Research On The Internal Flow Characteristic And Flow Interference In The Pelton Turbine

Based on Yusong hydraulic station,domestic hydraulic power manufactures are developing technologies for large-scale Pelton turbine,which has a 500 MW capacity and water head over 1000 m.At present,it is still lack of thorough study for the complexity multi-phase flow characteristic and accurate quantitative analysis of flow interference in the multi-nozzle Pelton turbine.Thus,it is of great practical significance to study the internal flow characteristic and flow interference in Pelton turbine.To analyze the unsteady multi-phase flow patterns and flow transition processes in a multi-nozzle prototype Pelton turbine,a numerical simulation method that applies to the confined flow,water jet flow and water sheet flow in the entire flow passage is developed.The unsteady air-water two-phase flow simulations are performed by adopting the SST k-? turbulence model and homogenous model.The sensitivities of the unsteady simulation results to mesh resolution and computational fluid dynamic solver time-step have been evaluated to discuss the effect of Courant-Friedrichs-Lewy condition and grid convergence index on the two-phase flow simulation.The numerical simulation of the hydrodynamic performance in the four-nozzle prototype Pelton turbine is carried out.The predicted results show that most of the hydraulic loss in the Pelton turbine appears during the rapid flow pattern transition process.During the transition process of confined flow to water jet at the nozzle outlet,the hydraulic loss decreases as the needle stroke increases and varies little with different water heads.A large proportion of hydraulic loss appears during the transition of water jet to water sheet flow inside the rotating buckets.The remaining kinetic energy carried by the outflow from the bucket under the nondesigned water head is the main reason for the efficiency loss in the Pelton turbine.The pressure fluctuation characteristic results in the rotating bucket show that as the water head increases,the water sheet spreads faster in the rotating bucket,which result in a shorter pressure fluctuation duration and a larger pressure fluctuation amplitude.Numerical simulations are carried out to analyze the flow mechanism of flow interference between the rotating bucket and water flow,and the quantitative effect of the flow interference on the unit’s hydraulic performance.The results show that during the bucket cut-in process,the damage for the water jet flow continuity leads to a negative pressure area at the contact place of bucket rear surface and water jet.The negative pressure area and squeezing action by the bucket rear surface would disturb the cutting water jet,which results in the occurrence of flow interference between the bucket rear surface and water jet.This flow interference intensified as the water head decreases.Thus it is the main reason for the hydraulic efficiency drops faster under low water head than the high water head condition.Another flow interference occurs between the bucket surface and outflow from the previous bucket.It is more likely to appear under a larger outflow angle or a higher water head condition.As the nozzle number increases,flow interference in the rotating bucket is more likely to occour in the multi-nozzle Pelton units.Based on the predicted output torque curve of a single bucket,a method is proposed to determine whether the flow interference would happen in a multi-nozzle unit.The quantitative results show that the flow interference in the bucket would cause a severe hydraulic efficiency drop and flow pattern change.Two water sheet formed by the successively two jet inflow would collide in the bucket and double the pressure fluctuation amplitude.