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关键词： 纳米粒子   纳米燃油   悬滴法   表面张力  

摘要： 以柴油为基液、溴化十六烷三甲基铵(Cetyltrimethyl Ammonium Bromide,CTAB)为助溶剂,通过两步法配制CNT(Carbon Nanotubc)、CeO_(2)及Co_(3)O_(4)纳米燃油。采用悬滴法测量柴油与纳米燃油的表面张力,探究纳米物质种类、粒径、质量分数及温度对纳米燃油表面张力的影响。研究发现,在纳米燃油液滴气液界面层内,纳米粒子与柴油分子之间的吸引力使液固分子体系总体的内聚力增强,因而表面张力增大;纳米燃油的表面张力随着粒子质量分数增加而增强,但随着温度升高而线性下降。大粒径的纳米粒子表面电荷密度降低,对表面电子束缚减小,电子游离所产生的与柴油分子之间的极化静电相吸作用更强,使液粒之间的范德华力增强,表面张力增大。在相同质量分数的条件下,非金属CNT的密度较小,纳米燃油中的粒子数目较多,此外其得电子能力更强,极易使周围柴油分子极化形成静电吸引,液粒间范德华力增强,因此CNT纳米燃油的表面张力最大;同为金属氧化物,Co_(3)O_(4)的分子量大于CeO_(2)而表现出较低的分子极性,其与基液燃油分子间的静电作用力也较弱,因而Co3O4纳米燃油的表面张力较低。本文测量了多种纳米燃油的表面张力,探讨了不同物质种类、浓度、尺寸的纳米介质及环境温度对燃油表面张力的影响,为纳米燃油在发动机缸内的液滴破碎与着火燃烧过程提供了重要的基础数据和理论支持。

关键词： 侧向微重力   板式表面张力贮箱   流体行为   液体晃动  

摘要： 侧向微重力是航天器在轨飞行时在东西位置保持和南北位置保持状态时所处的加速度环境.由于航天器贮箱内推进剂在侧向加速度环境下的重定位过程易产生晃动,因此侧向加速度环境对贮箱内推进剂管理装置(PMD)的管理能力的要求更高.为确保板式贮箱对推进剂的在轨管理能力,需要开展一系列的数值仿真与试验对PMD的管理能力进行验证.本文以板式表面张力贮箱为研究对象,采用VOF两相流模型对侧向微重力环境下重定位过程中贮箱内推进剂的运动进行数值模拟,并对推进剂运动中的晃动行为进行分析.最终,通过数值模拟结果验证了板式表面贮箱的管理能力,为微重力落塔试验与空间站试验提供参考依据.

关键词： Deep learning   Ultrasonography   Breast diseases   Diagnosis  

摘要： Background:The current deep learning diagnosis of breast masses is mainly reflected by the diagnosis of benign and malignant *** China,breast masses are divided into four categories according to the treatment method:inflammatory masses,adenosis,benign tumors,and malignant *** categorizations are important for guiding clinical *** this study,we aimed to develop a convolutional neural network(CNN)for classification of these four breast mass types using ultrasound(US)***:Taking breast biopsy or pathological examinations as the reference standard,CNNs were used to establish models for the four-way classification of 3623 breast cancer patients from 13 *** patients were randomly divided into training and test groups(n=1810 vs.n=1813).Separate models were created for two-dimensional(2D)images only,2D and color Doppler flow imaging(2D-CDFI),and 2D-CDFI and pulsed wave Doppler(2D-CDFI-PW)*** performance of these three models was compared using sensitivity,specificity,area under receiver operating characteristic curve(AUC),positive(PPV)and negative predictive values(NPV),positive(LR+)and negative likelihood ratios(LR-),and the performance of the 2D model was further compared between masses of different sizes with above statistical indicators,between images from different hospitals with AUC,and with the performance of 37 ***:The accuracies of the 2D,2D-CDFI,and 2D-CDFI-PW models on the test set were 87.9%,89.2%,and 88.7%,*** AUCs for classification of benign tumors,malignant tumors,inflammatory masses,and adenosis were 0.90,0.91,0.90,and 0.89,respectively(95%confidence intervals[CIs],0.87-0.91,0.89-0.92,0.87-0.91,and 0.86-0.90).The 2D-CDFI model showed better accuracy(89.2%)on the test set than the 2D(87.9%)and 2D-CDFI-PW(88.7%)*** 2D model showed accuracy of 81.7%on breast masses≤1 cm and 82.3%on breast masses>1 cm;there was a significant difference between the two groups(P<0.001).The accu

关键词： 卡门涡街   Fluent仿真   气固两相流   静电感应  

摘要： 针对现有静电感应式煤尘测量装置在低浓度、粒子运动速度较低情况下测量不精确且功能单一的问题,依据卡门涡街的原理,提出在测量管道中安装带狭缝圆柱发生体,从而提高煤尘粒子的运动速度,增大静电传感器的感应电荷量,提高煤尘浓度测量精度;同时,对测量管道内由于颗粒速度的周期性变化所感应出的静电信号周期性变化,进行快速傅里叶变换(FFT),求解颗粒的速度流量信息,实现对煤尘的多参数测量。仿真试验结果表明:依据感应电荷的周期性变化可以获得煤尘的流速(流量)和浓度参数。此装置结构简单,既可提高煤尘浓度测量精度,又能满足煤尘速度、流量测量的要求。

关键词： Navier-Stokes equations   Thermodynamics   Symmetry algebras   Differential invariants  

摘要： Symmetries and differential invariants of viscid flows with viscosity depending on temperature on a space curve are given. Their dependence on thermodynamic states of media is studied, and a classification of thermodynamic states is given. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Ecosystem functions   Habitat   Regulating   Provisioning   Cultural services   Biodiversity   Cities   Aquatic ecosystems  

摘要： Urban streams and ecosystems are highly affected by the intense development of cities and an increase of impervious surfaces. If these environments are close to their pristine state, they can be considered a nature-based solution promoting the integration of both green and blue elements in the cities, while providing a wide range of services (e.g. better aesthetics, air quality, leisure areas, and mitigation of climatic changes). In view of this, the current study aimed to develop a holistic assessment tool for urban streams that can be used to highlight their importance and support decision makers in the elaboration of measures to recover urban streams. The tool called Urban stream Assessment system (UsAs) includes the assessment of (1) biodiversity, (2) ecological functions and habitat that are inherent components of the ecosystems, and (3) provisioning, regulating, and cultural services, following The Millennium Ecosystem Assessment (MA) and the Common International Classification of Ecosystem Services (CICES). The tool includes 89 indicators belonging to 17 divisions (services and functions) selected from studies on urban areas and fluvial ecosystems. In addition, the tool introduces a method for the measurement of an indicator, rationale for the used indicator, positive and negative scope of the indicator, and reference to the proposed method. The tool incorporates also a step-by-step scoring system, which results in a global quality classification of streams. The UsAs tool was tested with a case study stream located in the city of Coimbra, Central Portugal. The final classification attributed to this stream was moderate, which is slightly higher than the ecological status, highlighting its potential ecosystem services (ES). However, the UsAs clearly showed a poor biodiversity (namely of aquatic invertebrates, amphibians, fish, aquatic mammals, and terrestrial insects), habitat conditions and the most compromised services that could be improved: water supply, a

关键词： Particle distribution   Circulating fluidized bed boiler   Visualization   Heat transfer enhancement   Fouling prevention  

摘要： A circulating fluidized bed boiler is designed and built to conduct visualization study by combining the circulating fluidized bed heat transfer technology and boiler. Water and polyformaldehyde particles are selected as the working media. The effect of the operating parameters such as the amount of added particles and feedwater flow rate on the fluidization and distribution of particles are investigated by using a charge coupling device. Results show that the particles can be normally fluidized and circulated at proper feedwater flow rate and amount of added particles. The solid holdup in the riser bundle increases with increased feedwater flow rate and amount of added particles. With increased feedwater flow rate, the nonuniform degree of particle distribution decreases. The amount of added particles has less influence on particle distribution than feedwater flow rate. The average solid holdup in the water wall tubes is close to that in the splayed flue tubes. (C) 2020 Elsevier B.V. All rights reserved.

关键词： 集群   形成构型   航路规划   无谓避碰   改进势场法   二重势能场  

摘要： 人工势场法由于其在构型组织能力上的不足,影响了该方法在集群航路规划上的应用,为此提出基于二重势函数法的集群航路规划法,通过第一重势能场形成集群到目标的可行路径,通过第二重势能场形成构型,从而实现集群航路规划.此外,针对人工势场法存在无谓避碰、陷阱问题等不足,通过引入碰撞危险度来确定障碍物影响距离以及虚拟障碍物,提出改进的人工势场法.所有改进措施均未改变简单且易于执行的传统势函数模型,为采用人工势场法实现集群航路规划提供了一种思路.仿真结果表明改进的人工势场法很好的克服了不足,所得路径也较为平滑,且以该方法为基础所构成的集群航路规划法在集群航路规划上具有较好的表现.

关键词： 压缩空气储能   轴流涡轮   弯叶片   优化设计   流动机理  

摘要： 应用于压缩空气储能系统(CAES)的轴流涡轮具有运行压力高、导叶展弦比低、端壁二次流影响大的特点.为进一步提高效率,本文引入导叶弯曲造型并进行优化设计,获得了最优导叶弯曲参数及其对流动损失的控制机理.研究结果表明:不同弯高下均存在一个最佳弯角值使等熵效率最大;随弯高增加,最佳弯角值逐渐减小.在不同弯高下,轴流涡轮质量流量均随着弯角先减小后增大,并在弯角为7°左右达到最小值.优化设计结果表明,当弯曲角和相对弯高分别为12.26°和0.31时,轴流涡轮等熵效率提高幅度最大为0.77％,此时质量流量仅增加0.1 kg/s.与应用于其他领域的涡轮不同,最优弯导叶对CAES轴流涡轮流场的改善效果主要集中在动叶通道内部:通过降低动叶轮毂和机匣表面附近进口气流角,消除动叶前缘根部滞止鞍点,限制马蹄涡影响范围;最优弯导叶也使叶顶前缘鞍点位置向下游移动,减低叶顶前缘吸力面附近马蹄涡分支的影响范围,延迟了叶顶间隙泄漏流的产生,减弱叶顶间隙泄漏流与上通道涡作用强度,最终降低流动损失.