Loss evaluation and aerodynamics investigation of an aggressive intermediate turbine duct under off-design conditions

To maximize the performance of the intermediate turbine duct (ITD) under off-design conditions, the loss generation in a one-half stage turbine was quantified using entropy generation and the global entropy generation rate. The numerical results solved by the unsteady Reynolds-averaged Navier–Strokes equations were first verified. Then, the aerodynamic losses within the high-pressure turbine stage were evaluated by efficiency loss under nine operating conditions composed of three rotor speeds and three rotor tip gaps. Finally, the disturbance modes caused by the upstream wake were captured by the dynamic mode decomposition method. Different from the influence of tip gaps, losses of the high-pressure turbine and the ITD are due to the swirl angle display an opposite trend. Under the influence of the interaction between the tip leakage flow and the shroud flow of the ITD, the viscous dissipation and turbulent dissipation increase with a larger tip gap owing to the dominant counter-rotating vortices and secondary flow occurring near the upstream of the ITD shroud. In addition, a large gap seems to enlarge the swirl component of the inflow angle, especially over 80% passage height, leading to greater dissipation losses in these areas. At the ITD inlet, two pairs of counter-rotating vortices at the shroud and the hub are, respectively, captured by the axial velocity mode. Large tip gaps enhance endwall vortices near the shroud and make the up vortex pairs merge into one pair.