Research On The Mechanism Of Performance Improving In Low Specific Speed Centrifugal Pump With Gap Drainage Blades

The low specific speed centrifugal pump is widely used, but its efficiency islow relatively. Besides, cavitation may occur in actual operations, which willdegrade the pump performance, and even affect the unit operation. Therefore, it is ofgreat significance to improve the pump hydraulic and cavitating performances forthe unit safe, stable and efficient operation.Early researchs prove that the gap drainage blade designed based on flowcontrol idea can improve hydraulic performance of the centrifugal pump under acertain range of working conditions. However, there are no clear understandings onits internal flow mechanism, and lack of consideration on its cavitationcharacteristics, and also without a set of effective numerical method which canaccurately describe the inner flow structure in the centrifugal pump with gapdrainage blades.In this paper, a set of reasonable multi-scale simulation method is to beconstructed, and then the related numerical and experimental researches aresystematically carried out on a low specific centrifugal pump with gap drainageblades, aiming to discover the internal flow mechanism. All these will providetheoretical, numerical and experimental basises for the optimal design, performanceprediction and flow control of the low specific speed centrifugal pump. The mainworks of the dissertation are as follows:Ⅰ. Scalable detached eddy simulation methodA scalable detached eddy simulation (SDES) method was put forward byintroducing the von Karman scale, which can automatically detect the RANS andLES regions based on the local flow fields. The method was tested and verifiedthrough multiple cases. The shortage of direct dependence on the grid for thetraditional DES method in identification simulation regions was improved by theSDES method. The method can also adaptively switch RANS/LES method to solve the steady or unsteady area.Ⅱ. PIV experimental research on the internal flow field in a centrifugal pumpA closed test unit was built, which is suitable for the experiments ofperformance, PIV flow fields and cavitation observation in centrifugal pump. And atypical pump model was built for PIV testing and cavitation insighting. Thehydraulic performance of the centrifugal pump with gap impeller is obviouslyimproved with the increasing of flow discharge. The PIV flow fields show that mainfluid tends to flow along the blade suction side in the passage of centrifugal impeller,and a vortex with opposite rotating direction occupies the pressure side passage.With the increase of flow discharge, the intensity of the vortex will be weakened bythe inhibition of main flow, and the vortex will be gradually constrained in the smallregion close to blade pressure side.Ⅲ. Numerical study on the cause of hydraulic performance improving with gapdrainage blade in a centrifugal pumpThe SDES method was verified with the experimental data obtained in thispaper, and then the differences of the pump internal flow characteristics werecompared and analyzed between the traditional impeller and gap one. Themechanism of the hydraulic performance improvement with gap drainage blade wasalso discussed. The gap blades guide some fluid flowing through the gap from theblade pressure side to the suction side, and the frontal passage inflow area isincreased for the design of some offset and overlapping small blades. These willmake the velocity distributions are more uniform, thereby the energy loss caused bythe strong shear turbulence in the impeller is reduced. In addition, the pump pressurepulsation intensity with gap impeller is generally lower than that of traditional one,which illustrates that the gap drainage blade may also improve the operation stabilityof the centrifugal pump.Ⅳ. Cavitation experiment in a centrifugal pumpTo examine the cavitating characteristics of the centrifugal pump with gap impeller, many sets of experiments were carried out under different flow discharges,such as tests of cavitation performance, observations of high-speed photography andphase-locking pictures. The results show that the gap impeller has better resistance tocavitation than traditional one under larger characteristic discharge, while undersmall flow conditions, the cavitation characteristics are nearly fair for the two kindsof impeller. For the gap impeller under large discharge conditions, cavitation occursmainly along the blade suction side, and the occupying volume will grow graduallyfrom the suction side to the pressure side with the decrease of cavitation number.While for the traditional impeller, cavitation occurs early at the blade pressure side,and it will grow from both sides to the passage middle at the inflow area when thecavitation number is decreased.Ⅴ. Numerical study on the cause of cavitation performance improving with gapdrainage blade in a centrifugal pumpThe SDES method was verified again by the experimental cavitation data, andthen it was applied to the cavitating simulation of the centrifugal pump withtraditional impeller or gap one under different discharge conditions. Some specialtopics are analyzed in details, such as the circumferential evolution characteristics incavitation flow field, the tongue cavitation phenomenon at large flow rate, thestaggered fixed cavitation phenomenon in gap impeller at small flow rate, and thecavitation suppression mechanism of the impeller with gap drainage blades. Theresults show that the main causes of the resistance to cavitation are combinations ofthe guiding flow of frontal small blades, the drainage of the gap tunnel, and thedisturbance from small blades to main ones. In the axial center plane of volute, theaverage static pressure along a circle decreases gradually when it is near to thevolute exit, so the cavitation intensity will be enhanced when near to the volute exit,while weakened when away from the volute exit. At large flow rate, the pumptongue cavitation occurs, which affects the drop of pump water head. The incidentangle around the blade leading edge will be changed at large flow rate, which will cause the region formation of partial low pressure and cavitation near the exit surfaceof the tunnel. The circumferential evolution characteristics of the tongue cavitationare determined by the change of the external environment pressure around thetongue. The changed incident angle around the blades determines the staggered fixedcavitation phenomenon for the gap impeller pump at low flow rate, which is theresults of the change of flow discharge, the formation of a certain volume ofcavitation, and the jet flow in gap tunnels.In a word, the scale adaptive SDES method offers an effective numericalmethod for the internal flow analysis in centrifugal pump. Both experimental andnumerical results show that the designed gap impeller based on flow control idea hasthe effects of guiding flow and drainage flow, which can improve the hydraulic andcavitation performances under the working conditions of a certain discharge rangefor the low specific speed centrifugal pump.