摘要
超低扬程的泵站常具有双向输水的需求,因而双向水泵装置在双向运行工况下都具有发生飞逸事故的风险。为研究泵装置在正反向飞逸过渡过程的差异性,该研究建立双向低扬程卧式轴流泵全过流系统,采用流体体积函数法分析上下游水气两相分布,利用力矩平衡方程推导叶轮实时转速,并结合熵产理论进行分析。结果表明:对比熵产计算与模型试验得到的扬程损失,在正反向工况下误差均在3%以内,验证了数值模拟及熵产计算的准确性;叶轮扭矩的波动频率受叶频控制,且正向飞逸工况下扭矩波动的幅值更高;总计算域中直接耗散熵产占主导地位,湍流耗散熵产次之,壁面耗散熵产最少;各计算域中叶轮的总熵产值最高,源于较大的速度梯度和强烈的动静干涉作用;轴流泵进入稳定飞逸状态下,正向飞逸工况扭矩的波动幅值明显高于反向飞逸工况;零流量时刻下,涡核与熵产率分布相似说明涡核的聚集与漩涡的演变是流场产生明显能量损失的原因。研究可为分析机组的瞬态特性提供一种思路。
A low-head pump station is often required to deliver water in two directions,resulting in the risk of runaway accidents under the bidirectional operation.This study aims to investigate the runaway transition of the pump under the Forward Runaway Condition(FRC)and Backward Runaway Condition(BRC),thereby establishing the full flow system of the low-head horizontal axial flow pump.The volume of fluid(VOF)was applied to determine the position of the free surface,and then to simulate the volume fraction of water and air in the upstream and downstream domains.The Shear Stress Transport(SST)k-ωturbulence model was selected to close the governing equations,where the eddy viscosity was modified to account for the transport of the principal turbulent shear stress.The Entropy Production Rate(EPR)mainly included the Entropy Production By Direct Dissipation(EPDD),Turbulence Dissipation(EPTD),and Wall Shear Stress(EPWS).The User-Defined Function(UDF)in the Fluent software was applied to control the real-time speed of the impeller using the torque balance equation.The Grid Convergence Index(GCI)was also calculated to verify the grid independence.A model test was conducted to verify the accuracy of three-dimensional simulation and entropy generation.The results show that the flow rate and rotation speed of the pump decreased first,and then increased,whereas,the torque generally presented a downward trend,while fluctuated around 0 value under FRC and BRC.Furthermore,the torque fluctuation amplitude under FRC was significantly higher than that under BRC in the runaway state,due to the strong Rotor-Stator Interaction(RSI)under FRC.The EPDD was dominant in the total simulation domain,followed by the EPTD and EPWS.Additionally,the total entropy production in the impeller was the highest in each simulation domain,due mainly to the larger velocity gradient and the stronger rotor-stator interaction.Additionally,the guide vane was located in the inflow direction of the impeller under FRC in the turbine or runaway state,where the smoother flow state and the lower EPDD and EPTD under FRC,compared with the BRC.As for the inlet conduit during the runaway state,the EPDD was slightly higher than the EPTD under FRC and BRC.However,the EPDD in the outlet conduit was much higher than the EPTD.More importantly,the upstream transformed from the outflow to inflow domain,and then the EPDD,EPTD,and EDWS were gradually close to zero,whereas,the downstream transformed from inflow to outflow domain,and then the EPDD,EPTD,and EDWS gradually increased during the runaway.There was a seriously unstable flow pattern in the inlet and outlet channel,leading to the strong vortices and reflux areas,particularly when the flow rate was zero(tQ=0).Correspondingly,the velocity gradient and turbulent kinetic energy were small at the low velocity,leading to the smaller total entropy production at the inlet and outlet conduit under FRC and BRC.Additionally,the span of velocity gradient in the downstream was larger under BRC than that under FRC,so did the EPDD in the runaway state(42.5 s).The vortex core gathered at the inlet side of the impeller blade under pump condition at tQ=0.Consequently,the distribution of vortex and EPR was similar at different blade-to-blade surfaces.The reason was that there was a large velocity gradient near the vortex core,particularly leading to a larger entropy yield,indicating that the vortex was the cause of energy loss.
作者
许哲
郑源
阚阚
黄佳程
Xu Zhe;Zheng Yuan;Kan Kan;Huang Jiacheng(College of Water Conservancy and Hydropower Engineering,Hohai University,Nanjing 210098,China;College of Energy and Electric Engineering,Hohai University,Nanjing 210098,China)
出处
《农业工程学报》
EI
CAS
CSCD
北大核心
2021年第17期49-57,共9页
Transactions of the Chinese Society of Agricultural Engineering
基金
国家自然科学基金项目(No.52009033)
江苏省自然科学基金项目(BK20200509)。
关键词
泵
流速
熵
飞逸过程
双向工况
自由液面
pumps
flow rate
entropy
runaway process
bidirectional condition
free surface
