Investigation On Effects Of Perforation At Blade Inlet On Cavitation Performance Of Centrifugal Pump

The low specific-speed centrifugal pump is widely applied in various sectors related to fluids transportation for it 's exclusive advantages of high head when delivering low flow rates.The loss is relatively larger than that of other specific speed pumps because of the unique impeller configurations of low specific-speed centrifugal pump.The rates of cavitation occurring at the impeller inlet are higher as much as possible,which are mainly responsible for the low efficiency,vibration and noise of low specific-speed centrifugal pump.Considering these special flow patterns in low specific-speed pump,it deserves extremely worthwhile concerns to study how to improve the performance of low specific-speed centrifugal pump for engineering applications.The low specific-speed centrifugal pump of KQW125-315 type is selected as basic objective in this thesis,ten cylinder holes with different diameter size located 1/30 impeller length from blade inlet were designed and the diameter varied from 1mm to 10 mm.The effects of perforation at blade inlet on the performance are comprehensively investigated based on experimental observations combined with numerical simulation.The main coverage and conclusions associated with the effects of perforating schemes on pump performance are going as follows:1.The performance experiment bed was designed and built firstly.Then,the associated external characteristics experiments and cavitation performance experiments were done in this rig.The experimental results were compared with the numerical simulation results.Experimental results had a good agreement with numerical results to verify that the turbulent model and cavitation model employed was reasonable.2.Under rated flow rate condition,steady flow characteristics in the model low specific-speed centrifugal pump and the centrifugal pump with 10 different cylinder holes were studied.Results indicated that with the mean diameter of cylinder holes 8mm,the head and efficiency of the low specific-speed centrifugal pump were increased manifestly.The cavitation performances were improved.The distribution scopes of low pressure region of suction surface,vortices in the impeller and diffuser regions of the volute,turbulent kinetic energy in the impeller and volute,and vapor in the impeller of the lower specific-speed centrifugal pump were the smallest with the cylinder hole mean diameter 8mm.3.Under rated flow rate condition,transient flow characteristics in the original lower specific-speed centrifugal pump and the lower specific-speed centrifugal pump with the cylinder holes mean diameter 8mm was studied.Results indicated that in the origina l centrifugal pump,the distribution scope of lower pressure region of the suction surface decreased first then increased with the time,which is near the baffle tongue of the volute.For the impeller with the cylinder hole mean diameter 8mm,the distribut ion scope of lower pressure region of the suction surface increased first,then decreased and increased again.In different time,the distribution scope of lower pressure region in the lower specific-speed centrifugal was all smaller than that of original low specific-speed centrifugal pump.Under cavitation inception and critical cavitation conditions,the distribution scope of vortices decreased firstly then increased in the passages which are near the baffle tongue of the volute.In the original and modified lower specific-speed centrifugal pump,places that vortices appeared and numbers of vortices regions were almost identical.The only difference of them was that the vortices distribution scope of the impeller with cylinder holes was smaller than that of the original centrifugal pump.In the passages which are near the baffle tongue of the volute,the distribution scope of turbulent kinetic energy and turbulent intensity increased firstly then decreased under cavitation onset and critical cavitation con ditions with the time.In the middle span of the impeller of the modified centrifugal pump,the distribution range of turbulent kinetic energy and turbulent intensity was smaller than that of the original lower specific-speed centrifugal pump.In the diffe rent time of one cycle,cavitation bubbles appeared at the inlet of the original and modified lower specific-speed centrifugal pump,which were attached bubbles.In the palaces which are near the cylinder holes,the bubbles became larger and they were gradually apart from the blade.4.Transient flow in the original and modified lower specific-speed centrifugal pump was investigated.There were 16 monitoring points which were placed at the pressure side,suction side,and volute to study the pressure fluctuations.Results indicated that for the suction surface,the maximum pressure of the monitoring points which was near the cylinder holes were higher than that of the original lower specific-speed centrifugal pump and the peak pressure in other monitoring point s was lower than that of the original lower specific-speed centrifugal pump.For the pressure surface,at the inlet of the impeller,the maximum pressure of the modified centrifugal pump was smaller than that of the original centrifugal pump.With the decr ease of inlet pressure,the variations were more obvious.At the outlet of the impeller with the cylinder holes,pressure fluctuation amplitude was larger than that of the original impeller.It became weaker gradually with the decrease of inlet pressure.F or the volute,pressure fluctuation amplitude of the volute which was matched with the modified impeller was smaller than that of the volute which was matched with the original impeller.