Submerged membrane bioreactors （SMBR） are widely used in wastewater treatment. The permeability of a membrane declines rapidly because of the formation of a cake layer on the membrane surface. In this paper, a multiple staining protocol was conducted to probe the four major foulants in the cake layer formed on a filtration membrane. Fluorescent images of the foulants were obtained using a confocal laser scanning microscope （CLSM）. The three dimensional structure of the cake layer was reconstructed, and the internal flow was calculated using computational fluid dynamics （CFD）. Simulation results agreed well with the experimental data on the permeability of the cake layer during filtration and showed better accuracy than the calculation by Kozeny-Carman method. B-D-Glucopyranose polysaccharides and proteins are the two main foulants with relatively large volume fractions, while a-D-glucopyranose polysaccharides and nucleic acids have relatively large specific surface areas. The fast growth of B-D-glucopyranose polysaccharides in the volume fraction is mainly responsible for the increase in cake volume fraction and the decrease in permeability. The specific area, or the aggregation/dispersion of foulants, is less important to its permeability compared to its volume fraction.
Film condensation is a vital phenomenon in the nuclear engineering applications,such as the gas-steam pressurizer design,and heat removing on containment in the case of postulated accident.Reynolds number in film condensation can be calculated from either the mass relation or the energy relation,but few researches have distinguished the difference between them at present.This paper studies the effect of Reynolds correlation in the natural convection film condensation on the outer tube.The general forms of the heat transfer coefficient correlation of film condensation are developed in different flow regimes.By simultaneously solving a set of the heat transfer coefficient correlations with Re_(mass) and Re_(energy),the general expressions for Re_(mass) and Re_(energy) and the relation between the corresponding heat transfer coefficients are obtained.In the laminar and wavefree flow regime,Re_(mass) and Re_(energy) are equivalent,while in the laminar and wavy flow regime,Re_(mass) is much smaller than Re_(energy),and the deviation of the corresponding average heat transfer coefficients is about 30% at the maximum.In the turbulent flow regime,the relation of Re_(mass) and Re_(energy)is greatly influenced by Prandtl number.The relative deviation of their average heat transfer coefficients is the nonlinear function of Reynolds number and Prandtl number.Compared with experimental results,the heat transfer coefficient calculated from Re_(energy) is more accurate.
Lei WuHai-Jun JiaXi-Zhen MaYang LiuXing-Tuan YangJun Wang
Ultrathin planar absorbers hold promise in solar energy systems because they can reduce the material, fabrication, and system cost. Here, we present a general strategy of effective medium design to realize ultrathin planar broadband absorbers. The absorber consists of two ultrathin absorbing dielectrics to design an effective absorbing medium, a transparent layer, and metallic substrate. Compared with previous studies, this strategy provides another dimension of freedom to enhance optical absorption; therefore, destructive interference can be realized over a broad spectrum. To demonstrate the power and simplicity of this strategy, we both experimentally and theoretically characterized an absorber with 5-nm-thick Ge, 10-nm-thick Ti, and 50-nm-thick SiO2 films coated on an Ag substrate fabricated using simple deposition methods. Absorptivity higher than 80% was achieved in 15-nm-thick （1/50 of the center wavelength） Ge and Ti films from 400 nm to near 1 btm. As an application example, we experimentally demonstrated that the absorber exhibited a normal solar absorptivity of 0.8 with a normal emittance of 0.1 at 500 ~C, thus demonstrating its potential in solar thermal systems. The effective medium design strategy is general and allows material versatility, suggesting possible applications in real-time optical manipulation using dynamic materials.
