Using shock waves to improve the sound absorbing efficiency of closed-cell foams

Producing closed-cell foams is generally cheaper and simpler than open-cell foams. However, the acoustic and filtration efficiency of closed-cell foam materials is generally poor because it is very difficult for fluid or acoustic waves to penetrate into the material. A new method using shock waves to remove the membranes closing the cell pores (known as reticulation) and thus to improve the acoustic and filtration behavior of closed-cell foam material is presented. Various shock treatments have been carried out on polyurethane and polyimide foams and the following conclusions were drawn: (1) reticulation efficiency increased and thus the airflow resistivity and tortuosity decreased when increasing the amplitude of the shock treatment; (2) the rigidity of the foam is decreased; (3) the process is reliable and repeatable and (4) obtained acoustic performance is comparable to classical thermal reticulation.     

A stochastic theory for deep bed filtration accounting for dispersion and size distributions

We develop a stochastic theory for filtration of suspensions in porous media. The theory takes into account particle and pore size distributions, as well as the random character of the particle motion, which is described in the framework of the theory of continuous-time random walks (CTRW). In the limit of the infinitely many small walk steps we derive a system of governing equations for the evolution of the particle and pore size distributions. We consider the case of concentrated suspensions, where plugging the pores by particles may change porosity and other parameters of the porous medium. A procedure for averaging of the derived system of equations is developed for polydisperse suspensions with several distinctive particle sizes. A numerical method for solution of the flow equations is proposed. Sample calculations are applied to compare the roles of the particle size distribution and of the particle flight dispersion on the deposition profiles. It is demonstrated that the temporal flight dispersion is the most likely mechanism forming the experimentally observed hyperexponential character of the deposition profiles.     

Experimental and numerical study of a gas cyclone with a central filter

This paper studies a novel gas cyclone with a cylindrical filter face installed in the center from the vortex finder to the bottom hopper. The experimental results show that this composite cyclone has a higher collection efficiency and a lower pressure drop than the original cyclone. The mechanisms for the improvement are analyzed by both physical experiments and numerical simulations. By measuring dust samples collected at different places it is revealed that the center filter can prevent fine particles from entering the inner vortex and escaping, which accounts for the increase of the collection efficiency. In addition, the flow field of the composite cyclone is simulated by computational fluid dynamics and compared with that of the original cyclone. The analysis shows that with the filter layer installed, the swirling flow disappears in the vortex finder, which decreases the kinetic energy dissipation and hence lowers the pressure drop.     

Unconditional Schauder decompositions and stopping times in the Lebesgue-Bochner spaces

We extend Troitsky's study of martingales in Banach lattices to include stopping times. Results from the theory of unconditional Schauder decompositions and multipliers are used to derive an optional stopping theorem for unbounded stopping times. We also apply these techniques to convergent nets of stopped processes, as well as to unconditional Schauder decompositions in vector-valued Lp-spaces (1<p<∞).     

A filtration model with hysteresis

We study a filtration model for which a hysteresis effect is accounted in the saturation versus pressure constitutive relation. The problem is formulated in a quasi-variational inequality. We prove the existence of a time global solution.     

科学地应对新冠疫情——洁净室相应技术措施的探讨

新冠肺炎疫情横扫全球,对人类健康和世界经济造成重大伤害。种种迹象表明新冠病毒还将相对长期地与人类共存。国内外的医疗卫生机构及政府主管部门,根据这种形势提出了各种防控疫情的措施。这些规定主要是针对公众场所制定,对其他环境有一定的参考价值。针对洁净室环境的具体情况和设施特点,分析、探讨了疫情之下,洁净环境应予关注的问题,提出了一些洁净室环境管控疫情值得注意的事项,供洁净室相关行业人士参考。     

Preparation and characterization of antimicrobial polycarbonate nanofibrous membrane

Electrospinning of polycarbonate (PC)/chloroform solution with quaternary ammonium salt (benzyl triethylammonium chloride, BTEAC) was investigated to develop antimicrobial nanofibrous membranes for ultrafiltration. With BTEAC additive, ultrafine PC fibers were continuously generated and densely mounted without the blockage of spinning tip on electrospinning. When small amounts of BTEAC were added to the PC solution, the average diameter was also decreased from several micrometers to submicron range. It was found that the conductivity of the PC solution was a major parameter affecting the morphology and diameter of the electrospun PC fibers as well as the electrospinnability of PC. The nanofibrous membranes electrospun from the PC solution with BTEAC exhibited better excellent antimicrobial activity than those prepared without BTEAC. The PC nanofibrous filter shows a good filtration efficiency to satisfy the criterion of HEPA filter, and the pressure drop of the PC filters are within the normal range. Therefore, PC nanofibrous membrane showed a great potential as a candidate for ultrafiltration, compared to a commercial HEPA filter.     

Characteristics of bio-diatomite dynamic membrane process for municipal wastewater treatment

The characteristics of bio-diatomite dynamic membrane (BDDM) process in municipal wastewater treatment were investigated with a laboratory-scale continuous-flow device. Experimental results indicate that the BDDM reactor was highly effective in removal of COD, NH₄-N and TN, and exhibited the advantages of good retention capacity for suspended solids (SS), short precoating time, high filtration flux, and easy backwash. In the precoating stage, a retention time of 25 min could reduce the effluent SS to non-detectable level. The filtration resistance of BDDM was composed of thickness-increase resistance and compaction resistance. At a low flux, microbial adhesion occurred on the interface between the BDDM and the stainless steel support mesh, which negatively impacted the system operation and backwash. However, microbial adhesion could be effectively minimized by increasing the filtration flux.     

Electrokinetic phenomena in fine-porous diaphragms at open chemical circuits

Classical electrokinetic phenomena are considered a coupling between electrical and hydraulic circuits where the diaphragm plays the role of an electromotive or a volumomotive force. Particular electrokinetic phenomena correspond to limiting conditions in those two circuits. Similarly, for fine porous diaphragms, equivalent chemical circuits are introduced to account for concentrational polarization. Classical electrokinetic phenomena turn out to be a particular mode occurring at closed chemical circuits. A new class of electrokinetic phenomena occurring at open chemical circuits is introduced. The examples of filtration potential and electro-osmotic pressure at charged chemical capacitors are considered in detail. The experimental conditions of observing the new phenomena and their advantages in terms of unambiguous interpretation of experimental data are then discussed.