Research On The Stall And Cavitation Flow Characteristics And The Performance Improvement Of Axial-flow Pump

Stall and cavitation are both typically unstable flow phenomena inside axial-flowpump. They would decrease the hydraulic performance of axial-flow pump anddestroy the operation stability of pump unit. For a long time, researchers have beentrying to study the mechanisms of unstable flow phenomena in axial-flow pump toimprove its hydraulic performance.In this dissertation, the unsteady flow caused by the stall and cavitation insideaxial-flow pump and its mechanisms were deeply studied with a construction ofcurvature corrected filter-based turbulance model. On the basis study above,corresponding flow control techniques were proposed to ameliorate the effect of theseunsteady flow. The techniques were verified effective to suppress the emerging ofunstable saddle zone and the separation flow on the blade suface at low flow rate,besides the cavitation was inhibited to some extent inside axial-flow pump.The main contents of this dissertation are as follows:I. Construction and validation of the curvature corrected filter-based turbulencemodelConsidering the multi-scale and unsteady flow characteristics inside axial-flowpump, as well as the effect of streamline curvature which caused by the curvy surfaceof the flow passage components and the rotation of the impeller, a curvature correctedfilter-based turbulence model(FBM-CC) was proposed. The FBM-CC model wastested and verified by the flow in a plane channel with a U-turn and athree-dimensional wingtip vortex flow. The numerical simulation results show that theFBM-CC model can not only capture more abundant scales of turbulent structure, butalso improve the sensitivity to the effect of streamline curvature and the numericalcalculation accuracy.II. Study of the flow characteristics of axial-flow pump under stall conditionsThe internal flow of an axial-flow pump under stall conditions were simulated byusing the FBM-CC model. Compared with the experimental results, the FBM-CC model could not only predict the pump hydraulic performance effectively, but alsocapture the typical flow characteristics inside impeller under stall conditions. Basedon the boundary vorticity dynamics theory, the flow fields inside impeller wereanalyzed by BVF flow diagnosis. The results show that the physical root for theseparated flow can be easily captured by the BVF flow diagnosis and the positivepeak regions of BVF on the surface of blade are the locations of unstable flow.Through studying the unsteady flow fields in an axial-flow pump under stallconditions, the flow structure which from the trailing edge of blade to the pressuresurface of the adjacent blade was founded and the impeller passege was blocked nearthe impeller hub. With the deteriorating of stall, the blocked zone enlarged until theimpeller passage almost was blocked. The flow rate decreasing or the tip clearanceenlarging would cause the back-rotating flow. The serious blocking of impellerpassage, caused by the back-rotating flow in the front of impeller and the seperatingflow on the surface of blade, not only reduce the hydraulic efficiency, but also lead tothe formation of saddle zone.III. Study of the internal flow characteristics of axial-flow pump under cavitationconditionsAdopting the homogeneous flow model combining with FBM-CC model and themass transport cavitation model, the unsteady cavitating flow around the DelftTwist-11hydrofoil was simulated and it was verified the FBM-CC can effectivesimulate the unsteady flow characteristics in three dimensional cloud cavitating flow.By analyzing the cavitation performance and the cavitating flow fields visualizationexperiments of an axial-flow pump, it was found that the cloud cavitation on the bladesurface has great influence on the hydraulic performance. The established numericalmethod was applied to simulate the cavitation flow in the axial-flow pump and theresults show that the established simulation method about unsteady cavitating flowcould not only effectively predict the NPSHc (indicating the critical of net positivesuction head) of axial flow pump, but also capture the typical characteristics ofcavitating flows phenomena inside impeller. The side-entrant jet formed by thegradient of adverse pressure would cause the cavitation bubble shedding from the blade surface. Shedding cavity would stimulate the introduction of multi-scales vortexbehind the trailing edge of blade and deteriorate the hydraulic performance andoperation of axial-flow pump.IV. Research on the performance improvement of axial-flow pumpTo restrain the rotating flow at the impeller inlet under stall condition, theJ-Groove flow control technology was adopted to modify the inlet pipe of axial-flowpump. Based on the FBM-CC model, the hydraulic performance and flowcharacteristics of axial-flow pump with and without the J-Groove were analyzed. Theresults show that the J-Groove could effectively weaken the intensity of vortex in thefront of impeller and improve the efficiency of blade under stall conditions. With themore stable inflow of the axial-flow pump with J-Groove, head performance curvecould maintain the stable trend with decreasing of flow discharge and the saddle zonecan be effectively suppressed.Based on the “flap” flow control technology, a new type of guide impeller wasput forward. The tradational blade was devided into the front small blade and the rearblade. The guide impeller was designed by optimization analysis and tested bynumerical analysis and hydraulic performance experiments about performance,vibration and pressure pulsation. The results show that the guide impeller could notonly increase the hydraulic efficiency of axial-flow pump under small flow rateconditions, but also improve the stability of the pressure pulsation and vibrationcharacteristics of axial-flow pump at the small flow rate conditions. The saddle zoneis also restrained to a certain degree. To study the flow mechanism in the guideimpeller, the internal flow was simulated by using the FBM-CC model. The numericalresults show that the guide impeller could control the flow separation at blade leadingedge and facilitate to improve the hydraulic efficiency of axial-flow pump at the lowflow rate.To suppress the cavitation in axial-flow pump, the obstacle method was appliedand studied by numerical simulation. The method is implemented by setting a barrieron the suction surface of impller near the blade trailing edge. The effects of wide andheight of the barrier on the cavitation flow and hydraulic parameters were simulated and analyzed respectively. It can be found that the height of barrier has a greatinfluence on the performance, but the wide of barrier has no significant influence on it.While too much height of the barrier would not only cause the local cavitation, butalso decrease the efficiency of axial-flow pump.