关键词： cashmere fibers   microwave vacuum drying   drying models   effective moisture diffusivity   mechanical properties  

摘要： Drying behavior of low temperature microwave vacuum-dried cashmere fibers and the drying rate at different times were investigated with different microwave powers, vacuum degrees and maximum drying temperatures to specify the optimum drying conditions in consideration of performance and drying efficiency in this study. The results show that higher microwave power can reduce the total drying time and significantly increase the drying rate in the initial stage. The degree of vacuum has a negligible effect on the drying behavior, and the influence of the three factors on effective moisture diffusivity is consistent with the drying behavior. Compared with hot-air-dried fibers, the mechanical properties of low temperature microwave-dried cashmere fibers could be significantly improved. In addition, excessive drying as delayed or overheated treatment will damage the fiber. The Page model is the most appropriate to describe the microwave vacuum drying of cashmere fibers.

关键词： Ni-based single crystal superalloys   Molecular dynamics simulation   Cyclic deformation   Fatigue behavior   Microstructural evolution  

摘要： The fatigue performance and deformation mechanism of Ni-based single crystal superalloys under cyclic tension-compression loading are studied by molecular dynamics simulations. The effects of temperature and strain rate on the cyclic deformation of superalloys are discussed. The results show that there are three different cyclic deformation mechanisms at different temperature ranges. In the low temperature range, dislocations and stacking faults shearing gamma' phase is the main deformation mechanism. In the high temperature range, Orowan bypassing and climbing mechanism dominates the deformation. While in the medium temperature range, the deformation mechanism gradually transforms from the dislocation and stacking faults shearing the gamma' phase to the Orowan bypassing and climbing. The simulation results also reveal that the superalloys exhibit a cyclic saturation stage after the initial cyclic hardening. The cyclic saturation stage is a dynamic equilibrium of dislocation proliferation and annihilation, indicating that the superalloys have excellent fatigue mechanical properties. Moreover, the dislocation density and the proportion of stacking faults increase with increasing strain rate, resulting in faster cyclic stability and higher stress amplitude for the superalloys. These results provide important information for understanding the fatigue mechanisms of superalloys from an atomistic perspective.

关键词： Spherical pendulum   Fractional damping   Nonlinear vibrations   Transient dynamics analysis  

摘要： The presented work deals with nonlinear dynamics of a three degree of freedom system with a spherical pendulum and a damper of the fractional type. Vibrations in the vicinity of the internal and external resonance are considered. The system consists of a block suspended from a linear spring and a fractional damper, and a spherical pendulum suspended from the block. The viscoelastic properties of the damper are described using the Caputo fractional derivative. The fractional derivative of an order of 0 < alpha <= 1 is assumed. The impact of a fractional order derivative on the system with a spherical pendulum is studied. Time histories, the internal and external resonance, bifurcation diagrams, Poincare maps and the Lyapunov exponents have been calculated for various orders of a fractional derivative. Chaotic motion has been found for some system parameters.

关键词： Enclosure fire   Liquid fuel   Air intake position   Extraction duct   Depression   Heat feedback  

摘要： The effects of air intake position on the burning behaviour of liquid pool inside a mechanically ventilated compartment are analyzed by using both an experiment and a physics-based model. A series of compartment fire experiments has been undertaken from a reduced scale enclosure with a length/height and width of 2 m. An external ventilation system consists of an air admission duct placed in low or high position inside the compartment, providing an Air Change Per Hour (ACPH) in a range from 1 to 4. Lowering the inlet duct enhances fuel-air mixing in the compartment. The experimental and numerical studies highlight a faster fire growth for a low air intake with a rise of ACPH, implying more dangerous fire with regarding the more important peak in heat release rate and heat feedback to liquid fuel surface. For air intake in high position, the direction of air jet inside enclosure is found to be orthogonal to the direction of the buoyancy-induced flow. This results in an air entrainment restriction towards the fire base and a decrease of the heat feedback to liquid fuel surface due to cooling effect on the hot smoke layer near the ceiling. Thus a high inlet contributes to a reduction by a factor of 40% in HRR, and the fire growth power is practically independent to ACPH. Moreover, for a high inlet, fire exhaust occurs more easily because both the heat feedback and air entrainment are weaker than these for a low inlet. As a consequence, changing air intake position from low to high leads to a change in fire regime from an under-ventilated fire to an over-ventilated one.

关键词： Drying kinetics   Fluidized bed drying   Reaction engineering approach   Vibrated fluidized bed  

摘要： The accurate modeling of vibrated fluidized bed dryers plays an important role in the design and optimization of industrial drying processes. Understanding the impact of vibration on the hydrodynamics of the bed, as well as the drying kinetics, is crucial for accurate modeling. The influence of vibration on the drying kinetics was investigated. It could be shown that vibration reduces the minimum fluidization velocity (umf) and thereby enables the operation of fluidized bed dryers at lower gas velocities. The reduction of umf is particularly pronounced at the resonance frequency of the bed material. However, once the powder was fully fluidized, no impact on the drying kinetics was observed. The Reaction Engineering Approach (REA) is one of the methods used to describe the drying kinetics with one material specific curve. In this work, it was shown that REA curves can be determined based on batch drying experiments in fluidized beds. Porous particles of Geldart groups A, B and D with particle sizes and densities similar to food products were investigated and their REA curves were experimentally determined. Comparison of the derived REA data with established material specific drying curves from literature shows high similarity. Hence, the REA data from fluidized bed experiments is suitable for the modeling and simulation of fluidized bed drying processes. (c) 2020 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

关键词： Polymorphism   Polymorphic transformations   Kinetics of transformation   Sorbitol   Milling   X-ray diffraction   DSC   Raman   Simulation  

摘要： In this paper, we present a kinetic investigation of the polymorphic transformation gamma -> alpha of sorbitol under milling in the objective to identify the microscopic mechanisms that govern this type of solid-state transformation. The milling was performed with a high energy planetary mill and the milled material was analysed by DSC, PXRD and Raman spectrometry. The transformation kinetics was found to be sigmoidal with a noticeable incubation time. Moreover, this incubation time was shown to shorten rapidly when seeding the initial form gamma with the final form alpha. The origin of the incuation period and its evolution upon seeding are puzzling as polymorphic transformations induced by milling are not expected to occur through a nucleation and growth process. To explain these puzzling kinetic features, we propose a two-step transformation mechanism involving local amorphisations due to the mechanical impacts, immediately followed by rapid recrystallizations of the amorphized fractions. The key point of the mechanism is that recrystallizations are oriented towards the forms gamma or alpha, depending on the crystalline form of neighbouring crystallites. This mechanism has been validated by numerical simulations which were able to reproduce all the experimental kinetic features of the polymorphic transformation (kinetic law and effects of seeding) upon milling.

关键词： Municipal sludges biochar   Cement-based composites   Mechanical properties   Hydration mechanisms   Sustainable waste management  

摘要： Biochar is a green material obtained from the thermochemical decomposition of biosolids under an anaerobic or oxygen-limited condition. Although sludge pyrolysis has been extensively studied in municipal sludge (MS) disposal, it is challenging to utilize the formed sludge biochar (SB) products. This study developed a novel and sustainable method for incorporating SB-derived biochar in cement-based composites, providing an economical and efficient way of recycling such wastes. A series of SB were prepared under different pyrolysis processes to replace 2% cement. Numerous mechanical, physical, microstructural and isothermal calorimetry analyses were used to study the performances of the SB incorporated cement composites. The results showed that the mobility of the composites declined slightly with the addition of SB. Pyrolysis regimes played a key role in influencing the strength, and the optimum performance was obtained at a pyrolysis regime of 500 degrees C and residual time of 60 min, which increased the 1 d and 28 d strengths by 29.3% and 5.8%, respectively. The addition of SB accelerated the formation of more hydration products, increased the hydration degree and led to a denser matrix. All these improvements were ascribed to the internal curing effect of sludge biochar and a decrease in the ratio of local water to cement. Thermal analysis by Krstulovic-Dabic Model indicated that the hydration mechanisms of biochar-cement composites belonged to nucleation and crystal growth (NG)- interactions at phase boundaries (I)-diffusion (D). The SB incorporated sample possessed more nucleation sites with a higher reaction rate constant at NG stage and shorter I stage. In addition, environmental risk assessment revealed that SB was a safe additive in cement-based composites. Therefore, pyrolysis of MS under appropriate temperature conditions has great potential to provide a green biochar in cement-based composites and thereby offers an alternative and viable way of disposal

关键词： Glass  

摘要： Polymer hybridization technique, consisting of an interlayer arrangement of different polymers, acts as the most economical and promising technique in augmenting the glass fiber-reinforced polymer composite's mechanical properties. This investigation focuses on the effect of cure kinetics on the flexural behavior of glass-polymer hybrid (GPH) composite, and also elucidates the comparative analysis on the mechanical behavior of glass-epoxy (GE) composite, glass-vinyl ester (GVE) composite, and GPH composite. The optimal postcuring temperature has been found to be 200 degrees C for GPH composite among the other postcuring temperatures conducted at 140, 170, and 230 degrees C. Among all these abovementioned composites, highest flexural strength and interlaminar shear strength properties have been recorded by the 200 degrees C postcured GPH composite leading to 10.87 and 18.76% increment, respectively, compared with GE composite. Furthermore, thermomechanical characterization has been done to know the viscoelastic behavior of the GPH composite postcured at different temperatures using dynamic mechanical thermal analysis. The fracture morphology of flexural tested composite samples demonstrated a combination of failure modes. Relevant information on the chemical restructuring and fracture morphology of experimented composite material using Fourier-transform infrared (FTIR) spectroscopy and Scanning electron microscopy (SEM) has also been studied.