Influence factors of clearance leakage flowrate and clearance hydraulic axial force of pump-turbine

Abstract

The clearance flow of pump-turbine has significant influence on hydraulic forces of the runner, which is related to safety of the unit. To clarify the influence factors, clearance flowrate and clearance hydraulic axial force of a prototype pump-turbine were analyzed by 3D CFD simulations. The results show that the rotational speed, and the smallest radial sizes of clearance inlet, outlet, and the seal ring sizes are the determinant factors for clearance leakage flowrate, while the rotational speed and the radial size at clearance inlet are the major factors for clearance axial force. The rotational speed of rotating surfaces mainly affects the strength of rotating shear flow, while geometric sizes mainly affect the hydraulic loss of leakage flow. By assuming that the clearance region as several ring pipes, we obtained the simplified calculation formulas for estimating clearance leakage flowrate and clearance axial force. The flowrate can be expressed as a function of the water head difference across the clearance region, the rotational speed, and the sizes of clearance inlet, outlet and sealing ring, while the clearance axial force is a function of the inlet pressure and the square of both radius and rotational speed. The formula calculations agree with the numerical simulations well. This paper can provide a reference for runner clearance size design.