The Decreasing of the Extrusion Time with Varying Ram Speed

In the extrusion process, friction and workpiece deformation usually cause an increase in the temperature of the extrudate. During the stroke, this temperature increase may reach a critical level and restrict the ram speed. In this study, extrusion processes, which were conducted under real production conditions, were simulated at different ram speeds using finite element modeling. In this way, the maximum workable constant ram speed was determined according to a defined critical extrusion temperature as a reference value. In the next stage, new ram-speed models were developed as alternatives to the maximum-workable, constant ram speed. The results showed that the time taken to extrude could be significantly shortened by using these models.     

平行隧道盾构施工对土体变形的影响分析

采用有限差分软件FLAC^3D,建立盾构隧道的三维数值模型,研究平行盾构隧道施工引起的地表沉降和土体水平变形规律。比较了单、双孔隧道施工地表沉降的特点,分析了平行盾构隧道施工的同步性、掌子面距离、注浆时间和土舱压力等对土体变形的影响,研究结果对实际工程具有一定的参考意义。     

Factors influencing the performance of Dynamic Decision Network for INQPRO

There has been an increasing interest in employing decision-theoretic framework for learner modeling and provision of pedagogical support in Intelligent Tutoring Systems (ITSs). Much of the existing learner modeling research work focuses on identifying appropriate learner properties. Little attention, however, has been given to leverage Dynamic Decision Network (DDN) as a dynamic learner model to reason and intervene across time. Employing a DDN-based learner model in a scientific inquiry learning environment, however, remains at infant stage because there are factors contributed to the performance the learner model. Three factors have been identified to influence the matching accuracy of INQPRO’s learner model. These factors are the structure of DDN model, the variable instantiation approach, and the weights assignment method for two consecutive Decision Networks (DNs). In this research work, a two-phase empirical study involving 107 learners and six domain experts was conducted to determine the optimal conditions for the INQPRO’s dynamic learner model. The empirical results suggested each time-slice of the INQPRO’s DDN should consist of a DN, and that DN should correspond to the Graphical User Interface (GUI) accessed. In light of evidence, observable variables should be instantiated to their observed states; leaving the remaining observable nodes uninstantiated. The empirical results also indicated that varying weights between two consecutive DNs could optimize the matching accuracy of INQPRO’s dynamic learner model.     

A FIRST STAGE IN THE DEVELOPMENT OF MICROMECHANICAL SIMULATIONS OF THE CRYSTALLOGRAPHIC PROPAGATION OF FATIGUE CRACKS UNDER MULTIAXIAL LOADING

Simulations of the nucleation of dislocations, glide and annihilation ahead of a fatigue crack growing along a localized slip band (a 'long' Stage I crack or a Stage II crack with a K value close to the threshold) are performed for the case of push–pull or reversed torsion loadings, ignoring, in a first approach, the effect of grain boundaries. The crack growth rates are deduced from the dislocation flux at the crack tip. An influence of the normal stress on the friction between the crack flanks as well as on the condition for dislocation emission is introduced. A slower Stage I growth rate is then predicted for reversed torsion, consistent with experimental data.     

Effects of friction and asymmetric inner sphere reorganization energy on the electron transfer reaction rate—Two-dimensional simulations

Molecular dynamic simulations are applied to study inner sphere reorganization effects on outer-sphere electron transfer (ET) reactions. The system studied is assumed to be a complex of the form A(H₂O)₆. The Hamiltonian describing the one-electron reduction reaction is based on the Schmickler and Koper model [W. Schmickler, M.T.M. Koper, Electrochem. Commun. 1 (1999) 402]. The resulting potential energy is a function of the generalized solvent coordinate and the change in the A-O distance. The first hydration shell in the oxidized and reduced states is described by two harmonic oscillators with frequencies ω₁ and ω₂. Three cases are studied in the simulations with respect to the frequencies ratio θ = ω₂/ω₁: 2/3, 1 and 3/2. An effect of different friction parameters applied to the two coordinates and the activation energy on the reaction rate is analyzed and compared for the three cases. A strong decrease in the reaction rate is observed when a very low friction in the inner sphere direction is assumed. The turnover for the three cases is found to increase when θ goes from 2/3 to 3/2, but it corresponds to the friction parameter which is much lower than the barrier frequency. The relation between the saddle point avoidance phenomenon and the reaction rate is qualitatively analyzed.     

Silicon thin-film solar cells on insulating intermediate layers

An interdigitated front grid structure for both the emitter and base was simulated and realized. This contact design is suitable for thin-film solar cells on insulating substrates or insulating intermediate layers. Confirmed efficiencies of up to 18.2% were achieved on a 46 μm thick epitaxial silicon layer which was grown on a SIMOX wafer with an implanted compact SiO₂ intermediate layer.Samples with and without a highly doped back surface field were prepared to study the influence of the back-side recombination velocity. L_eff values of 250 and 52 μm, corresponding to S_back values of 800 and 10⁵ cm/s, respectively, were measured, thus, underlining the importance of a low back-side recombination velocity. The optical confinement properties of the SiO₂ intermediate layers were calculated depending on the angle of the incident rays. An angle from the plane normal which is larger than 23° is necessary in order to achieve the condition of total internal reflection.Future work will focus on recrystallized Si layers on foreign substrates [1]. Since the surface of the silicon layer is fairly rough after the recrystallisation process, another set of masks was designed which is more tolerant to aligning accuracy. This is mainly relevant for the area where the base contacts are located between the locally diffused emitter. The technology for CVD Si-layer deposition, zone melting recrystallization (ZMR), as well as for a simplified solar cell process is under investigation.     

Effect of unequal fuel and oxidizer Lewis numbers on flame dynamics

The interaction of non-unity Lewis number (due to preferential diffusion and/or unequal rates of heat and mass transfer) with the coupled effect of radiation, chemistry and unsteadiness alters several characteristics of a flame. The present study numerically investigates this interaction with a particular emphasis on the effect of unequal and non-unity fuel and oxidizer Lewis numbers in a transient diffusion flame. The unsteadiness is simulated by considering the flame subjected to modulations in reactant concentration. Flames with different Lewis numbers (ranging from 0.5 to 2) and subjected to different modulating frequencies are considered. The results show that the coupled effect of Lewis number and unsteadiness strongly influences the flame dynamics. The impact is stronger at high modulating frequencies and strain rates, particularly for large values of Lewis numbers. Compared to the oxidizer side Lewis number, the fuel side Lewis number has greater influence on flame dynamics.     

TD-SCDMA HSPA+中的单双流MIMO技术的研究

多输入多输出(MIMO)技术是近年来移动通信的热门研究领域,它的特征在于无线发射机和接收机都引入了多根天线.MIMO技术除了通过空间分集方式获得容量的提升外,还可以通过空间复用方式,利用不同天线信道的独立性建立多个空间子信道来提高容量.但是,无论是基于空间分集的单流模式还是基于空间复用的双流模式,单独在TD-SCDMA...     

Design of an annular microchannel reactor (AMR) for hydrogen and/or syngas production via methane steam reforming

A bench-scale annular microchannel reactor (AMR) prototype with microchannel width of 0.3 mm and total catalyst length of 9.53 × 10⁻² m active for the endothermic steam reforming of methane is presented. Experimental results at a steam to methane feed molar ratio of 3.3:1, reactor temperature of 1023 K, and pressure of 11 bar confirm catalyst power densities upwards of 1380 W per cm³ of catalyst at hydrogen yields >98% of thermodynamic equilibrium. A two-dimensional steady-state computational fluid dynamic model of the AMR prototype was validated using experimental data and subsequently employed to identify suitable operating conditions for an envisioned mass-production AMR design with 0.3 mm annular channel width and a single catalyst length of 254 mm. Thermal efficiencies, defined based upon methane and product hydrogen higher heating values (HHVs), of 72.7–57.7% were obtained from simulations for methane capacities of 0.5–2S LPM (space velocities of 195,000–782,000 h⁻¹) at hydrogen yields corresponding to 99%–75% of equilibrium values. Under these conditions, analysis of local composition, temperature and pressure indicated that catalyst deactivation via coke formation or Nickel oxidation is not thermodynamically favorable. Lastly, initial analysis of an envisioned 10 kW autothermal reformer combining 19 parallel AMRs within a single methane-air combustion chamber, based upon existing manufacturing capabilities within Power & Energy, Inc., is presented.