The ductile-to-brittle transition temperature(DBTT)of high strength steels can be optimized by tailoring microstructure and crystallographic orientation characteristics,where the start cooling temperature plays a key ...The ductile-to-brittle transition temperature(DBTT)of high strength steels can be optimized by tailoring microstructure and crystallographic orientation characteristics,where the start cooling temperature plays a key role.In this work,X70 steels with different start cooling temperatures were prepared through thermo-mechanical control process.The quasi-polygonal ferrite(QF),granular bainite(GB),bainitic ferrite(BF)and martensite-austenite constituents were formed at the start cooling temperatures of 780℃(C1),740℃(C2)and 700℃(C3).As start cooling temperature decreased,the amount of GB decreased,the microstructure of QF and BF increased.Microstructure characteristics of the three samples,such as high-angle grain boundaries(HAGBs),MA constituents and crystallographic orientation,also varied with the start cooling temperatures.C2 sample had the lowest DBTT value(−86℃)for its highest fraction of HAGBs,highest content of<110>oriented grains and lowest content of<001>oriented grains parallel to TD.The high density of{332}<113>and low density of rotated cube{001}<110>textures also contributed to the best impact toughness of C2 sample.In addition,a modified model was used in this paper to quantitatively predict the approximate DBTT value of steels.展开更多
In this study,thermo-fluid characteristics of elliptical annular finned tube heat exchanger were numerically studied in detail.Transition SST model was utilized to simulate turbulent flow.Effects of air velocities,hor...In this study,thermo-fluid characteristics of elliptical annular finned tube heat exchanger were numerically studied in detail.Transition SST model was utilized to simulate turbulent flow.Effects of air velocities,horizontal to vertical fin diameter ratios,and fin densities were examined in detail.The simulations indicate superior performance of elliptical fin layout.It was shown that pressure drop of annular elliptical fin can be only one half of that of a circular annular fin while containing comparable heat transfer performance.The vertical elliptical annular fin may even contain a higher heat transfer performance over circular fin.Correlations are proposed to estimate the Nu number and pressure drop based on the annular circular fin.The maximum deviations between the proposed correlations and simulations regarding pressure drop and heat transfer coefficient are 5.6%and 3.2%,respectively.For further elaboration of the superiority of the elliptical layout from the second law perspective,normalized entropy generation was also studied.In all cases,the entropy generation rate in circular fin was higher than that of an elliptical fin.展开更多
The method of Zeng et al. (1991) employed diameter growth to estimate the transition probability of the matrix model in uneven-aged forest stands. In this paper the Weibull distribution for even-aged forest stands ins...The method of Zeng et al. (1991) employed diameter growth to estimate the transition probability of the matrix model in uneven-aged forest stands. In this paper the Weibull distribution for even-aged forest stands instead of uniform distribution chosen by Zeng is used. By comparing the results of the improved method with those of the original method of Zeng, it turns out that the improved method of Zeng given in this paper is more efficient.展开更多
Long-rod penetration in a wide range ol" velocity means that the initial impact velocity varies in a range from tens of meters per second to several kilometers per second.The long rods maintain rigid state when t...Long-rod penetration in a wide range ol" velocity means that the initial impact velocity varies in a range from tens of meters per second to several kilometers per second.The long rods maintain rigid state when the impact velocity is low,the nose of rod deforms and even is blunted when the velocity gets higher,and the nose erodes and fails to lead to the consumption of long projectile when the velocity is very high clue to instantaneous high pressure.That is,from low velocity to high velocity,the projectile undergoes rigid rods,deforming non-erosive rods,and erosive rods.Because of the complicated changes of the projectile,no well-established theoretical model and numerical simulation have been used to study the transition zone.Based on the analysis of penetration behavior in the transition zone,a phenomenological model to describe target resistance and a formula to calculate penetration depth in transition zone are proposed,and a method to obtain the boundary velocity of transition zone is determined.A combined theoretical analysis model for three response regions is built by analyzing the characteristics in these regions.The penetration depth predicted by this combined model is in good agreement with experimental result.展开更多
Al/Ni reactive multilayer foil(RMF)possesses excellent comprehensive properties as a promising substitute for traditional Cu bridge.A theoretical resistivity model of Al/Ni RMF was developed to guide the optimization ...Al/Ni reactive multilayer foil(RMF)possesses excellent comprehensive properties as a promising substitute for traditional Cu bridge.A theoretical resistivity model of Al/Ni RMF was developed to guide the optimization of EFIs.Al/Ni RMF with different bilayer thicknesses and bridge dimensions were prepared by MEMS technology and electrical explosion tests were carried out.According to physical and chemical reactions in bridge,the electrical explosion process was divided into 5 stages:heating of condensed bridge,vaporization and diffusion of Al layers,intermetallic combination reaction,intrinsic explosion,ionization of metal gases,which are obviously shown in measured voltage curve.Effects of interface and grain boundary scattering on the resistivity of film metal were considered.Focusing on variations of substance and state,the resistivity was developed as a function of temperature at each stage.Electrical explosion curves were calculated by this model at different bilayer thicknesses,bridge dimensions and capacitor voltages,which showed an excellent agreement with experimental ones.展开更多
为探究建成环境对城市轨道站点客流的非线性影响,以北京市轨道交通站点为研究对象,基于兴趣点(Point of Interest,POI)数据、手机信令数据、路网数据等多源数据,从社会经济与人口、土地利用、多模式衔接及站点特征4方面精细化刻画建成环...为探究建成环境对城市轨道站点客流的非线性影响,以北京市轨道交通站点为研究对象,基于兴趣点(Point of Interest,POI)数据、手机信令数据、路网数据等多源数据,从社会经济与人口、土地利用、多模式衔接及站点特征4方面精细化刻画建成环境,构建梯度提升决策树(Gradient Boosting Decision Tree,GBDT)模型揭示建成环境对不同时段及不同方向站点客流的相对贡献、非线性影响及阈值效应.研究结果表明:GBDT模型拟合效果优于普通最小二乘(Ordinary Least Squares,OLS)模型、自适应增强(Adaptive Boosting,AdaBoost)算法及随机森林(Random Forest,RF)模型;社会经济与人口属性对高峰时段客流的影响最大,对早晚高峰进出站客流的贡献均超过40%;土地利用属性对平峰进出站客流的影响最大,贡献为33.06%及49.10%;相对重要程度最高的变量对站点客流均表现出明显的非线性影响及阈值效应;距离市中心15~22 km的站点有更高的早高峰进站及晚高峰出站客流;高峰时段私家车接驳轨道交通的比例远大于平峰时段.研究结果可为城市轨道交通线网规划及城市土地利用布局提供理论支撑.展开更多
基金Project(2018XK2301) supported by the Change-Zhu-Tan National Independent Innavation Demonstration Zone Special Program,China。
文摘The ductile-to-brittle transition temperature(DBTT)of high strength steels can be optimized by tailoring microstructure and crystallographic orientation characteristics,where the start cooling temperature plays a key role.In this work,X70 steels with different start cooling temperatures were prepared through thermo-mechanical control process.The quasi-polygonal ferrite(QF),granular bainite(GB),bainitic ferrite(BF)and martensite-austenite constituents were formed at the start cooling temperatures of 780℃(C1),740℃(C2)and 700℃(C3).As start cooling temperature decreased,the amount of GB decreased,the microstructure of QF and BF increased.Microstructure characteristics of the three samples,such as high-angle grain boundaries(HAGBs),MA constituents and crystallographic orientation,also varied with the start cooling temperatures.C2 sample had the lowest DBTT value(−86℃)for its highest fraction of HAGBs,highest content of<110>oriented grains and lowest content of<001>oriented grains parallel to TD.The high density of{332}<113>and low density of rotated cube{001}<110>textures also contributed to the best impact toughness of C2 sample.In addition,a modified model was used in this paper to quantitatively predict the approximate DBTT value of steels.
文摘In this study,thermo-fluid characteristics of elliptical annular finned tube heat exchanger were numerically studied in detail.Transition SST model was utilized to simulate turbulent flow.Effects of air velocities,horizontal to vertical fin diameter ratios,and fin densities were examined in detail.The simulations indicate superior performance of elliptical fin layout.It was shown that pressure drop of annular elliptical fin can be only one half of that of a circular annular fin while containing comparable heat transfer performance.The vertical elliptical annular fin may even contain a higher heat transfer performance over circular fin.Correlations are proposed to estimate the Nu number and pressure drop based on the annular circular fin.The maximum deviations between the proposed correlations and simulations regarding pressure drop and heat transfer coefficient are 5.6%and 3.2%,respectively.For further elaboration of the superiority of the elliptical layout from the second law perspective,normalized entropy generation was also studied.In all cases,the entropy generation rate in circular fin was higher than that of an elliptical fin.
基金This paper was supported by the National Natural Science Foundation of China
文摘The method of Zeng et al. (1991) employed diameter growth to estimate the transition probability of the matrix model in uneven-aged forest stands. In this paper the Weibull distribution for even-aged forest stands instead of uniform distribution chosen by Zeng is used. By comparing the results of the improved method with those of the original method of Zeng, it turns out that the improved method of Zeng given in this paper is more efficient.
基金supported by the National Natural Science Foundation of China(No.11302031,11371069,11372053)
文摘Long-rod penetration in a wide range ol" velocity means that the initial impact velocity varies in a range from tens of meters per second to several kilometers per second.The long rods maintain rigid state when the impact velocity is low,the nose of rod deforms and even is blunted when the velocity gets higher,and the nose erodes and fails to lead to the consumption of long projectile when the velocity is very high clue to instantaneous high pressure.That is,from low velocity to high velocity,the projectile undergoes rigid rods,deforming non-erosive rods,and erosive rods.Because of the complicated changes of the projectile,no well-established theoretical model and numerical simulation have been used to study the transition zone.Based on the analysis of penetration behavior in the transition zone,a phenomenological model to describe target resistance and a formula to calculate penetration depth in transition zone are proposed,and a method to obtain the boundary velocity of transition zone is determined.A combined theoretical analysis model for three response regions is built by analyzing the characteristics in these regions.The penetration depth predicted by this combined model is in good agreement with experimental result.
基金National Natural Science Foundation of China(Grant No.11872013)for supporting this project.
文摘Al/Ni reactive multilayer foil(RMF)possesses excellent comprehensive properties as a promising substitute for traditional Cu bridge.A theoretical resistivity model of Al/Ni RMF was developed to guide the optimization of EFIs.Al/Ni RMF with different bilayer thicknesses and bridge dimensions were prepared by MEMS technology and electrical explosion tests were carried out.According to physical and chemical reactions in bridge,the electrical explosion process was divided into 5 stages:heating of condensed bridge,vaporization and diffusion of Al layers,intermetallic combination reaction,intrinsic explosion,ionization of metal gases,which are obviously shown in measured voltage curve.Effects of interface and grain boundary scattering on the resistivity of film metal were considered.Focusing on variations of substance and state,the resistivity was developed as a function of temperature at each stage.Electrical explosion curves were calculated by this model at different bilayer thicknesses,bridge dimensions and capacitor voltages,which showed an excellent agreement with experimental ones.
