Numerical Investigation of Unsteady Performance of a High-Pressure Transonic Turbine Stage of a Small Engine

The unsteadiness caused by relative motion between stator and rotor of a turbine stage and its effect on performance is still an active research area. For better understanding of flow and performance, a highly loaded low aspect ratio HP transonic turbine stage with application to a small engine is considered for the present study. The geometry has a stator-to-rotor pitch ratio of 2.63 and is designed to operate at a total-to-total pressure ratio of 2.78. Unsteady three-dimensional CFD simulations were performed with the help of the commercial software ANSYS CFX. Cases were simulated at five different operating conditions to study the influence of pressure ratio and rotor speed on the rotor–stator interaction. Simulations are performed at design speed for design point (DP), low and high pressure ratios (LP and HP, respectively), and two cases at the design pressure ratio, but at rotor speed higher (105%) and lower (90%) than that of design speed (named HS and LS). The stator and rotor in the LP case are unchoked at midspan and at a high subsonic condition, whereas they remain choked in the rest of the cases. The HP case represents an operating point where the turbine is close to the limit load condition. It is observed that the stator exit flow field is affected by the moving blade row, causing total temperature fluctuations upstream of the rotor, of about 10% of the stage total temperature drop locally. The magnitude of this fluctuation is determined by the pressure ratio and rotor speed. The magnitude of fluctuations of total temperature averaged over the interface plane remains relatively smaller; for the case of LP, these fluctuations are observed to be marginal.