeled reasonably well, although the downstream velocity decay of the larger droplets is underpredicted, which is attributed to a poor estimate of the radial fluctuating velocity of these droplets at the inlet boundary. Qualitative agreement is found between the predicted and experimental evolution of the droplet size distribution downstream of the nozzle. These results show that smaller droplets evaporate preferentially to larger droplets, because they disperse more quickly toward the edge of the jet, where the entrainment of fresh air from the surroundings produces a significant evaporative driving force. Droplet dispersion and evaporation rates are highly influenced by the rate of turbulence generation within the shear layer. This work demonstrates the potential of the Lagrangian approach for analyzing particle trajectories and drying within the more complex spray dryer system.     

ACS树脂粒径分布与性能的研究

详细研究了聚乙烯醇/活性磷酸钙(PVA/HAP)复合分散剂用量、氯化聚乙烯(CPE)含量及溶胀温度对ACS树脂的粒径分布、分子量、冲击性能及微观形貌的影响。结果表明:当复合分散剂用量为4.15%CPE含量为30%溶胀温度为40℃时,可以得到颗粒规整、粒径适宜、微观结构均匀、冲击性能良好的ACS树脂。     

DYNAMICS AND CONTROL OF CROSS-FLOW GRAIN DRYERS I. MODEL DEVELOPMENT AND TESTING

A general dynamic model for a cross-flow dryer is developed from fundamental balance equations. Six dimensionless groups that govern the process are defined and the resulting dimensionless equations are solved with the method of lines, using the Advanced Continuous Simulation Language software package ACSL (1986). The mesh influence on the numerical solution is investigated, and the assumption of quasi-steady state for the air is discussed. Application of the model to rice-drying experiments shows good agreement, and the results underline the potential of the model and the numerical solution strategy in obtaining critical physical insight towards the operation of cross-flow rice dryers.     

Development and Analysis of Three Diesel Particulate-Related Standard Reference Materials for the Determination of Chemical,Physical, and Biological Characteristics

The National Institute of Standards and Technology (NIST) has recently issued three Standard Reference Materials (SRMs) related to diesel particulate matter: SRM 1650a, Diesel Particulate Matter; SRM 2975, Diesel Particulate Matter (Industrial Forklift); and SRM 1975, Diesel Particulate Extract. These three materials have certified and reference concentrations for selected polycyclic aromatic hydrocarbons (PAHs) including many alkyl-substituted compounds. SRM 1650a is the reissue of an existing material and SRMs 2975 and 1975 represent new materials. The characterization of the PAH content of these three diesel particulate-related SRMs is presented with a discussion of the approach for the certification of PAH concentrations. In addition, the physical and biological characterization of these materials is discussed. A review of customer reported uses of these SRMs in the chemical, physical, and biological sciences is also provided.     

Polystyrene Suspension Polymerization: The Effect of Polymerization Parameters on Particle Size and Distribution

Effects of polymerization parameters are studied to better understand interactions between parameters in stirred-tank polymerization reactors and to derive a regression model correlating polymer particle size and particle-size distribution with agitator speed, agitator diameter, tricalcium phosphate, dodecyl benzene sodium sulfonate concentrations, and mass cycle period. The derived equations give way to prediction of the polymerization conditions in order to obtain desired particle size and particle-size distributions which covers particle sizes between 0.3 and 5 mm. The roles of some other parameters and limiting agitation parameters are also investigated.