Insight into fouling behavior of poly(vinylidene fluoride)(PVDF)hollow fiber membranes caused by dextran with different pore size distributions

Membrane fouling is the key problem that occurs in membrane process for water treatment. However, how membrane microstructure influences the fouling behavior is still not clear. In this study, fouling behavior caused by dextran was deeply and systematically investigated by employing four poly（vinylidene fluoride） （PVDF） membranes with different pore sizes, ranging from 24 to 94 nm. The extent of fouling by dextran was accurately characterized by pore reduction, flux decline, and the change of critical flux. The result shows that membrane with the smallest pore size of 24 nm experienced the smallest fouling rate and the lowest fouling extent. As the membrane pore size increased, the critical flux ranges were 105-114, 63-73, 38-44 and 34- 43 L. m 2. h t, respectively. The critical flux and fouling resistances indicated that the fouling propensity in- creases with the increase of membrane pore size. Two pilot membrane modules with mean pore size of 25 nm and 60 nm were applied in membrane filtration of surface water treatment. The results showed that serious ir- reversible membrane fouling occurred on the membrane with pore size of 60 nm at the permeate flux of 40.5 L.m 2.h 1. On the other hand, membrane with pore size of 25 nm exhibited much better anti-fouling per- formance when permeate flux was set to 40.5, 48 and 60 L-m 2-h- 1.

Influence of Specimen Size on Compression Behavior of Cement Paste and Mortar under High Strain Rates

Static and dynamic compression tests were carried out on mortar and paste specimens of three sizes（Ф68 mm×32 mm,Ф59 mm×29.5 mm and Ф32 mm×16 mm）to study the influence of specimen size on the compression behavior of cement-based materials under high strain rates.The static tests were applied using a universalservo-hydraulic system,and the dynamic tests were applied by a spilt Hopkinson pressure bar（SHPB）system.The experimentalresults show that for mortar and paste specimens,the dynamic compressive strength is greater than the quasi-static one,and the dynamic compressive strength for specimens of large size is lower than those of smallsize.However,the dynamic increase factors（DIF）has an opposite trend.Obviously,both strain rate and size effect exist in mortar and paste.The test results were then analyzed using Weibull,Carpinteriand Ba？ant＇s size effect laws.A good agreement between these three laws and the test results was reached on the compressive strength.However,for the experimentalresults of paste and cement mortar,the size effect is not evident for the peak strain and elastic modulus of paste and cement mortar.

Size effects on process performance and product quality in progressive microforming of shafted gears revealed by experiment and numerical modeling

As one of the indispensable actuating components in micro-systems,the shafted microgear is in great production demand.Microforming is a manufacturing process to produce microgears to meet the needs.Due to the small geometrical size,there are uncertain process performance and product quality issues in this production process.In this study,the shafted microgears were fabricated in two different scaling factors with four grain sizes using a progressively extrusion-blanking method.To explore the unknown of the process,grain-based modeling was proposed and employed to simulate the entire forming process.The results show that when the grains are large,the anisotropy of single grains has an obvious size effect on the forming behavior and process performance;and the produced geometries and surface quality are worsened;and the deformation load is decreased.Five deformation zones were identified in the microstructures with different hardness and distributions of stress and strain.The simulation by using the proposed model successfully predicted the formation of zones and revealed the inhomogeneous deformation in the forming process.The undesirable geometries of microgears including material unfilling,burr and inclination were observed on the shaft and teeth of gear,and the inclination size is increased obviously with grain size.To avoid the formation of inclination and material unfilling,the punch was redesigned,and a die insert was added to constraint the bottom surface of the gear teeth.The new products had then the better forming quality.