Aerodynamics design and off-design condition analysis of 1+1/2 counter-rotating turbine

Compared with the conventional turbines, counter-rotating turbines have the advantages of high efficiency, light weight and small gyroscopic torque, and have been widely used in military and civil aviation engines. The 3D aerodynamics design of 1+1/2 vaneless counter-rotating turbine (VCRT) was carried out by commercial software, and numerical simulation was completed. The results showed that when the mass flow rate was 4.13 kg/s, the overall expansion ratio was 5.823. The high pressure turbine (HPT) expansion ratio was 2.641, and the low pressure turbine (LPT) expansion ratio reached 2.205. The VCRT isentropy efficiency was 88.70%, and the total output power was 1815.2 kW, which met the design requirements. The flow loss analysis of the VCRT at the design condition showed that the intensity of the dovetail shock wave at the trailing edge of HPT rotor was the highest at the 50% span, and gradually weakened to the root and tip of the blade. There was a little flow separation on the pressure surface of the LPT inlet. Moreover, the off-design conditions of the 1+1/2 counter-rotating turbine were analyzed. It was found that with the increase of the VCRT expansion ratio, the intensity of shock wave at the trailing edge of the HPT rotor gradually increased, and the flow separation of LPT was weakened. Therefore, the isentropic efficiency of HPT decreased, and the overall isentropic efficiency and LPT isentropic efficiency increased first and then kept unchanged. With the increase of the LPT rotational speed, the mass flow rate of VCRT was almost constant, the overall isentropic efficiency increased first and then decreased, and the output power increased gradually.