Aerodynamics design of two-stage vane-less counter-rotating turbinec

It is one of the most efficient ways to greatly improve aero-engines' performance by utilizing vaneless counterrotating turbine (VCRT) technology. To supply sufficient power, VCRT turns to be high Mach number, large flow angle at high-pressure turbine (HPT) rotor exit, and low blade camber angle, which increase difficulties to turbine design. As the axial velocity ratio of HPT rotor is much larger than the conventional ones, the optimal selection of VCRT velocity triangles based on theoretical analysis is developed, and how the efficiency varied by HPT stator/rotor exit flow angle is also figured out. The key points to design a high efficient practicable VCRT are to select velocity triangles that are characterized by low flow coefficient, high outlet flow angle and large axial velocity ratio of HPT rotor. Meanwhile, performance comparison between convergent blade and convergent-divergent blade shows the latter is more appropriate for VCRT.