【作者】 陈超；

【导师】 李彦军；

【作者基本信息】 江苏大学 ， 动力工程（专业学位）， 2019， 硕士

【摘要】 随着南水北调、引江济淮等大型调水工程的建设,对泵站工程的研究也越来越重视,泵站运行状态主要有稳定运行状态和瞬变的过渡过程状态。压力脉动是泵装置稳态运行时的一个重要特征,是泵站机组导致破坏的重要因素。研究不同工况下的泵装置压力脉动规律对泵装置稳定状态下的运行具有重要意义。过渡过程是泵机组从一种稳态状态到另一种稳态状态的过程,过渡过程期间的各种不稳定因素对水力机械事故的产生具有较大影响。因此对低扬程泵装置不同运行状态下的过渡过程研究对泵站科学管理与运行具有一定的理论指导作用。本文以低扬程泵站为研究对象,基于CFD计算流体动力学,采用数值模拟与试验研究相结合的方法,对低扬程泵装置在不同扬程与空化工况下的压力脉动、三维过渡过程进行了研究,本文的主要研究内容与创新点如下:1.使用UG软件对全流道低扬程混泵装置进行了三维建模,使用ANSYS/ICEM软件对泵装置进行结构化网格的划分,使用ANSYS/CFX对某一低扬程混流泵装置全流道在最低净扬程(0.5m)、平均净扬程(3.94m)、设计净扬程(9.02m)与最高净扬程(11.25m)进行了未空化和发生临界空化两种空化工况下的数值模拟,选用RNG k-ε湍流模型与Zwart空化模型。研究结果表明:(1)在不同的空化情况下,叶片工作面进口处均有一个低压区,随着扬程的升高,低压区的面积逐渐减小,在最大扬程时在叶片背面进口处出现。发生临界空化时,叶片进口处的湍动能变化比未空化时剧烈。大流量低扬程(0.5m、3.94m)工况时空化现象较为严重的区域比较大,设计扬程与高扬程工况较不明显,说明当效率下降相同时,大流量工况汽蚀现象较为严重。(2)在不同的工况下,叶轮进口处压力脉动幅值均周期性地变化,波峰与波谷变化地规律性较为明显,一个叶轮旋转周期内有3个波峰与波谷。叶轮出口处受动静干涉作用明显,一个旋转周期内有明显的谐波现象。导叶出口处峰谷的幅值较进口处略低,不同工况下一个周期内的主波峰与波谷个数差异较大。叶轮进口处与导叶出口处的压力脉动主频均为1倍叶频,导叶出口处的主频幅值略低。叶轮出口处的主频均为叶频的整数倍频,幅值最大的频率为2倍或3倍叶频,叶轮进口处有更多的高频频率,发生临界空化时,主频附近有更多的谐频成份。2.对混流泵装置进行了外特性、汽蚀特性试验,采集了不同工况下的压力脉动数据,并与数值模拟值进行了对比。结果表明:未发生空化时,数值模拟值的扬程效率比试验值略高。发生临界空化时,最低净扬程时(0.5m)数值模拟效率值比试验值低。随着扬程的增加,试验值叶轮出口处主频附近幅值较小的谐频信号在对应的空化工况下均向高频区域移动,在不同倍数的叶片频率间均有幅值较小的次主频信号有规律地出现。3.使用Fluent软件对某一低扬程轴流泵装置进行了飞逸过渡过程、断电过渡过程、快速门不同开启速度启动过渡过程的三维数值模拟。研究结果表明:(1)飞逸过渡过程时,水泵叶轮叶片的轴向力与扭矩变化趋势相似,随着时间的推移呈现出先增大后减小的变化趋势。叶轮叶片叶道内并未出现明显的局部高压区或低压区,整个飞逸过渡过程中,水泵叶轮叶道内并未出现明显的脱流等不良流态。随着叶轮转速的增加,零散的漩涡逐渐在叶轮导水锥处汇聚成较大的漩涡,机组运行较不稳定。(2)断电过渡过程时,机组从断电开始计时约经历19.6 s左右的时间即达到该扬程下的飞逸转速(219.58 r/min),而该扬程下模型泵装置试验值换算成原型泵装置试验值为226.27 r/min,与数值模拟值基本一致。事故停机时,叶轮附近监测点的水压脉动幅值随时间不断增加,当飞逸工况下叶轮转速稳定后,压强变化逐渐趋于稳定。轴向力在停泵过渡过程中逐渐减小然后反向增加,容易产生“抬机”现象,工程中应注意机组在最高扬程停机时的安全稳定性。(3)快速门额定启动速度启动过渡过程时,叶轮转速较低时,容易发生“倒灌”现象。叶轮叶片的扭矩与轴向力均先增大后减小,叶轮轴向力方向与重力方向一致,不会出现“抬机”的可能。叶轮叶片局部高压区的压强值明显高于泵装置稳态运行时叶片工作面的压强值。当快速门开启速度增加时,外特性曲线变化规律与额定开启速度基本一致。提高快速门开启速度的启动方式能够明显改善启动过渡过程中叶轮叶片的最大轴向力,但最大倒灌流量会显著上升。更多还原

【Abstract】 With the construction of large-scale water diversion projects,such as South-to-North Water Transfer Project and Yangtze River Diversion Project,etc.More and more attention has been paid to the study of pumping station engineering.The operation state of pumping station mainly includes stable operation state and transient transition state.Pressure fluctuation is an important characteristic of steady-state operation of pumping unit and an important factor leading to damage of pumping station units.It is of great significance to study the pressure fluctuation law of pumping device under different working conditions for the stable operation of pumping device.Transition process is the process of pumping unit from one steady state to another steady state.Various unstable factors during the transition process have great influence on the occurrence of hydraulic machinery accidents.Therefore,the research on the transition process of low-head pumps under different operating conditions has a certain theoretical guidance for the scientific management and operation of pumping stations.Based on CFD computational fluid dynamics(CFD),the pressure fluctuation of low-lift pumping units under different lift and cavitation conditions is studied by means of theoretical analysis,numerical simulation and experimental study.Transition process of pump device is studied by combining numerical simulation with test.The main contents and innovations of this paper are as follows:1.Three-dimensional modeling of low-lift mixed-flow pump device with full-channel is carried out by UG.The structured grid of pump device is divided by using ANSYS/ICEM software.ANSYS/CFX is used to simulate the whole flow passage of a low-lift mixed-flow pump under the conditions of non-cavitation and critical cavitation at the lowest net head(0.5m),the average net head(3.94m),the designed net head(9.02m)and the highest net head(11.25m)with the RNG k-ε turbulence model and the Zwart cavitation model.The results show that:(1)Under different cavitation conditions,there is a low pressure zone at the inlet of blade working face.With the increase of head,the area of low pressure zone decreases gradually,and appears at the inlet of blade back at the maximum head.When critical cavitation occurs,the turbulent kinetic energy at the blade inlet changes more sharply than that without cavitation.Large flow and low head(0.5m,3.94m)conditions have more serious spatio-temporal phenomena,and the design head and high head conditions are not obvious.This shows that when the efficiency decreases the same,the cavitation phenomena in large flow conditions are more serious.(2)Under various head and cavitation conditions,the pressure fluctuation amplitude at the impeller inlet varies periodically,and the regularity of peak and trough variations is obvious.,there are three peaks and troughs in a rotating cycle of impeller.At the outlet of impeller by rotor stator interaction,there is a rotation cycle of a harmonic phenomenon.The amplitude of peak and trough at guide vane outlet is slightly lower than that at inlet,and the regularity and periodicity of pressure amplitude fluctuation in a rotating cycle are not obvious,and the number of main peak and trough varies greatly in different cycles.The main frequency of pressure fluctuation at impeller inlet and guide vane outlet is 1 fold of blade frequency,and the main frequency amplitude at guide vane outlet is slightly lower.The main frequencies at the impeller outlet are all integer multiple frequencies of blade frequencies.The maximum frequency is 2 or 3 times of blade frequencies.Because of the influence of static and dynamic interference,there are more high frequencies at the impeller outlet than at the impeller inlet,When critical cavitation occurs,there are more harmonic components near the main frequency.2.The external characteristics and cavitation characteristics of the mixed-flow pump are tested.The pressure fluctuation data under different working conditions are collected and compared with the numerical simulation results.The results show that:When no cavitation occurs,the head efficiency of numerical simulation is slightly higher than that of experiment.When critical cavitation occurs,the simulated value under design head(9.02m)is close to the experimental value,the simulated value at low head(3.94m)and high head(11.25m)is slightly lower than the experimental value,and the numerical simulation efficiency at the lowest net head(0.5m)is lower than the experimental value.Under different cavitation conditions,the main frequency domain of the test value at the impeller outlet is wider than that of the simulation value,and the frequency characteristic in the high frequency region is more obvious than that of the simulation value.With the increase of lift,theharmonic signal with smaller amplitude near the main frequency at the impeller outlet moves to the high frequency region under the corresponding cavitation conditions.3.Three-dimensional numerical simulation of a low-lift axial-flow pump was carried out by using Fluent software,including flight transition process,power-off transition process and start-up transition process of different opening speeds of the fast gate..Research results show that:(1)During the flight transition process,the axial force and the torque of the impeller blade of the pump are similar,and the change trend increases first and then decreases with the passage of time.There is no obvious local high pressure or low pressure zone in the impeller blade passage.During the whole flight transition process,there is no obvious bad flow pattern such as bleeding in the impeller blade passage of the pump.With the increase of impeller speed,the scattered eddies gradually converge at the impeller guide cone to form larger eddies,and the operation of the unit is more unstable.(2)In the process of power failure transition,the unit runaway speed(219.58 r/min)at the head is reached after about 19.6 seconds from the start of power failure,and the experimental value of model pump device is converted to 226.27 r/min at the head,which is basically consistent with the numerical simulation value.When the accident stops,the amplitude of water pressure fluctuation at the monitoring point near the impeller increases with time.When the impeller speed is stable under the flight condition,the pressure change tends to be stable gradually.Axial force gradually decreases and then increases in reverse during the transitional period of pump shutdown,which is prone to "lift" phenomenon.Safety and stability of the unit should be paid attention to when the unit shuts down at the highest lift in engineering.(3)The phenomenon of "backfilling" is easy to occur when the impeller speed is low when the transition process is started at the rated starting speed of the fast gate.The torque and axial force of impeller blade increase first and then decrease.The axial force direction of impeller is the same as that of gravity,and there is no possibility of "lifting machine".The pressure value in the local high pressure area of impeller blade is obviously higher than that in the working surface of the blade when the pump is running steadily.When the opening speed of the fast door increases,the change rule of the external characteristic curve is basically consistent with the rated opening speed.The maximum axial force of impeller blade duringstart-up transition can be significantly improved by increasing the opening speed of the fast gate,but the maximum backfilling flow will increase significantly.更多还原