中空纤维膜表面结构对脱氧效率的影响

采用聚偏氟乙烯(PVDF)中空纤维膜对水中的溶解氧进行了脱氧研究,考察了膜表面结构对脱氧效率的影响。通过改变内凝固浴的组成来改变膜的表面结构。在测定脱氧效率的同时,对不同内凝固浴下的中空纤维膜内表面结构进行了场发射电镜扫描(FE-SEM),并分别测定了中空纤维膜的孔径、透气系数以及力学性能。结果表明,随着内凝固浴中溶剂DMAc含量的增大,膜内表面开孔结构变大,表面变得粗糙,这种结构使得膜的透气系数增大,传质系数和脱氧效率提高。     

Computer simulation-based method to predict packing density of aggregates mixture

A key theory in concrete mix design is maximizing aggregate packing density (PD) of aggregate mixture. Different methods have been presented by researchers to estimate PD of aggregate mixture. One such method is computer simulation that has become increasingly common over the last decade; however, it is usually a time-consuming procedure. In the current study, a method based on computer simulation is proposed for estimating aggregate PD. In this method, aggregates with specific shapes, grading and PDs are substituted by monosized spherical aggregates. An equation is also presented for determining the diameter of equivalent monosized aggregates. The coefficient of friction between the equivalent monosized aggregates is determined in a way that the monosized aggregates will have a PD equal to that of actual aggregates. The proposed method is also used to simulate laboratory experiments conducted by the present authors and other researchers. Comparisons reveal the high accuracy of the proposed simple method in predicting the PD of aggregate mixtures.     

透析器中随机分布纤维管表面定浓度时传质模拟

通过数值模拟,研究了在透析器外壳内壁面的影响下,纤维表面定浓度时,中空纤维分布的随机性对透析器管外流场和浓度场的影响,同时考察了管外流体速度以及中空纤维填充密度对管外传质系数的影响.模拟结果表明:随机排布的方式对传质系数有较大的影响,但相比忽略壁面影响时的影响要小;Reynolds 数越大,传质系数也越大;在所研究的中空纤维填充密度范围内(10%～50%),随着填充密度的增大,透析器的管外传质系数先增大,后减小,变化幅度均较小,与忽略壁面影响的值比较发现,在填充密度小的时候壁面对传质有促进作用,填充密度大的时候则相反.     

Effect of particle-size distribution and specific surface area of different binder systems on packing density and flow characteristics of cement paste

The particle-size distribution (PSD) and specific surface area (SSA) of binders significantly affect the fresh and hardened characteristics of cement-based materials. An experimental investigation was undertaken to evaluate the influence of PSD and calculated SSA of various binary and ternary binder systems on flow characteristics, packing density, and compressive strength development of cement paste. The influence of dispersion state of the binder on packing density was evaluated using the wet packing density approach to determine the optimum water demand (OWD) needed to achieve maximum wet density. The modified Andreasen and Andersen (A&A), Rosin–Rammler (RR), and power law grading models were employed to optimize the PSD of binder system to achieve maximum packing density, while maintaining relatively low water demand. The incorporation of high-range water reducing admixture (HRWRA) is shown to decrease the OWD and increase the packing density resulting from greater degree of dispersion of the binder. The combined effect of lower OWD, greater packing density, and higher SCM reactivity results in higher compressive strength. The increase in SSA from 425 to 1600 m²/kg results in an enhancement in packing density from 0.58 to 0.72, while further increase in SSA from 1600 to 2200 m²/kg reduces the packing density from 0.72 to 0.62. Binder systems using a distribution modulus between 0.21 and 0.235 determined from the A&A model exhibited 18%–40% lower minimum water demand (MWD) to initiate flow, 8%–35% higher OWD to reach maximum wet density, and 15%–25% higher packing density compared to the binder with 100% cement. Binder systems with lower A&A distribution modulus resulted in higher relative water demand (RWD) required to increase fluidity, thus reflecting greater level of robustness. Good correlations were established between the A&A distribution modulus, SSA, RR spread factor, and power law distribution exponent.     

制膜条件对聚丙烯中空纤维微孔膜强度的影响研究

用热致相分离法制备了聚丙烯中空纤维膜,研究了制膜过程中等规聚丙烯熔融指数、初始浓度以及凝固浴温度等因素对所制备膜强度的影响,提出了用热致相分离法制备等规聚丙烯中空纤维膜的适宜工艺条件.对影响合成高分子膜强度的各因素进行了分析和讨论.     

短切碳纤维表面处理对木粉/高密度聚乙烯复合材料性能的影响

木粉（WF）填充增强高密度聚乙烯（HDPE）复合材料具有良好的环境效益,少量引入短切碳纤维（SCF）可进一步提高其力学性能。为改善SCF与WF/HDPE复合材料中塑料基体的界面结合,提高SCF在WF/HDPE复合材料中的增强作用,采用气相、液相及气液双效氧化3种表面处理方式处理SCF,通过挤出工艺制备短切碳纤维增强木粉/高密度聚乙烯复合材料（SCF-WF/HDPE）,探讨了不同处理方法对SCF-WF/HDPE复合材料性能的影响。SEM观察显示,表面处理增大了SCF的表面粗糙度,可提高其与基体的界面结合;动态力学性能分析证实碳纤维提高了存储模量。测试结果表明：表面处理过的短切碳纤维可使SCF-WF/HDPE复合材料的力学性能、热力学性能和蠕变性能均得到显著提高,其中气相表面处理的效果最好。对比WF/HDPE复合材料,SCF-WF/HDPE的拉伸强度提高了34.5%,弯曲强度提高了23%,冲击强度提高了54.7%。     

Effect of Annealing on Virgin and Recycled Carbon Fiber Electrochemically Deposited with N-type Bismuth Telluride and Bismuth Sulfide

N-type thermoelectric bismuth telluride (Bi₂Te₃) and bismuth sulfide (Bi₂S₃) were deposited on virgin carbon fiber (VCF) and recycled carbon fiber (RCF) substrates by electrodeposition. The effects of annealing on the surface morphology and the Seebeck coefficient of the Bi₂Te₃ and Bi₂S₃ films were investigated. A nearly stoichiometric N-type Bi₂Te₃ was obtained from an electrolyte solution of 8 mM of Bi(NO₃)₃.5H₂O and 10 mM of TeO₂, which displayed the highest Seebeck coefficient of ?20.01 and ?13.0 µV/K for VCF and RCF, respectively. The deposition of Bi₂S₃ was slightly off-stoichiometry, but the improvement was still significant with a Seebeck coefficient of ?16.3 and ?12.4 µV/K for VCF and RCF, respectively. The effect of varying the annealing temperature (275°C and 350°C) and annealing time (2 and 3 hours) was studied on a nearly stoichiometric N-type Bi₂Te₃. The result shows an improvement in the Seebeck coefficient by 1.51–1.24 times at 350°C for 2 hours.     

An analysis of synergistic and antagonistic behavior during BTEX removal in batch system using response surface methodology

The removal of benzene, toluene, ethylbenzene and xylene (BTEX) as quaternary mixtures were studied in batch systems using a well-defined mixed microbial culture. The synergistic and antagonistic effects of total BTEX removal (BTEXT-RE) due to the presence of mixed substrate was evaluated through experiments designed by response surface methodology (RSM). The low and high concentrations of individual BTEX were 15 and 75 mg l(-1), respectively. The results showed that, increasing the concentration of xylene increased the cumulative BTEX removal (BTEXT-RE), however the reverse occurred when benzene concentrations were increased from low to high levels. A mixed response of increasing and decreasing trend in the BTEXT-RE value was observed when either of toluene or ethylbenzene concentration was increased. When the concentrations of individual BTEX compounds were 30 mg l(-1), the BTEXT-RE was about 58%. Complete BTEXT-RE was achieved at optimal BTEX concentrations of 48.1, 45.6, 49.3 and 56.6 mg l(-1). The RSM approach was found efficient in explaining the main, squared and interaction effects among individual BTEX concentrations on the BTEXT-RE in a more statistically meaningful way.     

Theoretical consistency test of steam transport properties

The kinetic theory of polyatomic gases is used to test the mutual consistency of the thermal conductivity, viscosity and specific heat of low-pressure steam from 100° to 700°C. No inconsistency exists within the cited tolerances of the skeleton tables recommended by the Sixth International Conference on the Properties of Steam, but there is some basis for suspecting that the high-temperature thermal conductivity values are slightly low. Methods for tightening the consistency bounds are suggested.