15度斜式轴流泵装置水动力特性实验研究

15度斜式轴流泵装置是我国近年在低扬程、大流量泵站引入的一种新型泵装置,由于出水流道前端呈S形弯曲,流道内的二次流导致某些工况下压力脉动和振动较大,这一问题尚有待深入研究。本文针对浙江盐官泵站技术改造所研制的新型15度斜式轴流泵装置进行了水动力学特性的实验研究。研究发现,与泵段相比,斜式轴流泵装置的最优工况点向负叶片角度和小流量区偏移。泵装置在较小叶片角度下的空化特性优于大叶片角度;当装置空化余量低到使泵装置效率下降1%时,叶片背面出现占据近1/3叶道区域的空化区。斜式轴流泵的飞逸转速在较大负叶片角度时可达额定转速的1.73倍。斜式轴流泵压力脉动在导叶出口、出水弯管和出水流道内明显高于常规立式轴流泵;从水泵进口方向看,当叶轮逆时针方向旋转时,斜式轴流泵装置出水流道隔墩左侧流量大于右侧。本文研究成果为大型斜式轴流泵站优化设计和稳定运行提供了新的科学依据。     

高速潜水轴流泵大流量工况的空化特性

为了研究大流量工况下高速潜水轴流泵的空化特性,基于ANSYS CFX软件,选取Zwart、Kunz以及Schnerr-Sauer 3种空化模型进行大流量工况下高速潜水轴流泵外特性和泵内空化流动特性数值模拟。结果表明:大流量工况下Schnerr-Sauer空化模型预测的外特性变化趋势与试验值最为吻合,相较于另两种空化模型,Schnerr-Sauer空化模型模拟的叶片背面空泡体积分数较高;空化严重区域主要出现在叶片背面进口附近以及叶顶,同一空化数下,流量越大,叶片空化状况越严重;叶片载荷分布由叶片进口边到出口边呈先增大后减小的趋势;各流量下空泡首先出现在叶片背面进口前缘位置,随着空化数的减小,空泡体积分数沿着主流方向朝叶片后缘不断增大直至空泡占据整个叶片背面;叶片背面处的三角形云状空化尾缘空穴极不稳定,随着叶轮旋转,尾缘处空泡微团逐渐脱落,朝着相邻叶片不断移动,对相邻叶片的工作面产生侵蚀破坏,导致叶片载荷发生变化,对轴流泵水力性能产生影响。     

Experimental investigation and numerical analysis of unsteady attached sheet- cavitating flows in a centrifugal pump

This paper studies the attached sheet cavitation in centrifugal pumps. A pump casted from Perspex is used as the test subject. The cavitation bubbles were observed in the entrance of the impeller and the drops of the head coefficients were measured under different operating conditions. A Filter-Based Model (FBM), derived from the RNG k – ? model, and a modified Zwart model are adopted in the numerical predictions of the unsteady cavitating flows in the pump. The simulations are carried out and the results are compared with experimental results for 3 different flow coefficients, from 0.077 to 0.114. Under four operating conditions, qualitative comparisons are made between experimental and numerical cavitation patterns, as visualized by a high-speed camera and described as isosurfaces of the vapour volume fraction α_v = 0.1. It is shown that the simulation can truly represent the development of the attached sheet cavitation in the impeller. At the same time, the curves for the drops of the head coefficients obtained from experiments and calculations are also quantitatively compared, which shows that the decline of the head coefficients at every flow coefficient is correctly captured, and the prediction accuracy is high. In addition, the detailed analysis is made on the vapour volume fraction contours on the plane of span is 0.5 and the loading distributions around the blade section at the midspan. It is shown that the FBM model and the modified Zwart model are effective for the numerical simulation of the cavitating flow in centrifugal pumps. The analysis results can also be used as the basis for the further research of the attached sheet cavitation and the improvement of centrifugal pumps.     

特低扬程泵站水力性能研究

采用数值计算和物理模型试验方法研究特低扬程泵站的水力性能,并以苏州市东风新平面S形轴伸泵为例,对其内、外特性,包括装置的水力性能、流道沿程典型断面流速分布、空化特性及进出水流道的水头损失进行分析研究。通过对数值模拟计算及物理模型试验成果的逐项对比,证明数值模拟方法在特低扬程泵装置性能研究中具有良好的应用价值,特别是在高效率附近区域,数值模拟计算成果具有良好的精度。对断面流速分布和流道水力损失等内特性的进一步分析,揭示了不同工况下泵站性能差异的原因,为进一步优化流道型线提供了依据。     

斜式轴流泵装置出水流道偏流特性研究

斜式轴流泵是大流量低扬程提水领域的一种典型泵型,但其在某些工况下存在严重偏流问题,是影响斜式轴流泵站安全稳定运行的工程难题和关键科学问题。本文以浙江盐官泵站15°斜式轴流泵装置模型为研究对象,通过模型试验和数值模拟方法对装置内部瞬态流动特性进行研究,分析了不同流量工况下内部流动特征及偏流产生的原因,提出了解决偏流问题的抑制措施。研究结果表明:顺水流方向出水流道隔墩左侧流量高于右侧,最优流量工况下偏流比达2.33,有70%的流量从左侧流道流出;出水流道内的偏流是因水流在剩余环量的影响下发生偏转,通过弯管段时产生的;在出水流道弯管段设置偏流抑制措施可有效改善出水流道内部流态,抑制偏流的发生。本文研究成果已经应用到盐官泵站装置原型上,并得到了很好的应用,从而为解决大型斜式轴流泵站偏流问题,保证泵站安全稳定运行提供了参考。     

南水北调东线淮安二站泵装置模型试验研究

泵装置模型试验是检验和优化泵装置水力性能的重要手段。根据南水北调东线工程淮安第二抽水站的改造需要,为了进一步检验预测泵装置的水力性能,在河海大学水力机械多功能试验台上对淮安第二抽水站立式全调节轴流泵装置进行了能量、空化以及飞逸转速等方面的试验研究。试验结果表明,装置在水泵工况和飞逸工况下均能够安全、稳定地运行,而且也能够满足淮安第二抽水站改造的设计要求。对研究背景、该泵站的主要技术参数、试验的主要内容及其不确定度分析,装置模型的主要参数和试验条件、水泵工况模型的试验结果、原型装置的飞逸转速、原型的换算条件及结果等情况进行了介绍。     

Application of modified κ-ω model to predicting cavitating flow in centrifugal pump

Considering the compressibility of the cavity in the cavitating flow, this paper presents a modified κ-ω model for predicting the cavitating flow in a centrifugal pump, in which the modified κ-ω model and Schnerr-Sauer cavitation model were combined with ANSYS CFX. To evaluate the modified and standard κ-ω models, numerical simulations were performed with these two models, respectively, and the calculation results were compared with the experimental data. Numerical simulations were executed with three different values of the flow coefficient, and the simulation results of the modified κ-ω model showed agreement with most of the experimental data. The cavitating flow in the centrifugal pump obtained by the modified κ-ω model at the design flow coefficient of 0.102, was analyzed. When the cavitation number decreases, the cavity initially generates on the suction side of the blade near the leading edge and then expands to the outlet of the impeller, and the decrease of the total pressure coefficient mainly occurs upstream of the impeller passage, while the downstream remains almost unaffected by the development of cavitation.     

CAVITATING SUPPRESSION OF LOW SPECIFIC SPEED CENTRIFUGAL PUMP WITH GAP DRAINAGE BLADES

This paper aims to clarify the cavitation suppression mechanism of the gap structure impeller based on the analysis of cavitation characteristics in a low specific speed centrifugal pump. In order to obtain reliable and consistent numerical results, some numerical considerations and modeling methodology were demonstrated and researched, and a check of the time and space resolution were also conducted. Hence the predicted cavitation performance of the two centrifugal pumps were investigated and compared with experimental results, and they were in qualitative agreement. It was confirmed that the new gap structure impeller has a very good characteristic of inhibiting cavitation, especially in large flow area, the present numerical method can effectively capture the major internal flow features in the centrifugal pump, through the comparison of the two type impeller flow fields, the cavitation suppression mechanism of the gap impeller may be the combination effects of the small vice blade’s guiding flow and gap tunnel’s auto-balancing of pressure.     

A review of cavitation in hydraulic machinery

This paper mainly summarizes the recent progresses for the cavitation study in the hydraulic machinery including turbopumps, hydro turbines, etc.. Especially, the newly developed numerical methods for simulating cavitating turbulent flows and the achievements with regard to the complicated flow features revealed by using advanced optical techniques as well as cavitation simulation are introduced so as to make a better understanding of the cavitating flow mechanism for hydraulic machinery. Since cavitation instabilities are also vital issue and rather harmful for the operation safety of hydro machines, we present the 1-D analysis method, which is identified to be very useful for engineering applications regarding the cavitating flows in inducers, turbine draft tubes, etc. Though both cavitation and hydraulic machinery are extensively discussed in literatures, one should be aware that a few problems still remains and are open for solution, such as the comprehensive understanding of cavitating turbulent flows especially inside hydro turbines, the unneglectable discrepancies between the numerical and experimental data, etc.. To further promote the study of cavitation in hydraulic machinery, some advanced topics such as a Density-Based solver suitable for highly compressible cavitating turbulent flows, a virtual cavitation tunnel, etc. are addressed for the future works.     

Numerical investigation of the effect of rotation on cavitating flows over axisymmetric cavitators

The rotating axisymmetric cavitator is widely applied in underwater vehicles, and its rotational motion affects the cavitating flow over the cavitator. This study focuses on the effect of rotation on the flow structure in the cavity bubble. Unsteady 2-D/3-D numerical simulations of cavitating flows over axisymmetric cavitators are performed using the volume of fraction(VOF) method and the Sauer-Schnerr cavitation model. Firstly, the 2-D simulation of cavitating flow over a circular disk or a cone cavitator is carried out at various cavitation numbers(0.15, 0.175, 0.2, 0.225 and 0.25). The simulated cavity lengths and drag coefficients are compared with the experimental data, the theoretical estimations and the published numerical results. Then the 3-D simulations of cavitating flows over the same axisymmetric cavitators with different rotating speeds are performed using the sliding mesh model(SMM). The effect of rotation on the cavity shape and the internal flow structure is analyzed.     

On the numerical simulations of vortical cavitating flows around various hydrofoils

This paper reviews the numerical models of various cavitating flows around hydrofoils. Numerical models relating to cavitation flows, including mass transfer models and turbulence models, are summarized at first. Then numerical results and analysis of flow characteristics for the cavitating flows around twisted hydrofoils, truncated hydrofoil and tip leakage are discussed respectively. For mean flow fields, Reynolds averaged Navier-Stokes(RANS) simulation associated with a kind of nonlinear turbulence model is found to be an economic and robust numerical approach for different kinds of cavitating flows including cloud cavitation, tip cavitation and tip leakage cavitation. To predict the fluctuations of pressure and velocity, large eddy simulation(LES) and detached eddy simulation(DES) are two effective approaches. Finally, a few open questions are proposed for future research.     

某立式轴流泵 CFD 计算与模型试验性能比较分析

针对宣州区某立式轴流泵站水泵装置进行分析,验证水泵设计选型的科学性、合理性,尽量减少和避免如伴随冲击、二次流、汽蚀等有害物理现象,防止影响机组的安全运行。设计中采用 CFD 技术对立式轴流泵装置全流道进行了数值模拟计算,并针对进水流道、泵段、出水流道的内部流态进行了分析判断应用,从而得出立式轴流泵全工况外特性。结合水泵装置模型试验得到水泵试验外特性数据,对比试运行参数,验证了水泵基本性能。结果表明 CFD 计算能初步快速的判断避免不利流态,试运行的工况点水泵性能与 CFD 计算和模型试验结果较相近。因此,对大中型立式轴流泵站同时进行 CFD 计算和装置模型试验是必要的,均对水泵的性能可做出比较准确的判断。     

低扬程立式轴流泵出水流道基本流态及水力性能的比较

采用数值计算和模型试验的方法对低扬程立式轴流泵虹吸式和直管式2种不同形式的出水流道进行了比较,揭示了这2种出水流道的基本流态,测试了这2种形式出水流道的水力损失。结果表明:在低扬程的条件下,虹吸式出水流道内的水流转向更为有序、扩散更为平缓、水力损失更小,对于年运行时数较多的大型低扬程泵站,在上游水位变幅允许的条件下,应优先选用水力性能较好的虹吸式出水流道。     

Numerical research on the performances of slot hydrofoil

The paper presents an numerical study of the hydraulic and cavitating characteristics of a slot hydrofoil at the angle of incidence of o6. Results indicate that the performance of this slot hydrofoil is better than the original hydrofoil at non-cavitation condition, but deteriorates sharply once cavitation occurred. To improve the performance, a new splitting scheme was put forward and optimization research was carried out at the same indicent angle, numerical results show that the optimized slot hydrofoil achieves better hydraulic and cavitation performances.     

NUMERICAL INVESTIGATION OF PERFORMANCE OF AN AXIAL-FLOW PUMP WITH INDUCER

The interaction of flow through the inducer and impeller of an axial-flow pump equipped with an inducer has significant effect on its performance. This article presents a recent numerical investigation on this topic. The studied pump has an inducer with 3 blades mounted on a conical hub and a 6-blade impeller. The blade angle of the impeller is adjustable to generate different relative circumferential angles between the inducer blade trailing edge and the impeller blade leading edge. A computational fluid dynamics code was used to investigate the flow characteristics and performance of the axial-flow pump. For turbulence closure, the RNG k-ε model was applied with an unstructured grid system. The rotor-stator interaction was treated with a Multiple Reference Frame (MRF) strategy. Computations were performed in different cases: 7 different relative circumferential angles ( Δθ ) between the inducer blade trailing edge and the impeller blade leading edge, and 3 different axial gaps (G) between the inducer and the impeller. The variation of the hydraulic loss in the rotator was obtained by changing Δθ . The numerical results show that the pressure generated is minimum in the case of ( G = 3%D), which indicates that the interference between inducer and impeller is strong if the axial gap is small. The pump performances were predicted and compared to the experimental measurements. Recommendations for future modifications and improvements to the pump design were also given.     

Compressible effect on the cavitating flow: A numeric study

To understand the effect of the compressibility on the cavitating flow, a compressible, multiphase, single component Reynolds averaged Navier-Stokes(RANS) solver is used to study the cavitating flow on a wedge in the present work. A barotropic equation of status is used. A non-linear model for compressibility in the mixture is adopted to capture the effect of the compressibility within the complex cavitation bubbly mixtures. An unsteady cavitation phenomenon is found in the numerical simulation. The numerical results of local compressibility and Mach number in the bubbly mixture are given. The mechanism responsible for the unsteady shedding of the bubbly mixture is discussed based on the numerical results.     

潜水轴流泵装置特性模型试验

对上海市薛家泓泵站立式潜水轴流泵模型泵装置进行了能量特性和空蚀特性试验,得出了叶片角0°和±2°时的能量特性和空蚀特性试验结果,探讨了不同消涡导流措施对潜水轴流泵模型装置性能的影响。结果表明,在各工况能量试验中泵装置运行平稳、无振动;隔涡板和导水锥两种消涡导流方案对泵装置能量特性无明显差异;泵装置空蚀安全裕量较大,在常遇扬程条件下运行不会产生危害性叶形空蚀;提出了减少立式潜水轴流泵装置流经电动机灯泡体、Γ形弯头及出口等处水力损失的措施。     

绕扭曲翼型三维非定常空泡脱落结构的数值分析

该文以一种三维扭曲翼型为研究对象,采用均衡流空化模型和基于滤波器的湍流修改模式模拟了绕翼型非定常空化流动。计算结果显示,在一定条件下绕翼型的空泡流动存在两种脱落结构,即主体脱落和二次脱落。进一步的流场分析表明回射流和侧面射流是这两种空泡脱落结构产生的主要原因。计算结果与试验的比较说明,该文采用的数值模拟方法可成功模拟实验观测的空泡脱落现象。     

双吸泵小流量工况下的空化特性研究

为研究双吸泵在小流量工况下叶轮内部空化特性,同时进一步说明小流量工况相比于设计工况时的空化特性差异,结合均质两相流模型和SST k-ω湍流模型,对双吸泵小流量工况和设计工况下的全流道空化流场进行数值模拟,以分析不同流量工况下空化分布与发展情况,以及空化对各叶片载荷造成的影响。研究结果表明:适当减小双吸泵进口流量,有助于改善双吸泵的空化性能;在小流量工况下空化首先发生于叶片吸力面头部靠后盖板附近,而且此处的空泡体积分数最大,这一空化特征同设计工况有所差异;随着NPSH的降低,叶轮内空化不断加强,但是小流量工况下的空化强度始终不及设计工况;不同空化状态会导致叶片吸力面压力的变化,从而表现为叶片表面载荷分布的变化。     

PIV analysis and high-speed photographic observation of cavitating flow field behind circular multi-orifice plates

Based on a self-developed hydrodynamic cavitation device with different geometric parameters for circular multi-orifice plates, turbulence characteristics of cavitating flow behind multi-orifice plates, including the effects of orifice number and orifice layout on longitudinal velocity,turbulence intensity, and Reynolds stress, were measured with the particle image velocimetry(PIV) technique. Flow regimes of the cavitating flow were also observed with high-speed photography. The experimental results showed the following:(1) high-velocity multiple cavitating jets occurred behind the multi-orifice plates, and the cavitating flow fields were characterized by topological structures;(2) the longitudinal velocity at each cross-section exhibited a sawtooth-like distribution close to the multi-orifice plate, and each sawtooth indicated one jet issuing from one orifice;(3) there were similar magnitudes and forms for the longitudinal and vertical turbulence intensities at the same cross-section;(4) the variation in amplitude of Reynolds stress increased with an increase in orifice number; and(5) the cavitation clouds in the flow fields became denser with the increase in orifice number, and the clouds generated by the staggered layout of orifices were greater in number than those generated by the checkerboard-type one for the same orifice number. The experimental results can be used to analyze the mechanism of killing pathogenic microorganisms through hydrodynamic cavitation.