Experimental and numerical study of flow field structure in U-shaped channels with different bend sections

Abstract

Time-resolved particle image velocimetry is used to study the flow characteristics of rotating U-shaped channels with different types of bend sections: one with both inner and outer walls square, one with an inner circular wall and an outer square wall, and one with both inner and outer walls circular. The rotation number varies from 0 to 0.25, at a Reynolds number of 11 500. The present work aims at providing a detailed insight of the flow field occurring within a rotating U-shaped channel, typically resembling internal cooling channel embedded into first stages of turbine blades in aeroengines. A validated numerical simulation is carried out to determine the flow mechanism. A proper orthogonal decomposition and the Ω-criterion vortex identification method are used to study the vortical distribution and flow characteristics. The results show that a bend with both inner and outer square walls produces corner vortices on the outside of the bend section, and both the separation vortex and reattachment vortex are larger than those of the other two geometrical configurations. In the channels with a circular inner wall of the bend, separation is delayed, and both the separation vortex and reattachment vortex are smaller. When both walls of the bend are square, the peak Reynolds shear stress is twice than when they are both circular. With the increase in the rotation number, the size of vortical structures changes. The Coriolis force also changes the relative size of the secondary flow in the bend section, and the vortex near the leading surface becomes larger.