Research On Hydraulic Transient Characteristics Of Large-scale Siphon Flow Channel Pump Unit

Siphon outflow channels are widely used in the construction of large low head vertical axial flow pumping stations due to their direct and reliable flow break.For this type of pumping station,problems such as excessive starting head,vibration and overload can occur during pump start-up,while problems such as excessive flyaway speed can occur during pump shutdown.The existence of these problems is a hidden danger for the safe and stable operation of pumping stations.Therefore,it is of great significance to carry out research on the hydraulic transient characteristics of vertical axial flow pump units during start-up and shutdown.In this paper,on the basis of the reliability of the numerical calculation results verified by model tests,numerical calculations are carried out to analyse the starting and stopping process of a vertical axial flow pump unit with a siphon outlet water channel.(1)The transient calculation and analysis of the transition process of the unit,the numerical calculation and study of the discharge channel without vacuum breaker valve and the discharge channel with different orifice diameter vacuum breaker valve respectively,the calculation of gas-liquid two-phase and the tracing of the gas-liquid intersection in the discharge channel using the VOF method,revealing the discharge process of the discharge channel;with the help of the moment balance equation,the use of UDF technology to compile the function language to achieve the control of the impeller(2)Numerical calculations of the outflow channel exhaust process are carried out.(2)Numerical calculation of the start-up process of a unit without a vacuum breaker valve in the outflow channel.Through monitoring of the unit parameters,analysis of the flow pattern in the outflow channel and analysis of the evolution of the gas-liquid interface,the process of discharging the gas from the outflow channel by entraining the gas in the water stream is revealed.The total starting time of this solution is 95s and the maximum starting head is 7.21m.(3)Numerical calculation results of the starting process of vacuum break valves with different apertures of 400,500,and 600mm show that the parameter change rules during the starting process of each scheme are similar,and the total starting duration of each scheme is 66s,63 s and 62s,respectively.The scheme with vacuum breaking valve is shortened by nearly 30%,and the maximum starting lift of each scheme is 6.31,5.93 and 5.98m respectively,which is reduced by up to 18%compared with the scheme without vacuum breaking valve.(4)Numerical analysis of the flyaway process was carried out to monitor the trend and maximum values of pump speed and blade axial force.The unit flyaway went through three stages:pumping condition,braking condition and turbine condition.0.41s of the unit flow returned to zero and the maximum pump flyaway speed was 178.44rad/s;numerical study was conducted on the normal shutdown process with vacuum breaker valve.1.15s of the unit flow returned to zero and the maximum pump reversal speed was 64.007rad/s during the shutdown process.