Analysis And Prediciton Of The Internal Flow In The Vane Pump Impeller At Off Design Condition

Affected by the external working factors, vane pump usually works at off design condition. While that happens, the hydraulic efficiency will sharply decrease. And sometimes it may work in the unstable status. This will result in the great waste of the power energy, and also level down the stability of the hydraulic system. So it is a crucial problem for engineering applications to improve the hydraulic efficiency and the stability of the vane pump at off design condition.The internal flow in the vane pump impeller at off design condition is extremely complex. And it is fundamentally important for improving the performance of vane pump at off design condition to study the flow mechanism deeply and explore the effective methods for its prediction and control.This dissertation focuses on developing the effective flow predicting methods based on flow structure analysis of the internal flow in the vane pump impeller at off design condition. At first, validation and verification for CFD and the comparison among various unsteady modeling methods for turbulence are performed to find out the reason why these CFD methods are failed in calculating the flow in the vane pump impeller at off design condition. And then, the proper mathematic model is established based on the theoretical analysis in the rotating frame of reference. A series of numerical tests have been made to verify this model after that. Finally the study has been taken preliminarily for flow control technology at off design condition.The main contents of the dissertation are as follows.I.To analyze the reasons for the failure of predicting the flow in the vane pump impeller at off design conditionIn view of the fact that much research works have indicated the RANS method is failed in the predicting the internal flow structure in the vane pump impeller at off design condition, validation and verification studies are performed for the RANS work, It is found out that the errors from mathematic model are the key reason which would lead to the failure of the flow prediction. Then unsteady methods for turbulence modeling, such as URANS, Hybrid RANS/LES and LES methods, are used for the further study. The work identifies that the inappropriate use of the turbulence model is the major trouble in predicting the internal flow structure in the vane pump impeller at off design condition.Moreover, the results from LES method indicate that the rotation of the system not only affects the large scale flow structure by means of centrifugal force and Coriolis force, but also changes these turbulence fluctuations in the small scale size.II. To establish the proper mathematic model for predicting the complex internal flow structure in the vane pump impeller at off design condition.The theoretical analysis indicates that the transport equation for turbulence kinematics energy k in the rotating frame of reference is the same as in the inertial frame, while that for Reynolds stress is not. As a result, these turbulence models which are based on the turbulence kinematics energy k are not sensitive to the rotation of the system. That is the fundamental reason why these turbulence models are failed in predicting the internal flow structure in the vane pump impeller at off design condition.So based on the method developed by A.Hellsten and mechanism of transportation of Reynolds stress in rotating frame of reference, The Extend Intrinsic Mean Spin Tensor (EIMST) is introduced to redefine well known Richardson Number, and then to make turbulent models sensitive to the rotation.III. To verify the model developed in this dissertation by the CFD methodThe EIMST based rotation correction is added to the commercial CFD code ANSYS CFX 11.0 by using the CEL (CFX Expression Language) method, and verified by some typical examples. In the numerical test, the distribution of EIMST based Richardson Number in the impeller passage is more reasonable than the else before. And these corrected RANS and Hybrid RANS/LES models can capture the"alternative stall and unstall"phenomenon which matches with the PIV and LDV measurement under the condition of no guide vane. Also, qualitatively consistent with other experimental work, the calculated result has the low frequency characteristic in the spectrum And similar to the LES results in this dissertation, the frequency spectrum of turbulence fluctuations calculated by the corrected model is found to have a k^-(3) type in the inertial zone, but not as the Kolmogorov energy spectrum in the type of k^-5/3. This is coincided with the theory of turbulence in the rotating system. It manifests the fact that the rotation of the system has a great effect on the turbulence structure. All these results demonstrate that the correcting method developed in this dissertation is acceptable to describe the flow in the impeller passage especially at off design condition.IV. To analyze theoretically method of predicting the internal flow structure in the vane pump impeller at off design condition.Over all the numerical work in this dissertation, it signifies that as only these turbulence fluctuations in the small scale size are treated by sub-grid model, these turbulence fluctuations calculated by means of LES are sensitized to the effect of system rotating directly, so these results from LES method are acceptable. In the contrary, because turbulence fluctuations are calculated by means of modeling in RANS and Hybrid RANS/LES methods, and as mechanism of transportation for Reynolds stress in the rotating frame of reference is different from that in the inertial frame, these models are failed in predicting the flow in the rotating frame of reference. So as the EIMST is deduced from the transport equation of Reynolds stress in the rotating frame of reference, which represents the kinetic mechanism of the turbulence, the EIMST based Richardson Number can make the turbulence models sensitive to the system rotation.V. To study the flow control technology preliminarily especially for the off design condition with the developed model.By using corrected SST based SAS model, the study is performed preliminarily for a kind of flow control technology, which uses a cylinder to disturb the flow in the impeller passage. The results show that this method with proper parameters can be used to change flow structure of the impeller passage so as to improve the hydraulic efficiency of vane pump at off design condition. The final chapter of this dissertation is devoted to summary of the major work and put forward prospect of the further study.