| Centrifugal pumps are widely used in the field of in the aerospace,nuclear power and petrochemical,often operate in transient conditions,including rapid starting and stopping period.When these transient conditions occur,the flow rate and pressure in the centrifugal pump are prone to sudden changes,and the internal flow field is disturbed to produce dramatic changes,which affect the safe and stable operation of the centrifugal pump.However,the current reports of centrifugal pump internal flow characteristics are mainly based on the quasi-steady conditions,whose flow rates and rotational speeds do not change with time.The researches of centrifugal pumps flow stabilities during starting and stopping period are relatively small.Therefore,it is urgent to study the flow characteristics of centrifugal pumps during starting and stopping period.In this paper,the centrifugal pump is taken as the research object,the circulating pipeline system is established,and the numerical results are compared with the previous experimental results under quasi-steady conditions.The reliability of the numerical method is verified by comparing the head,efficiency and circumferential static pressure distribution of the volute.The application of energy change rate under quasi-steady conditions proves the feasibility of using energy change rate to analyze the flow stability of centrifugal pump in circulating pipeline system.Through the distribution of energy change rate and the evolution of mean value,the effects of valve openings and starting and stopping schemes on the flow characteristics of centrifugal pump during starting and stopping period were explored.The main conclusions are as follows:(1)The evolution of instantaneous energy field of centrifugal pump with linear starting period is studied.With the increase of impeller speed,the partial derivative of mechanical energy increases,the leading energy loss zone and the region of dominant energy loss and increase respectively,and the average value of energy change rate increases sharply.When the instantaneous speed exceeds half of the designed speed,the different opening of the valve begins to produce obvious differences in the stability of the flow field in the centrifugal pump,and the higher the value of the secondary change rate of energy,the higher the flow stability.At the moment when the design speed is reached,the value of the energy change rate of the centrifugal pump under the small valve opening degree is higher than that under the quasi-steady state condition,that is,the flow stability is higher,while the opposite is true under the large valve opening degree.(2)The instantaneous energy field evolution and pressure pulsation characteristics of centrifugal pump during linear stopping period are studied.In the linear stopping period,the flow rate lags behind the head.As the speed decreases,the gap between the flow rate and the quasi-steady flow rate increases,while the gap between the head gradually decreases.With the decrease of rotational speed,the stability of the flow field in the volute and impeller runner decreases,and the stability of the volute decreases earlier than that of the impeller passages,and the smaller the valve opening,the flow stability decreases obviously.When the impeller speed reaches 0rev/min,the decrease of the normal partial derivative of mechanical energy in the impeller passages will be slower than the flow direction partial derivative,thus leading to the increase of the value of the primary and secondary change rates of its energy.However,the stability of the flow in the centrifugal pump decreases,and the smaller the valve opening,the lower the flow stability.With the increase of the valve opening,the pressure pulsation in the centrifugal pump gradually decreases,and the vibration gradually weakens,especially inside the impeller and near the tongue.The degree of instability of the centrifugal pump gradually decreases(3)The effect of starting and stopping mode on the flow characteristics of centrifugal pump during fast starting and stopping period is studied.When the impeller speed increases and decreases,the instantaneous head and flow of the centrifugal pump are not sensitive to the change of start-stop mode.In the starting period,when the impeller rotates at more angles,the energy change rate of the flow field is closer to the quasi-steady condition when the impeller reaches the design speed.During starting and stopping period,when the instantaneous speed is higher than half of the design speed,the value of the secondary energy change rate is closer to the stability of the flow;when the instantaneous speed is less than half of the design speed and not close to 0rev/min,the value of the primary energy change rate is closer to the stability of the flow.The time domain distribution of pressure pulsation is positively correlated with the speed of impeller. |
