Effects Of Blade Number On Transient Flow Characteristics In The Centrifugal Pump Impeller

Centrifugal pump is a universal device that converts the energy of the prime mover into fluid energy and is widely used in various industries.Blades are one of the key components of centrifugal pumps,and their quantity significantly affects the flow characteristics inside the pump,including stall and pressure pulsation.At the same time,the operation of centrifugal pumps under transient conditions is a dynamic and complex process,manifested by the transient flow state and the complexity of pressure fluctuations.The mechanism by which the number of blades affects the transient flow characteristics inside centrifugal pumps is not yet clear,and further research is urgently needed.This thesis takes the centrifugal pump impeller as the research object,considering the influence of blade numbers(including 4,5,6,7),and adopts the Large Eddy Simulation(LES)method to obtain unsteady numerical results.Firstly,the reliability of the numerical method was verified under stable operating conditions.Then,under different transient operating conditions,the influence of blade number on the internal stall characteristics and pressure pulsation transient flow characteristics of centrifugal pump impellers was studied.The main research content and conclusions are as follows:1.A method for predicting the critical alternating stall of centrifugal pump impeller has been proposed and successfully used.The reliability of the prediction method was verified under constant flow conditions.The impeller was in an alternating stall state when the inlet flow rates were 0.25Q_dand 0.5Q_d,and in a non stall state when the inlet flow rate were 0.75Q_d.The availability of the prediction method under transient conditions was verified,and the effects of amplitude and speed on the critical flow rate of alternating stall were obtained.As the amplitude increases,the critical flow rate increases and decreases in the descending and ascending stages.As the speed increases,the critical flow rate increases in both the descending and ascending stages.2.The influence of blade number on the internal stall characteristics of centrifugal pump impellers is explored.The flow inside each blade impeller has undergone a development process from stability to flow separation,to stall,to flow separation,and finally to stability.The 6-blade impeller enters and exits the alternating stall state first compared to the 4-blade impeller.The stall vortex intensity in the flow channel of the6-blade impeller in the alternating stall section is much greater than that of the 4-blade impeller.The average vorticity field of 4 and 6 blade has a total vorticity of-73 and-86,the main influencing factor of total vorticity is negative vorticity.respectively.The 4and 6 blade impellers in the alternating stall section show a decrease in total entropy production during the flow decreasing stage and an increase in the rising stage.The total entropy production value of the 4-blade impeller at each flow point is lower than that of the 6-blade impeller,and the variation amplitude of the total entropy production with flow rate is smoother than that of the 6-blade impeller.The high entropy area on the impeller blade is less than that of the 6-blade impeller,indicating that the energy loss of the 6-blade impeller is higher than that of the 4-blade impeller.The distribution of stall vortices in the flow channel of odd blade impellers is irregular,and the stall intensity of the 5-blade impeller at the stall flow point is stronger than that of the 7-blade impeller.3.The influence of blade number on pressure pulsation and external characteristics of centrifugal pump impeller was analyzed.The low pressure at the inlet of the even blade channel is always stationary relative to the channel,while the evolution of the low pressure at the inlet of the odd blade impeller channel over time is transitive.The main frequency of pressure pulsation at the inlet of the impeller channel with even blades is the flow frequency,and the amplitude of the main frequency is alternately distributed.The odd blades are the flow frequency or blade frequency,and the distribution of the amplitude of the main frequency is irregular.The pulsation amplitude of the pressure surface in the impeller flow passage of each blade number is greater than that of the suction surface,and the pulsation amplitude in the impeller flow passage of even number blades and the main frequency amplitude of the overall pulsation are greater than those of odd number blades.When the number of blades is 5,the pressure fluctuation amplitude in the impeller flow channel is the smallest and the main frequency amplitude is the lowest.Under transient flow conditions,there is a negative correlation between the pump head and flow velocity,and the head lags behind the flow velocity.The fluctuation difference between different blade numbers and the centrifugal pump impeller head lag time is within 4%.