Analysis Of Time-requency Characteristics Of Attached Wall Vortex In Intake And Its Impact On Axial Flow Pumps

The open-type inlet pond of an axial flow pump station is prone to generating attached wall eddies,which can jeopardize the stable operation of the pump station.Studies have mainly focused on the evolution rules and formation mechanisms of attached wall eddies,but it is not yet clear how to identify them during the operation of pump stations and how they affect axial flow pumps.This paper systematically studies these issues using CFD.The Due to the fact that attached wall eddies are caused by strong separation flow and have typical unsteady characteristics,the turbulence model becomes a key factor in accurately predicting these patterns in CFD calculations.The Hybrid RANS/LES model combines the advantages of RANS and LES to accurately simulate complex turbulent problems with minimal computational resources.The VLES model is an excellent RANS/LES model that has displayed benefits in other flow cases and has good application prospects.Therefore,this paper investigates the applicability of VLES models in complex rotating fluid machinery internal flows and focuses on exploring the impact of core parameters on simulation results.Based on this,in order to identify attached wall eddies,this paper uses the wavelet analysis method to carry out time-frequency analysis of the pressure signal.This method overcomes the shortcomings of traditional Fourier transform in not being able to obtain frequency changes over time,and is suitable for the analysis of non-stationary attached wall eddy signals.By extracting and analyzing the time-frequency characteristics,and combining them with the internal flow field structure,the identification criteria of attached wall eddies are established,and the impact of attached wall eddies on axial flow pumps is revealed.The main conclusions obtained in this paper are as follows:The VLES model is suitable for predicting the internal flow in the inlet pond and axial flow pumps,and the results are better than those of RANS and other Hybrid RANS/LES models.The values of core parameters n and β in VLES have a significant impact on the prediction results,and the optimal values are n=2 and β=-0.002.The bottom attached eddy forms earlier than the back wall and side wall eddies,and has the greatest strength.The side wall eddy is the latest to form and has the smallest strength.The smaller the flow rate,the later the formation time of the three eddies.The main frequency range of the bottom attached eddy,back wall eddy,and side wall eddy is 0.56 Hz～1.5 Hz,and the energy of the main frequency increases with increasing water depth.At a flow rate of 0.8 Qd,a wide-band feature appears around the impeller rotation frequency of 36Hz at the measurement point in the inlet pond,which is due to the fact that the tangential velocity produced by impeller rotation is greater than the axial velocity when the flow rate decreases,and the rotational effect of the impeller is enhanced.After the attached wall eddy is fully developed,it will enter the diffuser,where the back wall eddy is relatively stable,while the area where the bottom attached eddy is located has periodic vortices of the same direction forming and merging.The main frequency of the impeller inlet and blade measurement points is consistent with the main frequency of the attached wall eddy in the inlet pond,because the attached wall eddy acts on the impeller after entering from the diffuser.To identify attached wall eddies during the actual operation of the pump station,pressure sensors can be installed inside a square area with a side length of 0.5 D at the bottom of the inlet pond.The frequency of the attached wall eddies can be monitored using the wavelet analysis method.The conclusions of this paper provide a scientific basis for the operation and monitoring of pump stations and have important scientific significance and practical engineering value.