| When the inlet backflow occurs in the centrifugal pumps,the flow entering the impeller oscillates periodically,and the pressure along the center line of the impeller inlet in the axial direction will reduce,causing the backflow vortex cavitation to occur,making the impeller inlet flow unstable.In order to study inlet backflow and its induced vortex cavitation,both numerical and experimental method were used to study the hydraulic performances,cavitation characteristics and dynamic characteristics of model centrifugal pumps with different impeller schemes.The main research contents and achievements of this thesis are as follows:1.The research background of backflow phenomenon and its induced vortex cavitation both at home and abroad were reviewed.Meanwhile,the current research problem of these two phenomenon was demonstrated.2.The steady numerical simulation of inlet backflow in the model centrifugal pumps were carried out.The effect of flow rates on the inception of inlet backflow,and the flow state in the impeller inlet,when the backflow phenomenon occurs were discussed by analyzing the velocity and pressure distribution in the inlet pipe.The results can be obtained as follows:(1)When the model pump operates under or lower than the optimal value,the inlet backflow will occur,and become more serious as the flow rate decreases.Among these 3 impellers,backflow phenomenon of C impeller is very obvious,and the A impeller is weak.The circumfluence and reverse axial velocity appear at the impeller eye due to backflow,and the pressure distribution is high at edge and low at center.(2)The reason of the backflow inception can be summarized as follows: When the operating flow rate decreases,the flow velocity in the axial component Vm decreases in the impeller.Therefore,some fluid flow towards the solid wall,where the local velocity and pressure in the region change,and forms backflow due to the effect of the centrifugal force.3.The steady numerical simulation of cavitation flow in the model centrifugal pump were carried out.The distribution of vapor fraction in the impeller with different cavitation number was studied.The development process of backflow vortex cavitation was discussed,and the flow velocity and pressure distribution in the backflow zone was investigated during the cavitation.The results can be obtained as follows:(1)Vapor distribution is affected by backflow.At the best efficiency operating point,cavities first appear on the inlet side of the blade near the shroud.At low flow rates,cavities were found first appear near blade root,and then developed gradually towards the impeller outlet.Backflow vortex cavitation occurs under some conditions.Due to the occurrence of the backflow.When the backflow vortex cavitation appeared,the cloud cavitation were obviously found both in the inlet pipe and impeller eye.For example,when the flow rate is 0.46 QBEPC and the cavitation number σ is 0.22~0.06,the volume fraction of backflow vortex cavitation cloud will increase,decrease and then disappear.(2)Backflow is also affected by cavitation.The backflow region will decrease gradually as the cavitation number σ decreases,and the reverse axial flow and circumferential velocity near the impeller inlet also decrease,and the pressure distribution gradually becomes even.4.The unsteady characteristics of the internal flow in the model centrifugal pump with the impeller C were studied based on unsteady numerical simulation.The variation of the backflow and its induced vortex cavitation in the inlet pipe with time was studied.The pressure pulsation characteristics of backflow and backflow vortex cavitation were analyzed based on domain and the phase correlation between different positions of pressure monitors.The results can be obtained as follows:(1)The backflow vortices are constantly generated in the impeller,developed upstream,and eventually disappear at the intersection with the main flow.At the operating condition of 0.36 QBEPC and with the cavitation number σ=0.82,the main frequency of in the inlet pipe is in the low frequency range of 0~fn,with higher amplitude near the pipe wall.The main frequency of the pressure pulsation in the impeller inlet is 4fn,with the secondary main frequencies lower than the shaft value.Base on the phase correlation analysis of pressure monitors,it can be obtained that there was rotating flow instabilities near the impeller inlet.At the operating condition of 0.46 QBEPC,and σ= 0.81,the main frequency of the pressure pulsation at the impeller inlet was 4fn,and the secondary main frequencies were lower than the shaft frequency,which have the characterizes of low frequency with high amplitude.Base on the phase correlation analysis,it can be revealed that there was both axial and rotational flow instabilities occurring at the impeller inlet.(3)Backflow vortex cavitation occurs near the center of impeller inlet in the axial direction and rotates with the impeller.Part of the cavities develop towards upstream continue to extend upstream and then disappear.At the operating condition of 0.46 QBEPC and σ=0.13,the main frequency of the pressure pulsation on the cross section plane 1 of impeller inlet is 4fn,and with the frequencies lower than the shaft one as the secondary main frequency.Base on the phase correlation analysis,it can be revealed that there was axial and rotating flow instability at the inlet of the impeller.In the operating condition,both 0.36 QBEPC,and σ=0.12,the main frequency of the pressure pulsation on the cross section plane 1 of the impeller is 4fn,and with the frequencies lower than the shaft frequency as the secondary main frequency.Base on the phase correlation analysis,it can be revealed that there are an axial instability flow at the inlet of the impeller.5.A closed test rig was set up to investigate hydraulic and cavitation performances of the model centrifugal pump,as well as pressure pulsation test was carried out.And the head Ψ and cavitation number σ of model centrifugal pump using different impeller are compared.It was found that A impeller has the best hydraulic performance and worst cavitation performance,while C impeller has the worst hydraulic performance and best cavitation performance.At the operating condition of 1.0QBEPA,the main frequencies of the pressure pulsation are 7fn.And at the operating condition of 0.6QBEPA,the main frequencies of the pressure pulsation are 7fn with lots of frequencies lower than the shaft frequency.Basing on the phase correlation analysis,it can be revealed that there is an axial instability flow.In the operating condition,both 0.6QBEPA,and σ1%,the pressure pulsation is 7fn and 14 fn,and with lots of frequencies lower than the shaft frequency.Basing on the phase correlation analysis,it can be revealed that there is an axial instability flow. |
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