| Our country vigorously advocates water saving and efficiency improvement,constantly rebuilding large water conservancy and hydropower projects,so as to achieve optimal allocation and efficient utilization of water resources.The double suction two-stage split case centrifugal pump is a special centrifugal pump with high power providing large flow rate and high lift.It plays an important role in large-scale irrigation projects.Meanwhile,high power also determines the characteristics of large energy consumption.The operation efficiency of the double suction two-stage split case centrifugal pump is related to the high efficiency and energy saving of the pump and the system.The unsteady flow inside the pump affects the stable operation of the pump and the system.The study of internal flow and unsteady characteristics of double suction two-stage split case centrifugal pump is very important.Supported by National Science & Technology Pillar Program of China(Grant No.2014BAB08B00),the investigation of internal flow and unsteady characteristics of a double suction two-stage split case centrifugal pump was carried out with numerical simulations and experiments.The main work and creative results of this dissertation are as follows:(1)All the meshes are generated by structural hexahedra in the computational domain of the double suction two-stage split case centrifugal pump using ANSYS-ICEM.The unsteady flows in the pump are simulated under different conditions using SST turbulence model,verified by experiments.Under design condition,the simulated head is lower than the design head 16%,and the experiment efficiency is lower than design efficiency as well.The fluid through the suction chamber swirls into the first stage impeller,then flows into the channels between stages.Influenced by the complex structure of channels and impeller rotation,the fluid flow into the next stage impeller with large swirl velocity,which providing bad quality inflow resulting in the low head.The static pressure distribution in channel between stages is symmetrical,which counteracts radial force and has a good balance effect.(2)Through time-frequency domain analysis,the pressure pulsation propagation law of the double suction two-stage split case centrifugal pump is studied.It is found that the blade passing frequency dominates the pressure pulsation in the pump.The pressure fluctuation in the suction dose not change obviously with flow.The pressure pulsation of the first stage impeller is affected by rotor-stator interaction.The dominant frequency appears at the two times of the blade passing frequency.The amplitude of the pressure pulsation in the dominate frequency and the harmonic frequency increases along with the flow.Due to the symmetrical structure of channel,the pressure pulsation appears the same regular pattern.Because of the complex internal flow,the dominate frequency of the double suction impeller changes along with the flow direction.The outlet of impeller is influenced by the interference of the tongue,and the amplitude of the pressure fluctuation coefficient at the dominate frequency increases from the inlet to the outlet.The pressure pulsation of the tongue near baffles is larger,and it decreases along with the flow.(3)Due to the bad performance of the pump,based on the guide vane design,two pairs of symmetrical baffles are added in the two side channels between stages to guide the flow.The method of simulations and experiments are the same with that before.It turns out that the head reaches the design head and the test efficiency is increased by 5% after adding the baffles.The baffle mainly affects the channels between stages and second stage impeller.The baffle block the original movement of the flow,which optimizes the inflow of the secondary impeller under the rotating effect of the impeller.It shows that the baffles are very effective and provide high quality inflow,thus the head and the efficiency increase a lot.(4)An evaluation method on flow loss based on entropy by using the second law of thermodynamics is applied to analyze the flow loss in the double suction two-stage split case centrifugal pump.The turbulence dissipation is the dominant flow loss,and the viscous dissipation can be neglected.Generally speaking,the losses for the pump with different channels mainly exist in the double suction impeller,spiral casing and the channels between stages.The influence of the baffle on the flow loss in the downstream is much greater than that of the upstream.The baffles weaken theprerotation of the inlet of the secondary impeller and reduce the flow loss and energy dissipation.(5)The influence of baffles on the operation stability of the pump is compared and analyzed.The baffles have a great influence on the downstream pressure pulsation.The amplitude of the pressure pulsation coefficient with baffles at the dominate frequency is significantly higher than that without the baffles.The pulsation at low frequency obviously decreases due to the drop of inlet swirl velocity.In terms of radial force,the baffles have a good effect on reducing radial force.At large flow rate,the baffles make against to the uniform distribution of velocity and pressure near the impeller outlet,leading to the increase of the radial force of the impeller. |
