Effect of Wall Surface Properties at Different Drying Kinetics on the Deposition Problem in Spray Drying

Current methods in alleviating the wall deposition problem in spray drying emphasize mainly controlling the stickiness of the drying particles and less attention is placed on the properties of the dryer wall. In this experimental study, the effect of wall surface properties on the deposition mechanism has been investigated. Properties considered in classifying different wall materials were surface energy, roughness, and dielectric properties. The model solution contained sucrose, representing low-molecular-weight sugars commonly encountered in spray drying of fruit and vegetable juices. The effect of wall properties on deposition was explored at different drying rates producing particles of different surface rigidity. Larger surface roughness produced higher deposition fluxes for particles with high impact velocity and moisture. Surface energy and surface roughness were found to have no significant effect for dry rigid particles at the middle and bottom elevation of the drying chamber. However, material with lower surface energy (Teflon) exhibited less deposition for rubbery particles at such elevations. Analysis shows that dielectric wall material (Teflon) tends to enhance deposition of dry particles because of attrition at the surface. Higher wall temperature was found to produce slightly more deposition. The results of this work give a general indication of the effect of wall material on the deposition problem and provide the fundamental understanding for further studies along this line. Proper selection of dryer wall material will provide potential alternatives for reducing the deposition problem.     

New Engineered Particles from Spray Dryers: Research Needs in Spray Drying

This article reviews developments in the simulations of spray dryer behavior, including the challenges in modeling the complex flow patterns inside the equipment, which are often highly transient and three-dimensional in nature. There appears to be considerable scope for using CFD simulations for investigating methods to reduce the rates of wall deposition and of thermal degradation for particles by modifying the air flow patterns in the chamber through small changes in the air inlet geometry. Challenges include building particle drying kinetics and reaction processes, as well as agglomeration behavior, into these simulations. The numerical simulations should be valuable supplements to pilot-scale testing, enabling more extensive and accurate optimization to be carried out than hitherto possible. New understanding of reaction processes and materials science, in combination with recent knowledge of the application of CFD to these problems, may enable new engineered powder products to be developed from the one-step spray-drying process.     

Measurements of turbulent mass transport of a circular wall jet

The results of a laboratory investigation on the turbulence characteristics of a circular three-dimensional turbulent wall jet are presented. Measurements were taken up to 50 nozzle diameters using combined particle image velocimetry and planar laser induced fluorescence. The results showed that the induced turbulence was still evolving in the present range and had not achieved similarity. While the turbulent intensity for both velocity and concentration increased downstream, the turbulent mass transport showed a decline over distance for both the streamwise and spanwise directions, implying weakening dispersion from the jet core.     

外墙渗水防治措施探讨

针对建筑物外墙渗水的原因,结合施工实践对防治措施的可行性进行探讨。     

钢管在线测厚和应用

钢管在线测厚是近年发展较快的一项高科技技术，该设备对生产高质量的无缝钢管具有较大帮助。叙述了钢管在线测厚的基本原理和在宝钢钢管分公司张减机上使用的实绩。     

WALL AND PARTICLE-SHAPE EFFECTS ON HEAT TRANSFER IN PACKED BEDS

The effective radial thermal conductivity and apparent heat transfer coefficient for a packed bed were experimentally determined for beds of spheres, full cylinders and hollow cylinders, for flow rates giving particle Reynolds numbers in the range 100-1000, and for tube to particle diameter ratios of 5-12. Over these ranges the radial Peclet number Pe_r showed significant dependence on solid conductivity, gas flow rate and particle shape, while the wall Biot number Bi showed significant dependence on tube to particle diameter ratio, gas flow rate and particle shape. These dependencies were predicted well by equations incorporating the effects of these variables into individual gas and solid phase parameters, which were then combined to give the effective or lumped parameters     

Flow modelling within non-homogeneous packings: The structural macro-correlations

The pressure drop and the fluid velocity profiles were investigated experimentally and numerically in the model packings of complex geometry. The numerical estimations were performed by means of the hybrid method involving the use of structural macro-correlations. In this paper it has been demonstrated on several examples that the structural macro-correlations allow to predict the macro-scale flow behaviour in non-homogeneous packings. Finally, the recent mechanistic approaches to flow modelling in packed tubes of low tube-to-particle diameter ratio have been discussed.     

Effects of microreactor wall heat conduction on the reforming process of methane

In this paper, the effect of wall conduction of an autothermal tubular methane microreformer is investigated numerically. It is found that the axial wall conduction can strongly influence the performance of the microreactor and should not be neglected without a careful a priori investigation of its impact. By increasing the wall thermal conductivity, the maximum wall surface temperature is decreased. Due to the complex exothermic–endothermic nature of the chemistry of reforming, the axial variation of the wall temperature is not monotonic. Methane conversion and hydrogen yield are strongly dependent on the wall inner surface temperature, hence the heat conduction through the channel wall. The equivalence ratio and the wall thickness also significantly affect the reforming effectiveness and must be carefully considered in reactor optimization. Furthermore, it is found that exothermic oxidation reaction mechanisms, especially partial oxidation, are responsible for syngas (hydrogen and carbon monoxide) production near the inlet. Farther downstream, in the oxygen deficient region, endothermic steam reforming is the main hydrogen producing mechanism. By increasing the thermal conductivity, steam reforming becomes stronger and partial oxidation becomes weaker. For all investigated inlet conditions, the highest hydrogen yield is obtained for no or very low conductive walls.     

基于温湿度变化对蒸压加气混凝土墙体变形影响试验研究

本文对加气混凝土墙体在温湿度变化下的开裂问题进行试验研究。通过调整内外墙体温度、湿度变化研究加气混凝土砌块应力应变的变化规律;利用ANSYS软件理论分析温差对墙体应力应变的影响。结果表明:随着内外墙体温差的增大,加气混凝土墙体的应变值越大;随着湿度的变化,应变值的变化不大。