Flow patterns and boundary conditions for inlet and outlet conduits of large pump system with low head

The flow patterns in the inlet and outlet conduits have a decisive effect on the safe, stable, and highly efficient operation of the pump in a large pumping station with low head. The numerical simulation of three-dimensional （3D） turbulence flow in conduits is an important method to study the hydraulic performance and conduct an optimum hydraulic design for the conduits. With the analyses of the flow patterns in the inlet and outlet conduits, the boundary conditions of the numerical simulation for them can be determined. The main obtained conclusions are as follows： （i） Under normal operation conditions, there is essentially no pre-swirl flow at the impeller chamber inlet of an axial-flow pump system, based on which the boundary condition at the inlet conduit may be defined. （ii） The circulation at the guide vane outlet of an axial-flow pump system has a great effect on the hydraulic performance of the outlet conduit, and there is optimum circulation for the performance. Therefore, it is strongly suggested to design the guide vane according to the optimum circulation. （iii） The residual circulation at the guide vane outlet needs to be considered for the inlet boundary condition of the outlet conduit, and the value of the circulation may be measured in a specially designed test model.

Cross influence of discharge and circulation on head loss of conduit of pump system with low head

The relationship between the head loss and the discharge and circulation of the conduit of a pump system with low head is an important problem with an obvious influence on the improvement of its hydraulic performance. The velocity circulation from the pump guide vane makes the relationship more complicated, which has to be understood comprehensively. The results indicate that, under the condition of zero circulation, the head loss of the inlet and outlet conduits is in proportion to the square of discharge. Under the condition that the Reynolds number is satisfied with the resistant square area, the conduit loss is in proportion to the square of discharge for the similar working points with different speeds in a certain rotational speed range, indicating that the pump system efficiency is constant. The outlet conduit loss of design discharge for a pump system with low head depends on the velocity circulation from the guide vane exit, and the relationship between the loss and the circulation is an open curve with an upward direction, meaning that there is an optimal circulation for the loss. Under the condition of various working points for a pump system with low head, the head loss of the outlet conduit is under the cross influence of both the discharge and the circulation. As a result, the relationship between the head loss and the discharge is almost linear, and the mechanism needs to be further studied.

Performance Investigation of a Simple Reaction Water Turbine for Power Generation from Low Head Micro Hydro Resources

Theoretical investigation has shown a simple reaction water turbine would perform better when it spins faster. And for the simple reaction turbine water turbine to spin faster under constant water head, its diameter should be smaller. This paper reports on a performance analysis based on the experimental data collected from different performance tests carried on two simple reaction water turbine prototypes. Two new designs of simple reaction water turbines and their manufacturing methods are reported. The two turbines under investigation have different rotor diameters Φ 0.243 m and Φ 0.122 m. In case of the simple reaction water turbine the water enters into the turbine axially and exits tangentially through nozzles located on the outer periphery of the turbine. Further this paper will discuss the performance characteristics of stationary turbine i.e. zero power produced and performance characteristics of turbine producing power. It was found that rotor diameter affects the maximum rotational speed of the simple reaction turbine for constant supply head. It was also found that faster the turbine spins its performance improves. The two turbines were tested between supply head range of 1 m to 4 m.

Pressure fluctuation propagation of a pump turbine at pump mode under low head condition

Pressure fluctuation at the vaneless space and vanes passages is one of the most important problems for the stable operation of a pump turbine. The fluctuation appears in any operating condition. Much research has been done on the pressure fluctuation of hydraulic machinery. However, the details of pressure fluctuation propagation of the pump turbine at the pump mode have not been revealed. The modem pump turbine with high water head requires the runner to be ＂flat＂, which would induce pressure fluctuation more easily than the low head pump turbine. In this article, a high head pump turbine model is used as the re- search object. As the pressure fluctuation at off-design point is more serious than at the design point, the low head condition is chosen as the research condition. Pressure fluctuation at the vaneless space and vanes passages is predicted by the computa- tional fluid dynamics method based on k-co shear stress transport model. The experiment conducted on the test rig of the Har- bin Institute of Large Electrical Machinery is used to verify the simulation method. It proves that the numerical method is a feasible way to research the fluctuation under this operating condition. The pressure fluctuation along the passage direction is analyzed at time and frequency domains. It is affected mainly by the interaction between the runner and vanes. In the circumferential direction, the influence of the special stay vane on the pressure fluctuation is got. The amplitude in the high-pressure side passage of that vane is lower than that in the other side. The study provides a basic understanding of the pressure fluctua- tion of a pump turbine and could be used as a reference to improve the operation stability of it.

Effects of a Low-Head Dam Removal on River Morphology and Riparian Vegetation: A Case Study of Gongreung River

The long term existence of a low-head dam in the river channel significantly affects river geomorphology and river ecosystem. Because more and more low-head dam structures have deteriorated in recent years, the attention for low-head dam removal is increasing as one of alternatives for river restoration. Thus, this study intends to investigate the impacts of low-head dam removal on river geomorphology and riparian vegetation with developing a quantitative method to predict the changes of river morphology as well as invasion, growth, expansion and destruction of riparian vegetation after a low-head dam removal. To verify the numerical simulation model, the low-head dam removal case in Gongreung River was employed with investigation of low-head dam removal responses on river geomorphology and riparian vegetation. Following the low-head dam removal, the results of monitoring and numerical simulation indicated that new sand bars has formed as well as increasing the extent of existing sand bars in the upstream of the low-head dam. The sand bars have been colonized in a year after the low-head dam removal by grass type plants. After a decade to several decades, the riparian vegetation in sand bars often developed to tree type plants in several low-head dam removal cases. As other cases, Gongreung River also showed the growth of tree type plants in 5 years after the removal.

Effects of Influential Parameters on Long-Term Channel Evolution Following Low-Head Dam Construction and Removal

The long-term existence of dam structures significantly modified the river channel. In accordance with a drastic increase of low-head dams under consideration for removal in recent years, it is important to predict the effects of low-head dam removal from the modified river channel by the low-head dam construction. This study intends to investigate the long-term channel evolution process following low-head construction and removal and to find out the influential parameters (sediment diameter, river bed slope, dam height) for those channel evolution by two-dimensional numerical simulation model. Following the low-head dam construction, sediment deposition rates in upstream of the low-head dam are varied with the influential parameters. The sediment deposition rates and sandbar formation with riparian vegetation settlement on sandbars have significantly affected for channel evolution following low-head dam removal. Especially the knickpoint formation and the types of vegetation (grass type and tree type) on the sandbars are critical factors for channel evolution following low-head dam removal. Through the numerical simulation results of low-head dam construction (50 years) and low-head dam removal (50 years), it is identified that the modified river channel by low-head dam may not be easily restored to pre-dam conditions following its removal especially in river geomorphology and riparian vegetation. Consequently, this study found that the reversibility following low-head dam construction and removal depends on the sediment deposition rates in upstream of the low-head dam.

深度学习迭代重建算法优化儿童头颅CT图像噪声和图像纹理的可行性

目的评价深度学习迭代重建(DLIR)算法与常规自适应迭代重建(ASIR-V)算法对儿童头颅外伤CT图像噪声和图像纹理的优化程度及图像显示效果的差异。资料与方法回顾性选取2020年12月7—11日首都医科大学附属北京儿童医院影像中心80例儿童头颅CT,扫描方案为低辐射剂量轴扫,电压120 kV,电流150~220 mA。将得到的原始数据重建为5 mm厚层与0.625 mm薄层的脑窗、骨窗图像,分别重建为50%ASIR-V、高权重DLIR图像(DL-H),共8组图像。应用4分制主观评价脑沟脑室、脑灰白质与颅骨显示情况,并统计各组图像的病变数量;客观评价测量基底节层面的灰质和白质的CT值和噪声值,并计算对比噪声比,同时在同层面测量模糊程度指数,比较两种图像重建方法的差异。结果相较于50%ASIR-V图像,2种层厚的DL-H均可以提升脑沟脑室、脑实质显示能力(W=5.5~22.2,P均<0.05),5 mm的50%ASIR-V与0.625 mm的DL-H图像脑沟脑室、脑实质显示能力差异无统计学意义(W=0.9、2.0,P=0.32、0.05)。骨质显示能力方面,所有图像均可以达到满分4.0分。5 mm的50%ASIR-V与DL-H图像均可以在80例患者中发现共35处病变,包括出血病变12处,颅内积气1处,骨折9处,头皮软组织肿胀13处。客观评分方面,DL-H图像噪声低于50%ASIR-V图像(t=21.4~35.7,P均<0.05),0.625 mm的DL-H与5 mm的50%ASIR-V图像噪声及对比噪声比差异无统计学意义(t=1.7~2.2,P均≥0.05)。模糊程度指数显示DL-H优于50%ASIR-V图像(t=6.1、10.0,P均<0.05),0.625 mm的DL-H与5 mm的50%ASIR-V模糊程度指数差异无统计学意义(t=2.6,P=0.28)。结论DLIR可以降低图像噪声,改善图像纹理,整体提升儿童头颅外伤CT图像质量,0.625 mm的DL-H图像质量接近5 mm的50%ASIR-V图像,可以达到诊断要求,使进一步降低儿童头颅外伤的辐射剂量成为可能。

低水头永磁同步发电机磁极结构的优化设计

文章基于低水头流水发电的工况特点和并网原理,建立了永磁同步发电机的数学模型,并提出了相应的解析算法。采取场路结合的方法,分析不同磁极结构对凸极率、磁阻转矩、电感和波形畸变率等参数的影响。利用上述设计分析方法,对一台30 kW的小型低水头永磁同步发电机进行了优化设计,试验结果表明,该优化设计方案具有可行性和正确性。

双低剂量技术在双能量头颈部CT血管成像中的应用

目的 探讨双低剂量扫描技术在双能量头颈部CTA扫描中的可行性。方法 前瞻性收集2021年1月~2021年5月间本院102名因头颈部血管疾病行头颈部CTA的患者纳入研究,随机分为CD、DLD两组,CD组行常规剂量扫描,DLD组行双低剂量扫描;分别对两组图像行MIP、VR、 MPR、 CPR、双能量去骨去钙化等后处理,用5分法对两组图像质量进行主观评价,选取颈总动脉起始部、颈内动脉起始部和大脑中动脉M1段测量血管的噪声、SNR、CNR,记录2组患者的x线剂量参数,包括CTDIvol(mGy)和DLP(mGy·cm),计算其有效剂量ED(mSv),亦计算2组患者的碘摄入量I(mg),对结果进行统计学分析。结果 CD组与DLD组的主观图像质量评分均≥3分,均能满足诊断的要求。2名医师对2组图像质量评价的一致性好(Kappa值分别为0.817、0.82 5)。2名医师对2组图像的主观质量评分的差异无统计学意义(Z=-0.572、-0.2 83,均P>0.05)。CD组与DLD组图像各层面血管CT值的差异均无统计学意义(t=-0.05、0.689、0.906,均P<0.05);颈内动脉起始部、大脑中动脉M1段层面血管噪声、SNR、CNR的差异均无统计学意义(t=-0.830~1.842,均P>0.05);颈总动脉起始部层面CD组噪声大于DLD组(t=2.855,P<0.05),SNR、CNR小于DLD组(t=-4.959、-5.041,均P<0.05)。DLD组患者C TDIvol、DLP及ED较CD组分别降低36.9%、35.3%、35.2%(t=186.138、37.753、37.753,均P<0.05)。DLD组患者碘摄入量较CD组降低57.1%(10.5g VS.245g)。结论 双低剂量双能量头颈部CTA扫描技术在不影响图像质量的前提下,降低了患者接受的辐射剂量和对比剂碘摄入量。

产钳保留原位方法娩出胎头且会阴不侧切母婴结局分析

目的:探讨在低位产钳助产中,采用产钳保留原位方法娩出胎头且会阴不侧切对产妇和新生儿结局的影响。方法:选择2021年1月至2023年4月在南京医科大学附属妇产医院经阴道分娩行低位产钳术且会阴不侧切产妇63例进行回顾性分析,依据取出产钳时间不同分为产钳保留原位方法娩出胎头组(观察组,37例)和提早取出产钳方法娩出胎头组(对照组,26例),比较两组产妇和新生儿结局差异。结果:两组产妇均无肩难产和会阴Ⅳ度裂伤。两组的会阴裂伤程度、阴道裂伤、阴道血肿、子宫颈裂伤、产后出血分娩并发症比较,差异均无统计学意义(P>0.05)。两组产妇全部活产,产钳助产至胎儿娩出时间比较差异无统计学意义(P>0.05)。两组新生儿均无重度窒息、颅内出血和死亡,新生儿钳痕、颜面部擦伤和轻度窒息占比差异均无统计学意义(P>0.05)。结论:产钳保留原位较提早取出产钳方法娩出胎头应用于会阴不侧切,不增加会阴损伤,不增加相关母婴并发症,是一项实用且安全的临床技术。

CFRP布加固广府古建木结构残损箍头榫节点抗震性能研究

采用不同方案的碳纤维(CFRP,Carbon Fiber Reinforced Plastics)布,对6个经过抗震性能试验的残损箍头榫节点试件进行加固,并进行抗震性能试验.结果表明:有3个加固后节点试件的峰值承载能力超越了加固前试件的峰值承载能力,加固后节点的峰值承载力主要与梁榫原始损伤程度及加固量直接相关.加固后节点的初始刚度明显大于加固前的节点,而加固后试件的极限转角普遍小于加固前,但大部分试件极限位移转角均能达到1/21以上,说明此方法加固的节点具有良好的抗震变形能力.加固梁上侧与下侧的CFRP布,对受拉损伤的梁起了重要的补强作用.梁侧面CFRP布在节点区转角较大时退出工作,而L型CFRP布也随着节点转角的增大而逐步退出工作.

超长溢流宽度大流量溢洪道泄洪水力特性研究

受地形、地质等因素的影响,超大库容、大泄量水利工程泄量分配与消能防冲问题十分复杂。尼雷尔工程采用堰上低水头、超长溢流宽度、大泄量溢洪道布置形式。为准确了解尼雷尔工程溢洪道实际泄洪能力和下游消能防冲与汇流等水力特性,通过模型试验与数值模拟相结合的方法,对不同方案下溢洪道的流量系数、消能率、坝后水流归槽情况、河道流速等进行了研究。研究结果表明:原设计低水头高堰溢洪道实际过流能力满足设计要求,采用数值模拟方法可以避免模型试验中缩尺效应的影响;坝面采用台阶高度1.5 m的消能方式消能效果最佳,护坦内部的侧向汇流也比较平稳顺畅,两侧来流在护坦内对撞消能后进入下游主河槽,坝后主河槽平均水深增大近30%,平均流速降低约20%。研究结果可为高坝大库容工程的泄洪消能设计提供参考。

基于改进YOLOv7-tiny的高效轻量遥感图像目标检测方法

针对现有遥感图像目标检测方法在受资源限制的小型设备中检测精度不足问题,提出了一种基于改进YOLOv7-tiny算法的高效轻量遥感图像目标检测方法。首先,针对遥感图像小目标分布密集问题,设计了一种低跨度的上下文解耦检测头模块,通过融合深层和浅层特征分别实现目标检测的分类和回归任务,有效解决了遥感图像小目标漏检和误检的问题。同时,针对遥感图像目标多尺度问题,设计了一种并行级联注意力机制,通过并行三分支网络与空间注意力模块相结合,增强了网络对多尺度目标特征的提取能力。此外,引入Focal-EIoU损失函数,提高模型泛化能力。对模型进行了对比实验、消融实验、部署实验和泛化实验,结果表明,在DIOR-5s和NWPU VHR-10数据集上的检测精度分别达到了85.4%、90.6%,相较原模型分别提高了2.6%、1.7%。且模型大小仅为19.1 MB,检测速度为64.1 fps,验证了算法的有效性。

面向会话的需求感知注意图神经网络推荐模型

针对现有基于图的会话推荐方法忽略了反馈数据中由于用户行为不确定性引起的噪声影响,存在无法准确和有效地捕捉用户偏好的问题,提出一种面向会话的需求感知注意图神经网络推荐模型(DAAGNNSR)。将具有时序性的会话数据构建为图,通过引入图神经网络学习图上节点嵌入表示;将提取的项目特征使用需求感知聚合器线性聚合为用户潜在需求矩阵,以自动削弱噪声干扰,同时用低秩多头注意力网络将该矩阵与全部项目特征进行逐项兴趣交互生成需求增强的项目表征;联合独立位置编码进一步分析项目间顺序关联,并且将生成的独立位置嵌入与项目表征进行线性融合;经过预测层生成推荐列表。将所提模型在Diginetica、Tmall和Nowplaying三个公共数据集上进行训练和测试,实验结果表明,该模型的推荐精度在各指标上均优于其他基线模型,与基于图上下文自注意力机制模型(GCSAN)相比,Diginetica上NDCG@10提高了5.6%,Tmall上Recall@10提高了6.4%;与基于图神经网络的SRGNN相比,Tmall上Precision@10提高了5.0%,推荐性能显著提升。

同轴分级低排放燃烧室头部适应性设计研究

本文以一种天然气低排放同轴分级燃烧室头部为对象,在总体结构不变的前提下,将其按照等马赫数准则缩放分别装配到西气东输型燃气轮机、发电用重型燃气轮机及航改型燃气轮机燃烧室上。采用数值模拟的方式探究提高头部适应性的方法,结果表明:在合理的范围内,降低中心两级当量比,可以在一定程度上提高燃烧室的燃烧效率,并且效率达标后,在保证值班级起到稳定火焰作用的前提下,通过降低值班级当量比可以实现降低NO_(x)和CO排放的目的。通过上述方法,达到西气东输型、发电用重型、航改型燃气轮机燃烧室效率≥99%、NO_(x)排放体积浓度≤25×10^(-6)、CO排放体积浓度≤50×10^(-6)的技术指标。

低水头混流式水轮机空化研究综述

随着全球节能减排和可再生能源需求的日益增加,低水头水轮机发挥着重要作用。低水头混流式水轮机的空化空蚀现象将导致过流部件表面材料损坏,影响发电安全与效率。通过查阅国内外相关文献可知,造成水轮机空化空蚀的关键因素分为内因与外因,内因为水轮机的材料硬度、塑性及材料的抗空蚀性能,外因为水轮机过流部件的表面粗糙度、水轮机淹没深度、水体温度和水体中的气体含量。针对两方面原因,国内外学者讨论了改善措施以及数值模拟在研究水轮机空化空蚀的应用,认为多相瞬态流动的CFD分析对于了解水轮机流动特性和性能非常有帮助。基于满足工程实况与学科发展的需要,本文提出了未来研究混流式空化空蚀的部分参考方向,为低水头混流式水轮机空化空蚀的研究提供了一定的借鉴。

向日葵盘天然低酯化果胶联合氯化钙制备凝固型酸奶的工艺优化

从向日葵盘中提取获得天然的低酯化果胶(LAHP),并将LAHP与CaCl_(2),复配作为稳定剂制备凝固型酸奶。采用单因素和正交试验对酸奶的制备工艺进行优化。结果显示:LAHP主要由半乳糖醛酸(80.62%±2.44%)构成,其干燥减重、酸不溶灰分、二氧化硫及铅含量均符合国家标准的要求,平均相对分子质量为2.12×10^(5),酯化度为25.33%±1.15%。凝固型酸奶的最佳生产工艺条件为以鲜奶质量为基准,LAHP添加量0.6%、CaCl_(2),添加量0.56%、白砂糖添加量8%、发酵时间8h、发酵剂接种量0.2%、发酵温度42℃。在此条件下制备的凝固型酸奶呈乳白色,无乳清析出,口感细腻醇厚,感官评分最高,理化及微生物指标均符合国家标准的要求。

铝合金缸盖低压铸造模具设计及凝固质量优化

针对摩托车发动机的铝合金单缸缸盖低压铸造成形时容易发生缩松、缩孔和气孔等缺陷问题,运用MAGMA软件模拟缸盖低压铸造过程温度和冷却率变化情况,预测缸盖容易产生的缺陷趋势及重点发生区域。在合理设计缸盖模具结构和侧模各部分冷却系统的同时,将数值模拟结果与智能化设备相结合应用,运用低压铸造机数字化智能技术进行凝固工艺过程的冷却时序控制和压力分段精细化控制,避免缸盖燃烧室重点区域凝固结晶时的缺陷发生,提高铝合金缸盖铸件内部组织成形致密度及低压铸造生产质量。

基于数值模拟的低水头闸坝工程下游冲刷动力研究

针对低水头闸坝工程长期面临下游河床冲刷问题,通过不同工况参数分析,构建闸室断面水工模型。同时,基于Fluent平台构建数值模型,采用边界设置以及动网格宏函数控制流体突变,从而改进模型。结果表明,粒径与冲刷流量对冲刷坑深度以及范围有直接影响;对下游冲刷坑流速进行分布数值模拟,二次开发模型能准确反映实际冲刷情况,相对误差0.092%,优于原有模型。研究成果可为水利工程的建设与维护提供技术参考。

低水头拦河坝(堰)生态性设计研究

在中小河流治理工程中,低水头拦河坝(堰)普遍存在强拦、社会功能为主、生态性不足的问题,引发河流演变新的矛盾。结合透水坝、柔性沙棘坝运行生态效果,提出生态设计弱作用理念,建立了河流与坝(堰)工程社会功能和生态功能协调的非线性多目标函数。依此理念和方法,研究改变传统坝(堰)结构,由实体型设计为开放型,有半拦河式、开孔式、梳齿式、网桩坝等多种布置型式。尤其是黄土高原小型淤地坝的新结构型式,能较好地维持沟壑环境,破解阶梯场地筑坝约束。弱作用及相关结构对其他水工建筑物的生态设计有借鉴价值。