Research On Flow In Centrifugal Impeller Based On LES/RANS Hybrid Model

The internal flow of the centrifugal pump is an anisotropic turbulent flow with high rotational speed and high curvature.The traditional Reynolds time-average RANS model is not accurate enough to predict the flow field,while the LES model has higher requirements for computational cost.The hybrid LES/RANS method combines the advantages of the two methods.It has obvious advantages in the calculation of high Reynolds number and large separation flow,and it can meet the accuracy requirements of flow field simulation while saving computing resources.In order to better simulate the internal flow field of the centrifugal impeller,the stress term that contains the helicity constraint is introduced into the original DES model to construct a model with helicity correction.The main work and research results of this paper are as follows:(1)Based on the original DES model,the stress term that contains the helicity constraint is introduced into the original DES model to construct a modified DES model with helicity correction.Using a user-defined function(UDF),the helicity correction module is written and dynamically loaded into the Fluent solver to solve the required flow field data.A classical rotating channel method is used to compare the flow velocity,turbulent kinetic energy,Reynolds stress,and helicity obtained by the classic DES model and the modified DES model.Comparing the velocity profile predicted by the two models with the experimental results,it is found that the velocity profile obtained by the modified DES model is closer to the experimental results,and the analysis results of each turbulence statistic are also consistent with the velocity profile,which verifies the reliability of the model with helicity correction for predicting the rotating flow field.(2)The model with helicity correction is used to verify the numerical simulation of the centrifugal pump,which is divided into external head verification and internal flow field verification.Comparing with the experimental values,it can be found that under low flow conditions(0.2Qd,0.5Qd),the model with helicity correction improves the head prediction accuracy by about 1%.On the whole,the head prediction accuracy error of the two models under different flow conditions does not exceed 7%.Through the comparison of the internal flow field results,it is found that the prediction of the relative velocity field is improved under the design condition(1.0Qd),mainly because the correction term is added so that there is no excessive sub-grid near the wall.Therefore,the ability of the wall rotation to drive the fluid to work is avoided.(3)Combining with the Reynolds stress transport equations,the high-order statistics inside the centrifugal pump are counted.It is found that under the design condition,the HELDES model predicts a larger strain production term near the impeller inlet(r=0.035m),so that the average energy transported from the average motion to the fluctuation motion is more,and near the impeller center region and the outlet(r=0.065m),the predicted strain production term is not much different,which results in a larger average energy difference between the inlet and outlet predicted by the HELDES model.That is,the HELDES model predicts a larger head under design condition.