Research On Flow Mechanism And Control Strategy Of Roof Attached Vortex In Closed Sump Of Pumping Station

Closed sump is widely used in a large number of pumping station for its unique advantage of dismissing free surface vortex under the circumstance of the implement of irrigation area reconstruction and small and medium-sized pumping station renewal in Jiangsu province.With the widespread use of closed sump,it is very urgent and necessary to control the roof attached vortex which is a new type vortex found in closed sump.In this case,this paper focus on the roof attached vortex and try to reveal the mechanism of roof attached vortex and control vortex with special structures funding from National Natural Science Foundation?No.51779214?,A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions ?PAPD?,Water conservancy science and technology project of Jiangsu Province?No.2014046,No.2016035?and the Innovation Project of Graduate Training of Jiangsu Province?KYLX16₁395?.The main contents is given below:?1?To get reliable results,several turbulent models which contains of Standard k-? turbulent model,RNG k-? turbulent model,Standard k-co turbulent model,SST zonal k-co based turbulent model and Baseline zonal k-? based turbulent model are used to close the governing equations to predict the hydraulic performance and flow pattern of closed sump.Comparing with the experiment results,the simulation results with Standard k-? turbulent model is more precise at hydraulic performance and flow pattern prediction than the other turbulent models,while comparing to experiment results.?2?Closed sump circulation test rig with transparent section is designed and set up.The efficiency uncertainty of test rig is ± 1.73%.High frequency pressure signal acquisition and processing methods during experiment are briefly introduced in this paper.Fifteen pressure sensors locate in the roof wall of closed sump behind the pump.The influence on pressure fluctuation characteristic of different sampling time and frequency is analyzed.The result shows that short sampling time result in low resolution of pressure fluctuation frequency spectrum which is not suitable for this experiment.So the sampling time is 1 s and the sampling frequency is 105 Hz in this paper.?3?There are 6 typical roof attached vortices shape observed during experiment in closed sump.The life course of roof attached vortex is divided into five evolutionary phases according to the morphological characteristics,which are brew phase,newly-born phase,development phase,shape forming phase and decaying phase.The damage to pump of roof attached vortex is largest at the shape forming phase for the vortex is sucked into pump.The amplitude of pressure fluctuation at the location near the pump back wall is significantly high during all stage of roof attached vortex occurrence condition,comparing with the other zone between pump and closed sump back wall.The main frequency of pressure fluctuation of most monitor points is rotation frequency at no-visible-vortex stage.The main frequency of monitor points around the vortex attached zone remain unchanged,and those monitor points' pressure fluctuation amplitude slightly oscillates.During the visible vortex stage,there are two characteristics of frequency spectrum at vortex attached zone.One is that the pressure fluctuation amplitude of low frequency caused by pressure decline is significantly increase,which is 1.11 Hz in this paper.The other is that the pressure fluctuation amplitude of blade pass frequency increases first and then decrease,corresponding to the development of vortex.During the newly-born phase,the main frequency of pressure fluctuation of vortex attached zone remain stable.It means that the perturbation of vortex is small at the newly-born phase.The amplitude of low frequency fluctuation significantly increases during the vortex development phase.Unsteady numerical simulation are used to predict the location and relatively strength of roof attached vortex.The prediction shows a great agreement with experiment.The formation mechanism of roof attached vortex.The preliminary research about the formation mechanism of roof attached vortex shows that the confluence boundary of two direction flow on the roof wall results in the formation of vortex.The experiment result shows that the characteristic frequency band of the confluence boundary is between 200 Hz and 400 Hz.?4?The coherent structure characteristic is found during the occurrence of roof attached vortex in different flow rate condition in the experiment.The relation between the number of roof attached vortex and flow rate parameter is positive correlation by observation and statistic during the experiment.The relation between the number of roof attached vortex and water level parameter is negative correlation by observation and statistic during the experiment.There is a kind of special low pressure fluctuation found near the pump back wall during every vortex occurrence condition.There is a positive correlation between the strength of special low pressure fluctuation and flow rate.On the contrary,the relation between special low pressure fluctuation and water level parameter is negative correlation.The amplitude of blade pass frequency of vortex occurrence zone is relatively higher on the roof.The amplitude of blade pass frequency reduces along with flow rate decrease and rise along with water level decrease.The height of roof has significant influence on roof attached vortex occurrence.On the one hand,the roof attached vortex is dismissed while the height of roof is down.On the other hand,the floor attached vortex is found in closed sump while the height of roof is down.The special low pressure fluctuation disappears while the height of roof is down.It means that this special low pressure fluctuation is a kind of precursors of roof attached vortex occurrence.The strength of blade pass frequency reduces along with the roof s height decrease,and the distribution of pressure fluctuation of blade pass frequency has a significant change on the no vortex condition,compared with that on the vortex condition.?5?The height of W shape back wall and location of middle division pier have a significant influence on the result of vortex elimination in closed sump.Unsuitable parameters of W shape back wall result in more vortex in roof.The recommend height of W shape back wall is 2.88 D and location of middle division pier is 0.950 D.The vortex elimination of different back wall shape is compared.The result shows that the vortex elimination is not obvious with the change of joining shape between back wall and side walls.The middle division piers structure could improve the flow pattern on the roof,so that the result of vortex elimination with middle division piers structure is better than other anti-vortex method.Trapezoid middle division piers is the recommend anti-vortex method in this paper.The result of vortex elimination of four kinds of wall grid structure is compared by simulation.The result shows that wall grid structure is a good anti-vortex method in closed sump.The recommend height of grid is 0.167 D.The result of vortex elimination with trapezoid middle division piers structure and wall grid structure is verified by experiment.