| The research is supported by National Natural Science Foundation of China (Grant No.51079062) project.With the development of society, science and technology, the stability and reliability of centrifugal pump operation is gradually important. While, the unsteady flow inside the pump is an essential factor that determines the stability of pump operation. In order to get a better understanding and researching about the internal flow in the pump, this paper takes the unstable flow pattern inside the impeller as the subject. The research method combining experiments and CFD has been applied to investigate the onset and development law of unstable flow within the impeller. Meanwhile, a new nonlinear turbulence model is generated based on SST k-co turbulence model, that aiming at simulating more precisely on the centrifugal pump because of its higher friction loss inside passage, higher adverse pressure gradient and bigger curvature of blade. Furthermore, a set of numerical method is set up for fitting the characteristics of centrifugal impeller passages based on OpenFOAM. The main research contents and creative achievements as below:1. The present situations on measurements in centrifugal pump, such as PIV measurement technique, pressure fluctuation experiment technique and vibration measurement technique, were summarized. The present research status and defects on linear eddy viscosity model were concluded. The development trends for combining explicit algebraic stress equation model with turbulence model of quadratic equation to solve the flow with rotation and curvature were illustrated.2. Onset and development rule of unstable vortex within the impeller were discovered using PIV technique. The effect law of unstable flow both on hydraulic performance curve and on absolute velocity distribution in impeller passages were quantitated and revealed separately. The main conclusions were following:(1) The flow was separated at0.6QBEP, developed at0.4QBEP, and spread almost the whole impeller passage at0.1QBEP In addition, flow in the passage near the tongue was the most unstable, and the vortex was also occurred in this passage firstly by comparing with rest of passages. With the flow rate decreasing, the vortex on the pressure side was increased and converged to a large-size vortex, in the end, the large-size vortex spread to the outlet of passage, and moved to the centre area of the passage.(2) The average circumferential component of absolute velocity vu2/u2at the impeller exit went up firstly, then down, and there was a hump on the above velocity distribution curve between0.1QBEP and0.6QBEP. Moreover, both vu2/u2and head fell steadily with the flow rate increased from0.6QBEP and1.0QBEP. Furthermore, the unstable flow was the main factor to cause the hump or flat area on the H-Q curve.(3) With the radius of the impeller passages increasing, the circumferential component of absolute velocity vu rose firstly, and then reduced. In addition, the changing law of vu was very similar under different working conditions, that was, it varied inversely as the flow rate. In contrast, the radial component of absolute velocity vm firstly decreased, and then increased with the radius of the impeller passages increasing. Meanwhile, the magnitude of vm was proportional to the flow rate.3. The experimental investigation was performed on pressure fluctuation of impeller exit in circumferential direction by utilizing the HSJ2010tester and high-frequency dynamic pressure transducers. The relationship between pressure pulsation and unstable flow in the impeller passages was summarized. The following3results were obtained:(1) The blade passing frequency and the shaft frequency were the domain frequencies, here, the blade passing frequency was induced by "jet-wake" structure and the shaft frequency was induced by the asymmetric flow in the impeller passages.(2) The periodic pressure pulsation amplitude, which caused by "jet-wake" structure, was gradually decreasing along with the gap between impeller exit and volute wall(the wall which directly against the impeller exit) increasing. In other words, the pressure pulsation amplitude, which was influenced by impeller outlet flow, was weakened along with the gap between impeller and volute wall was increasing.(3) There was a100Hz-145Hz broad-band frequency among the overall frequency domain of pressure pulsation near the impeller exit, and the broad-band frequencies were between about4times the shaft frequency and the blade passing frequency. In addition, the amplitude of above broad-band frequency was increasing with the scale of the unstable vortex enlarging.4. The experimental investigation was performed on vibration of centrifugal pump with unstable flow phenomena by utilizing the INV3020C data acquisition system, DASP-V10software and ICP acceleration sensors. Effects of unstable flow phenomena in impeller passages on vibration were summarized. Moreover, the correlation between vibration signals and pressure fluctuation signals among frequency domain was found. Here, their shaft frequency and blade passing frequency are the domain frequencies. For the vibration signals, there also existed a100Hz-145Hz broadband vibration, that was between about4times the shaft frequency and the blade passing frequency, caused by unstable vortex.5. A new computational program with nonlinear turbulence model which considering the effects of rotation and curvature, that was EASMRC, was developed firstly based on an open source code named OpenFOAM. The idea of the nonlinear turbulence model was given as follows:Firstly, the Reynolds stress and the eddy viscosity were calculated by explicit algebraic stress equation model. Then, the improved k equation and co equation in SST k-co turbulence model which considered the effects of rotation and curvature were used for governing equations closure. Finally, the mean spin tensor in the above equations was improved by the extended intrinsic mean spin tensor. Therefore, a new nonlinear turbulence model combining explicit algebraic stress equation model and SST k-co turbulence model was improved. Both a rotating turbulent duct flow and a90°curved duct flow is simulated for validation. The result showed that the calculation result of improved nonlinear turbulence model was closer to experimental results.6. The improved turbulence model (EASMRC) was added into MRFSimpleFoam and pimpleDyMFoam solver based on OpenFOAM respectively for the first time. Both steady simulation and transient simulation were performed on the pump internal flow. Then, the calculation results by EASMRC model was compared with the results by SST k-co turbulence model in OpenFOAM, the results by SST k-co turbulence model in CFX, the result by hydraulic performance test, the result by PIV measurement and the result by pressure pulsation measurement respectively. The results were followed:in terms of the pump hydraulic performance, the calculation accuracy of EASMRC was much better than original model both in OpenFOAM and CFX; in terms of the internal flow, EASMRC revealed that the evolution process of vortex in the impeller passages, which was more consistent with the PIV test; in terms of the pressure pulsation, EASMRC was much closer to test results from pressure pulsation charateristics, such as pressure fluctuation amplitude, pressure pulsation results on time domain, and on frequecy domain respectively,... |
PDF Full Text Request