The Influence Of Internal Flow Leakage On The Performance Of Twin Screw Hydraulic Turbine

Twin-screw hydraulic turbine is widely used in marine engineering,petroleum,environmental protection,electric power and other fields because of its advantages such as small pulsation,the ability to transport gas-liquid two-phase flow and the suitability to transport high viscosity medium.However,the internal leakage of the twin-screw hydraulic turbine is different in the actual operation,which results in the difference of volume efficiency.Taking two-thirds of tooth double screw hydraulic turbine as the research object,based on the principle of leak leakage and leakage passage of the geometric model for turbine flow mathematical derivation in detail,and using the UG software,SCORG and Pumplinx to double screw hydraulic turbine system for 3D modeling and numerical simulation,systematically studied the different structure parameters and the working condition of different internal flow field of double screw hydraulic turbine,axial force,leakage flow and the influence of volumetric efficiency.The main work and creative results of this research are as follows:(1)Based on the sealing characteristics of the screw turbine,the basic structure design of the twin screw hydraulic turbine was completed according to the mathematical formula of the screw profile and the tooth profile curve of the screw screw end face.(2)The basic principle of twin-screw hydraulic turbine leakage is analyzed,the theoretical formula of screw turbine leakage based on different clearance leakage channels is established by using the theory of fluid mechanics,and the calculation formulas of various performance parameters of twin-screw hydraulic turbine are given.(3)Numerical simulation was used to reveal the distribution law of flow field inside the twin-screw hydraulic turbine.It is found that the screw is divided into four sealing cavities of different pressure levels with the meshing line as the boundary,and decreases step by step with the movement of the sealing cavity.The high speed flow field is mainly concentrated in the circumferential gap,inter-tooth gap and inter-tooth gap area,among which the inter-tooth gap has the most obvious effect on the leakage.The negative rotor is more prone to deformation than the positive rotor,and the leakage phenomenon will be more serious.(4)For the first time,the influence of geometric dimensions such as intertooth gap,circumferential gap and gap width on the performance of the twin-screw hydraulic turbine was studied.The results show that the pressure distribution trend of the master and slave screw is consistent,and the pressure,velocity and leakage flow at the gap all increase with the increase of the gap.For the same screw,with the increase of the clearance,the leakage quantity tends to rise first and then fall,and the leakage quantity of the clearance between the teeth is much larger than that between the circumference and the side of the teeth.(5)The effects of inlet and outlet pressure difference,rotation speed and medium viscosity on the leakage of the twin-screw hydraulic turbine were studied for the first time.The results show that the velocity of leakage flow increases with the increase of inlet and outlet pressure difference and decreases with the increase of medium viscosity.The leakage flow increases with the increase of inlet and outlet pressure difference,rotating speed and medium viscosity,but decreases step by step along the screw axis.The volumetric efficiency decreases with the increase of inlet and outlet pressure difference,and increases with the increase of main screw speed and medium viscosity.In addition,it is found that the pressure difference between adjacent sealing Chambers is the main reason for the leakage of the screw turbine.