Suppression of secondary flows in an axial flow turbine rotor with a novel blade design concept

This paper presents a novel blade design concept about control of the secondary flow in axial turbines to improve the internal efficiency. A rotor blade for the high and intermediate pressure stage of a steam turbine was designed by one-dimensional inverse method incorporated with throughflow calculation. In the throughflow calculation, the axisymmetric Navier-Stokes equations are solved, assuming that the meridional flow is axisymmetric and viscous. To take into account the blade loading, a blade force is introduced as a body force to the governing equations. The blade force contains the inviscid blade effect only, namely, the pressure difference across a blade. The blade force acts in the direction perpendicular to a three-dimensional blade camber surface, which is constructed from camber lines stacked in blade height direction, so that the flow convects along the surface. The camber line is calculated by one-dimensional inverse method, based on a given blade loading distribution and the meridional velocity distribution from the throughflow calculation. The present throughflow calculation method was validated for a direct problem, using blade geometry of existing turbine rotor. The blade force with the camber surface in the present method was able to express the secondary flow inside the turbine by modifying the camber surface appropriately. As a result, the calculation result showed good agreement with the experimental result. The turbine rotor blade was redesigned with the present method, modifying the chord-wise blade loading distribution so as to suppress the development of the secondary flow. The way of modifying the chord-wise blade loading distribution is based on a new idea, which can be actualized by the present design method. It was confirmed that the secondary flow was successfully suppressed as intended in the designed rotor.