Research On Collaborative Optimization And Variable Load Dynamic Characteristics Of Low Head Tubular Turbine

In recent years,as the development of medium and high head hydropower resources has become saturated,attention has gradually shifted to low-head hydropower resources and tidal energy.Kaplan turbines have been widely used due to their superior hydraulic performance and economy.However,due to differences between the Kaplan model and the actual machine performance,it is not possible to fully demonstrate the operating performance of the actual machine through the model,which frequently leads to problems such as insufficient output and severe vibration in the unit after commissioning due to incorrect coordination relationships.In this study,based on dynamic mesh technology and considering the free liquid level of upstream and downstream reservoirs,a numerical method was used to optimize the coordination relationship of a Kaplan turbine under different heads.Non-steady-state numerical studies were carried out on the actual machine during load reduction and increase processes,and the flow characteristics and pressure pulsation characteristics inside the turbine during the load variation process were explored.The main results are as follows:(1)The numerical coordination relationship of a Kaplan turbine was optimized by fixing the blade angle and changing the guide vane opening,and the efficiency characteristics and internal flow characteristics of the turbine before and after the optimization were compared.The results show that changes in the coordination relationship have a greater impact on the water flow near the hub,and the change in coordination reduces the positive circulation near the hub at the outlet of the impeller,improving the backflow and vortex distribution state in the tailrace tunnel.The sensitivity of efficiency to changes in guide vane opening is higher under low-head conditions,and the coordination deviation is more obvious under low-head conditions.Through coordination optimization,the hydraulic losses in the impeller and tailrace tunnel were effectively reduced,resulting in increased efficiency at various operating points.(2)Non-steady-state numerical studies were conducted on a Kaplan turbine by simultaneously changing the blade angle and guide vane opening during load reduction and increase processes,and the internal flow characteristics during these processes were compared and analyzed.The results showed that during the load variation process,the drastic pressure changes inside the turbine mainly occurred in the impeller and guide vane areas.Changes in the guide vane opening and blade angle caused the deviation of the stagnation point of the water flow on the blade inlet,and the load reduction process aggravated the cavitation phenomenon on the back of the blade,while the load increase process resulted in a negative pressure zone at the front of the blade inlet.Analysis of the internal flow characteristics at the same load operating points during different load variation processes indicated that the difference in circulation between the inlet and outlet of the impeller and the difference in load between the front and back of the blade were larger during the load reduction process.The negative pressure zone inside the impeller was lower,indicating that the dynamic quality of the impeller stage during the load reduction process was relatively poor.As the load reduction process starts from the rated condition with better flow state,while the load increase process starts from the low-load condition with turbulent flow,the vortex distribution in the tailrace tunnel is overall better in the load reduction process than in the load increase process.(3)The time-domain and frequency-domain characteristics of pressure pulsation at various monitoring points inside the Kaplan turbine during load reduction and increase processes were compared and analyzed.The results showed that pressure pulsation inside the Kaplan turbine was mainly caused by low-frequency pressure pulsation induced by the tailrace tunnel vortex,impeller rotation frequency,and blade-passing frequencies.The main vibration area of the turbine was the impeller area,with the pressure pulsation transmitted to both upstream and downstream,decreasing in amplitude with increasing distance of transmission.Among them,low-frequency pressure pulsation had stronger transmission capability and posed a greater threat to the stability of the unit.Comparison of the different load variation processes showed that the main frequency of pressure pulsation during the load increase process was induced by the tailrace tunnel vortex,and its amplitude was much larger than that of the main frequency of the load reduction process.The tailrace tunnel vortex was the main reason for the more severe instability of the Kaplan turbine during the load increase process.