Prediction Of Start-up Process Model And Investigation Of Transient Characteristics Of Mixed-flow Pump

The mixed flow pump has the advantages of large flow rate and high head,which is widely used in the South-to-North Water Diversion Project,agricultural irrigation,water jet propulsion and launching torpedo.With the development of the application field of mixed flow pump,its transient performance has been widely concerned,especially in the process of start-up The transient change of pressure and flow rate during the startup process will cause great harm to the entire system.Therefore,theoretical analysis,numerical calculation and experimental study of the transient characteristics of a guide vane mixed-flow pump during start-up are carried out,which reveals the evolution of the internal flow field.The research results provide a reference for the hydraulic optimization for transient and further research on the transient flow characteristics of pump during start-up period.The main work and research results are as follows:(1)Based on torque-energy balance equation and the relative Bernoulli equation,the transient head expression of start-up is derived and the transient prediction model is established.The influence of start-up acceleration on the transient parameters including head,torque and efficiency under the circumstance of liner and exponential starting method is analyzed.Also,the saddle zone's effect is under consideration.The results show that the shock phenomenon of linear start-up is reflected at both the beginning and the end of the start-up,and weakens with the increase of start-up time.In the early stage of start-up,the external characteristics are mainly affected by acceleration while flow inertia is the dominant factor in the later stage.Under the circumstance of exponential start-up method,the shock phenomenon is mainly reflected in the initial start-up period.The shorter the start-up time is,the more likely the power overload will be caused.The transient effect of start-up process has always been dominated by acceleration.In addition,the saddle zone at rated speed has no significant effect on the exponential starting process,but has a significant effect on the linear starting process.(2)The pipe resistance characteristics and internal flow evolution of gate valve and elbow under steady and start-up transient conditions are compared and analyzed.The results show that the pressure drop in the gate valve and elbow is greater than that in the corresponding quasi steady state.,The pipeline loss is mainly caused by fluid inertia in the early stage of start-up while the hydraulic loss of gate valve and elbow is dominant in the later stage.With the increase of pump flow during start-up,the clearance flow behind the valve plate enhances that exacerbates the extrusion effect on the downstream main flow of the valve plate and the expansion angle of the main flow behind the valve plate decreases gradually.Due to the combined action of inertia and centrifugal force,the high pressure area uniform distributed at the inlet of elbow gradually shrinks to the outer diameter.The high-speed area is mainly affected by fluid inertia and high-speed fluid behind the valve and concentrated from the inner diameter area to the outer diameter.(3)The transient and quasi-steady velocity vectors of the mixed flow pump are compared and analyzed Additionally,the transient pressure evolution,leakage flow of tip clearance and the reverse flow characteristics of the inlet pipe are explored.The results show that the leakage flow circulates in the impeller at the initial period and the static pressure work increases due to the angular acceleration,so the hydraulic loss is smaller than the quasi steady state and the transient head is higher than the quasi steady value.With the increase of rotating speed,the acceleration inertia force weakens,generating the axial vortex and forming the inlet backflow and secondary outflow.The leakage flow at the trailing edge of tip spirals out along the pressure surface with the secondary flow.The low pressure area of suction surface gradually forms the separation vortex,which causes the entrainment effect on the leakage flow.The low pressure area is formed at the inlet of the pressure surface,and the hydraulic loss is large.Because the energy consumed by the fluid acceleration increases,the transient head is lower than the quasi steady head.In addition,due to the influence of the separation vortex,the inlet reverse flow strength and the entrainment effect of the reverse flow on the main flow increase firstly and then decrease.The distribution of the vortex core in each cross section(within the range of the circumfluence vortex)shrinks first and then disperses.(4)The measurement system of transient external characteristics and pressure vibration of mixed flow pump is established.The accuracy of theoretical analysis and numerical calculation is verified.The transient pressure characteristics under different start-up acceleration and pipe resistance are explored and the time-frequency evolution process of starting process is analyzed.The results show that the theoretical numerical methods used in this study are accurate and reliable.At the end of start-up,the shock phenomenon caused by the angular acceleration and the low operating condition is related to the pipe resistance and acceleration.When the pipe resistance is fixed,the greater the acceleration is,the more significant the pressure impact is.When the acceleration is fixed,the smaller the pipe resistance is,the inlet pressure impact gradually weakens or even disappears while the shock pressure in the tip of the impeller and the outlet is more obvious.For high start-up acceleration,pipe resistance has no effect on the transient pressure change during start-up,and the effect of pipe resistance on the pressure change begins to be obvious after the end of start-up.For low start-up acceleration,the effect of pipe resistance on start-up characteristics is significantly reflected.During the start-up process,the main frequency of impeller is blade frequency and its evolution trend is consistent with the change of speed.Under the condition of fast start-up,there is a maximum value of the main frequency scale caused by the shock impact at the end of start-up.