新型梳棉针布工艺性能分析

为合理配置针布型号,对新型梳棉针布的性能进行了试验研究,分析了锡林、道夫、盖板、刺辊新型针布的规格参数对梳理质量的影响,对比了原针布相对于新型针布在梳理质量上存在的差异,实践表明新型梳棉针布的梳理作用和质量优于原使用针布.     

Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods

Beetroot cubes were dehydrated by convective drying in hot air at 60 °C and by the combination of convective pre-drying (CPD) until moisture content 1.6, 0.6 or 0.27 kg/kg db and vacuum-microwave finish drying (VMFD) at 240, 360 or 480 W. The control samples were obtained by freeze-drying (FD). The drying kinetics of beetroot cubes was described with an exponential function. VMFD significantly reduced the total time of drying and decreased drying shrinkage in comparison with convective method. A critical moisture content divided the temperature profile of samples during VMFD into increasing and falling periods. At the falling temperature period a significant increase in the colour parameters L^∗, a^∗ and b^∗ was found. VM treated samples as well as FD ones exhibited lower compressive strength, better rehydration potential and higher antioxidant activity than those dehydrated in convection. Increasing the microwave wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the combined method.     

Analysis of giant pumpkin (Cucurbita maxima) drying kinetics in various technologies of convective drying

The paper analyses the results of the research on drying a variety of giant pumpkin (Justynka – 957). The research involved drying kinetics of pumpkin as well as changes in the volume of 10 mm pumpkin cubes, dried at the temperature of 80 °C, using free (natural) convection and in the forced convection in the tunnel dryer, with the drying factor speed of 1.2 m s⁻¹ and in the fluid bed dryer. Drying kinetics of pumpkin pieces dried in the tunnel dryer, fluid bed dryer and during two-stage initial drying in the tunnel, and in the fluid bed dryer were examined. Measurements results were used to verify the theoretical models of drying kinetics of the first period taking volume shrinkage into consideration, and the second period of the examined processes.     

Char cracking of medium density fibreboard due to thermal shock effect induced pyrolysis shrinkage

Pyrolysis experiments were conducted on medium density fibreboard (MDF) in inert atmosphere and different ambient pressures, to investigate the char shrinkage and cracking. It is found that the char cracking under uniform heat flux is a typical thermal shock process induced by unbalance shrinkage along the sample thickness during pyrolysis. To predict the number of char fissures, the critical stress criterion and energy conservation theory are used to develop mathematical models under plane constitutive stress state, which reveal that under the same surface degradation the number of char fissures (blisters) strongly relates to the pyrolysis depth at cracking time. Increasing external heat flux decreases the pyrolysis depth and increases the number of char fissures. Both experiments and numerical modelling are used to validate the models. The experimental results show that the horizontal shrinkage is 11% of original length and the micro-structure of char fissures of MDF is less uniform compared to the one of natural wood with a cellular pattern. The surface stresses after cracking are found similarly close to the tensile strength under different heat fluxes, while the surface stresses are very different assuming no crack, which indicates the cracking process reduces the surface stress to lower than the tensile strength. The modelled cracking times are different from the observed cracking time as the fissures are hard to identify at its initial stage and only when they have expanded to certain size the fissures are visually observed. Using the modelled cracking time, the number of char blisters can be well correlated with the pyrolysis depth.     

PLA纤维的性能研究

本文研究了PLA纤维的表面形态、物理机械性能、吸水性、在碱性洗涤剂中的收缩性,以及酸、碱和有机溶剂对PLA纤维的强力损伤程度。结果表明：PLA纤维的强度与形态和PET纤维相近,吸水率比棉小但比PET纤维大,PLA纤维在碱性条件下缩水率和强力损失速率大,PLA纤维抵抗酸碱和有机溶剂的能力大小为：丙酮〉盐酸〉碳酸钠〉氢氧化钠。     

稳健自适应波束形成器设计与仿真

针对自适应波束形成器在进行对角加载时存在选取对角加载值的不可靠问题,设计了一种稳健的自适应波束形成器。通过采用收缩方法(Shrinkage Method)获得了对协方差矩阵更精确的估计,以代替标准Capon波束形成器(SCB)中的采样协方差矩阵。与传统自适应波束形成器相比,能够根据给定数据完全自主的计算出对角加载值而不需要指定任何用户参数,有效的抑制干扰信号,改善输出信噪比,且具有良好的波束保形能力和较快的收敛速度,特别是在快拍数较少的情况下具有很好的性能。计算机仿真验证了设计方法的正确性和有效性。     

双筒高层建筑水平温差收缩应力计算

本文从现浇铨结构收缩和温度场分析入手,采用连续化方法,给出了计算双筒高层建筑水平温度收缩应力的一系列公式和一个应用实例,并在此基础上提出了这类结构考虑温差收缩影响的设计建议和准则。     

Effect of Co2O3 as sintering aid on perovskite BaCe0.8Y0.2O3-δ proton conductive membrane for hydrogen separation

BaCe_0.8Y_0.2O_3-δ proton conductor powder was prepared by sol-gel method, and the effects of sintering temperature and sintering aids addition on the mechanical properties and hydrogen permeability of BaCe_0.8Y_0.2O_3-δ proton conductor were investigated. XRD tests showed that when the addition of sintering aid Co₂O₃ reached 5%, the BaCe_0.8Y_0.2O_3-δ proton conductor still showed a good perovskite phase. The sintering temperature of the sample with sintering aid is significantly lower than that of the blank sample. SEM shows that the addition of Co₂O₃, the proton conductor grains are closely arranged, the mechanical properties are increased, and the hydrogen permeability is significantly improved.     

Influence of powder mixing processes and WC contents on microstructure,shrinkage behavior,and mechanical properties of WC-Si3N4 composites

WC-Si₃N₄ composite structural material with excellent mechanical properties is essential for achieving high-strength connections between WC and Si₃N₄. Proper powder mixing process can significantly improve the mechanical properties of sintered samples. This study investigates the effects of two different ball milling methods: (1) mixing WC powder, Si₃N₄ powder, and sintering aid powders and then ball milling for 24 h (BM24); and (2) ball milling WC powder, Si₃N₄ powder, and sintering aid powders separately for 12 h and then ball milling in same jar for 12 h (BM12 + 12), as well as WC content on sintering shrinkage behavior of mixed powders. Furthermore, the bonding between the layers of one-time sintered sample of the WC-Si₃N₄ composite structure was investigated. The results showed that the microstrains of powders prepared by BM24 mixing process were greater than those of the powders prepared by BM12 + 12 mixing process. Shrinkage displacements and shrinkage rates during sintering of BM12 + 12 powders were greater than those of BM24 powders to varying degrees. Moreover, sintering temperature at which shrinkage rate of composite powders reaches its maximum value gradually increases as WC content decreases, while maximum value of shrinkage rate gradually decreases. Additionally, mechanical properties of all BM12 + 12 samples were better than those of BM24 samples. WC-Si₃N₄ composite structure sample sintered with BM12 + 12 powders had a smooth transition between the layers without any defects.     

Mathematical modeling of eggplant drying: Shrinkage effect

In this paper two mathematical models with shrinkage effect describing eggplant drying are developed and discussed. The models are both modified diffusion equations and take differently into account changes of eggplant slice structure during drying. In the first model a diffusion coefficient variable with the water content is considered while in the second model a fictitious convective term is introduced. The two models are both suitable to describe the analyzed drying processes. Moreover, their equivalence is analytically demonstrated. Parameters values are estimated through a nonlinear regression procedure by comparison with the drying experiments carried out at different temperatures. Information about thickness evolution, derived from the models, are found to be in agreement with experimental data.