旋流泵含气混输数值计算及涡室流场探针测量

为探索旋流泵内部流场及气液两相流混输特性,该文通过32WB8-12型旋流泵外特性试验和泵最优工况流场五孔探针测量,得出泵性能曲线和5个测点静压ps、绝对速度v、圆周速度vu、径向速度vr和轴向速度vz分布情况。针对N-S方程中四项力在气液两相流动中的具体表现特性进行力学分析,说明了泵内部两相流动特点及关联因素之间约束关系。阐明了旋流泵含气混输抽吸和扬程形成的基本原理,及空化和气液两相流混输的不同特点。选择性能试验最优工况,基于改进重组化群k-ε湍流模型(RNG k-ε模型),应用Fluent软件对泵内部流场进行数值模拟,得到了3个轴面静压、速度矢量和含气率分布图。证明泵进口轴向运动为主流,气体主要集中在压力较低的进口区域。气相的引入,其微团与液相尺寸、形状及弹性模量的不同,造成两相流场惯性力、摩擦力和浮力发生变化,由此解释了外特性变化及气液抽吸与内部流动之间定性的因果关系。探讨了旋流泵内部气液两相流动速度场和过流通道发生畸变的基本原理,为建立旋流泵内部气液及空化流动模型提供实例。     

Depth-specific distribution and importance of nitrite-dependent anaerobic ammonium and methane-oxidising bacteria in an urban wetland

Anaerobic ammonium oxidation (anammox) and nitrite-dependent anaerobic methane oxidation (n-damo) are two recently discovered processes in the nitrogen cycle that are catalysed by anammox bacteria and n-damo bacteria, respectively. Here, the depth-specific distribution and importance of anammox bacteria and n-damo bacteria were studied in an urban wetland, Xixi Wetland, Zhejiang Province (China). Anammox bacteria related to Candidatus Brocadia, Candidatus Kuenenia and Candidatus Anammoxoglobus, and n-damo bacteria related to “Candidatus Methylomirabilis oxyfera” were present in the collected soil samples. The abundance of anammox bacteria (2.6–8.6 × 10⁶ copies g⁻¹ dry soil) in the shallow soils (0–10 cm and 20–30 cm) was higher than that (2.5–9.8 × 10⁵ copies g⁻¹ dry soil) in the deep soils, whereas the abundance of n-damo bacteria (0.6–1.3 × 10⁷ copies g⁻¹ dry soil) in the deep soils (50–60 cm and 90–100 cm) was higher than that (3.4–4.5 × 10⁶ copies g⁻¹ dry soil) in the shallow soils. Anammox activity was detected at all depths, and higher potential rates (12.1–21.4 nmol N₂ g⁻¹ dry soil d⁻¹) were observed at depths of 0–10 cm and 20–30 cm compared with the rates (3.5–8.7 nmol N₂ g⁻¹ dry soil d⁻¹) measured at depths of 50–60 and 90–100 cm. In contrast, n-damo was mainly occurred at depths of 50–60 cm and 90–100 cm with potential rates of 0.7–5.0 nmol CO₂ g⁻¹ dry soil d⁻¹. This study suggested the niche segregation of the anammox bacteria and n-damo bacteria in wetland soils, with anammox bacteria being active primarily in deep soils and n-damo bacteria being active primarily in shallow soils.     

半干旱黄土区土壤水分垂直分布规律的研究——以甘肃省兰州市孙家岔流域为例

以甘肃省兰州市孙家岔流域为研究对象,使用1981-1983年、2011年实测土壤水分资料,利用地统计法分析了该流域土壤水分的垂向变异特征.结果表明,球状模型对丰水年不同土地利用(梯田、荒坡、坡耕地、乔木林地)土壤水分的垂向分布拟合为极显著.在平水年球状模型仅对梯田、乔木林地的实验变异函数拟合较好,而对坡面地区的坡耕地和荒坡的拟合并不显著.枯水年梯田和林地的空间变异对模型仍然有一定的吻合,而坡耕地和荒坡土壤水分空间分布变异中有较大的随机性,即使在雨季时空间自相关也很弱.不同水文年下不同土地利用的土壤水分垂向变化表现出明显的季节变化趋势,变程为1.687 ～3.404 m,在该自相似范围内测得土壤特性的变异程度基本一致.     

大型低扬程泵装置优化设计与试验

为了对配置肘形进水流道和虹吸式出水流道的立式泵装置进行深入的研究,得到一种高效的立式泵装置,在肘形进水流道和虹吸出水流道型线数学模型基础上,开发了基于流道设计参数的优化设计软件,能快速进行流道型线的绘制,使流道的型线自动符合一维流速渐变的原则,并对虹吸出水流道的最优驼峰位置进行了理论分析。结合计算流体动力学技术,对待建的某大型低扬程泵站进行了肘形进水流道和虹吸出水流道的优化设计,得到了水力性能优良的进出水流道型线方案。根据泵装置水力模型比选试验,优选出了效率高、高效运行范围宽、无不稳定运行区、汽蚀性能好的高比转数导叶式混流泵211-80模型,在配置优化了的进出水流道的基础上使泵装置在扬程 5.4 m时模型装置效率达到了79.62%。该泵装置的优化设计方法与试验结果对相同装置型式的大型低扬程泵站建设具有重要的参考价值。     

基于数值仿真的泵阀变刚度弹簧设计与优化

目前往复泵锥形阀普遍采用定刚度系数弹簧,但是对于定刚度系数弹簧,单纯的通过调节刚度与预紧力并不能实现提高吸入性能的同时降低阀盘的落座速度与滞后高度,此外传统泵阀设计中,弹簧预紧力与刚度的确定往往是根据经验设计公式,这些公式是在特定条件下近似得到,简化过程中存在很大的计算误差。针对上述不足,该文建立了弹簧变刚度系数条件下阀盘运动规律的数学模型,论证了刚度渐减型弹簧可以提高往复泵的吸入性能,并基于阀盘运动规律的数值仿真模型,建立了以阀盘无冲击为约束条件,阀隙间最大水头损失最小为目标函数的优化模型,采用遗传算法与模式搜索相结合的优化方法对变刚度弹簧参数进行了优化求解,优化结果与传统设计方法确定的定刚度系数弹簧泵阀相比最大水头损失降低了20.85%。该优化设计方法对于指导泵阀弹簧参数设计具有一定的指导意义。     

A new approach for modelling vertical stress distribution at the soil/tyre interface to predict the compaction of cultivated soils by using the PLAXIS code

Soil compaction by agricultural machines can have adverse effects on crop production and the environment. Different models based on the Finite Element Method have been proposed to calculate soil compaction intensity as a function of vehicle and soil properties. One problem when modelling soil compaction due to traffic is the estimation of vertical stress distribution at the soil surface, as the vertical stress is inhomogeneous (non-uniform) and depends on soil and tyre properties. However, uniform stress distribution at the soil/tyre interface is used to predict the compaction of cultivated soils in most FEM compaction models. We propose a new approach to numerically model vertical stress distribution perpendicular to the driving direction at the soil/tyre interface, employing the FEM models of PLAXIS code. The approach consists of a beam (characterised by its geometric dimensions and flexural rigidity) introduced at the soil surface and loaded with a uniform stress with the aim to simulate the action of a wheel at the soil surface. Different shapes of stress distribution are then obtained numerically at the soil surface by varying the flexural rigidity of the beam and the mechanical parameters of the soil. PLAXIS simulations show that the soil type (soil texture) modifies the shape of the stress distribution at the edges of the contact interface: a parabolic form is obtained for sand, whereas a U-shaped is obtained for clay. The flexural rigidity of the beam changes the shape of distribution which varies from a homogenous (uniform) to an inhomogeneous distribution (parabolic or U-shaped distribution). These results agree with the measurements of stress distributions for different soils in the literature. We compared simulations of bulk density using PLAXIS to measurement data from compaction tests on a loamy soil. The results show that simulations are improved when using a U-shaped vertical stress distribution which replaces a homogenous one. Therefore, the use of a beam (cylinder) with various flexural rigidities at the soil surface can be used to generate the appropriate distribution of vertical stress for soil compaction modelling during traffic.     

不同叶顶间隙对斜流泵性能影响的数值分析

斜流泵具有高效,启动特性好,运行工况宽等特点。目前斜流泵设计时,无法定量评估叶顶间隙对性能影响的敏感性。为了揭示不同叶顶间隙值对斜流泵内部流场和性能的影响,给定叶顶间隙选取的范围。分别选取无叶顶间隙和叶顶间隙分别为0.5,1.0,1.5 mm共4种设计方案的斜流泵为对象,基于剪切压力传输模型(shear stress transport,SST k-ω)湍流模型,SIMPLEC算法与块结构化网格,对斜流泵内部流场进行数值模拟和试验验证。结果表明,叶顶间隙为0.5 mm时,可以有效抑制斜流泵的扬程-流量正斜率特性,此时斜流泵的效率值最高;无叶顶间隙时,斜流泵扬程-流量正斜率特性较为明显;叶顶间隙为1 mm时,数值模拟与试验结果吻合较好,SST k-ω模型可较好模拟斜流泵叶顶间隙区流动特征,性能预估结果具有一定的可信度。在小流量工况下,叶顶间隙为0.5 mm可有效抑制斜流泵的正斜率不稳定特性。小叶顶间隙0.5mm时,斜流泵水力性能最优;叶顶间隙增大时,叶顶泄漏流动逐渐显著,叶轮出口近壁区轴面流速和涡量分布规律显著变化,表明叶顶间隙直接影响叶轮轴面速度分布规律和叶片负荷分布规律,由于受壁面摩擦阻力和液体黏滞阻力的影响,叶轮轮毂和叶顶间隙侧的叶轮轴面速度较小;叶顶间隙增大时,叶轮轮毂和叶顶间隙侧叶片负荷急剧衰减,影响叶片的做功能力。同时,叶顶泄漏流动区域与叶片主流区域的掺混效应,使叶片轮缘的低速区扩展到叶轮流道内部的主流区域,引起叶轮流道内部主流流动的堵塞效应,产生二次流动、漩涡等流动不稳定现象。上述研究结果,揭示了叶顶间隙对斜流泵内部流场和性能的影响机理,为斜流泵叶顶间隙的选择提供了理论依据。     

基于不等扬程的离心式长轴泵的优化设计与试验

为使离心式长轴泵能够在不同工况下高效运行,该文以500GJC-32.3×3型离心式长轴泵为例,对其进行优化,首先根据传统方法估算离心式长轴泵叶轮参数,通过正交方法对离心式长轴泵叶轮进行优化设计,对正交试验结果进行极差分析,得到了叶轮几何参数对离心式长轴泵扬程和效率影响的主次顺序。综合考虑各参数对离心式长轴泵性能的影响,选取重要因素,基于不等扬程设计理论,采用控制变量法对叶轮进行多方案优化设计,对比不同方案计算结果可知:基于不等扬程理论优化设计的叶轮具有较好的水力性能,选择合适的后盖板无穷叶片数理论扬程系数,可使叶轮水力性能趋于最佳。对于该型离心式长轴泵,当后盖板无穷叶片数理论扬程系数取1.1时可获得较优的水力性能,对比较优方案的试验与计算结果可知:二者变化趋势相同,扬程、效率、轴功率的最大误差分别为4.02%、5.58%、3.59%,在(0.8~1.2)倍设计流量工况下,扬程、效率、轴功率的误差小。同时由试验可知:该型离心式长轴泵在设计流量时扬程大于97 m,效率高于82%,最高效率点出现在1.1倍设计工况附近为83.22%,曲线具有较宽的高效区和无过载特性,能够满足设计要求,在丰水期和枯水期均能高效稳定的运行,同时可降低电机的配套功率,减少一次成本投入。因此,该文的研究结果对离心式长轴的优化设计有较好的参考价值。     

球塞式液压泵配流轴平衡特性

在轴向压力线性分布的条件下,建立了配流副的流场模型,同时建立了配流轴支承系统在突变载荷下的动态仿真模型,在此基础上对球塞式液压泵配流轴的平衡特性进行了研究。结果表明配流轴支承系统具有压力反馈的闭环调节作用,配流轴在压力反馈作用下恢复到偏心率为0的平衡状态,且没有超调量。为高效球塞式液压泵配流副的设计提供了理论依据,同时为高功率密度球塞式液压元件的深入研究打下了基础。     

自吸喷灌泵自吸过程的非定常流动数值计算与验证

为了建立一种适合自吸喷灌泵在自吸过程中的气液两相流数值模拟方法,采用 ANSYS CFX 软件,进行仿真自吸喷灌泵自吸过程的数值计算,研究自吸过程中气液混合、气液分离及气液逸出现象,分析自吸过程中泵内部的速度、压力及含气率的变化规律,了解自吸过程的气液两相流特点。自吸泵的整个自吸过程分为3个阶段：自吸初期由叶轮旋转作用排水产生的吸气阶段（t≤0.5 s）、自吸中期由气水混合及气水分离作用产生排气功能的吸气阶段（0.5 s4 s）。自吸成功的关键就是叶轮的旋转迫使叶轮进口处少量的水混合着一部分气体沿着叶轮叶片压力面流动至叶轮出口处并进入正反导叶,然后沿着反导叶叶片压力面流出,表明在自吸过程中气水混合物总是沿着较高的压力面流动。通过采用摄影技术得到多级自吸喷灌泵在自吸初期及中期时泵出口段水柱高度的变化规律,发现试验结果与数值计算结果非常相近,不仅两者的变化规律基本相同,而且其结果相差不超过6%。